Tamm interface states in ZnSe/BeTe periodic heterostructures

The spectral response of the lateral optical anisotropy of periodic undoped type-II ZnSe/BeTe heterostructures with nonequivalent interfaces was studied by ellipsometry. The spectra revealed two types of features corresponding to optical transitions with energies lying in the bandgap. The position of features of the first type does not depend on the heterostructure period. Features of the second type shift toward lower energies with decreasing period of the heterostructure. This behavior is explained in terms of a model taking into account the existence of electronic and hole interface states and of a mixed-type interface state.     

Giant blue shift of photoluminescence in strongly excited type-II ZnSe/BeTe superlattices

A giant blue shift (≈0.5 eV) and a large decrease in the emission time of a spectral band corresponding to radiative recombination of spatially separated electrons and holes are observed in ZnSe/BeTe superlattices at high laser excitation levels. On the basis of numerical calculations, the observed defects are attributed to band bending arising in type-II structures at high carrier density. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 351–356 (10 September 1997)     

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.     

Picosecond carrier relaxation in type-II ZnSe/BeTe heterostructures

Relaxation of photoexcited carriers in the course of the formation of spatially separated layers of electrons and holes in type-II ZnSe/BeTe heterostructures has been studied based on a high time resolution investigation of fast luminescence kinetics. The hole escape times τ from the ZnSe layer have been measured in structures with different ZnSe layer thicknesses (τ = 2.5, 7.5, and 23 ps for thicknesses d = 10, 15, and 20 nm, respectively). It is shown that the increase in the time τ can be explained by the fact that the escape rate of photoexcited holes from the lowest above-barrier level in the ZnSe layer into the BeTe layer decreases as the thickness of the ZnSe layer increases.     

Optical anisotropy of surface-emitting ZnSe/BeTe LEDs

The electric field-induced in-plane optical anisotropy (Pockels effect) of ZnSe/BeTe multi-layered heterostructures has been studied by analysing the linear polarization of the spatially indirect photoluminescence. A pronounced quantum-confined Pockels effect was found in ZnSe/BeTe double-barrier structures. A tight-binding model is proposed which consistently interprets the experimental findings and suggests a new mechanism for the effect as realized in type-II heterostructures. The model takes into account that the ZnSe/BeTe heterosystem exhibits a type-II band alignment with large band offsets and that in the zinc-blende lattice the chemical bonds are oriented in 〈111〉 directions and, when shifted along the [001] principal axis, alternatively change their orientation in the (001) plane from [1 1 0] to [110] and vice versa. A light-emitting diode based on a single ZnSe/BeTe interface is demonstrated.     

Confined acoustic and propagating optical phonons in GaN/Ga1-xAlxN superlattices

The lattice vibrations in GaN/Ga_1-xAl_xN(001) superlattices are studied within a rigid-ion model. The elastic forces are characterized by a generalized three-parameter Keating model, whereas the microscopic electric field is fully included via an Ewald summation. The composition and the disorder in the barriers are described within a generalized REI model. Cubic symmetry is assumed for each single material layer in the superlattice. In contrast to the GaAs/Ga_1-xAl_xAs case we observe a confinement of the acoustic phonons in the frequency region between the zone-boundary phonons of GaN and AlN. In the region of optical phonons both propagating and confined modes are found. The results are explained in terms of the light common anion of the compounds.     

MOVPE Growth and optical characterization of ZnSe/ZnS strained layer superlattices

We have grown ZnSe/ZnS superlattices using low pressure MOVPE. The superlattices were grown onto a relaxed ZnSe buffer and were constituted of 45 periods. The well and barrier thicknesses were chosen from the calculation of critical thicknesses for coherent and free standing situations. Following this analysis we have grown samples with individual ZnSe, ZnS thicknesses of 3 to 9 monolayers.The optical properties of the samples were studied using photoluminescence, photoreflectance and reflectivity experiments. The results of the photoluminescence and photoreflectance experiments are consistent with a free standing model of the strain state, and a typical sketch of potential profile, describing the band structure of such superlattices is proposed. The photoluminescence linewidth is analyzed in terms of interface roughness. The asymmetry of the photoluminescence peaks is modelled using a density of state with a low energy exponential tail, due to the competition between radiative recombination and non-radiative recombination mechanisms. In some samples of lower crystalline quality, bound excitons were observed in the near band edge photoluminescence, which we attribute to impurity interdiffusion, which is favoured by crystalline defects.     

