The current high level of research on p-type doping of ZnO is driven by the possibility of important applications such as wide-gap electronics and spintronics, yet high-quality and reproducible p-type ZnO remains a formidable technological challenge.
In this contribution we discuss preparation of p-type ZnO by oxidation of thin films zinc nitride. Nitrogen incorporation is seen as the most prospective way to obtain p-type conductivity. The starting material was grown by rf reactive magnetron sputtering from high purity Zn target, in Ar-N₂ plasma. Crucial point during the deposition was to adjust both the N₂/Ar ratio and the working pressure so as to deposit Zn₃N₂ without any metallic Zn inclusions. For oxidation, samples were furnace annealed in O₂ flow Quartz and sapphire as well as lattice-matched GaN, ZnO and ZrB₂ were used as substrates. The effect of the growth direction - determined by growth conditions - on the structural perfection of the resultant ZnO films as well as on efficiency of nitrogen incorporation was studied. We present results of x-ray diffraction, secondary ion mass spectrometry and optical transmission measurements of the oxidized layers, indicating that formation of ZnO did take place.
We report on the appropriate conditions for the growth of p-type ZnO with Hall effect mobility of 20 cm²/Vs and carrier concentration of 2x10¹⁵ cm^-3. Further increase of p-type conductivity up to 5x10¹⁷ cm^-3 with mobility of 5 cm²/Vs was obtained by additional doping the starting material with Cr. We discuss the role of the latter in gettering of hydrogen which, possibly, deactivates the nitrogen acceptors. Finally we address the problem of stability of the films: we found that protection of the surface is necessary in order to prevent p-type conductivity degradation.

Work supported by grants from EC NANOPHOS (contract IST-2001-39112) and from the State Committee for Scientific Research PBZ-KBN-044/P03/2001.

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