Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

Elementary excitations in multiple quantum wire structures

We calculate the plasmon dispersion of electrons in multiple quantum wire structures. Wave function overlapping effects between different wires are neglected. The Coulomb interaction potential is calculated for a model with circular wire area. Analytical results for the excitation spectrum of electron multiple quantum wire structures are obtained within an one-subband model. Landau damping of intrasubband plasmons is discussed. Results for an electron superlattice within a two-subband model are presented and the coupling of intersubband plasmons with intrasubband plasmons is calculated. We compare the theoretical results with recent Raman measurements of intrasubband plasmons in Al _x Ga_1–x As/GaAs wire superlattices. The plasmon dispersion for boson multiple quantum wire structures also is calculated.     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

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.     

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.     

Plasmons in double quantum wells in a parallel magnetic field

Collective intraband charge-density excitations in the quasi-two-dimensional electron system of double GaAs/AlGaAs quantum wells in an external parallel magnetic field B_∥ are studied by inelastic light scattering. It has been found that the energy of the excitations under study (acoustic and optical plasmons) exhibits anisotropy depending on the mutual orientation of B_∥ and the excitation quasi-momentum k. It is shown theoretically that, in a strong parallel magnetic field, the effects associated with the finite width of the quantum wells dominate over the effects associated with interlayer tunneling and determine the anisotropy of plasmons. The experimental data are compared with a theoretical calculation.     

Intervalley electron scattering by phonons in (GaAs)<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>(AlAs)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(001) ultrathin superlattices

The electron scattering by short-wavelength and long-wavelength phonons in (GaAs) _m (AlAs) _n (001) superlattices with ultrathin layers (n, m = 1, 2, 3) has been investigated using the pseudopotential method and the phenomenological model of bonding forces. The deformation potentials have been found for intervalley electron transitions in the conduction bands of the superlattices and solid solutions of the corresponding compositions. It has been shown that, owing to the localization of the wave functions in the quantum wells Γ, L, and X, the intensity of intervalley electron transitions in the superlattices, as a whole, is higher than that of similar transitions in the solid solutions. As the content of light Al atoms in the superlattices increases, the deformation potentials monotonically increase for the X-X transitions and decrease for the L-L and X-L transitions. The potentials of the Γ-X and Γ-L transitions change nonmonotonically depending on the layer thickness due to the pronounced quantum-well effects in the deep Γ quantum wells of GaAs. The deformation potentials averaged over phonons and related valleys in the superlattices are close to the corresponding potentials in the solid solutions.     

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)     

Phonon dispersion curves of GaAsAlAs superlattices

The phonon properties of (GaAs)_m(AlAs)_n superlattices are studied with an eleven-parameter rigid-ion model. Short-range interactions up to the second neighbours are included, and the long-range Coulomb interaction is calculated exactly. Modes propagating both normal (k_// = 0) oblique (k_// ≡ 0) to the interfaces are studied. Anisotropy of zone center optical phonons is examined. The theoretical results are compared with the existing experimental data with favorable agreement.     

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.     

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.     

A study of the GaAs/AlAs(001) superlattice spectrum within the abrupt- and smooth-boundary models

The electron states and miniband spectra of (GaAs) _m (AlAs) _n (001) superlattices related to the Г- and X _z -valleys of conduction bands of bulk GaAs and AlAs crystals are considered within the framework of simplified abrupt- and smooth-heteroboundary models. The models are constructed on the basis of calculations made by the pseudopotential method. It is shown that the proposed models describe the results of the corresponding exact calculations in the approximation of the abrupt and smooth boundaries reasonably well. It is found that the difference between the smooth-boundary and abrupt-boundary models becomes pronounced in the case of superlattices with thin layers, where this difference results in a qualitatively different dispersion of lower minibands.     

Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

Suppression of the singularly localized states in dimer quasiperiodic Fibonacci superlattices

Using the transfer-matrix technique, we have numerically investigated the effect of introducing the dimer on the nature of the states across Dimer Fibonacci semiconductor superlattices on the miniband structure of the GaAs/Al_xGa_1?xAs superlattices. By the introduction of the dimer model, the transmission spectra reveal the appearance of a miniband structure with a concomitant disappearance of the singularly localized states. This behavior is due to the interaction between the states of the dimer wells inside the potential and, therefore, the system is seen by the particle as two overlapped ordered structures.     

