Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN
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Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN M. A. Reshchikov and H. Morkoç Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284, U.S.A. ABSTRACT We studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700-800oC) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ~1.8 and ~2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 µs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands. INTRODUCTION In spite of considerable progress in GaN-based light-emitting and electronic devices, understanding and identification of point defects in unintentionally doped GaN is lagging [1]. The yellow luminescence (YL) band dominates the visible part of the PL spectrum in most GaN samples grown by different techniques. However, with increasing III/V ratio the relative contribution of the YL band decreases, the layers become highly resistive, and two other defectrelated PL bands, at ~1.8 eV and ~2.35 eV, begin to dominate the low-temperature PL spectrum of GaN grown by MBE [1,2,3]. We have labeled these PL bands as the RL2 and GL2 bands, respectively [1]. We observed the RL2 and GL2 bands in about 50 GaN samples grown by MBE on sapphire under Ga-rich conditions. Hofmann et al. [4] apparently observed the RL2 band (and did not observe the GL2 band) in the MBE-grown GaN contaminated with oxygen and carbon in the upper 1019 cm-3 range and suggested that VNCN donors and VGaON or VGaSiGa acceptors might be responsible for this band. Skromme and Martinez [5] apparently observed the RL2 (at 1.73 eV) and GL2 (at 2.35 eV) bands after implanting Mg into GaN. We assume that these PL bands are caused by implantation-induced native defects, rather than being directly related to the Mg impurity. Note that we also observed the GL2 band in Mg-doped GaN grown by MBE under Ga-rich conditions, along with the broadened blue band [1]. In this work, we undertake a detailed investigation of the properties of the RL2 and GL2 bands in GaN grown under Ga-rich conditions by MBE. Unusual transient behavior of both PL bands after pulsed excitation of PL and dramatic decrease of the RL2 band lifetime with increasing temperature indicate that internal transitions at some point defects created by excess of Ga might be responsible for these PL bands.
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