Pinholes, Dislocations and Strain Relaxation in InGaN
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Internet Journal Nitride Semiconductor Research
Pinholes, Dislocations and Strain Relaxation in InGaN B. Jahnen1, M. Albrecht1, W. Dorsch1, S. Christiansen1, H. P. Strunk1, D. Hanser2 and Robert F. Davis2 1Institut
für Werkstoffwissenschaften, Lehrstuhl VII, Universität Erlangen-Nürnberg, of Materials Science and Engineering, North Carolina State University,
2Department
(Received Friday, July 17, 1998; accepted Monday, October 19, 1998)
We analyse by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) the strain relaxation mechanisms in InGaN layers on GaN as dependent on the In content. At the experimentally given thickness of 100 nm, the layers remain coherently strained, up to an In concentration of 14 %. We show that part of the strain is reduced elastically by formation of hexagonally facetted pinholes. First misfit dislocations are observed to form at pinholes that reach the InGaN/GaN interface. We discuss these results in the framework of the Matthews-Blakeslee model for the critical thickness considering the Peierls force for glide of threading dislocations in the different slip systems of the wurtzite lattice.
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Introduction
The growth of misfitting heteroepitaxial layers generally is influenced by processes that relax the built-up strain. Well known are misfit dislocations that form beyond a critical layer thickness for plastic relaxation. However, also at smaller thicknesses, elastic relaxation may take place by formation of sinusoidal undulations, islands and/or pits [1], [2]. For many optoelectronic devices like single or multiple quantum wells however, twodimensional, pseudomorphically grown structures are required. Thus, the limits of lattice mismatch and thickness must be known. In this work, we analyse the strain relaxation mechanisms that occur in the system InGaN/GaN (0001). We will find that, as a consequence of glide geometry and high Peierls forces, plastic relaxation hardly occurs. Thus, elastic relaxation is expected to dominate, and we will exemplify that pinholes [3], [4], [5], [6] (pits in the shape of open inverted pyramids) take part. In addition, in the case of pinholes as deep as the heteroepitaxial InGaN layer, stress concentration at the apex of the pinholes leads to limited formation of misfit dislocations in the heteroepitaxial interface, i. e. in the basal plane of the wurtzite lattice. We show that these deep pinholes are the only source of misfit dislocations up to the investigated InGaN layer thickness of 100 nm and within the investigated range of In concentrations (up to 14 %).
Experiment
Three InGaN/GaN single heterostructures were grown by MOVPE. Figure 1 shows a schematic of the layer structure. (0001)-SiC was used as a substrate, and an AlN layer and a GaN layer of a total thickness of about 1 µm were used as a buffer layer. On top of that, 100 nm thick InGaN layers with In contents of 6 %, 8 % and 14 % were grown. The growth temperatures for the AlN layers, the GaN layers and the InGaN layers were 1100 °C, 1000 °C and 780 °C, r
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