Khalid Shahzad

Correlation between radiative transitions and structural defects in zinc selenide epitaxial layers

We present low‐temperature photoluminescence and transmission electron microscopy data to show that two transitions I0V at ∼2.774 eV and Y0 at ∼2.60 eV, frequently observed in unintentionally doped zinc selenide epitaxial layers, are directly related to structural defects. It is shown that these transitions are strong in those samples which have very low background impurities and high density of structural defects and weak in those cases that have either high background impurities or low density of structural defects.

Novel technique for p‐type nitrogen doped ZnSe epitaxial layers

We report a novel technique to obtain p‐type ZnSe layers doped with nitrogen. The layers were grown in a low‐pressure metalorganic vapor phase epitaxy system using ammonia as the dopant source. A rapid thermal anneal was used to enhance the activation of the nitrogen acceptors. Net acceptor concentration values as high as 3×1016/cm3 were obtained from capacitance‐voltage measurements and the profile was uniform over the thickness of the epitaxial layers. The 7 K photoluminescence spectrum was dominated by the acceptor bound exciton peak; the donor‐acceptor pair spectra were also observed.

Stimulated emission via inelastic exciton‐exciton scattering in ZnSe epilayers

Stimulated emission at 6 K by optical excitation has been investigated in ZnSe epilayers. A detailed examination of the spectra below threshold indicates that the stimulated emission is due to inelastic exciton‐exciton scattering, in contrast with some recent reports that lasing is due to electron‐hole plasma luminescence modified by self‐absorption. A red shift of the lasing line at high pump intensities has also been modeled in terms of the exciton‐exciton and band filling mechanisms, giving excellent agreement with the data.

Exciton line broadening in ZnSxSe1−x epilayers grown on GaAs by molecular‐beam epitaxy

ZnSxSe1−x epilayers grown on GaAs substrates by molecular‐beam epitaxy were examined for exciton line broadening in photoluminescence as functions of x(0.0≤x≤0.49) and temperature (6–300 K). The variation of linewidth with x is partly explained by alloy broadening effects and partly attributed to the density of defects near the epilayer surface, which in turn is a function of x and layer thickness. The increase in linewidth with temperature is modeled up to 200 K by taking into account scattering due to acoustic and optical phonons. The band gap is observed to be a quadratic function of x at both low (6 K) and high (300 K) temperatures.

Effects of strain on the optical and vibrational properties of ZnSe-ZnSxSe1-x strained-layer superlattices

In this review article, we describe detailed optical properties of ZnSe-ZnSxSe1-x strained-layer superlattices (SLS) grown on GaAs substrates by molecular beam epitaxy. Photoluminescence and excitation measurements were carried out to study effects of the strain and carrier confinement in this strained layer system. For the case where the total thickness of SLS is very small (∼1000A) compared to its critical thickness, the structure grows pseudomorphic with the ZnSe buffer layer. In this case the ZnSe well layers are not strained and all the blue shift in the optical spectra is attributed to the carrier confinement effects only. At the other extreme, for the case of a SLS with very large total thickness (∼4 μm), we show that it can be treated as free standing with the ZnSe layers under in-plane biaxial compression and the ZnSxSe1-x layers under biaxial tension. In the intermediate cases of total thicknesses, we show that SLSs are not fully relaxed to their equilibrium states by measuring the strains directly in the well layers. We also present photoluminescence data to show that SLSs grown on ZnSe buffer layers can exist in a continuous range from a perfectly coherent state to a totally free standing state as the total thickness of the SLS increases. In addition, we also observe the effect of strain produced by the SLS on the buffer layers. We also discuss in detail the depth dependence of the strains in a given SLS or a heteroepilayer by using Raman scattering measurements performed under laser excitation below and above the band gap. We find that the strain values near the top surface are driven by a stronger relaxation of the in-plane lattice constants towards equilibrium. Empirical values for the band offsets are obtained from the analysis of optical response as a function of the sample parameters. Theoretical calculations of the band offsets, based on the model-solid approach, were also performed and are found to agree with the experimental observations to within 0.05 eV. They indicate that all possible ZnSe-ZnSxSe1-x interfaces will exhibit a very small conduction band offset. We also carried out resonant Raman-scattering experiments at low temperatures to study the optical modes of these SLS systems. The observed Raman modes can be classified into two groups: one corresponds to vibrations with amplitudes localized either in the ZnSe or in the ZnSxSe1-x layers. These are confined modes. In the second group, the phonons with amplitudes in both layers are included, namely, interface vibrations and folded optical modes. The measured dependence of the confined and delocalized phonons on sample parameters (individual layer thicknesses, superlattice period, total superlattice thickness, and interfacial strain) and on resonant excitation is presented and discussed in detail. The trends that were established are compared with the theoretical predictions of lattice dynamic models for multilayer structures. Experimental data on the lattice dynamic properties of ZnSxSe1-x alloys are included for comparison purposes and to distinguish unequivocally superlattice effects. These data were gathered with ternary layers grown and measured under similar conditions as the superlattices. Concomitant with vibrational characteristics, this work has also yielded an insight into other intrinsic properties of SLSs. The interplay between confinement and strain in the renormalization of phonon frequencies was elucidated from an experimental point of view and this information was applied to characterize the nature of superlattice transition from pseudomorphic to free standing.

