Superlattice Band Gap Research Articles

Properties of HgTe–Hg0.15Cd0.85Te superlattices grown by molecular-beam epitaxy

The structural and optical properties of a series of molecular-beam epitaxy (MBE) grown HgTe–Hg0.15Cd0.85Te superlattices (SL) have been studied. Vertical cross-section transmission electron microscopy studies show no evidence of Hg interdiffusion, confirming the high quality of these multilayered structures. Transmittance and reflectance measurements were performed on each sample and analyzed to obtain optical absorption coefficients. The absorption coefficient versus photon energy spectra show consecutive rises and plateaus characteristic of two-dimensional quantum structures. A two-band tight-binding model was used to analyze the absorption data and identify the quantum transitions. The observation of both light-hole and heavy-hole transitions allowed the determination of both the superlattice band gap and the valence-band offset. The offset was found to be 300 meV (±20 meV) for the superlattices studied, which extrapolates to 350 meV (±20 meV) for the HgTe–CdTe binary interface.

Superlattice k

A quantitative comparison is presented of two realistic superlattice k\ensuremath{\cdot}p electronic structure calculations. The first is an analytic approach based on an extended bulk Kane model; the second is an extended-basis treatment, developed by McGill and collaborators, based on bulk pseudopotential calculations. Both approaches are applied to HgTe/CdTe superlattices. Energies, wave functions, effective masses, and oscillator strengths are found to agree within 10%. The limited-basis approach based on the Kane model is seen to be adequate for superlattices whose bulk constituents have direct gaps in the conduction- and valence-band regions near the superlattice band gap.

Light hole interband transitions in HgTe-HgCdTe superlattices

The light hole to conduction band optical transition has been identified in the room-temperature absorption spectra of several high quality HgTe-HgCdTe superlattices, in addition to the familiar heavy hole to conduction band transitions. The observation of the light hole transition, coupled with a more accurate determination of the superlattice layer thicknesses, allows the superlattice band gap and the HgTe-HgCdTe valence band offset to be determined more precisely than previously possible. A two-band tight-binding model was used to calculate the transition energies to compare with the optical data. The valence band offset for the HgTe-Hg0.15 Cd0.85 Te interface was determined to be 300±25 meV.

Thermal stability of sawtooth-shaped HgCdTe superlattices

Steps in optical absorption spectra due to transitions to different subbands have been observed in HgCdTe-based nonrectangular superlattices. The sawtooth-shaped superlattice band gap shows much less thermal degradation than the conventional rectangular superlattice. The improved thermal stability can be attributed to the enhanced quantization effect in which the dependence of superlattice band gap on quantum well size is less sensitive for a sawtooth quantum well than for a rectangular quantum well.

Band offset of the ZnSe–ZnTe superlattices: A fit to photoluminescence data by k⋅p theory

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k⋅p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k⋅p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.

Band structure of ZnSe-ZnTe superlattices.

We report the first study of the band structure of ZnSe-ZnTe superlattices. Our calculations are based on second order k\ensuremath{\cdot}p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. Since the valence-band offset for this system is not well known, we have investigated the dependence of the superlattice band gap on the valence-band offset. Based on the assumption that the photoluminescence from the superlattice is due to an isoelectronic ${\mathrm{Te}}_{1}$-bound exciton in ZnSe, we have fitted the experimental photoluminescence data with k\ensuremath{\cdot}p theory to obtain the best value of the valence-band offset. The value we find is 0.975\ifmmode\pm\else\textpm\fi{}0.098 eV. Alternatively, assuming that the photoluminescence is due to band-to-band transitions we obtain a valence-band offset of 1.196\ifmmode\pm\else\textpm\fi{}0.134 eV. We have also calculated the superlattice band gap as a function of the constituent-material-layer thicknesses for the first valence-band offset quoted. We expect these results to be important in gaining an understanding of the value of the band offset, and the nature of the photoluminescence from this system.

Optical and electronic properties of HgTe-CdTe superlattices

A survey of current issues concerning the electronic and optical properties of HgTe-CdTe superlattices is given. The superlattice has been conjectured to provide certain advantages relative to the HgCdTe alloy for infrared detectors. A comparison of theoretical predictions with recent measurements is made. The conduction electrons are expected to be partially localized near the HgTe-CdTe interfaces, to a degree dependent on the superlattice band gap. The valence band offset between HgTe and CdTe is still not determined precisely. Measurements of the optical absorption coefficient as a function of photon energy indicates that expected structure due to energy subbands does occur, indicating the presence of abrupt interfaces. Recent measurements of resonant tunneling currents through double barrier structures also imply abrupt interfaces, and may help in determining the valence band offset.

