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

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Journal of Crystal Growth 275 (2005) 141–149

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Metastable group II sulphides grown by MBE: surfacemorphology and crystal structure

K.A. Priora,�, C. Bradforda, L. Davida, X. Tangb, B.C. Cavenetta

aSchool of Engineering and Physical Sciences, Brewster Building, Heriot-Watt University, Edinburgh, EH14 4AS, UKbCentre de Recherche sur l’Hetero-Epitaxie et ses Applications, Centre National de la Recherche Scientifique (CRHEA-CNRS), rue

Bernard Gregory, Parc Sophia Antipolis, F-06560 Valbonne, France

Available online 8 December 2004

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

using a simple MBE growth procedure that can increase the thickness of these metastable layers to over 130 nm. In this

paper, we review the growth method and features which arise during the growth of both MgS and MnS, namely the

development parallel surface ridges and the loss of the zinc-blende crystal structure.

r 2004 Elsevier B.V. All rights reserved.

PACS: 81.05.Dz; 81.10.Aj; 81.15.Ef; 81.30.Kf; 68.55.Jk

Keywords: A1. Low-dimensional structures; A3. Molecular beam epitaxy; B1. Sulfides; B2. Semiconducting II–VI materials

1. Introduction

II–VI semiconductors differ from III–V semi-conductors in that they can be found in a largenumber of different crystal structures, includingthe rocksalt (NaCl) structure. However, using athin film growth technique such as molecular beamepitaxy (MBE) or metal-organic chemical vapourdeposition (MOCVD), it is possible to grow

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.10.078

ng author. Tel.: +44 131 451 3035; fax:

6.

ss: [email protected] (K.A. Prior).

compounds and alloys in the zinc-blende (ZB)phase when it is not their stable (lowest energy)structure. Examples are the growth of CdSe [1] andCdS [2] on (1 0 0) substrates where the four-foldperiodicity of the underlying layer causes adoptionof the metastable ZB structure rather than thelower energy wurtzite structure. In this case, thesymmetry of the overlayer is changed, but bothstructures have four-fold co-ordination and thelocal bonding remains unchanged.

For compounds with NaCl as their stablestructure, an additional difference is that theatoms are six-fold co-ordinated. It is not obvious

d.

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Table 1

Summary of values obtained from thick MgS and MnS

epitaxial film layers

Compound MgS MnS

Lattice constant (nm) 0.562270.0002 0.555970.0002

Strain to GaAs 5.6� 10�3 1.7� 10�2

Poisson’s ratio 0.425 0.475

Residual zinc content (%) 0.5–2 3

Maximum thickness (nm) 134 132

K.A. Prior et al. / Journal of Crystal Growth 275 (2005) 141–149142

that growth of one of these compounds will form aZB epitaxial layer on an appropriate substrate. Aswe shall demonstrate in this paper, this does occur,and considerable effort has gone into exploring thegrowth of metastable ZB compounds particularlyMgS and MnS, which can be lattice matched toGaAs.

Initial attempts to grow MgS by MBE producedZB layers only 0.96 nm thick before changes in theRHEED pattern were observed, believed tocorrespond to a change in the crystal structure toNaCl [3]. Subsequent growth by MOCVD wasmore successful, with ZB MgS layers up to 10 nmthick [4]. Attempts by MBE to grow MnSproduced layers up to 50 nm before the ZB crystalstructure was lost [5,6].

Subsequently, we have shown that, by modify-ing the MBE growth process, much thicker layersof both MgS [7,8] and MnS [9,10] can be grown inthe ZB structure. We have successfully produced alarge number of different multilayer structuresexploiting the properties of these novel semicon-ductors. In particular, MgS is nearly latticematched to both GaAs and ZnSe, and has a bandgap of �5 eV, allowing us to produce structures inwhich highly confined excitons can be studied in2D (MgS/ZnSe quantum wells) [7,8] or 0D (MgS/CdSe quantum dots) [11,12].

