ZnSe-ZnS Strained-layer Superlattices Research Articles

Effects of temperature and ZnSe well-layer thickness on PL in ZnSe/ZnS strained-layer superlattices

A study of the photoluminescence (PL) spectra in ZnSe–ZnS strained layer superlattices (SLSs) with various ZnSe well-layer thicknesses was investigated by PL measurements. An intense excitonic-emission line, and no emissions due to deep levels, were observed. As the ZnSe well-layer thickness is decreased, the peak of the line largely shifted towards the higher-energy side. This behavior may be related to the quantum size effect. In the temperature dependence of PL intensity, this is the thermal quenching process, which may be related to the thermal release of excitons in the quantum well. As the thickness of the ZnSe well-layer is decreased, the activation energy increased.

Growth of znse/zns strained-layer superlattice on si substrates by atomic layer epitaxy

High-quality ZnSe-ZnS strained-layer superlattices were successfully grown on Si substrate by atomic layer epitaxy using a metal-organic chemical vapor deposition system. The characteristics of the ZnSe-ZnS strained-layer superlattices were investigated. Using photoluminescence measurements, an intense excitonic emission line and no emission due to deep levels were observed. As the ZnSe well-layer thickness increased, the peak of the line shifted markedly towards the lower-energy side. This behavior may be related to the quantum size effect. As the thickness of the ZnSe well-layer decreased, the activation energy increased. The ZnSe thickness is less than the three-dimensional exciton Bohr radius in bulk ZnSe. It is proposed that the dependence of the activation energy is due to two-dimensional exciton behavior in quantum wells.

Raman scattering study of the longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices

Raman scattering studies were performed on ZnSe–ZnS strained-layer superlattices with different strains. In the optical phonon regime, the Raman spectra of the ZnSe–ZnS superlattices with a superlattice axis along [001] show optical phonons confined in the ZnSe well and ZnS barrier layers, and display a two-mode behavior corresponding to ZnSe-like and ZnS-like modes. We report the experimental results, by means of Raman scattering of confined longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices. The Raman frequency shift dependence on the layer thickness, superlattice period, and interface strain is presented and discussed.

Structural characteristics and higher-order zone-folded phonon modes in ZnSe-ZnS strained-layer superlattices

We have observed higher-order (up to the 5th order) zone-folded phonon modes in ZnSe-ZnS strained-layer superlattices by means of Raman scattering spectroscopy. The structural characterizations of both the fluctuation of the periodicity and the roughness of the interface were obtained from the images of the transmission electron microscopy. The Raman spectrum of the zone-folded modes is well explained by the theoretical calculation which takes account of the two structural characteristics. Furthermore, we found that the strain does not play a dominant role in the Raman spectrum of the zone-folded modes.

The study of lattice strain and high-resolution transmission electron microscopy in ZnSe-ZnS strained-layer superlattices

The structural properties of ZnSe-ZnS strained-layer superlattices (SLSs) grown on GaAs (100) by atmospheric-pressure metalorganic chemical vapor deposition (AP-MOCVD) were studied using high-resolution transmission electron microscopy (HRTEM). The type and distribution of the defects in ZnSe-ZnS SLS are related to the surface quality of GaAs substrate, of the buffer layer and the thickness of each layer for ZnSe and ZnS in SLS. In this work we noticed that there are stacking faults (SFs), mismatch defects (MDs) and microtwins (MTs) in ZnSe 1− x S x buffer layer at interfacial steps due to the GaAs surface not being smooth, and the dislocation can penetrate the ZnSe-ZnS SLS layer.

Optical absorption of ZnSeZnS strained-layer superlattices

Optical absorption coefficient for vertical valence-band-to-conduction-band transitions in ZnSeZnS strained-layer superlattices is studied theoretically. The calculations of the electronic states near the band edges are carried out by using the Kronig-Penney model with the Luttinger-Kohn Hamiltonian. It is shown that the overall features of the absorption spectrum observed by Shen et al. are explained fairly well, and it is predicted that a change of polarization for incident photons produces a significant modification in the absorption spectrum.

MOVPE Growth and optical characterization of high-quality ZnSe-ZnS superlattices using a novel zinc adduct precursor

ABSTRACTA novel zinc precursor adduct tetramethylmethylene diamine: dimethylzinc (TMMD:DMZn) has been used to grow ZnSe-ZnS strained layer superlattices by low pressure MOVPE at 300'C. The use of this new precursor leads to improved material purity and lower homogeneous gas phase premature reaction than in the case of dimethylzinc. Optical characterization has been made between liquid helium and room temperature. The 2K photoluminescence lines range between 2.85 eV and 3.15 eV. The 2K photoluminescence is characterized by a linewidth at half maximum ranging from 13-45 meV. Additional reflectivity and photoreflectance experiments have been carried out to measure light-hole excitons. A variational calculation of the Rydberg energies has been performed using a sophisticated trial function. The experimental data is in agreement with the theoretical value when a small conduction band offset is used.

