High Resistivity Al x Ga 1-x N Layers Grown by MOCVD
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 36
High Resistivity AlxGa1-xN Layers Grown by MOCVD A. Y. Polyakov, M. Shin Department of Materials Science and Engineering, Carnegie Mellon University D. W. Greve Department of Electrical and Computer Engineering, Carnegie Mellon University M. Skowronski Department of Materials Science and Engineering, Carnegie Mellon University R. G. Wilson Hughes Research Laboratory This article was received on May 25, 1996 and accepted on November 15, 1996.
Abstract Undoped Alx Ga1-xN layers with good surface morphology and very low electron concentration have been grown by MOCVD on sapphire substrates. The observed electrical and optical properties depend strongly on the growth temperature. Layers grown at 1000 °C exhibited low resistivity and strong optical absorption below the bandgap. In contrast,layers grown at 1050 °C had low carrier concentrations and good mobilities. Virtually no optical absorption near the band edge was observed as opposed to the usual situation in Alx Ga1-xN. The electrical properties of these layers can be explained by the presence of donor centers whose energy increases with composition, and deeper lying compensating defects. The interaction of these centers renders the samples with x0, very different results were obtained for these two series. The A series showed electron concentrations of order 1018 cm-3 for most of the composition range, with the electron concentrations decreasing for x>0.5. Similar behavior has been reported in Alx Ga1-xN by a number of researchers [3] [4] . The mobilities in this series were quite low (≈10 cm2/Vsec). In contrast, the B series exhibited much lower electron concentrations- indeed, at x=0.05-0.08 the electron concentration was not measurable in our apparatus. The electron concentration reached a peak value of near 1015 cm-3 before declining for x>0.3. Mobilities in this series of samples are much higher (200-500cm 2/Vsec). We will now discuss possible interpretations for these results in terms of unintentional impurities and native defects. We first consider the possibility that the poor characteristics of the A series can be explained by higher impurity concentrations. The most likely donor and acceptor contaminants in MOCVD are Si, C, and O. The concentrations of these impurities measured by SIMS are presented in Table 1. Carbon is likely to be the major acceptor impurity as our system has never been used to grow p-type material; and silicon and oxygen the dominant donor impurities. The results in Table 1 show a higher concentration of carbon acceptors in the A series, inconsistent with the observed higher electron concentrations. Similarly, the silicon concentrations appear to be approximately the same (except forAlN samples), and the oxygen contamination is greater in the B series. While the measured impurity concentrations are high enough to account for the high electron concentration in the A series, the results of the B series can only be explained if there is an extremely large chan
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