Recombination at surface states in GaN
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Recombination at surface states in GaN M. A. Reshchikov, P. Visconti,1 K. M. Jones, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. 1
Also with: Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano, Lecce, Italy and INFM-Unita` di Lecce, Dipartimento di Ingegneria dell`Innovazione, University di Lecce, Lecce, Italy
R. J. Molnar MIT, Lincoln Laboratory, Lexington, MA 02420, U.S.A. C. W. Litton Air Force Research Laboratory, Wright Patterson AFB, OH 45433, U.S.A. ABSTRACT We have studied radiative and nonradiative recombination at surface states in GaN, including as-grown samples and those treated with acids or bases. The surface states manifested themselves in two ways: (i) a reversible increase of the photoluminescence (PL) intensity after ultraviolet (UV) illumination in vacuum and (ii) appearance of new PL bands after treatment with acid or base and subsequent exposure to air. It has been established that the GaN surface physi-sorbs species from air (presumably oxygen) which induce surface states acting as nonradiative recombination centers. It has been found that nonradiative recombination of photogenerated carriers via surface states comprises more than 70% of the recombination in some GaN samples. Another type of the surface state, which participates in radiative recombination, has been found in GaN samples with Ga polarity after brief etching of the surface with hot acid or base and subsequent exposure to air. In such samples, a broad PL band emerges in the blue region of the spectrum at low temperatures. The blue band has been attributed to transitions of photogenerated electrons from donors in the near-surface depletion region to the surface states introduced by the above-mentioned procedure. The changes in the GaN surface caused by etching were examined by atomic force microscopy. In some samples the blue band appeared even when no evidence of the layer etching was found except for quite shallow etch pits formed at dislocation sites. The emerging blue band can be related to the surface states formed on the a-planes of etch pits. INTRODUCTION The importance of GaN and related III-nitride compounds for optical and hightemperature/high-power electronic devices has been recently recognized, however the influence of interfaces and nonideal surfaces on device performance is still not understood well. High density of surface states may degrade the electrical and optical device characteristics. The surface states in n-type GaN result in upward band bending of about 1 eV [1-4]. The bending can be changed by wet chemical cleaning or by adsorption of O2 or activated H2 [1,5]. Surface states near the valence band, detected with the angle-resolved photoemission, have been attributed to dangling bonds [5]. Ishikawa et al. established that the as-grown GaN surface is covered by a contamination layer with a thickness of ~ 2 nm consisting of Ga2O and adsorbed carbon and hydrocarbon [6]. Previously we observed a broad blue band in the photoluminescence (PL) spectrum of G
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