xAs Superlattices Research Articles

Superlattice with two δ-type defects: electronic structure and density of states

The electronic-level structure and local density-of-states (LDOS) distributions are investigated for an infinite GaAs/AlxGa1−xAs superlattice (SL) with two strongly spatially confined defect layers (the so-called δ-defects). The energy spectrum is determined by using a multi-matching procedure within the envelope-function approximation, while the LDOS is computed via combined factorization and direct methods in the framework of the Green's function formalism. In general, insertion of two δ-defects leads to the appearance of two localized states inside the minigaps, whose energy positions and space-charge distributions can be tuned by adjusting the defect parameters. A substantial change in the LDOS distribution is also observed. In particular, the region consisting of n periods between the defected SL layers exhibits—for large enough δ-defect strengths—an energy spectrum of n quasi-discrete resonances within the miniband range, whereas outside this region the LDOS takes the form characteristic of semi-infinite SL's terminated at the defect sites.

New developments in superlattice transport: quenching of miniband conduction in high magnetic fields

DC miniband transport is studied in undoped GaAs/Al xGa 1−xAs superlattices at low temperatures and at high magnetic fields applied perpendicular to the tunnel barriers. A strong suppression of miniband conduction is observed under these conditions. We propose a qualitative semi-classical Drude model to explain the observed results.

Temperature range for re-emission of carriers in GaAs/Ga 1− xAl xAs superlattices

The photoluminescence from n-type doped GaAs/Ga 0.57Al 0.43As single quantum well (QW) and GaAs/Ga 0.54Al 0.46As superlattice (SL) heterostructures grown by molecular beam epitaxy was studied in the temperature range 10< T<300 K. The temperature dependence of the QW emission energy follows the band-gap shrinkage of bulk GaAs. The integrated intensity observed under indirect excitation of the GaAs/Ga 1− x Al x As SL drops exponentially by three orders of magnitude above 30 K, with an activation energy of 139 meV in the range 25–45 K, 305 meV in the range 75–160 K and 578 meV in the range 200–300 K. These energies are comparable to the effective barrier height of a light-hole, of an electron and of electron–hole pairs, respectively. The decrease of the integrated intensity is therefore attributed to thermal re-emission of confined carriers out of the QW. Near room temperature, the decrease of the integrated intensity is because of the thermally activated nonradiative recombination.

Analysis of mechanisms for electron scattering in GaAs/AlxGa1−x As superlattices with doped quantum wells for longitudinal resonant current flow in high electric fields and at low temperatures

Formulas are derived for, and a numerical analysis made of, the dependence of the transverse phase relaxation time on electron energy for resonant current flow through GaAs/AlxGa1−xAs superlattices with doped quantum wells. The parameters are chosen to be close to those of superlattices used for creating photodiodes for operation at λ⋍10 µm. The analysis is limited to the interactions of electrons with neutral atoms and impurity ions at low temperatures. Resonant current flow is ensured by an electric field that brings the ground state and the first excited state of the “Stark ladder” into resonance with neighboring, weakly interacting quantum wells.

Density of states of a terminated superlattice with periodically arranged δ -defects

Surface electronic structure and the local density of states (LDOS) of a periodicallyδ-doped GaAs/AlxGa1−xAs superlattice (SL), terminated by an AlyGa1−yAs substrate, are investigated using the wave-function multi-matching procedure within an effectivemass approximation and a Green-function formalism based on factorization and direct techniques. The considered system is described in terms of a semi-infinite Kronig-Penneytype of model interleaved with a periodic array ofδ-function-like potentials, representing deep-center defect sheets. It is shown that the spectrum of surface-localized states and the corresponding space-charge distributions within particular SL minibands can be tailored by varying the terminating conditions (i.e., the substrate parameters) as well as the properties of inserted defects (i.e., the weight and position ofδ-potentials). In this respect, importance of a symmetric/nonsymmetric arrangement ofδ-dopants inside SL barriers and the related effect of miniband crossing/anticrossing are emphasized.

A study of miniband conduction in Wannier–Stark superlattices at high magnetic fields

We investigate dc miniband transport in undoped GaAs/AlxGa1−xAs superlattices. High magnetic fields are applied to the samples in the growth direction at low temperature. A strong suppression of miniband conduction is observed under these conditions. We propose a qualitative semi-classical Drude model that can explain the observed results.

Self-consistent analysis of persistent photoconductivity data in Si δ-doped AlxGa1−xAs (x=0.32) superlattices

Hall measurements were performed on a series of Si δ-doped AlxGa1−xAs (x=0.32) superlattices. The DX center has been detected in these structures through the observation of persistent photoconductivity at low temperature. Electrical properties of this defect are investigated as a function of the δ-doping content. The results have been analyzed using the large lattice relaxation model. We have calculated the minibands for the δ-doping AlxGa1−xAs:Si superlattices studied as well. Evidence that the Si DX center is present in these structures and has a negative-U character will be given in this study.

