Impurity cyclotron resonance in InGaAs/GaAs superlattice and InGaAs/AlAs superlattice grown on GaAs substrates

Various temperature measurements of cyclotron resonance (CR) under pulsed ultra-high magnetic field up to 160 T were carried out in InGaAs/GaAs superlattice (SL) and InGaAs/AlAs SL samples grown by molecular beam epitaxy on GaAs substrates. Clear free-electron CR and impurity CR signals were observed in transmission of CO₂ laser with wavelength of 10.6 μm. A binding energy of impurities in these SLs was roughly estimated based on the experiment as result, and we found it was smaller than the previous experimental result of GaAs/AlAs SLs and theoretical calculation with a simple model.     

Cyclotron resonance in (GaAs)n/(AlAs)n superlattices under ultra-high magnetic fields

We present a review on recent study of the type I to type II transition in short-period superlattices (SLs) of GaAs/AlAs by means of cyclotron resonance (CR) in pulsed high magnetic fields. The behavior of CR varies depending on the thickness of the GaAs and AlAs layers. In CR of (GaAs)_n/(AlAs)_n, the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the Γ point was observed for n>15. We estimated electron masses on X and Γ point in the SLs by using the empirical sp³ tight-binding method including second-nearest-neighbor interaction. These calculations have shown good agreement with the experimental results. Moreover, it was found that the angular dependence of the CR peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence in the SLs, the longitudinal and transverses electron masses of AlAs at the X point were deduced to be m_t=0.21m₀ and m_l=1.04m₀, respectively.     

Impurity cyclotron resonance in InGaAs/AlAs superlattice under ultra high magnetic fields

We have carried out cyclotron resonance (CR) measurements of (InGaAs)₈/(AlAs)₈ superlattice (SL) to investigate electronic properties of the SL under pulsed ultra-high magnetic fields. The magnetic fields up to 160 T were generated by using the single-turn-coil technique. Clear CR signals were obtained in the transmission of far-infrared laser through the SL at room temperature and lower temperature. We observed a shift of CR peak to lower magnetic field caused by transition from free-electron CR to impurity CR below 90 K. Compared with the previous works of GaAs/AlAs SL, the peak shift was small and the transition temperature was low. This result suggests that a binding energy of the impurity in the InGaAs/AlAs SL is smaller than the GaAs/AlAs SL.     

Band discontinuity in GaAs/AlAs superlattices with InAs strained insertion-layers

We have theoretically investigated the valence-band discontinuity (ΔE_v) at the (100) GaAs/AlAs interface with the InAs strained insertion-layer. The theoretical calculation is carried out by the self-consistent tight-binding method with the sp³s* basis in the (GaAs)₅/(InAs)₁/(AlAs)₅/(InAs)₁ [100] superlattice. ΔEv at the GaAs/InAs(1ML)/AlAs interface is calculated to be 0.50 eV, which is practically equal to ΔEv = 0.51 eV at the GaAs/AlAs interface with no InAs layers. The insertion of the InAs monolayer changes the detail of valence charge density at the GaAs/AlAs interface but does not change ΔE_v. The result of calculation is in consistent with our experimental measurement by using the x-ray photoelectron spectroscopy.     

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)_n/(BAs)_n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.     

Photoreflectance and photoluminescence study of short period (GaAs)n/(AlAs)n superlattices with n = 1 – 15

Photoreflectance and photoluminescence measurements were performed in (GaAs)_n/(AlAs)_n (n = 1 – 15) short-period superlattices in the temperature range from 25 K to 275 K. Weak signals of photoreflectance associated with the critical point of the pseudodirect transition, weakly allowed direct transition arising from the zone-folding effect, have been observed as well as main signals associated with the direct allowed transitions. This assignment is supported by the measurement and analysis of the temperature dependence of the photoluminescence intensity.     

FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

Phonon-plasmon interaction in tunneling GaAs/AlAs superlattices

The phonon-plasmon interaction in tunneling GaAs _n /AlAs _m superlattices (m=5and 6≥n≥0.6 monolayers) was studied by Raman scattering spectroscopy. The interaction of optical phonons localized in GaAs and AlAs layers with quasi-three-dimensional plasmons strengthens as the thickness of GaAs quantum wells decreases and the electronic states in the superlattices become delocalized due to tunneling. It is assumed that the plasmons also interact with the TO-like phonon modes localized in quantum islands or in thin ruffled layers.     

Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

Properties of (BaTiO₃)₁/(BaZrO₃) _n ferroelectric superlattices (SLs) with n = 1?7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the Γ₁₅ polar phonon and phonons at the M point (for SLs with even period) or at the A point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed z-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.     

Determination of the roughness of heteroboundaries from photocurrent spectra of short-period AlAs/GaAs superlattices

Photocurrent spectroscopy is used to study the nature of the roughness of heteroboundaries in (AlAs)_m/(GaAs)_n short-period superlattices (m=3−5, n=10−13) grown by molecularbeam epitaxy. The formation of minibands broadens the optical spectra of superlattices in comparison with isolated quantum wells; therefore to analyze the degree of perfection of the boundaries we used the decay of the minibands into a series of discrete Wannier-Stark levels in an electric field parallel to the superlattice axis. Exciton lines were observed in the photocurrent spectra in an electric field corresponding to direct and indirect (in space) transitions between the Wannier-Stark levels. Comparison of experimental data with calculation indicates that even in the better structures, in addition to monotonic variation of the thickness of the layers over area, roughnesses in the heteroboundaries one monolayer in height are present with characteristic lateral dimension not exceeding 10 nm. Fiz. Tverd. Tela (St. Petersburg) 39, 2085–2089 (November 1997)     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

Spectroscopic studies of real space indirect symmetric GaAs/AlAs short period superlattices

The band structure and the optical transitions of symmetric ultra-short period (GaAs) _m /(AlAs) _n superlattices (n=m is the number of monolayers) have been studied using different spectroscopic techniques, namely photoacoustic spectroscopy, reflectivity and luminescence, photoluminescence excitation and high excitation intensity time-resolved luminescence. Direct observation of the transition from type I to type II energy band alignment is reported for superlattices whose configuration consists of more than 12 monolayers (i.e. m=n>12). The radiative recombination processes associated with the real space indirect transitions have been investigated as a function of the density of photogenerated electron-hole pairs, revealing an unusual density-dependent behaviour. A tentative interpretation in terms of a condensed electron-hole state at the indirect gap is given, which accounts for the long decay time of the type II luminescence at high excitation rate.     

不同量子阱宽度的InP基In0.53GaAs/In0.52AlAs高电子迁移率晶体管材料二维电子气的性能研究

用Shubnikov-de Haas(SdH)振荡效应，研究了在1.4 K下不同量子阱宽度(10—35 nm)的InP基高电子迁移率晶体管材料的二维电子气特性.通过对纵向电阻SdH振荡的快速傅里叶变换分析，得到不同阱宽时量子阱中二维电子气各子带电子浓度和量子迁移率.研究发现，在Si掺杂浓度一定时，阱宽的改变对于量子阱中总的载流子浓度改变不大，但是随着阱宽的增加，阱中的电子从占据一个子带到占据两个子带，且第二子带上的载流子迁移率远大于第一子带迁移率.当量子阱宽度为20 nm时，处在第二子能级上的电子数与处在 关键词： 量子阱宽 二维电子气 Shubnikov-de Haas振荡 高电子迁移率晶体管     

Time-resolved spectra and kinetics of the exciton photoluminescence in different types of GaAs/AlAs superlattices

The temperature dependence of the recombination dynamics of excitons is investigated by time-resolved photoluminescence spectroscopy in(GaAs)_n / (AlAs)_n superlattices, where n denotes the layer thickness in monolayers, for different types of band structures. In direct-gap superlattices with a layer thickness of the order or larger than the exciton Bohr radius, the carrier dynamics is dominated by the transfer from light-hole to heavy-hole excitons. When the layer thickness becomes smaller than the exciton radius, the dynamics of free excitons is controlled by localization. vb In quasi-direct and indirect-gap superlattices, the influence of lateral potential fluctuations due to interface roughness completely governs exciton recombination.     

Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

Local density valence band offset in GaAs/AlAs: Role of interface dipole

The valence band offset, ΔE_V ,at the lattice-matched GaAs/AlAs(001) interface is derived from highly precise self- consistent all-electron local density band structure calculations of the (GaAs)_n(AlAs)_n(001) superlattices (with n ⩽ 3). Using the core levels as reference energies, we find that ΔE_V = 0.50 ± 0.05 eV, in very good agreement with recent experimental results (ΔE_V = 0.45 − 0.55 eV). The dependence of ΔE_V on the superlattice thickness is studied and related to the interface charge redistribution which produces an interface dipole potential estimated to be ∼ 0.14 eV.     

Optical phonon damping in the ultrathin-layer GaAs/AlAs superlattices

We performed the precision measurements of the near-normal far-infrared reflection and transmission spectra of the MBE-grown (GaAs)_n/(AlAs)_n (n =  1, 2, 4) superlattices. The results obtained show a noticeable (almost twofold) increase in the AlAs-phonon damping when n is decreased from 4 to 1. We consider possible physical mechanisms of this increase and compare our results with those obtained by other authors. The main implication of the work is that the increase in the TO-phonon damping may be related to interface broadening whose role becomes more important when the superlattice period decreases.     

Wannier–Stark resonances in Zener tunneling diodes with GaAs/AlAs superlattices

We present DC transport measurements of the valence to conduction band (Zener) tunneling current in ap–i–ndiode with an ultrathin intrinsic layer containing a (GaAs)₅/(AlAs)₂multi-quantum well structure. According to recent theoretical predictions, the DC current should show maxima as a function of the reverse bias voltage that reflect the formation of Wannier–Stark resonances. So far, Wannier–Stark resonances have only been observed optically and never in a regime of strong Zener tunneling. Experimentally, we find the second derivative of the current-voltage characteristics to show a weak oscillatory structure indeed, indicating the existence of Wannier–Stark resonances in Zener tunneling.     

Spontaneous one-dimensional lateral alignment of multistacked InGaAs quantum dots on GaAs (n 1 1)B substrates

In_0.45Ga_0.55As/GaAs multistacking quantum dot (QD) structures were fabricated on a GaAs (n 1 1)B (n=2–4) substrate by metalorganic vapor-phase epitaxy. QDs spontaneously aligned in the [0 1 1] direction were observed on stacked QDs, whereas QDs were randomly distributed in the initial In_0.45Ga_0.55As layer growth. The formation mechanism of this self-alignment was studied by changing the number of In_0.45Ga_0.55As/GaAs multilayers and crystallographic arrangement. Photoluminescence spectra showing clear polarization dependence indicate carrier coupling in the QD arrays. This growth technique results in spontaneously aligned InGaAs QDs without any preprocessing technique prior to growth.     

Structural and optical properties of In0.5Ga0.5As/GaAs quantum dots in an In0.1Ga0.9As well using repeated depositions of InAs/GaAs short-period superlattices for the application of optical communication

We report structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) in a 100 Å-thick In_0.1Ga_0.9As well grown by repeated depositions of InAs/GaAs short-period superlattices with atomic force microscope, transmission electron microscope (TEM) and photoluminescence (PL) measurement. The QDs in an InGaAs well grown at 510 °C were studied as a function of n repeated deposition of 1 monolayer thick InAs and 1 monolayer thick GaAs for n=5–10. The heights, widths and densities of dots are in the range of 6–22.0 nm, 40–85 nm, and 1.6–1.1×10¹⁰/cm², respectively, as n changes from 5 to 10 with strong alignment along [1 −1 0] direction. Flat and pan-cake-like shape of the QDs in a well is found in TEM images. The bottoms of the QDs are located lower than the center of the InGaAs well. This reveals that there was intermixing—interdiffusion—of group III materials between the InGaAs QD and the InGaAs well during growth. All reported dots show strong 300 K-PL spectrum, and 1.276 μm (FWHM: 32.3 meV) of 300 K-PL peak was obtained in case of 7 periods of the QDs in a well, which is useful for the application to optical communications.