Structure and Electrophysical Properties of the Transparent

Conducting Zinc and Indium Oxides Films

V.A. Kulbachinskii

V.G. Kytin, O.V. Reukova, D.S. Glebov, D.D. Melnik, A.R. Kaul, L.I. BurovaM.V. Lomonosov Moscow State University, Moscow, Russia

Yu.M. Galperin, A.G. Ulyashin

University of Oslo, Oslo, Norway

Outline

1) Transparent conducting oxides

2) Zinc oxide: - crystal structure; - electronic structure; - defects and dopants

- MOCVD grown undoped - Ga doped ZnO films- Co doped ZnO films

3) Indium oxide (In2O

3):

- crystal structure- electronic structure

- properties of Sn doped In2O

3

4) Summary

2

Transparent conducting oxides and their applications

3

Crystal structure of ZnO

Zn

O

Rocksalt

Zinc blende

Wurtzite

Basic structures of ZnO*

a=3.250 Åc=5.206 Å

4

Band structure of ZnO wurzite

Band structure of ZnO calculated by non-local (solid lines) and local (dashed lines) empirical pseudo potential method*

Brillouin zone of wurtzite ZnO

Electron effective mass: mII≈m

┴≈(0.25-0.3)m

0*

5

Defects in ZnO

Native donor defects:- Oxygen vacancies V

O

- Zinc interstitials Zni

Calculated formation energies of native defects in ZnO*

Native acceptor defects:- Zn vacancies Zn

V

- O interstitials Oi

Zn rich conditions O rich conditions

6

Structure of undoped ZnO films grown on R-Al2O

3 and ZrO

2(Y

2O

3)

(111) substrates by oxygen assisted MOCVD

XRD data of ZnO films grown on R-Al2O

3 and ZrO

2(Y

2O

3) (111) substrates: a) and b) on R-Al

2O

3 θ-

scan and φ-scan; c) and d) on ZrO2(Y

2O

3)(111) θ-scan and φ-scan

Epitaxial films with orientation determined by substrate 7

Structure of undoped ZnO films grown on MgAl2O

4 (111) substrates

by oxygen assisted MOCVD

a b

XRD data of ZnO films grown on MgAl2O

4(111) substrates: a) θ-scan; b) φ-scan

Epitaxial films with 2 in-plain orientations of ZnO with respect to substrate

8

Structure of undoped ZnO films grown by water assisted MOCVD

20 25 30 35 40 45 50 55 60

r-Al2O

3

*(110)

* (101) *

(002)

lg(I

nten

sity

), a

.u.

2degrees

*(100)

r-Al2O

3* - ZnO

20 25 30 35 40 45 50 55 60 65 70

CuK

log

(Int

ensi

ty),

a.u

.2deg.

YSZ(111)

YSZ(222)

ZnO(002)

ZnO(101)

XRD θ-scans of ZnO films grown on r-Al2O

3 and ZrO

2(Y

2O

3) (111) substrates at 300 0C by water

assisted MOSVD

No visible structure from ZnO in φ-scans

Polycrystalline films with chaotic orientation of crystallites

9

Surface morphology of undoped ZnO films grown by oxygen and water assisted MOCVD

rms 4.68 nm rms 40.78 nm

H2O assisted MOCVD O2 assisted MOCVD

AFM images of the surface of ZnO films grown on ZrO2(Y

2O

3) (111) substrates at 600 0C

Surface of ZnO films grown by water assisted MOCVD is smoother than by oxygen assisted MOCVD

10

Surface morphology of undoped ZnO films grown by water assisted MOCVD at different temperatures

SEM image ZnO film grown on R-Al

2O

3 at 300 0C

SEM image ZnO film grown on R-Al

2O

3 at 500 0C

11

M(H) at room temperature for the films deposited by water-assisted CVD onr-sapphire substrates at 300 °C (R_W_300) and at 500 °C (R_W_500).

