Interface corrugation effect on the polarization anisotropy of photoluminescence from (311)A GaAs/AlAs short-period superlattices

Studying GaAs/AlAs superlattices containing a quantum-well-wire array revealed photoluminescence polarization anisotropy for samples with GaAs layers less than 21 Å thick. It was found that polarization for a thickness of more than 40 Å was mainly due to valence band anisotropy, whereas polarization for a thickness of less than 21 Å was equally attributable to both valence band anisotropy and anisotropy associated with interface corrugation. For a GaAs layer thickness of less than 21 Å, a blueshift of the Γ electron-Γ heavy hole transition was observed. In this transition, the position of the peak of photoluminescence from the GaAs/AlAs (311)A superlattices containing a quantum-well-wire array is shifted toward higher energies compared to the (311)B and (100) superlattices containing no quantum-well wire with the same GaAs layer thickness. The conclusion was made that a blueshift is observed in GaAs/AlAs superlattices with GaAs layers less than 21 Å thick and a red-shift is observed when the thickness is larger than 43 Å.     

Raman study of confined TO phonons in GaAs/AlAs superlattices grown on GaAs (311)A and B surfaces

The use of Raman scattering in different polarization geometries makes it possible to observe the splitting of transverse optical (TO) phonon modes confined in GaAs/AlAs superlattices grown on faceted GaAs (311)A surfaces. The frequencies of TO modes with atomic displacements in the direction along the facets were observed to be higher than in the transverse one. Increased splitting, up to 3.5 cm ^− 1, was observed for (311)A superlattices when the average thickness of the GaAs layers was 6 monolayers or less. The splitting was absent in superlattices grown on (311)B surfaces under the same conditions. The effect of splitting is reputed to be caused by corrugation of GaAs/AlAs (311)A interfaces and formation of lateral superlattices or arrays of quantum wires, depending on the GaAs layer thickness.     

Photoluminescence and the structure of heterointerfaces of (311)A-and (311)B-oriented GaAs/AlAs superlattices

GaAs/AlAs superlattices grown simultaneously on GaAs substrates with the (311)A and (311)B orientations have been studied by photoluminescence and high-resolution transmission electron microscopy with a Fourier analysis of images. A periodic interface corrugation is observed for (311)B superlattices. A comparison of the structure of (311)A and (311)B superlattices indicates that the corrugation occurs in both cases and its period along the $[01\overline 1 ]$ direction is equal to 3.2 nm. The corrugation is less pronounced in (311)B superlattices, wherein it exhibits an additional modulation (long-wavelength disorder) with the characteristic lateral size exceeding 10 nm. The vertical correlation of regions rich in GaAs and AlAs, which is well observed in (311)A superlattices, is weak in (311)B superlattices due to the occurrence of long-wavelength disorder. The optical properties of (311)B superlattices are similar to those of (100) ones and differ radically from those of (311)A superlattices. As distinct from (311)B, strong photoluminescence polarization anisotropy is observed for (311)A superlattices. It is shown that it is the interface corrugation rather than the crystallographic (311) surface orientation that determines the optical properties of (311)A corrugated superlattices with thin GaAs and AlAs layers.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

Experimental discovery of the anisotropy of phonon-plasmon modes in GaAs/AlAs(100) superlattices

Doped (n-type) GaAs/AlAs superlattices with thicknesses of the GaAs and AlAs layers from 1.7 to 6.8 Å and 13.6 Å, respectively, have been studied by means of Raman spectroscopy. The use of a microattachment for Raman backscattering studies has allowed for the observation of the modes with the wave vector directed both across and along the superlattice layers (in the scattering from the side face of the superlattice). The theoretically predicted anisotropy of mixed phonon-plasmon modes caused by the anisotropy of the electron effective mass in the type II superlattices has been experimentally discovered.     

Anisotropy of long-wavelength optical phonons in GaAs/AlAs superlattices

The angular anisotropy of optical phonons in GaAs/AlAs (001) superlattices is investigated by Raman scattering spectroscopy. Scattering configurations allowed for phonons with wave vectors oriented along the superlattice layers and normally to them are used. For phonons localized in GaAs layers, the theoretically predicted mixing of the LO1 longitudinal modes with TO1 transverse modes in which atomic displacements occur along the normal to the superlattice is observed experimentally. These modes possess noticeable angular anisotropy. For transverse modes in which atoms move in the plane of the superlattice, the angular anisotropy is small.     

Observation of LO-phonon localization in GaAs quantum wires on faceted (311)A surfaces

The localization of longitudinal optical phonons in GaAs/AlAs lateral superlattices and quantum wires grown on faceted GaAs (311)A surfaces are investigated by means of Raman scattering spectroscopy. The frequencies of the localized phonons are found to decrease as the average thickness of the GaAs layer is decreased from 21 to 15 Å. As the GaAs thickness is decreased further to 11.3 and 8.5 Å, the frequencies of the localized phonons increases sharply in connection with the formation of an array of quantum wires. The frequencies calculated in a two-dimensional chain model agree with the experimental values. This makes it possible to interpret the increase in the frequencies of localized phonon states as being the result of the quantization of phonons in the array of one-dimensional objects. The results obtained support the model of GaAs (311)A surface faceting with a facet height of 10.2 Å. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 942–946 (25 June 1996)     

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)     

Cylindrical GaAs quantum wires incorporated within chrysotile asbestos nanotubes: fabrication and polarized optical absorption spectra

GaAs has been injected into chrysotile asbestos channels. GaAs quantum wires (nanocylinders) with diameters ∼ 6 nm have been observed in the channels by means of transmittance electron microscopy. Polarized optical absorption spectra of asb-GaAs (chrysotile asbestos containing GaAs wires) have been studied. A high anisotropy of the absorption has been observed, and intersubband transitions in the visible light region due to a strong quantum size effect have been found.     

