Microstructure and Physical Properties of GaN Films on Sapphire Substrates
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ABSTRACT Transmission electron microscopy (TEM), x-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were applied to study the correlation between the microstructure and physical properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. When the density of the threading dislocation (TD) increases about one order of magnitude, the yellow luminescence (YL) intensity is strengthened from negligible to two orders of magnitude higher than the band edge emission intensity. The full width of half maximum (FWHM) of the GaN (0002) peak of the XRD rocking curve was widened from 11 min to 15 min, and in Raman spectra, the width of E2 mode is broadened from 5 cm-1 to 7 cm-1. A “zippers” structure at the interface of GaN/sapphire was observed by high-resolution electron microscope (HREM). Furthermore the origins of TD and relationship between physical properties and microstructures combining the growth conditions are discussed.
I. INTRODUCTION GaN is a promising material for optoelectronic and electronic devices such as bluegreen light emitting diodes, laser diodes and high temperature, high frequency, high power transistors. Although blue current-injected laser diodes have been fabricated by S. Nakamura [1], there are still many problems under debate. The origins of the YL emission remain unclear while it is commonly observed in almost all undoped and n-doped GaN epilayer grown by conventional methods [2-5]. The generation of TD in the epilayer of which density is high as 108-1010 cm-2 [6-8], and the nature and the role of initial nitridation of sapphire surface and the low-temperature growth GaN (AlN, InN) buffer layer which affects the GaN-based materials epitaxy growth [9-13] are ambiguous too. Several groups reported the relationships between GaN microstructures and XRD, PL, and Raman spectra, respectively [11,14-17]. But there are few works to try to resolve above problems by studying the microstructures and physical properties combining the growth conditions, such as the initial nitridation time and buffer layer thickness, and so on. In this paper, different properties of two kinds of GaN samples are grown by metalorganic chemical vapor deposition (MOCVD) under different conditions. Two sets of GaN films in TEM, XRD, PL and Raman measurements are investigated. According to the results, the origins of TD and YL are discussed.
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Table 1 Growth conditions of the sample A and B Sample A Sample B Nitridation time (min) 3 1.5 Buffer growth temperature (oC) 500 500 Buffer thickness (nm) 50 25 Epilayer Growth temperature (oC) 950 950 20 28 TMG flow (µmol/min) NH3 flow (l/min) 1.65 2.35 0.9 1.7 Epilayer thickness (µm)
II. EXPERIMENTS The GaN films were grown by LRH-MOCVD on (0001) sapphire substrates. Details of the growth processes are given in Ref. 18. Table 1 shows the growth conditions of two typical samples which named as sample A and sample B. Transmission electron microscope (high-
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