AlGaN/GaN HFETs with different undoped GaN thicknesses were grown on Fe‐doped freestanding GaN substrates by conventional MOVPE. To realize a high drain current, thick undoped GaN is found to be necessary. SIMS measurement shows that Fe is redistributed into the epilayer, by which the scattering center is generated at the channel when the thickness of the undoped GaN is insufficient. We also observed a similar Fe profile in the GaN/sapphire template placed on the side of the Fe‐doped GaN substrate during growth. Therefore, Fe in the Fe‐doped GaN substrate is redistributed not only through a solid but also through vapor. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We evaluated AlGaN‐based 355 nm UV laser diodes prepared under two different Mg activation conditions. The annealing processes for Mg activation in p‐layers were carried out under N2 or O2 ambient to investigate the effect of the ambient gas on the laser characteristics. The threshold current densities and operating voltages of the UV laser diodes were improved by annealing under O2 compared with those under N2. We then estimated the injection efficiencies of the laser diode structures by considering the internal quantum efficiencies of optical excitation and electrical excitation. The internal quantum efficiency of the electrically excited spontaneous emission from the laser structure annealed under O2 reached 50% at a carrier density of 7.0×1018 cm‐3, while the structure annealed under N2 required a 1.8‐fold higher carrier density of 1.2×1019 cm‐3 to reach the same internal quantum efficiency. In addition, the internal quantum efficiency estimated from optical excitation reached 50% even at a carrier density of 3.0×1018 cm‐3. This implies that the injection efficiencies in the UV laser diode structures annealed under N2 and O2 were 25% and 45%, respectively. Mg activation by O2 annealing is effective for increasing the injection efficiency. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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AlGaN/GaN heterostructure field-effect transistors (HFETs) were fabricated by metalorganic vapor phase epitaxy (MOPVE) on c-plane Fe-doped GaN (GaN:Fe) substrates with different growth conditions of GaN buffer layers. The GaN buffer layers were grown at a V/III ratio of 272 at 300 Torr to realize highly resistive GaN buffer layers with automatic carbon doping. HFETs with carbon-doped GaN buffer layers showed the best characteristics among all the samples in this study. The carbon concentration was 5 Â 10 16 cm À3 . In the sample with a gate-drain length of 50 m, the off-state breakdown voltage was 2 kV or higher and the on-resistance was 18.5 mÁcm 2 . We found that the carbon-doped GaN buffer layers are necessary for high breakdown voltages even using semi-insulating GaN:Fe substrates. #

We fabricated and evaluated GaN/AlGaN multi‐quantum wells (MQWs) and ultraviolet laser diodes (UV LDs) on high and low dislocation density underlying layers by epitaxial lateral overgrowth (ELO) method. We analyzed the internal quantum efficiency (IQE) versus carrier concentration characteristics quantitatively by excitation intensity dependent photoluminescence method. The IQE of the MQWs on the ELO AlGaN is 75% when the carrier density is 1x1019 cm‐3. We demonstrated the UV LD on the ELO AlGaN. However, the UV LD on flat AlGaN did not operate. Also, we investigated the internal loss (αi) and the IQE multiplied the injection efficiency by changing the reflectivity of the facets. The results showed that the internal loss is 6 cm‐1, and the IQE multiplied by the injection efficiency is 18% (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)