Surface Study of In2O3 and Sndoped In2O3 thin films with 100 and 111 orientations

1
Surface Study of In2O3 and Sn-doped In2O3 thin
films with (100) and (111) orientations
AVS 2007, Seattle, Washington, USA

• 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
2
Motivation
AVS 2007, Seattle, Washington, USA

• 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 polycrystalline form
• Orientation most studied is (100)
• Few surfaces studies on any other low index
  orientation

3
Characterization
AVS 2007, Seattle, Washington, USA

• Substrates and films where characterized using in
  situ RHEED, LEED and XPS
• Also samples where characterized using UPS at
  the 3m TGM beamline at Center for Advanced
  Microstructures and Devices, Baton Rouge Louisiana

4
Preparation
AVS 2007, Seattle, Washington, USA

• 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
5
RHEED Substrate Characterization, YSZ (111)
AVS 2007, Seattle, Washington, USA
No Treatment
110
110
211

• After treating substrate in air at 1350 C during
  30 min a high surface quality is obtained

6
In2O3 Crystal Structure
AVS 2007, Seattle, Washington, USA
Oxygen
Indium

• BixByite
• BCC a 1.0117 nm
• (100) has a polar character
• (111) is not polar

7
In2O3 Films
AVS 2007, Seattle, Washington, USA

• Films
• UHV 5 10-10 mbar base pressure
• Molecular Beam Epitaxy
• Indium e-beam evaporated at 0.1 nm/min
• Evaporation in Oxygen Plasma at 15mA and O2 at 5
  10-6mbar
• Sn was co-evaporated using a Knudsen cell
• Growth temperatures at 450, 550 and 800C, highest
  temp gives best results

8
RHEED In2O3 (111) Film
AVS 2007, Seattle, Washington, USA
850C
850C
750C
110
110
211

• Films grow epitaxial at 850C

9
LEED
AVS 2007, Seattle, Washington, USA

• YSZ(111) substrate and In2O3 at 103eV

YSZ(111)
In2O3 (111)
10
LEEDIn2O3 ITO (100)
AVS 2007, Seattle, Washington, USA

• In2O3 (100) facets
• Sn doped In2O3 at different Sn concentrations
  from 11 to 3 results in stabilization of the
  surface
• 9 Sn shown

11
ARXPS
AVS 2007, Seattle, Washington, USA

• Surface sensitive at higher polar angles.
• Forward Focusing

12
ARXPS of In2O3 (100) and Forward Scattering
Analysis
AVS 2007, Seattle, Washington, USA
13
Sn-doped In2O3 (100)
AVS 2007, Seattle, Washington, USA

• Sn possibly substitutes In
• Sn segregates to the surface

14
Sn-doped In2O3 (111)
AVS 2007, Seattle, Washington, USA

• Sn substitutes In
• Sn does not segregate to surface

15
Band Structure
AVS 2007, Seattle, Washington, USA
In2O3
ITO

• Large band gap (optically transparent)
• Conduction band is highly dispersing (good
  conductivity)
• Second band gap in conduction band -gt
  transparent optical conductor

O. N. Mryasov and A. Freeman Phys. Rev B 64
233111 (2001)
16
UPS of Valence Band
AVS 2007, Seattle, Washington, USA

• Gap states for In2O3 (possibly defect states)
• Sn induces additional gap states, much more for
  (100) orientation
• VBM is at 2.6 eV below EFermi for both, In2O3 and
  ITO

already observed by A. Klein et al. Phys. Rev.
B 73 245312 (2006) for polycrystalline ITO
17
In2O3 ITO (100)
AVS 2007, Seattle, Washington, USA

• Gap State and Resonant Photoemission of gap state

18
Conclusions
AVS 2007, Seattle, Washington, USA

• Sn stabilizes the (100) surface so it doesnt
  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