Layer Thickness Dependence of Strain in GaN grown by HVPE
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Layer Thickness Dependence of Strain in GaN grown by HVPE Gyu Gwang Sim1, P. W. Yu1, D. C. Reynolds2, D. C. Look2, Sang Soo Kim3, and D.Y. Noh3 1 Department of Information and Communications, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea 2 Semiconductor Research Center, Wright State University, Dayton, Ohio 45435 3 Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea
ABSTRACT
Wurzite GaN epilayers on sapphire substrates usually suffer from biaxial compressive strain due to the mismatches of the thermal expansion coefficients and the lattice constants between GaN layers and sapphire substrates. We have investigated the layer thickness effects on strain and transition energies by photoluminescence (PL), photoreflectance (PR) and X-ray diffraction (XRD). Samples used in this study are grown by hydride vapor phase epitaxy (HVPE) and have the layer thickness of 0.76, 2.6, 5.3 and 48 µm. The PL and PR spectra showed the redshift of the transition energies with increasing layer thickness. This is attributed to strain-induced energy shift. The layer thickness dependence of strains is directly observed by XRD. The strain along the c -axis (εzz) decreased with increasing layer thickness. This indicates the strain is relaxed with layer thickness. From strain variation with layer thickness, we suggest that strain relaxation process is rapid at the initial stage of growth and becomes slower as the layer grows. The full width at half maximum (FWHM) of PL spectra and theta rocking curves decrease with increasing layer thickness. This indicates the crystal quality improves as the strain is reduced. Since the strain effect is very small at the layer thickness of 48 µm, we expect zero strain for thicker layers that can potentially be used as substrates for homoepitaxy.
INTRODUCTION
GaN layers are usually grown by heteroepitaxy on sapphire substrate. The mismatches of the thermal expansion coefficients and the lattice constants between GaN and sapphire usually introduce the biaxial compressive strain in GaN layers. [1] Though it is difficult to grow high quality GaN layers due to these mismatches, homoepitaxial growth can be used in order to overcome the difficulties. Since HVPE provide very high growth rate (up to ~100µm/h) [2] and easy availability of thick layers, thick layers grown by HVPE can potentially be used as substrates for homoepitaxtial growth allowing the feasibility of high quality layers. In this paper, we have investigated the layer thickness effects on strain and transition energies of the GaN layers on sapphire grown by HVPE by using Photoluminescence (PL), phtoreflectance (PR) and X-ray diffraction (XRD).
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EXPERIMENT
Samples used in the experiment are undoped wurzite GaN epitaxial layers grown by HVPE on sapphire substrate. They have the layer thickness of 0.76, 2.6, 5.3 and 48 µm. In order to investigate the layer thickness effects on strain and transition energies PL, PR and XRD measurements were used. PL spectra
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