Superlattice Band Gap Research Articles

Tight-binding calculations of energy gaps in (001)-(InAs)n(InSb)m strained superlattices

Tight-binding calculations of electronic structures for (001)-(InAs)n(InSb)m strained layer superlattices are presented. The dependences of the superlattice band gap on the band offsets between InAs and InSb are examined for three different types of biaxial strains. It is found that the band gap depends strongly on the band offset, and that for m=1 the ordering lowers the band gap with respect to the random alloy. A comparison with the photoluminescence data for the energy gaps of (n×1) strained-layer superlattices is discussed. In addition, the electronic structures of strained InAs quantum wells are calculated, and interpretations are provided for the observed type-I to type-II band alignment transition at n=5 in a quantum well formed by (n×1) strained layer superlattices and AlSb barriers. Changes of energy gaps with layer thicknesses in strained layer superlattices with n=m and n=8−m are also studied.

Electronic and Optical Properties of Orientational Superlattices in Gainp Alloys

AbstractWe demonstrate the formation, and the electronic and optical properties of a novel type of semiconductor superlattice in spontaneously ordered GaInP alloys. The most frequently observed ordered structure in MOCVD grown GaInP has CuPt symmetry where the ordering directions occur in the two [111]B directions, corresponding to two distinct ordered variants. A new type of superlattice, termed an orientational superlattice, emerges as the ordered domains are stacked in a sequence whereby the ordering direction switches alternatively from the [111] direction in one domain to the [111] direction in the next domain. The novelty of this type of superlattice lies in that there is neither a band-gap nor an effective mass discontinuity along the superlattice axis. When the GaInP epilayer is grown on an exact (001) or [111]A tilt GaAs substrate, the two ordered variants are equally favorable. Thus, ordered domain twins appear in ordered GaInP epilayers. We present a comparitive study between the single-variant ordered structure and the double-variant ordered superlattice structure, using TEM and time-resolved differential absorption. We show that for a same order parameter, the band-gap of an orientational superlattice is higher than that of a single-variant ordered structure, and the in-plane optical anisotropy between the [110] and [110]B directions is greatly enhanced due to the superlattice effect. The experimental results are explained in terms of the band structure of the orientational superlattice.

Evaluation of low-temperature interdiffusion coefficients in Hg-based superlattices by monitoring the E1 reflectance peak

We show that variations of the E1 reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E1 peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100°C for approximately six months, and then at 150°C for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern.

Calculated electronic structure of GaAs/Ge2 (001) superlattices

We use the semiempirical tight binding method with an sp3s* basis and second-nearest-neighbor interactions to investigate the electronic structure of (GaAs)m/(Ge2)n (001) superlattices (SLs) with 1⩽(m,n)⩽20. We have found no correspondence between the calculated band structures of these (GaAs)/(Ge2) SLs and those for (GaAs)1−x(Ge2)x random alloys. The inclusion of second-nearest-neighbor, compared with the nearest-neighbor interactions, raised the SL band gap for (m,n)>1, while lowering that for m=n=1. For the (GaAs)20/(Ge2)20 (001) SLs our calculation gives a band gap energy of 0.79 eV, while for (GaAs)1/(Ge2)1 the band gap energy is 0.11 eV. For small values of m, the (GaAs)m/(Ge2)n SLs are predicted to have indirect band gaps regardless of the valence band offset used in the calculation. For larger values of m, however, the predicted SL band gaps become direct for large values of valence band offset. For ΔEv=0.85 eV we find a direct to indirect band gap transition for m between 14 and 16. In the indirect-gap SLs, the electrons and holes are confined in the Ge layers, while the direct-gap SLs have holes confined in the Ge layers and electrons in the GaAs layers.

Electronic structures and band offsets of heterocrystalline superlattices (3C-AlN)3n/(2H-AlN)2n and (3C-SiC)3n/(2H-SiC)2n (n=1,2,3).

The electronic structures and the band offsets of heterocrystalline superlattices (3C-AlN${)}_{3\mathit{n}}$/(2H-AlN${)}_{2\mathit{n}}$ and (3C-SiC${)}_{3\mathit{n}}$/(2H-SiC${)}_{2\mathit{n}}$ (n=1, 2, and 3) are studied by ab initio calculations. The results indicate that the band lineups at both (3C-SiC)/(2H-SiC) and (3C-AlN)/(2H-AlN) interfaces are of type-II with small valence-band offsets and large conduction band offsets. The effects of lattice relaxation on the band offsets of the (3C-AlN)/(2H-AlN) system are also investigated. It is found that the band gaps of the heterocrystalline superlattices decrease rapidly with the increase of the slab thickness. This abnormal band-gap behavior is shown to be related to the internal electric fields induced by the intrinsic spontaneous polarization of the 2H structures. \textcopyright{} 1996 The American Physical Society.

