Growth and Chemical Substitution of Transparent p-Type CuAlO 2
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Mat. Res. Soc. Symp. Proc. Vol. 623 ©2000 Materials Research Society
subsequent annealing experiments have focussed on films deposited exclusively by RF sputtering at room-temperature. By changing substrates from sapphire to (100) oriented YSZ substrates, and annealing at lower temperatures and oxygen partial pressures, we can reproducibly make phase-pure c-oriented thin films up to about 2000k' in thickness. All the p-TCO candidates under investigation today -- including CuAlO2 -- still have conductivities four or five orders of magnitude lower than what has been achieved with n-type TCO's. We are attempting to reduce that gap with CuA1O2 , and to that end, are conducting various single-species doping experiments (with oxygen, nitrogen, magnesium, and potassium). To date, the only species we know was actually incorporated into the films (without inducing a substantial loss of phase-purity) has been Mg, but it only made the films more resistive. EXPERIMENT Amorphous copper-aluminum-oxide samples were grown on (100) YSZ substrates by radio-frequency sputtering of a one-inch diameter CuAlO 2 target. The target was mounted 3.3 cm above the substrate. After reducing the base pressure to below 10 4 Torr, a fixed flow rate (10.5 sccm) of N2 gas was admitted into the chamber and the valve separating the chamber and the turbo pump was throttled to maintain a constant total pressure of 65 mT. The films were deposited at a rate of 70A/min using an emmitted power of 50W. The film thickness was determined by means of a Sloan Dektac 3 stylus profilometer. Ex-situ anneals were performed in a quartz tube furnace with the samples placed on an alumina sled. Rotameters and a digital flowmeter (Sierra Designs model 820) were used to monitor the ratio of the manually regulated flow rates between argon and oxygen. Using this method, a partial pressure of lOmT of oxygen was maintained in the tube during the anneal. The furnace temperature, as measured by a thermocouple inside the tube, was ramped up from 2000 C to 940' C at a rate of 10 degrees per minute, then held at 940' C for 1.5 hours before gradually cooling to 200' C. Optical measurements were performed on a Cary 5G UV-vis NIR spectrophotometer in dual beam mode with a fixed spectral width of 8nm in the near-infrared and 2nm in the UV and visible portions of the spectrum. Four-point conductivity and Hall measurements were taken on a Bio-Rad model HL550OPC using a van der Pauw electrode configuration with pressed indium contacts. Crystallinity was determined by x-ray diffraction (XRD) using Cu Ka radiation in a Scintag diffractometer. RESULTS
Growth In our initial work [2], CuAlO 2 films were grown on c-plane sapphire substrates, because A120 3 is well lattice-matched to CuAlO 2, and has a high melting point. However, due to the
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persistence of the CuAI20 4 impurity phase, we suspected that at the 10500 C annealing temperatures the aluminum in the substrate was contaminating the films. Figure 1 shows a film which worst-case demonstrates the point. The top panel is the XRD
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