GaN and AlN Layers Grown by Nano Epitaxial Lateral Overgrowth Technique on Porous Substrates
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In order to overcome this difficulties, we have proposed [3] to grow GaN layers on porous substrates. The porous substrate materials have been successfully applied to grow GeSi [4], SiC and AlN [5], CoSi2 [6] on silicon. Porous SiC buffer has been used to reduce defect density in SiC epitaxial layers [7]. In this paper we describe preliminary results on GaN and AlN growth on porous substrates. Usually, pores have nanometer size. GaN and AlN layers were grown over these pores and we call this technique nano epitaxial overgrowth technique (NELOG). It is important that NELOG technique does not require any mask. This technique may be easily scaled for large area substrates.
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Figure 1. Al0.3Ga0.7N alloy layer grown by HVPE technique using ELOG approach. After the growth, sample was treated by reactive ion etching to visualise defects. Area 1 corresponds to AlGaN layer grown over SiO2 mask and area 2 corresponds to AlGaN grown directly on 6H-SiC substrate (window region). It is clearly seen that etch pit density in the alloy grown over the masked area (1) is much lower than that for AlGaN grown on non-masked area (2). Experimental procedure and results Porous GaN layers were formed by anodization of single crystal GaN epitaxial layers grown on SiC substrates by HVPE. 6H-SiC (0001)Si face oriented wafers were used as initial substrates. The HVPE technology employed for GaN layer deposition and properties of GaN layers grown by HVPE have been described elsewhere [8]. The layers were grown at 1000oC at atmospheric pressure. The anodization technique used to fabricate porous GaN layers has been described in ref. [9]. Thickness of porous GaN layers ranged form 1 to 10 microns. Pore size ranged from 50 to 200 nm. GaN epitaxial layers up to 120 microns thick were grown by the same HVPE technique on porous GaN (Fig. 2). The layers had smooth surface; no traces of porous structure were detected. Reflection high-energy electron diffraction showed sharp Kikuchi lines indicating high crystal quality of the surface of grown layers. Results of Raman spectroscopy measurements revealed reduction of residual stress in GaN layer grown on porous GaN in comparison with GaN layer grown directly on 6H-SiC substrate (Fig. 3). The stress estimated by Raman spectroscopy measurements was about 1.3 GPa for layer grown on SiC and about 0.2 GPa for the layer grown on porous GaN.
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Photoluminescence measurements detected a blue shift of the edge peak (Fig. 4) for GaN grown on porous GaN that also may be attributed to stress reduction. X-ray diffraction measurements showed that GaN layers were of high crystal quality having the full width at a half maximum (FWHM) of ω-scan and ω−2Θ-scan x-ray rocking curves for (0002) reflections of about 400 arc sec and 40 arc sec, respectively. Preliminary experiments have been done on AlN deposition on porous substrates. AlN layers were grown by HVPE technique [10] on porous SiC substrates. Porous SiC substrates were fabricated by surface anodization of commercial 6H-SiC (0001) wafers. Thickn
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