Gallium nitride (GaN) has a bandgap of 3.4eV at room temperature. The compound has played a major role in the development of modern light-emitting diodes and is today used in back-illuminated liquid-crystal displays in devices ranging from mobile phones to TV screens. GaN-based LEDs emitting blue and ultraviolet (UV) light have also been used in DVDs, where the shorter wavelength of the light allows higher data-storage densities.

One of the main advantages of GaN is that even with large numbers of dislocations, sometimes exceeding 1010/cm2, the material continues to emit intense light – a property that makes it unique among the other III-V materials, such as GaAs, GaP and InP. (Dislocations form thanks to the mismatch between epitaxial layers and underlying substrates and normally adversely affect the properties of electronic and optoelectronic materials and device structures). This particular feature, among other material characteristics, allowed researchers to make GaN-based blue LEDS in the 1990s, a feat that earned Isamu Akasaki, Hiroshi Amano and Shuji Nakamura the Nobel prize for Physics in 2014.

GaN looks set to further revolutionize electronics and optoelectronics with the recent demonstration of an electrically pumped “inversionless” polariton laser operating at room temperature made from a bulk GaN-based microcavity diode.

Improving GaN sample quality

To allow for further progress in the development of III-nitride-based power electronics and high brightness LEDs, researchers need to be able to make good quality, single-crystalline GaN substrates. In the last decade, many good growth techniques have been developed, including ammonothermal growth and hydride vapour phase epitaxy. Although the former allows for GaN wafer growth over an inch in diameter, the relatively low growth rates (of around 50 µm/day) make it difficult to produce samples for commercial use. Higher growth rates can be achieved using HVPE (up to 500 µm/hour), but samples produced in this way are often poorly crystalline. Besides producing threading dislocations, GaN grown by HPVE also contains V-shaped defects or pits on the surface that can reach millimetres in size.

Now, a team led by Ion Tiginyanu at the Academy of Sciences, Technical University and State University of Moldova, has found that it can produce hexagonal concentric architectures on GaN by photoelectrochemically etching bulk GaN grown by HVPE. These structures, which form in several crystallographic directions during growth and cover the V-shaped defect or pits, improve the crystalline quality of the as-produced material and so its electrical and optical properties.

“We showed that we were able to modulate the physical and optical properties of GaN on the nanoscale by growing pencil-like nanocrystals that emit light of various colours along their sharp tips (see figure),” says Tiginyanu. “Our growth technique combined with electrochemical etching represents a powerful tool for designable 3D nanostructured gallium nitride with potential applications in optoelectronics, photonics, biomedicine,” he tells nanotechweb.org.

Full details of the research can be found in ECS Journal of Solid State and Technology doi: 10.1149/2.0091605jss.