GaAs AlAs Short Period Superlattices Research Articles

New Editorial Board member of physica status solidi (b)

physica status solidi (b)Volume 243, Issue 5 p. 969-969 Editorial BoardFree Access New Editorial Board member of physica status solidi (b) First published: 27 March 2006 https://doi.org/10.1002/pssb.200690011AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract physica status solidi is proud to announce the nomination of a new member of the Editorial Board of pss (b), Bernard Gil, Montpellier, France. Information on other Editorial Board members can be found in phys. stat. sol. (a) 201, 826, 2865 (2004); 202, 347 (2005) and phys. stat. sol. (b) 241, 974, 2873 (2004); 242, 495 (2005). Bernard Gil, Montpellier Born 1957, Bernard Gil received his Ph.D. in 1981 and joined the “Centre National de la Recherche Scientifique” (CNRS) the year after. He obtained his “Doctorat d'Etat” in 1985. He has authored more than 200 scientific papers and contributed to organize some 30 international conferences. He is also directing the Groupe d'Etude des Semiconducteurs (GES), a joint research laboratory between the CNRS and the University of Montpellier II. Among the scientific contributions of Gil are specific properties of excitons bound to non-cubic centres in GaP, the symmetry of the conduction band of short-period GaAs–AlAs superlattices, the effect of strain fields on phonons and on the valence band physics in GaN epitaxial films, excitonic recombination properties in GaN and InGaN quantum wells and dots, and the excitonic fine structure in ZnO films and ZnO–ZnMgO quantum wells. Bernard Gil has been member of the Editorial Board of physica status solidi (b) since March 2006. e-mail: gil@GES.univ-montp2.fr Volume243, Issue5Special Issue: Current Trends in Electronic StructureApril 2006Pages 969-969 RelatedInformation

Ordered arrays of vertically correlated GaAs and AlAs quantum wires grown on a GaAs(311)A surface

We study GaAs–AlAs short-period superlattices (SPSLs) grown on a GaAs(311)A surface using plan-view transmission electron microscopy (TEM). A strong in-plane compositional modulation with a period of 3.2 nm along the [01̄1] direction is revealed by TEM under chemically sensitive imaging conditions and in high-resolution TEM. Our results confirm the formation of highly ordered vertically aligned arrays of GaAs and AlAs quantum wires formed via self-organized growth. Bright photoluminescence (PL) at room temperature in the green and yellow spectral range is observed.

Electron mobility and charge correlation in silicon doped GaAs–AlAs short period superlattices

The silicon deep donor (so-called DX center) is known to exhibit a bistable charged state DX−/d+ in GaAs and related compounds. We investigate the electron mobility as a function of the carrier concentration μ(n) in a silicon doped GaAs–AlAs short period superlattice at the temperature of liquid nitrogen, i.e., in the regime of metastability of the DX center. To vary the electron density, two methods are utilized: gradual photoexcitation of the DX donors or appropriate thermal capture cycles after complete photoionization of the DX donors. We observe the multivalued character of μ(n) and hysteresis effects when tuning of the metastable DX center occupancy is achieved with various procedures. Similar behavior has been previously observed in the silicon doped AlGaAs alloy and has been explained as the result of changing the degree of the donor charge spatial correlation. Our results show the influence of the photon energy when the electron concentration is varied by illuminating the sample with above- or below-bandgap light. This is due to radically different mechanisms of persistent photoconductivity in these two spectral domains. Moreover, the strong mobility enhancement we observed in the interband regime is explained by DX charge transfers which are specific for superlattice structures.

Pressure sensors based on silicon doped GaAs–AlAs superlattices

We show how GaAs–AlAs short period superlattices, in place of AlGaAs thin layers, improve the performances of n-type III–V semiconductors as pressure sensing material. Pressure induced electron capture on relaxed silicon donor sites (so called DX center) is responsible for the high pressure coefficient of resistance (30% per kbar). In comparison to AlGaAs, band gap engineering is employed to optimize both pressure and temperature sensitivities of GaAs–AlAs pseudoalloys between 0 and 200 °C under pressures up to 2000 bars. An electrical characterization is made by performing resistance and Hall effect measurements as functions of hydrostatic pressure and temperature on two microstructures forming the monolithic transducer. The heterostructures consist of (GaAs)9–(AlAs)4 superlattices doped with silicon at concentrations of 1.4×1017 and 2×1018 cm−3, respectively. Accurate pressure measurements (resolution less than 0.2 bar) are performed on two resistors patterned on these microstructures. Monolithic microsensors can be designed on such a stacked GaAs–AlAs two-resistor microstructure.

