Yoichi Kawakami

Excitonic and edge emissions in MOCVD-grown ZnS films and ZnSe-ZnS superlattices☆

Abstract Excitonic lines and edge-emission bands in metalorganic chemical vapour deposition (MOCVD) heteroepitaxially-grown ZnS films on (100) GaAs substrates have been extensively investigated at 4.2 K using He-Cd (325 nm) and Xe-Cl excimer (308 nm) lasers. Distinct edge-emission bands were observed; the LO-phonon-replicated 3.641 and 3.666 eV bands are ascribed to a donor-acceptor pair and free-to-bound acceptor transition, respectively, from time-resolved spectral measurements. The Na acceptor level is tentatively assigned to be about 170 meV above the valence band. ZnSe-ZnS and ZnSe-ZnSSe superlattices, which were for the first time prepared by low-pressure MOCVD, have been characterized by the well-width dependence of the excitonic lines and by optical absorption. We observed heavy-hole and light-hole excitons in the absorption spectra of the ZnSe-ZnS superlattices.

Effects of strain and temperature on excitonic emissions in ZnSeZnS strained-layer superlattices grown by low-pressure MOCVD

Abstract The dominant excitonic-emission line in low-pressure metalorganic chemical-vapour deposition (MOCVD)-grown ZnSeue5f8ZnS strained-layer superlattices (SLS) has been extensively characterized by the temperature dependence of its linewidth, peak energy and emission intensity. The emission line of the ZnSe well shows a band tail on its low energy side. It is tentatively suggested that the n = 1 heavy-hole free and light-hole free excitons, which are separated from each other by about 200 meV, were clearly observed in the absorption spectra at 4.2 K. Taking into account the variations of both the band offsets and the bandgap of the ZnSe well and the ZnS barrier layers as a function of strain, the Kronig-Penney analysis has been carried out in order to interpret quantatively the energy shifts of the n = 1 heavy-hole free exciton absorption peak to the high-energy side with decreasing ZnSe well width.

Fabrication and photoluminescence properties of ZnTe and CdZnTe films by low pressure metalorganic chemical-vapour deposition

Abstract Unintentionally-doped heteroepitaxial ZnTe and CdZnTe films have successfully been grown on (100)GaAs substrates at about 350°C using a low-pressure metalorganic chemical-vapour deposition (MOCVD) method by cracking (CH 3 ) 2 Te. Photoluminescence spectrum at 4.2 K of ZnTe layers is dominated by a neutral-acceptor-bound-exciton line (I l a′ ) at 2.366 eV and the characteristic oxygen-bound exciton band around 1.9 eV. The free-exciton line from the epitaxial layer is greatly shifted toward lower photon energy with respect to that of bulk ZnTe. From the thickness dependence of both the emission peak and linewidth of the I l a′ line, it is evident that the epitaxial film is strained. We tentatively suggest that tensile stress induced by the different thermal expansion coefficients between ZnTe and GaAs strongly affects the optical properties in the films. In a mixed crystalline Cd 0.33 Zn 0.67 Te film, a dominant emission band appears at 2.035 eV at 4.2 K and is ascribed to a donor-acceptor pair luminescence as indicated from the excitation intensity dependence of its peak position.

Interface properties and the effect of strain of ZnSe/ZnS strained-layer superlattices

Abstract The ZnSe/ZnS alternately layered system is one of the fundamental examples of the wide-gap II–VI compound strained-layer superlaticee (SLS) used to understand the basic physics associated with optical, electronic and structural properties. Interface properties of ZnSe/ZnS SLSs have been extensively investigated by means MeV transmission electron microscopy (TEM), ion Rutherford backscattering (RBS)/channeling, X-ray diffraction (XRD), X-ray photoelectron emission spectra (XPS) and cross-sectional TEM measurements. It is shown that the ZnSe/ZnS SLS can be grown coherently up to a layer thickness of about 100 A and to exhibit good crystalline quality. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being induced in each layer can be quantatively evaluated; the ZnSe well is certainly under compressive strain, while the ZnS barrier is under tensile strain. Band offsets in the conduction and valence bands in the strained layers are experimentally determined to be about 0.25 and 0.73 eV by XPS, respectively, and are theoretically considered as a function of the well width thickness using the ‘model solid theory’. The channeling in the SLS shows much higher dechanneling yields than along the growth direction. This may be derived from the small angle change in the inclined crystal directions at each interface which result in significant dechanneling, and can be discussed in terms of the bond relaxation model or beam steering effects. The periodic oscillatory structure of the superlattices in RBS/channeling spectra has been observed in this SLS system. The interface properties can be understood in terms of the excitonic linewidth broadening which is influenced by either the interface roughness or by well width fluctuation. Photoluminescence and absorption spectroscopies of the ZnSe/ZnS and ZnSe/ZnSSe SLS are used to characterise the interface, and to clarify the energy separation of the degenerate hole bands due to a compressive stran in the ZnSe quantum well. The effects of ion irradiation and thermal stability during annealing in SLS are briefly introduced with a view to providing the fundamental physical properties of the creation of lattice defects and impurity diffusion. A Fibonacci SLS sequence with a quasi-periodic superlattice system demonstrates the possibility of new functional devices.

Characterization of epitaxial ZnS films fabricated by sputtering in controlled H2S vapor

Abstract ZnS films doped with Mn(ZnS:Mn) were deposited on a variety of substrates by RF sputtering in controlled hydrogen sulfide vapor. In order to evaluate the interface characteristics of the grown layer, the effects of substrate orientation on the Mn 2+ photoluminescence band appearing in homoepitaxially grown ZnS:Mn films were investigated. The crystallinity of the films was also assessed by 2 MeV transmission electron microscopy. It is tentatively suggested that the incorporation of hydrogen sulfide gas can improve the crystallinity and interface properties of the epitaxial films.

Interface characteristics and excitonic emissions in ZnSe-ZnSSe superlattices fabricated by low-pressure MOCVD

Abstract ZnSe-ZnSSe and ZnSe-ZnS strained-layer superlattices (SLS), exhibiting excellent optical properties, have for the first time been prepared on (100) GaAs substrates by a conventional low-pressure metal-organic chemical-vapor-deposition (MOCVD) technique. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being included in each layer can be quantitatively evaluated. The excitonic line and its spectral shape in the SLS, depending upon the composition of S in the ZnSSe barrier layer, are investigated at 4.2 K; with decreasing the barrier height the ground-state exciton binding energy is considerably lowered and its linewidth becomes narrow. We have found light-hole and heavy-hole exciton transitions in the absorption spectrum and in particular the existence of the light-hole exciton is confirmed by the excitation spectrum.

In Situ Monitoring And Control For MBE Growth Of Optoelectronic Devices

Self-organized CdSe/ZnSe quantum dots (QDs) were fabricated on the cleavage-induced GaAs (110) surface in ultra high vacuum (UHV) by molecular beam epitaxy (MBE). CdSe layer showed the Stranski¿Krastanow (S-K) growth mode. QWs and QDs emissions originated from the wetting layer and island structures, respectively, were observed in photoluminescence (PL) spectra. This is a evidence of S-K type where island structures are self-formed on the two-dimensional wetting layer as a result of the transition of the growth mode. The state filling effect in the QDs was also observed by employing excitation power dependence on the PL intensity. By using the microscopic PL spectroscopy, the broad PL peak of QDs was resolved into a number of sharp peaks. These peaks are attributed to the recombination of excitons localized at the individual QDs indicating that the fabricated CdSe islands have quasi-zero-dimensional δ-function like density of states.