Gallium nitride (GaN) substrates are strongly required for the fabrication of high-quality electronic and optoelectronic devices. These include blue laser diodes with high reliability and white as well as UV LEDs with high efficiency. Up to now, the growth of of sufficiently large sized GaN crystals from melt, as it is common e.g. for GaAs substrates, has not been achieved. For UV LEDs, AlGaN layers with low defect densities are required. The generally high defect density is caused by hetero epitaxy on sapphire substrates and can be reduced by the growth of thick layers of several 10 µm.
The HVPE technique is currently accepted worldwide to be one of the promising techniques for this purpose. It allows for large-size crystal growth with rates exceeding 400 µm/h. This technique uses liquid gallium which is transported to the substrate after being transformed into gallium chloride by reaction with hydrogen chloride at temperatures of about 900°C. On the substrate gallium nitride is formed by reaction of gallium chloride and ammonia at temperatures of about 1050°C. The use of metallic aluminium allows for the growth of AlGaN layers. Similarly, the high growth rates of the HVPE technique are advantageous. Therefore, growth processes for all AlGaN compositions are in development.
A single wafer Aixtron HVPE horizontal reactor is used for basic investigation of AlGaN growth processes. A vertical Aixtron HVPE reactor is dedicated to the growth of GaN bulk crystals which are then processed into GaN substrates by FBH's industrial partner Freiberger Compound Materials GmbH. Both reactors are equipped with LayTec in-situ reflectometers for optimization of growth parameters.