| HAO HATA KATA TAI MIEL NI UT ALAT DAN MATA US009893227B2

( 12 ) United States Patent Sampath et al .

( 10 ) Patent No . : US 9 , 893 , 227 B2 ( 45 ) Date of Patent : Feb . 13 , 2018

( 54 ) ENHANCED DEEP ULTRAVIOLET PHOTODETECTOR AND METHOD THEREOF

( 58 ) Field of Classification Search CPC . . . . . . . . . . . . . . . . HO1L 31 / 107 ; HO1L 25 / 167 ; HOIL

27 / 14643 ; HO1L 31 / 02024 ; ( Continued )

@ ( 71 ) Applicant : U . S . Army Research Laboratory ATTN : RDRL - LOC - I , Adelphi , MD ( US )

( 72 ) Inventors : Anand Venktesh Sampath , Montgomery Village , MD ( US ) ; Michael Wraback , Germantown , MD ( US ) ; Paul Shen , North Potomac , MD ( US )

( 56 ) References Cited U . S . PATENT DOCUMENTS

4 , 731 , 641 A * 3 / 1988 Matsushima . . . . . . . . . . . HOLL 31 / 18 257 / 189

4 , 935 , 795 A 6 / 1990 Mikawa ( Continued )

FOREIGN PATENT DOCUMENTS ( 73 ) Assignee : The United States of America as

represented by the Secretary of the Army , Washington , DC ( US )

EP EP

0150564 A3 5 / 1986 0082787 A16 / 1987

( Continued ) ( * ) Notice : Subject to any disclaimer , the term of this

patent is extended or adjusted under 35 U . S . C . 154 ( b ) by 0 days .

OTHER PUBLICATIONS

( 21 ) Appl . No . : 15 / 181 , 074 Konstantinov , A . O . et al . , “ Ionization rates and critical fields in 4H silicon carbide ” , Jul . 7 , 1997 , Applied Physics Letters 71 , No . 1 , pp . 90 - 92 . *

( Continued ) ( 22 ) Filed : Jun . 13 , 2016 ( 65 ) Prior Publication Data

US 2016 / 0284919 A1 Sep . 29 , 2016 Primary Examiner — Olik Chaudhuri Assistant Examiner — Pauline Vu ( 74 ) Attorney , Agent , or Firm — Lawrence E . Anderson

( 63 ) Related U . S . Application Data

Continuation - in - part of application No . 14 / 285 , 964 , filed on May 23 , 2014 , now Pat . No . 9 , 379 , 271 .

( Continued ) ( 51 ) Int . Cl .

HOIL 31 / 107 ( 2006 . 01 ) HOIL 31 / 028 ( 2006 . 01 )

( Continued ) ( 52 ) U . S . CI .

CPC . . . . . HOIL 31 / 1075 ( 2013 . 01 ) ; HOIL 31 / 0336 ( 2013 . 01 ) ; HOIL 31 / 03044 ( 2013 . 01 ) ; HOIL 31 / 03046 ( 2013 . 01 ) ; YO2E 10 / 544 ( 2013 . 01 )

( 57 ) ABSTRACT A photodiode for detecting photons comprising a substrate ; first semiconducting region suitable for forming a contact thereon ; a first contact ; a second semiconducting region comprising an absorption region for the photons and being formed of a semiconductor having one or more of a high surface recombination velocity or a high interface recombi nation velocity ; a second contact operatively associated with the second region ; the first semiconducting region and the second semiconducting region forming a first interface ; the second semiconducting region being configured such that reverse biasing the photodiode between the first and second contacts results in the absorption region having a portion

( Continued )

Anti - reflective coating 39

BIASING ; - The bias range of operation will depend upon the entire structure . For a 500 nm thick region 32 the bias is * 150V . The E - field in the region 32 is anticipated between 2 - 3 MV / cm during operation ,

35 : 117

38 "

0 * = 2E18 cm3 AlGaN 1 -

. . = 2 x 1018 cm ? SiC , - 35 nm - thick

36 31

- - .

n = 2E46 cm sic , - 500 nm 1 - - -

33 p - metal contact layer

D - Sic 33 n - type , Si - face 4H - SiC substrate 34

with a 49 miscut

US 9 , 893 , 227 B2 Page 2

depleted of electrical carriers and an undepleted portion at the reverse bias point of operation ; the undepleted portion being smaller than the absorption depth for photons ; whereby the depletion results in the creation of an electric field and photogenerated carriers are collected by drift ; and a method of making .

20 Claims , 18 Drawing Sheets

. . . . H 01L 31 / 1075 Related U . S . Application Data

( 60 ) Provisional application No . 62 / 174 , 710 , filed on Jun . 12 , 2015 , provisional application No . 61 / 827 , 079 , filed on May 24 , 2013 .

( 51 )

2006 / 0202297 A1 * 9 / 2006 Ishimura . . . . . . . . . . HO1L 31 / 02161 257 / 436

2006 / 0263923 A1 * 11 / 2006 Alfano . . . . . . . . . . . . . . . . . . . B82Y 10 / 00 438 / 48

2006 / 0273421 A1 * 12 / 2006 Yasuoka . . . . . . . . . . . . HO1L 31 / 0232 257 / 438

2007 / 0093073 A1 * 4 / 2007 Farrell , Jr . . . . . . . . . . . . . . B82Y 20 / 00 438 / 763

2007 / 0194300 A1 * 8 / 2007 Ibbetson . . . . . . . . . . . . . . . . . HO1L 29 / 88 257 / 30

2007 / 0267653 A1 * 11 / 2007 Yoneda . . . . . . . . . . . . . . . HO1L 31 / 0304 257 / 186

2008 / 0191240 A1 * 8 / 2008 Yagyu 257 / 186

2008 / 0230862 A1 * 9 / 2008 Singh . . . . . . . . . . . . . . . . . . HOIL 27 / 1446 257 / 436

2008 / 0283822 Al * 11 / 2008 Yui . . . . . . . . . . . . . . . . . . . . . . . . . HOIL 33 / 06 257 / 13

2009 / 0065900 A1 * 3 / 2009 Saito . . . . . . . . . . . . . . . . . . . . . . . HO1L 33 / 16 257 / 615

2009 / 0315073 A1 * 12 / 2009 Shi . . . . . . . . . . . . . . . . . HO1L 31 / 02027 257 / 185

2009 / 0325339 Al * 12 / 2009 Niiyama . . . . . . . . . . . . . . B82Y 20 / 00 438 / 72

2010 / 0006780 A1 * 1 / 2010 Metcalfe . . . . . . . . . . GO2F 2 / 004 250 / 504 R

2010 / 0187550 A1 * 7 / 2010 Reed . . . HO1L 33 / 18 257 / 98

2010 / 0220761 Al * 9 / 2010 Enya . . HOIS 5 / 3202 372 / 45 . 01

2011 / 0001392 A1 * 1 / 2011 Masmanidis . . . . . . . . B81B 3 / 0021 310 / 316 . 03

2011 / 0024768 A1 * 2 / 2011 Veliadis . . . . . . . . . . . . HO1L 27 / 14609 257 / 77

2011 / 0291108 Al * 12 / 2011 Shen . . . . . . . . . . . . . . HO1L 31 / 03687 257 / 77

2011 / 0291109 Al * 12 / 2011 Wraback . . . . . . . . . . . HOIL 31 / 1075 257 / 77

2012 / 0016538 Al * 1 / 2012 Waite . . . . . . . . . . . . . . . . GO1C 21 / 20 701 / 3

2013 / 0016203 A1 * 1 / 2013 Saylor . . . . . . . . . . . . . . . G06K 9 / 00604 348 / 78

2013 / 0032860 A1 * 2 / 2013 Marino . . . . . . . . . . . . . HO1L 29 / 66462 257 / 194

2014 / 0084146 A1 3 / 2014 Boisvert 2015 / 0311375 A1 10 / 2015 Shen et al .

Int . Cl . HOIL 31 / 0304 ( 2006 . 01 ) HOIL 31 / 0336 ( 2006 . 01 ) Field of Classification Search CPC . . . . . . . . . . . HO1L 31 / 02027 ; HO1L 31 / 1075 ; HOIL

31 / 0304 ; HOIL 31 / 03044 ; HOIL 31 / 03048 ; HOIL 31 / 0376 ; HOIL 31 / 03765 ; HOLL 29 / 66113 ; HOIL

31 / 03046 ; HOIL 31 / 105 ; HO1L 31 / 03687 ; HO1L 31 / 1804 ; HO1L 31 / 1812

See application file for complete search history .

( 58 )

. . . . . . . . . . . . . . . . . . .

( 56 ) References Cited U . S . PATENT DOCUMENTS

FOREIGN PATENT DOCUMENTS EP EP WO

1833095 A1 * 9 / 2007 . . . . . . . HO1L 31 / 03529 2175497 A2 4 / 2010

WO 2013058715 A1 * 4 / 2013 . . . . . . . . . . . HOIL 31 / 107

OTHER PUBLICATIONS

4 , 984 , 032 A * 1 / 1991 Miura . . . . . . . . . . . . . HOLL 31 / 03042 257 / 186

5 , 212 , 395 A * 5 / 1993 Berger . . . . . . . . . . . . . HO1L 31 / 02161 136 / 256

5 , 311 , 047 A * 5 / 1994 Chang . . . . . . . . . . . . . HOIL 31 / 1105 257 / 187

5 , 394 , 005 A * 2 / 1995 Brown . . . . . . . . . . . . . . HOIL 31 / 03528 257 / 461

5 , 569 , 942 A * 10 / 1996 Kusakabe . . . . . . . . . . . HOIL 31 / 1075 257 / 186

6 , 137 , 123 A * 10 / 2000 Yang . . . . . . . . . . . . . . . . HO1L 31 / 03046 257 / 184

6 , 326 , 654 B1 12 / 2001 Ruden 6 , 838 , 741 B2 1 / 2005 Sandvik 7 , 049 , 640 B2 5 / 2006 Boisvert 7 , 902 , 545 B2 * 3 / 2011 Csutak B82Y 20 / 00

257 / 14 8 , 269 , 222 B2 9 / 2012 Shen et al . 8 , 269 , 223 B2 9 / 2012 Wraback

2003 / 0226952 Al * 12 / 2003 Clark . . . . . . . . . . . . . . . HO1L 31 / 02027 250 / 214 . 1

2004 / 0108530 A1 * 6 / 2004 Sandvik . . . . . . . . . . . HO1L 31 / 02322 257 / 290

2004 / 0251483 A1 * 12 / 2004 Ko . . . . . . . . . . . . . . . . . . HOIL 31 / 1075 257 / 292

2005 / 0006678 A1 * 1 / 2005 Tanaka . . . . . . . . . . HOIL 31 / 1075 257 / 292

2005 / 0098884 AL 5 / 2005 Cheng 2005 / 0167709 A1 * 8 / 2005 Augusto . . . . . . . . . . . HO1L 27 / 14643

257 / 292 2005 / 0218414 Al * 10 / 2005 Ueda . . HOIL 21 / 02082

257 / 94 2006 / 0001118 A1 * 1 / 2006 Boisvert . . . . . . . . . . . HO1L 31 / 1075

257 / 438 2006 / 0017129 A1 * 1 / 2006 Nakaji . . . . . . . . . . . . . . HOLL 31 / 03046

257 / 438 2006 / 0186501 A1 * 8 / 2006 Ishimura . . . . . . . . . . . . . . . HOIL 31 / 107

257 / 436

Xiangyi Guo et al . , “ Demonstration of ultraviolet separate absorp tion and multiplication 4H - SiC avalanche photodiodes ” , Jan . 1 , 2006 , in IEEE Photonics Technology Letters , vol . 18 , No . 1 , pp . 136 - 138 . * X . Guo et al . , “ Performance of Low - Dark - Current 4H - SiC Ava lanche Photodiodes With Thin Multiplication Layer " , Sep . 2006 , in IEEE Transactions on Electron Devices , vol . 53 , No . 9 , pp . 2259 2265 . * Sandvik , P . et al . , " SiC deep ultraviolet avalanche photodetectors ” , Oct . 2010 , General Electric Global Research Niksayuna NY . * Lin Qi , K . , et al . , “ UV - Sensitive Low Dark - Count PureB Single Photon Avalanche Diode , ” IEEE Transactions on Electron Devices , vol . 61 , No . 11 , 3768 - 3774 , Nov . 2014 . Ariane L . Beck , et al . “ Quasi - Direct UV / Blue GaP Avalanche Photodetectors , ” IEEE Journal of Quantum Electronics , vol . 40 , No . 12 , 1695 - 1699 , Dec . 2004 . Poster presented at the IEEE Lester Eastman Conference on High Performance Devices on Aug . 7 , 2013 entitled “ Aluminum Gallium Nitride / Silicon Carbide Separate Absorption and Multiplication Avalanche Photodiodes . "