Interface and layer thickness dependence of the effective mass in [formula omitted] superlattices studied by high field cyclotron resonance

The effective mass has been studied in a series of semiconducting and semimetallic InAs GaSb superlattices as a function of superlattice period and band gap. The mass is found to be determined almost entirely by the superlattice period, and is relatively insensitive to both the ratio of the InAs to GaSb thickness, and the structure of the interface layer, which was grown as both a monolayer of InSb and GaAs. Using pulsed magnetic fields of up to 180 T it was possible to observe spin-split cyclotron resonance at room temperature for all samples studied. On cooling the spin-splitting was found to be temperature dependent. This is attributed to the importance of electron-electron interactions which couple the two spin transitions as observed recently in high purity GaAs GaAlAs heterojunctions at low temperatures.

Comparative study of band-structure calculations for type-II InAs/InxGa1-xSb strained-layer superlattices.

Short-period InAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Sb superlattices may allow strong optical transitions in the long-wavelength infrared (g10 \ensuremath{\mu}m) spectral region. Absorption calculations can be difficult, however, because of the strongly type-II interface and because of the large lattice mismatch. We present a comparative study of band-structure calculations for strained-layer type-II InAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$Sb superlattices grown on GaSb. The energy of superlattice band gaps (${\mathit{E}}_{\mathit{g}\mathit{s}}$) and the cutoff wavelengths (${\ensuremath{\lambda}}_{\mathit{c}}$) are computed in the empirical tight-binding, effective-bond-orbital, and 8\ifmmode\times\else\texttimes\fi{}8 k\ensuremath{\cdot}p models. In the empirical tight-binding model (ETBM) the strain is included by scaling the matrix elements according to Harrison's universal 1/${\mathit{d}}^{2}$ rule and by appropriately modifying the angular dependence. The bond-orbital model (EBOM) and k\ensuremath{\cdot}p calculations include the strain via the deformation-potential theory. We find in all cases that the superlattice band gap decreases rapidly with increasing x and that the proper inclusion of strain is critical in the ETBM. Our results compare favorably with existing experiments. In addition, we compare directly the results of the EBOM and k\ensuremath{\cdot}p models. Contrary to expectations, the two models give quite different results for InAs/InSb superlattices.

InAs/InxGa1-xSb Type II Strained Layer Superlattices for Long Wavelength Infrared Detection Applications

AbstractShort period InAs/InxGa1−xSb superlattices (SLs) may allow strong optical transitions in the long wavelength infrared (> 10 μm) spectral region. Absorption calculations can be difficult, however, because of the strongly type - II interface and because of the large lattice mismatch. We propose that a long wavelength response can be achieved for substantially thinner layers of SLs if the In composition in InxGa1−xSb is properly chosen. This will misalign the bands through strain effects and further reduce the superlattice bandgap. Band structure calculations are reported for InAs/InxGa1−xSb type - II SLs grown on GaSb substrate by using an empirical tight-binding model (ETBM). All of the structures considered here are assumed to be well within the critical strain thickness. Particular care is taken to incorporate the strain effects accurately in the ETBM formalism by modifying the overlap integrals according to the bond lengths and bond angles. We compute the band structure and the cutoff wavelengths of InAs/InxGa1−xSb (001) SLs and compare the results with the existing magnetooptical and photo conductivity data. In addition, we compare the ETBM with the k.p and effective bond orbital models.

Optical study of vertical transport in Cd0.82Mn0.18Te/CdTe superlattices.

We have performed cw and time-resolved photoluminescence measurements on a 30/30 ${\mathrm{Cd}}_{0.82}$${\mathrm{Mn}}_{0.18}$Te/CdTe superlattice containing two enlarged wells with different widths, namely, 60 and 200 \AA{}. The distance between the two enlarged wells is 2400 \AA{} and the wider one is located 0.96 \ensuremath{\mu}m away from the sample surface. Excitation above the alloy band gap allows us to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed. Mobilities of the order of 4\ifmmode\times\else\texttimes\fi{}${10}^{4}$ ${\mathrm{cm}}^{2}$/V s and 3\ifmmode\times\else\texttimes\fi{}${10}^{2}$ ${\mathrm{cm}}^{2}$/V s are estimated for the two processes, respectively, at low temperatures (20 K). Transfer of excitation between the two wells is also observed when exciting resonantly the thinner enlarged well, i.e., below the superlattice band gap. This transfer is assigned to carrier hopping on localized states of the superlattice.

Photoluminescence of HgTe/CdTe superlattices under high hydrostatic pressures.

