Single-phase aluminum nitride films by dc-magnetron sputtering
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Single-phase aluminum nitride films were deposited onto fused quartz and single-crystal sapphire by current-controlled, reactive, dc magnetron sputtering from an aluminum metal target. Optical and structural properties were observed to correlate systematically with the composition of the sputter gas over a wide range of nitrogen partial pressures. A transition in the electrical conductivity of the deposited films occurred as a function of N2 partial pressure. This transition is driven by the condition of the target surface. When the N2 partial pressure was high and the target surface was substantially covered with A1N*, the deposited film was insulating, stoichiometric A1N. When the N2 partial pressure was low and the target surface was substantially AF, the deposited film was conducting, substoichiometric A1N*.
I. INTRODUCTION
Aluminum nitride is one of a series of Group IIIA metal nitrides which are potentially useful for optoelectronic applications. Devices based on A1N, GaN, InN, and their alloys can theoretically cover photon energies corresponding to the range encompassing the near infrared, the visible, and the vacuum ultraviolet. A1N is a highly insulating semiconductor with a direct bandgap of 6.0 eV.1 Growth of bulk A1N for ceramic applications has been widely studied.2 A1N has also been produced by reactive sputtering3"8 and ion beam deposition.910 The properties of sputtered films of A1N/A1 cermets have been found to correlate closely with the condition of the target surface.4'5 In general, A1N films tend to be multiphase material including both Al° and A1N,4'5'9-10 which tends to make their properties difficult to interpret. In fact, the electrical conductivity of A1N has been found to be strongly affected by included Al° filaments, inducing bistable resistance11 and superconductivity.12 In this study, the dynamics of the reactive magnetron sputtering process are correlated with the properties of A1N films. The general characteristics of reactive sputtering have been well-studied.4'13"16 At high sputtering power or low reactive gas flow, a metal target will be sputtered at a very high rate due to the high secondary electron emissivity of most metals.14 Usually, this high sputtering rate causes all or most of the reactive gas to be simply trapped against the walls of the sputtering chamber, and a highly metallic film to be produced. At low sputtering power or high reactive gas flow, a compound (for example, a metal oxide if oxygen13'14 is the reactive gas) forms on the surface of the target and the sputtering rate is, in most cases, reduced due to the lower secondary electron emissivity of J. Mater. Res., Vol. 5, No. 11, Nov 1990
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the compound relative to the metal. Here, the careful control of sputtering dynamics has been utilized to deposit thin films of A1N with widely varying composition and properties. II. EXPERIMENTAL
Single-phase A1N films were produced by reactive magnetron sputtering onto single-crystal sapphire and fused quartz substra
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