Magnetic properties of undoped ZnO films grown by water assisted MOCVD at different temperatures

12

Resistivity of undoped ZnO films grown by oxygen assisted MOCVD

0 50 100 150 200 250 300 3501

10

100

1000

10000

100000

1000000

10000000

100000000

T, K

ρ, m

Ohm

cm

substrate R-Al2O

3

substrate C-Al2O

3

substrate MgAl2O

4 (111)

deposited at 500 0C

0 50 100 150 200 250 300 3501

10

100

1000

10000

100000

1000000

T, Kρ

, mO

hm c

m

substrate ZtO2(Y

2O

3)(111)

substrate R-Al2O

3

substrate C-Al2O

3

substrate MgAl2O

4(111)

deposited 600 0C

Lowest resistivity have the most ordered films grown on R-Al2O

3 and ZrO

2(Y

2O

3)(111) substrate

Highest resistivity have the films with 2 different orientation of crystallites grown on MgAl2O

4(111)

substrates

13

Hopping conductivity in undoped ZnO films grown by oxygen assisted MOCVD

0.2 0.3 0.4 0.5 0.6 0.73

5

7

9

11

T-1/4, K-1/4

ln(r

, mO

hmc

m)

T0=260 K

T0=2500 K

substrate C-Al2O

3

deposited at 500 0C

substrate MgAl2O

4(111)

deposited at 600 0C

r=r[(T 0

T )1 /4]

Mott's law:

T 0≈21.2

k B r03 g (EF )

14

Resistivity of undoped ZnO films grown by water assisted MOCVD

50 100 150 200 250 300 3501

10

100

1000

10000

T, K

ρ, m

Ohm

cm

substrate R-Al2O

3

substrate ZrO2(Y

2O

3)(111)

deposition at 600 0C

0.22 0.24 0.26 0.28 0.3 0.32 0.34 0.363

4

5

6

7

8

9

T-1/4, K-1/4

ln(r

, mO

hm c

m)

substrate R-Al2O

3

substrate ZrO2(Y

2O

3)(111)

T0=207000 K

T0=899000 K

deposition temperature 600 0C

Variable range hopping conductivity in a wide temperature range

15

Magnetoresistance of undoped ZnO films grown by oxygen assisted MOCVD

0 1 2 3 4 5 6 7 8-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

B, T

Δρ

/ρ(0

)

substrate R-Al2O

3

Dep. temp. 500 0C

substrate R-Al2O

3

Dep. temp. 600 0C

substrate MgAl2O

4(111)

Dep. temp. 600 0C

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

substrate R-Al2O

3

substrate ZrO2(Y

2O

3)(111)

deposition temperature 600 0C

B, TDr

/r(0

)

16

ZnO:Ga films

2θ, degree

Inte

nsity

, a.

u.

X-ray data of ZnO:Ga films deposited at 600 0C on ZrO2(Y

2O

3)(111) substrate by oxygen assisted

MOCVD: a) 1.7 at. % Ga; b) 3.6 at. % Ga.

Shift of ZnO peak positions correspond to increase of lattice constant with increase of Ga content

17

Resistivity of ZnO:Ga films grown by oxygen assisted MOCVD

ZnO:Ga on R-Al2O

3 substrate ZnO:Ga on ZrO

2(Y

2O

3)(111)

substrate

Resistivity decreases first with an increase of Ga content

0 50 100 150 200 250 300 3500

5

10

15

20

25

30

35

T, K

ρ, m

Ohm

cm

0 at. % Ga

5 at. % Ga

7.2 at. % Ga

0 50 100 150 200 250 300 3500

2

4

6

8

10

12

14

16

T, Kρ,

mO

hm c

m

0 at. % Ga

1.7 at. % Ga

6.8 at. % Ga

18

Magnetoresistance of ZnO:Ga films grown by oxygen assisted MOCVD

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

B, T

Dr/

r(0) 0 at. % Ga

5 at. % Ga

7.2 at. % Ga

substrate R-Al2O

3

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

B, TDr/

r(0)

0 at. % Ga

1.7 at. % Ga

6.8 at. % Ga

substrate ZrO2(Y

2O

3)(111)

19

Resistivity of ZnO:Ga films grown by water assisted MOCVD

0 50 100 150 200 250 300 3500.01

0.1

1

10

100

1000

Т, К

r, O

hm c

m

subsrate ZrO2(Y

2O

3)(111)

7 at. % Ga

subsrate ZrO2(Y

2O

3)(111)

