Metastable II–VI sulphides: Growth, characterization and stability

Among the II–VI compounds, many sulphides have the stable rocksalt structure, but can also be grown by MBE in the metastable zinc blende (ZB) form in epitaxial thin films, including MgS and MnS. By using a metal rich growth regime where sulphur is supplied from the evaporation of ZnS, we have shown that layers of both MgS and MnS up to 140nm thick can be grown in the ZB structure, allowing the production of different multilayer structures exploiting the properties of these novel semiconductors. This review concentrates on developments in MgS, which we have studied in greater detail than the other compounds and has proved a very versatile component of multilayer structures. These include the development of new metastable MgS-rich ZnMgSSe quaternary alloys, which we have been able to incorporate into structures with MgS and have used to develop a new lift off technology. Comparison of the growth of MgS, ZnMgS and ZnMgSSe has allowed us to determine the incorporation coefficients of Zn and Mg, which are approximately 0.03 and 0.4, respectively. The Zn flux appears to play a crucial role in the development of the film, by forming a ZnMgS surface layer, aiding transport of material across the surface and increasing the energy barrier to conversion to the stable rocksalt phase. TEM shows that occasionally both ZB and RS phases may be present simultaneously. The small RS inclusions seem to stabilise the ZB phase against relaxation and do not propagate through the crystal. Examination of the totally converted films suggests that they are composed of different rocksalt domains and may not have transformed via the mechanism which has been suggested as the lowest route for bulk MgS.