Combined Raman/HREELS study of ZnSeZnS strained-layer superlattices

Abstract ZnSeZnS strained-layer superlattices have been investigated by combining Raman scattering and high-resolution electron energy loss spectroscopy (HREELS). The superlattices were grown on GaAs(001) by MOCVD without a buffer layer. Epitaxial growth of the superlattices was confirmed by the polarization selection rule of Raman scattering. Energy differences between the bulk (ZnS, ZnSe) LO phonons and the superlattice LO phonons indicate that strain is mainly localized in the ZnS layers. In HREELS the loss peak originating from ZnS must be assumed to be appreciably weaker than that from ZnSe. This result does not agree with the theoretical prediction based on the dielectric theory. We conclude that the reduction of the electron scattering intensity results from large strain in the ZnS layers.

Combined Raman/HREELS study of ZnSeZnS strained-layer superlattices

ZnSeZnS strained-layer superlattices have been investigated by combining Raman scattering and high-resolution electron energy loss spectroscopy (HREELS). The superlattices were grown on GaAs(001) by MOCVD without a buffer layer. Epitaxial growth of the superlattices was confirmed by the polarization selection rule of Raman scattering. Energy differences between the bulk (ZnS, ZnSe) LO phonons and the superlattice LO phonons indicate that strain is mainly localized in the ZnS layers. In HREELS the loss peak originating from ZnS must be assumed to be appreciably weaker than that from ZnSe. This result does not agree with the theoretical prediction based on the dielectric theory. We conclude that the reduction of the electron scattering intensity results from large strain in the ZnS layers.

Optical properties of the subband in ZnSe-ZnS strained-layer superlattices on transparent subtrate CaF2 under high excitation

Good quality ZnSe-ZnS strained-layer superlattices (SLSs) on transparent substrate CaF2(111) have been successfully Fabricated by atmospheric pressure metallorganic chemical vapor deposition (Ap-MOCVD). The satellite peaks observed by X-ray diffraction measurement were indicative of superlattice structures. Photoluminescence (PL) spectra of SLS measured at 77–300 K showed strong excitonic emission. The n=2 heavy-hole excitonic emission in the SLS, either with ZnS buffer layer or without buffer layer were obtained. The emission intensity of n=1 heavy-hole exciton was compared with the emission intensity of an n=2 heavy-hole exciton for different excitation levels. Excitonic saturation for the n=1 exciton and excitonic thermal dissociation for the n=2 exciton were observed under high excitation. In absorption spectra n=1, 2 heavy hole are observed for narrow-well samples.

Absorption spectra of ZnSe-ZnS strained-layer superlattices grown on (001) GaAs by molecular beam epitaxy

The absorption spectra of ZnSe-ZnS strained-layer superlattices were measured at temperatures ranging from 16 K to room temperature. Three excitonic absorption peaks corresponding to 1E-1HH, 1E-1LH, and 1E-3HH transitions were observed for the first time.

Band alignments in Zn(Cd)S(Se) strained layer superlattices

The authors estimate, by reviewing the available photoluminescence data, the band alignments at heterojunctions of Zn(Cd)S(Se) semiconductors. The photoluminescence peak energies of ZnSe-ZnS, CdS-ZnS and CdSe-ZnSe strained layer superlattices (SLS), with a range of well and barrier widths, have been calculated using Kronig-Penney theory and assumed values of the band offsets which yield acceptable fits to both the experiments and data taken from the literature. For ZnSe-ZnS SLS, fair agreement is obtained between calculation and experiment by assuming, in line with previous work, a negligible conduction band offset. At the same time, good estimates of the photoluminescence peak energies of CdS-ZnS and CdSe-ZnSe superlattices are obtained by assuming a negligible valence band offset in each case. The authors construct a band alignment diagram which predicts that CdSe-CdS and CdS-ZnSe are type-II SLS with electron confinement in the CdS layers; these predictions are borne out by experiment.

New characterization method of biaxial stress by Raman scattering: demonstration in ZnSe-ZnS strained-layer superlattices

We proposed a new characterization method of anisotropic stress by means of Raman scattering, demonstrated in ZnSe-ZnS strained-layer superlattices. We used the Raman microprobe technique with the incident light parallel as well as perpendicular to the interface plane. The optical phonon modes split into two types, i.e., a singlet and a doublet, by the built-in biaxial stress. The method of this work will become a new characterization tool for studying the anisotropic stress in semiconductor heterostructure.

