Growth and Chemical Substitution of Transparent P-Type CuAlO2

CuAlO₂ is one of several materials currently being investigated for application as a p-type transparent conducting oxide. In this paper we report a method of making c-axis oriented CuAlO₂ thin films which eliminates previously reported phase-purity and surface morphology problems. Thin film precursors of CuAlO₂ were deposited on YSZ (100) substrates by room-temperature radio frequency sputtering. Subsequent annealing in 10mT of oxygen at temperatures as low as 800°C resulted in phase-pure, textured CuAlO₂. The films were p-type and transparent with a gap of 3.5eV, but typical carrier concentrations were low (on the order of 10¹⁶ cm^-3). Oxygen anneals at 700°C in 1 atm of O₂ raised this to 10¹⁸ cm^-3 in some samples. In order to increase the carrier concentration further, we are testing chemical substitution on the metal sublattice. Initial experiments were done with Mg as a dopant, either by direct solid-state synthesis for bulk materials or by alternating deposition of MgO and CuAlO₂ during the pulsed laser deposition of thin films. At Mg molar concentrations of 1% or less, the bulk material was phase-pure CuAlO₂ by x-ray diffraction, but was mixed-phase CuAl₂O₄ and CuO when the Mg concentration was increased to 2%. Thin films sputtered from the 1% Mg substituted target crystallized to CuAlO₂ after a 940°C anneal at 10T O₂, but were more than 30 times more resistive than the undoped films.