Photoluminescence Study of Defects in GaN Grown by Molecular Beam Epitaxy
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Photoluminescence Study of Defects in GaN Grown by Molecular Beam Epitaxy Michael A. Reshchikov, Manhong H. Zhang, Jie Cui, Paolo Visconti1, Feng Yun, and Hadis Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 1 Also with Istituto per lo Studio di Nuovi Materiali per l’ Elettronica, CNR, 73100 Lecce, Italy ABSTRACT Defect related photoluminescence (PL) in unintentionally doped GaN layers grown by molecular beam epitaxy (MBE) was studied. Under certain growth conditions, we observed new defect-related bands: a red band with a maximum at about 1.88 eV and a green band with a maximum at about 2.37 eV. The quenching of these bands with increasing temperature took place with an activation energy of about 120-140 meV at temperatures above 100 K. Moreover, the red band exhibited an increase of PL intensity with an activation energy of 1.2 meV in the range of 10-60 K. The observed behavior is explained by invoking a configuration coordinate model and that we speculate the defects to be partially nonradiative and related to Ga atoms. INTRODUCTION Unintentionally doped GaN usually contains high concentration of different defects, affecting the quality of devices. These are dislocations, inversion domains and other extended defects, as well as point defects, such as vacancies, self-interstitials, impurities and various kinds of complexes. The defect concentration depends on purity of the sources used and the growth environment; yet there are some native defects, which are naturally formed in thermodynamic equilibrium conditions and their concentration depends on the growth method and parameters of growth. Identification of defects in GaN remains a challenging problem and is controversial. Photoluminescence (PL) is a powerful tool for studying point defects in wide band-gap semiconductors. The most common defect observed in PL spectrum in undoped GaN grown by all the techniques is a broad band with a maximum at about 2.2-2.3 eV, the so-called yellow luminescence (YL) band. After extended debates on the type of transitions resulting in YL and the nature of associated defect, most investigators attributed the YL to a deep acceptor, presumably gallium vacancy (VGa) or its complex with shallow donors (VGaSiGa and VGaON). It is widely believed that the broad PL bands observed are related to broad distribution of defect states in the band gap. However it is well known that strong electron-phonon coupling, typical for deep defects, is responsible for broadening of the related PL [1]. Recently, it has been demonstrated that broad PL bands with maxima at 2.21, 2.50 and 2.88 eV (yellow, green and blue bands) in undoped GaN can be well described in terms of the configuration coordinate (CC) model and do not require the assumption of a broad distribution of the defect states in the gap [24]. In this work, we studied two new broad bands, which appeared in the PL spectrum of undoped GaN samples grown by molecular beam epitaxy (MBE) with radio-frequency (RF) plasma as the nitrogen source. EXPERIMENTAL DETAILS Undoped and sl
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