Aspects of this work have been reviewedrecently [13], with emphasis on the growth methodand wide range of structures developed incorpor-ating MgS or MnS. Here, we review the structuralproperties of the metastable sulphides. Following abrief summary of the growth method, the mor-phology of the layers is described in Section 3. InSection 4, we discuss the development of thesurface morphology with the formation of parallelridges. The loss of the ZB crystal structure isdescribed in Section 5 and methods of transform-ing the crystal structure from ZB to NaCl will bediscussed in Section 6.

2. MBE growth method for metastable sulphides

The method used to grow layers of metastablesulphides is the same for the two compoundsinvestigated in detail (MgS and MnS). Samples are

grown in a Vacuum Generators V80 H MBEsystem on GaAs (1 0 0) substrates, on which thin(20–50 nm) buffer layers of ZnSe have beendeposited. Conventional K-cell sources are usedfor Zn, Se and the metals Mg, Mn and Cr. Sulphuris provided by evaporation of ZnS, which evapo-rates congruently producing both Zn and S2 fluxes.The MBE system contains no other source ofsulphur than the ZnS cell, and the backgroundsulphur pressure is kept as low as possible, helpedby a liquid nitrogen cooled shutter in front of theZnS source. As a result, sharp RHEED streaks areobserved from the GaAs substrates after oxidedesorption and during the subsequent cooling tobelow 300 1C which occurs under an applied fluxof zinc [14]. This gives reproducible nucleation of2D growth of ZnSe, as shown by streaky RHEEDpatterns.

Growth of the metal sulphide layers occurstypically at substrate temperatures of around230–270 1C. This temperature is higher than thatused for the growth of ZnS [15] and under theseconditions the growth rate of ZnS is practicallyzero. With an applied Mg or Mn flux, however, weobtain a growth rate for the metal sulphide ofapproximately 0.15 mm h�1. Although the growthoccurs under sulphur rich conditions, residual zinccontamination in the layers is low and is sum-marised with other relevant material data inTable 1.

3. Stability of metastable sulphide epitaxial layers

MgS and MnS are metastable and the ZBcrystal structure is easily lost. This can be easilyobserved during growth by RHEED, and after

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Fig. 1. MgS RHEED patterns from the ½0 1 1� azimuth of layers (a) 10 nm thick, (b) 60 nm thick and (c) showing the development of

inclined facets after the loss of the ZB structure.

K.A. Prior et al. / Journal of Crystal Growth 275 (2005) 141–149 143

growth by X-ray double crystal rocking curve(DCRC) analysis and atomic force microscopy(AFM) [16].

Fig. 1 shows RHEED patterns for a series ofMgS layers grown at a substrate temperature of240 1C. A thin layer, approximately 15 nm thick isshown in Fig. 1a. The long RHEED streaksindicate that growth is 2D. RHEED oscillationshave never been observed during the growth ofMgS, suggesting that the growth method is step-flow. AFM measurements have been performed onsamples which displayed this RHEED patternduring growth, after first depositing a thin (5 nm)ZnSe cap to prevent oxidation. The sample surfaceis flat with a mean roughness value (Ra) of0.435 nm, comparable to that of a ZnSe epilayer.Sample thicknesses and growth rates have beenobtained in multilayer structures of MgS or MnSwith ZnSe by using the X-ray Interferencetechnique (XRI) which is described in detailelsewhere [17]. This technique also indicates thatthe high quality and extreme flatness of these

layers is maintained in the multilayer structures, asdo photoluminescence spectra obtained from ZnSe[7,8] or CdSe [11,12] quantum wells with MgSbarriers.

As the thickness of the MgS or MnS epitaxiallayer is increased to approximately 20 nm, theRHEED pattern becomes spotty in the ½1 1 0�azimuth, indicating a transition to a roughersurface (Fig. 1b). This is also observed in anAFM scan, Fig. 2, which shows ridges manymicrons long running parallel to [1 1 0]. Theseridges have a regular periodicity of 50 nm and aretypically 0.5 nm high.

The epitaxial layer is still ZB at this stage. Thisis shown in the DCRC trace in Fig. 3a, whichclearly shows to the right of the GaAs substratepeak a weak signal from a MgS layer in the ZBcrystal structure. Loss of the ZB crystal structure isobserved by RHEED with the formation ofinclined streaks, as shown in Fig. 1c. Transitionbetween the two structures is fast (o1 s), and hasallowed us to determine values for the maximum

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Fig. 2. AFM scan of a 20 nm thick ZB MgS surface showing

1D periodic features parallel to [1 1 0].

