UV induced dielectric loss in AlN ceramics
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The dissipation factor of aluminum nitride ceramics (AIN), doped with various concentrations of oxygen, is probed at 1 kHz before and after exposure to UV radiation. The results of this study show that UV photogenerated carriers, which are trapped at charged oxygen impurity related defect sites, are responsible for the observed dielectric loss. A simple energy level diagram is presented that outlines trapped carrier distributions within the AIN bandgap and is consistent with the experimentally observed changes in dissipation factor as a function of time, UV exposure, and exposure to visible light, which liberates carriers from charged trap sites.
I. INTRODUCTION Aluminum nitride (AIN) has received significant attention over the past 10 years as a new substrate material for electronic applications. AIN is uniquely suited for this application because of this material's high thermal conductivity,1'2 close thermal expansion match to silicon,3 and relatively low dielectric constant.4 A number of studies, beginning with the work of Slack in 1973, have illustrated the sensitivity of key properties of AIN to the presence of impurities, particularly oxygen.1 For example, the controlling effect that oxygen has on thermal conductivity of the AIN lattice has been documented in a number of studies.1'5"10 In addition to thermal properties, oxygen in AIN has been shown to influence significantly extended defect formation,5 unit cell volume,1'5 and a range of optical properties, including luminescence,5'11'12 UV absorption,13 and UV induced photoabsorption of visible light.13'14 From these studies, the following picture of oxygen accommodation in the AIN lattice emerges. At low concentrations (below approximately 0.75 at. %), oxygen atoms are incorporated substitutionally on nitrogen atom sites, with subsequent formation of aluminum atom vacancies for charge compensation, as originally proposed by Slack et al.1>7 This defect is shown schematically in Fig. 1. Modeling of phonon scattering from this defect has shown that a significant coupling exists between the strongly phonon scattering aluminum atom vacancy VA1 and the surrounding AIN lattice.5'14 It has been proposed that this coupling occurs via a Coulomb interaction between the (negatively charged) vacancy site and the (positively charged) substitutional oxygen site O N . 5 ' 14 When oxygen-doped AIN is exposed to UV radiation, photogenerated carriers are trapped at these charged, impurity-related defect sites populating a defect level within the 6.2 eV band gap. 13 Subsequent exposure to visible radiation elevates these trapped and highly 2734 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 10, Oct 1993 Downloaded: 28 Mar 2015
localized carriers to extended states near the band edge, where efficient radiative recombination can occur. Thus, after UV irradiation, visible light is absorbed by AIN until the UV induced centers have been completely depopulated,13 at which point the sample becomes transparent to visible radiation. At high oxygen concentration (above 0.7
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