New Type of Sensitive Infrared and Submillimeter Radiation Photodetectors

Dmitry Khokhlov

Moscow State University, Moscow, Russia

Acknowledgements N.B.Brandt B.A.Akimov L.I.Ryabova S.N.Chesnokov I.I.Ivanchik D.E.Dolzhenko

D.Watson J.Pipher

A.Nikorich E. Slyn’ko

B.Volkov K.Kikoin

A.Artamkin A.Kozhanov N.Matrosov

Outline

1. Undoped lead telluride-based alloys. 2. Effects appearing upon doping. a) Fermi level pinning effect. b) Persistent photoconductivity. 3. Theoretical models 4. Pb1-xSnxTe(In)-based infrared photodetectors. a) Radiometric parameters. b) Comparison with Si(Sb) and Ge(Ga). c) Spectral response. d) “Continuous” focal plane array. 5. Summary.

Undoped Lead Telluride-Based Alloys

PbTe: narrow-gap semiconductor: 1. Cubic face-centered lattice of the

NaCl type 2. Direct gap Eg = 190 meV at T = 0 K

at the L-point of the Brillouin zone 3. High dielectric constant 103. 4. Small effective masses m 10-2

me.

Pb1-xSnxTe Solid Solutions:

0 0.1 0.2 0.3 0.4

-100

0

100

200

Ev

Ec

E, meV

x

Origin of free carriers: deviation from stoikhiometry 10-

3.As-grown alloys: n,p 1018-1019 cm-3

Long-term annealing: n,p > 1016 cm-3

Effects Appearing upon Doping

Fermi Level Pinning Effect.

7 1018

cm-3

NIn

p

n

PbTe(In), NIn > Ni

Doping with In, Ga, Tl

Ev

Ec

190 meV

130 meV

70 meV

Tl

In

Ga

70 meV

Consequences

1. Absolute reproducibility of the sample parameters independently of the

growth technique. Therefore low production costs.

2. Extremely high spatial homogeneity.

3. High radiation hardness (stable to hard radiation fluxes up to 1017 cm-

2)

Shubnikov – de Haas Magnetoresistance Oscillations in

PbTe(In)

Fermi Level Pinning in the Pb1-xSnxTe(In) Alloys.

0,0 0,1 0,2 0,3 0,4

-100

-50

0

50

100

150

200

semiinsulating state

p-type metal

n-type metal

Ev

Ec

EF

E, meV

x

Persistent Photoconductivity

0 5 10 15 20 2510-2

10-1

100

101

102

103

104

105

3'

2'

32

4'

41'

1

R, Ohm

100/T, K-1

Temperature dependence of the sample resistance Rmeasured in darkness (1-4) and under infrared illumination (1'-4') in alloys with x = 0.22 (1, 1'), 0.26 (2, 2'), 0.27 (3, 3') and 0.29 (4, 4')

Photoconductivity Kinetics

> 104 s

Ordinaryphotoconductor

Persistent photoconductor

tradiationoff

radiation on

Long lifetime of the photoexcited electrons is due to a barrier between local and extended electron states – DX-like impurity centers.

Model of the mixed valence

2In2+In3+ + In+

neutr. donor accept.

Еion(left) = Е(1) + Е(1)

Еion(right) = Е(2) + Е(1)

Е(2) = Е(1) + UUsually U>0“Negative-U” center: U<0

In3+

In+

In2+

e-

e-

e-

In3+In3+

In3+In3+

e-

In2+

Model for long-term processes

Configuration-coordinate diagramEtot = Eel + Elat =

= (Ei-)n +2/20

(n = 0,1,2)

E2 – ground local state;

E1 – metastable local state

Alternative explanation

Ec

Ev

s2p1

s1p2

s0p1pl2 s1p2 s2p1 s0p3

s0p1pl

2

One local level pl is formed for N >> 1 (up to 103 - 104) s0p3 centers

2 In2+ In+ + In3+

Quenching of the Persistent Photoconductivity

1. Thermal quenching: heating to 25 K and cooling down: too slow process.

2. Microwave quenching: application of microwave pulses to the samplesf = 250 MHz, P = 0.9 W, t = 10 sQuantum efficiency of the photodetector may be increased up to 102 in some special regime of the microwave quenching.

