The reduction of undesirable gas phase reaction between trimethylaluminum and NH3 was achieved by spatially separating the precursors using N-2 purge line during AlN growth by metal organic chemical vapor deposition (MOCVD). Under this condition, it was shown that the growth pressure has a strong impact on the surface morphology independent of pre-reaction. For 0.8-mu m-thick AlN grown on (0001) sapphire substrates, increasing pressure from 200 to 500Torr drastically increased a root-mean-squared surface (r.m.s.) roughness from 0.48 to 33nm. This morphological change was previously attributed to the pre-reaction. Less pre-reaction also allowed us to investigate the pressure dependence of impurity (carbon and oxygen) incorporation in AlN as the growth rate was no longer affected by the pressure. Unlike GaN, the carbon level almost doubled with increasing pressure from 200 to 500Torr. By optimizing the surface morphology (r.m.s. roughness from 33 to 0.62nm) at 500Torr, the carbon concentration in AlN decreased from 5x10(18) to 7x10(17)cm(-3). Although there was no improvement in the structural quality, this uniquely designed MOCVD could further improve the material quality of AlN by reducing the impurity level. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

We observe terahertz (THz) emission from c-plane InN and In0.15Ga0.85N films on GaN templates due to surface-normal transport with a strong anomalous in-plane transport component. Analysis of the rotational dependence of the THz emission associated with this in-plane transport indicates that an electric field exists along the [1-100] m-axis correlated with the underlying template miscut. Calculations show that the field is correlated with strain-induced polarization fields at the heterointerface related to step-like charges at interfaces of the epilayers and the miscut templates. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mid-ultraviolet LEDs grown on AlN substrates with a range of quantum efficiencies and wavelengths spanning 250 to 280 nm have been investigated by time-resolved photoluminescence and electroluminescence. Through scaling of room temperature internal quantum efficiencies across all devices, radiative and nonradiative lifetimes are also estimated. General trends observed include an increase in PL lifetime for longer wavelength and higher external quantum efficiency devices, consistent with the increase in the estimated nonradiative lifetime with increasing wavelength. Despite these trends, the external quantum efficiency of the devices increases only weakly with increasing wavelength from 258 to 279 nm, suggesting that optimization of radiative lifetime and injection efficiency also play an important role. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

UVC light output of 66 mW at 300 mA CW has been achieved from LEDs on AlN substrates with extensive photon extraction. Proper vessel design allows for efficient irradiation of a water sample for purification.

Temperature-dependent carrier transport is investigated using ultrafast spectroscopy in a p-GaN/iInGaN/n-GaN solar cell with heavily-doped layers to compensate for polarization charges at the hetero-interface. We observe a flip in the transport direction at 110 K.