Aquamarine Luminescence Band in Undoped GaN
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E9.8.1
Aquamarine Luminescence Band in Undoped GaN M. A. Reshchikov,1 L. He,1 R. J. Molnar,2 S. S. Park,3 K. Y. Lee,3 and H. Morkoç 1 1 Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 2 MIT Lincoln Laboratory, Lexington, MA 02420, U.S.A. 3 Samsung Advanced Institute of Technology, P.O.Box 111, Suwon, Korea ABSTRACT We report a new defect-related photoluminescence (PL) band peaking at 2.56 eV at 15 K in undoped GaN layers grown by molecular beam epitaxy (MBE) on GaN templates. The maximum of the aquamarine luminescence (AL) band shifts to higher energies by about 100 meV with increasing temperature from 15 to 300 K. In spite of the fact that the shape and the peak position of the AL band are close to the characteristics of the green luminescence band in a freestanding GaN template, we were able to delineate these two bands for their markedly different behavior with temperature and excitation intensity. The origin of defect responsible for this PL band remains unknown. INTRODUCTION Recent development of high-quality GaN templates for GaN epitaxy [1] increased the likelihood for attaining nitride materials with very low threading dislocation density. However, point defects due to contamination or less then optimal growth conditions can in fact seriously degrade the properties of devices fabricated even on high crystalline quality templates. Point defects in GaN are commonly studied by PL, and yet almost no one defect-related PL band is identified in unintentionally doped GaN [2]. In addition to strong near-band-edge emission due to exciton transitions, the PL spectrum from undoped GaN almost always contains a broad yellow luminescence (YL) band with a maximum at about 2.2 eV. In high-purity GaN grown by hydride vapor-phase epitaxy (HVPE), the YL band is replaced by the green luminescence (GL) band at about 2.5 eV under certain experimental conditions [3,4]. The GL band is attributed to another charge state of the defect responsible for the YL band [4]. In several GaN layers grown by MBE on high-quality GaN templates, we observed a new broad band at about 2.6 eV labeled as AL band for its aquamarine color. In this work, we present the results of a preliminary investigation of this new PL band. Its identification is yet unclear, but certainly different from those for the YL and GL bands. EXPERIMENTAL DETAILS Nominally undoped 1-2 µm-thick GaN layers were grown by MBE on GaN templates using radio-frequency plasma as the nitrogen source. The substrates were either 200 µm-thick freestanding templates or 10 µm-thick layers on a c-plane sapphire, both grown by HVPE. The AL band was observed only in samples grown at relatively low temperatures (~600°C) under very Ga-rich conditions. Often, the AL band appeared in the layers whose surfaces were covered with Ga droplets. However, removal of excess Ga from the surface did not affect the AL band intensity, indicating its bulk origin.
E9.8.2
Steady-state PL was excited with a He-Cd laser (325 nm), dispers
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