Effect of the coherence of free electron-hole pairs on excitonic absorption in GaAs/AlGaAs superlattices

The effect of photoexcited free carriers on the absorption spectra dynamics of GaAs/AlxGa1−xAs superlattices is investigated experimentally by the pump-probe method. A sharp change in the shift of the excitonic resonance energy from the low-to the high-energy direction is found to occur at the moment that the electromagnetic radiation of the pump and probe beams overlap in the case of band-band excitation. This phenomenon is explained in a model of scattering of high-energy electron-hole pairs. The dephasing time of free high-energy particles is experimentally estimated to be several tens of femtoseconds.

Intersubband optical nonlinearity and bistable behavior of semiconductor superlattices

The optical nonlinearity and bistability (OB) induced by intersubband transitions (ISBTs) in semiconductor superlattices are studied by means of the Kronig–Penney model and the density-matrix method. The OB state equation in a Fabry–Perot configuration is derived. It is shown that the ISBT-induced OB can be realized over a wide frequency range, and its bistable domain and hysteresis loop are much larger than those of a two-level system. Numerical illustration of the OB behavior in a GaAs/AlxGa1−xAs superlattice is also given.

Hot-electron power loss in a doped GaAs/AlGaAs superlattice at intermediate temperature studied by infrared differential spectroscopy

The power loss of electrons in a strongly coupled n-type GaAs/AlxGa1−xAs superlattice is studied by analyzing the temperature and electric-field dependence of the interminiband absorption. Electrons are heated by an electric-field pulse and the resulting change of the infrared absorption spectrum is monitored by a step-scan Fourier transform spectrometer operated in a time-resolved, gated mode. The measured power loss is higher than predicted by a simple three-dimensional calculation including acoustic (deformation potential and piezoelectric) and polar-optical phonon emission. Possible explanations for this, such as relaxation via folded acoustic phonons or coupled plasmon–phonon modes, are discussed. The energy relaxation time, which can be extracted from the power balance, decreases from 300 ps at 15 K to 20 ps at 48 K.

Wannier–Stark localization in strained barrier GaAs/InXAl1−XAs superlattices

We report the observation of the Wannier–Stark localization effect in short period GaAs/ InXAl1−XAs superlattices with strained barriers. The superlattices, each of which is contained in the intrinsic region of a p-i-n diode structure, consist of GaAs (3.0 nm) wells and strained shallow InXAl1−XAs (0.9 nm) barriers (X=0, 0.1, 0.2, and 0.3) grown on GaAs by molecular beam epitaxy. In spite of the use of strained barriers, the Wannier–Stark localization effect is clearly observed for all samples at room temperature. Even the superlattice sample with the highest In content of X=0.3 exhibits distinct photocurrent spectra showing several peaks associated with Wannier–Stark ladder transitions as well as Franz–Keldysh oscillations. It is found that the transition intensities are consistent with theoretically calculated oscillator strengths based on the simplified tight-binding model. By increasing the In content X, the miniband width increases and the absorption peak energy due to the zeroth order ladder (e1-hh1 and e1-lh1) transitions decreases because of the reduced barrier height. The transition energies are consistently explained by taking modulation effects into account on the valence subbands due to the compressively strained barriers. The above results show that the use of a large strain effect on the barriers is possible in Wannier–Stark localization effect type devices.

Nonequilibrium lateral photoconductivity of GaAs/AlxGa1−xAs quantum-well superlattices at low temperature

The photoconductivity of GaAs/AlxGa1−xAs (x=0,36) superlattices grown by molecular beam epitaxy have been investigated at T=77K. It was found that the photocurrent relaxes to “dark” current-volt characteristics j(V) even in the case of temporally constant sample illumination. The heating from 77 K to 293 K leads to the restoring of original light characteristics. The photocurrent dependence on time j(t) consists of several exponent with Г=5–50 c. Phenomenon was explained by re-charge of DX-centers in barriers. Peaks associated with those impurities were found in the low temperature photoluminescence spectrum.

Spatial and dynamical properties of optical phonons in Al xGa 1− xAs and GaAs/Al xGa 1− xAs superlattices: Beyond the mean field approach

Spatial and dynamical properties of optical phonons in Al x Ga 1− x As alloys and GaAs/Al x Ga 1− x As quantum wells are studied. An isotopically disordered harmonic crystal model is developed to study spatial properties of optical phonons in Al x Ga 1− x As alloys. In the GaAs/Al x Ga 1− x As system, transmission of the GaAs optical phonons through an Al x Ga 1− x As barrier is studied using three-dimensional lattice dynamical model, fully accounting for the random arrangement of Ga and Al atoms in the alloy barrier. In the Al x Ga 1− x As alloy, a localized-to-extended transition is found as a function of x. In GaAs/Al x Ga 1− x As quantum wells, the transmission coefficient of the GaAs LO phonons through the alloy barrier remains significant below x < 0.3. These results are in good qualitative agreement with the experimentally deduced coherence length of LO phonons from nonequilibrium hot phonon populations.

