Scanning tunneling microscopy observation of surface reconstruction of GaN on sapphire and 6H-SiC
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ABSTRACT We report studies of the surface structure of MBE-grown GaN layers on sapphire (0001) and 6H-(0001) SiC substrates. A different set of reconstructions is observed for nitrogen-face and gallium-face layers. The gallium-face has so far only been grown on MOCVD GaN/ sapphire substrates, while the nitrogen-face has been obtained on SiC and bare sapphire substrates. INTRODUCTION It is well known that the quality of GaN epitaxial layers is strongly influenced by the initial stages of growth. Recent work has shown that several types of inhomogeneities are generated in the initial stages of growth. Zincblende GaN has been detected near the initial interface [1] and also crystallites which are tilted with respect to the surface [2]. Typically these structures do not propagate through the film. However, under some conditions inversion domain boundaries (IDB) can be formed which can result in rough surfaces, while under other circumstances layers of a single polarity result [3]. Consequently, it is important to understand the factors which determine the polarity of GaN layers. In this paper, we will summarize our recent studies of the surface structure of GaN layers with differing polarity. Nitrogen-face and gallium-face layers have been grown by MBE on bare sapphire and MOCVD GaN on sapphire surfaces, respectively. We will show that each face is characterized by a unique set of surface reconstructions which have been observed by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Initial results on 6H-(0001)SiC substrates will also be presented. EXPERIMENT The studies described here were performed in a combined MBE/ surface analysis system designed and built at Carnegie Mellon. The growth chamber contains the gallium effusion cell, an SVTA RF plasma nitrogen source, and in situ RHEED. Samples approximately 1 cm 2 is size are mounted with spring clips on a molybdenum block. After growth, samples are transferred under vacuum to the analysis chamber for STM. The base pressures of the growth and analysis chambers were in the 10-il torr range. Growths were performed on bare (0001) sapphire substrates; sapphire substrates with a 2 gim GaN layer grown by MOCVD at 1050 'C using a low temperature GaN buffer layer; and 6H(0001) SiC substrates. The growth procedures on the various substrates were as follows: o Bare sapphire substrates were solvent-cleaned, heated to 1000 'C, and exposed to a nitrogen plasma for 30 minutes. After cooling to 685 'C with the plasma on, growth was initiated by turning on the gallium flux. The substrate temperature was slowly raised to 775 'C for the main part of the growth. A streaky ×x1 RHEED pattern was observed after a few hundred Angstroms of growth. After growth of 2000 A, the sample was annealed at 363
Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
800 'C for 15 minutes, resulting in a lxI RHEED pattern.
GaN/ sapphire substrates were loaded into the growth chamber and heated to " The MOCVD 0
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800 C under a nitrogen plasma
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