x-rays techniques as a powerful tool for characterisation of thin film nanostructures

X-rays techniques as a X-rays techniques as a powerful tool for powerful tool for characterisation characterisation

of thin film nanostructuresof thin film nanostructures  

ElElżżbieta Dynowskabieta DynowskaInstitute of Physics Polish Academy of Sciences,

al. Lotników 32/46, Warsaw, Poland

dynow@ifpan.edu.pl 

Workshop on Semiconductor Processing for Photonic Devices, Sept. 30 – Oct. 2, Warsaw, Poland

1. Introduction

2. Basics General information about nanostructures What we want to know about thin layers? How to get this information?

3. Selected X-ray techniques X-ray reflectivity X-ray diffraction

4. Synchrotron radiation – new possibilities

5. Summary

OutlineOutlineOutlineOutline

x

Homoepitaxial layer – the layer and substrate are the same material (the same lattice parameters).

Heteroepitaxial layer – the layer material is different than the substrate one (different lattice parameters).      

Thin layer – the dimension in the z-direction is much smaller than in the x and y, respectively.

single crystal thin layer having the

crystal structure and orientation of

single crystal substrate on which it

was grown.

x

y

z

(A) Epitaxial layerz

y

0

0

(B) Polycrystalline layers –

orientations of small crystallites are

randomly distributed with respect to

layer surface(C) Amorphous layers - lack of long-distance ordering of atoms

Lattice mismatch –

f = (alayer - asubs )/ asubs

Critical thickness hc – thickness below which the layer grows pseudomorphically the cubic unit cell

of layer material is tetragonally distorted: alz alx = aly = as (the layer is fully

strained). hc decreasing when f increasing.

Layer relaxation - alxy alz al relax = abulk

alayeras

ayax

az

What we want to know about thin layers?What we want to know about thin layers? 

Crystalline state of layer/layers

(epitaxial?; polycrystalline?; amorphous? …)

crystal quality;

strain state;

defect structure;

chemical composition (in the case of ternary compounds layers);

thickness

surface and interface roughness, and so on…

 How to get this information?How to get this information?

 By means of X-ray techniquesBy means of X-ray techniques

Because X-ray techniques are the Because X-ray techniques are the most important, non-destructive most important, non-destructive

methods of samplemethods of sample characterizationcharacterization

Why?Why?

Selected X-ray techniqueSelected X-ray techniquess

X-ray reflectivity Small-angle regionRefraction index for X-rays n < 1: Roughness investigation

x

z

n = 1- + i

Layer thickness determination

i i

t

The distance between the adjacent interference maxima can be approximated by:

i / 2t

kIkR

kT

c

i

~10-5 in solid materials (~10-8 in air); - usually much smaller than .

2i

Si rough wafer - simulation

Example: Example: superlattice

Si/{Fe/Fe2N}x28/GaAs(001)

28 times

repeated

GaAs

Fe

Fe2N

Si cap-layer

Inte

nsi

ty

i (deg)

c0.3

(2) - superlattice period

(2) –

cap-layer

Experiment

Simulation

Results of simulation

10.4 nm

4.52nm

126.6nm

All superlattice

X-ray diffraction wide-angle region

d’hkl

Bragg’s law:

n = 2d’sin

d’/n = d

= 2d sin

Geometry of measurement

Detector

Incidentbeam 

Diffractedbeam 

2

/2 coupling

/2 coupling

DetectorIncidentbeam 

Diffractedbeam 

2’

Crystalline state of layer/phase analysis

PossibilitiesPossibilities

25 30 35 40 45 50 55 6010

100

1000

10000

100000

202

NiA

s-ty

pe

004

NiA

s-ty

pe

w 00

4sf

222

006

Al2O

3

10

2 N

iAs-

type

101 N

iAs-

type

002

NiA

s-ty

pe

w 00

2sf

111

ZnMnTe/MnTe/Al2O

3

Sample 082703A T

Zn= 225oC

Inte

nsity

(cpu

)

2 theta (deg)

35 40 45 50 55 60 650

2000

4000

6000

8000

10000

12000

MnTe hex. 00.4 k

kk

Al2O

3 substrate

00.6

MnTe hex. 00.2

Imax

= 270000 cps

Inte

nsity

[cps

]

2 theta [deg]

MnTe/Al2O3

FeK radiationCuK1 radiation

ZnMnTe/MnTe/Al2O3

24,0 24,2 24,4 24,6 24,8 25,0 25,2 25,4

100

1000

10000

ZnMnTe/MnTe/Al2O

3

Sample 082703A T

Zn= 225oC

ZnMnTe sfaleryt 111

ZnMnTe wurcyt 002

Inte

nsity

(cps

)

2 theta (deg)

Crystal quality

„Rocking curve”

Detector

32,6 32,8 33,0 33,2 33,4100

1000

10000

100000 ZnSe/GaAs 131101004 rocking curve

FWHM = 112 arcsec

ZnSeLayer

FWHM = 21 arcsec

GaAsSubstrate

Omega (deg)

Inte

nsity

(cps

)

Lattice parameter fluctuations

Mosaic structure

?