25 at. % Ga

subsrate R-Al2O

3

21 at. % Ga

0.2 0.3 0.4 0.5 0.6 0.7-3-2-101234567

Т-1/4, К-1/4ln

(r, O

hm c

m

subsrate ZrO2(Y

2O

3)(111)

7 at. % Ga

subsrate ZrO2(Y

2O

3)(111)

25 at. % Ga

subsrate R-Al2O

3

21 at. % Ga

Variable range hopping conductivity in investigated temperature range

r=r[(T 0

T )1/ 4] T 0≈

21.2

k B r03 g (EF )

20

Magnetoresistance of ZnO:Ga films grown by water assisted MOCVD

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.85.4955.5005.5055.5105.5155.5205.5255.5305.5355.5405.545

B, Т

r, O

hm m

substrate ZrO2(Y

2O

3)(111)

7 at % Ga

0 1 2 3 4 5 60.0500.0510.0520.0530.0540.0550.0560.0570.0580.0590.060

B, T

r, O

hm m

substrate R-Al2O

3

21 at. % Ga

Estimate of r0 and g(E

F) from positive magnetoresistance and ρ(T)

Substrate Ga content, at. % r0, nm g(E

F), 1019 cm-3eV-1

R-Al2O

325 7 13

ZrO2(Y

2O

3)(111) 7 8 3.8

ZrO2(Y

2O

3)(111) 21 7 25

4/3

02

240

2

2016

50ln

T

TBre=

ρ

Bρ

21

ZnO:Co films deposited by oxygen assisted MOCVD

20 25 30 35 40 45 50 55 60 65 70

ZnO(002)

lg(I

nte

nsi

ty),

a.u

.

2deg.

ZnO:Co (7,7 at. % Co)

c-Al2O3

(006)

0 60 120 180 240 300

с-Al2O3

ZnO:Co (7,7 at. % Co)

Inte

nsi

ty, a

.u.

, deg.

Epitaxial films with oriention determined by substrate

22

Structure of ZnO:Co films deposited by water assisted MOCVD

20 25 30 35 40 45 50 55 60

ZnO(110)

ZnO(102)

ZnO(100)

ZnO(101)

Al2O3 (006)

ZnO(002)

lg (

Inte

ns

ity

), a

.u.

grad.

ZnO:Co(1,5 at. % Co)

20 25 30 35 40 45 50 55 60

ZnO(102)

ZnO(110)

ZnO(100)

ZnO(101)

ZnO(002)

Al2O3 (006)

deg.

lg (

Inte

nsi

ty),

a.u

.

12,4 at. % Co

No peaks in φ-scans. Polycrystalline structure with chaotic orientation of crystallites.

23

EXAFS (extended X ray absorption fine structure ) spectra and Co state in ZnO:Co films

Cobalt substitutes Zn up to 33 at. % content

EXAFS-спектроскопия — новый метод исследования вещества, позволяющий определять структурные параметры ближнего окружения атомов с выбранным Z, спектры которых изучаются. Среди этих параметров — межатомные расстояния, координационные числа, амплитуды тепловых колебаний. Существование дальнего порядка в исследуемых образцах не требуется. В зависимости от применяемой методики получения спектров можно анализировать ближнее окружение атомов, расположенных либо в объеме образца, либо на его поверхности.

24

Magnetic properties of ZnO:Co film

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

-1 ,0

-0 ,5

0,0

0,5

1,0

1,5

Zn0.937

Co0.063

O

M,

10

-12 (

A. m

2 )

B (T)

Zn0.985

Co0.015

O

25

Magnetoresistance of ZnO:Co films

0 1 2 3 4 5 6

0 .0 4

0 .0 6

0 .0 8

8 .0 x1 0-4

1 .0 x1 0 -32 .5 x1 0

-4

2 .5 x1 0-4

2 .6 x1 0 -4

2 .6 x1 0-4

rО

мм

B (Т л)

Zn0.937

C o0.063

O

Zn0.985

C o0.015

O

Zn O

B, T

ρ,

Oh

m c

m

0 1 2 3 4 5 6 70

0.050.1

0.150.2

0.250.3

0.350.4

0.450.5

B,T

Dr/

r(0)

oxygen assisted MOCVD1.5 at. % Co

water assisted MOCVD1.9 at. % Co

substrate R-Al2O

3

Value of positive magnetoresistance increases with an increase of Co content

Value of positive magnetoresistance is larger for the films grown by oxygen assisted MOCVD

Possible origin of positive magnetoresistance: reduction of the density of states at Fermi energy in magnetic field

26

In2O

3

Two inequivalent positions of In cations

27

Кубическая структура типа биксбита пространственной группы

3Ia

.

