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

* 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 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

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

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

ARXPS of In2O3 (100) and Forward Scattering Analysis

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

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

YSZ(111) substrate and In2O3 at 103eV 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

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 73 245312 (2006) VBM In2O3 (eV) ITO (100) 2.6 (111) 2.7 2.8

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

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

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