MAGNETIC AND ELECTRONIC ASPECTS OF MnSe/ZnSe SUPERLATTICES NEAR MONOLAYER LIMIT

Selected electronic and magnetic properties are studied in the strained layer MnSe/ZnSe superlattice, recently grown by molecular beam epitaxy, with individual MnSe layers in a zincblende structure approaching the monolayer limit. The luminescence spectra show a well defined superlattice bandgap ; however, both magneto-optical and direct magnetization measurements indicate that the role of interfaces is crucial in determining the magnetic behavior of the thin layers of MnSe. Anomalous characteristics are interpreted in terms of frustration against antiferromagnetic ordering by microstructure effects at the heterointerfaces.

HgTe–CdTe superlattices and Hg1−xCdxTe grown by low-temperature metalorganic chemical vapor deposition

We report low-temperature metalorganic chemical deposition of Hg1−xCdxTe and HgTe–CdTe superlattices using a thermal precracking technique. The success of the precracking technique depends on decomposing the incoming metalorganic reactants at a high temperature while allowing the epitaxial growth to occur at a lower temperature. The precracking technique enables one to grow Hg1−xCdxTe and HgTe–CdTe superlattices at temperatures of 200 to 250 °C. The Hg1−xCdxTe epilayers show infrared transmission with sharp cutoffs. The as-grown Hg0.7Cd0.3Te is n type and has a room-temperature mobility of 12 200 cm2/V s with carrier concentration of 2.7×1017 cm−3 and 77 K mobility of 27 000 cm2/V s with carrier concentration of 1.0×1017 cm−3. For the HgTe–CdTe superlattices, HgTe layer thicknesses as small as 60 Å were obtained. All the samples examined using transmission electron microscopy had sharp HgTe–CdTe interfaces. The HgTe–CdTe superlattice structures also exhibited sharp infrared transmission cutoffs. In addition, compared to the band gap of Hg1−xCdxTe alloy with the same composition, a narrowing of the superlattice band gap was observed as predicted by theoretical studies.

Tuning the effective bandgap of a silicon doping superlattice

In this paper, we examine the tunability of the effective bandgap and carrier concentration in a silicon-doping superlattice with narrow n+-doped layers and wide p-doped layers. Quantitative tunability parameters are defined and their variation with dopant concentration and temperature are calculated self-consistently in the effective mass and Hartree approximations. It is found that the tunabilities increase as doping is increased or as temperature is decreased.

Nonlinear optical response of [111] growth axis strained-layer superlattices

The presence of internal strains in [111] growth axis strained-layer III–V superlattices leads to the generation of large polarization fields (exceeding 100 kV/cm) because the constituent semiconductors are piezoelectric. These strain-induced polarization fields produce a substantial reduction of the superlattice band gap and cause a spatial separation of the electrons and holes in the confining layers in such a way as to screen the internal fields. When free carriers are present to screen the strain-induced piezoelectric fields, the electronic structure (energy band gap, in-plane effective masses) and the optical response (absorption edge, optical constants) of the strained-layer superlattice depend on the density of free carriers. In the case where the electrons and holes are photogenerated, the superlattice photoabsorption profile can be modified by variations of the intensity of the optical excitation. The nonlinear optical response of strained-layer superlattices owing to free-carrier screening of strain-generated internal fields is illustrated here for the case of Ga1–xInxAs–Al1–yInyAs strained-layer superlattices.

Electronic structure of (001) and (111) growth axis InAs–Ga1−xInxSb strained-layer superlattices

The electronic structure and optical properties of InAs–Ga1−xInxSb strained-layer superlattices grown along the (001) and (111) axis are investigated with the use of a realistic k⋅p theory. We assume that this superlattice is of the type II, i.e., the conduction band minimum of InAs lies at an energy below the valence band maximum of the alloy Ga1−xInxSb. We consider alloy compositions from x=0.1 to x=0.4, corresponding to lattice mismatches ranging approximately from 1% to 3%. Deformation potentials effects cause a substantial reduction of the band gap and allow the possibility to reach the technologically important 12 μm infrared region for very thin superlattices. This result is significant because type II superlattices exhibit optical matrix elements which decrease rapidly with increasing superlattice layer thickness. Reasonably large optical matrix elements can be obtained in the 12 μm region by properly tailoring the strain in the superlattice layers. In fact, optical matrix elements in InAs–Ga1−xInxSb strained-layer superlattices are shown to be comparable to those of HgTe–CdTe superlattices in the 12 μm region. Strained-layer superlattices grown along the (111) axis exhibit smaller quantum confinement energies and consequently appear to be slightly more advantageous than their (001) counterparts for optical applications. Furthermore, the presence of internal strain-induced piezoelectric fields in (111) growth-axis strained-layer superlattices produces a further reduction of the superlattice band gap. These results are significant and reveal that properly strained type II superlattices constitute a novel class of systems with useful materials properties for long-wavelength detection applications.

Quantitative theory of optical properties of HgTe-CdTe superlattices

We have performed full-scale calculations of the optical properties of (001) HgTe-CdTe superlattices, of period ∼60 Å. We present the superlattice band gaps and oscillator strengths for the principal transitions as a function of HgTe layer width, in good agreement with experiment. The valence-band offset is 370 meV. Our results highlight the importance of the degree of localization of interface states in determining the optical properties of the superlattice.

