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L11.3.1

Excitons bound to surface defects in GaN M. A. Reshchikov, D. Huang, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. ABSTRACT Sharp intense peaks are sometimes detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN samples in the photon energy range of 3.0 – 3.46 eV. Some of these peaks can be attributed to excitons bound to dislocations and inversion domains, whereas some others originate from the GaN surface because they can be affected essentially by surface treatment. In our samples, grown by molecular beam epitaxy on sapphire substrate, the 3.42 eV peak always disappeared after removing the surface layer by etching for a few seconds in hot phosphoric acid. Atomic force microscopy images confirmed that such light etching modifies the surface morphology, although the etched depth is negligibly small. Moreover, intensities of two other peaks (at 3.32 and 3.35 eV) were observed to depend on sample etching, as well as on the length of subsequent exposure to air. The 3.32 and 3.35 eV peaks evolved with time of UV illumination, increasing by several times and demonstrating memory effect at low temperature. We attribute the 3.42 and 3.35 eV peaks to bound excitons, whereas the 3.32 eV peak is tentatively attributed to a surface donor-acceptor pair transition. INTRODUCTION Structural and surface defects are known to affect markedly electrical and optical properties of semiconductor devices. It is well known that dislocations can bind excitons and be responsible for intense photoluminescence (PL) lines detected well below the bandgap energy [1]. However, excitons can be bound also to other types of structural defects, in particular to surface adatoms [2]. In high-quality undoped GaN, only free excitons and excitons bound to shallow donors and acceptors comprise the low-temperature PL spectrum at photon energies between 3.0 and 3.5 eV [3], along with the characteristic series of peaks due to shallow donor-acceptor pair (DAP) transitions [4]. In less pure samples, sharp unidentified peaks are often detected in the range of 3.0 – 3.46 eV in addition to the well-established transitions in pure samples. The origin of these additional peaks is controversial. For example, the commonly observed 3.42 eV peak [5-8] has been attributed to recombination between electrons bound to oxygen donor and free holes [5], DAP-type transitions involving a very shallow unidentified acceptor [6], and exciton bound to structural defects [7] or to c-axis screw dislocations [8]. Sharp lines at 3.31 and 3.36 eV were repeatedly reported [9-14] and attributed to the cubic phase inclusions formed by stacking faults [9,10] or to excitons localized at extended defects in GaN [11-14]. However much less is known about other peaks. In this paper, we discuss in detail the behavior of the 3.32, 3.35 and 3.42 eV peaks in undoped GaN and correlated the appearance of these peaks with surface morphology, crystal structure and sample history. EXPERIMENTAL DETAILS Undoped GaN layers with thicknesses in the