Synthesis of nanocrystalline Cu deficient Cu
x
CrO
2
– a high figure of merit p-type
transparent semiconductor
L. Farrell, E. Norton, C. M. Smith, D. Caffrey, I. V. Shvets, K. Fleischer
School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices
(CRANN), Trinity College, The University of Dublin, Dublin 2, Ireland
E-mail:
The material system of CuCrO
2,
a delafossite, is well known as one of the best performing p-
type transparent conducting oxides. We report details on a low temperature facile growth
method for CuCrO
2
by spray pyrolysis. The dependence of the growth on the precursors, the
temperature and oxygen partial pressure is examined. The decomposition routes are critical to
obtain the best performing films. The thermopower and electrical measurements indicate p-
type films with conductivity ranging from 1-12 Scm
−1
depending on the growth conditions.
This p-type conductivity is retained despite the nanocrystallinity of the films. The figure of
merit of these films can be as high as 350 μS, which is the best performing p-type TCO by
solution methods to date.
Comprehensive analysis of the thin films by X-Ray Diffraction, Raman spectroscopy, X-ray
absorption spectroscopy and X-ray photoelectron spectroscopy was used to identify the
material as a highly Copper deficient Cu
x
CrO
2
, with best performance in terms of the figure
of merit found for x=0.4. Interestingly, the material’s low growth temperature is compatible
with flexible substrates, glass (Corning Willow® glass) and plastic sheets (Kapton®
polyimide). We discuss the optical and electrical properties and morphology of films grown
on variety of glass and flexible substrates.
XRD pattern and Raman spectra of a Cu
0.4
CrO
2
film grown by spray pyrolysis. Raman data
show very poor local crystallinity (very broad modes) while XRD data confirms some long
range order compatible with nano-crystalline CuCrO
2
.
Combined with XPS and XAS data the
film stoichiometry is interpreted as highly copper deficient, defective material showing good
optical and electrical properties for a p-type TCO. This is achieved by a CVD like growth
mode in Spray pyrolysis where the film is grown in Cu- and Cr- terminated alternating
layers, leading to the delafossite structure.
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