M. Behringer

Device Properties of Homo- and Heteroepitaxial ZnSe-Based Laser Diodes

The characteristics of ZnSe-based laser diodes grown on GaAs and ZnSe substrates are discussed. There is no significant difference observed in the dynamic behavior and in the operating voltages between the two cases. The degradation mechanism is similar with the developing of dark line defects and a 1/t-like decrease in light intensity at constant current for t →∞. The width of the dark line defects is in homoepitaxy almost constant in time, although their number is higher. This difference is also reflected in the lifetimes of our devices during lasing, which is in heteroepitaxy three minutes and about one second in homoepitaxy, for both in cw operation at room temperature.

The growth of HgSe by molecular beam epitaxy for ohmic contacts to p-ZnSe

Abstract The structural properties of HgSe grown by molecular beam epitaxy (MBE) are investigated for different lattice mismatches to the substrate and various growth conditions. The growth rate is shown to depend strongly on the growth temperature above 100°C as well as on the Hg/Se flux ratio. It has been found that the crystalline perfection and the electrical properties are mainly determined by the layer thickness, especially for the growth on highly lattice mismatched substrates. Changes in the surface morphology are related to growth parameters. Differences between the electrical behavior of MBE-grown and bulk HgSe are discussed. The electrical properties of HgSe contacts on p-ZnSe are investigated as a function of different annealing procedures.

Quantum confined Stark effect of II-VI heterostructures suitable as modulators in the blue–green spectral region

We present a systematic and quantitative investigation of the quantum confined Stark effect in (Zn, Cd)Se/Zn(S,Se) quantum well structures at room temperature. For this purpose, differential transmission spectroscopy is performed on two samples with different thicknesses of the active layers (4×5 nm and 10 nm) and compared to model calculations. We observe a Stark shift of 18 meV in the heavy-hole exciton peak for an electric-field change from 82 to 175 kV/cm for the 4×5 nm sample. In contrast, the 10 nm sample shows a rather weak and more Franz–Keldysh-like signal. Furthermore, we have analyzed the spectral behavior of the linewidth enhancement factor αH. Values between 5.1 and 0.2 are found at the energy of absolute maximum of the absorption change for the 4×5 nm sample.

On the growth and doping of blue-green emitting ZnSe laser diodes

Abstract Structural, optical and electrical properties of green and blue emitting room temperature laser diodes grown by molecular beam epitaxy are presented. The influence of the growth start at the GaAs ZnSe heterointerface is discussed based on cross sectional and plan view transmission electron microscopy studies. GaAs buffer layers are necessary for reproducible layer growth and stable room temperature lasing. Doping problems like the ratio of the net acceptor concentration to the overall nitrogen incorporation for binary, ternary and quaternary layers and laser diodes are discussed as well as the problem of ohmic contacts.

Temperature-dependent measurements on ZnSe heterostructures by high-resolution X-ray diffraction

Thermally induced strain in ZnSe epitaxial layers on GaAs occurs due to the different expansion coefficients of the single materials. This strain is directly investigated by high-resolution X-ray diffraction in the temperature range from 10 to 600 K. The strain is measured for a thick relaxed and a thin pseudomorphic layer. Furthermore, the lattice constants of ZnSe and GaAs bulk materials are determined at variable temperatures. It was observed that the thermal expansion is quite linear for GaAs and ZnSe only in the temperature range from 250 to 600 K. The measured temperature-dependent lattice constants and the derived expansion coefficients are input parameters for a model to calculate the thermally induced strain in the epitaxial layers. Good agreement could be obtained between the experimental data and the model.

Lateral index guiding in ZnCdSe quantum well lasers by selective implantation-induced disordering

Local implantation-induced disordering of ZnCdSe quantum wells is applied to generate lateral index guided II–VI lasers. Lateral selectivity of implanted nitrogen ions is achieved by metal stripes also used for contacts. Secondary ion mass spectroscopy and luminescence prove that quantum well intermixing can be generated without subsequent thermal treatment. Lateral index guiding in II–VI lasers after implantation is demonstrated by recording the near-field pattern, yielding a lateral index step of the order of 10−3. First full index-guided injection lasers with an implantation-induced lateral confinement are processed. Threshold current density is reduced by 40% and emission characteristics of these lasers are strongly improved.

Doping-dependent Mg diffusion in ZnMgSSe/ZnSSe-structures

Diffusion of magnesium in ZnMgSSe/ZnSSe-structures is investigated. The superlattices structures is partly implanted and annealed. Annealing atmosphere, time and temperature are varied. Secondary-ion mass spectroscopy investigations demonstrate that magnesium diffuses via group II lattice sites. Diffusion enhancement results for p-type doping as well as additional group II vacancy generation. Diffusion coefficients for the different annealing and doping conditions are determined.

Optical gain characteristics and excitonic nonlinearities in II–VI laser diodes

Gain characteristics and lasing of (Zn, Cd)Se/Zn(S, Se)/(Zn, Mg)(S, Se) separate confinement heterostructures are investigated by optical-gain spectroscopy and by high-excitation spectroscopy under quasi-stationary conditions. The optical gain is measured by means of the stripe-length method whereas excitonic bleaching under lasing conditions is analyzed through two-beam photoluminescence excitation (PLE) spectroscopy. At low lattice temperature, we find a rather low threshold density of 15–20 kW/cm2 for both laser structures and the excitonic enhancement is still preserved at the onset of lasing. The red shift of the gain maximum with respect to the low-density PLE exciton peak increases with temperature. The results indicate a considerable influence of Coulomb correlations even at the high densities necessary for stimulated emission and are more adequately explained by a strongly correlated electron-hole plasma model that goes beyond the simple “excitonic lasing” concept.

Electroabsorption in low-dimensional and bulk-like Zn1−xCdxSe

We investigated and theoretically interpreted the electroabsorptive properties of Zn x Cd 1-x Se quantum wells with a thickness of 5 and 20 nm representing the limits of the quantum-confined Stark-effect and the Franz-Keldysh effect, respectively. We demonstrate an electroabsorptive waveguide modulator with a contrast ratio of 100 : 1 and electro-optical bistability in a quantum-well laser diode.

Structural properties of homoepitaxial and heteroepitaxial ZnSe-based laser structures

High-resolution X-ray diffraction has been used for the structural characterization of hetero- and homoepitaxial ZnSe-based laser structures. A common problem of both kinds of conventional laser structures containing sulphur is the formation of composition gradients in the thick quaternary layers. In addition to depth gradients within the single cladding layers, a composition shift between the n-type and the p-type side of the laser structures has been observed. Clear differences between homoepitaxial and heteroepitaxial samples were found with respect to the strain relaxation.