The University of Nottingham has developed a technique to grow bulk cubic GaN, avoiding the natural hexagonal structure, and has extended its work to AlGaN.

"The hexagonal form has strong internal piezo fields which makes it very difficult to design vertical devices," scientist Dr Sergei Novikov told Electronics Weekly. "Cubic GaN also has high mobility, especially in the p type."

According to Novikov, the only way to get a non-polar GaN substrate currently is to saw a thick hexagonal wafer vertically and use it edge-on.

As 5mm is the practical thickness limit for GaN growth, the technique produces only thin strips of substrate - less suited to standard semiconductor processing techniques.

Working with Sharp Labs in Oxford, Novikov in Nottingham is using plasma assisted molecular beam epitaxy (MBE) to grow cubic GaN - specifics of the process were patented in 2005.

Starting with a GaAs substrate, the Nottingham Nitrides Research Group first grows a GaAs buffer layer by MBE, followed by the GaN. The GaAs can later be etched away to leave a free-standing GaN substrate.

MBE is famously slow. "It is like a snail," said Novikov, "but before we grew 0.3µm/hour maximum. Now we have reached 4µm/h so you can grow a 50µm substrate in a day."

The cubic form is metastable with growing crystals tending towards hexagonal structure as they get taller.

Before the Nottingham/Sharp work, the thickest previously reported (1999) cubic GaN layer Novikov can find was grown by vapour-phase epitaxy, and was already 40 per cent hexagonal when it reached 10µm thick.

"We can grow 2in. [50mm] wafers up to 100µm thick," he said.

Now Nottingham has applied the knowledge gained with GaN to cubic AlGaN growth, with success last month.

"This wasn't demonstrated before," said Novikov. "It is initially cubic and we don't know the hexagonal fraction yet."

Free-standing cubic GaN wafers of around 30µm can easily be handled. "Wafers in the 30-50[micron]m range can be used as substrates for further growth of cubic GaN-based structures and devices," said the University of Nottingham.