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To investigate the impact of oxygen on the band structure at the Ni/p-type GaN interface, the crystal structure and nanoscale impurity distributions were evaluated using transmission electron microscopy and three-dimensional atom probe (3DAP) analysis, respectively. These measurements revealed that the oxygen region existed approximately 5 nm from the GaN surface and that the oxygen concentration was equal to or higher than the Mg acceptor concentration. The band bending and photoelectron spectrum were then simulated using the Mg and oxygen concentration profiles obtained by 3DAP to consider the impact of the interfacial oxygen donors on the photoelectron spectrum measured using hard X-ray photoelectron spectroscopy (HAXPES). The precise band bending was then determined by fitting the simulated spectrum onto the experimental measurements. This showed that the oxygen donors at the interface modulated the band structure and decreased the energy barrier by at least 0.1 eV, which demonstrates the importance of considering the existence of oxygen at the interface. It is, therefore, essential to use techniques like 3DAP and HAXPES to evaluate both the nanoscale impurity distributions and the resulting band structure to fabricate higher-performance devices.

We report the generation of a picosecond optical pulse with 2.2 nJ pulse energy at blue-violet wavelengths using a GaN-based mode-locked laser diode (MLLD) and a semiconductor optical amplifier (SOA). The picosecond optical pulse generated by MLLD at a frequency of 812 MHz was amplified effectively by SOA. We optimized SOA with a widely flared waveguide structure to generate a high optical pulse energy.

We have demonstrated passively mode-locked operation of a GaInN bisectional laser diode in a dispersion-compensated external cavity. With negative group velocity dispersion in the external cavity, we obtained femtosecond optical pulses after spectral filtering with a bandpass filter. The pulse duration was as short as 200 fs with a time-bandwidth product of 0.41. This is the shortest pulse ever reported for a mode-locked GaInN laser diode.

We directly generated 1.6-ps optical pulses with a peak power of 20 W at a repetition of 1 GHz using a blue-violet GaInN mode-locked laser diode incorporating a flared waveguide operating in single transverse mode. The flared waveguide enabled optical pulses to be generated with peak powers of over six times higher than those produced using a straight waveguide while preserving the pulse duration.

We evaluated saturation energies and absorption recovery time dependent on reverse-bias voltage for a waveguide-coupled GaInN multiquantum well saturable absorber in a bisectional laser structure. When the applied reverse-bias was increased from 5 to 20 V, the saturation energy at 405 nm monotonically increased from 5 to 14 pJ; the linear absorption coefficient at 405 nm showed the same dependence. These dependences are different from those observed in an AlGaAs multiquantum well. The absorption recovery time was shortened to 3 ps when a highly reverse-bias voltage of 20 V was applied.

We have generated single-transverse-mode optical pulses with 100 W peak power and 3 ps duration at 1 GHz repetition from a blue-violet GaInN mode-locked laser diode (MLLD) and a semiconductor optical amplifier (SOA) without the use of any pulse compression. The generation of clean optical pulses without subpulse components from the MLLD and the reduction in amplified spontaneous emission in the SOA by incorporating a flare waveguide structure resulted in effective amplification of optical pulses to produce over 100 W peak power.