We have measured the photoluminescence (PL) of HgTe/CdTe superlattices under high hydrostatic pressures up to 30 kbar at liquid-nitrogen temperature. We observed several photoluminescence peaks with energies ranging from \ensuremath{\sim}130 to \ensuremath{\sim}700 meV. The most prominent peak at \ensuremath{\sim}130 meV, which has been attributed to the recombination across the superlattice band gap, moves higher in energy with a pressure coefficient of \ensuremath{\lesssim}1 meV/kbar. Other peaks, whose origins are not well established, have higher energies and their pressure coefficients are in the range of 0--2 meV/kbar. A calculation based on the envelope-function approximation gives a pressure coefficient of at least \ensuremath{\sim}6.5 meV/kbar for the superlattice band gap. This is far outside the error bars of the measured pressure dependence of the main peak, \ensuremath{\lesssim}1 meV/kbar. Varying the input parameters for the calculation, including the valence-band offset, changes the result of the calculation by less than 10%. Possible explanations for this disagreement, including a modification of the current model of HgTe/CdTe superlattice bands and a reinterpretation of the PL peaks, are examined.

Structural and electronic properties of GaP-AlP (001) superlattices.

We present the results of ab initio pseudopotential calculations for studying the structural stability and the electronic structure of short-period (GaP${)}_{\mathit{m}}$(AlP${)}_{\mathit{m}}$ superlattices with m ranging from 1 to 6, composed of lattice-matched indirect-gap semiconductors. Both the bulk and epitaxial superlattices ordered in the CuAu-I, CuPt, and chalcopyrite structures are found to be unstable against phase segregation into their binary constituents at T=0. The bulk formation enthalpies are found to be similar to those for epitaxial superlattices grown on (001) GaP. The band gap of superlattices tends to decrease as the superlattice period increases. The ultrathin (001) superlattices with the superlattice period of m=1 and 2 show indirect-gap behavior while the direct band gap occurs for m\ensuremath{\ge}3. Details of the electronic structure of superlattices are discussed based on the band-pushing and charge-confinement effects. The oscillator strength of the optical transition from the valence-band maximum to the conduction-band minimum state at the \ensuremath{\Gamma} point is found to be much stronger for even numbers of m. Both the monolayer and bilayer superlattices can be direct-gap semiconductors if substrates are selectively chosen with lattice constants above 5.48 and 5.47 \AA{}, respectively.

Enhanced band-gap luminescence in strain-symmetrized (Si)m/(Ge)n superlattices.

We report on band-gap luminescence in strain-symmetrized, (Si${)}_{\mathit{m}}$/(Ge${)}_{\mathit{n}}$ superlattices grown on a step-graded, alloy buffer with a reduced dislocation density, using Sb as a surfactant. The luminescence efficiency for a (Si${)}_{9}$/(Ge${)}_{6}$ and (Si${)}_{6}$/(Ge${)}_{4}$ superlattice is strongly enhanced compared with a corresponding ${\mathrm{Si}}_{0.6}$${\mathrm{Ge}}_{0.4}$ alloy reference sample. The luminescence signals can be attributed to interband transitions of excitons localized at potential fluctuations in the superlattice. The observed systematic shift of the band-gap luminescence to lower energies with increasing period length compares well with results of a simple, effective-mass calculation. An increasing superlattice band gap and a reduction in luminescence intensity is observed if the Si and Ge layers are interdiffused by thermal annealing. The band gap for a (Si${)}_{6}$/(Ge${)}_{4}$ superlattice was also measured with absorption spectroscopy. The absorption coefficient, as determined by direct transmission, is in the order of ${10}^{3}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ about 0.1 eV above the band gap.

Tailoring of the Valence Band Offset in the ZnSe/ZnTe Strained-Layer Superlattices Using Strong Built-in Strain Fields

ZnSe/ZnTe strained-layer short period superlattices have been grown by MOVPE. Using well adapted buffer layers, we could tune the internal built-in strain of the individual layers. We studied both free standing superlattices and strained-layer superlattices lattice matched to ZnTe. We observe that the tuning of the valence band offset by the internal strain can produce a switching of the superlattice band gap from type II e1hh1 to type I e11h1. Photoreflectance investigations enabled us to measure excited state transitions allowing us to obtain the strain free valence band offset with considerable accuracy.

Compatible laser emission and optical waveguide modulation at 1.5 μm using Wannier–Stark localization

We investigate the electroabsorption properties of an InGaAs-InAlAs superlattice optical waveguide. When reverse biased, the structure exhibits large extinction ratios over short waveguide lengths with very low drive voltages by using low-energy oblique transitions below the superlattice band gap. Although the structure has been optimized for modulation, laser emission is observed under forward bias. The peak emission wavelength stands in the ‘‘blue-shift’’ region which opens a way to straightforward laser-modulator monolithic integration.

