TEM investigation of defect reduction and etch pit formation in GaN
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Y5.22.1
TEM investigation of defect reduction and etch pit formation in GaN Angelika Vennemann, Jens Dennemarck, Roland Kr¨oger, Tim B¨ottcher, Detlef Hommel and Peter Ryder Institute of Solid State Physics, University of Bremen P. O. Box 330440, 28334 Bremen, Germany
ABSTRACT GaN samples of this study were chemically wet etched to gain easier access to the dislocation sturcture. The scanning electron microscopy and transmission electron microscopy investigations revealed four different types of etch pits. After brief etching, several dislocations with screw component showed large etch pits, which may be correlated with the core of the screw dislocation. By means of SiN x micromasking the dislocation density could be reduced by more than one order of magnitude. The reduction of threading dislocations in the SiN x region in GaN grown on h0001i sapphire is due to bending of the threading dislocations into the {0001} plane, such that they form dislocation loops if they meet dislocations with opposite Burgers vectors. Accordingly, the achievable reduction of the dislocation density is limited by the probability that these dislocations interact. Edge dislocations bend more easily on account of their low line tension. This results in a preferential bending and reduction of dislocations with edge character. INTRODUCTION GaN is the most important material for optoelectronic devices emitting in the blue-green spectral range. Nevertheless, the heteroepitaxy of GaN is still hampered by the high threading dislocation (TD) density commonly found in these structures. Much effort has been made to analyze these dislocations, as it is known that they can act as nonradiative recombination centers [1]. An important approach to improving the emission characteristics is the defect reduction of the grown film. In particular, in situ methods such as the deposition of submonolayer SiN x or thin AlN [2] interlayers, are interesting due to their convenient application. The basis of SiN x micromasking applied in this work is the role of Si as an antisurfactant for GaN. This role of Si in GaN-based materials was first reported by Tanaka et al. [3]. Until now, several groups used thin in situ deposited interlayers to reduce the dislocation density [4-9]. For a model of the reduction mechanism see [5,7]. In this work a TEM analysis of the TDs in the SiN x region is given. Another method of characterizing TDs in GaN is defect-selective etching, which reveals not only the dislocation density but also, by selective etching, the dislocation character [9-12]. The etch pit formation seems to be dependent on the etchant used. In this paper the defect-selective etching of GaN with a solution of hot H2 SO4 :H3 PO4 is analyzed with the aim to correlate the etch pit shape with the dislocation character.
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EXPERIMENTAL DETAILS About 3 µm thick GaN films containing a fractional monolayer of SiN x used as a micromask were grown on h0001i oriented sapphire substrates by metal organic vapor phase epitaxy (MOVPE). The buffer and cap layer GaN were gro
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