Optical properties of GaN with Ga and N polarity
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Optical properties of GaN with Ga and N polarity M. A. Reshchikov, D. Huang, F. Yun, P. Visconti,a T. King, and H. Morkoç Virginia Commonwealth University, Richmond, VA 23284, U.S.A. a
Also with Instituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, Via Arnesano, Lecce, Italy
J. Jasinski b and Z. Liliental-Weber Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. a
Also with the Institute of Experimental Physics, Warsaw University, Warsaw, Poland
ABSTRACT We compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low density of inversion domains (1x107 cm-2). For N-polar GaN the situation is the reverse: the density of dislocations and inversion domains are 5x108 and ~1x1011 cm-2, respectively. One of the important conclusions derived from the combined PL and TEM study is that inversion domains do not seem to affect the radiative efficiency very adversly, whereas dislocations reduce it significantly. INTRODUCTION Wurtzite GaN is a polar crystal. When it is grown on c-plane of sapphire substrates, the crystal direction [0001] of the resulting GaN film can be either parallel or antiparallel to the growth direction, leading to epilayers with Ga polarity (the (0001) plane is Ga terminated) or N polarity (the (000-1) plane is N terminated). In MBE growth, Ga-polar films are usually obtained on AlN buffer layers and N-polar films - on GaN buffers [1]. Previous investigations showed that the films with different polarities have very different structural and electronic properties [2]. Ga-polar films have relatively smooth and stable surface, which is very difficult to etch. The PL properties of Ga-polar films grown by MOCVD also seem better than N-polar films [3,4]. It is often believed that the overall quality of the Ga-polar films is much better than N-polar films. However, N-polar films may be important for some device applications [5,6]. In this paper, we analyze the structural and optical characteristics of Ga- and N-polar GaN films grown by MBE. EXPERIMENTAL DETAILS Undoped GaN layers with thickness from 1 to 3 µm were grown on c-plane of sapphire by MBE using radio frequenc
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