Lattice dynamics and elastic properties of zinc-blende MgS

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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...

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confidence: 87%

Growth by MBE and characterization of metastable group II sulphides

Prior

Bradford

David

et al. 2004

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“…In spite of this importance, experimental and theoretical information regarding the elastic con-stants and mechanical properties of MgS is scarce. Moreover, to the best of our knowledge, there are only two investigations so far which report the elastic constants and bulk modulus for ZB MgS [9,10]. While Wolverson et al [9] have calculated elastic constants, C 11 and C 12 , by using density functional theory (DFT) and used them to calculate the zone-center LO phonon frequency and compared with experimental data obtained from their own investigated Raman spectra, Chung et al [10] have reported only the elastic constants.…”

Section: Introductionmentioning

confidence: 96%

Mechanical, elastic and anharmonic properties of zinc blende MgS compound

Jha

Talati

2003

The mechanical, elastic and anharmonic properties of zinc blende MgS compound have been investigated using a three-body force potential arising from the charge transfer effects due to the deformation of the electronic shell of overlapping ions. The agreement between the results for elastic constants and bulk modulus and the available data is generally satisfactory. Values for the zone center LO phonon frequency, Debye temperature and specific heat at constant volume have also been obtained. In the case of anharmonic properties, such as third-order elastic constants, physically significant and systematic values have been obtained similar to other compounds of the zinc blende structure.

“…MnS has a similar reduced mass to ZnS and the LO phonon frequency of this material is also similar , 356 cm -1 . In this respect it is unlike MgS, where the LO phonon frequency of 421 cm -1 is far lower than the value predicted by this model of approximately 500 cm -1 [8].…”

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confidence: 49%

Characterization of heterostructures containing MnS grown by MBE

David

Tang

Beamson

et al. 2004

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MnS can be grown lattice matched to GaAs and ZnSe in the metastable zinc blende structure. Recently, layers of MnS over 100nm thick have been produced which are thick enough for structural characterisation. In this paper we report measurements obtained from X-ray diffraction, Raman and X-ray Photoelectron Spectroscopy. X-ray diffraction has been used to obtain a value of Poisson's ratio of 0.475. This is unusually high and has implications for the stability of the material. Raman spectroscopy has been used to determine a value for the LO phonon frequency of 380 cm -1 which is in line with the value estimated from its reduced mass. XPS has been used to determine a value for the Valence Band offset. Unlike previous measurements on this compound we have found that the MnS growth must start with Mn, and not S. This presumably reflects the differences between our growth method and that used previously to grow zinc blende MnS.1 Introduction Semiconductors that do not have the usual zinc blende (ZB) or wurtzite crystal structures as their lowest energy configurations are interesting for a number of reasons. Many of these compounds contain transition metals, including materials that are ferromagnetic at room temperature. These compounds have potential spintronic applications if they could be grown epitaxially in the ZB crystal structure. Growth of such compounds on a ZB substrate allows the formation of the metastable ZB form of the compound up to a critical thickness, above which the compound reverts to its stable structure. In the case of manganese sulphide (MnS), the stable crystal structure is rocksalt. Conventional MBE growth has previously yielded layers only a few nm thick of metastable ZB when grown on GaAs (001) substrates [1].Recently, at Heriot-Watt, we have devised a new MBE growth method that is particularly effective at producing increased thicknesses of metastable ZB sulphides. This technique was developed for the growth of ZB magnesium sulphide (MgS) [2] but we have demonstrated that MnS layers of up to 132 nm can be grown in the ZB phase [3]. This is over four times thicker than the samples reported previously grown using conventional MBE [1].Due to the availability of thicker layers, the properties of MnS can be measured in more detail. Here, we describe the use of X-Ray Interference techniques to provide structural information of the layers grown and the determination of Poisson's Ratio, as well as the measurement of the Valence Band (VB) offset between ZnSe and MnS using the Daresbury RUSTI XPS system. Raman spectroscopy has also been carried out on MnS layers at the University of Bath and the LO phonon frequency has been determined.