Lattice constant variation in GaN:Si layers grown by HVPE
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Lattice constant variation in GaN:Si layers grown by HVPE A. Usikov, O.V. Kovalenkov, M.M. Mastro, D.V. Tsvetkov, A.I. Pechnikov, V.A. Soukhoveev, Y.V. Shapovalova, G.H. Gainer Technologies and Devices International, Inc., 12214 Plum Orchard Dr., Silver Spring, MD 20904, USA ABSTRACT The structural, optical, and electrical properties of HVPE-grown GaN-on-sapphire templates were studied. The c and a lattice constants of the GaN layers were measured by x-ray diffraction. It was observed that the c and a lattice constants vary non-monotonically with Si-doping. The proper selection of Si-doping level and growth conditions resulted in controllable strain relaxation, and thus, influenced defect formation in GaN-on-sapphire templates. It was also observed that HVPE homoepitaxial GaN layers grown on the templates have better crystal quality and surface morphology than the initial templates. INTRODUCTION One of the challenges remaining in the epitaxial growth of III-N materials is the absence of a suitable lattice-matched substrate. Currently, sapphire and SiC substrates are widely used in nitride-based technology [1,2]. Device quality III-nitride materials are commonly grown on sapphire by metal organic chemical vapor deposition (MOCVD). Blue, green and white light emitting diodes (LEDs), violet laser diodes (LDs), field effect transistors, and ultraviolet photodiodes have been developed. These structures, however, suffer from a high density of threading dislocations and strain–induced problems that strongly affect device performance. GaN grown on sapphire with a low temperature nucleation layer still has a dislocation density as high as 109 to 1010 cm-2. These dislocations are a major cause of device degradation, particularly for LDs, which operate at high current density [3]. Due its high growth rate and high material quality, hydride vapor phase epitaxy (HVPE) is the method of choice for fabricating GaN quasi-bulk material and GaN templates for use as substrates for III-N device structure growth. The homoepitaxial growth of GaN-based device structures on GaN templates can improve the crystal quality, and thus, device performance. Other benefits of GaN and AlGaN templates are reduced growth time and precursor consumption, reduced downtime and maintenance, and increased productivity of the MOCVD reactors. The purpose of this work is to study the optical and structural properties of HVPE-grown GaN-on-sapphire templates and HVPE GaN layers grown homoepitaxially on the templates. The influence of Si-doping on the structural properties of GaN templates was also studied. This is important because structural defects can penetrate from the template to the upper growth structure. EXPERIMENTAL DETAILS Details of the HVPE technique for fabricating GaN-on-sapphire templates were previously reported [4]. The GaN-on-sapphire templates consist of a 2.5 to 30 µm-thick GaN layer grown on c-plane 2” diameter sapphire substrates at 1000 to 1100 ºC at atmospheric pressure. The flow
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rates of GaCl and ammonia were controlle
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