P. I. Kuznetsov

Hexagonal ZnCdS epilayers and CdSSe/ZnCdS QW structures on CdS(0001) and ZnCdS(0001) substrates grown by MOVPE

Abstract Optimization of growth conditions allows us to grow mirror-like hexagonal ZnCdS layers and CdSSe/ZnCdS quantum well (QW) structures on CdS(0001) and ZnCdS(0001) substrates by MOVPE. ZnCdS epilayers with high cathodoluminescence (CL) intensity at room temperature (RT) have also been obtained. An intense QW emission line has been observed in CdS/ZnCdS QW structures. With increasing excitation intensity the line shifted to shorter wavelength, which is a manifestation of an internal piezoelectric effect. An additional short-wavelength line appears in low-temperature CL spectra of CdSSe/ZnCdS QW structures due to localization of charge carriers by nonuniformity of alloy composition or QW thickness.

Layer structure of Zn1−xCdxSe films grown by vapor-phase epitaxy from metal-organic compounds on Cd0.92Zn0.08S(0001) substrates

The structure of Zn1−xCdxSe films, which were grown by vapor-phase epitaxy from metal-organic compounds on a Cd0.92Zn0.08S(0001) substrate, was investigated by X-ray diffractometry. For both cubic and hexagonal phases, asymmetric reflections were selected. These reflections make it possible not only to reliably determine the presence of these phases in the film, but also to estimate the dimensions of coherent X-ray scattering regions and (or) variation in the lattice parameters in the intergrowth plane. The ZnSe films preferentially consist of twinned interlayers of the cubic phase, 200–250 Å thick, and with a low content of the hexagonal phase. In contrast, the hexagonal phase with a small number of cubic interlayers predominates in CdSe films. The thickness of the interlayers of the cubic phase in Zn1−xCdxSe decreases, whereas the concentration of the hexagonal phase increases for low x values with an approximately identical development of both phases for x=0.15–0.20.

Exciton emission of crystalline Zn(S)Se thin films arranged in microcavities based on amorphous insulating coatings

A technology for the production of fully hybrid microcavities on the basis of Zn(S)Se films and amorphous insulating SiO2/Ta2O5 coatings is proposed. The influence of all stages of the manufacturing cycle on the structure of exciton states in the Zn(S)Se films is demonstrated. This influence is reduced to four main effects: the appearance of a fine structure of emission lines related to free excitons; a decrease in the relative contribution of excitons bound at neutral acceptors to the exciton-emission spectrum; a shift of the emission lines related to exciton–impurity complexes and free excitons to lower frequencies; and a decrease in the splitting between emission lines related to heavy and light excitons. Samples of fully hybrid microcavities, in which the high structural and optical quality of Zn(S)Se films is retained, are fabricated.

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm2) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.

Laser CRT as a light source for display technology

The notion ofusing a laser CRT as monochromatic light source for passive display technologies is discussed. Optimism for such an application is based on the high efficiency (more than O %) of red lasers obtained in GaInP/AlGaInP multi-quantum well (MQW) structures. A miniature vacuum tube designed as a light source is presented. Technological issues of achieving high efficiency in green and blue using wide band gap II-VI compound MQW structures is also presented. To date, maximum output power of 3.2 and 1 W was achieved at wavelengths of 535 and 462 nm respectively under longitudinal pumping by an electron beam with electron energy of 4O keV at room temperature.

Ultrafast nonlinear response of heterostructures based on zinc chalcogenides

Nonlinear optical response of periodic structures based on ZnSe/ZnS heterostructures using interband excitation of a ZnSe sublattice by 1 50 fs laser pulses is reported. A considerable shift of reflection spectrum and large relative reflection changes were observed in a wide spectral range corresponding to the transparency region of ZnSe far from the intrinsic absorption onset. Evaluated refraction index change is about -0.02 with the relaxation time being about 3 picoseconds. The nonlinear refraction is supposed to be controlled by population induced absorption changes in ZnSe single crystals and relevant refraction index modification via Kramers-Kronig relations. The nonlinearity relaxation time is supposed to trace a transition from non-equilibrium to quasi-equilibrium distribution of electrons and holes within ZnSe conduction and valence bands, respectively, rather than electron-hole recombination time. The nonlinearity mechanism does not reduce to just population dependent absorption saturation but essentially results from the specific distribution function in the first instance after excitation.

E-beam longitudinal pumped laser on MOVPE-grown hexagonal CdSSe/CdS MQW structure

En CdSSe/CdS multi quantum well structures were grown by metalorganic vapor phase epitaxy (MOVPE) on a CdS substrate misoriented by 12−15o from (0001) towards (1010). Microcavities with dielectric oxide mirrors were fabricated on the basis of these structures. Lasing in the 535−590 nm spectral range was achieved on different structures under longitudinal electron beam pumping at room temperature. The relatively high threshold of the laser and the blueshift from the spontaneous emission peak to lasing wavelength were explained by taking into account the type−II band alignment of the active region.

Excitons in heteroepitaxial CdSe/CdS structures

X-ray diffractometry and low-temperature exciton spectroscopy are used to study heteroepitaxial CdSe/CdS layers grown at temperatures of 350–485 °C by MOCVD. The high-temperature samples are found to display the exciton and x-ray diffraction spectra characteristic of hexagonal Wurtzite (W) structures, while the low-temperature samples display the features characteristic of the cubic structure of sphalerite (ZB). A number of the samples have x-ray spectra characteristic of structures with stacking faults (SF), which represent a separate crystalline phase in the structures studied here. It is found that the individual crystalline phases are spatially separated.