Investigation of DX center in silicon doped GaAs–AlAs short period superlattices

Electrical properties of various GaAs–AlAs short period superlattices uniformly doped with silicon are investigated. Hydrostatic pressure is used to capture electrons from the conduction miniband onto the deep donor states associated with the large lattice relaxation of the Si donor in GaAs, AlAs, and at the GaAs–AlAs interface. The ionization energies and densities of different deep donors are determined. We find that a two-band model of conduction, assuming the existence of three donor states (discrete structure of the Si-DX center), accounts for the Hall data in all samples. Application of hydrostatic pressure allows us to vary the miniband energy in some samples and leads to improved determination of the donor energies and concentrations. When the AlAs growth rate is half of the GaAs growth rate, the fraction of DX states decreases in AlAs, whereas the fraction of DX states increases at the interface indicating a strong Ga/Al cation exchange across the interface.

Negative magnetoresistance and electron localization in GaAs-AlAs superlattices

We report the negative magnetoresistance measured in GaAs-AlAs short-period superlattices. The samples were doped with silicon so that the electrical conductivity was controlled by the DX silicon donor. We take advantage of the metastable character of the DX states at low temperature to tune the conductivity by increasing the photoionization of the ground state in the persistent photoconductivity regime. The magnetic correction is generally interpreted in terms of weak localization and electron-electron interaction according to the Kawabata and Altshuler theories of the magnetoresistance at low magnetic field in semiconductors. However, we find that these models, previously developed in pure 3D or 2D systems, cannot be strictly applied in superlattices showing anisotropic diffusion and low mobility. Conversely, we show that the self-consistent approach to electron localization recently proposed by Bryksin and Kleinert (BK) in anisotropic systems accounts properly for the experimental results.

Combined HREELS, Raman and X-ray diffraction study of short-period GaAsAlAs superlattices

We have investigated short-period GaAsAlAs superlattices with layer thickness down to a few monolayers (ML) by combining HREELS, Raman spectroscopy and X-ray diffraction. The superlattices were grown on semi-insulating GaAs(100) substrates by molecular beam epitaxy (MBE). The experimental results are analyzed by the dielectric theory of HREELS. The transverse-optical (TO) phonon frequencies and the damping constants of GaAs and AlAs of individual samples, required for the theoretical calculations, were determined by Raman-scattering measurements. For the sample with a layer thickness of 18 ML an excellent agreement was obtained between the theory and experiment. However, the theory fails to reproduce the experimental intensity ratio between the GaAs-like and AlAs-like loss peaks when the layer thickness becomes shorter (6 and 12 ML). From X-ray diffraction measurements we found that the layer thickness fluctuation due to interface steps becomes large relative to the period for the superlattices with very thin layers. The thickness fluctuation significantly affects the HREEL spectra of the superlattices with layer thickness less than a few ML.

Power nonlinearities in the luminescence spectrum of [formula omitted] superlattices

We report on a time-correlated nonlinear photoluminescence study of GaAs AlAs short period superlattices at low to medium excitation densities (10 13 – 10 16 cm −3). The nonlinearity of PL intensity at doubling the excitation density is 1–10% and depends on luminescence wavelength revealing the internal structure of the luminescence line. The observed nonlinearities have been attributed to three main phenomena: repopulation of localized excitons by photogenerated hot excitations, filling of the (localized) exciton states and population of higher energy states by carrier-carrier collisions. Observed time-correlations allow one to differentiate between these contributions and establish the characteristic densities and time constants.

Short period superlattices under hydrostatic pressure

Hydrostatic pressure (0–13 kbar) has been used to probe the Bloch transport in GaAs AlAs and Ga 0.47In 0.53As Al 0.48In 0.52As short-period superlattices between 77 K and room temperature. We show that for both types of superlattices, the width of the lowest Γ conduction miniband decreases with increasing pressure due to the increase of the effective mass. Moreover, in GaAs AlAs short period superlattices, an additional effect of the increasing pressure is the change of the relative energy separation between the lowest Γ miniband and the lowest X states and the resulting transfer of carriers from Γ to X.

