No abstract

Continuous-wave operation at room-temperature has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on low-dislocation-density n-GaN substrates with a backside n-contact. The current, current density and voltage at the lasing threshold were 144 mA, 10.9 kA/cm2 and 10.5 V, respectively, for a 3 µm wide ridge-geometry diode with high-reflection dielectric coated mirrors. Single-transverse-mode emission was observed in the far-field pattern of the LDs and the beam full width at half power in the parallel and perpendicular directions was 6° and 25°, respectively.

The origin of the internal loss in ridge‐type laser diodes (LDs) fabricated using selective re‐growth is investigated through a systematic device characterization and additional optical measurements. We found that the internal loss of this LD is mainly caused by the absorptive layers at the re‐growth boundary and Mg‐doped GaN layer. The internal loss can be significantly reduced through a re‐design of the LD structure to avoid these absorptive regions by shifting the perpendicular optical field to the n‐cladding side. The re‐designed LDs had a very low threshold current of 10 mA and superior gain characteristics. These results indicate, that the InGaN‐quantum‐well (QW) active layer has a large differential gain and fewer non‐radiative defects. The fabrication method of this LD, i.e. epitaxial growth on low‐dislocation‐density GaN substrates combined with a process without dry‐etching, is responsible for the high quality of the QWs.

The uniaxial stress effects on valence band structures in GaN are investigated by reflectance spectroscopy. It is observed that the energy separation between A and B valence bands increases with the application of uniaxial stress in the c plane. The experimental results are analyzed on the basis of the k⋅p theory, and deformation potential D5 is determined as −3.3 eV. It is indicated that the uniaxial strain effect could be utilized for improving GaN-based laser performance.

Nonresonant carrier tunneling is investigated by time-resolved and time-averaged optical methods for a series of samples with various barrier thicknesses. The electron tunneling times decrease exponentially with the decrease of barrier thickness from 8 to 3 nm, and the trend is well described by a semiclassical model. Additional efficient hole tunneling is observed in the 3 nm barrier sample, and the time constant is of the order of 50 ps.