MOCVD Growth of GaN on bulk AlN Substrates
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277 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
substrates were placed besides the AIN substrate in both runs. After growth, the samples were examined by optical microscopy, x-ray diffraction and low temperature photoluminescence (PL). RESULTS AND DISCUSSIONS a. GaN layers grown on c-plane sapphire In Fig. 1, the surface morphologies of GaN grown on c-plane sapphire without (a) and with (b) an AIN buffer layer are shown. The growth conditions may not be optimal in either case, but the difference is still significant. For the sample without a buffer layer, the GaN film was composed of three dimensional islands with random shapes, resulting in poor surface morphology. On the other hand, the GaN surface grown with the AIN buffer layer showed a much improved morphology, consisting of hexagonal shaped islands with extremely smooth facets. Because of the better lattice match and chemical compatibility, a thin AIN buffer layer seems to help the GaN wet the substrate in its early nucleation stage, as has also been reported by other authors [1-3].
Figure 1:
a) b) Surface morphologies of GaN grown on the c-plane orientation sapphire substrates, a) GaN grown directly on sapphire, b) GaN grown on sapphire with an AIN buffer layer. The bar shown at upper right corner of the figures represents 25 tim.
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Photon energy(elv)
Phobon Energy (oy)
a) b) Figure 2: Photoluminescence spectra at 7K of (IaN directly grown on sapphire (a), and grown on sapphire with an AIN buffer layer (b). The PL were conducted at 7 K. The difference in the quality of these two films was further demonstrated in lowtemperature PL experiments. Fig. 2 shows the PL spectra at 7K for GaN films grown on sapphire and the insets show the high-energy portion of the spectra. For GaN grown directly on sapphire (Fig. 2a), the PL reveals a donor-bound-exciton peak (BE) located at 3.470 eV, with a FWHM of 278
30 meV. The broad yellow peak located around 2.2 eV is attributed to a deep-level (DL) related recombination which is undesireable [10, 11]. The intensity ratio of the BE to DL peak is about 50. On the other hand, GaN grown on sapphire with an AIN buffer layer exhibits a BE peak at 3.477 eV with a FWHM of only 16 meV, and the BE to DL intensity ratio of about 300 (see Fig. 2b). The BE peak shifts 7 meV towards the higher energy compared to the one in Fig. 2a. This energy difference is probably due to the strain difference in these two layers. The narrower BE peak and weak DL emission indicate a much better GaN layer grown with an AIN buffer layer than the one directly grown on sapphire. These results illustrate that a buffer layer which has similar material properties to GaN is necessary for an epitaxial system like GaN/sapphire with a 16% lattice mismatch. This is consistent to the earlier reports [1-3] and indicates the necessity of a better lattice-matched substrate like AIN to reduce the growth complexity and improve the layer quality.
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aN
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GaN(0004)
AIN(0004)
20 (degree)
Figure 3: Mor
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