Manifestation of structural defects in photoluminescence from GaN
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Manifestation of structural defects in photoluminescence from GaN M. A. Reshchikov,1 J. Jasinski,2 F. Yun,1 L. He,1 Z. Liliental-Weber,2 and H. Morkoç1 1 Virginia Commonwealth University, Richmond, VA 23284 2 Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT Sharp peaks of unidentified nature are detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN in the photon energy range between 3.0 and 3.46 eV. These PL lines are commonly attributed to excitons bound to yet unidentified structural defects. We analyzed X-ray diffraction data in a large set of GaN samples grown by molecular beam epitaxy in order to find any correlation between these unusual PL peaks and the GaN crystal structure. Moreover, in selected samples exhibiting such peaks, cross-sectional transmission electron microscopy was taken in an effort to detect the presence and density of various structural defects. The preliminarily results indicate that most of unusual PL lines in GaN (Y lines) are not directly related to the observed structural defects, such as edge, screw, mixed dislocations, or stacking faults. However, there exists the possibility that point defects trapped at dislocations or other structural defects are responsible for these PL lines. INTRODUCTION A series of sharp intense peaks is sometimes observed in the low-temperature photoluminescence (PL) spectrum of unintentionally doped GaN in the photon energy range between 3.0 and 3.46 eV [1]. These “unusual” peaks are named Y lines [1] due to the similarity of their properties to those of the Y lines in II-VI compounds [2]. Most of these peaks (Y2 - Y7 at 3.42, 3.38, 3.35, 3.34, 3.32, and 3.21 eV, respectively) are typically observed in Ga-polar films [1]. Combined analysis of PL and transmission electron microscopy (TEM) revealed that the Y2 peak appeared (at 3.41 eV) in GaN samples with high density of stacking faults [3,4]. Stacking faults can be considered as layers of cubic phase surrounded by wurtzitic phase, and they are expected to bind excitons in GaN [5]. Threading screw dislocations were also suggested as the origin of the 3.41 eV peak [6], and indeed, the sample with high density of threading screw dislocations exhibited the 3.41 eV line [3], although in another work [4] the density of screw dislocations was lower in the samples showing the 3.41 eV line as compared to control samples lacking this line. In this work we carried out a comprehensive investigation with PL, TEM, and X-ray diffraction (XRD) of the samples with different densities of structural defects and different contribution of the Y lines in the low-temperature PL spectrum. EXPERIMENTAL DETAILS Undoped GaN layers with thicknesses ranging from 1 to 3 µm were grown on c-plane sapphire by molecular beam epitaxy (MBE). Different buffer layers were employed to reduce the strain due to the GaN/sapphire lattice mismatch. Among several hundred GaN layers, some 50 samples exhibited Y lines in the low-temperature PL spectrum. PL experiments were carried out in the temperature
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