Structural Defects in Mg-doped GaN and AlGaN grown by MOCVD
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Structural Defects in Mg-doped GaN and AlGaN grown by MOCVD S. Tomiya1, S. Goto2, M. Takeya2 and M. Ikeda2 1 Material Analysis Dept. Technical Solution Center, Sony Corporation 2 Development Center, Sony Shiroishi Semiconductor, Inc. ABSTRACT We have investigated the structural defects formed in Mg-doped GaN and AlGaN epitaxial layers grown by metal organic chemical vapor deposition. These defects have an inverse pyramidal shape and appear when the Mg concentration [Mg] is higher than ~4x1019/cm3. The density of the defects increases as [Mg] increases, but the size of the defects becomes smaller as [Mg] increases. The density of the defects also has a strong correlation with the hydrogen concentration in the epitaxial layers. Transmission electron microscope analysis reveals that the defects have an inversion operation to the matrix and that their boundaries are Mg-rich. We also propose a model for defect formation. INTRODUCTION Minimizing the power consumption of GaN-based laser diodes (LDs) is of critical importance for extending the device lifetime [1]. One of the key issues is the optimization of the Mg doping conditions for GaN and AlGaN layers in order to lower the resistivity. During the course of our study of LDs, inverse pyramidal-shaped defects (which are hereafter referred to simply as “pyramidal defects”) were often found in Mg-doped layers by transmission electron microscope (TEM) observations. The pyramidal defects may increase the resistivity of the p-type layers; therefore, these should be eliminated. Such defects have recently been reported by several researchers [2-4], but the formation mechanism of the pyramidal defects is not fully understood. In this paper, we report on the relationship between the pyramidal defects and the atomic concentration of Mg [Mg]. We also briefly discuss the formation of the defects. EXPERIMETAL DETAILS The samples investigated here were grown on (0001) c-face sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Trimethylgallium (TMG), trimethylaluminium (TMA), bis-methylcyclopentadienyl-magnesium [(MeCp)2Mg] and ammonia were chosen as the Ga, Al, Mg, and N precursors, respectively. In order to study the effects of Mg doping on the defect structure, Mg step-doped structures were grown, as shown in Fig.1. The growth temperature was ~1000 ºC. Growth was carried out under an H2-ambient. TEM was performed on films that were mechanically polished and ion milled to electron transparency. The microstructure was observed using an accelerating voltage of 200kV and was analyzed by conventional diffraction contrast, multiple (0002) dark field imaging and high resolution lattice imaging. The defects were also analyzed by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) operating at 200kV. The Mg and H concentrations were measured by secondary ion mass spectrometry (SIMS). O2+ and Cs+-ion beams were used as the primary ion sources to detect Mg and H ions, respectively. Ion-implanted standard samples were also measur
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