Raman scattering from confined LO phonons and dispersion relation in GaAs/AlAs superlattices

The Raman scattering from both GaAs and AlAs confined LO phonons for several GaAs/AlAs superlattices is presented. The GaAs confined LO phonons were observed for two narrow GaAs layer samples at room temperature. The frequencies of observed phonons fit fairly well with the theoretical dispersion curves.     

FIRST-PRINCIPLE SELF-CONSISTENT PSEUDOPOTENTIAL CALCULATION OF THE ELECTRONIC STRUCTURES OF SHORT-PERIOD (GaAs)m(AlAs)n SUPERLATT1CES

With the local density approximation, the band structares of the short-period (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated by using the first-principle self-consistent pseudopotential method. The results show that the (GaAs)₁(AlAs)₁ superlattice is an indirect semiconductor, and the lowest conduction band state is at point R in the Brillouin zone; the (GaAs)₂(AlAs)₁ superlattice is a direct semiconductor and the lowest conduction band state is at point Γ. The squared matrix elements of transition are calculated. The pressure coefficients of energy gaps of the (GaAs)₁(AlAs)₁ and (GaAs)₂(AlAs)₁ superlattices are calculated and compared with those obtained by hydrostatic pressure experiments.     

Anisotropy of phonon-plasmon modes in GaAs/AlAs(311) superlattices

Doped GaAs/AlAs superlattices grown on the (311)A and (311)B surfaces have been studied using Raman spectroscopy. Phonon and phonon-plasmon modes with different directions of the wave vectors in the superlattice plane (i.e., the modes propagating in different lateral directions) have been observed in back-scattering from the superlattice face with the use of a Raman scattering accessory. Lateral anisotropy of mixed phonon-plasmon modes associated with structural anisotropy of the superlattice grown on the faceted (311)A surface has been experimentally revealed for the first time.     

Polaronic shift of the Wannier-Mott exciton binding energy in a quantum wire with allowance for the phonon confinement effect

Within the framework of the Li-Low-Pines model the interaction of a Wannier-Mott exciton with polar optical phonons in a cylindrical semiconductor wire is studied, taking into account the phonon confinement effect. An analytical expression for the exciton binding energy with allowance for the polaronic effect is obtained. Numerical calculations of the binding energy are carried out for AlAs/GaAs/AlAs and ZnSe/CdSe/ZnSe wires with a various degree of polarity of quantum wire materials. The polaronic shift of the binding energy of light and heavy hole excitons is calculated.     

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.     

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.     

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.     

Polaron effects on third-harmonic generation in cylindrical quantum-well wires

Polaron effects on third-harmonic generation (THG) in cylindrical quantum wires with a finite confining potential are investigated. The THG coefficient is obtained by using the compact-density-matrix approach and an iterative method, and the numerical results are presented for GaAs/AlAs cylindrical quantum wires. The results show that the THG coefficient is greatly enhanced and the peak shift to the aspect of high energy when considering the influence of electron-phonon interaction.     

Optical detection of magnetic resonance and of exciton-level anticrossing in quantum wells and superlattices

Recent studies of excitonic and electron-hole recombination in GaAs/AlAs quantum wells and superlattices are reported using microwave-optical spectroscopy of low-dimensional structures developed by the authors. Fiz. Tverd. Tela (St. Petersburg) 41, 888–890 (May 1999)     

Ab Initio calculations of the anisotropic dielectric tensor of GaAs/AlAs superlattices

The static dielectric properties of (001)(GaAs)(p)/(AlAs)(p) superlattices have been calculated as a function of their period p for 1< or = p < or =12, starting from density-functional theory. The interplay between quantum confinement and local field effects is shown to be crucial. For light polarized in the growth direction it leads to the otherwise surprising justification of the use of a classical effective medium theory, even for the smallest periods. Only the inclusion of both contributions allows in ab initio and in semiempirical calculations to reproduce the experimentally observed birefringence.     

GaAs/AlAs super-flat interfaces in GaAs/AlAs and GaAs/(GaAs) (AlAs) quantum wells grown on (4 1 1)A GaAs substrates by MBE

Effectively atomically flat GaAs/AlAs interfaces over a macroscopic area (“super-flat interfaces”) have been realized in GaAs/AlAs and GaAs/(GaAs) (AlAs) quantum wells (QWs) grown on (4 1 1)A GaAs substrates by molecular beam epitaxy (MBE). A single and very sharp photoluminescence (PL) peak was observed at 4.2 K from each GaAs/AlAs or GaAs/(GaAs) (AlAs) QW grown on (4 1 1)A GaAs substrate. The full-width at half-maximum (FWHM) of a PL peak for GaAs/AlAs QW with a well width ( ) of 4.2 nm was 4.7 meV and that for GaAs/(GaAs) (AlAs) QW with a smaller well width of 2.8 nm (3.9 nm) was 7.6 meV (4.6 meV), which are as narrow as that for an individual splitted peak for conventional GaAs/AlAs QWs grown on (1 0 0) GaAs substrates with growth interruption. Furthermore, only one sharp peak was observed for each GaAs/(GaAs) (AlAs) QW on the (4 1 1)A GaAs substrate over the whole area of the wafer (7 7 mm ), in contrast with two- or three-splitted peaks reported for each GaAs/AlAs QW grown on the (1 0 0) GaAs substrate with growth interruption. These results indicate that GaAs/AlAs super-flat interfaces have been realized in GaAs/AlAs and GaAs/(GaAs) (AlAs) QWs grown on the (4 1 1)A GaAs substrates.