GaN crystals were synthesised in a horizontal reactor system using ammonia as a nitrogen source. In the first part of this project a parameter study was performed to evaluate a suitable growth window, concerning e.g. process temperature, gas flows or partial pressures of the process gases. These first experiments were carried out in a horizontal reactor with a silica ampoule using a quartz crucible. Applying the optimum parameter set it was possible to grow GaN single crystals and free-standing GaN with a thickness of 0.1 mm. Pure hexagonal GaN was confirmed by XRD and EDX measurements, neither a cubic phase content nor any other binary or ternary phase was found, which is in agreement with the Raman spectroscopy data and the results from cathodoluminescence and PL measurements. IntroductionRecently there is a high activity in research, targeting a suitable growth process for the production of GaN volume crystals, which could be used as a substrate material for nitride based devices. Compared to the conventional semiconductor materials there are severe problems growing GaN from a melt, due to its high vapour pressure and the low solubility of nitrogen, respectively GaN, in liquid gallium.Currently the HVPE technique is favoured for the fabrication of 300-400 µm thick GaN wafers sliced from a 5 mm boule, as it was shown by [1]. However, the HVPE technique is expensive compared to a conventional solution growth process and is restricted to relatively small boule lengths. Beside HVPE there are other techniques like the ammonothermal method, which is sometimes used, but not well established up to now. The first process exhibiting a GaN single crystal of the order of several millimetres in size was a high pressure process [2], which is difficult to be scaled up for industrial use. Although the crystals from high pressure solution growth (HPSG) are relatively large, up to about 15 mm in diameter, they are very thin (≈100 µm) platelets due to the high growth anisotropy between the a-and the cdirection of the GaN lattice under this conditions. For a volume crystal, suitable for industrial wafering, a lower growth anisotropy would be necessary. Several attempts have been made to find a way for growing GaN from a solution using various metal fluxes [3][4][5], from which the sodium flux method turned out to be the most promising [5] so far. The sodium flux method also resulted in crystals up to 6 mm in size, but suffers strongly from the evaporation of the solvent due to its high vapour pressure and possible Namelt inclusions in the crystals. Moreover, this method still requires some pressure on the order of 5 MPa. To avoid a high pressure process it is possible to choose an alternative nitrogen source. Ammonia is the most simple choice and it was shown by [6] that GaN could be formed under room pressure conditions, although the reaction proceeds very fast and is difficult to be controlled. GaN crystals grown by one of the above techniques mainly grew by spontaneous nucleation in the liquid rather than by a seeded proc-
By using reflectivity and temperature resolved Hall measurements the electrical properties of low pressure solution grown (LPSG) GaN are determined. Hall measurements show that the material is degenerate. The reflectivity spectra are governed by the free electron gas in accordance with this finding. The charge carrier concentration is about 4x1019 cm -3 and the mobility 70…80 cm 2 /Vs. These results are compared to gallium nitride synthesized by other solution or vapour phase growth techniques.
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