This relatively large value compared to the previous measurement

This relatively large value compared to the previous measurement on sapphire (0.61°) [11] can be attributed to the AlN buffer layer epitaxial

quality and to the nucleation on the defects. HRTEM cross-section observations have been performed to investigate the epitaxial relationship in between the GaN wire/AlN buffer/Si substrate. The observation was made with a JEOL 3010 (JEOL Ltd., Tokyo, Japan) operating at 200 kV along the zone axis. Figure 3a shows the base of a GaN wire grown on Si with an AlN buffer layer of 10-nm nominal thickness. As shown by the detailed view of Figure 3b, four distinct layers are observed. A 2-nm-thick ACY-1215 price amorphous (or nanocrystallized) layer is observed directly on top of the Si substrate. This layer can be attributed to the spontaneous SiN x formation resulting from the high-temperature growth of the AlN buffer on silicon as already reported by Radtke et al. [15]. The AlN seeds probably nucleate through this non-continuous thin silicon nitride layer, and a planar growth develops laterally to form an almost single-crystalline AlN epitaxial layer for further growth. To confirm these assumptions, the in-plane epitaxial relationships have been studied at the European

Synchrotron Radiation Facility (ESRF, Grenoble, France) on the French BM32 CRG beamline with a 0.1204-nm wavelength. Grazing incidence X-ray diffraction (GIXRD) has been performed with 0.18° incidence to check the AlN

epitaxy on SiN x /Si (111). The usual orientations [17] have been measured corresponding to the AlN //Si and AlN //Si alignments. These measurements check details Lumacaftor confirm also the complete registry of GaN wires with the AlN layer (see for example the scans along the Si direction shown in Figure 2c,d). The AlN layer has been formed at high temperature (approximately 1,100°C) in the 10- to 50-nm range to sufficiently protect the surface and maintain the epitaxy. The study of the epitaxial relationship at lower growth temperature and different thicknesses could be interesting in further studies. Figure 2 X-ray diffraction measurements of GaN wires grown on Si (111) with an intermediate AlN layer. (a) Symmetric Θ-2Θ scan performed on a laboratory setup (approximately 0.179 nm Co-wavelength) and indexed with Si, GaN and AlN Bragg Kα1 reflections. Dots and squares correspond respectively to the Kα2 and Kβ excitation wavelengths. The broad and low intensity peak around 51° (see the triangle) is attributed to a diffraction tail of the Si substrate. (b) Rocking curves (Δω-scan) of the GaN (0002) and (0004) peaks. (c,d) Grazing incidence X-ray diffraction performed at ESRF along the silicon direction (approximately 0.1203 nm wavelength and 0.18° incidence). Figure 3 HRTEM imaging of the GaN/AlN/Si interface (a,b). Observation along the zone axis showing the materials stacking.

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