Optical absorption in α-C : H multilayer periodic structures

A new type of amorphous semiconductor superlattice is obtained. The optical properties, which are in the range 0.85–1.45 nm, are presented. The optical band gap of the amorphous superlattices are determined. It is shown that a blue shift in the optical band gap with decreasing well layer thickness may be explained by quantum size effect in one-dimensional periodic potential.

Optical characterization of AlAs/GaInAs multiple quantum wells

We report on the optical characterization by photoluminescence, photoluminecence excitation, and photoreflectance spectroscopies of a series of AlAs/GaInAs multiple quantum wells. Samples have been grown by molecular-beam epitaxy on {100} GaAs substrates, and consist of ten GaInAs wells of different widths confined by ∼12 nm-thick AlAs barriers. The composition of the Ga 1− x In x As layers varies from x = 0.08 to x = 0.15, so that the strain within the wells is accordingly increased as x increases. The experimental energies of the optical transitions compare well to results of k·p band calculations. Both type I and type II superlattice band gaps are observed and discussed.

Wannier-Stark localization in a 1.55 mu m InGaAs/InAlAs superlattice waveguide modulator structure

The authors present an optical waveguide modulator structure based on Wannier-Stark localization in a InGaAs-InAlAs superlattice. Optical waveguide transmission below the superlattice bandgap displays expected F/sup -1/ oscillatory behavior leading to various modulation schemes. An 11 dB extinction ratio was obtained by applying a 0.7 V drive voltage to a 100 mu m long waveguide device operating at 1.55 mu m under a transverse-electric (TE)-polarization mode. On-state attenuation was 5 dB. Lower open-state attenuation (3 dB) can be obtained simultaneously with a higher extinction ratio (13 dB) but in that case a larger drive voltage (1.6 V) is needed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical etalon effects and electronic structure in silicon-germanium 4 monolayer: 4 monolayer strained layer superlattices

Si/Ge short period superlattices containing four monolayers of Si and four monolayers of Ge have been grown on Si(100) substrates by molecular beam epitaxy. The superlattices have been investigated structurally by secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and Raman spectroscopy, and electronically by electroreflectance (ER), Resonant Raman (RR) and photo current (PC) spectroscopies. TEM and Raman measurements show good layer quality, and also that strain is clearly present in the Ge layers. While ER is a technique that has been widely used in the literature, the authors show by comparison with SIMS and TEM observations that such ER measurement may be complicated by an optical etalon effect when the epitaxial layer is sufficiently thick. Allowing for this by a suitable design of sample, they are able to observe the critical points in the superlattice at 2.4 and 2.8 eV, which they have confirmed with RR measurements. The RR also shows clearly that the critical points must be superlattice related. Optical transitions too weak to observe using ER or RR can be observed with PC below the bandgap of silicon, and are related to weak absorption at the superlattice bandgap of approximately 0.8 eV. The observed electronic transitions in this and the 2.4-2.8 eV energy range are compared in energy and oscillator strength with theoretical predictions.

Optimization of growth parameters for direct band gap SiGe superlattices

The criteria affecting the direct/indirect nature of the lowest energy transition across the band gap of a SiGe superlattice are discussed, and a prescription for obtaining a direct gap system is given. A systematic study of the optical properties of Si nGe n superlattices as a function of period and substrate is performed in order to optimize the energy and transition strength. Specific structural parameters are presented for two systems in which the lowest energy transition is direct and of a magnitude large enough to be of practical use. The range of energies for which direct transitions are possible is found to cover an almost continuous spectrum from 50 meV to around 900 meV, the lowest energy transitions being between conduction subbands. Of particular interest to optical fibre applications is the prediction that, with a suitably chosen substrate, the Si 5Ge 5 superlattice has a direct optical band gap corresponding to a wavelength of 1.55 μm.

Infrared Photoconductivity and Photoluminescence from InAs/Ga1–xInxSb Strained-Layer Superlattices

ABSTRACTWe have examined spectrally resolved photoconductivity and photoluminescence from InAs/Ga1–xInxSb strained-layer superlattices, which have been proposed as infrared detectors in the 8-14 μm region. Our measurements indicate that the energy gaps of the strained–layer superlattices are substantially smaller than those of InAs/GaSb superlattices with similar layer thicknesses, in agreement with previous theoretical predictions. Measurements on InAs/Ga1–xInxSb superlattices with x=0 and 0.25 and layer thicknesses of 25 – 45 A indicate superlattice band gaps of 3 – 15 μm, in excellent agreement with gaps calculated by a two band k · p model. Our results demonstrate that far-infrared energy gaps are compatible with the thin layers necessary for strong optical absorption in type-IT superlattices, and suggest that InAs/Ga1–xInxSb superlattices are promising candidates for far-infrared detection.