Materiales

APPLIED PHYSICS LETTERS 95, 192105 2009

Undoped vacuum annealed In2O3 thin films as a transparent
conducting oxide
A. Dixit,1 C. Sudakar,1 R. Naik,1 V. M. Naik,2 and G. Lawes1,a
1

Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA
Department of Natural Sciences, University of Michigan–Dearborn, Dearborn, Michigan 48128, USA

2

Received 22September 2009; accepted 21 October 2009; published online 10 November 2009
We have investigated the structural, optical, and electrical properties of both as-grown and vacuum
annealed In2O3 thin films. In contrast to the insulating as-prepared samples, vacuum annealed In2O3
films exhibit a metallic electrical conductivity with increased carrier concentration and mobility. We
attribute the excesscarriers to an oxygen deficiency introduced during vacuum annealing.
Remarkably, these carrier densities seem to be stable under ambient conditions for at least two years.
Optical spectroscopy measurements show a large optical transparency, greater than 80%, for both
the as-prepared and vacuum annealed In2O3 films. © 2009 American Institute of Physics.
doi:10.1063/1.3262963
Transparent conductingoxides TCO combine the properties of high optical transparency at visible wavelengths
with high electrical conductivity. These materials are important for many optoelectronic applications including photovoltaic and photoelectrochemical devices, liquid crystal displays and light emitting diodes.1 These materials should have
a wide bandgap, above roughly 3 eV to avoid optical excitations in thevisible wavelengths, but also a high electrical
conductivity. The coexistence of these two contraindicated
material properties has been identified in a number of metal
oxides including ZnO,2 CdO,3 SnO2,4 and In2O3.3 A high
electrical conductivity in these metal oxides is usually produced by doping higher valence cations, which act as donor
source for excess electrons, resulting in n-typeconduction.
Among the most common n-type TCOs used for solar cells
and various optoelectronic applications are Al doped ZnO,5 F
doped SnO2,6 and Sn doped In2O3.7
Among various metal oxides, indium oxide based TCOs
typically show the lowest resistivity.8 There remains some
debate on the value of bandgap of In2O3. Early studies found
a direct optical bandgap of 3.75 eV with an onset of indirectphonon assisted transitions starting at 2.6 eV. More
recent theoretical9 and x-ray photoelectron spectroscopy
studies show that band-edge transitions in In2O3 are forbidden because of the even parity for states at both the top of
valence band and the bottom of conduction band, which prohibits dipole transitions. These studies also proposed a strong
optical transition from 0.8 eV below the top ofthe valence
band to the conduction band. These theoretical studies
coupled with recent optical measurements on high quality
epitaxial indium oxide thin films10 suggest a lower intrinsic
direct bandgap value 2.6 eV in contrast to previously accepted value 3.75 eV.
This proposed small bandgap for indium oxide may play
a critical role in establishing the performance of In2O3 as a
TCO. It hasbeen argued that surface charges pin the Fermi
level of In2O3 0.4 eV above the minimum of the conduction
band, which accounts for the coexisting large carrier concentration with large optical bandgap and may also explain the
tendency for In2O3 to express n-type conductivity.11 It is also
a

Electronic mail: glawes@wayne.edu.

0003-6951/2009/95 19 /192105/3/$25.00

suggested that doped In2O3can support a high carrier density
without forming compensating acceptor defects, even for
large dopant concentrations.12 This may explain why In2O3
has more stable, higher electrical conductivity than other
undoped/doped oxides. Previous studies on Sn doped and
undoped In2O3 annealed under a variety of conditions show
that this system can often develop significant electrical conductivity...