Laser-frequency dependence of Raman modes in GaAsAlAs superlattices

The excitation frequency dependence of the Raman spectra in short-period GaAsAlAs superlattices is studied. The off-resonance Raman spectra can be explained by a disorder-induced breakdown of wave-vector conservation leading to the contribution of small magnitude, random orientation wave-vector phonons. As extreme resonance is approached, double-resonance effects cause phonons with large magnitude, preferentially in-plane wave-vectors to dominate the Raman spectrum.

Determination of the energetical separation of DX states in GaAs and in AlAs by using planar-doped superlattices

We present Hall measurements performed in Si-doped GaAs-AlAs short period superlattices under hydrostatic pressure. The planar doping technique allows us to isolate the DX 0 state in GaAs layers and the DX 3 state in AlAs layers. The period of the superlattice was adjusted to make the DX 0 level resonant in the conduction miniband and the DX 3 level in the gap of the superlattice. By applying hydrostatic pressures up to 12 kbars, we determined the energetical shift between the two states to be around 130 meV.

Non-linear transport properties of miniband conduction in the presence of crossed electric and magnetic fields: A semi-classical approach

In this paper we compare a series of perpendicular transport measurements for short period GaAs-AlAs superlattices in presence of a magnetic field applied parallel to the layers with the semi-classical calculations of the miniband conduction properties. The theoretical approach takes into account the local solution of the Boltzmann equation in the presence of scattering mechanisms such as phonons and interface fluctuations. The Poisson and drift-diffusion equations have been self-consistently solved to obtain the current density vs the applied electrical field characteristics, which is directly compared with the experimental measurements. A good agreement is found at 300 K for all values of the electric and magnetic fields. At low temperatures a pronounced disagreement is observed when the electric field F is greater than the critical value F c that determines the negative differential resistance regime in the current-voltage characteristics. We discuss the physical origin of this difference.

Electronic states in GaAs-AlAs short-period superlattices: energy levels and symmetry

We have made a comprehensive photoluminescence (PL) study of the low lying conduction band states of (GaAs) m /(AlAs) n type-II short-period superlattices (SL) with m, n ⩽ 4, in order to determine their energy and symmetry. The symmetry is found from the shift and splitting of the levels under uniaxial stress, and from the no-phonon oscillator strengths determined by time-resolved PL. Our samples are found to be true superlattices obeying the optical selection rules predicted by the space group symmetry, which determines whether a transition is indirect or pseudo-direct. However, selection rules depending on parity with respect to reflection in the growth plane are not obeyed. The results are compared with theoretical calculations from the literature. When strain due to lattice mismatch is taken into account, the ordering of the levels is found to agree with the most recent calculations, except for the case m = n = 1. In this SL the lowest conduction band state is found to derive from the bulk X valley, rather than from the L valley as predicted. We confirm that this discrepancy can be resolved if there is an ordered interchange of a certain fraction of the Ga and Al atoms, and our data support the theoretical prediction that such an ordered intermixed SL may in fact be more stable than either the perfect SL or the random alloy.

Self-consistent tight-binding calculations of band discontinuity in [formula omitted] superlattices controlled by group-IV-element insertion layers

We have calculated the band discontinuities in GaAs AlAs short period superlattices (SL's) with group-IV-element insertion layers. The calculations are performed by using a self-consistent tight-binding method. In the ( GaAs) 5( Ge 2) ( AlAs) 6 [001] SL, the (Ge 2) insertion layers induce a dipole potential of 1.35eV. The resulting valence-band discontinuity ΔE v is 1.86eV creating a type-II band lineup. We also found that the (Ge 2) insertion layers induce the impurity-related localized states in the band-gap.

High field magneto-optical study of the band structure of GaAsAlGaAs and GaAsAlAs short period superlattices

Magneto-absorption and magneto-luminescince spectra in (GaAs) m /(Al x Ga 1−xAs) n (type I) and (GaAs) m /(AlAs) n (type II) short period superlattices were measured in pulsed high magnetic fields up to 40T applied both parallel (B |) and perpendicular (B ⊥) to the layers. Well- defined oscillatory structures were observed in type I samples with ( m, n)=(6,6) and (7,5) for both field directions. For B |, the Landau level energies were calculated for both the conduction and valence band on the basis of the Kronig-Penny model, and excellent agreement was obtained with the experimental data. This result shows that the effective mass approximation is still adequate for these ( m, n). In type II samples with ( m, n)=(3,5) and (5,6), weak oscillations were observed only for B ⊥. A very large anisotropy was observed in magneto-luminescence spectra, showing that the lowest conduction band is of the X z character.

