Microstructure of ELO-GaN Layers Grown by Hydride Vapor Phase Epitaxy
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Microstructure of ELO-GaN Layers Grown by Hydride Vapor Phase Epitaxy Silvija Gradecak1, Volker Wagner2, Marc Ilegems2, Fabienne Bobard1 and Pierre Stadelmann1 1 Centre Interdépartemental de Microscopie Electronique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland 2 Institut de Micro et Optoélectronique, Département de Physique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ABSTRACT Electron microscopy techniques are applied to investigate structural properties of GaN layers selectively grown by hydride vapor phase epitaxy on crystalline and amorphous GaN seed layers deposited on (0001)Al2O3 substrates. Optimalization of the growth conditions lead to a reduction both of the stacking fault concentration and c-axis tilting in the laterally grown regions. During the lateral growth threading dislocations from the seed layer bend from vertical direction of propagation. Bending behavior depends on the type of the dislocation and on the shape of the GaN film in the initial stage of the growth. Optical properties of laterally grown regions are correlated with the high point-defect incorporation that is revealed by high-resolution electron microscopy. INTRODUCTION The majority of GaN is grown in a form of thin epitaxial films on variety of substrates. The crystalline quality and surface morphology of GaN films are improved by the deposition of a low-temperature buffer layer before subsequent high temperature epitaxy [1,2]. Defect density is further reduced with epitaxial lateral overgrowth (ELO) [3-5]. Hydride vapor phase epitaxy (HVPE) provides very large growth rates [6] and is suitable for ELO because of the good selectivity and large growth rate anisotropy [7]. The GaN thick films are grown by a two-step HVPE ELO in order to obtain layers with low dislocation density and with good crystalline quality for subsequent epitaxial regrowth. In this work we investigate the microstructure of these films using electron microscopy techniques. Firstly, we investigate the influence of the growth parameters on the defects’ configuration within the GaN films in the initial stage of the growth. Secondly, we use optimized growth conditions and investigate dislocation behavior in the upper layers of the films. Finally, we correlate spatial distribution of the luminescence observed in cross-sections of the films [8] with the defect structure. EXPERIMENTAL DETAILS Crystal growth Samples are prepared by a two-step ELO HVPE process. A 2 µm thick crystalline or amorphous seed layer is deposited on (0001)Al2O3 substrate using metal-organic vapor phase epitaxy (MOVPE). A SiO2 mask is deposited onto the seed layer and stripe openings (windows) are made using standard photolithographic techniques. The stripe pattern (window openings 5 µm and periodicity 15 µm) is aligned along 〈1-100〉GaN since this direction provides the best selectivity [9]. Subsequent regrowth is carried out using a horizontal HVPE Aixtron reactor at I3.23.1
atmospheric pressure and 1050°C. The gallium source is GaCl formed by the r
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