Temperature Dependent Morphology Transition of GaN Films
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w of about 5 slm. All samples were grown for 1 hour which resulted in a GaN layer thickness of about 1.5 gm. AFM and SEM were applied to study the surface morphology of the samples. RESULTS AND DISCUSSION For the naked eye, lower deposition temperatures result in matt-white samples, whereas at higher deposition temperatures the layers look colourless and mirror-like. In figure 1 the specimen appearance is given as function of growth temperature and pressure. It shows that the transition from matt-white to mirror-smooth appearance occurs between 1000'C and 1015'C for samples grown at 50 mbar, and at 1035'C for those grown at 20 mbar. To ensure that changes in polarity are not the cause for the observed morphology transition, the polarity of the deposited GaN layers is determined by anisotropic etching [3]. For all samples it is found that growth occurred in the [0001] direction (Ga-face). 60 -
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c'0 Figure 1. The appearance of about 1.5 jim thick GaN layers versus growth temperature for two different pressures
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A matt-white
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A morphology transition 0970
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temperature (°C)
One sample, grown at 20 mbar and 1035'C, forms an excellent specimen to study the morphology transition, since it exhibits a mixture of both appearances. This sample, which will be referred to as sample 'B', has a matt-white appearance at the edges, where the deposition temperature has been slightly lower than at the central region, and is mirror-like and transparent at the centre. To determine the temperature difference during growth along sample B, the centre of a complete matt-white sample grown at 1030'C and 20 mbar and slightly thicker as sample B is used as a reference. AFM examination of this sample shows a morphology comparable to that on sample B at a position halfway between the periphery and centre. This indicates, taking the thickness difference into account, that the growth temperature near the edges of sample B has been 1027 ± 3 'C. For which can be concluded that a temperature gradient of only about 8VC can cause a dramatic change in morphology. In figure 2 an AFM image from the periphery of sample B is shown. The main characteristic of the surface is formed by large and irregularly shaped pits of about 300-600 nm wide, with crystallographically oriented side faces and with a density of about 1.2 x 109 cm 2 . The depth of the pits is at least 250 nm, however, the sides are too steep to give reliable depth measurements using AFM. SEM observations showed that the angle between the pit walls and the (0001) face is about 600. From the crystal structure of wurtzite-GaN and its lattice constants [4] the theoretical crystal morphology of GaN can be determined using the 'connected net' theory [5,6]. It states that {hklm} faces parallel to a network of atoms interconnected by strong bonds, within a slice thickness dhktm, determine the morphology of a crystal. Three faces, {0001}, {1 1011 and
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1100), are found to be parall
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