A. L. Gurskii

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.

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

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.

Near-band-edge photoluminescence of MOVPE-grown undoped and nitrogen-doped ZnSe

Abstract Optical properties of undoped and nitrogen-doped ZnSe GaAs epilayers grown by MOVPE were investigated by using CW HeCd laser excitation or by pulsed N2-laser radiation in the temperature range between 10 and 300 K and at excitation intensities in the range of Iexc = 10−1−106 W/cm2. The presence of deep donor states with an ionization energy of ED ≈ 60 meV and acceptor states with EA ≈ 80 meV as well as the existence of trap levels with a concentration comparable to the amount of incorporated nitrogen were detected. The concentration of the electron-hole plasma (EHP) is saturated at Iexc > 200 kW/cm2 in undoped and moderately doped layers due to plasma expansion through the layer into the substrate. High doping levels as well as the presence of a barrier layer between ZnSe and the substrate leads to a further increase of the EHP concentration. The overheat of the near-surface layer which is much higher compared to the average temperature of the irradiated area leads to a high temperature gradient (ΔT ∼ 150 K) in the 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.

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.

The role of impurity bands and electron}phonon interaction in formation of near-band-edge PL spectra of compensated ZnSe

Abstract A modification of the potential fluctuation model is proposed to explain the experimentally observed dependencies of the band shape and maximum position of the impurity luminescence in ZnSe samples highly doped with nitrogen on the doping level, excitation intensity, temperature, and decay time. In the proposed model, transitions from the localized and delocalized states of the donor impurity band and the conduction band tail appearing at high-doping level due to overlap of the wave functions of donor states, to the acceptor levels are considered instead of the donor–acceptor pair recombination. The influence of the electron–phonon interaction parameter on the spectral shape was taken into account.

Nature of the impurity bands of the edge luminescence of highly doped compensated ZnSe:N

A modification of the model of potential fluctuations is suggested to explain the experimentally observed dependences of the form and position of the maxima in the impurity luminescence spectra of ZnSe specimens, highly doped with an acceptor nitrogen impurity, on the degree of doping, excitation level, temperature, and decay time. In this model, analysis of the recombination of donor-acceptor pairs is replaced by consideration of transitions from localized and delocalized states of the donor impurity band and density tail of the conduction-band states, which appear in strong doping because of the overlapping of wave functions of the donor states, to the acceptor levels. The influence of the parameter of the electron-phonon interaction on the spectrum shape is considered. The model allows one to offer a consistent explanation at the qualitative level for the features observed experimentally.