D. J. Olego

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.

Effect of N doping on the structural properties of ZnSe epitaxial layers grown by molecular beam epitaxy

The structural properties of ZnSe doped with N, in the concentration range of 1×1018–2×1019 cm−3, were characterized by transmission electron microscopy, x‐ray diffraction, and Raman spectroscopy techniques. The relaxation of the lattice mismatch induced compressive strain between ZnSe and GaAs is less for N doped layers for a given ZnSe thickness. The smaller amount of strain relaxation with N doping results in layers that contain residual compressive strain up to thicknesses of at least 1.7 μm. In addition, the misfit dislocation array becomes a regular rectangular grid when N is incorporated in ZnSe layers. The ZnSe lattice constant, as measured by x‐ray diffraction, decreases as the N concentration increases. The reduction in lattice constant, however, is greater than can be explained by the shorter Zn‐N bond distance of model predictions. We attribute the excess lattice contraction to the generation of point defects accompanying N doping. The Raman spectra display a broadening of the linewidth as the...

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.

Strains in Si‐on‐SiO2 structures formed by oxygen implantation: Raman scattering characterization

Low‐temperature Raman scattering measurements were carried out to characterize Si‐on‐SiO2 structures formed by oxygen implantation and subsequent furnace or lamp annealing. The experiments were conducted with 413.1 nm laser light to probe only the thin Si layers at the top of the structures. The Raman spectra of the furnace‐annealed samples are red shifted and broadened when compared with a virgin Si surface. The shifts and broadenings decrease with increasing annealing temperatures but they are still present in samples annealed above 1250 °C for 3 h. No shifts or broadenings affect the Raman peaks of the layers, which were lamp annealed at 1405 °C for half an hour. The red shifts indicate that the recrystallized Si layers are under tensile strains, whose origin is attributed to oxide precipitates. Quantitative estimates of the strains and associated stresses are obtained from the measured Raman shifts.

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.

ZnSe(100): The surface and the formation of Schottky barriers with Al and Au

A study of the ZnSe(100) surface and of its interfaces with Al and Au is presented. We find that the (2×1) reconstructed Se‐rich surface is terminated with a full monolayer of dimerized Se, whereas the C(2×2) reconstructed Zn‐rich surface corresponds to a half‐monolayer of nondimerized Zn atoms. These atomic configurations and corresponding surface electron affinities are consistent with the requirement of dangling bond saturation and fully accounted for by the electron counting rule. For the metal/ZnSe interfaces, we find that Au forms an abrupt junction, whereas Al reacts and forms Al2Se3, releasing Zn in the process. The stabilized Fermi level position is 2.1 eV above the valence band maximum for Al and 1.15 eV for Au, irrespective of doping type and in qualitative agreement with the Schottky limit for the barrier heights. The Au/p‐ZnSe Schottky barrier height can be reduced by 0.25 eV by introducing a 2–3 ML Se interlayer between the metal and the semiconductor.

Structural properties of nitrogen-doped ZnSe epitaxial layers grown by MBE

We have used transmission electron microscopy (TEM) and high-resolution x-ray diffraction (HRXRD) techniques to investigate the structural properties of ZnSe doped with nitrogen, in the concentration range of 1 × 1018 to 2 × 1019cm−3. The nitrogen-doped layers contain substantial residual compressive strain at layer thicknesses where undoped ZnSe would be completely relaxed. The residual strain is clearly observed both in the inequality of the lattice constants (measured by HRXRD) parallel and perpendicular to the growth direction, and in the reduction of the misfit dislocation density (measured by TEM) relative to undoped ZnSe. In addition to the reduction in dislocation density, the misfit dislocations form a regular rectangular grid, rather than the irregular array seen in undoped ZnSe. The effective relaxed ZnSe lattice constant, as measured by x-ray diffraction, decreases as the nitrogen concentration increases. For the highest nitrogen concentration, this reduction in lattice constant, however, is greater than can be explained by the shorter Zn-N bond distance of theoretical predictions.