Hot electron cooling in parabolic and modulation doped quantum wells and doped superlattices

Hot electron cooling in variously structured and doped quantum wells and superlattices has been studied by low temperature steady-state photoluminescence. A parabolic quantum well realized by thickness grading of Al_0.3Ga_0.7As and GaAs epitaxial layers deposited by molecular beam epitaxy with electron level spacings of ∼25 meV did not show increased electron plasma temperatures compared to thick epitaxially deposited GaAs or square quantum wells with electron level spacings greater than the LO phonon energy of GaAs; this implies that mechanisms involving intersubband Δk ≠ 0 transitions and interfacial recombination are dominant in the parabolic structure. Investigations as a function of carrier concentration in modulation-doped quantum wells and n-type superlattices with strong miniband formation indicate that increasing the carrier concentration in either structure above ∼ 5 × 10¹⁷ cm^-3 significantly increases the electron plasma temperatures, even under low light excitation, suggesting that such structures may be suited for high efficiency hot electron photovoltaic and photoelectrochemical cells.     

Metal enhanced photoluminescence of near-infrared CdTexSe1−x quantum dots

High-quality alloyed near-infrared CdTe_xSe_1?x quantum dots were synthesized by a modified organometallic method. The emission wavelength of the alloyed quantum dots were turned from visible to near-infrared range by changing the composition of the precursor. The photoluminescence intensity of the alloyed quantum dots was further enhanced by coupling through localized surface plasmons from Au nanoparticles. The alloyed CdTeSe quantum dots coupled with Au nanoparticles exhibited a 4 times photoluminescence enhancement than that of bare CdTeSe quantum dots by turning the localized surface plasmons resonant absorption of Au nanoparticles consistent to the excitation wavelength. This method will be beneficial for the potential applications in the biological imaging and detection.     

Evidence for interlayer donor–acceptor recombination in type II GaAs/AlAs superlattices

Steady-state and time-resolved photoluminescence of (GaAs)₇(AlAs)₉type II superlattices grown simultaneously by molecular beam epitaxy on (311)A and (100) GaAs substrates, intentionally undoped or uniformly doped with silicon, has been studied. It is shown that at temperatures T >  30 K, the dominant line in the photoluminescence spectra of superlattices is caused by donor–acceptor recombination between the donors located in the AlAs layers and the acceptors in the GaAs layers. The sum of the binding energies of the donors and acceptors in the pairs has been determined. A spectrally-dependent linear polarization of the donor–acceptor line along the direction of the interface corrugation of the superlattice has been discovered in the spectra of (311)A-oriented superlattices.     

Splitting of transverse optical phonon modes localized in GaAs quantum wires on a faceted (311)A surface

The energy splitting of fundamental localized transverse optical (TO1) phonon modes in GaAs/AlAs superlattices and quantum wires grown by molecular-beam epitaxy on a faceted (311)A GaAs surface is observed by Raman spectroscopy. The form of the Raman scattering tensor makes it possible to observe the TO_x and TO_y modes separately, using different scattering geometries the y and x axes are the directions of displacement of the atoms and are directed parallel and transverse to the facets on the (311)A surface). Enhancement of the splitting of the TO1_x and TO1_y modes is observed as the average thickness of the GaAs layers is decreased from 21 to 8.5 Å. The splitting is probably due to the effect of the corrugation of the GaAs/AlAs (311)A hetero-interface on the properties of localized phonon modes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 45–48 (10 July 1997)     

Resonant electron tunneling in GaN/Ga1−x AlxN(0001) strained structures with spontaneous polarization and piezoeffect

Electron tunneling through the GaN/Ga_1?x Al_xN(0001) wurtzite strained structures is investigated by the pseudopotential and scattering matrix methods. It is shown that the results of multiband calculations at low aluminum concentrations (x<0.3) are adequately described within the single-valley model in the envelope wave function method accounting for the dependences of the effective mass on the energy and strain. Upon electron tunneling through two-barrier structures, sharp resonance peaks are observed at a barrier thickness of several monolayers and the characteristic collision time in the resonance region is equal to ～1 ps. The internal electric fields associated with spontaneous and piezoelectric polarizations lead to a “red” or “blue” shift in the resonance energy according to the thickness and location of barriers with respect to the polar axis. In the (GaN)_n(Ga_1?x Al_xN)_m superlattices, the internal fields can form the Stark ladder of electronic states at a small number of ultrathin layers even in the absence of external fields.