3Ia

Band structure of In2O

3

28

Band structure of In2O3

Brillouin zone of In2O3

Investigated In2O

3:Sn films

Deposition method: magnetron sputteringTargets: 1) Oxide target (baked In

2O

3 and SnO

2 9:1); 2) metal target In-Sn alloy

Substrate: glass

TargetDeposition

temperature, 0C Film thickness

oxide RT 80

oxide 230 80

oxide 230 75

Valence band XPS spectra of In2O

3:Sn films

Bandgap states in films deposited from metal target

29

X-ray phoemission data about O state in In2O

3:Sn films

High resolution O 1s spectra acquired after slight sputtering using low energy (500 eV) Ar+

30

XPS spectra of In and Sn states in In2O

3:Sn films

High resolution In 3d XPS spectra of ITO films deposited at different conditions

High resolution Sn 3d XPS spectra of ITO filmsdeposited at different conditions

Estimate of the film composition determined from XPS data

Target Deposition temperature In content, at. % Sn content, at. % O content, at. %

oxide RT 47.41 3.65 0.96

oxide 230 0C 47.66 3.50 0.95

metal 230 0C 47.93 4.04 0.92

31

Resistivity of In2O

3:Sn films deposited from oxide target

0 50 100 150 200 250 3000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

deposited at RT

deposited at 230 0C

deposited at 230 0C 30 min H plasma

deposited at 230 0C 5 min in H plasma

T, K

r, m

Ohm

. cm

The conductivity of the films deposited at 230 0C is higher than the conductivity of the films deposited at room temperature. This correlates with the better

crystallinity of the film deposited at 230 0CTreatment in H plasma leads to the increase of conductivity. The Effect is larger

for 5 min treatment than for 30 min treatment 32

Magnetoresisitance of In2O

3:Sn films deposited from oxide target

-5 -4 -3 -2 -1 0 10

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

ln(B, T)

Δσ

s/G

0

deposited at RTL

= 230 nm

deposited at 230 0CL

= 360 nm

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.0040

-0.0035

-0.0030

-0.0025

-0.0020

-0.0015

-0.0010

-0.0005

0.0000

B, T

Dr/r

(0) deposited at 230 0C

deposited at RT

Negative magnetoresistance is explained by weak localization theory

D σ2D

G0

≈− ln(4 L

l B )−1.96 l B=√ e B

33

Resistivity and magnetoresistance of In2O

3:Sn films deposited from

metal target in oxygen deficit conditions

2D variable range hopping conductivity. Large localization length r0>35 nm. Negative

magnetoresistance could be caused by increase of localization length in magnetic field

34

Summary

- Electron mobility in ZnO and In2O

3:Sn films correlates with degree of

crystallinity: the better is the crystallinity the larger is the electron mobility.

- Electron transport in highly crystalline ZnO, ZnO:Ga and films is bandlike.

- Electron transport in polycrystalline ZnO, ZnO:Ga films and oxygen deficient In

2O

3:Sn films is hopping.

- Electron concentration in ZnO:Ga films is essentially smaller than concentration of Ga atoms.

- Electron transport in ZnO:Co films is hopping at low temperatures.

-Increase of Co content in ZnO films leads to increase of paramagnetic susceptibility and large positive magnetoresistance at low temperatures. This magnetoresistance could be explained by Zeemann splitting of electronic energy levels in magnetic filed.

- Conductivity of In2O3:Sn films deposited from oxide target is larger than conductivity of ZnO:Ga films grown by oxygen assisted MOCVD due to larger electron concentration.

-The increase of substrate temperature from RT to 230 °C leads to essential increase of the electron mobility and film conductivity. 35