Free-standing zinc-blende (cubic) GaN, AlN and AlGaN layers grown by molecular beam epitaxy

The group III-nitride semiconductors (AlN, GaN and InN and their solid solutions) are being increasingly used for amber, green, blue and white light emitting diodes (LEDs), for blue/UV laser diodes (LDs) and for high-power, high-frequency and high temperature electronic devices. However, one of the most severe problems hindering progress in this field is the rarity of suitable substrates. AlN, GaN, InN and their solid solutions are commonly grown on non-lattice matched substrates e.g. sapphire, GaAs or SiC, but bulk GaN and AlN substrates would be much better for the highest-quality nitride-based devices. For AlGaN-based devices for ultra-violet optoelectronics and for high frequency applications at high power levels, AlN substrates would be ideal. AlN substrates have higher radiation hardness and superior thermal conductivity compared to GaN. There is a measurable difference in the lattice parameters of GaN and AlN, therefore for several device applications AlGaN substrates would be preferable to either GaN or AlN. Success in producing cubic AlN and AlGaN substrates would mean our technology could be extended commercially to water sterilization, bioterrorism detection, satellite communication and data storage devices.The group III-nitrides normally crystallise in the hexagonal (wurtzite) structure. The unique feature of wurtzite group III-nitrides, in comparison with conventional III-V semiconductors, is the existence of very strong electric fields inside the crystal structure. These reduce the optical emission intensity in quantum wells, due to charge separation. The electric fields can be eliminated in wurtzite material by growing in non-polar directions. However, a direct way to eliminate electric fields would be to use non-polar (100) oriented zinc-blende (cubic) III-nitride layers. The thermodynamically metastable cubic GaN and AlN layers have, so far, received less attention than the more familiar hexagonal films. However, interest in zinc-blende nitrides is now rapidly increasing for three main reasons: 1) the absence of electric fields in cubic (100) nitrides; 2) the ability to cleave cubic (100) nitrides in the perpendicular cleavage planes and 3) the enhanced mobility of the carriers (particularly p-type). Recently we have demonstrated for the first time that it is possible to grow free-standing zinc-blende GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE) with potential applications as substrates. We are not aware of any data or publications to-date on free-standing zinc-blende AlN or AlGaN layers. The main aims of this project are feasibility studies of the growth of free-standing zinc-blende (cubic) AlN and AlGaN layers by PA-MBE and a comprehensive analysis of their structural, optical and transport properties. This is the first step towards developing commercially viable cubic nitride substrates.