Physical Properties of Silicon Doped Hetero-Epitaxial MOCVD Grown GaN: Influence of Doping Level and Stress
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Figure 1. a) Free electron concentration (cm-3), Hall measurements, versus [SiH4] for GaN layers grown at 1045°C with constant NH3 and TMG partial pressures. The slope of the linear relationship is 0.95. b): free electron concentration (cm-3), Hall measurements, versus the reciprocal growth temperature (K-1) for two [SiH4] (8x10-10 and 1.92x10-8). RESULTS AND DISCUSSION To evaluate SiH4 as precursor for n-type doping of GaN, the dependency of the free electron concentration (n) on the input mol fraction silane ([SiH4]) and growth temperature was studied. Figure 1 shows n (cm-3) versus [SiH4] as determined by HallVan der Pauw measurements at 300K in experiments where all parameters were kept constant except from [SiH4]. In figure 1 n increases linearly with [SiH4] with a slope of 0.95. Values up to 1x1019cm-3 have been reached easily without observation of any saturation of the free carrier concentration. In contrast to III/V semiconductors like GaAs [2], saturation of n is not observed. A problem forms the drastic change in morphology (see figure 2) due to the large [SiH4]. For example, no reliable values for the layer thickness could be obtained for [SiH4] = 8.2x10-7 although the estimated value for n nicely fits in the graph. The hexagonal symbol in figure 1 represents a layer grown without a low temperature buffer leading to an N-face layer [3] as confirmed by free etching experiments using a 20% KOH solution [4]. As is well known, the donor incorporation in N-face layers is higher than in Ga-face layers [5]. This is clearly illustrated by the experiment in which two layers are compared grown with the same [SiH4]. The N-face layer, grown without a low temperature buffer layer, has a 4 times higher silicon incorporation (5.4x1018 cm-3 compared to 1.7x1018 cm-3) compared to the epilayer grown on the buffer layer. This is illustrated in figure 1a. To reveal the temperature behaviour of the silicon doping of GaN process using SiH4, experiments were performed at different temperatures keeping [SiH4] constant. Plotting n versus T -1 reveals the apparent activation energy of the process (see figure 1b). The obtained values, around 8 kcal/mol, point to a diffusion limited incorporation process, which is supported by the 1:1 relationship between n and [SiH4]. The electron mobilities are a measure of the electrical quality of the material. Values ranging from 320 cm2V-1s-1 for n = 5.3x1017 cm-3 till 192 cm2V-1s-1 for n = 9.2x1018 cm-3 were obtained indicating a compensation grade of about 0.4 [6,7]. To distinguish between the contribution of a possible interfacial layer and the epilayer, temperature dependent Hall measurements have to be performed. The samples described in this paper consist of an epilayer deposited on a highly defective low temperature buffer layer with a thickness of 20 nm as is revealed by TEM measurements. The influence of this buffer layer on the
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Figure 2. SEM photograph of the morphology of the GaN sample grown with [SiH4] = 8.2x10-7, magnification 5200x, marker represents 1 µm. e
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