In
this Letter, we report on deep UV laser emitting at 249 nm based
on thin GaN quantum wells (QWs) by optical pumping at room temperature.
The laser threshold was 190 kW/cm2 that is comparable to
state-of-the-art AlGaN QW lasers at similar wavelengths. The laser
structure was pseudomorphically grown on a c-plane
sapphire substrate by metalorganic chemical vapor deposition, comprising
40 pairs of 4 monolayer (ML) GaN QWs sandwiched by 6 ML AlN quantum
barriers (QBs). The low threshold at the wavelength was attributed
to large optical and quantum confinement and high quality of the material,
interface, and Fabry-Pérot facet. The emissions below and above
the threshold were both dominated by transverse electric polarizations
thanks to the valence band characteristics of GaN. This work unambiguously
demonstrates the potentials of the binary AlN/GaN heterojunctions
for high-performance UV emitters.
Wurtzite BAlN alloys are emerging ultrawide bandgap III-nitride semiconductors promising for optical and electronic devices. Yet the boron compositions of the grown alloys have been limited. In this Letter, we report on the demonstration of a thick single-phase wurtzite BAlN film with a boron composition over 20%. The growth was conducted at 1010 C and 150 Torr with continuous flows of group-III precursors and ammonia with a growth rate of 2.2 lm/h by metalorganic chemical vapor deposition. The boron composition was studied by x-ray diffraction (XRD), secondary neutral mass spectrometry (SNMS), and Rutherford backscattering spectrometry (RBS). The XRD 2h scan exhibited the clear wurtzite BAlN peak 1.82 larger than the AlN peak, indicating the boron composition of 30.9% based on the lattice constants of wurtzite AlN and BN. The SNMS and RBS experiments, independent of strain and defects, revealed that the boron content was 22%. The microstructures of the wurtzite BAlN film were further studied by transmission electron microscopy, showing an initial 5 nm thick layer free of crystal twinning followed by widespread crystal twinning with lattice rotations of 60 clockwise and anticlockwise. The optical transmission experiment manifested that the bandgap of the smaller-lattice BAlN film was 5.1 eV, smaller than that of larger-lattice AlN. This trend was the opposite of the conventional InGaAlN but consistent with theoretical predictions. This study would greatly facilitate the research of material, physics, and devices incorporating the wurtzite BAlN alloys.
A new method has been established and employed to create a random nanophotonic crystal (NPhC) structure without photolithography on the unpolished side of a single-side-polished sapphire substrate. This nano structure has potential use in enhancing the light-extraction efficiency (LEE) of deep ultraviolet light-emitting diodes (DUV-LEDs), and has never been built for DUV-LED applications before. Two mask layers in the nano scale (Au and SiO2) were used to create the NPhC and observed using scanning electron microscopy to have an average height of 400 nm and various sizes from 10 to 200 nm. Finally, a conventional DUV-LED and a DUV-LED device with NPhC were simulated using 2D Lumerical Finite-Difference Time-Domain (FDTD) for comparison. The results show that the LEE of the DUV-LED device with this NPhC integrated was significantly directly enhanced by up to 46% and 90% for TE and TM modes, respectively, compared to the conventional DUV-LED device. Thus, this NPhC is believed to be a new, key technique to enhance the LEE of DUV-LEDs.
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