Photoluminescence and Excitation Spectra of Deep Defects in GaN
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Photoluminescence and Excitation Spectra of Deep Defects in GaN Michael A. Reshchikov,1 Richard J. Molnar 2 and Hadis Morkoç1 Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 2 MIT, Lincoln Laboratory, Lexington, MA 02420, U.S.A.
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ABSTRACT Deep defects responsible for broad bands in the red-to-green range of the photoluminescence (PL) spectrum of undoped and Si-doped GaN grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied by employing PL and PL excitation (PLE) methods. In HVPE grown samples, a red luminescence (RL) and a green luminescence (GL) bands were observed, respectively, at about 1.9 and 2.4 eV. Similar in positions but different in properties red and green bands (RL2 and GL2, respectively) dominated in the samples grown in Ga-rich conditions by MBE with radio frequency plasma as a nitrogen source (RF-MBE). A yellow luminescence (YL) with a maximum at about 2.2 eV dominated in the samples with ammonia used as a nitrogen source (NH3-MBE). It has been established from the variation of temperature, excitation intensity and excitation wavelength that the abovementioned five bands are related to different deep-level defects. INTRODUCTION PL is one of the most powerful tools for studying point defects in GaN. However the PLE is rarely employed, especially for deep defects. The sole investigation of the PLE spectrum for the YL in bulk C-doped GaN has been carried out by Ogino and Aoki [1]. However later studies of the YL at below-gap excitation [2,3] seem to be in conflict with the model proposed in Ref. 1. PLE can shed light on electronic structure of the point defect in question and help delineate different defects with similar PL bands. The most common defect observed in PL spectrum of n-type GaN grown by any technique is a broad band with a maximum at about 2.1-2.4 eV, the so-called YL band, the exact position and shape of which are somewhat sample dependent. Theoretical calculations predict that a gallium vacancy (VGa) and its complexes with shallow donors or hydrogen may be responsible for the deep defects revealed in the PL spectrum of n-type GaN [4,5]. In this work, we studied PL and PLE spectra of five bands revealed in the red-to-green range of n-type GaN: GL and RL in the HVPE grown samples, two bands with similar positions but very different characteristics, which we denoted GL2 and RL2, in the RF-MBE grown samples and the “usual” YL band in the NH3-MBE grown samples. EXPERIMENTAL DETAILS Undoped and Si-doped GaN layers for study were grown on c-plane of sapphire by HVPE and MBE techniques. Five samples analyzed in this work (see Table I) were chosen for high intensities of the particular PL bands, however the main features of PL and PLE spectra were reproduced in a large set of samples. Characterization of the samples grown by RF-MBE in Ga-rich conditions can be found in Ref. 6. A He-Cd laser with excitation photon energy ( ω exc) of 3.81 eV was used for above-gap excitation of PL and a second harmonic of the Ti-sapphire laser was used for PL and PL
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