Comparative study of structural properties and photoluminescence in InGaN layers with a high In content
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occurring after annealing at 950°C [6]. This enhanced stability is due to the high elastic strain experienced in thin heterostructures and quantum wells, which results in a significant decrease of the critical temperature for single phase stability [8]. In all of the aforementioned studies, typical precipitate sizes of (several) tens of nanometer were reported, hence too large to result in efficient luminescence [3]. In this paper, we present a correlation between the PL-energy of single InxGa1-xN thick films and their overall and local In-content. In particular, we will focus on the effect of local variations of the composition on the nanoscale, resulting in a phase segregation of the ternary nitride. EXPERIMENTAL Two InxGa1-xN layers, each with a laterally varying composition, were grown at 780°C by metalorganic chemical vapour deposition on a thick GaN buffer / Al2O3 substrate. The thickness of the layers is about 250 nm. The exact In-content and thickness, as well as the crystalline quality, was mapped by Rutherford backscattering and channeling spectrometry (RBS/C), using a beam spot of approximately 1 mm2. The crystallinity, azimuthal orientation and the phases present in the sample were further monitored with low (Rigaku system with a rotating anode) and high (Bruker D8 discover) resolution XRD in θ−2θ geometry. PL mapping was performed at low temperature (< 30 K) using an Ar+ laser with a spot diameter of 100 µm. Samples for transmission electron microscopy (TEM) were prepared in cross sections and studied with a Philips CM 30 FEG electron microscope, operating at 300 kV. Electron dispersive X-ray analysis (EDX) was performed with a Link instrument on a Philips CM 20 microscope.
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Energy (M eV)
Figure 1. Random (o), aligned (∆) and simulated (solid line) RBS spectra of the In0.25Ga0.75N/GaN/Al2O3(0001) layer. All TEM results shown further have been obtained from this part of the specimen.
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RESULTS AND DISCUSSION
PL Emission (Arb. Units)
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Photon Energy (eV)
Figure 2. Low temperature PL spectrum of the same sample as shown in Fig. 1. Using RBS, the exact In-fraction of the InxGa1-xN layers was mapped at a large number of distinct points on the samples, x-values in the range from 0.2 to 0.4 were found [9]. Figure 1 shows the random and aligned RBS spectra of a particular area of the sample which will be discussed in detail below. From the random spectrum, a composition of In0.25Ga0.75N and a thickness of 290 nm are deduced. The spectrum measured with the incoming beam aligned along the direction indicates that the minimum yield χmin of the In0.25Ga0.75N layer is 20 % (this is the ratio of the backscattering yields of the aligned and random spectra, and is a measure for the crystalline quality of the layer). The value of χmin is found to increase with the In-content
Figure 3. TEM image of a cross section of the InGaN layer. Rows of
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