Mitsubishi expands non-polar substrates
by Richard Stevenson in Montreux, Switzerland
Mitsubishi Chemical Corporation is aiming to extend its lead in the non-polar GaN field with 2-inch substrates by 2010, according to the company s Toshinari Fujimori.
Fujimori, speaking in Montreux, Switzerland, on October 6 revealed that Mitsubishi has already produced 12 mm x 20 mm non-polar GaN substrates in the lab. He told the audience gathered at the International Workshop for Nitride Semiconductors meeting that 18 mm x 18 mm pieces should follow in the near future.
Non-polar substrates enable the growth of high-quality lasers and LEDs that are free from strong internal electric fields.
These fields are an obstacle to the development of conventional green lasers. Fujimori is optimistic that growth on non-polar GaN will lead to the fabrication of the first GaN-based green laser diodes.
Mitsubishi constructed the HVPE reactor it produces bulk GaN from itself, and makes non-polar substrates by slicing the boules it produces vertically.
Fujimori said that clogging of the reactor s exhaust line is one the problems that the researchers have had to overcome, alongside the need to eliminate pits and a rough surface through optimization of the growth condition.
He presented studies on a 52 mm diameter, 5.8 mm thick free-standing boule to demonstrate the present status of Mitsubishi s technique.
Cathodoluminescence measurements on horizontal slices from this boule revealed that the dislocation density is highest next to the interface with the HVPE growth substrate. The slice nearest to this interface had a dislocation density of 5x106 cm-2, compared to 1x106 cm-2 for the slice nearest the boule s top surface.
Secondary ion mass spectrometry measurements showed very low levels of impurities other than silicon, which was present at a concentration of 1.5x1017 cm-3.
Fujimori also revealed that Mitsubishi had also been developing a “chemical equilibrium reaction” for GaN boule growth that promises to slash the cost of non-polar GaN substrates.
Very few details about this technology were disclosed, but Fujimori told the audience that the company would say more in the future.