US 9 , 893 , 227 B2 Page 3

( 56 ) References Cited

OTHER PUBLICATIONS

Shao , Zhen , et al . , “ High - Gain AIGN Solar - Blind Avalanche Photodiodes , ” IEEE Electron Device Letters , vol . 35 , No . 3 , pp . 372 - 374 , Mar . 2014 . Sun , L . et al . , “ AIGN Solar - blind Avalanche Photodiodes with High Multiplication Gain ” , Applied Physics Letters - 97 ( 191103 ) Nov . 2010 DOI : 10 . 1063 / 1 . 3515903 . Suvarna , P . , “ Design and Growth of Visible - Blind and Solar - Blind II - N APDs on Sapphire Substrates , ” J . Electron . Mater . 42 , 854 - 858 ( 2013 ) . X . Bai , et al . “ High Detection Sensitivity of Ultraviolet 4H - SIC Avalanche Photodiodes ” , IEEE J . Quantum Electron 43 , 1159 ( 2007 ) . L . E . Rodak , A . V . Sampath , C . S . Gallinat , Y . Chen , Q . Zhou , J . C . Campbell , H . Shen , and M . Wraback . “ Solar - blind AlxGal - xN / AIN / SiC photodiodes with a polarization - induced electron filter . " Applied Physics Letters 103 No . 7 ( 2013 ) : 071110 . Y . Zhang , et al . , “ GaN ultraviolet avalanche photodiodes fabricated on free - standing bulk GaN substrates , ” Phys . stat . sol . ( c ) 5 , No . 6 , 2290 - 2292 ( 2008 ) . Anand V . Sampath , et al . , “ P - type Interface Charge Control Layers for Enabling GaN / SIC Separate Absorption and Multiplication Avalanche Photodiodes ” , Applied Physics Letters , 101 , ( 2012 ) 093506 . A . V . Sampath , L . E . Rodak . Y . Chen , Q . Zhou . J . C . Campbell , H . Shen and M . Wraback , “ Enhancing the Deep Ultraviolet Perfor mance of 4H - SiC Based Photodiodes " ECS Transactions , 61 ( 4 ) 227 - 234 ( 2014 ) . S . Soloviev , A . Vert , J . Fronheiser , P . Sandvik , “ Positive Tempera ture Coefficient of Avalanche Breakdown Observed in a - Plane 6H - SiC Photodiodes , " Materials Science Forum vols . 615 - 617 ( 2009 ) pp . 873 - 876 .

Handin Liu , et al . “ 4H - SIC PIN Recessed - Window Avalanche Photodiode With High Quantum Efficiency ” IEEE Photonics Tech nology Letters 20 , No . 18 , ( 2008 ) 1551 . E . Munoz , “ III nitrides and UV detection , " J . Phys . : Condens . Matter , 13 , 7115 - 7137 ( 2001 ) . Sun , L . et al . “ AlGaN solar - blind avalanche photodiodes with high multiplication gain , ” J . Electron . Mater . 42 , 854 ( 2013 ) . McClintock , R . , et al . “ Avalanche multiplication in AlGaN based solar - blind photodetectors , ” Appl . Phys . Lett . , 87 , ( 241123 ) ( 2005 ) . Tut , T . et al . “ AlxGal - xN - based avalanche photodiodes with high reproducible avalanche gain , ” Appl . Phys . Lett . , 90 , ( 163506 ) ( 2007 ) . Yoo , Dongwon , et al . “ Al x Ga 1 - x N Ultraviolet Avalanche Photodiodes Grown on GaN Substrates . ” Photonics Technology Letters , IEEE 19 , No . 17 ( 2007 ) : 1313 - 1315 . X . Bai , “ High Detection Sensitivity of Ultraviolet 4H - SIC Ava lanche Photodiodes , ” IEEE J . Quantum Electron 43 , 1159 ( 2007 ) . H . Liu , “ 4H - SIC PIN Recessed - Window Avalanche Photodiode With High Quantum Efficiency , ” IEEE Photonic Technol . Lett . 20 , 1551 ( 2008 ) . A . Sciuto , “ High responsivity 4 H — Si C Schottky UV photodiodes based on the pinch - off surface effect , ” Appl . Phys . Lett . 89 , 081111 ( 2006 ) . X . Xin , “ Demonstration of 4H - SiC visible - blind EUV and UV detector with large detection area ” Electron . Lett . , 41 ( 2005 ) . A . V . Sampath , et al . “ High quantum efficiency deep ultraviolet 4H - SiC photodetectors , ” Electronics Letters , vol . 49 , Issue 25 , Dec . 5 , 2013 , p . 1629 - 1630 . Electronic archiv : “ New Semiconductor Materials . Characteristics and Properties , ” NSM Archive — Silicon Carbide ( SIC ) Band struc ture , w . © 1999 - 2014 . Ahrenkiel , R . K . , et al . , “ An optical technique for measuring surface recombination velocity ” Solar Energy Materials & Solar Cells 93 ( 2009 ) 645 - 649 .

* cited by examiner

10

U . S . Patent

BIASING : - The bias range

19 Semi

of operation will depend

transparent

upon the entire structure .

metal contact

For a 500 nm thick in

region the bias is ~ 150V . The E - field in the i - region

is anticipated between 2 - 3

multiplications MV / cm during operation . Ilayer 12 15

LXII .

11 na 2018 cm 3 SIC , ~ 35 nm

Preferred thickness range is between 20 - 120 nm . Preferred doping range is between 9E17 to 5E18 The thickness and doping of the illuminated

n - Sic layer are such that it will be nearly depleted at high bias near avalanche breakdown , i . e . , the thickness of the neutral and quasi - neutral regions in the nt region will become smaller than absorption length of photons in the deep ultraviolet spectral

range .

OXER

Feb . 13 , 2018

RAI

. . * *

12

. .

- .

- - -

-

.

. .

- -

. .

-

.

n = 1E16 cm - 3 Sic , ~ 500 nm

- .

- -

Sheet 1 of 18

.

- -

- - - - -

-

-

. . . .

-

- -

- -

- -

. . -

13 p - metal contact layer

The effective minority diffusion length of holes has not been

experimentally validated yet . Current experimental data suggests that it is ~

50 nm or shorter

14

FIG . 1

Grown on an n - type , Si - face , 4H - SIC

substrate with a 49 miscut

US 9 , 893 , 227 B2

U . S . Patent Feb . 13 , 2018 Sheet 2 of 18 US 9 , 893 , 227 B2

Gain 20

dark

220 nm illumination

160

120 TTTTT 80 Reverse Bias ( V )

40

0 Www ww .

?? ?? ?? Current Density ( A / cm ” ) FIG . 2

U . S . Patent atent

0 . 2

i

146 V

146 V

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* *

* * * w * *

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* * * *

143V

* * * * * 2 *

. . *

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Feb . 13 , 2018

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Responsivity ( Amps / watt )

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* * * *

280 280

* - - - * * - -

* * * *

130 V

* * *

320

320 360 Wavelength ( mm )

Sheet 3 of 18

125V

* * * * * * * * * * * * *

. . . * * * * * * *

* *

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FIG . 3

200

5 V

op 240 280 320 360 400 Wavelength ( nm )

US 9 , 893 , 227 B2

80 70

30 nm

25 nm

atent

- 15 nm 5 nm 5 nm

Feb . 13 , 2018

Quantum Efficiency ( % )

tttttt Om

20

Sheet 4 of 18

O tempore 200 240

360

400

280 320 Wavelength ( nm )

FIG . 4

Calculated Q . E . of a n - i - p Sic diode with increasing depletion of the top illuminated n - layer .

Nanometers abbreviated as nm .

US 9 , 893 , 227 B2

p - i - n

U . S . Patent atent

90

00

W

•

w

wo w

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ee * S

* * * *

* * * * * * * * *

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Feb . 13 , 2018

Gain

*

n - i - p w

Sheet 5 of 18

*

Sheet 5 01 18

* *

*

O t

. 200 240 280 320 360

FIG . 5

Wavelength ( nm ) Calculated gain in a Sic p - i - n diode as a function of photon wavelength at

constant reverse bias for illumination of the n - side down and p - side down .

US 9 , 893 , 227 B2

.

atent

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• • • • • . .

. . .

|

??? Responsivity ( A / w )

70v - - 100V • • • 110V

- 120 = = = = 130V

140v = 150v • • • 160v

Feb . 13 , 2018

• • • • • • • • • • • 1

. .

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. . .

Sheet 6 of 18

tent

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_ g ,

Calculated t ,

FIG . 6

0 . 0 + TTTTTTTT

200 240 280320 _ 360 _ 400 wavelength ( mm ) Modeling Gain Enhancement in DUV . Calculated responsivity in the DUV - APD with increasing bias .

US 9 , 893 , 227 B2

p - Sic p - Sic

i

- SiC i - SiC

n - Sic

atent

Math

2 . 5X10°

DUV Absorption

Feb . 13 , 2018

1 . 5X10

e w

Electric Field ( V / cm )

miattoon

Sheet 7 of 18

5 . 0X10

0

1000

2000

3000

4000

5000

6000

7000

Distance ( 8 )

FIG . 7

US 9 , 893 , 227 B2

Electric Field distribution in DUV - APD

U . S . Patent

630 Anti - reflective coating 39

BIASING : - The bias range of operation will depend upon the entire structure . For a 500 nm thick region

32 the bias is ~ 150V . The E - field in the region 32 is

anticipated between 2 - 3 MV / cm during operation .

T & RUR 35

17

38

Feb . 13 , 2018

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APARTXA #

nt = 2E18 cm - 3 Alog Ga 0 .

2N

1 * = 2

nta 2 x 1018 cm 3 Sic , ~ 35 nm - thick

31

*

za

. * * *

n = 1516 cm 3 Sic ,

500

www . si

Sheet 8 of 18

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YYYYYY YYYY p - SiC

33

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n - type , Si - face 4H - Sic substrate 34 with a 49 miscut

US 9 , 893 , 227 B2

FIG . 8

the absorption depth of photons at the wavelength of interest for detection .

U . S . Patent

a quasi - neutral layer and a neutral layer

Optional semi - transparent metal or conducting or semi conducting transparent layer 88

Feb . 13 , 2018

- 54 | a second region where

absorption occurs ; further comprising at the reverse bias operating point a quasi neutral layer , a neutral layer , a

quasi - neutral layer , a neutral layer , and a depletion layer

Optional multiplication layer

Sheet 9 of 18

A first p - type layer located above the substrate suitable for forming a p - metal contact SUBSTRATE

US 9 , 893 , 227 B2

FIG . 9

60

U . S . Patent

the absorption depth of photons at the wavelength of interest for detection .

t

a quasi - neutral layer and a neutral layer

Optional semi - transparent metal or conducting or semi conducting transparent layer 68

65

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Feb . 13 , 2018

64 | a second region where

absorption occurs ; further comprising at the reverse bias operating point a quasi neutral layer , a neutral layer , a

quasi - neutral layer , a neutral layer , and a depletion layer

Optional multiplication layer

Sheet 10 of 18

62

A first n - type layer located above the substrate suitable for forming a n - metal contact

61

SUBSTRATE

US 9 , 893 , 227 B2

FIG . 10

U . S . Patent Feb . 13 , 2018 Sheet 11 of 18 US 9 , 893 , 227 B2

substrate 2 , 719 um

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - . . . . . . . . . . . . . . . . . - - - -

Measured AI , N impurity concentration p * - SIC

Depth ( um )

theranh 0 . 572 um . vt etw . . . . graded n - Sic n - Sic

0 . 035um 0 . 117um - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 11 X - : - - : - - - - - - - - - - - - - - - - - - - - - - : - : - : - : - : - : - : - : - : - : - : - : - : - : - : - : - . - . - . - . - . - . - - -

???????????????????????????????????????????????????????????????????????? ecor de ouderen de & D ecor on the one 1013 14 & 1019 1020 1019 1015 1024 ?????? ?? ?? ?? ???? ) Al ( cm N ( cm )

nt - SiC

FIG . 11

100

U . S . Patent

106

104

N * - transparent window layer 105

1109

Feb . 13 , 2018

second region in which the absorption of photons occurs and from where

photogenerated carriers are swept out .

n - layer 103

|

r107

First interface 109

p - layer

102

Sheet 12 of 18

Substrate 101

FIG . 12

US 9 , 893 , 227 B2

U . S . Patent Feb . 13 , 2018 Sheet 13 of 18 US 9 , 893 , 227 B2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

117

n - layer Substrate 111 ?

p - transparent window layer 115 p - layer 113 112

First interface 119 P116 7116

114 110

second region in which the absorption of photons occurs and photogenerated carriers from where

are swept out .

FIG . 13

atent

50 nm n - A10 . 80 Gao . 20N 50 nm Al2 . 80G20 . 20N 120 nm AIN

Feb . 13 , 2018

350 nm n - SIC 2000 nm n - SIC

Sheet 14 of 18

Bulk n - SiC Substrate FIG - 14

US 9 , 893 , 227 B2

US 9 , 893 , 227 B2

Wavelength ( nm )

CIC 1 200 220 240 260 280 300 320 340 360 380 400

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Feb . 13 , 2018

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in - sic , 480 nm thick , [ N ] = 1x1016 cm

m

Sheet 16 of 18

p - Sic , 2000 nm thick , [ Al ] = 2x1018 cm3 123 Sic Substrate

124

US 9 , 893 , 227 B2

FIG . 16

U . S . Patent Feb . 13 , 2018 Sheet 17 of 18 US 9 , 893 , 227 B2

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U . S . Patent

FIG . 18

Boundary interface in FIG .

