Ultrathin-layer Superlattices Research Articles

Valley-Mixing Effects in AlAs/GaAs (001) Superlattices

The effects of ΓX-valley mixing in (AlAs)n(GaAs)n (001) superlattices (SLs) on the electronic properties are theoretically investigated versus the SL period (n = 1–25) and the band offsets. The calculations are based on the empirical sp3s* tight-binding model, which includes only nearest-neighbour interactions. The results show that the highest state of the valence band (VB) is always confined to the GaAs slabs, whereas the bottom state of the conduction band (CB) shows different behaviours as it is sensitive to band-mixing effects. It is due to these effects in the ultrathin-layer SLs (n ≤ 8) that the electrons become localized in the AlAs X-valley and the heterostructure becomes of type-II. Whereas, for thick-layer SLs (n ≥ 9), the GaAs wells become completely separated. As a result, the electrons are localized in the GaAs slabs and the SL is of type-I. The estimated critical layer thickness is about 26 Å (nc = 9), which is consistent with the photoreflectance experiments. The valley-mixing effects are shown to be efficient only for small conduction band offset (CBO ≤ 200 meV) and are essential for the existence of type-II behaviour. The relevance of our work to the photonic device applications is furtherly discussed.

Exciton localization in GaAs/AlAs superlattices grown on high-index oriented surfaces

In this work we have investigated the temperature dependence of photoluminescence (PL) properties of GaAs/AlAs ultrathin-layer superlattices (UTLS) grown by molecular beam epitaxy on high index surfaces. The temperature dependence reveals an anomalous decrease of the PL linewidth with the increase of the temperature, which is explained in terms of the exciton thermal activation energy transfer between nanometre-steps formed in the corrugated interfaces.

Atomic-scale characterization of interfaces in the GaAs/AlGaAs superlattices

We present a new interpretation of the Raman spectra of GaAs/AlAs ultrathin-layer superlattices based on the microscopic analysis of the optical vibrational modes. The difference between normal and inverted interfaces is responsible for the lack of the inversion symmetry of the layers with respect to the central plane, therefore confined modes can no longer be considered as even or odd ones. All the optical vibrational modes, independently of their quantum index, are now active in Raman scattering.

Influence of atomic-scale roughness on Raman selection rules in the ultrathin-layer (GaAs)n1(AlAs)n2 superlattices.

An interpretation of the Raman spectra of GaAs/AlAs ultrathin-layer superlattices based on the microscopic analysis of the optical vibrational modes has been reported. The difference between normal and inverted interfaces is responsible for the lack of the inversion symmetry of the layers with respect to the central plane, therefore confined modes can no longer be considered as even or odd ones. All the optical vibrational modes, independently of their quantum index, are now active in Raman scattering. Our results show the evidence of the strong difference between the distribution of the Ga and Al atoms at the normal and inverted interfaces.

Characterization of AlAs/GaAs ultrathin-layer superlattices grown by migration enhanced epitaxy at low temperatures

This paper describes the effect of As coverage during migration enhanced epitaxy (MEE) on the structural and optical quality of (AlAs) 10(GaAs) 10 ultrathin-layer superlattices (ULSs) grown at low temperatures. The As coverage during MEE was in-situ determined by analyzing the oscillation waveform of reflection high energy electron diffraction intensity. Experimental results on Raman scattering and photoluminescence spectroscopy have revealed that high quality ULSs can be grown under a slightly As-excess condition although the optical quality deteriorates with increasing the excess of As adsorption. When the ULSs are grown completely free from excess As adsorption, on the other hand, the width of the ULS structure tends to be high with a considerable fluctuation. The increased width has been attributed to the presence of column III droplets on the surface after the irradiation of Ga and Al beams.

Raman scattering analysis of InAs/GaSb ultrathin-layer superlattices grown by molecular beam epitaxy

Raman scattering analysis has been performed to study characteristic phonon modes in InAs/GaSb ultrathin-layer superlattices (ULSs) grown by molecular beam epitaxy. Either interface bond InSb or GaAs was selectively made in the ULSs by controlling the beam supply sequence during growth. In addition to the acoustic and optic phonons modulated by ULS structure, the Raman spectra were composed of two different types of new phonon mode localized at each interface of InSb or GaAs. These experimental data have been understood in agreement with the theoretical analysis.

