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

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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
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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

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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

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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
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RHEED Substrate Characterization
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In2O3 Crystal Structure

• BCC a 1.0117 nm
• Substrate lattice mismatch is 1
• (100) has a polar character
• (111) is not polar

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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

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RHEED In2O3 Film
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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

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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

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ARXPS of In2O3 (100) and Forward Scattering
Analysis
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Sn-doped In2O2 (100)

• Sn segregates to the surface

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Sn-doped In2O2 (111)

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

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LEED

• YSZ(111) substrate and In2O3 at 103eV

YSZ(111)
In2O3 (111)
2x2
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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

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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)

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In2O3 ITO (100)

• Gap State and Resonant Photoemission of gap state

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Compare VB ITO (100) (111)

• Point is Sn derived states doesnt show so
  clearly

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Conclusions Outlook

• 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
• Do absorption experiments to see if Sn derived
  states move to the conduction band on (111)
  orientation