J. H. C. Hogg

Optically pumped stimulated emission in ZnS/ZnCdS multiple quantum-wells, MBE-grown on GaP

Abstract Optically pumped stimulated emission is observed in a series of ZnS Zn 1−x Cd x S multiple quantum-well (MQW) structures, grown on GaP substrates by MBE. We report lasing from a ZnS Zn 0.97 Cd 0.03 S MQW at wavelengths as low as 333nm, the shortest yet reported in a semiconductor heterostructure. The lasing threshold decreases for deeper wells and reaches 6.5 kW.cm−2 at 8K and 80 kW.cm−2 at 300K for the MWQs with a Cd-composition of 0.2. The results are compared to known values for similar structures grown on GaAs by MOCVD, which reveals the potential of the GaP-substrate MBE technology for the widest bandgap II–VI heterostructures incorporating ZnS-based ternary alloys.

Chemical diffusion of Hg in CdTe

Abstract Proton-induced X-ray emission has been used to characterize Hg interdiffusion from the vapour phase into CdTe between 360°C and 550°C. The measured interdiffusivities are described by D=6.6 exp (−1.91 eV /kT) cm 2 s −1 for Hg concentrations of 3% or less. The results differ significantly from earlier measurements based on electron microprobe and Rutherford backscattering techniques but are in good agreement with very recent radiotracer experiments. Possible causes for the differences with the earlier measurements are discussed.

Dependence of the critical thickness on Si doping of InGaAs on GaAs

The formation of misfit dislocations during the initial stages of relaxation of In0.04Ga0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution double crystal x-ray topography during molecular beam epitaxy growth. Relaxation is initially anisotropic with the fast B(g) dislocations being nucleated before the slow A(g) set. On doping with Si up to a maximum concentration of 4×1018 atoms/cm3, an increase in critical thickness was observed for both dislocation sets. The data can be fitted to an extension of the Matthews–Blakeslee model that includes a lattice friction force varying linearly with the dopant concentration.

High resolution x-ray diffraction and scattering measurement of the interfacial structure of ZnTe/GaSb epilayers

The surface and interface structures of ZnTe epilayers grown by molecular beam epitaxy on GaSb (001) substrates under different conditions have been investigated by high resolution x-ray diffraction and grazing incidence scattering. Reciprocal space mapping around the symmetrical diffraction reciprocal point 004 and asymmetrical diffraction point 115 showed that the ZnTe epilayers, in the samples investigated, were fully strained to the substrate. The crystalline quality of the ZnTe epilayer grown on a substrate annealed in a Zn flux was very good, while evidence for an interfacial layer, of thickness varying from 2–20 nm, was found when the substrate was annealed in a Te flux prior to growth. This is attributed to Ga2Te3 formation at the interface. The interfacial layer roughens the interface and surface, and both crystal truncation rod measurements and grazing incidence x-ray reflectivity show the surface roughness to be about 4 nm. Such a rough surface and interface is also inferred from the broader ...

SPECTROSCOPIC EVIDENCE FOR DIFFERENT LASER GAIN MECHANISMS IN OPTICALLY PUMPED ZNCDS/ZNS QUANTUM WELL STRUCTURES

The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.

Pulsed laser annealing of CdTe/CdMnTe superlattices

Abstract A CdTe/CdMnTe multiple quantum well with L z = 75 A, L b = 150 A and 18 periods has been used to investigate, with photoluminescence and X-ray diffraction, thermal processes that occur following pulsed laser annealing with a XeCl laser. With increasing power, increasing diffusion of Mn ion from the barrier layers into the wells occurs. A threshold of 80 mJ cm −2 is found which is assigned to the point at which the surface melts. Thereafter, the melt depth increases and evidence for significant diffusion is found in the wells immediately beneath the melt front. Eventually, complete homogenization of the MQW stack occurs. At 180 mJ cm −2 evidence is found for the melt front having extended throughout the stack, corresponding to a melt depth of ≈ 0.4 μm. These result are in accord with a model which assumes the laser pulse is absorbed entirely at the surface and that the temperature rise beneath the surface occurs by the diffusivity of the heating pulse. In addition there is some indication that segregation of Mn ions has occurred at the surface following recrystallization.

The refractive indices of alloys

We have studied alloy layers with low Cd concentrations of x = 0.017, 0.05, 0.09 and 0.17, grown by molecular beam epitaxy on GaP substrates, by means of reflectivity and ellipsometry measurements at room temperature. The refractive indices in the transparent region, from about 350 nm to 800 nm, are presented with a three-parameter dispersion relationship with the wavelength. The results should provide essential information in the design of /ZnS waveguide structures.

Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures

Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D.

Near band-edge emission in strained MBE-grown ZnS

Abstract We report, for the first time, on the strain related features of the near band-edge luminescence of ZnS epilayers and find emissions that can be related to both heavy-hole (hh) and light-hole (lh) character. Layers with different relative strain show well resolved peaks due to the recombination of excitons at neutral donors (D 0 ,X) and acceptors (A 0 ,X), as well as from free-to-bound transitions due to donors (D 0 ,h) and acceptors (e,A 0 ). The deepest emission (e,A 0 ) follows the calculated shift of the hh band-edge, although manifestation of hh character is unexpected under the conditions of tensile biaxial strain. For the shallower excitonic emissions, the strain-induced lh shift is comparable to their smaller binding energy even for intermediate relative strain. Consequently bound lh exciton (BX lh ) transitions dominate in the higher strained layers. Simple considerations based upon the hydrogenic description of excitons confirm these observations.

In-situ direct measurement of activation energies for the generation of misfit dislocations in the InGaAs/GaAs(001) system

Abstract In-situ X-ray topography (XRT) studies of misfit dislocation generation and movement in epitaxial InGaAs strained-layer structures on (001) GaAs are described. Examination of the changes in dislocation structure during a series of successive post-growth in-vacuo sample anneals has, for the first time, yielded activation energies of 0.7 and 0.8 eV for the formation of α-and β-misfit dislocations (MDs) by the initial glide of substrate threading dislocations (TDs) in the InGaAs epilayer. The introduction of MDs by this method is supplemented by the presence of an additional MD generation process. The activation energy for this is found to be comparable to that required to initiate the glide of a TD. The XRT studies have also confirmed the existence of MD cross-slip events, where α to β cross-slip was found to have an activation energy of 1.2 eV and to be much more common than the reverse β-α cross-slip process.