I.P. Marko

Blue InGaN/GaN multiple-quantum-well optically pumped lasers with emission wavelength in the spectral range of 450–470 nm

Optically pumped lasing in the wavelength range of 450–470 nm in InGaN/GaN multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy was achieved and investigated. The energy and power per pulse of the laser were 80 nJ and 10 W correspondingly for one facet at room temperature. The far-field patterns of the laser emission consisted of three light spots near the angles of +30°, −15°, and −45°. The highest operating temperature was 450 K. The photoluminescence and photoluminescence excitation spectrum structures suggest that the quantum dots inside the quantum wells are involved in the recombination mechanism.

Multiple quantum well InGaN/GaN blue optically pumped lasers operating in the spectral range of 450-470 nm

Lasing under optical pumping by N 2 -laser radiation in InGaN/GaN multiple quantum well heterostructures grown in AIXTRON MOVPE reactors was achieved and investigated in the wavelength range of 450-470 nm. The laser operation wavelength depends most strongly on V/III ratio during quantum well barrier growth. The total energy and power per pulse of the laser were 300 nJ and 40 W, respectively, with differential quantum efficiency of 3% at room temperature. The laser threshold increases exponentially with increasing operation wavelength which is mainly due to the decreasing efficiency of the spontaneous emission and due to an increase of its spectral width.

Influence of UV Light-Assisted Annealing on Optical Properties of InGaN/GaN Heterostructures Grown by MOVPE

The influence of ultraviolet light-assisted annealing on the optical properties of InGaN/GaN single quantum wells (SQW), and thick single and double heterostructures (SH, DH) was investigated. It was shown that annealing promotes an increase of the photoluminescence (PL) intensity from the active layers of the SQW and double heterostructures by as much as one order of magnitude. This is effected mainly by a diminishing of the defect concentration and a smoothing of the potential fluctuations in the upper cladding layer. The operating temperature of the DH lasers was lifted from 220 up to 300 K after annealing. Using nitrogen laser irradiation during annealing led both to a PL efficiency increase and to a phase separation in the SH with a thick active layer.

HIGH-TEMPERATURE OPTICALLY PUMPED LASING IN ZNMGSSE/ZNSE HETEROSTRUCTURES GROWN BY METALORGANIC VAPOR PHASE EPITAXY

Lasing and optical properties of ZnMgSSe/ZnSe-, ZnMgSSe/ZnSSe/ZnSe-, and ZnMgSSe/ZnMgSSe/ZnSe-based single- and multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy were studied, and the characteristics were found to depend on the excitation intensity Iexc, temperature, and well width. Laser action under transverse optical pumping was achieved only for well widths Lz⩾4 nm and optical confinement factors Γ>0.04. In separate confinement heterostructures, lasing with the lowest threshold (Ithr=10–30 kW/cm2 at T=78 K) was achieved and device characteristics were studied up to T=577 K.

Thermal stability of ZnMgSSe/ZnSe laser heterostructures

Optically pumped lasing and photoluminescence of ZnMgSSe/ZnSe quantum wells grown by metal-organic vapour phase epitaxy were studied in the temperature interval of 13-650 K. Thermal annealing at high temperatures deteriorated the lasing properties which, however, could be almost completely recovered by subsequent optical excitation above the lasing threshold. The technique of spectrum imaging in a transmission electron microscope with an energy filter and a two-dimensional detector was applied to ZnMgSSe/ZnSe multiple quantum well laser structures. It was found that the main degradation mechanism of ZnMgSSe/ZnSe quantum well heterostructures at temperatures higher than 450 K is the diffusion of S atoms from the barriers into the quantum wells which leads to increasing point defect concentrations in the active layers of the lasers. It was established that stimulated lasing decreases the defect concentration and reduces the laser threshold in the optically pumped ZnMgSSe/ZnSe quantum well lasers after their thermal degradation at 450-650 K.

High‐Temperature Lasing in InGaN/GaN Multiquantum Well Heterostructures

Lasing, stimulated emission and photoluminescence in InGaN/GaN multiquantum well (MQW) heterostructures were investigated in a wide temperature interval from the liquid nitrogen temperature up to 600 K. Laser action was achieved up to T = 585 K in a heterostructure consisting of ten quantum wells with a width of 10 nm. Laser thresholds at T = 78, 300, and 585 K were 25, 100, and 550 kW/cm2, respectively. The pulse energy of the InGaN/GaN MQW laser at room temperature was 600 nJ and the average power was 0.6 mW. The characteristic temperature value of T0 = 164 K was derived from the temperature dependence of the lasing threshold.

