Cubic crystal offers GaN solution
III-nitride (III-N) devices are being held back by a shortage of bulk GaN for lattice-matched epitaxial growth. Meanwhile, those few that are available command a high price, along with a hexagonal wurtzite structure that is not ideal for all types of device. Although the high-polarity fields that exist in the substrate and the subsequent epilayers – due to a mix of piezoelectric and spontaneous polarizations – aid HEMT design, they cut recombination efficiency and therefore the light output in optoelectronic devices.
This weakness has fuelled interest in the growth of non-polar light-emitting III-N structures. In these devices, polarization effects are eliminated due to growth on the non-polar orientations of the crystal, such as the m-plane. However, these non-polar hexagonal bulk crystals and templates are difficult to produce, they are even more expensive than their polar equivalents and their quality, while satisfactory, is far from perfect.
An alternative, more promising method for eliminating the polarization effects in GaN-based devices involves the use of non-polar (100) oriented zinc-blende III-N layers. Although these thermodynamically metastable cubic GaN layers have received relatively little attention, they also have two major advantages over both polar and non-polar wurtzite GaN: they can easily be cleaved on {110} crystal planes for device fabrication, thanks to the geometry of the crystal structure; and they offer a carrier mobility that is an order of magnitude higher, due to the increased crystal symmetry.
Cubic GaN epilayers have already been produced on cubic platforms, such as GaAs and SiC, using specialized growth conditions and MBE, HVPE and MOCVD techniques. The MBE approach is the most promising because it produces the lowest content of hexagonal GaN inclusions. In comparison, the higher process temperature required for HVPE, and especially MOCVD, produces rapidly increasing hexagonal content with further GaN growth.
Up until now the thickness of cubic GaN has been limited to 1 µm or so, but research by our team at the University of Nottingham, UK, has shown that MBE can produce far thicker material. This has enabled us to form free-standing GaN, which is a promising forerunner to making cubic substrates.
We grew our undoped, thick cubic GaN films on semi-insulating GaAs (001) substrates by plasma-assisted MBE (PAMBE), using arsenic as a surfactant to initiate the growth of the cubic phase. The growth rate for these films is 0.3 µm/h, which is not particularly fast but is comparable to that used for forming bulk hexagonal GaN crystals from liquid gallium at high pressures.
Examples of our free-standing cubic GaN layers produced by PAMBE include an 8 µm thick piece that has a surface area greater than 1 cm2 (figure 1). This film is transparent with a shape that reveals its cubic microstructure. It is also strong – it can be easily handled when its thickness is increased to 30 µm or more, enabling it to be used as a substrate for cubic GaN-based structures and devices.
We have investigated the properties of our material using a range of ex situ techniques, such as X-ray diffraction, reflection high-energy electron diffraction (RHEED), transmission electron microscopy, photoluminescence (PL) and nuclear magnetic resonance spectroscopy. All of these techniques, including RHEED (figure 2), were unable to identify any hexagonal inclusions within the first 10 µm of this material. PL measurements determined that the film s bandgap is 3.25 eV at room temperature and 3.30 eV at 4 K.
When we increased this thickness towards 50 µm, hexagonal inclusions formed in the film with a concentration of up to 10%. We plan further investigation into the nature and density of this and other types of defects in cubic GaN and to work in partnership with Sharp Laboratories of Europe in a program funded by the UK s Department for Business, Enterprise and Regulatory Reform that is aimed at refining our technology for commercialization. This effort will focus on increasing the material s size and growth rate.