1 , 8 , 9 , 10

absorption depth ( varies with

photon energy )

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diffusion length

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area of depletion ( varies with bias )

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The minimal depletion width ( distance ) extends from the first interface to at

least the sum of the absorption depth and the diffusion length from the boundary interface .

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Enlarged view

US 9 , 893 , 227 B2

ENHANCED DEEP ULTRAVIOLET addition to being large and fragile , PMTs require the use of PHOTODETECTOR AND METHOD expensive filters to limit the bandwidth of detection . While

THEREOF APDs can be more compact , lower cost and more rugged than the commonly used PMTs , commercially available

CROSS - REFERENCE TO RELATED devices such as silicon ( Si ) single photon counting APDs APPLICATIONS ( SPADs ) have poor DUV single photon detection efficiency .

Group III - nitride avalanche detectors can presumably be This application claims priority of U . S . Provisional Appli - widely functional between 200 nm and 1900 nm ( i . e . infra

cation 62 / 174 . 710 filed Jun . 12 . 2015 , entitled “ Enhanced red to ultraviolet radiation ) . Aluminum gallium nitride Deep Ultraviolet Photodetector and Method Thereof " by 10 ( Al , Ga1 - XN ) photodetectors can take advantage of a sharp Anand Sampath , Paul Shen and Michael Wraback filed Jun . and tunable direct band gap to achieve high external quan 12 , 2015 , the contents of which are incorporated herein by tum efficiency and avalanche multiplication in Al Gal - xN reference . This application also claims priority to U . S . patent based p - i - n diodes has been reported . See , e . g . , L . Sun , J . application Ser . No . 14 / 285 . 964 , filed May 23 , 2014 . entitled Chen . J . Li , H . Jiang , “ AlGaN Solar - blind Avalanche Pho “ Variable Range Photodetector and Method Thereof " by 15 todiodes With High Multiplication Gain , ” Appl . Phys . Lett . . Paul Shen , et al . ( ARL 13 - 27 ) , which was published as U . S . 97 , ( 191103 ) ( 2010 ) and P . Suvarna , M . Tungare , J . M . Pub . Appl . No . 2015 / 0311375 on Oct . 29 , 2015 , and which Leathersich , P . Agnihotri , F . Shahedipour - Sandvik , L . D . in turn claims priority to U . S . Provisional Patent Application Bell , and S . Nikzad , “ Design and Growth of Visible - Blind Ser . No . 61 / 827 , 079 entitled “ Photodetector With Polariza - and Solar - Blind III - N APDs on Sapphire Substrates , ” J . tion Induced Electron Filter And Method Thereof . " filed 20 Electron . Mater . 42 , 854 ( 2013 ) ) , both of which are herein May 24 , 2013 , both of which are incorporated herein by incorporated by reference . reference . However , this approach is limited by the difficulty in

doping high AIN mole fraction alloys p - type , and a very GOVERNMENT INTEREST large breakdown electric field for high AIN mole fraction

25 that implies higher voltage operation and greater suscepti The embodiments herein may be manufactured , used , bility to dark current associated with defects in the material .

and / or licensed by or for the United States Government Therefore , a need remains for low cost , compact , high without the payment of royalties thereon . sensitivity , low dark current / dark count rate photodetectors

operating in the ultraviolet spectrum . BACKGROUND OF THE INVENTION 30 Silicon Carbide ( SiC ) photodetectors have emerged as an

attractive candidate for DUV pin photodetectors and APDs I . Field of the Invention due to their very low dark currents , small k factor , and high The present invention relates to photodetectors and meth gain . State - of - the - art SiC APDs employing a recessed top

ods for making such photodetectors . window exhibit peak quantum efficiency ( QE ) of 60 % at 268 II . Description of Related Art 35 nm and k factor of - 0 . 1 , and dark current of 90 PA at a gain In avalanche photodiodes ( APDs ) or photodetectors , of 1000 . See , X . Bai , X . Guo , D . C . Mcintosh , H . Liu , and

incoming light is used to generate carriers ( i . e . , free elec J . C . Campbell , “ High Detection Sensitivity of Ultraviolet trons or holes ) that are collected as current . Semiconductor 4H - SiC Avalanche Photodiodes ” , IEEE J . Quantum Elec materials are selected for photodiodes based upon the wave - tron 43 , 1159 ( 2007 ) , herein incorporated by reference . length range of the radiation that is desired to be utilized or 40 These dark currents are more than three orders of magnitude detected . APDs are operated at high reverse - bias voltages lower than what has been reported for Al , Ga N based where avalanche multiplication takes place . The multiplica - APDs . See , e . g . , L . Sun , J . Chen , J . Li , H . Jiang , “ AlGaN tion of carriers in the high electric field depletion region in Solar - blind Avalanche Photodiodes With High Multiplica these structures gives rise to internal current gain . In linear tion Gain , ” Appl . Phys . Lett . , 97 , ( 191103 ) ( 2010 ) , herein mode operation , the output photo - induced current within the 45 incorporated by reference . However , the responsivity of APD is linearly proportional to the illuminating photon flux , these devices diminishes at wavelengths shorter than 260 nm with the level of gain increasing with reverse bias . Impor - due to increasing absorption and carrier generation in the tantly , the dark current that flows through the APD also tends illuminated doped layer of this device and the short effective to increase with increasing reverse bias . When biased suf - diffusion length of minority carriers in this region in the ficiently above the breakdown voltage , commonly referred 50 presence of a high density of surface states through which to as excess bias , the APD can have sufficient internal gain these photogenerated carriers recombine . so that an incident photon can induce a large and self - In general , the short wavelength response in pin detectors sustaining avalanche . This operating scheme is often associated with detection of photons having energies much referred to as Geiger mode and the diode along with greater than the band gap of a semiconductor having a high enabling circuitry may be referred to as a single photon - 55 density of surface states is hindered by the absorption of counting avalanche photodiode ( SPAD ) . In this mode , a these photons near the surface of the heavily doped illumi non - photogenerated carrier may also be excited leading to nated layer ( p - or n - type ) . As a result , photo - generated avalanche current that is referred to as a dark count . In carriers are trapped by surface band bending and are lost to practice , the detection efficiency and dark count rate for the surface recombination ; the carrier transport in this layer may SPAD increase with increasing excess bias . 60 be characterized as diffusion , associated with the spatial

High sensitivity deep ultraviolet ( DUV ) photodetectors gradient in photogenerated carriers , with a significantly operating at wavelengths shorter than 280 nm are useful for reduced diffusion length for minority carriers over what various applications , including chemical and biological would be expected in the bulk that can be described as a identification , optical wireless communications , and UV shorter effective diffusion length . sensing systems . Often , these applications require very low 65 A number of groups have explored Schottky and metal light level or single photon detection and , as a result , semiconductor - metal ( MSM ) 4H - SiC photodetectors to photomultiplier tubes ( PMTs ) are widely used . However , in address this issue by enabling more efficient collection of

US 9 , 893 , 227 B2

carriers through photogeneration of these carriers primarily One preferred embodiment photodiode for detecting pho within the depletion region of the device . A . Sciuto , et al . , tons comprises : “ High responsivity 4H - SiC Schottky UV photodiodes based a substrate ; on the pinch - off surface effect , ” Appl . Phys . Lett . 89 , 081111 a first semiconducting region operatively associated with the ( 2006 ) ( herein incorporated by reference ) , report a peak QE 5 substrate suitable for forming a contact thereon ; of 29 % at 255 nm for vertical Schottky diodes fabricated on a first contact operatively associated with the first semicon n - type 4H - SiC using the pinch - off surface effect to increase ducting region ; the direct optical absorption area in the detector . X . Xin , F . a second semiconducting region comprising an absorption Yan , T . W . Koeth , C . Joseph , J . Hu , J . Wu , and J . H . Zhao : region for the photons having a predetermined energy range ; “ Demonstration of 4H - SiC UV single photon counting ava - 10 the second region being formed of a semiconductor having lanche photodiode , " Electron . Lett . , 41 1192 ( 2005 ) ( herein one or more of a high surface recombination velocity or a incorporated by reference ) reported large - area , 2x2 mm , high interface recombination velocity ; n - 4H - SiC Schottky diodes with QE of ~ 20 % at 200 nm . a second contact operatively associated with the second However , high efficiency single photon counting capable region ; APDs at wavelengths shorter than 260 nm have not been 15 the first semiconducting region and the second semiconduct demonstrated to date . One challenge has been to realize a ing region forming a first interface ; device design that mitigates the effects of surface recombi the second semiconducting region being configured such nation in these devices while maintaining sufficiently low that reverse biasing the photodiode between the first and dark currents at high bias to allow avalanche breakdown . second contacts results in the absorption region having a

Therefore , a need remains for low cost , compact , high 20 portion depleted of electrical carriers and an undepleted sensitivity , low dark current / dark count rate photodetectors portion at the reverse bias point of operation ; the undepleted that operate in the ultraviolet spectrum . portion being smaller than the absorption depth for photons

having a predetermined energy range ; SUMMARY OF THE PRESENT INVENTION whereby the depletion results in the creation of an electric

25 field and photogenerated carriers are collected by drift . The present invention is directed to , inter alia , an n - n - p As second preferred embodiment photodiode that elimi

photodetector wherein the thickness and doping of the nates or minimizes surface recombination of photogenerated illuminated n + - SiC layer are such that it will be nearly carriers generated by photons having a predetermined depleted at high bias near or above avalanche breakdown , energy range comprises i . e . , the thickness of the neutral and quasi - neutral regions in 30 a substrate ; the n + region will become larger than absorption length of a first semiconducting region operatively associated with the the high energy photon of interest and / or the diffusion length substrate suitable for forming a contact thereon ; of holes . The preferred thickness of the nt illuminated area a first contact operatively associated with the first semicon or region is between 20 - 120 nm . Preferred doping range is ducting region ; between 9x1017 to 5x1018 cm - with nitrogen atoms doping . 35 a second region comprising an absorption region for the Alternatively , an n - n ' - n - p structure is employed to allow photons having a predetermined energy range ; the second that the illuminated n ' - region is nearly depleted at the region being formed of a semiconductor having a high operating bias of the photodetector , typically near or above surface or interface recombination velocity ; avalanche breakdown . The n ' - layer consists of an n - type a second contact operatively associated with the second doped layer where the dopant concentration varies with 40 region ; layer thickness such that n ' is approximately equal to n at the the first semiconductor region and the second region form n / n ' interface and decreases or grades to that of the n at the ing a first interface ; the second region being configured such n ' / n interface . An illustration of this is shown in FIG . 11 that reverse biasing the photodiode between the first and where the n - type dopant , N , concentration is varied from second contacts results in depletion of the second region at approximately 2x1018 cm - 3 to approximately 1x1016 cm3 45 the reverse bias point of operation from the first interface to over a thickness of approximately 85 nm . Importantly , the at least one of the absorption depth and the sum of the free carrier concentration implied by the description of a absorption depth and effective diffusion length from the layer as be n , i - or p is essentially the same ; that is it is boundary of the second region ; sufficiently low such that the layer is depleted around zero whereby the depletion results in the creation of an electric

50 field and photogenerated carriers are collected by drift . Optional use of Al , Ga ( 1 - x ) N alloys as a transparent n - type The present invention is also directed to Amethod making

window enables an increase in the collection of electron - a photodiode for detecting photons comprising : hole pairs due to photogeneration of these carriers by high providing a substrate ; energy photons in the high - field n - SiC region . A peak QE providing a first semiconducting region operatively associ of 76 % at 242 nm has been measured on n - AlGaN / AIN / n - 55 ated with the substrate suitable for forming a contact SiC / p - SiC n - i - p diodes that is attributed to the minimization thereon ; of the effects of surface states and absorption in heavily providing a first contact operatively associated with the first doped layers currently hindering homogeneous SiC devices . semiconducting region ; See , L . E . Rodak , A . V . Sampath , C . S . Gallinat , Y . Chen , Q . providing a second semiconducting region comprising an Zhou , J . C . Campbell , H . Shen , and M . Wraback . “ Solar - 60 absorption region for the photons having a predetermined blind Al , Ga N / AN / SiC photodiodes with a polarization - energy range ; the second region being formed of a semi induced electron filter . ” Applied Physics Letters 103 no . 7 conductor having one or more of a high surface recombi ( 2013 ) : 071110 , herein incorporated by reference . Impor - nation velocity or a high interface recombination velocity ; tantly , use of an n - Al , Ga / 1 - N n - type window may greatly providing a second contact operatively associated with the reduce the thickness or remove the need for the illuminated 65 second region ; n - SiC layer . In this case the structure would be n * - AlGaN / the first semiconducting region and the second semiconduct i - SiC / p - Sic . ing region forming a first interface ;

nh

bias .

