MgS has been grown by molecular beam epitaxy in the zincblende crystal structure on GaAs ͑100͒ substrates using a technique where the sources are Mg and ZnS. Layers up to 134 nm thick have been grown without any degradation in the crystal structure. The lattice constant was found to be 0.5619Ϯ0.0001 nm and Poisson's ratio was estimated to be 0.425. The success of this growth technique has allowed the fabrication of MgS/ZnSe/MgS quantum wells that show sharp photoluminescence and transmission spectra indicating less than 1 ML fluctuations of the well widths. The small inhomogeneous broadening of the samples has allowed magneto-optical studies of the exciton absorption where the observation of higher excited exciton states have been observed and the exciton binding energies, E X , have been measured directly, notably E X (1s -2s) Ͼh LO in a 5 nm well. The full width at half maximum of the heavy-hole absorption transitions for this sample has been measured as a function of temperature and no broadening of the excitonic transitions has been observed up to 150 K showing that the exciton-LO phonon scattering has been suppressed.

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.

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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.

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The optical properties of MgS/CdSe quantum structures grown by molecular beam epitaxy are characterized by photoluminescence ͑PL͒ spectroscopy. The increase in the CdSe thickness from 1 to beyond 3 ML results in the formation of, at first, quantum wells ͑QWs͒ and then quantum dots ͑QDs͒ 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.

Many group II sulphides semiconductors have the rocksalt structure as their stable crystal structure. In the case of MgS and MnS it has been demonstrated that these compounds can be grown in the metastable zinc blende structure. Recently, at Heriot-Watt we have shown that a simple MBE growth procedure can be used to increase the thickness of these metastable layers to over 130 nm. In this paper, we summarise some of the results we have obtained and describe some of the structures which we have grown using these materials. The aim is to demonstrate that the growth method is simple, and that MgS in particular is a material with useful properties that can be incorporated into a variety of structures. Finally, we describe some potential uses of these compounds which we have not yet explored, and suggest other compounds which may be successfully grown with this method.1 Introduction It is possible, using a variety of methods, to produce compounds in crystal states which are not their lowest energy configurations. Perhaps the best-known example of this is carbon, where the well-known diamond structure is metastable with respect to the graphite structure. In the case of II-VI compounds, using thin film growth techniques such as MBE and MOCVD it is possible to grow compounds and alloys in the zinc blende (ZB) phase when this is not the lowest energy state. Examples are the growth of CdSe [1] and CdS [2] on (100) oriented substrates where the fourfold periodicity of the underlying layer causes adoption of the ZB phase rather than the lower energy wurtzite phase. II-VI compounds are different to III-Vs in the variety of crystal structures that they can adopt. In addition to ZB and wurtzite, the NaCl (rocksalt) structure is found. The lowest energy crystal structure which is adopted can be related to the ionicity f i of the compound and for f i < 0.625 II-VI compounds adopt the ZB structure [3]. Above this value the wurtzite structure occurs, with a transition to the rocksalt structure when f i < 0.78 [4].Unlike the compounds which have wurtzite as their stable phase that can be made to adopt the ZB structure on (001) surfaces, there is no symmetry requirement which can force the adoption of ZB instead of rocksalt. In this case both structures are cubic, and the differences are that atoms incorporated into a ZB layer have 4 nearest neighbours instead of 6 as is found in the rocksalt structure.In recent years, effort has gone in to exploring the growth of metastable ZB compounds, particularly MgS and MnS, which have similar lattice constants to GaAs. MgS is of particular interest as its Philips' ionicity is 0.786, placing it close to the rocksalt stability boundary [5]. Initial attempts to grow MgS by MBE produced ZB layers only 0.96 nm thick before changes in the RHEED pattern were observed which were believed to correspond to a change in the crystal structure to rocksalt [6]. Subsequent MOCVD growth obtained layers up to 10 nm thick in the ZB crystal structure [7] which were of high quality and could be incorporated in MgS...