Optical properties of strained-layer superlattices with growth axis along

We show that large piezoelectrically induced electric fields strongly modify the optical properties of [111] growth axis strained-layer superlattices with growth axis along [111]. These internal fields reduce the superlattice band gap, change optical matrix elements, and cause a spatial separation of photoexcited electrons and holes so as to screen the fields. Very large (${\mathrm{\ensuremath{\chi}}}^{3}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}1}$ esu) optical nonlinearities occur because of the free-carrier screening of the strain-generated internal electric fields.

Electronic structure of

A k\ensuremath{\cdot}p theory is used to investigate the electronic structure of semiconductor superlattices grown along the [001] and [111] axes. The present work considers the case of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{In}}_{\mathrm{x}}$As-${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{In}}_{\mathrm{y}}$As superlattices. We specifically treat three alloy composition pairs: a lattice-matched case (x=0.53, y=0.52), a case where the Ga-containing layers are in biaxial tension with a 0.8% lattice mismatch (x=0.53, y=0.64), and a case where the Ga-containing layers are in biaxial compression with a 1.5% lattice mismatch (x=0.53, y=0.30). We analyze the effects of the growth axis on the electronic structure of the superlattice from a consideration of the subband dispersion both parallel and perpendicular to the growth direction. Apart from point-group symmetry considerations, a major factor which differentiates the electronic structure of [001]- and [111]-growth-axis superlattices is the presence of large (exceeding 100 kV/cm) internal strain-induced electric fields in strained-layer superlattices grown along the [111] axis. These internal electric fields are directed along the [111] growth axis and are generated by the internal strain because the constituent semiconductors are piezoelectric. In [001]-growth-axis strained-layer superlattices, the orientation of the lattice-mismatch-induced strains is such that these fields are not present. We demonstrate that the strain-induced electric fields result in sizeable Stark shifts on the superlattice electron and hole subbands and lead to a substantial reduction of the superlattice band gap. Moreover, these strain-induced internal electric fields modify the superlattice wave functions and cause a spatial separation of electrons and holes within the confining superlattice layers. This latter effect greatly modifies the interband optical matrix elements. It also leads to a screening of the strain-induced internal electric fields by photogenerated free carriers which causes nonlinear (i.e., intensity-dependent) optical response of [111]-growth-axis strained-layer superlattices.

Theoretical study of the electronic properties of semimagnetic superlattices

We present the first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. We explicitly consider the Cd0.8Mn0.2Te/Cd0.7Mn0.3Te superlattice system. We show magnetic field splittings both including (zero spin temperature) and neglecting (high spin temperature) the exchange interaction. We find that the exchange interaction dominates the magnetic effects. We present calculations of the derivative of the superlattice band gap with magnetic field as a function of the superlattice layer thickness. We show calculations of the change in superlattice band gap with magnetic field for several magnetic fields as a function of temperature.

Band offsets and the optical properties of HgTe-CdTe superlattices

The effect of variation in the band offset on the band gap and optical properties of the HgTe-CdTe superlattice is examined. If we define the valence-band offset ΔEν to be the energy of the valence-band edge of the HgTe with respect to the valence-band edge of the CdTe, both the band gap and optical properties are smooth functions about ΔEν≊0.

Strain effects in HgTe-CdTe superlattices grown on CdTe substrates

The effects of strain on the band structure, the band gap, and optical properties of the superlattice grown on CdTe substrates are discussed. The presence of strain modifies the band gap and optical properties of the HgTe-CdTe superlattice in a minor way. However, strain does change the band structure as compared to that in the unstrained superlattice. For wave vectors perpendicular to the layers, the top of the valence band is more light-hole-like in constrast to the unstrained case where the top of the valence band is more heavy-hole-like. For wave vectors parallel to the layers, the bands remain unchanged.

GaAs sawtooth superlattice laser emitting at wavelengths λ>0.9 μm

A new type of semiconductor superlattice laser grown by molecular beam epitaxy is realized in GaAs. The active region of the injection laser consists of alternating n and p Dirac-delta doped layers resulting in a sawtooth-shaped conduction-band and valence-band edge. The band gap of this new GaAs superlattice is smaller than the GaAs bulk band gap. Room-temperature operation of broad-area GaAs sawtooth superlattice (STS) injection lasers is demonstrated at a wavelength of 905 nm. The threshold current density of the STS laser is 2.2 kA/cm2.

GaAs saw-tooth superlattice light-emitting diode operating monochromatically at λ ≥ 0.9 μm

Edge-emitting diodes with a GaAs saw-tooth superlattice as the active region operating with high efficiency at 300 K are reported. The saw-tooth superlattice active region of the device consists of alternating n- and p-type Dirac-delta-doped GaAs layers grown by molecular beam epitaxy. The superlattice bandgap energy is lower than that of the GaAs host material. The new superlattice diode emits monochromatic light at wavelengths λ ⩾ 0.9 μm.