Growth by MBE and characterization of metastable group II sulphides

Abstract

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...

Metastable group II sulphides grown by MBE: surface morphology and crystal structure

“…Previously, we observed the loss of the ZB crystal structure and assumed that transformation had occurred directly to the stable phase, NaCl [7,8,13]. At present, however, we have no direct evidence for the formation of this phase, rather than an orthorhombic metastable structure.…”

“…Aspects of this work have been reviewed recently [13], with emphasis on the growth method and wide range of structures developed incorporating MgS or MnS. Here, we review the structural properties of the metastable sulphides.…”

Determination of the lattice constant of CrS from Mn1–xCrxS MBE epitaxial layers

“…This requires a value for the lattice parameter of CrS, a CrS , in order to calculate the Cr mole fraction, x, in the Mn 1-x Cr x S layers. Initially a value of 5.806 Å was used for a CrS based on the previous estimates made at Heriot-Watt [4]. Figure 1 shows the 004 XRI and simulated data for the structure grown with T Cr = 1010 °C (Sample 1).…”

“…Preliminary calculations were carried out at Heriot-Watt using published atomic radii to calculate the lattice parameters of the first row transition metal sulphides and then taking into consideration the known lattice parameter of MnS. This indicated that the lattice parameter could be as large as 5.806 Å [4]. However, ab initio calculations carried out by I. Galanakis et al [5] have determined the lattice parameter of CrS to be 5.04 Å, while H. Shoren et al [6] calculated it as 5.309 Å.…”

Magnetic field induced changes of the magnetic phase transition in metastable zinc blende MnS layers grown by MBE

“…The films are, however, usually polycrystalline and exhibit a mixture of different crystallographic phases. We have shown earlier that MnS can be grown in the metastable zinc blende (ZB) phase with ZnSe cladding layers [6,7]. Bulk ZB MnS is a fcc type III antiferromagnet, but the effect of reducing the layer thickness and the presence of the interfaces on the magnetic phase transition are unknown.…”