Surface Study of In2O3 and Sn-doped In2O3 thin films with (100) and (111) orientations

Erie H. Moralesa), M. Batzillb) and U. Diebolda)

a) Department of Physics, Tulane University, New Orleans, LA 70118

b) Department of Physics, University of South Florida, Tampa, FL 33620

NSF # CHE 0715576, CHE 010908

Motivation

• Sn doped In2O3 is a Transparent Conducting Oxide

• Besides being used in solar cells finds application in Organic Light Emitting Diodes as hole injector

• Mostly used in polycristalline form• Orientation most studied is (100) • Few surfaces studies on any other low index

orientation

Characterization

• Substrates and films where characterized using in situ RHEED, LEED and XPS

• Also sample where characterized using UPS at Center for Advanced Microstructures and Devices, Baton Rouge Louisiana

Preparation

• Substrate– YSZ Yttrium Stabilized Zirconia, (Y 9%)– Cubic body centered, cube-on-cube epitaxy

with In2O3

– Lattice parameter • YSZ is 0.5125 nm • In2O3 is 1.0117 nm

– Substrate prepared by high temp treatment at 1350 C*

* Hiromichi Ohta et al. Appl. Phys. Lett. 76 19 (2000) 2740-2742

RHEED Substrate Characterization

In2O3 Crystal Structure

• BCC a = 1.0117 nm• Substrate lattice

mismatch is 1%• (100) has a polar

character• (111) is not polar

In2O3 Films• Films

– UHV 5 10-10 mbar base pressure– Molecular Beam Epitaxy – In e-beam evaporated at 0.1 nm/min– Oxygen Plasma Assisted at 15mA – O2 at 5 10-6mbar– O2 at 10-5 mbar– Sn was co-evaporated using a Knudsen

cell– Growth temperatures at 450, 550 and

800C, highest temp gives best results

RHEED In2O3 Film

LEEDIn2O3 & ITO (100)

• In2O3 (100) facets• Sn doped In2O3 at

different Sn concentrations from 11% to 3 % results in stabilization of the surface

• 9% Sn shown

• Surface sensitive at higher polar angles. When rotating sample photoelectron would need to travel longer distance to surface. Considering IMFP only photoelectrons closer to surface manage to be detected

ARXPS

ARXPS of In2O3 (100) and Forward Scattering Analysis

-5 5 15 25 35 45 55 65

2x104

3x104

4x104

Sn-doped In2O2 (100)

• Sn segregates to the surface

0 10 20 30 40 50 60 708

10

12

14

16

18

Sn / (

Sn +

In)

(%)

Polar Angle

Sn doped In2O3(100)

Sn-doped In2O2 (111)

0 10 20 30 40 50 601

2

3

4Sn-doped In

2O

3 (111)

Sn / (

Sn +

In)

%

Polar Angle 0 10 20 30 40 50 60

8.0x102

1.2x103

1.6x103

2.0x103

2.4x103

0.0

2.0x104

4.0x104

6.0x104

8.0x104

Polar Angle

Sn 3d

In 3d

• Sn does not segregate to surface, I measured this yesterday!!!, nice!

LEED• YSZ(111) substrate and In2O3 at 103eV

YSZ(111) In2O3 (111)

2x2

UPS• Point is Sn derived states in the Band Gap• Point is to correlate it to Sn segregation observed in XPS and the fact that UPS is surface sensitive corroborating

Sn migration to the surface or Sn terminated surface

12 10 8 6 4 2 0

Undoped In2O

3 (100) 39.77

37.77 35.69 33.61 31.60 29.62 27.67 25.75 23.83 21.83

BE(eV)12 10 8 6 4 2 0

Sn-doped In2O

3 (100)

39.77 37.77 35.69 33.61 31.60 29.62 27.67 25.75 23.83 21.83

BE(eV)

Valence Band Maximum

• Still an open question the measured VBM at 2.6 eV smaller than 3.7eV

• Optical BG Direct and Indirect meas. by Weiher and Ley J. Appl. Phys 37 1 (1966)

• UPS meas. by A. Klein et al. Phys. Rev. B 73 245312 (2006)

VBMIn2O3

(eV)

ITO

(eV)

(100) 2.6 2.6

(111) 2.7 2.8

In2O3 & ITO (100)

4 3 2 1 0

(100) In2O

3

ITOh = 30

21 24 27 30 33 36 39

1000

1200

1400

1600

1800

2000

(100)

Arb

. U.

Photon Energy (eV)

• Gap State and Resonant Photoemission of gap state

Compare VB ITO (100) & (111)• Point is Sn derived states doesn’t show so clearly

4 3 2 1 0

(100) In2O

3

ITOh = 30

4 3 2 1 0

(111) In2O

3

ITOh = 30

Conclusions & Outlook

• Sn stabilizes the (100) surface so it doesn’t facet• Sn replaces substitutionally In sites• There are clear Sn derived states in Band Gap• The position of the VBM is an open question• Less clear Sn derived states in (111)

corroborated by UPS and ARXPS• Do absorption experiments to see if Sn derived

states move to the conduction band on (111) orientation