Oxidation of Aluminum Nitride for Defect Characterization
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Oxidation of Aluminum Nitride for Defect Characterization J.H. Edgar, Z. Gu, and K. Taggart, Kansas State University, Department of Chemical Engineering, Durland Hall, Manhattan, KS 66506-5102 J. Chaudhuri, L. Nyakiti, and R.G. Lee,Texas Tech University, Department of Mechanical Engineering, Lubbock, TX 79409 R.Witt, EBSD Analytical Inc., 2044 N 1100 E, Lehi, UT 84043 ABSTRACT The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects. INTRODUCTION Despite the progress achieved in the performance of group III nitride devices, there remains tremendous potential for improving device efficiency, power, and lifetime and for creating entirely new devices by employing native GaN and AlN substrates. The advantages of these substrates include better lattice constant and coefficient of thermal expansion match, reduced defect densities and background impurity concentrations, and the ability to support epitaxy on any crystals plane, including nonpolar planes [1,2]. Currently, these advantages remain largely unrealized due to the difficulties associated with the bulk crystal growth of GaN and AlN; the extremely high nitrogen vapor pressure of GaN, and the high thermal stability of AlN. Bulk AlN crystals are particularly attractive for supporting Al-rich AlxGa1-xN epitaxial layers for ultraviolet emitters and detectors [1]. Steady progress has been reported over the past ten years in producing bulk AlN crystals via the sublimation-recombination technique. Durable materials (tungsten and tantalum carbide) have been identified that are capable of withstanding high temperatures necessary for crystal growth [3-5]. The dependence of the growth rate on process parameters (temperature, temperature gradient, and pressure) has been modeled theoretically and verified experimentally [6]. To realize the full benefit of AlN substrates, their defect densities must be as low as possible, so the epitaxial layers they support can have similarly low defect densities. Mymrin et al [2]
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