Phase-transition Cubic GaN Doubles UV Efficiency
Emission efficiency of optimised cubic GaN is measured to be ~29 percent
Novel photonic materials are critical for energy conversion, communication, and sensing. GaN materials (also known as III-Nitrides) have proved to be one of the most promising of these in recent years as recognised in the 2014 Nobel Prize in Physics.
In the latest issue of ACS Photonics, a research team led by Can Bayram at the Innovation COmpound semiconductoR (ICOR) Laboratory, at the University of Illinois at Urbana-Champaign, has detailed a new phase of GaN materials: Cubic.
GaN and its alloys are the semiconductors of choice for driving the solid state lighting revolution through the white LEDs. Such LEDs are hexagonal (wurtzite) phase semiconductors and conventionally grown on the polar (0001) plane of the thermodynamically stable hexagonal (h-) phase of GaN.
However, the lack of inversion symmetry in h-crystal structure leads to strong spontaneous and piezoelectric polarisation fields (~MV/cm) along the <0001> growth direction. Such large electric fields have many detrimental effects in both photonics (reducing the radiative recombination rates in LEDs due to the quantum confined Stark effect ) and electronics (dictating a normally-on operation in HEMTs).
In contrast, the cubic phase of GaN (c-GaN) is inherently polarisation-free in the <100> growth direction due to its centro-symmetric atomic arrangement. Additionally, c-GaN has benefits such as high carrier mobility, high p-type conductivity, high electron drift velocity, small Auger losses, high optical gain, high radiative efficiency, and cleavage planes.
Most notably, the bandgap of c-GaN (Eg = 3.22 eV) is 0.2 eV smaller than that of h-GaN (Eg = 3.42 eV).
The conventional high–indium–content h-InGaN green LED is four times less efficient than the blue and red LEDs; this creates the so called 'green gap', because the available semiconductors cannot emit in this part of the spectrum efficiently. And to date, the green gap has prohibited the implementation of full spectrum solid state LEDs for display and lighting applications.
Cubic phase materials are interesting because cubic LEDs reduce the necessary indium content by ~10 percent (because of the 0.2 eV lower bandgap) and they can quadruple radiative recombination dynamics by virtue of their zero polarisation.
But the most important issue hampering the use of c-GaN is that it it is metastable when grown directly on conventional cubic substrates (Mg, GaAs, 3C-SiC), limiting its practicality. GaN grown via such approach results in highly defective (>1010 cm-2) and mixed h-/c- phase materials as GaN thermodynamically prefer the h-GaN crystal formation.
An alternative approach is to use hexagonal-to-cubic phase transition to synthesise single crystalline stable c-GaN. Now, Bayram's team has reported the hexagonal-to-cubic phase transition process in GaN enabled through aspect ratio patterning of a silicon substrate. It has produced high phase purity and high crystal quality c-GaN via MOCVD on a CMOS-compatible on-axis nanopatterned silicon(100) substrate.
The emission efficiency of this optimised cubic GaN, thanks to its polarisation-free nature, is measured to be ~29 percent. This is in sharp contrast to ~12 percent, ~8 percent, and ~2 percent of conventional hexagonal GaN on sapphire, hexagonal free-standing GaN, and hexagonal GaN on Si. This makes the technology suitable for next generation photonic devices.
This recent demonstration of high internal quantum efficiency from phase transition cubic GaN is a critical step towards bridging the green gap in the visible spectrum. Furthermore, cubic phase GaN materials can serve as enablers in polarisation-free photonics, room-temperature ferromagnetism, high-temperature spintronics, normally-off transistors, and single-photon emitters.
'High internal quantum efficiency ultraviolet emission from phase-transition cubic GaN integrated on nanopatterned Si(100)' by Richard Liu et al: ACS Photonics, 08 Jan 2018