Hydrostatic-pressure-induced type-I → type-II conversion in ZnSe-ZnS strained-layer superlattices

A conversion from type-I to type-II under hydrostatic pressure in band lineups of a wide-bandgap ZnSe-ZnS strained-layer superlattice is observed for the first time. This type conversion results from a Г-point crossover of ZnSe well and ZnS barrier conduction bands and occurs at approximately 31 kbar. From the crossover pressure, the conduction-band offset is estimated to be 68.8 meV. This value is consistent with the result of a Kronig-Penney analysis of our photoluminescence excitation measurements and can be supported by semi-quantitative calculations that modify the model-solid theory.

Excitonic transition of (ZnSe-ZnS)/CaF 2 strained-layer superlattices under lower excitation

A photoluminescence (PL) study of ZnSe-ZnS strained-layer superlattice (SLS) on a transparent substrate, CaF 2, grown by atmospheric pressure metalorganic chemical vapour deposition (AP-MOCVD) is presented. Two absorption bands and three emission bands in the SLS were observed for the first time under low excitation. The origin of some bands might be ascribed to the excitonic transition of n = 1 heavy and light hole.

Optical properties of [formula omitted] strained-layer superlattices

We present a study of the optical properties of ZnSe ZnS strained-layer superlattices (SLSs) by absorption, transient luminescence and Raman scattering measurements. In the absorption measurements, three absorption peaks corresponding to 1E-1HH, 1E-1LH and 1E-3HH transitions were observed up to room temperature. From the transient luminescence measurements, the exciton lifetime and exciton formation time were determined. We have also observed folded longitudinal acoustic (LA) phonon modes in the SLSs by room temperature Raman scattering measurements.

Type conversion under hydrostatic pressure in ZnSe-ZnS strained-layer superlattices.

A conversion from type-I to type-II in a ZnSe-ZnS strained-layer superlattice under hydrostatic pressure is observed. The hydrostatic-pressure dependence of the integrated intensity and the linewidth of the n=1 heavy-hole exciton emission spectra is represented by a couple of straight lines with a clear kink at about 31 kbar. Moreover, the emission peaks appear below the band-gap energy of the ZnSe well layers above 31 kbar. All of these features are well explained by the type conversion associated with the \ensuremath{\Gamma}-\ensuremath{\Gamma} conduction-band crossover between the ZnSe well and ZnS barrier layers.

Biaxial splitting of optical phonon modes in ZnSe-ZnS strained-layer superlattices

Raman scattering studies were performed on ZnSe-ZnS strained-layer superlattices with the incident light parallel as well as perpendicular to the interface plane. We found for the first time that the optical phonon modes split into two types, that is a singlet and a doublet, by the built-in biaxial stress. A new method to characterize the directional stress is demonstrated.

Determination of refractive index and study of absorption in wide gap II–VI semiconductor superlattices

The refractive indicies of ZnSe-ZnTe and ZnSe-ZnS strained-layer superlattices (SLSs) were determined by double-beam reflectance measurements at 300 K. A discontinuity was found in the index spectra at about the energy equal to the transition energy E 1(e−h) between n=1 confined electron and hole levels. Large difference was found between the indices of the SLSs and those of the compositional bulk materials. Using the effective infinite-well model within effective mass approximation and taking into account the large strain effect, we also calculated the shifts of the exciton peaks with applied electric fields for ZnSe-ZnS multiple quantum wells (MQWs).

II–VI widegap superlattices

We review our recent results of the excitonic properties in ZnSeZnS and Cd xZn 1−xSZnS strained-layer superlattices (SLSs). The most important physical insights in the II–VI widegap superlattices are to understand the relationship between the optical properties of quasi-two-dimensional exciton and strain because the well layer frequently receives biaxial compression or tension. The strain thus causes the significant shifts of the bandgap and splitting of the valence band. Semi-quantative calculations lead to an expectation that ZnSeZnS SLS always exhibits a type I band lineup within 100 Å thicknesses of the ZnSe well at a constant ZnS barrier width of several tens angstrom. This is in good agreement with the experimental results of exciton absorption and its luminescence excitation spectra. The Cd 0.3Zn 0.7SZnS SLSs with a range of well widths can produce intense excitonic emissions around 3.4 eV at room temperature due to the quantum confinement of excitons in the ternary CdZnS well. In order to elucidate localisation and relaxation processes of excitons, we have for the first time reported a multiple-LO-phonon emission process in the excitation spectra. The electric-field studies suggest that the concomitant decrease in intensity and the energy downshift of the exciton line may originate from the quantum confined Stark effect.