Yasuhide Kuroda

Extremely‐low‐threshold and high‐temperature operation in a photopumped ZnSe/ZnSSe blue laser

A ZnSe/ZnSSe blue semiconductor laser was demonstrated to show very low threshold by direct photopumping of the ZnSe active layer. The lowest threshold at the excitation wavelength of 445 nm was 10 kW/cm2 at 300 K which is equivalent to the current density of 3.6 kA/cm2. This is the lowest threshold ever reported in II‐VI photopumped lasers and is approaching the theoretically calculated threshold. The characteristic temperature of the threshold, which characterizes the temperature dependence of the threshold in the exponential form, was 124 K up to the measured 400 K and was comparable to III‐V double‐heterostructure lasers. The differential quantum efficiency remained the similar level up to 400 K and its decrease observed at 400 K was within 23% of the efficiency at 300 K.

Blue‐light stimulated emission from a localized state formed by well‐barrier fluctuation in a II‐VI semiconductor superlattice

A well‐defined exciton absorption peak from a localized state was observed for the first time. The localized state was formed by fluctuation of well‐barrier interfaces in a ZnSe/ZnSSe superlattice structure. Stimulated emission was observed from the localized state up to 100 K, and the physical origin is discussed from the temperature dependence of the stimulated emission peak. This opens the possibility to form a natural quantum box structure by controlling the well‐barrier fluctuation.

High Output Power (>20 W) and High Quantum Efficiency in a Photopumped ZnSe/ZnSSe Blue Laser Operating at Room Temperature

High output power of 24 W was observed from a photopumped ZnSe/ZnSSe blue laser operating at room temperature. A differential quantum efficiency greater than 25% was estimated in the lasing region of the photopumped wafer. The improved lasing property is attributed mainly to the improved waveguiding structure verified from the far-field measurement. The other is the introduction of a ZnSe/ZnSSe superlattice buffer which improved the crystalline property of the active layer grown above. The lowest threshold power was 50~70 kW/cm2 for a cavity length of 220 µm under pulsed operation.

Optical and structural characterizations of ZnSe/ZnSSe superlattices grown by metalorganic chemical vapor deposition

ZnSe/ZnSSe superlattices (SLs) grown on GaAs substrates were studied with transmission electron microscopy and x‐ray diffraction. Defects were observed in the initial growth stage, but they were recovered with the growth of short‐period SL. The temperature dependent decrease of photoluminescence (PL) intensities improved in the short‐period SL. Clear excitonic absorption peaks were observed in photocurrent spectra and the blue shift of the absorption peaks by the quantum confinement was in reasonable agreement with the calculated peak shift. In a SL with the well width of 93 A, a biexciton PL peak was observed from the very low excitation power of less than 1 mW/cm2. Details of the biexciton properties are discussed.

Near-Room-Temperature Photopumped Blue Lasers in ZnSxSe1-x/ZnSe Multilayer Structures

Photopumped blue laser oscillation was achieved up to near room temperature (280 K) in a ZnS0.18Se0.82/ZnSe multilayer structure grown by MOVPE. This significant improvement in the lasing temperature is attributed to the higher heterobarrier which prevents thermoionic emission and to the multilayer structure which has an average lattice constant matched to the GaAs substrate at the growth temperature of 515°C. The threshold excitation optical power density below 200 K where the thermoionic emission is negligible is less than 50 kW/cm2, which corresponds to a current density of 14 kA/cm2.

Photopumped ZnSe/ZnSSe blue semiconductor lasers and a theoretical calculation of the optical gain

A ZnSe/ZnSSe blue semiconductor laser was demonstrated to show a very low threshold of about 10 kW/cm2 with photopumping at 300 K, which is equivalent to a current density of 3.8 kA/cm2. The optical gain in ZnSe blue semiconductor lasers was calculated theoretically for various band structures, such as a bulk band structure and a quantum well (QW) structure. In a common cation system such as the ZnSe/ZnSSe heterostructure, the band offset is localized in the valence band and the conduction band is nearly flat. Therefore, the conduction band is bulk-like and the valence band is QW-like. This kind of mixed band structure was also treated. The measured lasing threshold is compared with the calculations and the influence of the carrier confinement on the lasing threshold is discussed.

Room‐temperature stimulated emission in optically pumped narrow ZnSe/ZnSxSe1−x multiple‐quantum‐well structures

Room‐temperature stimulated emission in ZnSe/ZnS0.18Se0.82 multiple‐quantum‐well structure is reported. An optical excitation threshold around 300 kW/cm2 at 300 K and 8.4 kW/cm2 at 10 K was measured. An unsaturated optical gain coefficient of the order of 5×102 cm−1 has been measured at low temperature.

Doping of nitrogen in ZnSe films: improved doping properties in ZnSe/ZnSSe periodic layered structures grown on GaAs by MOVPE

Abstract Nitrogen (N) doping behaviour in ZnSe films was found to be significantly improved in a ZnSe/ZnSSe periodic layered structures. The photoluminescence spectra were dominated by the neutral acceptor bound exciton (I 1 ) line which indicates that the N was incorporated into the ZnSe film as a shallow acceptor. The donor-acceptor pair emission which has usually dominated the observed spectra was well suppressed and remained at a much lower level. It was also found that the I 1 intensity was sensitively dependent on the S mole fraction of the undoped ZnSSe layers in the periodic structure and that the maximum intensity was observed at a S mole fraction of 18% where the equivalent ZnSSe alloy with the S mole fraction averaged in the layered structure is lattice matched to the GaAs substrate.

Lasing Properties and Lasing Mechanism in a ZnSe/ZnSSe Multiple Quantum Well Heterostructure

Lasing properties in a ZnSe/ZnSSe multiple quantum well heterostructure were studied quantitatively at room temperature with photopumping. The internal optical absorption loss was measured in II-VI heterostructure for the first time and it remained at the low value of 9 cm-1. The internal quantum efficiency of about 40% and the differential quantum efficiency of 28% were estimated. The lasing peaks were red shifted by about 50 meV in comparison to photoluminescence peaks measured with low excitation, and the energy separation was almost independent of the temperature. The lasing mechanism was studied by measuring the temperature and excitation-level dependencies of the emission spectra.

Radiative recombination processes in ZnSe/ZnSexSe1−x multiple-quantum-well structures

Abstract Radiative recombination processes in highly excited ZnSe/ZnS 0.18 Se 0.82 multiple-quantum-well structures have been investigated as a function of the well width and of the lattice strain. Excitonic scattering processes dominate the photoluminescence spectra. An optical amplification process is observed, up to room temperature, due to inelastic exciton-exciton scattering. An optical excitation threshold around 300 kW/cm 2 at 300 K and 8.4 kW/cm 2 at 10 K was measured for the stimulated emission.