Optically‐Pumped Lasing of Doped ZnSe Epitaxial Layers Grown by Metal‐Organic Vapour‐Phase Epitaxy

Laser action in undoped, nitrogen and chlorine doped ZnSe epitaxial layer has been achieved and investigated at pulse optical excitation by N2 laser radiation with a frequency of 1000 Hz from liquid nitrogen temperature up to near room temperature at 270 K. The highest value of ZnSe laser energy and power were E = 5 x 10 -8 J and P = 5 W at I exc = 800 kW/cm 2 . The laser line positions in doped samples ZnSe: Cl (λ = 450.7 nm) and ZnSe:N (λ= 451.2, 451.5, 452.0 and 455.5 nm at different excitation intensities) are shifted to the high wavelength side compared to undoped ZnSe (λ= 449.8 nm). It was shown that doping ZnSe with both acceptor or donor impurities as well as using a ZnMgSSe barrier layer between the ZnSe layer and the substrate is favourable to increase PL efficiency and to decrease the laser threshold. The lowest threshold value (130 to 150 kW/cm 2 ) was achieved in ZnSe:N grown with hydrogen carrier gas. The results obtained from measurements of the laser line positions as well as from the evaluation of the nonequilibrium carrier concentration proves that the ZnSe lasing mechanism under excitation by the N2 laser radiation is the recombination in an EHP. We found that under high excitation intensity a degradation of the excited region of the ZnSe takes place causing a decrease of the emission intensity. Under lower excitation power the PL intensity enhances during the course of irradiation.

Time‐ and Temperature‐Resolved Photoluminescence of GaN:Mg Epitaxial Layers Grown by MOVPE

Time-integrated and time-resolved photoluminescence (PL) spectra as well as the luminescence transients of moderately doped GaN:Mg samples grown by MOVPE were studied between 80 K and 380 K at pulse excitation by a nitrogen laser beam in order to clarify the mechanism of the large blue shift of the 2.8 eV PL band above room temperature. Based on the performed study, the new band at 3.05 eV dominating in PL spectra above room temperature is attributed to the donor-to-valence band recombination. The corresponding donor ionization energy is about 290 meV. The blue shift of the spectra is therefore explained as a result of ionization of shallow acceptor states involved together with deep donors in donor-acceptor recombination forming the 2.8 eV band below room temperature.

Luminescence and lasing in GaN epitaxial layers and InGaN/GaN quantum well heterostructures under optical and electron-beam excitation

Abstract Interrelation between stimulated and excitonic emission intensity of GaN epitaxial layers and yellow luminescence intensity as well as correlation between photoluminescence and laser properties of InGaN based multiple quantum well heterostructures was investigated. It was found among all studied undoped GaN epitaxial layers that the higher intensity of the yellow luminescence and so the higher concentration of the yellow luminescence related centres the higher is the excitonic, electron–hole plasma and stimulated emission intensity. It was shown that a small Stokes shift and a high ratio of the luminescence intensity from InGaN quantum well layers to the photoluminescence intensity from GaN barrier layers indicate high laser quality of the multiple quantum well heterostructures. The lowest full width at half maximum of the laser line was 0.04 nm, the highest operating temperature was 585 K, the lowest threshold was 100 kW cm −2 , the highest characteristic temperature was 164 K and the highest wavelength was 442.5 nm. The far-field patterns of the laser emission from the MQW lasers consist of two approximately symmetrical high brightness spots localized at angles α =±30–35°.

Optically pumped transverse lasers based on ZnMgSSe/ZnSe and InGaN/GaN heterostructures

ZnSe/ZnMgSSe and InGaN/GaN heterostructure based lasers under optical transverse pumping by pulsed N2-laser radiation were investigated in a wide spectral, temperature and excitation intensity range for various types of heterostructures which differed in the epitaxial layer composition, layer sequence and thickness. The spectral- angular distribution of the laser emission of the ZnSe/ZnMgSSe separate confinement heterostructures and the influence of the excitaion intensity and the cavity length on the laser mode structure were investigated. It was found that the main degradation mechanism of the ZnSe/ZnMgSSe multiple quantum well heterostructures at temperatures higher than 400 K is the diffusion of S atoms from the barriers into the quantum wells which leads to increasing point defect concentration in the active layers of the laser. The recovery of the laser threshold of the ZnSe/ZnMgSSe MQW-SCHs degraded during thermal annealing after the action of the inherent laser radiation is attributed to a significant decrease of the point defects in the active layers. Laser action of InGaN/GaN multiple quantum well heterostructures in the blue spectral region has been obtained for the first time and investigated. Temperature tuning of InGaN/GaN MQW laser emission from the violet to blue has been realized. The influence of the photoluminescence characteristics on the laser parameters of the InGaN/GaN MQW optically pumped lasers operating in the blue spectral region is investigated.