Yu.G. Sadofyev

MBE growth and characterization of ZnTe epilayers and ZnCdTe/ZnTe structures on GaAs(1 0 0) and ZnTe(1 0 0) substrates

Abstract Deposition of an amorphous ZnTe seed layer with a thickness of 10xa0nm followed by its solid-phase crystallization was done before the beginning of molecular beam epitaxy of ZnTe layers on GaAs(1xa00xa00) substrates. RHEED patterns have proved that a creation of three-dimensional formations is avoided, and two-dimensional growth occurs during the early stage of epitaxy. Cathodoluminescence (CL) and X-ray measurements have evidenced a higher quality of ZnTe layers grown with an amorphous ZnTe seed layer. ZnTe substrates made of ZnTe crystals grown from vapor phase have been used for the epitaxy of ZnTe layers and ZnCdTe/ZnTe structures also. CL comparison of homo- and heteroepitaxial layers and structures shows the superiority of epilayers and structures grown on ZnTe(1xa00xa00) substrates.

Multiphonon relaxation in ZnSe thin films and ZnSe/ZnCdSe MQW structures

The photoluminescence and resonant Raman scattering spectra of thin films and multiple-quantum-well (MQW) structures were investigated at room temperature. The light emission spectra of MQWs consist of an intensive band of the quantum well luminescence and additional narrow bands. The energy shift of these additional bands was equal to a multiple of the longitudinal phonon value of the strained ZnSe barriers. The intensity of these bands is resonance amplified on approach of the excitation energy to the MQW luminescence. We make the supposition that this process consists of several stages: absorption of stimulating light by quantum wells; exchange of energy between hot electrons and barriers and relaxation of the system through LO-phonon generations in the barriers.

Band alignment in ZnCdTe/ZnTe and ZnCdSe/ZnSe SQW structures grown on GaAs(100) by MBE

Molecular-beam-epitaxy-grown ZnTe/CdZnTe/ZnTe and ZnSe/ZnCdSe/ZnSe strained single-quantum-well structures with non-doping layers were investigated by cathodoluminescence (CL) and deep-level transient spectroscopy (DLTS). The activation energies for deep levels in ZnTe and ZnSe buffer layers grown on GaAs were determined by the DLTS spectra. Moreover, an additional DLTS peak that depends on the quantum well (QW) parameters and correlates with the QW emission line position in the CL spectra was observed. This peak is interpreted as an emission of electrons from a ground level in the QW. Obtained DLTS and CL results were used for the estimation of the conduction band offset parameter QC.

Effect of CdTe monolayer insertion on CdZnTe/ZnTe quantum well characteristics

The effect of CdTe monolayer (ML) insertion on the structural and luminescence characteristics of ZnCdTe/ZnTe quantum well (QW) was investigated by a high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL) methods. The structures were grown by MBE on just-oriented or 3° off (001) GaAs substrate and contained Zn0.6Cd0.4Te QW with or without CdTe 1ML insertion embedded in the middle of QW. HRXRD(004) diffraction profiles and reciprocal space maps in the vicinity of the (004) reflection were measured. CdTe insertion led to the shift of QW-related peak to larger angels as well as to significant broadening and disappearance of fringes. The low-temperature PL investigations showed that CdTe insertion resulted in a blue shift of the QW PL peak position, in the two times narrowing of the QW PL band and one order of value increase of its intensity.

Anti-Stokes photoluminescence and structural defects in CdSe/ZnSe nanostructures

Abstract An efficient anti-Stokes photoluminescence (ASPL) in multistacked CdSe/ZnSe nanostructures with quantum dots (QDs) has been found under the excitation below the QD ground states. An efficiency of ASPL is a few percents of the Stokes one. The dependencies of anti-Stokes emission spectra on temperature and excitation density have been studied. It is shown that excitation mechanism of this luminescence at low temperature can be explained by the two-step two-photon absorption process involving deep levels of cation vacancy related defects as intermediate states. It is found that with temperature increase resonant excitation in QD photoluminescence band may occur and contribution of thermal excitation of carriers becomes actual.

Optical investigations of the influence of point defects on quantum dots in CdSe/ZnSe heterostructures

Optical properties of single-layer and multistack CdSe/ZnSe self-assembled quantum dot (QD) heterostructures have been investigated. It is found that QDs at the interface can accumulate cation vacancy-related defects. In this case the level of defects is associated with the quantized heavy-hole level of the QDs. It is shown that study of the excitation spectra of the defect-related band enables one to obtain information about optical transitions in QDs.

Far-infrared analysis of lattice vibrations in ZnSe/ZnCdSe superlattices

The results of the first measurements of far-infrared lattice reflection spectra of ZnSe/ZnCdSe superlattices grown on a GaAs substrate by molecular-beam epitaxy are presented. It is shown by a dispersion analysis of the experimental spectra measured at temperatures 300 and 10 K that, in the superlattices under study, instead of two TO modes corresponding to two different layers ZnSe and ZnCdSe of the superlattice only one TO mode is observed which has a frequency between the frequencies of the two layers. The existence of a single transverse mode in the lattice reflectivity spectrum of the ZnSe/ZnCdSe superlattice has been explained by a convergence of the vibrational modes under the effect of internal elastic strains.

Deep-level transient spectroscopy and cathodoluminescence of CdxZn1-xTe/ZnTe QW structures grown on GaAs(100) by MBE

Abstract MBE-grown ZnTe/CdZnTe/ZnTe strained single quantum well (SQW) and multiple quantum well (MQW) structures with non-doping layers were investigated by cathodoluminescence (CL) and deep-level transient spectroscopy (DLTS). The DLTS method with electrical current relaxation was used because the as-grown structures were compensated. The activation energies of 0.21 and 0.58xa0eV for deep levels in ZnTe buffer layers grown on GaAs were determined by the DLTS spectra. Besides, an additional DLTS peak ( E t =34 –133xa0meV) that depends on the quantum well parameters and correlates with the QW emission line position in the CL spectra was observed. This peak is interpreted as an emission of electrons from a ground level of dimensional quantization in a conduction band. Obtained DLTS and CL results were used for an estimation of a conduction-band offset parameter Q C .

Optical characterization of CdZnTe/ZnTe heterostructures modified by electron or X-ray irradiation

Abstract The effect of electron and X-ray irradiation on the optical characteristics of CdZnTe/ZnTe quantum-size structures has been investigated. A comparison between the results of both irradiations has shown an essential role of electron excitation in radiation enhancement of Cd diffusion that is one of the reasons for degradation of the II–VI-based quantum-size structures. High defect density in the barrier layer and high stress level are considered to serve as additional reasons for degradation of the CdTe-based quantum size structures under irradiation.

E-beam irradiation effect on CdSe/ZnSe QD formation by MBE: deep level transient spectroscopy and cathodoluminescence studies

CdSe/ZnSe structures containing 1 or 15 thin (3–5 monolayers) CdSe layers were studied by cathodoluminescence (CL) and deep level transient spectroscopy (DLTS). The DLTS spectra consisted of peaks from deep levels (DLs) and an additional intense peak due to electron emission from the ground quantized level in the CdSe layers. Activation energy of this additional peak correlated with an energy of the CdSe-layer emission line in the CL spectra. Electron-beam irradiation of the structure during the growth process was found to influence the DLTS and CL spectra of the CdSe layers, shifting the CdSe-layer emission line to the long-wave side. The obtained results are explained using the assumption that e-beam irradiation stimulates the formation of quantum dots of various sizes in the CdSe layers.