US 9 , 893 , 227 B2 the second semiconducting region being configured such contact and showing the absorption region for the photons of that reverse biasing the photodiode between the first and the predetermined energy range . second contacts results in the absorption region having a FIG . 13 is an illustration of an alternate preferred embodi portion depleted of electrical carriers and an undepleted ment comprising a n - p - - P + photodiode illuminated through portion at the reverse bias point of operation ; the undepleted 5 the transparent P + window suitable for forming an Ohmic portion being smaller than the absorption depth for photons contact and showing the absorption region for the photons of having a predetermined energy range ; whereby the depletion the predetermined energy range . results in the creation of an electric field and photogenerated FIG . 14 is a schematic illustration of a SiC / AIN / AL , carriers are collected by drift . Ga ( 1 - x ) N nip photodiode demonstrating enhanced DUV pho

These and other embodiments will be described in further 10 toresponse . FIG . 15 is the measured DC spectral response of SiC / detail below with respect to the following figures . AIN / AL , Ga ( 1 - x ) N nip photodiode at varying reverse bias BRIEF DESCRIPTION OF THE DRAWING voltage ( closed figures ) . The DC spectral response for a

homogeneous SiC pin photodiode is shown for comparison A better understanding of the present invention will be 15 ( dashed line ) . FIG . 3 illustrates the DC responsivity of

had upon reference to the following detailed description , SiC / AIN / AlxGa ( 1 - x ) N nip photodiode at zero , 5 , 10 , 15 , 20 , 22 , 24 , 25 , 30 , 35 and 40 volts reverse bias . wherein like reference characters refer to like parts through FIG . 16 is an illustration of an alternate preferred embodi out the several views , and in which : ment n - AlGaN / n - SiC / p - SiC photodiode employing an n - Al FIG . 1 is a schematic illustration of a preferred embodi - 20 hematic illustration of a preferred embodi - 20 GaN transparent window layer

ment of the present invention consisting of an n - n p - SiC FIG . 17 is a plot of the measured photoresponse for the photodiode . At the reverse bias operating point , the illumi fabricated n - AlGaN / n - SiC / p - SiC photodiode demonstrating nated n - layer is substantially depleted so that deep ultravio - enhanced deep ultraviolet response over a homogenous Sic let photon are substantially absorbed within the depletion p - i - n diode employing an illuminated recessed window . region within this layer so that concomitant photogenerated 25 FIG . 18 is a schematic diagram depicting the absorption carriers are collected more efficiently by drift , depth , effective diffusion length and the depletion width .

FIG . 2 illustrates the measured reversed bias voltage characteristics of the preferred embodiment of FIG . 1 . DETAILED DESCRIPTION OF PREFERRED

FIG . 3 is a graphical illustration of the measured photo EMBODIMENTS OF THE PRESENT response of the embodiment of FIG . 1 with increasing 30 INVENTION reverse bias . The inset of FIG . 3 ( upper right ) illustrates the measured responsivity between 260 - 380 nm with increasing The embodiments of the invention and the various fea bias . tures and advantageous details thereof are explained more

FIG . 4 is a graphical illustration obtained by calculating fully with reference to the non - limiting embodiments that the expected unity - gain photo - response from an n - i - p SiC 35 are illustrated in the accompanying drawings and detailed in diode having a 35 nm thick illuminated n - doped region with the following description . It should be noted that the features increasing depletion of this layer . illustrated in the drawings are not necessarily drawn to scale .

FIG . 5 graphically illustrates a calculated gain in a Sic Descriptions of well - known components and processing p - i - n and n - i - p diode as a function of photon wavelength at techniques are omitted so as to not unnecessarily obscure the constant reverse bias where p - i - n refers to illumination of 40 embodiments of the invention . The examples used herein are p - layer and n - i - p refers to illumination of n - layer intended merely to facilitate an understanding of ways in

FIG . 6 illustrates the calculated responsivity in a deep which the embodiments of the invention may be practiced ultraviolet avalanche photodetector similar to the preferred and to further enable those of skill in the art to practice the embodiment of FIG . 1 with increasing reverse bias . embodiments of the invention . Accordingly , the examples

FIG . 7 illustrates the Electric Field distribution in a deep 45 should not be construed as limiting the scope of the embodi ultraviolet avalanche photodetector similar to the preferred ments of the invention . Rather , these embodiments are embodiment of FIG . 1 . The depth for full absorption of provided so that this disclosure will be thorough and com representative DUV photons to be detected within the n - i - p plete , and will fully convey the scope of the invention to structure is shown by the shaded region . those skilled in the art . In the drawings , the dimensions of

FIG . 8 is an illustration of an alternate preferred embodi - 50 objects and regions may be exaggerated for clarity . Like ment comprising a transparent n - type doped AlGaN layer numbers refer to like elements throughout . As used herein operatively connected to an n - n - - p - SiC diode of the present the term “ and / or ” includes any and all combinations of one invention . or more of the associated listed items .

FIG . 9 is an illustration of an alternate preferred embodi - The terminology used herein is for the purpose of describ ment comprising a p - type layer adjacent to the substrate 55 ing particular embodiments only and is not intended to limit showing the neutral and quasi - neutral regions within the the full scope of the invention . As used herein , the singular illuminated n - type layer . forms “ a ” , “ an ” and “ the ” are intended to include the plural

FIG . 10 is an illustration of an alternate preferred embodi forms as well , unless the context clearly indicates otherwise . ment comprising an n - type layer adjacent to the substrate It will be further understood that the terms “ comprises ” showing the neutral and quasi - neutral regions within the 60 and / or " comprising , " when used in this specification , specify illuminated p - type layer . the presence of stated features , integers , steps , operations ,

FIG . 11 is a measured doping profile for a preferred elements , and / or components , but do not preclude the pres embodiment comprising an illuminated n - type layer with ence or addition of one or more other features , integers , graded or varying Nitrogen doping with layer thickness . steps , operations , elements , components , and / or groups

FIG . 12 is an illustration of an alternate preferred embodi - 65 thereof . ment comprising a p - n - - N + photodiode illuminated through It will be understood that , although the terms first , second , the transparent N + window suitable for forming an Ohmic etc . may be used herein to describe various ranges , elements ,

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components , regions , layers and / or sections , these elements , art and will not be interpreted in an idealized or overly components , regions , layers and / or sections should not be formal sense unless expressly so defined herein . limited by these terms . For example , when referring to first FIG . 1 shows a preferred embodiment of an n - n - p SiC and second ranges , these terms are only used to distinguish avalanche photodiode 10 that exhibits high responsivity at one range from another range . Thus , a first element , com - 5 wavelengths shorter than 280 nm , gain and low dark current ponent , region , layer or section discussed below could be at high reverse bias , just short of avalanche breakdown . As termed a second element , component , region , layer or sec - a result , these devices are ideal for realizing a highly tion without departing from the teachings of the present sensitive single - photon - counting avalanche photodiode invention . ( SPAD ) in the deep ultraviolet spectrum ( a < 260 nm ) for the

Furthermore , relative terms , such as “ lower ” or “ bottom ” 10 first time , based on SiC . and “ upper ” or “ top , " may be used herein to describe one The SiC n - n - - p structure or photodiode 10 includes an element ' s relationship to other elements as illustrated in the approximately 35 nm thick n + - layer 11 doped with 2x1018 Figures . It will be understood that relative terms are intended cmnitrogen atoms overlying a 480 - 500 nm thick n layer , to encompass different orientations of the device in addition or i - layer , 12 doped with 1x1016 cm - 3 nitrogen atoms which to the orientation depicted in the Figures . For example , if the 15 in turn overlies a 2000 nm thick p - layer 13 doped with an device in the Figures is turned over , elements described as aluminum concentration of 2x1018 cm3 . The layer structure being on the “ lower ” side of other elements would then be is grown on an n - type , Si - face , 4H - SiC substrate 14 with a oriented on “ upper ” sides of the other elements . The exem - 4º miscut . The avalanche photodiodes 10 have a 50 - 250 plary term “ lower ” , can therefore , encompass both an ori - um - diameter circular mesa 15 and seven degree beveled entation of " lower ” and “ upper , ” depending of the particular 20 sidewalls 16 . orientation of the figure . Similarly , if the device in one of the A top n - type metal contact 17 includes a Ti 10 nm / Al 100 figures is turned over , elements described as “ below ” or nm / Ni 30 nm / Au 50 nm metallization scheme while a p - SiC “ beneath ” other elements would then be oriented " above " Ohmic contact 18 includes a layer of Ni 25 nm , Ti 35 nm , the other elements . The exemplary terms “ below " or Al 100 nm , and Ni 80 nm . A thin semi - transparent metal " beneath ” can , therefore , encompass both an orientation of 25 contact 19 was deposited in a top window region to ensure above and below . Furthermore , the term " outer ” may be used lateral E - field spreading in the optical absorption area , and to refer to a surface and / or layer that is farthest away from its transmission over the spectral range of interest should be a substrate . about 50 % .

This description and the accompanying drawings that Importantly , the thickness of the n - layer 12 may be varied illustrate inventive aspects and embodiments should not be 30 to modify the gain and dark current within the diode for a taken as limiting — the claims define the protected invention . given reverse bias . The thickness of the n - layer 12 may Various changes may be made without departing from the range from 250 nm to 960 nm , with the lower end of the spirit and scope of this description and the claims . In some range resulting in reduced reverse bias required for ava instances , well - known structures and techniques have not lanche breakdown but likely reduced gain and increased been shown or described in detail in order not to obscure the 35 dark current . In contrast , the higher end of the range will invention . Additionally , the drawings are not to scale . Rela - likely result in higher gain and lower dark current but with tive sizes of components are for illustrative purposes only a higher reverse bias required for avalanche breakdown . and do not reflect the actual sizes that may occur in any FIG . 2 shows the measured reversed bias IV characteris actual embodiment of the invention . Like numbers in two or tics of the fabricated deep ultraviolet avalanche photodiode more figures represent the same or similar elements . Ele - 40 ( DUV - APD ) . The measured dark current increases slowly ments and their associated aspects that are described in below 120V bias and then increases more rapidly to ~ 153V . detail with reference to one embodiment may , whenever It is important to note that the diameter of the tested devices practical , be included in other embodiments in which they were large , - 200 um , in comparison to what has been are not specifically shown or described . For example , if an investigated for GaN based APDs . See , for example , Y . element is described in detail with reference to one embodi - 45 Zhang , D . Yoo , J . Limb , J . Ryou , R . D . Dupuis , and S . Shen , ment and is not described with reference to a second " GaN ultraviolet avalanche photodiodes fabricated on free embodiment , the element may nevertheless be claimed as standing bulk GaN substrates , ” Phys . stat . sol . ( c ) 5 , No . 6 , included in the second embodiment . 2290 - 2292 ( 2008 ) , herein incorporated by reference .

Embodiments of the present invention are described FIG . 3 shows the measured spectral response from a herein with reference to cross - section illustrations that are 50 fabricated deep ultraviolet avalanche photodetector 10 under schematic illustrations of idealized embodiments of the 100 nW illumination as function of increasing reverse bias present invention . As such , variations from the shapes of the from 5 to 146V , and thus short of avalanche breakdown at elements in the illustrations are to be expected . Thus , - 150 - 160V . At low bias the photoresponse peaks at ~ 265 embodiments of the present invention should not be con nm , corresponding to a unity - gain quantum efficiency of strued as limited to the particular shapes of regions illus - 55 - 41 % that is reduced due to absorption in the semi - trans trated herein but are to include deviations in shapes . Thus , parent top metal contact , and drops off at shorter wave the layers or regions illustrated in the figures are schematic lengths . These results are typical of what is generally in nature and their shapes are not intended to illustrate the observed for SiC APD devices . At increasing reverse bias , precise shape of a layer or region of a device and are not the responsivity increases and the peak response shifts to intended to limit the scope of the present invention . 60 ~ 212 nm at the highest bias investigated , i . e . 146V . This Unless otherwise defined , all terms ( including technical change in the shape of the spectral response can be explained

and scientific terms ) used herein have the same meaning as by considering two phenomena that occur with increasing commonly understood by one of ordinary skill in the art to reverse bias , 1 ) the depletion of the top - illuminated n + - layer which this invention belongs . It will be further understood and 2 ) the spectrally inhomogeneous gain in this structure . that terms , such as those defined in commonly used diction - 65 At low reverse bias there is no increase theoretically aries , should be interpreted as having a meaning that is expected in the measured photocurrent responsivity of an consistent with their meaning in the context of the relevant n - n - p diode structure due to the very low E - field within the

