On the nature of the oxygen-related defect in aluminum nitride
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I. INTRODUCTION
The high solubility of oxygen in the aluminum nitride (A1N) lattice1'2 and the controlling effect of this impurity on phase and polytypoid formation,3"6 optical properties,7"11 and thermal conduction12"16 have critical importance from both a scientific and technological viewpoint. Over the past two decades, the character of this impurity has been probed utilizing a wide range of techniques, including luminescence,101117 x-ray diffraction,1418 NMR,19 thermal conduction measurements,13'14'20'21 and electron microscopy.5'22"25 The results of these measurements have clearly established that (i) at low concentrations oxygen dissolves into the A1N lattice, creating an impurity level in the A1N optical band gap;6"9 (ii) this dissolved oxygen has a deleterious effect on thermal conduction in the A1N lattice10"14; and (iii) at high concentrations, oxygen-related structures such as polytypoids are formed.1"5 The first and most extensive modeling of the oxygen-related defect in A1N is due to the work of Slack.13'14 In these studies, Slack proposed that oxygen substitutes for nitrogen in the A1N lattice with the subsequent formation of an aluminum vacancy as a charge balancing mechanism. Slack supported this model with the results of x-ray diffraction lattice parameter measurements, and also showed that this picture was consistent with observed trends in thermal resistance (as a function of oxygen content) when modeled utilizing the substitutional impurity approach developed by Abeles.26 The work described in this series of papers has been the basis for interpreting a wide range of subsequent experimental findings, and thus will be discussed in more detail in a later section. In addition, Slack has had a significant impact by producing the first large, low oxygen content single crystal of A1N.13-27 This sample, which has been an important reference point for evaluJ. Mater. Res., Vol. 5, No. 8, Aug 1990
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ation of polycrystalline A1N ceramics, will serve as an end-point measurement for many of the results discussed in this paper. The manifestations of the oxygen-related defect on optical properties in A1N have been investigated extensively by Pastrnak et a/.8'10 and others.7'11'17 The results of these studies have shown that an oxygen-related impurity level is created in the A1N optical band gap which can be readily observed in luminescence measurements. The origin of this luminescence peak has been assigned to donor-acceptor pair transitions by Pastrnak10 and to transitions within the optical bandtail states by Harris and Youngman.11 In each case, a broad, rather temperature insensitive luminescence peak is observed experimentally. In addition, up to this point trends in the luminescence peak intensity or spectral position as a function of oxygen dopant concentration have not been well established. In the work presented here, luminescence, thermal conductivity, and x-ray diffraction lattice parameter measurements will be utilized to show that (i) at low ox
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