1. 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 , CHE

2. 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

3. 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

4. * Hiromichi Ohta et al. Appl. Phys. Lett. 76 19 (2000) 2740-2742
Preparation
Substrate
YSZ Yttrium Stabilized Zirconia, (Y 9%)
Cubic body centered, cube-on-cube epitaxy with In2O3
Lattice parameter
YSZ is nm
In2O3 is nm
Substrate prepared by high temp treatment at 1350 C*
* Hiromichi Ohta et al. Appl. Phys. Lett (2000)

5. RHEED Substrate Characterization

6. In2O3 Crystal Structure BCC a = 1.0117 nm
Substrate lattice mismatch is 1%
(100) has a polar character
(111) is not polar

7. 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 mbar
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

8. RHEED In2O3 Film

9. LEED In2O3 & 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

10. ARXPS
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

11. ARXPS of In2O3 (100) and Forward Scattering Analysis

12. Sn-doped In2O2 (100)
Sn segregates to the surface

13. Sn-doped In2O2 (111)
Sn does not segregate to surface, I measured this yesterday!!!, nice!

14. YSZ(111) substrate and In2O3 at 103eV
LEED
YSZ(111) substrate and In2O3 at 103eV
YSZ(111)
In2O3 (111)
2x2

15. 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

16. Valence Band Maximum VBM In2O3 (eV) ITO (100) 2.6 (111) 2.7 2.8
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 (2006)
VBM
In2O3
(eV)
ITO
(100)
2.6
(111)
2.7
2.8

17. Gap State and Resonant Photoemission of gap state
In2O3 & ITO (100)
Gap State and Resonant Photoemission of gap state

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

19. 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