Galina N. Semenova

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.

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.

Resonant stokes and anti-stokes Raman scattering of light in CdSe/ZnSe nanostructures

Raman scattering spectra of CdSe/ZnSe multilayer nanostructures with a CdSe insert of a nominal thickness of 2.1 monolayers were studied. A heavy dependence of the intensity and of the frequency position of the multiphonon Stokes and anti-Stokes LO bands on the exciting photon energy was detected. The results obtained are interpreted as a resonance with various exciton transitions in the CdSe insert and barrier ZnSe layers. A difference between the Stokes and anti-Stokes frequencies of LO bands observed as the resonance conditions are varied confirms the inhomogeneous nature of the photoluminescence band of CdSe quantum dots.

Investigation of inhomogeneous broadening of CdSe/ZnSe nanoisland photoluminescence band by resonant excitation methods

Abstract This paper presents the results of photoluminescence and Raman scattering investigations of CdSe/ZnSe nanostructures. Using resonant excitation of Cd-containing nanoislands both Stokes and anti-Stokes emission are observed. The mechanism of anti-Stokes luminescence, including two-photon two-step excitation through the local states of defects and quantized states of islands, is proposed. The inhomogeneous broadening of the photoluminescence band is shown to be largely due to the size distribution of islands and/or to the variation of their Cd-content.

Depth inhomogeneity of ZnTe, CdZnTe, ZnSe epilayers grown on (001)GaAs of MBE

In this work we report the depth inhomogeneity study of MBE grown ZnSe, CdZnTe and ZnTe/(001) GaAs epilayers of different thickness by x-ray and depth resolved photoluminescence methods. Step etching and different wavelength excitation were used for this purpose. It is shown that all these epilayers consist of three regions with different extended defect and impurity concentrations: (i) near the interface one with high density of misfit dislocations and impurities concentration; (ii) the region with low extended defect and impurity concentration and (iii) near the top surface region with higher extended and point defect concentration. The deterioration of near top surface region increases with epilayer thickness. Influence of GaAs substrate preparation regimes on ZnSe layer growth and optical properties as well as the study of interdiffusion of Ga and Zn across the wafer-epilayer interface have been investigated. The possibility to use thin intermediate ZnTe layer with solid phase crystallization for blocking of the interdiffusion in ZnTe layers and improvement of epilayer photoluminescence characteristics have been explored.

Effect of irradiation on quantum-size layer properties grown on semi-insulating GaAs

The effect of electron (E = 1.8 MeV), γ-(60Co) and X-ray irradiation on the structural and optical properties of a (Ga,Al)As and a (Zn,Cd)Te quantum wells (QWs) grown on semi-insulating (SI) GaAs is studied. The high radiation hardness of the A3B5 QWs (no changes in recombination characteristics) after irradiation up to a dose of approximately 2•109 rad was found, while SI GaAs substrate manifested the standard effect of the charge carrier removal under such irradiation dose. A2B6 quantum-sized structures (QSS) tends to degrade under lower irradiation dose. The emission bands transformation after irradiation up to a dose ≥ 2•109 rad have been observed in ZnTe buffer layer and CdZnTe QWs. The role of Cd diffusion and internal strain in radiation enhanced ternary alloys QWs degradation is discussed. The calculation of PL peak energy shift is presented in assumption of the well profile change as a result of the radiation enhanced Cd diffusion.

Lateral and depth inhomogeneities in Zn-based heterostructures grown on GaAs by MBE

The near-surface morphology, crystalline quality and contamination as well as depth and lateral homogeneity of background impurities and defects distribution in undoped ZnSe and ZnTe films grown by molecular beam epitaxy on GaAs(001) substrates were studied. The improvement of structural quality with the increase of film thickness was observed by X-ray diffraction and atomic force microscope. But sufficient depth inhomogeneity of point and extended defect distribution is present in all films independently on their thickness and increases after deposition of quantum wells. Transversal photovoltage measurements confirm the laminar inhomogeneity of investigated films.

Molecular-beam epitaxial growth of CdZnTe/ZnTe QW structures and superlattices on GaAs (100) substrates for optoelectronics

The effect of the 5 nm thick ZnTe intermediate layers obtained by solid crystallization at growth temperature on the optical properties of ZnTe epilayers grown by molecular beam epitaxy (MBE) on (100) GaAs substrates has been investigated by low temperature photoluminescence and reflectance spectroscopy. Reduction of nonradiative center concentration and improvement of ZnTe epilayer photoluminescence characteristics have been achieved using of solid phase crystallized intermediate layers. At the same time defect depth nonuniformity was found to occur in ZnTe epilayers with and without such intermediate layers. Use of such surfactant layer and optimized technology conditions on early stage of growth makes possible to obtain CdZnTe/ZnTe quantum wells and super lattices with high luminescence efficiency for further application.

Photoluminescence investigation of Dy incorporation into InP during liquid phase epitaxy

The low-temperature photoluminescence and x-ray structural investigations of the properties of InP epilayers grown from indium melt with rare earth element dysprosium (Dy) addition are presented. The Dy addition influence on intensity, linewidth and spectral position of the near-band-gap emission and of vacancy-impurity bands at 0.75 - 1.1 eV is reported. The obtained data of the stoichiometry changes in InP epilayers grown with addition of Dy are considered. Low background doping level with free electron concentration below about 1014 cm-3 at the room temperature for InP epilayers was achieved, that witnessed of the Dy strong gettering effect. It was also ascertained that Dy incorporated into the grown layers in various phases forms (like the inclusions) at lowest using concentration (CLDy approximately 0.01 at.%).