Photoluminescence in wurtzite GaN containing carbon
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Photoluminescence in wurtzite GaN containing carbon M. A. Reshchikov, R. H. Patillo, and K. C. Travis Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, U.S.A.
ABSTRACT We studied photoluminescence (PL) from a set of GaN layers grown on sapphire substrates by metalorganic chemical vapor deposition with the concentration of carbon varied by the growth conditions. One of the remarkable features in these samples is the extremely low intensity of the shallow donor-acceptor pair band. Analysis of the PL data gives the shallow acceptor concentration of less than 1014 cm-3 in most of the C-doped GaN layers. This result shows that C does not form a shallow acceptor, CN, in appreciable concentrations in wurtzite GaN. As for the YL band, there is no clear correlation between its intensity and the degree of Cdoping. The question of identification of the deep acceptor responsible for the YL band in undoped and C-doped GaN still remains to be solved. INTRODUCTION While carbon in cubic GaN introduces a shallow acceptor level that is responsible for ptype conductivity [1], its role in wurtzite GaN is not well established [2]. Some researchers expect that C introduces a shallow acceptor that may be responsible for the ultraviolet luminescence (UVL) band, also known as the shallow donor-acceptor pair (DAP) band, in unintentionally doped or Si-doped GaN [3,4]. First-principle calculations also predict [5] that CN is a shallow acceptor with low formation energy in wurtzite n-type GaN, especially when Garich conditions of growth are employed, and therefore it may be responsible for the UVL band in GaN. There are many reports that C-doping greatly enhances the yellow luminescence (YL) band (a broad band peaking at about 2.2 eV in the majority of GaN samples) and presumably is involved in the defect responsible for this band [2]. However, the fact that there is no clear correlation between the concentration of C and the intensity of the YL band was also noted [6]. Armitage et al. [7] suggested that C-related defects cause the YL band in C-doped GaN while a gallium vacancy is responsible for the YL band in undoped GaN. On the other hand, firstprinciples calculations of Wright [5] show that the YL band is unlikely caused by a carbonrelated defect. In this work, we studied photoluminescence (PL) from a set of GaN layers grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) with concentration of carbon varied by the growth conditions. Our results suggest that concentration of CN, assumed to be a shallow acceptor in wurtzite GaN [5], is very low. We observed a strong YL band but could not find a correlation between its intensity and concentration of C. EXPERIMENTAL DETAILS A set of six GaN layers on c-plane sapphire substrates was prepared by EMCORE Corporation and studied by several research groups within the Wood-Witt Initiative “Defects in GaN”. The layers were grown by MOCVD method and contained different amounts of carbon impurity controlled by growth conditions such as g
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