-1500 -1000 -500 0 500 1000 1500 2000 2500

(a)

(b)

(c)

Inte

nsity

(de

t. co

unts

)

Th\2Th (arcsecs)

Fig. 3. 400 DCRC curves from samples with the following

structures: (a) ZnSe (5 nm)/MgS (67 nm)/ZnSe (5 nm), (b) ZnSe

(5 nm)/MgS (25 nm)/ZnSe (10 nm)/MgS (134 nm)/ZnSe (5 nm)

and (c) ZnSe (50 nm)/MgS (125 nm)/ZnSe (10 nm).

Fig. 4. AFM scan of 200 nm rocksalt MgS surface showing the

formation of cracks along /1 1 0S.


thickness (Table 1). After the transition, DCRCshows no signal from ZB MgS (Fig. 3c). AFMscans of samples that have transformed show thatthe layers have many deep cracks running along/1 1 0S directions (Fig. 4), penetrating the entirethickness of the layer down to the GaAs substrate.

4. Morphology of ZB epitaxial sulphide layers

In the previous section, the surface morphologyof the metastable epitaxial layers was shown toevolve into a series of parallel ridges, as shown inFig. 2. Similar ridge structures have also beenobserved in other materials with the epitaxiallayers under tensile strain, for example InAlAs [18]and InGaAs [19–21] grown on InP. In these alloys,tensile strain is found to cause significant aniso-tropy within the layers, with relaxation occurringpreferentially along [1 1 0] rather than ½1 1 0�[18,20,21].

Anisotropic relaxation has been suggested inMgS as being responsible for the formation of thesurface ridges [16]. The mechanism suggested isthat the anisotropic relaxation of the semiconduc-tor creates a network of mismatch dislocationsoriented preferentially along one of the /1 1 0Sdirections. Mismatch dislocations are known torepel adsorbed atoms diffusing over the surface,thereby acting as templates for the confined

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nucleation of nanostructures from adatoms [22].Thus, the dislocation network increases theanisotropy of the diffusion coefficient for materialtransport on the surface.

For InGaAs, an alternative explanation hasbeen suggested [19]. Here the height of the ridgesincreases with layer thickness, compatible with amodel involving stress driven mass transport ofmaterial from the regions between the ridges. Thespontaneous generation of surface undulationswith a preferred periodicity under stress is knownas the Asaro–Tiller–Grinfield instability [23,24].

We are unable to grow very thick layers ofmetastable sulphides and monitor the developmentof the ridges. However, it is possible to distinguishbetween the two possible mechanisms. If theepitaxial layer has relaxed anisotropically, bothmechanisms predict that 1D ridges form on thesurface, but in orthogonal directions. In the case ofthe mechanism we suggested previously [16], theridges will be parallel to the strained direction,while the Asaro–Tiller–Grinfield instability pre-dicts they are orthogonal. X-ray reciprocal spacemapping measurements on partially relaxed layerswill be needed to distinguish between the twomechanisms.

5. Transformation of ZB to rocksalt

Transformation of ZB crystals to NaCl has beenstudied experimentally, under high pressure [25]and theoretically [26–28] for many III–V and II–VIsemiconductors. The transformation process in-volves a change in co-ordination number fromfour to six for all atoms. It is accomplishedwithout any diffusion and is a martensitic transi-tion [29].

Different diffusionless mechanisms are possiblefor transforming ZB into NaCl. However, Sowa[26] has shown that all these pathways must occurvia a lower symmetry intermediate with a spacegroup that is a subset of the space groups of bothZB and NaCl. Fig. 5 shows a diagram of oneschematic pathway involving a transformationmechanism similar to that determined for ZnS[28]. This fulfils the criteria of Sowa, proceedingvia an orthorhombic intermediate. The diagrams

show the strains applied and the orientation of theresulting NaCl lattice with respect to the ZBsubstrate. It should be noted that this is only onepossible pathway and the correct (lowest energy)route for MgS has not been established. However,all pathways transform via an orthorhombicintermediate and result in the same relativeorientations of the structures as shown here.