US 9 , 893 , 227 B2 10

depletion region and therefore the lack of gain associated gain for carriers photogenerated in the n - layer is a hole while with impact ionization . This understanding is consistent that for carriers photogenerated in the p - layer is an electron , with the relatively constant response observed in the n - n - p and the impact ionization coefficient of holes is ~ 10x that of structure at wavelengths longer than ~ 280 nm initially with electrons , the significantly shorter absorption length for increasing reverse bias ( FIG . 3 inset ) . However , a measur - 5 DUV photons results in the generation of holes that have a able increase in the short wavelength response even initially maximized path length through the gain region . In contrast , with increasing bias can be explained by an increase in the longer wavelength photons are absorbed more uniformly depletion width of the n - n - p with increasing reverse bias . throughout the structure , resulting in gain from a mix of While both the n - side and p - side depletion width are photogenerated electrons and holes traveling , on average , a expected to increase with increasing bias , the increase on the 10 shorter distance that necessarily results in lower gain . A n - side will be larger due to the lower doping in the n - region . calculation on the gain expected for an n - side ( n - i - p ) and a The increase in depletion width , primarily on the n - side , is p - side illuminated ( p - i - n ) SiC diode having a 480 nm thick expected to have more significant impact on the collection of i - region demonstrates this trend , as shown in FIG . 5 . The carriers generated by deep ultraviolet photons that are “ spectrally inhomogeneous gain ” shown in FIG . 5 results strongly absorbed in the top illuminated n - layer , and are 15 from gain arising primarily from impact ionization multi generally lost to surface recombination . Increasing the plication of holes and their spatial distribution in the device depletion width should enhance the DUV photoresponse by structure associated with the wavelength dependent absorp increasing the number of photo - generated holes collected tion depth for photogeneration of carriers . The absorption through drift as well as improving the efficiency for collec - depth refers to a thickness within a layer wherein 1 - 1 / e tion of carriers generated in the neutral region by diffusion 20 ( e = natural logarithm = 0 . 368 ) ; i . e . , approximately 63 % of the through a reduction of the distance these carrier need to photons at the detection wavelength of interest are absorbed diffuse through the top n - layer . This interpretation is further according to Beer ' s Law . For an n - i - p structure , the gain at supported by calculating the expected quantum efficiency 200 nm is ~ 310 while at 360 nm it is ~ 75 . In contrast for a from an n - n " - p SiC diode with increasing depletion of p - i - n structure the opposite is the case , gain at 360 nm is ~ 6x n - layer as shown in FIG . 4 . Consistent with experimentally 25 greater than gain at 200 nm . This phenomena is related to 1 ) observed trends , the response in the DUV spectral range is the ionization rate of the principal carrier that is being found to increase , while it remains relatively constant in the multiplied in the gain region and 2 ) where the photons are near ultraviolet spectral range . Further support for this absorbed within the structure , which effects the distance explanation is found in the qualitative agreement between photogenerated carriers travel inside the gain region . As the measured response observed at ~ 70 V in the DUV - APD 30 holes have a higher ionization rate than electrons in 4H - SIC and that of a metal - i - p structure that has been previously and deep ultraviolet photons are absorbed very close to the reported in A . V . Sampath , L . E . Rodak , Y . Chen , Q . Zhou , surface , carriers generated by photons at these wavelengths J . C . Campbell , H . Shen ; M . Wraback , “ High quantum have higher gain in the n - i - p structure . This phenomenon is efficiency deep ultraviolet 4H - SiC photodetectors ” , Elec partially responsible for the emerging 212 nm peak observed tronics Letters , Volume 49 , Issue 25 , 5 Dec . 2013 , p . 35 in FIG . 3 at biases above 130V . Calculation of the expected 1629 - 1630 , herein incorporated by reference . It is important response in our DUV - APD shown in FIG . 6 demonstrates to note that the photoresponse of the DUV - APD was mea - the same trends observed experimentally at high bias near sured in the spectral range between 200 - 400 nm due to avalanche breakdown . Specifically , the near flat spectral limitations in the experimental setup and that this device has response observed at moderate bias over a wide spectral sensitivity at wavelengths shorter than 200 nm . It is also 40 range from 200 - 280 nm becomes a more peaked response important to note that these devices were biased short of around 220 nm . avalanche breakdown due to limitations in the experimental The operating reverse bias point for the photodiode will test setup for probing the devices . These devices can in generally be application specific . For applications requiring principal be reverse biased at or above avalanche break high sensitivity , the diode may be biased to have high gain down . 45 in the multiplication layer 12 . For operation in a single

For increasing bias above - 70V an increase in photore - photon counting mode , the diode may be operated near or sponse is observed over a wider spectral range . This increase above the avalanche breakdown voltage . This corresponds can be attributed to on - set of gain within the DUV - APD due to an E - field in the multiplication region 12 of ~ 2 - 3 MV / cm to impact ionization . However , this enhancement is signifi - for SiC or a reverse bias of ~ 150V for a 500 nm thick region cantly stronger in the DUV spectral range resulting in a shift 50 12 . The doping in this region will generally be sufficiently in the peak response to ~ 212 nm at the highest bias inves - low to allow for a large and uniform electric field throughout tigated . The spectral inhomogeneity in the observed gain can such as n = 1x1010 cm for the photodiode 60 . be explained by considering the ~ 10x larger impact ioniza - The thickness and doping of the illuminated n - region 11 tion coefficient for holes ( . ) over electrons ( an ) as well as will be designed so that it is sufficiently depleted at the the significantly stronger absorption of photons in the deep 55 reverse bias operating point such that DUV photons are ultraviolet spectral range . Previously we have shown that the absorbed within this region . However , region 11 cannot be photocurrent generated in an n - i - p SiC structure ( i . e . illu - biased to full depletion as this will cause a large increase in minated from the n - side ) in the DUV spectrum is dominated dark current in the photodiode that can reduce detector by carriers generated from photons absorbed in the top - performance . For photodiode 10 the n - region preferred illuminated n - layer , while the near band - gap response has 60 thickness range is between 20 - 120 nm and preferred n - type significant contribution from carriers generated in both the i - doping range in this region is 9x1017 to 5x1018 cm - 3 . The and bottom p - doped regions of the structure , as reported in thickness of the n - layer 12 may be varied to modify the gain A . V . Sampath , L . E . Rodak . Y . Chen , Q . Zhou . J . C . and dark current within the diode for a given reverse bias . Campbell , H . Shen and M . Wraback , “ Enhancing The Deep The thickness of the n - layer 12 may range from 250 nm to Ultraviolet Performance Of 4H - SiC Based Photodiodes ” 65 960 nm , with the lower end of the range resulting in reduced ECS Transactions , 61 ( 4 ) 227 - 234 ( 2014 ) , herein incorpo - reverse bias required for avalanche breakdown but likely rated by reference . As the principal carrier associated with reduced gain and increased dark current . In contrast , the

US 9 , 893 , 227 B2 12

higher end of the range will likely result in higher gain and Gallinat , Y . Chen , Q . Zhou , J . C . Campbell , H . Shen , and M . lower dark current but with a higher reverse bias required for Wraback . “ Solar - blind Al , Ga - N / AIN / SIC photodiodes avalanche breakdown . with a polarization - induced electron filter . ” Applied Physics

One advantage of this design is the low noise in the DUV Letters 103 , no . 7 ( 2013 ) : 071110 , herein incorporated by spectrum that is expected due to the spatial separation of the 5 reference . absorption and multiplication regions in the structure for FIG . 8 shows a schematic of an alternate preferred photons in this spectral range , as illustrated in FIG . 7 . As embodiment n * - Al , Ga N / n - nº - p - SiC structure 30 com DUV photons are primarily absorbed and photocarriers prising an approximately 100 nm thick n * - AlGaN layer 38 generated in the lower E - field n * - region 11 of this structure having an AIN mole fraction of - 80 % and doped with due to strong absorption in this spectral range , while carrier 10 5x1018 cm - 3 silicon atoms , a 35 nm thick nt - SiC layer 31 multiplication primarily occurs in the n - - SiC region 12 , doped with 2x1018 cm - nitrogen atoms , a 480 - 500 nm thick single carrier hole multiplication can be achieved in this n - SiC layer 32 doped with 1x1010 cm - nitrogen atoms and wavelength range due to the long transit path of holes a 2000 nm thick p - SiC layer 33 doped with an aluminum through the high electric field multiplication region , as concentration of 2x1018 cm - 3 . The structure is grown on an compared to electrons , which have a significantly shorter 15 n - type , Si - face , 4H - SiC substrate 34 with a 4° miscut . path primarily through a diminishing electric field in the Avalanche photodiode devices with 50 - 250 um - diameter n + - SiC region . As a result , this structure should yield lower circular mesas 35 and seven degree beveled sidewalls 36 excess noise for operation in this spectral range without the were fabricated . The top n - type metal contact 37 comprises use of a thick doped charge layer traditionally employed in a Ti 10 nm / A1 100 nm / Ni 30 nm / Au 50 nm metallization a separate absorption charge multiplication avalanche pho - 20 scheme while the p - SiC Ohmic contact 38 comprises Ni 25 todiode , which can result in loss of detection efficiency nm / Ti 35 nm / Al 100 nm / Ni 80 nm . An anti - reflective resulting from diffusive transport in this region . See , for coating 39 was deposited in the window region to minimize example , S . Soloviev , A . Vert , J . Fronheiser , P . Sandvik , losses associated with reflection of the illuminated light “ Positive Temperature Coefficient of Avalanche Breakdown from the semiconductor surface . Observed in a - Plane 6H - SiC Photodiodes , ” Materials Sci - 25 The alternate preferred embodiment of the present inven ence Forum Vols . 615 - 617 ( 2009 ) pp 873 - 876 , herein incor - tion shown in FIG . 8 comprises a heterostructure operable to porated by reference . extend the deep ultraviolet ( DUV ) response of SiC below Moreover , the replacement of the semi - transparent win - 260 nm . Since the direct bandgap of aluminum gallium

dow metal required to improve the lateral E - field spreading nitride ( Al , Ga / 1 - r ) N ) can be engineered from 3 . 4 eV to 6 . 1 in the current structure with a conductive and transparent 30 eV depending on the AIN to GaN mole fraction , these alloys wide band gap n - type semiconductor such as AlGaN should are employed as UV transparent , n - type contact windows improve on the unity gain quantum and single photon ( for example , the transparent window in FIG . 8 ) to increase detection efficiencies of the current demonstrated device . the absorption of high energy photons and concomitant The structure of this improved heterostructure is shown in photogeneration of carriers in the high electric field deple FIG . 8 . 35 tion region of the nip structure , as compared to the previ As described in US 2015 / 0311375 paragraph 57 , the use ously described structure with the semi - transparent metal

of Al , Ga ( 1 - xN alloys as a transparent n - type window contact that reduces the photon flux absorbed within the increases the collection of electrons created by absorption of detector due to losses in the illuminated semi - transparent high energy photons in the high - field n - SiC region . Peak metal contact . As described in the following , photo - gener QE of 76 % at 242 nm has been measured and attributed to 40 ated carriers in the Al Ga ( 1 - x ) N windows do not contribute to the minimization of the effects of surface states and absorp - photoresponse and can therefore be used to adjust the high tion in heavily doped layers currently hindering homoge - energy cutoff wavelength of this device . See , for example , L . neous SiC devices . E . Rodak , A . V . Sampath , C . S . Gallinat , Y . Chen , Q . Zhou , US 2015 / 0311375 discusses a SiC / AIN / AL , GaN nip J . C . Campbell , H . Shen , and M . Wraback . “ Solar - blind

photodiode as shown in FIG . 14 where the diode is illumi - 45 Al , Ga - N / AIN / SIC photodiodes with a polarization - in nated through the n - Al , Ga - N layer . These diodes exhibit a duced electron filter . ” Applied Physics Letters 103 , no . 7 peak QE of 76 % at 242 % that is significantly larger than that ( 2013 ) : 071110 , herein incorporated by reference . of a conventional homogenous n - i - p SiC diode with a thick Preferred thickness range of the n - type AlGaN contact illuminated n - layer as shown in FIG . 15 . This has been window layer is between - 50 - 600 nm thick . The Al com attributed to the reduction of the losses of carriers generated 50 position is preferably greater than 60 % to insure transpar by these DUV photons to recombination via surface states in ency in the deep ultraviolet spectral range between 200 - 260 the traditional n - i - p - SiC or p - i - n - SiC APD structure , where nm . The n - type AlGaN having a ratio of aluminum to these carriers are generated in the illuminated highly doped gallium of greater than 80 % , The n - type AlGaN preferably layer and are collected primarily through diffusion . The has a free electron concentration from 1x1017 cm3 to presence of surface states and the associated surface band 55 1x1019 cm and having a thickness between 50 - 470 nm bending acts to significantly reduce the effective diffusion This enhancement of the DUV response using a transpar length of these carriers so that they are lost to surface ent window semiconductor is more clearly demonstrated in recombination . See , for example , A . V . Sampath , L . E . a device structure that does not have the barrier layer such Rodak , Y . Chen , Q . Zhou , J . C . Campbell , H . Shen ; M . as a p - i - SiC / n - Al Gal - N photodiodes . FIG . 16 shows a Wraback , “ High quantum efficiency deep ultraviolet 4H - SiC60 schematic of an alternative preferred embodiment compris photodetectors ” , Electronics Letters , Volume 49 , Issue 25 , 5 ing an approximately 470 nm - thick n - Al Ga - N layer 128 Dec . 2013 , p . 1629 - 1630 . Instead , the wider band gap having - 90 % AIN by mole fraction , a 480 - 500 nm thick illuminated n - Al , Ga ( 1 - x ) N and AlN layers enables these n - SiC layer 121 doped with 1x1016 cm - 3 nitrogen atoms photons to be absorbed in the high E - field , low doped , and a 2000 nm thick p - SiC layer 123 doped with an n - SiC region where they are collected more efficiently by 65 aluminum concentration of 2x1018 cm3 . The structure is drift resulting in significant enhancement in DUV photore - grown on an n - type , Si - face , 4H - SiC substrate 124 with a 4° sponse . See , for example , L . E . Rodak , A . V . Sampath , C . S . miscut . Avalanche photodiode embodiments 120 with