<< I

3I2I

1

I3

I2

I1

t

quenching pulses

Pb1-xSnxTe(In)-Based

Infrared Photodetectors Single photodetector operating in the regime of the periodical accumulation and successive fast quenching of the photosignal, quantum efficiency stimulation regime.

operating temperature 4.2 K; wavelenghth 18 m (defined by the filter); operating rate 3 Hz; area 300*200 m; current sensitivity > 107 A/W; minimal power detected < 10-16 W (sensitivity

of the measuring electronics was only 10-7 A).

Experimental setup 1 - 300 K or 77 K

blackbody; 2 - 300 K window; 3 - 77 K filter; 4 - 4.2 filter and a

stop aperture; 5 - cold filter on the

filter wheel; 6 - filter wheel; 7 - sample; 8 - liquid helium can.

Comparison with Si(Sb) and Ge(Ga)

-0,5 0,0 0,5 1,0 1,5 2,0 2,5

0,0

5,0x10-3

1,0x10-2

1,5x10-2

2,0x10-2

amplifier overload

77

300

I, A

V, V

= 14 m; state of the art Si(Sb) BIB

Pb1-xSnxTe(In) photodetector: dark current at the minimal possible bias of 40 mV (red point)

Kinetics of Response of the Pb1-xSnxTe(In) Photodetector at = 90 m and 116 m, at 40 mV Bias

0 10 20 30 40 50 60 70 800,0

2,0x10-2

4,0x10-2

6,0x10-2

8,0x10-2

1,0x10-1

1,2x10-1

1,4x10-1

1,6x10-1

77 K, 89 m

77 K, 115 m300 K, 89 m

300 K, 115 m

I, A

t, s

Pb1-xSnxTe(In): SI103A/W

State of the art Ge(Ga): SI = (3.3-3.5) A/W

Extremely important:

E=(90, 116)m < Ea

Photoresponse is due to excitation from metastable excited local states.The cutoff wavelength red may be much higher than 220 m.

Ea E

ground

Emetastable

Ec

Ev

Photoresponse at 176 m and 241 m

Strong photoresponse at both 176 and 241 m

= 241 m is higher than red = 220 m observed for Ge(Ga) 0 200 400 600 800

0,0

1,0x10-2

2,0x10-2

3,0x10-2

4,0x10-2

5,0x10-2

6,0x10-2

Cur

rent

, A

Time, s

241 m

176 m

Focal-Plane “Continuous” Array.

Local infrared illumination leads to local generation of photoexcited free electrons.

Spatial characteristic scale < 100 m

radiation "spot"photodetector

h

Ideas of Readout

n rows

m columns

1.Contact readout

Columns and rows are insulated at the intersections

Disadvantages:

Sophisticated multiplexing;

n+m electrical leads

Ideas of Readout

Incident radiation

Scanning beamof electrons

Semitransparentelectrode

Active Pb1-xSnxTe(In)layer

2. Electron beam readout

Disadvantages:

Difficulties with high vacuum and low temperatures;

Not clear how electrons will affect conductivity;

Heated cathode needs to be screened from the sample

Ideas of Readout

3. Laser beam readout

1 - semitransparent electrodes

2 - active Pb1-xSnxTe(In) layer

3 - fluoride buffer layer, 4 – wide-gap

semiconductor layer, 5 - short-wavelength laser 6 - incident infrared

radiation flux 1 12 3 4

56

A

Summary

Pb1-xSnxTe(In) photodetectors have a number of advantageous features that allow them to compete successfully with the existing analogs:

Internal accumulation of the incident radiation flux,

Possibility of effective fast quenching of an accumulated signal

Microwave stimulation of the quantum efficiency up to 102

Possibility of realization of a "continuous" focal-plane array

Possibility of application of a new readout technique

High radiation hardness