A. Balocchi

Growth of zinc blende MgS/ZnSe single quantum wells by molecular-beam epitaxy using ZnS as a sulphur source

Zinc blende MgS has been grown on GaAs by molecular beam epitaxy using a novel method where the sources were Mg and ZnS. A reaction at the surface results in the formation of MgS layers with a Zn content estimated by secondary ion mass spectrometry and Auger spectroscopy to be between 0.5% and 2%. Double crystal x-ray rocking curve measurements of ZnSe/MgS/ZnSe layers show layers with good crystallinity. Using this growth technique layers up to 67 nm thick have been grown. Photoluminescence measurements of MgS/ZnSe/MgS single-quantum-well structures show that the confinement of the heavy hole excitons can be as large as 430 meV for a 1.7 nm well.

Epitaxial liftoff of ZnSe-based heterostructures using a II-VI release layer

Epitaxial liftoff is a post-growth process by which the active part of a semiconductor heterostructure, the epitaxial layer, is removed from its original substrate and deposited onto a new substrate. This is a well established technique in GaAs-based heterostructures where epitaxial liftoff can be achieved by exploiting the contrast in the etch rates of GaAs and AlAs in hydrofluoric acid. We report here successful epitaxial liftoff of a ZnSe-based heterostructure. We find that a metastable layer of MgS acts as a perfect release layer based on the huge contrast in the etch rates of ZnSe and MgS in hydrochloric acid. Epitaxial liftoff of millimeter-sized ZnSe samples takes a fraction of the time required for GaAs liftoff. Photoluminescence experiments confirm that the liftoff layer has the same optical characteristics as the original wafer material.

Photoluminescence properties of MgS/CdSe quantum wells and quantum dots

The optical properties of MgS/CdSe quantum structures grown by molecular beam epitaxy are characterized by photoluminescence (PL) spectroscopy. The increase in the CdSe thickness from 1 to beyond 3 ML results in the formation of, at first, quantum wells (QWs) and then quantum dots (QDs) by Stranski–Krastanov growth. The PL temperature dependence measurements reveal that, in the QWs, excitons localized by potential fluctuations principally govern the PL properties, which is in strong contrast to the QD PL properties.

Excitons with large binding energies in MgS/ZnSe/MgS and ZnMgS/ZnS/ZnMgS quantum wells

The wide bandgap II-VI semiconductors have unique properties which allow the possibility of suppressing the exciton-phonon scattering up to room temperature in quantum well structures designed so that the exciton excitation E1s?2s>h?LO. In particular, magnetic field and temperature dependent measurements are used to study the exciton binding energies and to investigate the exciton-LO phonon scattering processes of high quality ZnSe quantum wells in MgS grown by MBE. The small inhomogeneous broadening of the exciton transitions in these samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS the exciton binding energy in a 5?nm well is found to be 43.9?meV, which is the largest reported for this material system. The FWHM of the heavy hole absorption transitions measured as a function of temperature shows that the scattering of the excitons by the LO?phonons is partially suppressed. These results are compared with ZnS quantum wells where the exciton g-values have been measured and the exciton binding energies have been deduced from the exciton diamagnetic shifts. The results show the possibility of suppressing exciton-LO phonon scattering in these structures.

Excitonic properties of MgS/ZnSe quantum wells

Magnetic field and temperature dependent measurements are used to study the excitonic properties of high quality ZnSe quantum wells in MgS barriers grown by molecular beam epitaxy. The small inhomogeneous broadening of the samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS a value of 43.9 meV was measured for the exciton binding energy which is the largest reported for this material system. The full width at half maximum of the heavy hole transitions is measured as a function of temperature and the broadening of the excitonic transitions in narrow quantum wells is reduced compared to the ZnSe bulk value due to the expected reduction in the LO-phonon scattering.

Growth and characterisation of MgS/CdSe self-assembled quantum dots

MgS has a very large bandgap of /spl sim/ 5eV and can form an excellent barrier material for wide-gap II-VI quantum structures. Although its stable crystal structure is rocksalt, our group has recently established a novel molecular beam epitaxy (MBE) technique that allows us to grow zinc-blende MgS lattice matched to GaAs substrates to thicknesses greater than 130 nm [1]. The lattice parameter of zinc-blende MgS is almost the same as that of ZnSe. Accordingly, strain between MgS and CdSe is almost identical to that between ZnSe and CdSe and a transition from 2D to 3D growth is expected with increasing CdSe coverage. Using the barrier material MgS with CdSe dots instead of ZnSe has two advantages. Firstly, large band discontinuities, which are estimated to be 2.1 and 0.9eV for the conduction and valence bands, respectively, will provide strong carrier confinement. Secondly, interdiffusion of MgS and CdSe is inhibited due to the immiscibility of these two materials. The clear material boundary between the dots and the barrier will enhance the confinement even further.

Photoluminescence Studies of the Formation of MgS/CdSe Quantum Dots Grown by Molecular Beam Epitaxy

The optical properties of MgS/CdSe quantum structures grown by molecular beam epitaxy are characterized by photoluminescence (PL) spectroscopy. The increase in the CdSe thickness from 1 to beyond 3 monolayers results in the formation of, at first, quantum wells (QWs) and then quantum dots (QDs) by Stranski-Krastanov growth. The PL temperature dependence measurements show clear difference in the optical properties of QWs and QDs.

Creating excitons in II-VI quantum wells with large binding energies

The wide bandgap II-VI semiconductors have unique properties which allow the possibility of suppressing the exciton-phonon scattering up to room temperature in quantum well structures designed so that the exciton excitation E/sub 1s/spl rarr/2s/>h/spl nu//sub LO/. High quality ZnSe quantum wells in MgS and ZnS quantum wells in ZnMgS have been grown by MBE and these have excellent optical properties. Magnetic field and linewidth temperature dependent measurements have been used to determine the exciton binding energies and to investigate the exciton-LO phonon scattering processes. The results show the possibility of suppressing exciton-LO phonon scattering in these structures.