C. Bradford

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.

Characterization of MBE grown II–VI semiconductor thin layers by X-ray interference

X-ray interference (XRI) is a simple and powerful technique used to characterise thin layers buried within epitaxial films of GaAs/AlGaAs and InGaAs/GaAs. XRI measurements are double crystal rocking curves made on structures of the type A/B/C/B, where A is the substrate, B a different semiconductor and C the thin layer of interest, (possibly the same as A). B/C/B forms an X-ray interference film with the thicknesses of B much greater than that of C. Here, the Pendellosung fringes from B are strongly modulated by the introduction of C. XRI is also ideal for investigating materials which can not be grown thick enough for DEKTAK measurements, in which the lattice constant is not known accurately, or the layer is unstable in air and must be capped.

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.

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.

Development of an epitaxial lift-off technology for II-VI nanostructures using ZnMgSSe alloys

An epitaxial lift-off technique for removing wide bandgap II-VI heterostructures from GaAs substrates has previously been demonstrated using lattice-matched MgS as the sacrificial layer. However, using MgS as an etch release layer prevents its use as a wide bandgap barrier in the rest of the structure. Here, we describe the use of the etch-resistant alloy Zn.2Mg.8S.64Se.36 which we have developed as a replacement for MgS. We demonstrate that this alloy can be grown by MBE together with MgS in heterostructures and used as a barrier for ZnSe. A ZnSe quantum well with Zn.2Mg.8S.64Se.36 barriers shows no decrease in photoluminescence intensity after the etching process but shows a shift in emission wavelength associated with the changing strain state.

Ferromagnetic phase transition in zinc blende (Mn,Cr)S-layers grown by molecular beam epitaxy

We studied the magnetization of zinc blende Mn1-xCrxS films embedded between diamagnetic ZnSe layers grown by molecular beam epitaxy with chromium mole fractions x ≤ 0.7. These ternary semiconductors exhibit an increasing ferromagnetic contribution with increasing x caused by competing antiferromagnetic and ferromagnetic coupling. As a result, whereas MnS in the zinc blende phase is a pure antiferromagnet, it was found that with increasing x zinc blende Mn1−xCrxS became a ferromagnet. The ferromagnetic phase transition dominates in case of x greater than about 0.5. Hence, we conclude that metastable zinc blende CrS will be a ferromagnetic material with half-metallic character in contrast to the analogous stable NiAs-structure which exhibits an antiferromagnetic phase transition.

Growth and Spectroscopy of CdSe: Mn Quantum Dots

In this paper we report the growth and optical properties of ZnSe/CdSe:Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 109 cm−2 were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn2+ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at <0.2 meV.

UV Raman microscopy of zincblende magnesium sulphide

The first Raman scattering studies of zincblende magnesium sulphide (MgS) have been carried out and lead to the identification of its zone-centre LO phonon frequency. Several overtone and combination bands are observed and lead to the identification also of its zone-centre TO phonon frequency. It is demonstrated that the Raman scattering is strongly resonant at the excitation energy used (5.08 eV), thereby confirming present estimates of the band gap of zincblende MgS. Finally, the phonon frequencies observed are compared to the results of calculations.

Growth and spectroscopy of II-VI CdSe quantum dots

In this paper we review the recent progress in the growth and spectroscopy of CdSe quantum dots. In particular, atomic layer epitaxy (ALE) has been used to grow ZnSe/CdSe and ZnSe/CdSe:Mn magnetic quantum dots. For samples grown without a ZnSe capping layer, dot densities of the order of 10/sup 9/ cm/sup -2/ were measured by atomic force microscopy (AFM). In the capped samples, the ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV and in the high Mn concentration samples a spectrally broad emission at 2.15 eV from the internal Mn/sup 2+/ transition. Single dot spectroscopy was carried out by confocal microscopy and the PL linewidth was measured as a function of Mn concentration. Also, CdSe/MgS quantum dots have been grown successfully by molecular beam epitaxy using a thermally activated reorganization process that occurs during growth interruption. Unlike the ZnSe/CdSe dots the PL measurements show emission from both QDs and the wetting layer, with emission energies ranging between (2.3 and 3.8 eV). AFM topography and /spl mu/m-PL measurements also show evidence of quantum dot structures and power dependent PL measurements carried out on the dots give a value of 30 meV for the bi-exciton binding energy at 77 K.