Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

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Direct comparison of optical properties and carrier dynamics of InGaN multiple quantum well (MQW) laser diode structures grown on pendeo epitaxial (PE)-GaN and sapphire substrates is reported. A strong increase in quantum efficiency and a dramatic reduction in stimulated emission threshold are observed for InGaN MQWs on PE-GaN substrates as compared to MQWs on sapphire substrates. Based on temperature-dependent time-resolved optical analysis, the authors find that a significant increase in nonradiative lifetime due to suppressed dislocation density plays an important role in enhancing optical properties of InGaN MQWs grown on PE-GaN substrates, resulting in radiative-process dominant emission even at room temperature.

PACS 42.55.Px, 81.05.Ea, 81.15.Kh High performance laser diodes were fabricated on lateral epitaxial overgrown GaN layers on sapphire substrates. The threshold current of the LDs was strongly dependent on the dislocation density. A low threshold voltage was obtained using highly Mg doped contact layers. The lifetime of LDs was also influenced by the operation voltage. Threshold current density and threshold voltage were 3.74 kA/cm 2 and 4.8 V, respectively. The LDs showed a lifetime of 1,000 hs at 50 °C under automatic power controlled conditions of 30 mW.

An optical loss of GaN-based blue-violet laser diodes (BV-LDs) was measured by taking the intensity decay of edge emitting luminescence with respect to the distance from cleaved edge of a wafer to the position where an excitation laser was focused. Amplified spontaneous emission (ASE) was also investigated by tuning the power of an excitation laser on BV-LD wafers. Measurements were performed on wafers with different thicknesses of InGaN optical confinement layers (OCLs). The threshold power of ASE intensity was minimized at an optimum thickness of InGaN OCL. We also found that optical loss of wafers was determined by absorption of an InGaN layer in thicker OCL structure. From experimental data and fittings, we obtained 40 cm−1 for InGaN absorption at 405 nm. The optical field confined in OCL region was reasonably high enough to affect the overall modal loss in devices. Therefore, the optical losses still remained even though the Mg-doped GaN regions are far enough from the active layers. The crystal quality of an InGaN layer should be an important aspect to improve the performance of BV-LDs.

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (Na=5×1017 cm−3). The as-deposited contact reveals nonlinear current–voltage (I–V) characteristics. However, the contacts show considerably improved I–V behavior when annealed at temperatures of 350–550 °C for 1 min in air ambient. For example, the specific contact resistance as low as 7.2×10−5 Ω cm2 is obtained from the samples annealed at 550 °C, whose transmittance is measured to be 82.5% at a wavelength of 470 nm. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the annealed contacts are described and discussed.

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Recent progress on the fabrication processes of blue‐violet laser diodes (BV‐LDs) which reduced input power and enhanced heat dissipation has been discussed. In order to reduce operating current of the BV‐LDs, a novel ridge waveguide structure with two different side angles was developed, and ridge width and thickness between active layer and etched surface wais optimized, as well. For lowering of operating voltage, we optimized the thickness of the highly Mg doped p‐GaN contact layer, where the ohmic metals were contacted, as well as pre‐treatment process before deposition of p‐electrode. Improving the fabrication processes for high power AlInGaN‐based BV‐LDs as well as optimizing design and growth conditions for BV‐LD structures resulted in a threshold current density and a threshold voltage of 3.3 kA/cm2 and 4.4 V, respectively. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)