Equidistant spectrum of localized states due to fluctuations of the parameters in quantum-well structures

Narrow equidistant peaks are recorded in the microphotoluminescence spectra of GaAs/AlxGa1−xAs superlattices with thin barrier layers. An approximate method, consisting of the reduction of a three-dimensional problem to a one-dimensional problem, is developed for calculating the electronic spectrum of quantum dots of arbitrary shape. It is shown on the basis of this method that the observed peaks may be due to the formation of electronic states of a new type, which are produced as a result of the overlapping of fluctuations of the thicknesses of adjacent wells in structures with thin barriers. The equidistant spectrum of the new states is accounted for by the nearly quadratic dependence of the diameter of the overlap regions on the coordinate in the plane of the layers.

Time-resolved optical investigations of bloch oscillations in semiconductor superlattices

We present a complementary study of the interband and intraband dynamics of optically excited Bloch oscillations in GaAs Al xGa 1−xAs superlattices. Distinct differences in the frequency and dephasing of the oscillations give evidence for Bloch oscillations performed by electrons in continuum states and by excitonic wavepackets. The dephasing time of the electronic continuum states is found to be exceptionally long as compared to the excitonic dephasing time under identical excitation conditions. These observations are confirmed by THz emission experiments, where Bloch oscillations are detected under optical excitation well above the fundamental band gap. The experimental observations suggest that the electronic coherence may be partially maintained during relaxation and momentum scattering processes.

Small-signal analysis of novel semiconductor superlattice electron-wave interference devices

The linear response of a novel AlxGa1−xAs superlattice electron-wave interference diode (EWID) is numerically investigated using the time-dependent Schrödinger equation. This device is based on analogies between electromagnetic waves in dielectrics and quantum mechanical electron waves in semiconductors. These analogies provide a basis for a new class of highly functional devices which use above-band transport. Recent experimental results and theoretical models showed that the EWID has direct current characteristics similar to the resonant tunneling diode. This first quantum mechanical calculation for the EWID alternating current characteristics shows that the device negative differential resistance (NDR) persists up to about 10 THz. By examining three different EWID designs, it is shown that device parameters, such as the number of layers, have a strong effect on high-frequency performance. The important property of NDR, combined with expected high current densities and possible integration with optoelectronic devices, makes the EWID a good candidate for high speed applications.

Electronic surface states in GaAs/Ga 1 − xAl xAs superlattice: effect of surface location

Two Green function techniques (the direct matching formalism and the interface response theory) are used to investigate the electronic structure of a semi-infinite GaAs/Ga 1 − x Al x As superlattice being in contact with a Ga 1 − y Al y As substrate. The effect of the superlattice termination on the formation of a surface state as well as on the density-of-states distributions is studied in a systematic way by varying the position of the superlattice surface (i.e. substrate/superlattice interface) within a superlattice period. It has been found that the occurrence, the position and the localization properties of surface states are very sensitive to the way the superlattice is terminated—they occur in particular mini-gaps only for some ranges of the outermost layer thickness. When this thickness is close to the value at which surface states merge into the mini-bands, the density-of-states distributions exhibit the most pronounced modifications.

D- states in GaAs/AlxGa1-xAs superlattices in a magnetic field.

D- states in GaAs/AlxGa1-xAs superlattices in a magnetic field.

Electric-field effects on above-barrier states in a GaAs/AlxGa1-xAs superlattice.

Electric-field effects on above-barrier states in a GaAs/AlxGa1-xAs superlattice.

The effect of controlled impurity incorporation on interfacial roughness in GaAs/Al xGa 1− xAs superlattice structures grown by metalorganic vapor phase epitaxy

Oxygen impurities are known to degrade the bulk and heterointerfacial quality of Al x Ga 1− x As/GaAs layers grown by metalorganic vapor phase epitaxy (MOVPE). We have intentionally added oxygen at the interfaces of Al 0.3Ga 0.7As/GaAs superlattice layers to examine its effect on interfacial and surface roughness in these structures. Small angle X-ray diffraction and atomic force microscopy (AFM) were used to quantify and compare both small scale interfacial roughness and large scale surface roughness. Interfacial oxygen incorporation was found to increase with increasing substrate misorientation angle off (100) toward the (110) and this was reflected in increased interfacial roughness. Large scale surface structures with spacings of ∼2.0 μm were observed using AFM for growth on nominally (100) substrates. The existence of these features is also indicated in X-ray correlated roughness measurements. A mechanism of preferential oxygen incorporation at surface step edges is proposed to explain the evolution of roughness in the superlattice layers.