US 9 , 893 , 227 B2 13 14

50 - 250 um - diameter circular mesas 125 and seven degree field at the hetero - interface and can potentially result in beveled sidewalls 126 were fabricated . The top n - type metal reduced dark current generation associated with heteroepi contact 17 comprises a Ti 10 nm / Al 100 nm / Ni 30 nm / Au 50 taxially generated defects . This may also make the structure nm metallization scheme while the p - SiC Ohmic contact 18 more easily manufacturable , as the tolerances for designing comprises Ni 25 nm / Ti 35 nm / A1 100 nm / Ni 80 nm . An 5 the top n + - region for optimal DUV efficiency over a com anti - reflective coating 129 was deposited in the window parable nt - n - - p - SiC structure as shown in FIG . 1 while region to minimize losses associated with reflection of the avoiding the metal contact punch - through effect would be illuminated light from the semiconductor surface . For com - relaxed . parison , a 4H - SiC homojunction avalanche photodiode For photodiode 30 the thickness and doping of the n - re employing an illuminated recessed window to improve the 10 gion 31 will be designed so that it is either fully depleted to EQE is provided from Handin Liu , Dion Mcintosh , Xiao - the interface with the transparent window 38 or sufficiently gang Bai , Member , IEEE , Huapu Pan , Mingguo Liu , and Joe depleted at the reverse bias operating point such that DUV C . Campbell . “ 4H - SiC PIN Recessed - Window Avalanche photons are absorbed within this region . The depletion width Photodiode with High Quantum Efficiency ” IEEE PHO - in photodiode 30 would minimally extend from its interface TONICS TECHNOLOGY LETTERS 20 , NO . 18 , ( 2008 ) 15 with the multiplication region 32 to at least within a distance 1551 . This SiC APD structure consists of a 2 um - thick nt of its interface with the transparent window layer 38 that is ( 4 . 5x1018 cm - 3 ) layer , a 480 nm - thick p - ( N = 1x1016 cm - 3 ) equal to either 1 ) and absorption depth for the predetermined layer , a 200 nm p ( Na = 2x1018 cm " ) layer , and a 200 nm p + energy range or 2 ) the sum of the absorption depth for the ( N = 1x1019 cm - 3 ) layer ; the thickness of the top layer was predetermined energy range and the effective diffusion reduced to ~ 35 nm in the illuminated window through dry 20 length of carriers For photodiode 30 a preferred n - layer 31 etching to reduce absorption in this region . The III - Nitride thickness is between 20 - 120 nm and preferred n - type doping heterojunction and SiC homojunction devices were passi - range is 9x10 ' ' to 5x1018 cm . vated with 200 nm and 220 nm thick silicon dioxide layers , However , it should be noted that use of the n - type AlGaN respectively , which also act as an anti - reflection coating . transparent window can reduce the thickness of or eliminate

The photoresponse of the p - i - SiC / n - Al , GaN and 25 the need for the more heavily doped n + - SiC layer as in homogenous SiC photodiodes are shown in FIG . 17 . The preferred embodiment 120 . recessed window SiC photodiode has a peak EQE of ~ 60 % The operating reverse bias point for this photodiode will at 260 nm and a sharp drop off in response at shorter generally be application specific . For applications requiring wavelength ( grey shaded curve ) . In contrast , the p - i - SiC / n - high sensitivity , the diode may be biased to have high gain Al , Ga - N photodiode has a flatter response in DUV that 30 in the multiplication layer 32 . For operation in a single exceeds 60 % over a spectral range from 215 - 256 nm and a photon counting mode , the diode may be operated near or sharp cut - off at shorter wavelengths ( black curve ) . This above the avalanche breakdown voltage . This corresponds enhancement is a ~ 10x enhancement in performance over to an E - field in the multiplication region 32 of ~ 2 - 3 MV / cm comparable homogenous SiC photodiodes at the shortest for SiC or a reverse bias of ~ 150V for a 500 nm thick region wavelength and is attributed to the photogeneration of 35 32 . The doping in this region will generally be sufficiently carriers within the depletion region of the heterojunction , low to allow for a large and uniform electric field throughout where these carriers are more efficiently collected through such as n = 1x10 - 16 cm - 3 for the photodiode 30 . drift over a typical p - i - n SiC photodiode structure that While the preceding has focused on SiC and the collection strongly relies on carrier diffusion . of carriers generated by photons having energies corre

The sharp short wavelength cutoff observed at 210 nm is 40 sponding to wavelengths shorter than 260 nm , the underly attributed to the loss of photogenerated carriers within the ing principles are applicable to any semiconductor having a neutral n - Al , GaN " window " layer ; this results from the large surface recombination velocity and potentially a large short effective carrier diffusion lengths within this region surface band bending and where the carriers to be collected associated with the presence of dislocations arising from are generated near the surface of material . In this case , it is lattice mismatch inhibits the collection of photo - generated 45 desirable to design the thickness and doping levels of the carriers through diffusion . This conclusion is consistent with illuminated semiconductor layer such that this layer is what we have previously observed for the collection of sufficiently depleted so that the thickness of the remaining photo - generated carriers within the GaN absorption region neutral and quasi - neutral regions are reduced to become near of a GaN / SiC separate absorption and multiplication APD or shorter than the absorption depth of the photon to be for which drift in an electric field is required . Anand V . 50 detected at the reverse bias operating point of interest for the Sampath , Ryan W . Enck , Q . Zhou , D . C . McIntosh , H . Paul detector . This results in the beneficial case where carriers Shen , J . C . Campbell , and Michael Wraback , “ p - type Inter - generated by these photons may be collected more effi face Charge Control Layers for Enabling GaN / SiC Separate ciently through 1 ) drift within the depletion region and / or 2 ) Absorption and Multiplication Avalanche Photodiodes ” , reduced distance for carriers to diffuse from the illuminated Applied Physics Letters , 101 , ( 2012 ) 093506 This is further 55 neutral and quasi - neutral regions to the depletion region for supported by measurement of the reflection spectrum from collection . As a result , the performance of the detector will the heterojunction shown in FIG . 17 where the damping of improve in the spectral range of interest . As the neutral and the reflection oscillation associated with absorption in the quasi - neutral region of the illuminated layer thickness n - Al , Ga - N layer correlates well with drop off in the short reduces , the lateral conductance of the layer may decrease so wavelength response of the heterojunction photodiode . 60 as to prevent uniform lateral electric field spreading across

An additional benefit of an N - n - n - - p structure such as the detection area of the photodetector . In this case , a n + - AlGaN / n - nº - p - SiC is that the n - type AlGaN layer ( the semi - transparent metal contact layer can be deposited in the N - layer ) can be made arbitrarily thick to improve lateral illumination area of the detector to improve the lateral conductivity and lateral electric field spreading that will electric field spreading . Another approach is to employ a improve uniformity over the detection area while also pre - 65 sufficiently conductive semiconductor that is transparent in venting punch - through of the E - field to the top metal con - the spectral range of interest and make a metal contact to it tact . The presence of the nt - SiC layer will reduce the electric around the periphery of the detection area to prevent absorp

15 US 9 , 893 , 227 B2

16 tion losses within the metal . The design choice of illumi - The choice of which structure to employ may be influ nating the n - or p - layer will be made based upon a number enced by the semiconductor material selected and its prop of consideration such as whether a particular carrier , elec erties . For example , as the ionization rate of holes is ~ 10x tron or hole , has the higher ionization rate . larger than that of electrons for 4H - SiC an illuminated

A schematic of such a device is shown in FIG . 9 for a 5 n - layer is desirable to fully absorb photons having energies device comprising a preferred embodiment n - n - p structure much greater than the band gap and generate holes for 50 deposited upon a substrate 51 and where the second multiplication within the multiplication region of the ava region n - layer 54 is illuminated with light . Optional multi - lanche photodiode . However for silicon , that also has a high plication layer 52 between the first and region is shown for density of surface states , the ionization rate of electrons is when photodiode 50 is designed to have gain . It further 10 - 100x greater than that of holes so that an illuminated comprises a first layer 52 that is doped p - type and suitable p - layer is likely more desirable . for making a metal contact . While not shown , it is assumed Lastly , the use of a doped and conductive wider band gap that the device is properly reverse biased at the operating semiconductor in the illuminated layer within a heterojunc point of interest for detection . For example , the device may tion N - n - p or P - po - n can greatly reduce complexity of be biased below the avalanche breakdown voltage and 15 design , where by convention the capital letter refers to the operating in linear mode with or without gain or Geiger wider band gap semiconductor material . This is due to the mode . Alternatively the device may be biased above the fact that the wider band gap layer may be selected so as to avalanche breakdown voltage as is commonly done in be transparent to the photons at the short wavelength range Geiger mode to improve single photon detection efficiency . of interest such that the photons are absorbed in the high At this reverse bias the illuminated n - layer 54 will partially 20 electric field i - region near the hetero - interface . As a result , deplete so that the thickness of neutral and quasi - neutral the carriers generated by photons in the short wavelength region 55 within will be thinner than that of the n - layer 54 . range of interest will be collected more efficiently by drift Since the width of the neutral and quasi - neutral region 55 over a comparable homojunction detector . For the case of will now be near or shorter than the absorption depth of the polar semiconductors such as AlGaN and SiC there is also photon of interest for detection , the carriers generated by 25 an advantage associated with positive polarization induced absorption of these photons will be more efficiently col - charge at the AlGaN / SiC heterointerface in a n - AlGaN / n - lected . The absorption depth refers to a thickness within a SiC / p - SiC heterojunction detector , as it will act to prevent layer wherein 1 - 1 / e ( e = natural logarithm = 0 . 368 ) ; i . e . , the depletion of the n - AlGaN layer and prevent punch approximately 63 % of the photons at the detection wave through to the metal contact . However , as lattice mismatch length of interest are absorbed according to Beer ' s Law . The 30 at hetero - interfaces can result in the generation of defects depletion width in photodiode 50 would minimally extend that can result in deleterious dark current , the use of the from its interface with the first layer 51 / optional multipli - n - layer in the N - n - n - p ( or correspondingly the p - layer in a cation region 52 to at least within a distance of its interface P - p - p - n ) can act to suppress these effects by reducing the with its boundary / the optional transparent window layer 58 electric field at the hetero - interface . This design may also that is equal to either 1 ) and absorption depth for the 35 have lower excess noise due to the spatial separation of the predetermined energy range or 2 ) the sum of the absorption principal absorption region ( the n - layer for an N - n - n - p ) and depth for the predetermined energy range and the effective the multiplication region ( the n - - layer for an N - n - n " - p ) diffusion length of carriers This is due to more efficiently within the diode . collecting these carriers through drift and / or the reduced Referring now to FIG . 12 , shown is preferred embodiment length that carriers need to diffuse to the depletion region to 40 100 comprising a substrate 101 , p - layer 102 , n layer 103 and be collected as current in the detector . As the lateral resis - transparent window layer 105 , which is formed predomi tance of the neutral and quasi - neutral region 55 may now be nately of a N + material , i . e . , an n - type material with excess large so as to prevent a uniform lateral electric field distri - doping and transparent at the detection wavelength of inter bution across the device area , an optional transparent semi est potentially utilizing a wider band gap material than layer conductor or semi - transparent metal 58 is shown adjacent 45 103 . Layer 105 is transparent at the predetermined photon the illuminated n - layer 54 . If a transparent wider band gap energy range and is also suitable for making an operative semiconductor is employed , then the structure can be connection to a contact 106 . The preferred embodiment described as an N - n - n - p diode where the capital N is used photodiode 100 eliminates or minimizes surface recombi to refer to the wider band gap semiconductor layer . It is nation of photogenerated carriers generated by photons important to note that in general the doping of n - layer 54 can 50 having a predetermined energy range . The region 104 has a vary with thickness such that it increases from it interface short absorption depth within an effective diffusion length of with the p - layer 52 / optional multiplication region 53 to its the surface or interface of the n - layer 103 for the photons boundary / interface with optional layer 58 . This increase having a predetermined energy range . The n layer or second may be of some arbitrary function such as for example region 103 is formed of a semiconductor having a high linear , continuous , graded , abrupt , step or some combination 55 surface or interface recombination velocity . The first semi of these . conductor region 105 and the n layer 103 ( second region )

This approach can also be employed for an alternate form a first interface 109 such that the second region is preferred embodiment p - p - n structure 60 where the p - type depleted at the interface between the second region and the doped second layer 64 is now illuminated as shown in FIG . transparent region at the reverse bias point of operation . The 10 . The structure 60 includes a substrate 61 and an optional 60 region 103 is depleted to include the region 104 ; and the layer 63 where multiplication occurs and an n - type first layer depletion extends from the first interface through the absorp 62 suitable for making a metal contact . This case is analo - tion region 104 through the region denoted 103 at the gous to the previous case except that the types of doping in operating bias . The depletion results in the creation of an the illuminated layer and region adjacent the substrate are electric field and photogenerated carriers are collected by now switched . While not shown , it is assumed that the 65 drift and the photons at the predetermined energy are device is properly biased at the operating point of interest for detected as current . The current will flow between the detection . terminals 106 and 107 .