Symmetry of the lowest conduction band states in very short-period GaAsAlAs superlattices

The symmetry of the lowest conduction band minimum (CBM) in type-II (GaAs) n (AlAs) n short-period superlattices (SL) was investigated by photoluminescence (PL) decay, PL excitation (PLE) and PL under uniaxial compressive stress. The results show that for n ⩾ 4, the CBM is in the center of the SL Brillouin zone (BZ) and the wave function is mainly derived from the bulk X z state. For n ⩽ 3, the CBM is away from the center of the SL BZ, and derives mainly from the bulk X x, y states. Interface disorder plays an essential role in both the transition probabilities and the energy levels.

Science and technology of interfaces

The advent of submicrometer technologies, the growth of ultrathin layer films and superlattice structures, and the increasing incorporation of quantum well structures in devices have given new impetus and prominence to the science and technology of interfaces. In homoepitaxy and heteroepitaxy of lattice-matched materials and in the heteroepitaxy of dissimilar materials, the issues of interface morphology, planarity and thermal stability are essential for achieving and controlling the artificially structured device materials. Interface-induced strain effects have led to several new phenomena such as: 100% increase in the elastic moduli (“supermodulus effect”) of thin metallic films (CuNi. AuNi, CuPd), the control of the wavelength of emission in thin semiconductor films (InAlAs-InAs-InAlAs), the enhancement of device performance (“pseudomorphic layer growth”) of a number of devices (lasers, transistors etc.), the observation of new transport phenomena in semiconductor heterostructures (indirect-direct switching in short-period GaAsAlAs superlattices). Some examples are discussed in this paper.

Γ- and X-miniband structure in GaAsAlAs short period superlattices

In this work, we present a review of our recent study [Phys. Rev. Lett. 63 (1989) 2284] of the anisotropy in the bandstructure of a strongly coupled GaAsAlAs superlattice by the application of a magnetic field. We report the observation of hybridisation of the zone centre Γ-state with zone folded X-states in the superlattice dispersion, which is demonstrated by the presence of an energy gap in the Landau level diagram for cyclotron motion through the layers. A simple model is developed to treat Γ-X mixing at interfaces in the envelope function approximation and the observed mixing strength is examined in terms of theoretical predictions.

Exciton radiative lifetime in short period GaAs-AlAs superlattices of type II

From an extensive study of time-resolved photoluminescence in the temperature range 1.8–50 K on a dozen of samples of type II GaAs-AlAs(001) superlattices, we have identified the region where the states derived from X z -valleys are lower than those derived from X xy -valleys. This is in agreement with a simple model involving the competition between (i) the difference in the confinement energies of X-type states in AlAs layers related to their effective mass anisotropy, and (ii) X-valley splitting of about 19 meV caused by the residual uniaxial stress in AlAs layers related to the small lattice mismatch with GaAs. We present an analysis of the luminescence decay curves in order to determine the variation of radiative lifetime of excitons involving X z - or X xy -electrons, as a function of AlAs and GaAs layer thicknesses. The parity of X z -states in the superlattice and the mixing of X x - and X z -components into the wavefunction of these excitons localized by interface disorder play a significant role in this analysis; they contribute both to non-exponential decay.

Interlayer Γ-X scattering in staggered-alignment AlGaAs-AlAs ternary alloy multi-quantum-well structures

Abstract The interlayer Γ-X scattering rate of photoexcited electrons was measured in staggered-alignment type II AlGaAs-AlAs ternary alloy multi-quantum-well structures. The scattering process was directly probed by the femtosecond pump-and-probe spectroscopy. The mean Γ-X scattering time of electrons across the interface at 4.2 K was determined to be 1.0–1.2 ps, which is longer than that observed recently in GaAs-AlAs short period superlattices and is 20 times longer than that observed in bulk GaAs. The slowing mechanism is ascribed to the small penetration of the evanescent Γ electrons into the AlAs barrier layers.