21 arcsec

112 arcsec

Strain state & defect structure

Cubic unit cell of substrate:

Cubic unit cell of layer material

Strain tetragonal deformation of cubic unit cell: Pseudomorphic

case

Partially relaxed

Relaxed

as

alayer

ayax

az

az ax = ay = asub

az ax = ay asub

axay

az

alayer

az = ax = ay = alayer

The reciprocal lattice maps

S = [100]

Reciprocal lattice:

pseudomorphic

Origin

001

002

003

004

101

100

102

200 300

201

202

P = [001]sample

The sample orientation can be described by two vectors:

P - vector which is the direction normal to the sample surface;

S – any other vector which is not parallel to the P vector and lies in the horizontal plane.

|H|102 = 1/d102

Mosaic structure

relaxedLattice

parameter fluctuations

xd00l

z

Symmetric case

dhhldz

dx

Asymmetric case

Examples In0.50Al0.50As/InP

004

224 224

004

(a) (b)

2

222

2 a

lkh

d

1 For cubic system: For tetragonal

system:2

2

2

22

2 c

l

a

kh

d

1

chemical composition

Vegard’s rule:

If AB and CB compounds having the same crystallographic system and space group create the ternary compound A1-xCxB then its lattice parameter a ACB depends linearly on x-value between the lattice parameters values of AB and CB, respectively.

aAB

aCB

abulk

x0 1

aACB

x

In the case of thin layers arelaxed must be taken for chemical composition determination from Vegard’s rule:

)/(21

)/(2

1112

1112

cc

accaa

xyzrelaxed

c12, c11 – elastic constants of layer material

28.4 28.6 28.8 29.0 29.2 29.4 29.6 29.8Omega (°)

100

1K

10K

100K

counts/s

S

1-3.x00

59.6 59.8 60.0 60.2 60.4 60.6 60.8 61.0 61.2 61.42Theta/Omega (°)

1

10

100

1K

10K

counts/s

S

1-6.x00

40 60 80 100 120 140 160 180Qx*10^4 (rlu)

4980

5000

5020

5040

5060

5080

Qz*10^4 (rlu) 1-7m.A00

1.7

3.4

6.6

12.8

24.8

48.2

93.7

181.9

353.3

686.0

1332.1

2586.7

5022.9

9753.5

18939.6

5420 5440 5460 5480 5500 5520Qx*10^4 (rlu)

6140

6150

6160

6170

6180

6190

6200

6210

6220

Qz*10^4 (rlu) 1-18m.A00

1.6

2.9

5.3

9.6

17.4

31.5

57.0

103.0

186.3

336.9

609.3

1101.9

1992.8

3604.0

6517.9

Heterostructure: ZnMnxTe/ZnMnyTe/ZnMnzTe/ZnTe/GaAs

004 rocking curveZnTex

yz

004 /2

004

335 relaxed

pseudomorphic

Ti/TiN/GaN/Al2O3 under annealing

0 200 400 600 80010

2

103

104

105

106

SIM

S s

igna

l [ c

/s ]

Time [ s ]

TiGa

N

GaN/Ti as-deposited

Time [ s ]

0 200 400 600 80010

2

103

104

105

106

TiGa

N

GaN/Ti

900oC, 30s, N

2

SIM

S s

igna

l [ c

/s ]

Towards an ohmic contactsTowards an ohmic contacts

Secondary Ion Mass Spectrometry (SIMS)

XRD

NbN/GaN/Al2O3

45 50 55 60 65 70

k

k

kkAl2O3

NbN 10.1

GaN 00.2 GaN/NbN

1000oC, 30s, N2

2 (deg)

45 50 55 60 65 700

2000

4000

6000

8000

10000

12000

14000

k

k

GaN/NbN as-deposited

kkAl

2O

3

NbN 10.1

GaN 00.2

Inte

nsi

ty [ c

ps

]

2 (deg)

0 200 400 60010

1

102

103

104

105

106

107

108

GaN/NbNas-deposited

N

Nb

Ga

SIM

S S

ignal [ c/s

]

Time [ s ]

0 200 400 60010

1

102

103

104

105

106

107

108

GaN/NbN

900oC, 30s, N2

N

Nb

Ga

SIM

S S

ignal [ c/s

]

Time [ s ]

XRD

XRD

(SIMS)

(SIMS)

30 40 50 60 70

0

50

100

150

200

250

300

pN2=5-7x10-3mbar

PRF=1.9W/cm2,

ptot.=1x10-2mbar,

400 Zn3N2

Inte

nsi

ty

2 [ deg ]

30 40 50 60 70

200

400

600

800

1000 pN2

=2.5x10-3mbar

400

Z

n3N

2

PRF=1.9W/cm2,

ptot.=1x10-2mbar,

222

Zn

3N2

440

Zn

3N2

332

Zn

3N2

321

Zn

3N2

Inte

nsi

ty

2 [ deg ]30 40 50 60 70

100

200

300

400

500

600

700

pN2

=2x10-3mbar

PRF=1.9W/cm2,

ptot.=1x10-2mbar,

Zn

400

Z

n3N

2

440

Zn

3N2

321

Zn

3N2

Inte

nsi

ty [

a.u

.]