Anisotropy of optical constants of ZnSe/BeTe heterostructures with no common atoms at the interfaces

The in-plane anisotropy of the refractive index and absorption coefficient in the blue-green spectral region in ZnSe/BeTe type-II heterostructures with no common atoms at the interface has been studied by ellipsometry in reflection. It was established that the relative difference (anisotropy) in the refractive index and absorption coefficient remains nonzero throughout the range covered and reaches 0.6% for the refractive index and 85% for the absorption coefficient. It was found that, in contrast to excitonic transitions involving a heavy hole, the anisotropy in absorption for the light-hole exciton is larger in magnitude and is of the opposite sign.     

Formation of metastable above-barrier hole states in ZnSe/BeTe type II heterostructures under high-density optical excitation

A considerable slowing down of the luminescence kinetics of the direct optical transitions has been discovered in ZnSe/BeTe type II heterostructures under high-density optical pumping by femtosecond laser pulses. The effect is attributed to the potential barrier that appears due to the strong band bending at a high density of spatially separated photoexcited carriers and forms a metastable above-barrier hole state in the ZnSe layer. This yields a longer energy relaxation time of the holes migrating to the adjacent BeTe layer. The experimental results agree well with the numerical calculations.     

Interaction of electrons with polar optical phonons in semiconductor superlattices

On the basis of the microscopic theory of lattice dynamics, simulation of the electric potentials created by optical phonons in semiconductor superlattices is performed. It is shown that the spatial distribution of the amplitudes of electric potentials differs from that in a dielectric continuum predicted by the conventional macroscopic model without dispersion. A modified macroscopic continuum theory is proposed that takes into account the dispersion of short-range interatomic forces and allows one to obtain analytical expressions for the potentials of electron-phonon interaction.     

Surface phonons in superlattices

We report the existence of surface localized phonons for a superlattice consisting of alternating slabs (parallel to the surface) of two different crystals. The superlattice has a larger periodicity in the direction perpendicular to the slabs and therefore many phonon branches in the folded Brillouin zone. In the gaps existing between these phonon branches appear the surface localized modes.     

Influence of chemical composition on confinement and interface character of optical phonons in GaAs/Ga1−xAlxAs superlattices

The influence of alloying on optical vibrations in GaAs/Ga_1−xAl_xAs superlattices is theoretically studied. To calculate frequencies and effective atomic displacements two different approaches are applied. The random-element-isodisplacement model is generalized for layered structures under inclusion of the macroscopic electric field. Parallel to it vibrational properties are studied by means of numerical simulations of alloying and randomness. Two effects, confinement versus folding and interface character, are discussed in particular.     

Acoustic phonons localized at an interface between superlattice and liquid

The vibrational modes localized at a superlattice–liquid interface are studied theoretically. We calculated the phonon transmission rate and found the peculiar peak due to the interface-localized mode. The magnitude of this peak is governed by the number of constituent bilayers (N) of superlattice. We derived the equation determining N=N_max for which the transmission rate takes the maximum value for a given combination of the superlattice and liquid. When N=N_max, the interface-localized mode has the same feature inside the superlattice as the surface-localized mode generated in the semi-infinite superlattice.     

Anisotropy of optical phonons in semiconductor superlattices: Raman scattering experiments

Experiments on Raman scattering in the “forward” geometry, permitting observation of anisotropy of the optical phonons, are performed on specially prepared short-period GaAs/AlAs superlattice structures with the substrates removed and the surfaces covered with an antireflective layer. The experimental data agree well with the computational results obtained for the angular dispersion of optical phonons in superlattices on the basis of a modified continuum model. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 50–55 (10 July 1998)     

Excitonic processes and lasing in ZnSSe/ZnSe superlattices

We report the results of a systematic investigation of the stimulated emission in ZnSSe/ZnSe superlattices, by time resolved luminescence and high excitation intensity magaetoluminescence. The stimulated emission is found to have excitation origin from localized states around the thresold, and free-carrier origin at carrier densities well above the phase space filling thresold of the exciton.     

Confined longitudinal and transverse phonons in GaAs/AlAs superlattices

The phonon spectrum of GaAs/AlAs superlattices along the (001) growth direction is calculated using a new approach based on a realistic unified treatment of the interactions in both bulk constituents, which naturally lends itself to dealing with superlattice geometries. Results for a (GaAs)₃(AlAs)₃ superlattice are shown with emphasis on different aspects of confinement.