Micro-Raman scattering in ultrathin-layer superlattices: Evidence of zone-center anisotropy of optical phonons

Raman spectra of (001) (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$, 1\ensuremath{\le}m, n\ensuremath{\le}7 superlattices have been measured in backscattering along x'\ensuremath{\parallel}[1\ifmmode\bar\else\textasciimacron\fi{}10], x\ensuremath{\parallel}[010], and z\ensuremath{\parallel}[001] directions with a microprobe. Confined longitudinal (LO) and transverse (TO) optical phonons with q either parallel or normal to the z axis have been studied in all the independent scattering geometries. A strong anisotropy is observed when the direction of the transferred momentum is changed from the z axis to x' or x. The most evident feature is that the Raman spectra in the z(x'x')z\ifmmode\bar\else\textasciimacron\fi{} geometry are dominated by ${\mathrm{LO}}_{1}$ confined phonons, whereas no signal at this energy is present in the y'(x'x')y\ifmmode\bar\else\textasciimacron\fi{} ' geometry. Modes evolving from ${\mathrm{TO}}_{1}$ and ${\mathrm{LO}}_{\mathit{n}}$ (n odd) phonons give rise to structures at intermediate frequencies in the x'(y'y')x\ifmmode\bar\else\textasciimacron\fi{} ' spectrum. In contrast to ${\mathrm{LO}}_{1}$, the measured ${\mathrm{TO}}_{1}$ energy remains constant, showing the isotropy of the lower-energy branch of this doubly degenerate mode. These results are explained by microscopic lattice-dynamical calculations performed in an ab initio scheme, which properly accounts for angular dispersion and mode mixing. We have also found that in the ultimate limit of confinement, when only one principal mode per branch can be considered, the phonon frequencies can be reproduced by the macroscopic dielectric continuum model, provided that the effect of phonon confinement is considered.

Intrinsic luminescence in GaAs/AlAs ultrathin-layer superlattices

The improved structural perfection of GaAs/AlAs ultrathin-layer superlattices (UTLSs) having a configuration close to the type-I/type-II (direct/indirect) cross-over allowed us to separate the intrinsic luminescence due to quasi-free excitons from the luminescence due to localized excitons. The observed narrow linewidth made feasible the accurate determination of the energies of the direct (Γ) and indirect (X) minima and their dependence on the UTLS parameters. The measured values are in excellent agreement with recent calculations.

Carbon acceptor luminescence in type-I GaAs/AlAs ultrathin-layer superlattices

The luminescence associated with residual carbon acceptors in type-I (direct-gap) ultrathin-layer superlattices (UTLS) with well and barrier widths of 22.7<L z<25.2 A and 11.5<L b<14.0 A, respectively, is composed two lines reflecting the on-center and on-edge state of the impurities. In these narrow wells the on-center acceptor binding energy increases to 60 meV in agreement with theoretical calculations for GaAs single quantum wells using a valence band offset of 500 meV. While the binding energy of the on-center state does not vary significantly within the studied L z and L b range, the on-edge state shows a strong dependence on the very narrow barrier width. This increase of the acceptor binding energy makes the energy position of the impurity-related luminescence in UTLS very sensitive to the actual barrier height. Investigation of the impurity-related luminescence thus provides a versatile tool to determine the band offset ratio at the heterojunction.

Strain Analysis on MBE Grown InAs/AlSb Ultrathin-Layer Superlattices Using Raman Scattering

ABSTRACTThis paper describes an experimental analysis of strained InAs/AlSb ultrathin-layer superlattices (ULSs) grown by MBE. Interface bonds of InSb or AlAs were selectively made in the ULSs by controlling the beam supply sequence during growth. Structural analysis was performed for ULSs on both InAs and AlSb buffer layers using Raman scattering. This analysis revealed that each constituent in the ULSs was relaxed individually only when the AlAs interface bond was combined with the AlSb buffer layer. Other combinations of the interface bond and buffer layer, however, resulted in a free-standing superlattice structure. These experimental results are consistent with the interface dependence of electron mobility reported for InAs/AlSb single quantum well structures.

Raman scattering from InAs/AlSb ultrathin-layer superlattices grown by molecular beam epitaxy

Experimental analysis by Raman scattering has been performed to study characteristics phonon modes in InAs/AlSb ultrathin-layer superlattices (ULS's) grown by molecular beam epitaxy. Zone-folded longitudinal acoustic and confined optical phonons have been observed in the Raman spectra of these ULS's. In addition to them, we have observed new phonon modes localized at the heterointerface for the first time. It is also shown that the excitation of localized modes depends on the types of interface bonds, InSb- or AlAs-like, formed in the ULS's.

Infrared reflectivity by transverse-optical phonons in (GaAs)m/(AlAs)n ultrathin-layer superlattices.