2 [ deg ]

20% N2 Zn3N2 + Zn 25% N2 polycryst. Zn3N2

50% - 70% N2 monocryst.

30 40 50 60 700

50

100

150

200

250

300 pN2=9x10-3mbar

400 Zn3N2

PRF=1.9W/cm2,

ptot.=1x10-2mbar,

Inte

nsi

ty

2 [ deg ]

GaN, Al2O3, ZnO

N2>80% polycryst. & amorph.

Deposition of Zn3N2 by reactive rf sputtering

Zn3N2

40 45 50 55 60 95 100

200

400

600

800

1000

Zn3N

2 d=650nm on GaN

oxidation @ 600oC, 15 min. 00.4

GaN

00.4

Zn

O

00.6

Al 2O

3

00.2

Zn

O00.2

GaN

Inte

nsi

ty

2 [ deg ]

polycrystalline ZnO on sapphire and quartz

40 45 50 55 600

2000

4000

6000

8000

ZnO

2 [ deg ]

Inte

nsi

ty (

a. u

.)

100

101

002

Zn3N2(50%N2)/ZnOsput./quartz

oxidation 600oC

45 50 55 600

200

400

600

800

1000

ZnO

Zn3N

2 d=650nm on Al

2O

3

oxidation @ 600oC,

15 min.

2 (deg)

101

102

002

KK

006 Al2O

3

Inte

nsi

ty (

cps)

40 50 60 700

100

200

300

400

500

600

700

800

ZnO

Zn3N

2 d=650nm on quartz

oxidation @ 600oC, 15 min.

110 102

101

101 K

002

100

Inte

nsi

ty

2 [ deg ]

ZnO:N by oxidation of Zn3N2 microstructure

highly textured ZnO on GaN and ZnO

30 40 50 60 70 80 90

100

1000

10000

Zn

O 1

01

ZnTe:N/ZnTe/GaAs

oxidized 6000C, 20 min.

Ga

As

00

4 (

K)

Zn

O 1

03

+ T

e 1

13

Z

nT

e 0

04

(K

)

Ga

As

00

4 (

K)

Zn

Te

00

4 (

K)

Te

20

1Te

11

0T

e 1

02

Zn

O 0

02

Zn

O 1

00

Z

nT

e 0

02

(K

)

Te

10

1

Zn

Te

00

2 (

K)

Inte

nsi

ty [

a.u

.]

2 [ deg ]0 1 2 3 4 5 6 7

1018

1019

1020

1021

1022

As

Ga

TeO

Zn

Depth [ m ]

Co

nce

ntr

atio

n o

f N

an

d H

in Z

nO

[a

t/cm

3 ]

100

101

102

103

104

105

106

107

108

SIM

S S

ign

al o

f Ga

, As, Z

n, T

e, O

an

d H

[ c/s ]

ZnTe:N/ZnTe/GaAs oxidised 6000C

ZnO by oxidation of ZnTe/GaAs

Te inclusions in ZnO film

XRD(SIMS)

Synchrotron radiationSynchrotron radiation

)bandwidth%1.0)(areasourcemm()mrad(

sec/PhotonsBrilliance

22

50 55 60 65 70 75

0

2

4

6

8

10

2 =

0.3

1o

Si1-x

Gex

bottom layer

"-1" "+1"

Ge004

Si (substrate) +Si

0.95Ge

0.05 (SL

0)

004

Inte

nsity

(a.u

.)

2 (deg)

Si substrate (001)

Si, 115 nm, 780 C

Si, 10 nm, 480 C

Si, 24 nm, 450 C Si, 2nm, 250 C

Ge, 1nm, 250 C

7 times

repeated

High resolution electron microscopy (HREM) –

JEOL-4000EX (400 keV)

55 60 65 702Theta/Omega (°)

0.01

0.1

1

10

100

1K

10K

100K

1M

10M

counts/s dorby-dla jarka.Z01

Example: superlattice of self-assembled ultra-small Ge quantum dots

Experimental diffraction pattern Simulated diffraction pattern

Si

004Ge

004 2 = 0.314o

Si0.8Ge0.2

bottom layer

„-1”„-2”

C

Results: HREM XRD

superlattice period C..... 33.5 nm 33 nm,

thickness of Ge............... 1.8 nm 2.0 nm

thickness of SiGex

bottom layer..................... 6.7 nm 6.7 nm

Compositon........................ ---- x 0.2

50nm

Hasylab (Hamburg), W1.1 beamline: X’Pert Epitaxy and Smoothfit software

Acknowledgements

I would like to express my gratitude to my colleagues for their kind help:

Eliana Kaminska

Jarek Domagala

Roman Minikayev

Artem Shalimov