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
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
4x10
3x10
2x10
4
4
4
-5
5
15
25
35
45
55
65
Sn-doped In2O2 (100)
Sn doped In2O3(100)
Sn / (Sn + In) (%)
18
16
14
12
10
8
0
10
20
30
40
50
60
70
Polar Angle 
• Sn segregates to the surface
Sn-doped In2O2 (111)
4
Sn / (Sn + In) %
Sn-doped In2O3 (111)
In 3d
3
2.4x10
Sn 3d
4
8.0x10
3
2.0x10
3
4
6.0x10
3
1.6x10
4
4.0x10
2
3
1.2x10
4
2.0x10
2
1
8.0x10
0
10
20
30
40
Polar Angle 
50
60
0.0
0
10
20
30
40
50
60
Polar Angle 
• 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
Undoped In2O3 (100)
12
10
8
Sn-doped In2O3 (100)
39.77
37.77
35.69
33.61
31.60
29.62
27.67
25.75
23.83
21.83
6
BE(eV)
4
2
0
12
10
8
39.77
37.77
35.69
33.61
31.60
29.62
27.67
25.75
23.83
21.83
6
BE(eV)
4
2
0
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)
VBM
In2O3
(eV)
ITO
(eV)
(100)
2.6
2.6
(111)
2.7
2.8
In2O3 & ITO (100)
• Gap State and Resonant Photoemission of gap state
2000
(100)
In2O3
ITO
h = 30
1800
1600
Arb. U.
(100)
1400
1200
1000
4
3
2
1
0
21
24
27 30 33 36
Photon Energy (eV)
39
Compare VB ITO (100) & (111)
•
Point is Sn derived states doesn’t show so clearly
(100)
4
3
In2O3
ITO
h = 30
2
1
In2O3
ITO
h = 30
(111)
0
4
3
2
1
0
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