19 January 2018
Hexagonal-to-cubic phase transition in GaN via aspect ratio nano-patterning of silicon substrate
© Semiconductor Today Magazine / Juno PublishiPicture: Disco’s DAL7440 KABRA laser saw.
A research team led by professor Bayram of the University of Illinois at Urbana-Champaign (UIUC) Department of Electrical and Computer Engineering’s Innovation COmpound semiconductoR (ICOR) Laboratory has reported using aspect ratio nano-patterning of a silicon substrate to enable a hexagonal-to-cubic phase transition in the metal-organic chemical vapor deposition (MOCVD) growth of gallium nitride (Richard Liu et al, ‘High internal quantum efficiency ultraviolet emission from phase-transition cubic GaN integrated on nanopatterned Si(100)’, ACS Photonics (2018); DOI: 10.1021/acsphotonics.7b01231).
For light-emitting diodes, compared with incumbent hexagonal-phase GaN (which has an energy bandgap of 3.42eV), cubic-phase GaN has a 0.2eV lower bandgap of 3.22eV. This reduces by ~10% the indium content necessary in InGaN to achieve the wavelength required. Cubic-phase material can also quadruple radiative recombination dynamics by virtue of the zero polarization.
Due to its polarization-free nature, the room-temperature internal quantum efficiency (IQE) of optimized cubic GaN is measured to be ~29% (at an ultraviolet emission wavelength of 380nm), in sharp contrast to about 12%, 8% and 2% for conventional hexagonal GaN-on-sapphire, hexagonal free-standing GaN, and hexagonal GaN-on-Si, respectively.
Graphic: Phase transition of hexagonal GaN (red) to cubic GaN (blue) through the composite scanning electron microscopy images of these nano-grooves on the left. Temperature-dependent internal quantum efficiency (IQE) of the UV band-edge emission (λ = 380nm) from the right-most nano-groove (magenta circle), which has pure cubic GaN on the surface, is shown on the right with a 3D drawing of the nano-groove as the inset. Room-temperature IQE of phase-transition cubic GaN is 29% (double of that of the conventional hexagonal GaN).
The demonstration of high internal quantum efficiency from phase-transition cubic GaN is reckoned to be a critical step towards bridging the ‘green gap’ in the visible spectrum.
Furthermore, cubic-phase GaN materials can serve as enablers in polarization-free photonics, room-temperature ferromagnetism, high-temperature spintronics, normally-off transistors, and single-photon emitters, add the researchers.