In the past decade, Group III-nitride materials have been widely used for
visible and ultraviolet light emitting diodes and blue, violet laser diodes.
Most of these optoelectronic devices are typically fabricated on the conventional
polar (0001) c-plane oriented substrate materials. Devices grown on the polar
substrate orientation suffer undesirable spontaneous and piezoelectric polarization
resulting in significant band bending in the quantum well. This reduces radiative
recombination efficiency and lowers device performance.
Now the technical team at Oxford
Instruments-TDI, led by Dr. Alexander Usikov, has made significant progress
in solving this problem, and is working closely with a leading LED manufacturer
to fabricate these semi-polar GaN layer for optoelectronic devices. This would
increase radiative recombination efficiency and device performance.
In order to diminish these polarization effects, growth of GaN-related devices
along semi-polar and non-polar directions has been studied intensely. Using
hydride vapour phase epitaxy (HVPE), the team has grown high quality, semi-polar
(11.2) oriented GaN on (10.0) m-plane sapphire with an intermediate layer between
the sapphire substrate and the GaN layer.
The semi-polar (11.2) GaN layers were grown in the temperature range from 930
to 1050°C in an inert argon ambient at atmospheric pressure. Gallium and aluminum
were used as metallic source materials and hydrogen chloride (HCl) and ammonia
(NH3) as the active gases for the HVPE process. The epitaxial growth of GaN
was performed at approximately 60µm/ hour using an intermediate layer deposited
on m-plane sapphire followed by an undoped GaN layer. The growth procedure results
in high quality, semi-polar GaN layer with thickness up to 30 µm.