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Threading screw dislocations in wurtzite GaN are studied using Heyd-Scuseria-Ernzerhof hybrid functional with in generalized Kohn-Sham scheme. Several types of core structures, both stoichiometric and non-stoichiometric, are constructed from the unconventional dislocation line position. We computed the formation energies and compared their stabilities systematically. For the open core type structure, the equilibrium diameter is ˜ 4.9 A. With the intermediate value of Ga chemical potential, the Ga-filled type dislocation core, where N atoms near the dislocation line are removed and only Ga atoms exist, becomes the most stable structure. This dislocation core does not introduce deep empty states into the band gap, but introduces deep filled states originated from the Ga–Ga wrong bonds of Ga atoms located along the dislocation core. This leads to the band gap narrowing and degrades the electronic properties such as breakdown voltage. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The scattering and dephasing processes that accompany the initial photoexcitation of electron-hole pairs in the semiconductor GaN are studied by sub-picosecond time-resolved photoluminescence. The rise time of the carrier recombination at the band edge after excitation 750 meV above the band gap is analyzed as a function of initial photoexcited carrier density and sample temperature. To elucidate the findings, simulations were performed based on a two band model with a non-parabolic conduction band and using a generalized Monte-Carlo technique. Results showed a small dependence in the rise time on the initial carrier density but a strong dependence on the sample temperature, where simulation and experiment scaled proportionately. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We employ an optically detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in an AlGaN/GaN heterojunction p-i-n diode to measure the electron velocity overshoot in GaN at room temperature. It has been found that electron velocity overshoot occurs at electric fields as low as 105 kV/cm, with the peak transient velocity increasing with E up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. At higher fields, the increase in transit time with increasing field suggests the onset of negative differential resistance due to intervalley transfer. The existence of transient velocity overshoot at fields lower than the calculated peak steady-state velocity suggests that it occurs while the electrons are primarily in the r valley. Full zone Monte Carlo calculations imply that the overshoot is associated more with band nonparabolicity in the Γ valley than with intervalley transfer at fields less than 325 kV/cm, and, in conjunction with theoretical calculations employing a semiclassical transport model, confirm the importance of this nonparabolicity for the determination of the temporal shape of the transient velocity overshoot curves.

We report the development of AlGaN-based deep ultraviolet LEDs by plasma-assisted molecular beam epitaxy. The devices were evaluated under bare-die configuration, placed outside an integrated sphere, and were found to have optical power of 1.3 mW and 1.2 mW at 300 nm and 277 nm, under pulsed driving current of 200 mA and 500 mA, respectively. The external quantum efficiency (EQE) of the 300 nm LED is 0.16% at drive current of 90 mA. These data suggest that AlGaN alloys grown by MBE under excess Ga are capable of producing deep UV-LEDs with high IQE. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)