17 US 9 , 893 , 227 B2

18 Referring now to FIG . 13 , shown is preferred embodiment surface and higher ionization rate of holes over electrons

110 comprising a substrate 111 , n - layer 112 , p layer 113 and within Sic . This design also takes advantage of these transparent window layer 115 , which is formed predomi - phenomena to realize low excess noise in the spectral range nately of a P + material , i . e . , an p - type material with excess of interest via single carrier multiplication within the devel doping and transparent at the detection wavelength of inter - 5 oped structure without the use of a charge layer . This can est potentially utilizing a wider band gap material than layer lead to further improvements in detection efficiency over a 113 . Layer 115 is transparent at the predetermined photon conventional separate - absorption - charge - multiplication energy range and is also suitable for making an operative structure . connection to a contact 116 . The preferred embodiment As used herein in the drawings , specification . abstract photodiode 110 eliminates or minimizes surface recombi - 10 and claims ) , the term “ light ” means electromagnetic radia nation of photogenerated carriers generated by photons tion , unless specifically noted to the contrary . In the draw having a predetermined energy range . The absorption region ings , the symbol à means electromagnetic radiation . Within 114 is within an effective diffusion length of the surface or the light spectrum , the solar blind region refers to the region interface for the photons having a predetermined energy of the light spectrum wherein , due to absorption of sunlight range . The player or second region 113 is formed of a 15 by the atmosphere , the potential interfering effect of sunlight semiconductor having a high surface or interface recombi - does not occur ; i . e . , normally considered to be less than 280 nation velocity . The first semiconductor region 115 and the nm at low elevations . p layer 113 ( second region ) form a first interface 119 such As used herein , the terminology “ layer " includes “ region ” that the second region 113 is depleted at the interface and is not limited to a single thickness of a material covering between the second region 113 and the transparent region 20 or overlying another part or layer , but encompasses a region 115 at the reverse bias point of operation . The region 113 is having a variety of configurations and / or thicknesses . depleted to include the region 114 ; and the depletion extends As used herein , the terminology “ multiplication layer " or from the first interface 119 through the absorption region “ multiplication region " means a layer or layers or region in 114 at the operating bias . The depletion results in the which the carriers predominantly multiply . The carriers may creation of an electric field and photogenerated carriers 25 be either holes and / or electrons . within the absorption region 114 are collected by drift and As used herein , the terminology “ absorption layer ” , the photons at the predetermined energy are detected as “ absorption region ” , “ absorber " , " absorber region " means a current . The current will flow between the terminals 116 and layer or layers or region in which photons are predominantly 117 . absorbed and photogenerated carriers created . Absorption

It can thus be seen that the present invention provides an 30 and multiplication may occur in the same layers ( or regions ) . approach for realizing a highly efficient SPAD in DUV As used herein , the terminology “ spectrally inhomoge spectrum at wavelengths shorter than 280 nm , as the key neous gain ” in a representative structure is shown in FIG . 5 . transducing element within a SPAD module is the avalanche FIG . 5 shows that photons of different wavelengths generate photodiode . Currently system designers requiring high sen - carriers that have different gain associated with them . For an sitivity and low noise UV detectors for spectroscopy and 35 n - i - p structure , the gain at 200 nm is ~ 310 while at 360 nm single photon counting have the option of employing PMTs it is ~ 75 . In contrast for a p - i - n structure the opposite is the or UV enhanced Si avalanche photodetectors . PMTs have case , gain at 360 nm is ~ 6x greater than gain at 200 nm . This significant shortcomings , including high cost ( $ 2000 with phenomena is related to 1 ) the ionization rate of the principal power supply and cooling ) , bulky packaging , susceptibility carrier that is being multiplied in the gain region and 2 ) to magnetic fields , requiring high voltage for operation 40 where the photons are absorbed within the structure that ( > 1000 V ) and cooling for high sensitivity . UV enhanced Si effects the distance photogenerated carriers travel inside the avalanche photodetectors can provide high gain , but can gain region . As holes have a higher ionization rate that have high dark current and significant long wavelength electrons in 4H - SiC and deep ultraviolet photons are response that can make them suboptimal for applications absorbed very close to the surface , an n - i - p device operating such as biological agent detection . Commercially available 45 at these wavelengths has higher gain . This phenomenon is Si based single photon counting detectors ( SPADs ) have not partially responsible for the emerging 212 nm peak observed been specified for operation below 300 nm . The present in FIG . 3 at biases above 130V where one starts seeing invention provides an approach for developing a semicon - effects of gain . ductor based SPAD with efficient detection in DUV spectral As used herein in the drawings , specification . abstract range between 200 - 280 nm . The preferred embodiments are 50 and claims ) , the term “ light ” means electromagnetic radia advantageous for system designers because : tion , unless specifically noted to the contrary . In the draw

The preferred embodiments of the present invention can ings , the symbol means electromagnetic radiation . Within be operated at room temperature , while PMTs often require the light spectrum , the solar blind region refers to the region thermoelectric cooling depending upon the sensitivity of the light spectrum wherein , due to absorption of sunlight required . 55 by the atmosphere , the potential interfering effect of sunlight PMTs require the cathode detection material and dynode does not occur ; i . e . , normally considered to be less than 280

gain medium to be encased within a vacuum sealed tube . nm at low elevations . This packaging is inherently more fragile than that As used herein , the terminology “ layer " includes “ region ” employed for semiconductor based detectors . and is not limited to a single thickness of a material covering

The present invention provides a novel approach for 60 or overlying another part or layer , but encompasses a region increasing the deep ultraviolet response in a SiC based having a variety of configurations and / or thicknesses . avalanche photodiode in a more robust fashion . This is As used herein , the terminology “ multiplication layer " or accomplished by extending the depletion region of the “ multiplication region " means a layer or layers or region in illuminated n - type doped layer under high reverse bias which the carriers predominantly multiply . The carriers may where the device exhibits substantial gain . It also leverages 65 be either holes and / or electrons . the inhomogeneous gain associated with strong absorption As used herein , the terminology “ absorption layer ” , of these DUV photon at the illuminated semiconductor “ absorption region ” , “ absorber ” , “ absorber region ” means a

19 US 9 , 893 , 227 B2

20 layer or layers or region in which photons are predominantly readily modify and / or adapt for various applications such absorbed and photogenerated carriers created . Absorption specific embodiments without departing from the generic and multiplication may occur in the same layers ( or regions ) . concept , and , therefore , such adaptations and modifications As used herein , the terminology “ potential ” with respect should and are intended to be comprehended within the

to “ electrostatic potential ” refers to voltage potential . 5 meaning and range of equivalents of the disclosed embodi As used herein , the terminology InGaN , ( InGaN or ments . It is to be understood that the phraseology or termi

InxGal - xN refers to the binary compound GaN or a ternary n ology employed herein is for the purpose of description and compound of InGaN having arbitrary mole fraction of Inn . not of limitation . Therefore , while the embodiments herein As used herein , the terminology AIGAN , ( Al ) GaN or have been described in terms of preferred embodiments ,

AlxGal - xN refers to the binary compound GaN ( when x = 0 ) 10 those skilled in the art will recognize that the embodiments or a ternary compound of AlGaN having arbitrary mole herein can be practiced with modification within the spirit fraction of AIN . and scope of the claims . As used herein , the terminology ( Al ) ( In ) GaN or ( In ) ( AI )

GaN refers to the binary compound GaN or ternary or The invention claimed is : quaternary III - Nitride semiconductor compound having 15 1 . A photodiode for detecting photons in deep ultraviolet arbitrary mole fractions of InN and / or AIN . energy range comprising : As used herein , the terminology “ approximately ” means a substrate ;

something is almost , but not completely , accurate or exact ; a first semiconducting region operatively associated roughly . with the substrate suitable for forming a contact

As used herein , the terminology ( In ) AIN refers to the 20 thereon ; binary compound AIN or ternary compound having arbitrary a first contact operatively associated with the first mole fractions of InN . semiconducting region that collects first carriers ; As used herein the terminology “ p - metal contact ” means a second semiconducting region comprising an absorp

a metal contact to a p - type layer . tion region for the photons in the deep ultraviolet As used herein the terminology " n - metal contact ” means 25 energy range impinging upon an absorption interface

a metal contact to an n - type layer . of the absorption region ; the second semiconducting As used herein , the terminology player means p - type region being formed of a semiconductor having one

doped semiconductor layer . or more of a high surface recombination or a high As used herein , the terminology n + layer means an n - type interface recombination for carriers generated by

doped semiconductor layer with increased doping concen - 30 deep ultraviolet photons and being suitable for form tration . ing a contact thereon ; As used herein , the terminology p * layer means a p - type a second contact that collects second carriers opera

doped semiconductor layer with increased doping concen tively associated with the second semiconducting tration . region ; As used herein , the terminology n layer means a n - type 35 an intermediate semiconducting region between the

semiconductor that has sufficiently low doping so that it is first semiconducting region and the second semicon mostly depleted at zero bias m ducting region wherein carrier multiplication occurs

As used herein , the terminology p » - layer layer means a by impact ionization ; p - type semiconductor that has sufficiently low doping so that a first interface between the first semiconducting region it is mostly depleted at zero bias 40 and the intermediate semiconducting region ; As used herein , the terminology N - layer refers to a a second interface between the second semiconducting

semiconductor layer having n - type doping and is transparent region and the intermediate semiconducting region ; at the wavelength of interest for detection doping of the second semiconducting region being less As used herein , the terminology P - layer refers to a semi than doping of the first semiconducting region such

conductor layer having p - type doping and is transparent at 45 that depletion occurs to a greater extent towards the the wavelength of interest for detection absorption interface upon which the photons As used herein , the absorption depth refers to a thickness impinge in the second semiconducting region than in

within a layer wherein 1 - 1 / e ( e = natural logarithm = 0 . 368 ) ; the first semiconducting region upon increasing i . e . , approximately 63 % of the photons at the detection reverse bias ; the second semiconducting region hav wavelength of interest are absorbed according to Beer ' s 50 ing a doping concentration such that reverse biasing Law . the photodiode between the first and second contacts

As used herein the effective diffusion length is the dis produces an undepleted portion adjacent to the tance from the depth from the absorption depth at which the absorption interface having a thickness less than an carriers are created to the distance such that a majority of absorption depth from the absorption interface so carriers reach the depletion region prior to recombining . 55 that recombination of carriers is mitigated , and a Diffusion is the net movement of carriers from a region of depleted portion ; the undepleted portion being high concentration to a region of low concentration of located adjacent to the depleted portion such that a carriers . majority of the photons impinging upon the second

It is understood that an absorption depth and the effective semiconducting region are absorbed in the depleted diffusion length ( see FIG . 26 ) are parameters that describe a 60 portion and converted to carriers and such that with distribution of carriers and that the initial distribution is the increasing reverse bias , the depleted portion defined by the photon absorption probability according to , increases in thickness towards the absorption inter for example , Beer ' s Law , as appreciated by persons of face such that the undepleted portion becomes ordinary skill in the art . smaller than the absorption depth and therefore deep

The foregoing description of the specific embodiments 65 ultraviolet photo - generated carriers are collected will so fully reveal the general nature of the embodiments more efficiently instead of being lost to recombina herein that others can , by applying current knowledge , tion at the absorption interface ;

40

US 9 , 893 , 227 B2

CO

the second semiconducting region having the doping electron concentration from 1x1017 cm - 3 to 1x1019 cm - and concentration that varies with thickness such that the having a thickness in a range of 50 - 470 nm . doping concentration increases from the second 7 . The photodiode of claim 1 wherein the thickness of the interface to the absorption interface resulting in a second semiconducting region is between 20 - 120 nm and diminishing electric field from the second interface 5 preferred n - type doping concentration range is 9x1017 to to the absorption interface ; the undepleted portion 5x1018 cm - 3 . being sufficiently doped to create an N or P type 8 . The photodiode of claim 1 wherein the first semicon material and remain undepleted during reverse bias ducting region comprises p - type silicon carbide with an ing ; aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm3 whereby the second carriers generated will have a short 10 1 and wherein the intermediate region comprises n - type sili path to the absorption interface and the first carriers con carbide with a nitrogen atom doping in a range from will have a long path to the first semiconducting 1x1015 cm - 3 to 1x1016 cm - 3 and a thickness in a range from region which enables a gain to be greater for the deep ultraviolet photons than for photons not in the deep 250 - 1000 nm and wherein the second semiconducting ultraviolet energy range . s region comprises a first portion comprising n - type silicon