Observation of excitonic features in ZnSeZnMgSSe multiple quantum wells by normalized Kelvin probe spectroscopy at low temperatures

The surface photovoltage spectra (SPS) of a series of ZnSeZnMgSSe multiple quantum wells of different well thickness (15–120 A) have been obtained as a function of temperature (48K < T < 300 K) using a contactless Kelvin probe. In contrast to previous SPS measurements our data were taken using normalized incident light intensity. This normalization procedure has made it possible to fit the numerical derivative of the spectra (with respect to photon energy) to an appropriate lineshape factor to accurately determine the energies of the excitonic transitions. From the splitting between the fundamental conduction to heavy (11H)- and light (11L)-hole transitions as a function of well width we have confirmed the conduction band offset deduced by K. Shahzad et al. [Appl. Phys. Lett., 67, 659 (1995)]. We also have observed an interesting temperature dependence of the intensities of the 11H and 11L SPS features for different well widths.

Low‐temperature growth of ZnSe by molecular beam epitaxy using cracked selenium

The growth of zinc selenide by molecular beam epitaxy using a cracked selenium source is studied. It is found that high quality growth can be achieved at substantially lower substrate temperatures than has been possible using uncracked selenium sources. It is determined from reflection high‐energy electron diffraction observations that the use of cracked selenium produces growth dominated by a two‐dimensional mechanism at substrate temperatures as low as 225 °C and that exposure of the GaAs substrate to cracked selenium prior to the initiation of growth has a substantial effect on the GaAs substrate and the early stages of ZnSe growth.

Room‐temperature photopumped blue lasing in ZnSe‐ZnS0.06Se0.94 double heterostructures

We present results on room‐temperature photopumped lasing action in the blue region (∼4650 A) of the spectrum for double heterostructures with ZnSe active layers. The confining layers are ZnS0.06Se0.94, providing lattice match to the GaAs substrate. The room‐temperature lasing, observed under pulsed excitation, yields threshold pump intensity as low as 58 kW cm−2 for an active layer thickness of 1500 A.

Strained‐layer interfaces between II–VI compound semiconductors

II–VI multilayer structures are eminently suitable for various optoelectronic devices covering from the infrared to the ultraviolet spectral range. We present a theoretical study of the band lineups at interfaces involving ZnS, ZnSe, ZnTe, CdTe, and HgTe based on the ‘‘model solid’’ theory. We also experimentally investigate the strains and band offsets in ZnSe/ZnSx Se1−x strained‐layer superlattices, using Raman and photoluminescence spectroscopy. The conduction‐band offset at such interfaces is found to be small. Theoretical predictions for other interfaces include a large type‐II offset in ZnSe/ZnTe, and a small average valence‐band discontinuity in ZnTe/CdTe.

Distortion of excitonic emission bands due to self‐absorption in ZnSe epilayers

In this letter we investigate the influence of self‐absorption on the excitonic emission spectra of ZnSe epitaxial layers grown by molecular beam epitaxy on GaAs substrates. We observe that, very often, samples grown under identical conditions show a widely ranging ratio of neutral donor‐bound‐exciton to free‐exciton intensities in photoluminescence (PL). We present experimental evidence to show that these differences can arise from the variations in the epilayer thickness and detailed PL experimental conditions and do not necessarily represent the layer‐to‐layer purity fluctuations, as the spectral line shapes may suggest.