Raman Mapping and Finite Element Analysis of Epitaxial Lateral Overgrown GaN on Sapphire Substrates
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Raman Mapping and Finite Element Analysis of Epitaxial Lateral Overgrown GaN on Sapphire Substrates M. Benyoucef,1 M. Kuball,1 B. Beaumont,2 V. Bousquet,2 and P. Gibart 2. 1
H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom. Centre de Recherches sur l’Hétéroépitaxie et ses Applications (CRHEA-CNRS), Rue Bernard Grégory, Parc Sophia Antipolis, 06560 Valbonne, France. 2
ABSTRACT Using micro-Raman scattering and finite element (FE) analysis, stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates using a two-step growth method were investigated. Nearly full stress relaxation at the top ELO GaN surface can be achieved by increasing the thickness of ELO GaN to about 50 µm. Reductions in stress variation between window and overgrown regions can be achieved by using a double ELO GaN growth at a much smaller ELO thickness. Increased compressive stress at the coalescence boundary of two adjacent wings of ELO GaN was related to the presence of voids in this area. In the double ELO growth, stress near the top surface was mainly attributed to the presence of voids on top of the upper dielectric mask. INTRODUCTION GaN and related compounds are the most promising materials for ultraviolet optoelectronics as well as high-power electronic devices. Due to the lack of large area GaN single crystals, the growth of GaN has been developed on foreign substrates such as sapphire, SiC, and Si. Large differences in lattice parameters and thermal expansion coefficients exist between GaN and these substrates, which result in high defect densities ranging from 109 to 1010 cm-2 [1]. Epitaxial lateral overgrowth (ELO) has proven to be a powerful technique for reducing this dislocation density by 3-4 orders of magnitude. This technique allowed the fabrication of blue laser diodes (LDs) with lifetimes of 10 000 hours at room temperature in continuous wave operation [2]. The ELO technique has been applied in both metalorganic vapor phase epitaxy (MOVPE) [3-5] and hydride vapor phase epitaxy (HVPE) [6, 7]. Knowledge of stress fields in ELO GaN is of importance for the understanding of dislocation generation and propagation in ELO GaN, and consequently, for improvements of the process procedures. We report on the Raman scattering and finite element (FE) analysis of stress fields in ELO GaN substrates. Nearly full stress relaxation can be achieved by increasing the thickness of ELO GaN to about 50 µm. Stress variations between window and overgrown regions can be reduced by using a double ELO GaN (D-ELO GaN) growth at a much smaller ELO thickness. EXPERIMENTAL DETAILS Micro-Raman mapping experiments were performed on single and D-ELO GaN samples using a confocal Renishaw micro-Raman system, with the 488 nm-line of an Ar-laser as
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excitation source. Micro-Raman spectra were recorded in Z(X,.) Z back-scattering geometry from the top surface of the ELO GaN. A 50x objective was used to focus and collect the scattered laser
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