Carbon-related Deep States in Compensated n-type and Semi-Insulating GaN:C and their Influence on Yellow Luminescence
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Carbon-related Deep States in Compensated n-type and Semi-Insulating GaN:C and their Influence on Yellow Luminescence A. Armstrong,1 D. Green,2 A. R. Arehart,1 U.K. Mishra,2 J.S. Speck2 and S. A. Ringel1 1 Department of Electrical and Computer Engineering The Ohio State University Columbus, OH 43210 USA 2 Materials and Electrical and Computer Engineering Department University of California, Santa Barbara Santa Barbara, CA 93106 USA
ABSTRACT The physical origin of the yellow luminescence in MBE-grown GaN co-doped with C and Si was investigated. Deep level optical spectroscopy (DLOS), deep level transient spectroscopy (DLTS), and photoluminescence (PL) were used to study the deep level spectrum as a function of C incorporation. In the absence of C co-doping, samples were n-type and demonstrated a weak yellow luminescence band, likely related to VGa. For increasing C codoping, samples became semi-insulating concurrent with increased intensity of the yellow luminescence and the concentration of C-related deep acceptors. The DLOS results were used to develop a configuration-coordinate model for a C-related deep level with optical ionization energy of 3.0 eV and Franck-Condon shift of 0.4 eV that is consistent with the observed yellow luminescence and DLTS results. From these findings, a general model for independent mechanisms of the yellow luminescence related to VGa for n-type GaN and C for n-type and semi-insulating GaN:C:Si is discussed. INTRODUCTION The pervasive yellow luminescence (YL) band observed in GaN films centered near 2.2 eV has been attributed to many different defects. Though not applicable to every reported instance of YL, a VGa-related model has become prevalent for n-type GaN. Theoretical investigations indicate that VGa-related defects have positions in the bandgap consistent with the YL [1], and experimental studies provide evidence of deep levels at these positions [2,3]. Over the last three decades, there has been mounting evidence for another mechanism for the yellow luminescence in GaN involving C [4,5,6,7,8,9,10]. However, a defect whose concentration tracks C incorporation and has a deep level position in the bandgap consistent with yellow luminescence near 2.2 eV has yet to be demonstrated. To address this issue, we employed deep level optical spectroscopy (DLOS), deep level transient spectroscopy (DLTS), and PL to track the evolution of the deep level spectrum of MBE-grown GaN:C:Si as a function of [C], and we applied this combined experimental methodology in the context of the configuration-coordinate (CC) model to provide a self-consistent explanation of the YL in GaN:C as a function of C incorporation. Ogino and Aoki first put forward the idea of C-related YL for GaN microcrystals grown from NH3 and molten Ga, finding a broad YL band centered around 2.2 eV appearing only for specimens doped with C. Based on extensive photoluminescence (PL) study, Ogino and Aoki [4] invoked the CC model to interpret the YL in GaN:C as a transition involving an unspecified shallow donor and a deep
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