Optical and vibrational properties of doped zinc selenide epitaxial layers

Abstract In this review article, we present photoluminescence and/or Raman scattering properties of ZnSe epitaxial layers which were doped with either nitrogen, oxygen, indium, or lithium. In particular, we present photoluminescence data due to recombination at discrete donor-acceptor pairs in ZnSe epitaxial layers implanted with nitrogen with a dose of 1013 cm-2. The experimental pair data is explained with the help of a theoretical computation of a type-II spectrum involving In donors substituting for Zn and N acceptors for Se. We also report two-hole transitions involving up to 6S 3 2 states of the nitrogen acceptor. The experimental values of the energy positions of these excited states are in good agreement with those obtained using an effective-mass calculation. A sharp line superimposed on the broad donor-acceptor pair band, whose peak position has a constant separation from the excitation energy, is also observed. This separation is ≈0.9 meV larger than the 1S-2S energy spacing for the nitrogen acceptor. We show that this line could be either due to resonant inelastic scattering of the exciting photons by the acceptor impurities, or due to selective excitation of the discrete donor-acceptor pairs. We also present photoluminescence and excitation data to show that oxygen substituting isoelectronically in ZnSe gives rise to a pair of transitions A+ (2.7895 eV) and B (2.7877 eV) as a result of the exchange interaction between the trapped electron-hole pair. The former is attributed to total angular momentum J= 1, Г 4 representation and is electric-dipole allowed while the latter is assigned to J=2 belonging to Г3+Г5 representation and is electric-dipole forbidden. Based on this model, we explain several experimental observations including varying PL intensity of the B line from sample to sample, relatively rapid disappearance of the B line as function of increasing temperature and strong longitudinal-optical (LO) couplings of A+ and B lines. Raman spectroscopy was used to investigate the coupling mechanisms between LO-phonons and electronic excitations in n-type ZnSe layers. The layers, grown by molecular-beam epitaxy, were intentionally doped below the Mott criterion for the insulator-metal transition. The nature of the electron-phonon interaction is determined by the degree of electron localization, which was effectively changed by temperature and donor concentration. The LO-phonons couple to plasmons when electrons are thermally excited into the conduction band or at donor sites. In both cases, unbound phonons are observed. From the renormalized phonon frequencies at high temperature, values of free electron concentration as a function of temperature were established. They are in excellent agreement with Hall-effect determinations. At low temperatures, the phonon Raman profiles are asymmetric and show Fano-type line shapes. The electronic continuum responsible for the phonon self-energies at low temperatures was identified as Raman scattering by bound electrons. In addition, Raman spectroscopy was used to establish the presence of a free hole gas and its coupling to the longitudinal optical phonons in Li-doped ZnSe epilayers. The phonon spectra shift to higher frequencies and broaden with increasing acceptor concentration and temperature, as was the case for indium-doped layers, in accordance with the expectation for coupled phonon-plasmon modes. Values for the concentration and mobility of the holes were obtained from an analysis of the spectral lineshapes. They agree with those determined by other methods. A linear relationships was found between the spectral broadening and the hole concentration. In addition, electronic Raman scattering (ERS) from holes bound to Li acceptors was also studied in ZnSe epilayers and correlated with the net acceptor concentration determined by capacitance versus voltage measurements. The ERS spectra reveal several transitions between the ground 1s and the excited states of the Li acceptors as well as transitions to a continuum of delocalized valence band states. Values of excitation energies for the bound hydrogenic states and the ionization energy of the acceptors were measured. The strength of the ERS signal normalized to the phonon scattering depends linearly on net acceptor concentration.

Electronic Raman scattering from acceptors and correlation with transport properties in Li‐doped ZnSe layers

Electronic Raman scattering (ERS) from holes bound to Li acceptors was studied in ZnSe layers and correlated with the net acceptor concentration NA−ND determined by capacitance versus voltage measurements. The layers were grown by molecular beam epitaxy on GaAs substrates and were doped in situ to NA−ND concentrations ranging from high 1015’s to low 1017’s cm−3. The ERS spectra reveal several transitions between the ground 1S and shallower S and P bound states of the Li acceptors as well as transitions to a continuum of delocalized valence‐band states. Values of excitation energies for the bound hydrogenic states and the ionization energy of the acceptors were measured. The strength of the ERS signal normalized to the phonon scattering depends linearly on NA−ND. This relationship can be exploited in contactless characterization of p‐type ZnSe.

Optoelectronic properties of ZnSeGaAs interfaces: Role of interface chemistry and structural defects

Abstract Electric-field induced Raman scattering by longitudinal optical phonons was applied to study band bendings at the heterojunction interface between ZnSe epitaxial layers and GaAs substrates. Instabilities of the interface were observed and are attributed to atomic exchanges triggered by the formation of bonds between Ga and Se. The effects of the instabilities on the optoelectronic properties of the interfaces were distinguished from those of the misfit dislocations.