Matsushita patents GaN growth method
Matsushita Electric Industrial s method for making GaN crystals, which it says produces high quality material under milder conditions than existing techniques, has gained a US patent.
US patent 7288152, granted on October 30, focuses predominantly on bulk crystal growth aimed at making substrates, but does include a brief description of device fabrication.
Earlier this year the Japanese electronics manufacturer unveiled what its said were the first commercial white power LEDs produced on GaN substrates, under its Panasonic brand (see related story).
Matsushita s patented technique employs a pressurized atmosphere of nitrogen, in which a gallium/sodium melt is heated. The gallium and nitrogen react to form GaN, which remains dissolved in the melt until crystallized out at specific temperatures and pressures.
The company s best example of the process takes place between 830°C and 950°C at pressures that vary with temperature but are consistently below 120 atmospheres. The crystal quality that Matsushita claims depends on maintaining conditions within a different 15 atmosphere window for each temperature. The authors say that these conditions suppress defect-causing heterogeneous nucleation.
The seed from which the larger crystal is grown is placed into the melt as a thin film on another substrate, usually sapphire.
Although the firm doesn t record any detailed data about defect density, it uses the GaN material s transparency as one indicator of quality, and claims to have produced perfectly transparent crystals. Other quality data includes a 49° full-width, half maximum for the GaN s X-ray rocking curve, and an impurity level of less than 10ppm was measured by secondary ion mass spectrometry.
The patent s authors point out that before the gallium/sodium melt, or flux, method was developed, typical liquid-phase GaN growth conditions would be 1200°C and 8000 atmospheres. Matsushita is not the first to use a flux-based method, but it says previous efforts only produced 1.2 mm crystals.
In this case, Matsushita published a photo of a 17 mm crystal in its patent application, and recorded a 1.5 mm-thick transparent crystal grown at 12.5 µm/h.