2 . The photodiode of claim 1 wherein the second semi carbide with a nitrogen doping increasing from 1x1016 cm3 is formed of SiC and wherein the first to 1x1018 cm over a thickness in a range from 50 to 100

carriers are holes and the second carriers are electrons , and nm and a second portion comprising n - type silicon carbide wherein the second semiconducting region has the doping with a nitrogen doping up to 5x1018 cm over a thickness concentration of n - type dopant impurities that increases 20 in a range from 10 to 40 nm and wherein upon the reverse from the second interface to the absorption interface upon biasing the undepleted portion having the thickness less than which the photons impinge by a gradual increase or a the absorption depth forms in the second portion that variable rate increase wherein in the second semiconducting enhances the gain for the deep ultraviolet photons relative to region only photon absorption and carrier generation occurs that for the photons not in the deep ultraviolet energy range . and not multiplication of carriers . 25 9 . The photodiode of claim 1 wherein the first semicon

3 . The photodiode of claim 1 further comprising one of a ducting region comprises p - type silicon carbide with an semi - transparent metal conductor region or a transparent aluminum doping in a range from 1x1018 cm - - 1x1019 cm3 semiconducting region at the deep ultraviolet energy range and wherein the second semiconducting region comprises a adjacent to the second semiconducting region suitable for first portion comprising n - type silicon carbide with a nitro forming an n - metal contact that uniformly spreads an elec - 30 gen doping increasing from 1x1016 cm - 3 to 1x1018 cm3 tric field laterally within the photodiode ; and wherein the over a thickness in a range from 50 to 100 nm and a second second semiconducting region is formed of SiC and has the portion comprising n - type silicon carbide with a nitrogen doping concentration of n - type dopant impurities that varies doping concentration in a range of 1 - 5x1018 cm - 3 over a with thickness . thickness in a range of 10 to 40 nm and wherein upon the

4 . The photodiode of claim 1 wherein the first semicon - 35 reverse biasing , the undepleted portion having the thickness ducting region comprises p - type silicon carbide with an less than the absorption depth forms in the second portion aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm - that enhances the gain for the deep ultraviolet photons and wherein the intermediate region comprises n - type sili - relative to that for the photons not in the deep ultraviolet con carbide with a nitrogen atom doping in a range from energy range such that recombination of carriers at the 1x1015 cm - 3 to 1x101 cm - 3 and a thickness in a range from 40 absorption interface is mitigated . 250 - 1000 nm and wherein the second semiconducting 10 . The photodiode of claim 1 wherein the first semicon region comprises a first portion comprising n - type silicon ducting region comprises p - type silicon carbide with an carbide with a nitrogen doping increasing from 1x1016 cm - 3 aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm3 to 1x1018 Cm - over a thickness in a range from 50 to 100 and wherein the second semiconducting region comprises nm and a second portion comprising n - type silicon carbide 45 n - type silicon carbide with a nitrogen atom doping in a range with a nitrogen doping in the range of 1 - 5x1018 cm - 3 over from 1x1015 cm - 3 to 1x1016 cm - 3 and a thickness in a range a thickness in a range of 10 to 40 nm and wherein upon the from 250 - 1000 nm and further comprising a transparent reverse biasing , the undepleted portion having the thickness semiconductor conducting region comprising AlGaN having less than the absorption depth forms in the second portion a ratio of aluminum to gallium of greater than 80 % , and such that the recombination of carriers at the absorption 50 having a free electron concentration from 1x1017 cm - to interface is mitigated . 1x1019 cm - and having a thickness between 50 - 470 nm .

5 . The photodiode of claim 1 wherein the second semi - 11 . A photodiode for improved collection of carriers conducting region comprises n - type silicon carbide having a generated by photons in deep ultraviolet energy range com first portion having a thickness in a range from 50 to 100 nm prising : and a second portion adjacent to the absorption interface 55 a substrate ; having a thickness in a range of 10 to 40 nm ; and wherein a first semiconducting region operatively associated with upon the reverse biasing , the undepleted portion having the the substrate suitable for forming a contact thereon ; thickness less than the absorption depth forms in the second a first contact operatively associated with the first semi portion such that the recombination of carriers at the absorp conducting region that collects first carriers ; tion interface is mitigated . 60 a second semiconducting region comprising a boundary

6 . The photodiode of claim 1 further comprising a trans upon which the photons impinge and an absorption parent semiconducting region that is transparent at the deep region for the photons having the deep ultraviolet ultraviolet energy range adjacent to the absorption interface energy range ; the second semiconducting region being of the second semiconducting region and suitable for mount formed of a semiconductor having a high surface or ing the second contact and wherein the transparent semi - 65 interface recombination at an absorption boundary for conducting region comprises AlGaN having a ratio of alu carriers generated by deep ultraviolet photons and minum to gallium of greater than 80 % , and having a free suitable for forming a contact thereon ;

US 9 , 893 , 227 B2 23 24

a second contact that collects second carriers operatively bias the depletion occurring in the n - type silicon carbide of associated with the second semiconducting region ; the second semiconducting region and the p - type silicon

the first semiconducting region and the - second semicon - carbide of the first semiconducting region increases ; the ducting region forming a first interface ; doping of the increase in the depletion of the n - type silicon carbide of the second semiconducting region being less than doping 5 second semiconducting region being larger due to lower of the first semiconducting region such that depletion doping in the n - type silicon carbide of the second semicon occurs to a greater extent towards the boundary upon d ucting region ; the increase in depletion of the n - type silicon which the photons impinge in the second semiconduct - carbide of the second semiconducting region extends within ing region than in the first semiconducting region upon the absorption depth of the photons resulting in the increasing reverse bias ; the second semiconducting 10 improved collection of the carriers generated by the photons region having a doping concentration such that reverse being absorbed in the depletion rather than lost to surface biasing the photodiode between the first and second recombination ; whereby increasing the depletion enhances contacts - produces an undepleted portion adjacent to the deep ultraviolet photoresponse . absorption boundary having a thickness less than an 15 . The photodiode of claim 11 further comprising one of absorption depth from the absorption boundary so that 15 a semi - transparent metal conductor region or a transparent recombination of carriers is mitigated , and a depleted semiconducting region at the deep ultraviolet energy range portion ; the undepleted portion being located adjacent adjacent to the second semiconducting region suitable for to the depleted portion such that a majority of the forming an n - metal contact that uniformly spreads an elec photons impinging upon the second semiconducting tric field laterally within the photodiode ; and wherein the region are absorbed in the depleted portion and con - 20 second semiconducting region is formed of SiC and has a verted to carriers and such that with increasing reverse doping concentration of n - type dopant impurities that varies bias , the depleted portion increases in thickness with thickness . towards the absorption boundary such that the unde - 16 . The photodiode of claim 15 wherein the transparent pleted portion becomes smaller than the absorption semiconducting region - comprises AlGaN having a ratio of depth and therefore deep ultraviolet photo - generated 25 aluminum to gallium of greater than 80 % , and having a free carriers are collected more efficiently instead of being electron concentration from 1x1017 cm - to 1x1019 cm - and lost to recombination at the absorption interface . having a thickness in a range of 50 - 470 nm .

12 . The photodiode of claim 11 wherein the second 17 . The photodiode of claim 11 wherein the thickness of semiconducting region formed of SiC and wherein the first the second semiconducting region is between 20 - 120 nm carriers are holes and the second carriers are electrons and 30 and preferred n - type doping concentration range is 9x1017 to wherein the second semiconducting region has a doping 5x1018 cm - 3 . level of impurities that increases from the first interface to an 18 . The photodiode of claim 11 wherein the first semi opposing boundary of the second semiconducting region by conducting region comprises p - type silicon carbide with an a gradual increase or a variable rate increase . aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm3

13 . The photodiode of claim 11 wherein the first semi - 35 and wherein the second semiconducting region comprises a conducting region comprises p - type silicon carbide with an first portion of n - type silicon carbide with a nitrogen doping aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm3 increasing from 1x1010 cm - to 1x1018 cm - over a thick and wherein the second semiconducting region comprises a ness in a range from 50 to - 100 nm and a second portion first portion comprising n - type silicon carbide with a nitro comprising n - type silicon carbide having a doping of 1x1018 gen doping increasing from 1x1016 cm - 3 to 1x1018 cm - 3 40 cm - 3 - over a thickness in a range from 10 to 40 nm . over a thickness in a range from 50 to 100 nm and a second 19 . The photodiode of claim 11 wherein the second portion comprising n - type silicon carbide with a nitrogen semiconducting region is doped in a range from 1x1015 cm - 3 doping concentration up to 5x1018 cm and having a thick - to 1x1016 cm < 3 and has a thickness in a range from 250 - 1000 ness in a range of 10 to 40 nm ; and wherein with increasing nm and further comprising a transparent semiconducting bias , the depletion occurring in the n - type silicon carbide of 45 region that is transparent at a predetermined photon energy the second semiconducting region and the p - type silicon range ; the transparent semiconducting region being suitable carbide of the first semiconducting region increases ; the for mounting the second contact and adjacent to the second increase in the depletion of the n - type silicon carbide of the semiconducting region and wherein the transparent semi second semiconducting region being larger due to lower conducting region comprises AlGaN having a ratio of alu doping in the n - type silicon carbide of the second semicon - 50 minum to gallium of greater than 80 % , and having a free ducting region ; the increase in depletion of the n - type silicon electron concentration from 1x1017 cm ^ 3 to 1x1019 cm - 3 and carbide of the second semiconducting region extending having a thickness in a range of 50 - 470 nm . within the absorption depth of the photons resulting in the 20 . A method of making a photodiode for detecting improved collection of the carriers generated by the photons photons in deep ultraviolet range that eliminates or mini rather than lost to surface recombination ; whereby increas - 55 mizes surface recombination of photogenerated carriers at ing the depletion enhances deep ultraviolet photoresponse an absorption interface comprising the following steps , not relative to the near ultraviolet response . necessarily in the following order :

14 . The photodiode of claim 11 wherein the first semi providing a substrate ; conducting region comprises p - type silicon carbide with an providing a first semiconducting region operatively asso aluminum doping in a range from 1x1018 cm - 3 - 1x1019 cm 3 60 ciated with the substrate suitable for forming a contact and wherein the second semiconducting region comprises a thereon ; first portion comprising n - type silicon carbide with a nitro providing a first contact operatively associated with the gen doping increasing from 1x1016 cm - 3 to 1x1018 cm - 3 first semiconducting region that collects first carriers ; over a thickness in a range from 50 to 100 nm and a second providing a second semiconducting region formed of a portion comprising an n - type silicon carbide with a nitrogen 65 semiconductor having a high surface recombination or doping in a range of 1x1018 cm 3 to 5x1018 cm - 3 over a a high interface recombination and comprising an inter thickness in a range of 10 to 40 nm ; whereby with increasing face upon which photons impinge ,

26 US 9 , 893 , 227 B2

25 providing a second contact that collects second carriers majority of the photons impinging upon the second

operatively associated with the second semiconducting semiconducting region are absorbed in the depleted region ; portion and converted to carriers and such that with the

providing an intermediate semiconducting region increasing reverse bias , the depleted portion increases between the first semiconducting region and the second 5 in thickness towards the absorption interface such that semiconducting region wherein carrier multiplication the undepleted portion becomes smaller than the occurs by impact ionization , wherein a second interface absorption depth and therefore deep ultraviolet photo is between the second semiconducting region and the generated carriers are collected ; intermediate semiconducting region ; the second semiconducting region having the doping

the second semiconducting region being doped less than 10 concentration that varies with thickness such that the doping of the first semiconducting region such that doping concentration increases from the second inter depletion occurs to a greater extent towards the inter face to the absorption interface resulting in a dimin face upon which the photons impinge in the second ishing electric field from the first second interface to the semiconducting region than in the first semiconducting absorption interface ; the undepleted portion being suf region upon increasing reverse bias ; the second semi - 13 ficiently doped to create an N or P type material and conducting region having a doping concentration such remain undepleted during reverse biasing ; that reverse biasing the photodiode between the first whereby the second carriers generated will have a short and second contacts produces an undepleted portion path to the absorption interface and the first carriers adjacent to the absorption interface having a thickness will have a long path to the first semiconducting region less than an absorption depth from the absorption 2 which enables a gain to be greater for the deep ultra interface so that recombination of carriers is mitigated , violet photons than for photons not in the deep ultra and a depleted portion ; the undepleted portion being violet energy range . located adjacent to the depleted portion such that a * * * * *