We report far-infrared reflectivity measurements on (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices (SL's) with systematically varied layer thicknesses in the range 1\ensuremath{\le}m,n\ensuremath{\le}7. Taking advantage of an appropriate choice of the total SL thickness D\ensuremath{\le}0.3 \ensuremath{\mu}m, we measure the frequencies of AlAs-like ${\mathrm{TO}}_{1}$ confined phonons from the peak of the reststrahlen band. The GaAs-like ${\mathrm{TO}}_{1}$ frequencies are obtained by fitting reflectivity spectra to the SL dielectric-response-theory model. Microscopic calculations of confined TO frequencies are performed within an ab initio scheme and successfully compared with the experimental data.

Raman scattering by LO phonons in (GaAs)n1/(AlAs)n2 ultrathin-layer superlattices.

Raman spectra of (GaAs${)}_{\mathit{n}1}$/(AlAs${)}_{\mathit{n}2}$ ultrathin-layer superlattices were measured at room temperature and under off-resonance conditions. The experimental results show that there are two effects in ultrathin-layer superlattices: the confinement effect of LO phonons and the alloy effect. It is found that the relative intensity of the disorder-activated TO mode can give a measure of the alloy effect. The Raman spectra of one-monolayer superlattices measured in various scattering configurations are very similar to those of the ${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As alloy, and thus the alloy effect is prominent. However, in the case of monolayer number n\ensuremath{\ge}4, the confined effect is prominent, while the alloy effect is only shown as an interface effect.

Custom-Designed HI-V Semiconductor Mimstmctures Wed to the Ultimate Physical Limit : U I ~ - L a y e r GaAs/AUs ~uperhtticesa nd Delta- (Monolayer) Doping in GaAs/Al,Ga,-As Structures

Two prototype artificially-layered semiconductor structures are presented in which the concept of microscopical structuring of solids is scaled to its ultimate physical limit normal to the crystal surface. In both the(G~AS),/(AIASm)~o no layer superlattice and in the delta- (or monolayer)doped G~AS/AI~G~,-s~truAcStu res, which have been grown by molecular beam epitaxy, the characteristic material lengths have reached a spatialextent nornlal to the surface of less than the lattice constant. The (GaAs),/(AIAs), ultrathin-layer superlattices exhibit novel optical properties due tothe indirect-gap nature ofthe constituent AlAs layers. The minority-carrier lifetimes can be tailored over four orders of magnitude by appropriately designing the superlattice configuration. This feature opens up new fieldsof application in lasers and in nonlinear photonic and optoelectronic devices. The narrow buried doping channel in delta-doped GaAslayers andin GaAs/AI,Gal-xAs structures leads to a significant improvement of the electrical properties. Based on this concept, non alloyed ohmic contacts,field-effect transistors with ;cry high transconductance, unpinned GaAs surfaces for MOS devices, and electron mobilities as high as 10' cm2v-'s-Ihave been fabricated.

Stability and electronic structure of ultrathin-layer superlattices: (GaAs)n/(AlAs)n.

A first-principles total-energy calculation has been performed for ultrathin-layer (001) superlattices, (GaAs${)}_{\mathrm{n}}$/(AlAs${)}_{\mathrm{n}}$ (n layers of GaAs alternating with n layers of AlAs, with n=1 and 2). The total energies for the superlattices are higher than the averaged total energies of GaAs and AlAs by about 5 meV per atom. This small difference in energy results from the efficient screening of the valence electrons at the As interface layers. The calculated total energy of the alloy semiconductor ${\mathrm{Ga}}_{0.5}$${\mathrm{Al}}_{0.5}$As, in turn, is higher than that of the superlattices by 88 meV per atom. An indirect gap in (GaAs${)}_{1}$/(AlAs${)}_{1}$ and a direct gap in (GaAs${)}_{2}$/(AlAs${)}_{2}$ are inferred from the calculated energy bands.

Molecular Beam Epitaxy of Artificially Layered III-V Semiconductors: Ultrathin-Layer (GaAs)m (AlAs)m Superlattices and Delta (δ-) Doping in GaAs

After a brief summary of the key aspects of molecular beam epitaxy (MBE) of III-V compound semiconductors we present two examples for the application of MBE to modify the bulk properties of semiconductors through bandgap engineering in artificially layered structures. In both the GaAs/AlAs short-period and ultrathin-layer superlattices as well as in the monolayer or δ-doping in GaAs the concept of artificially layered semiconductor structures is scaled down to its ultimate physical limit normal to the crystal surface, as the constituent layers have a spatial extent of less than the lattice contant of the respective bulk material.

Raman scattering from (AlAs)m(GaAs)n ultrathin-layer superlattices.

A Raman scattering experiment was performed on ultrathin-layer superlattices (AlAs${)}_{m}$(GaAs${)}_{n}$ (1,n6). It was confirmed that LO phonons are confined in respective layers even in ultrathin-layer superlattices. The dependence of observed LO-phonon frequency on the AlAs- and GaAs-layer thickness is in good agreement with a linear chain model.