Although the NaCl structure is considerablydenser than ZB, Fig. 5 shows that the transforma-tion mechanism requires contraction only alongone axis, producing a small expansion along thetwo orthogonal axes. The loss of symmetry at theorthorhombic intermediate also gives rise tomultiple domains. A mosaic of such domainswould show a similar pattern of deep cracks tothose observed in Fig. 4.

A review of recent experimental work onstructural transformations of semiconductors athigh pressure [25] has concluded that in manycases the transformation from ZB occurs to a lowsymmetry crystal structure. These low symmetrystructures appear to be metastable intermediates,rather than transition states, and transformationfrom them to NaCl can be slow. Under pressure,therefore, the crystal can be frozen in a lowsymmetry structure.

Previously, we observed the loss of the ZBcrystal structure and assumed that transformationhad occurred directly to the stable phase, NaCl[7,8,13]. At present, however, we have no directevidence for the formation of this phase, ratherthan an orthorhombic metastable structure.

Any transformation requires a considerabledistortion of the unit cell and changes in the localbonding environment around each atom. In Table2, the elastic constants C11 and C12 calculated forMgS [30] are compared with values for GaAstogether with parameters derived from them.From the elastic constants, we derive a value forPoisson’s ratio, n, in excellent agreement with thevalue obtained experimentally by DCRC inTable 1. In comparison, the value of n for GaAsis 0.31 [31] and is typical of other III–V and II–VIcompounds.

The unusually high value of n for MgS derivesfrom the fact that C11 and C12 are much closer inmagnitude than in GaAs. The quantity C11–C12 is

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known as the elastic shear constant [32] and is ameasure of the resistance of the crystal to shearalong a h0 1 1i direction lying in a 1 1 0f g plane [29].

c′ (a)

(c)

In MgS this quantity is substantially smaller thanin GaAs, and the structure is much less stable toshear. These directions are the parallel to the

a′

b′

(b)

(d) aR

bRcR

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Table 2

Elastic constants for GaAs (experimental values [31]) and MgS

(calculated values [30]) and quantities derived from them

Compound GaAs MgS

C11 (GPa) 119 74

C12 (GPa) 54 54.7

n 0.31 0.425

C11–C12 (GPa) 65 19.3

C1 (eV) 2.29 0.67


strains shown in Fig. 5 along the orthorhombicaxes a0 and b0.

A quantity related to the elastic shear constant isC1, the bond bending force constant, defined as

C1 ¼ a3ðC11 � C12Þ=32; (1)

which is also much smaller for MgS. Sensitivity toshear and a small bond bending force constant areconsistent with the model of structural deforma-tion outlined above.

Values of C11 and C12 have not been calculatedfor MnS, but may be estimated from the MgSvalues. Rewriting C11–C12 in terms of n:

C11 � C12 ¼ C111 � 2nð Þ

1 � nð Þ: (2)

From this, we estimate changing n from 0.425 to0.475 will decrease both C11–C12 and C1 by afactor of three, assuming similar values for C11 forboth compounds.

6. Onset of transformation and stability of the ZB

phase

Once a maximum critical thickness of a meta-stable sulphide has been deposited, the structure

Fig. 5. Two unit cells of MgS, using a non-standard tetragonal unit c

respectively small black and large grey circles. (a) The anion and cation

Mg atoms have very short bond lengths along a0. The lattice expands

six-fold co-ordination is clear, but the bond angles are distorted. The

The unit cell contracts along b0 and c0. (d) The final NaCl structure

conventional ZB axes, the cubic axes of the NaCl crystal (aR, bR and

orthogonal but have been distorted during the transformation.

rapidly changes. Here, we consider reasons for theeventual loss of stability.

One possible reason is that pseudomorphic MgSand MnS layers on GaAs substrates are strainstabilised. Both materials are under slight tensilestrain (Table 1). Ekbundit et al. [33] havecalculated the stabilities of different MgS struc-tures. They calculated that the ZB and wurtzitephases can be stabilised under a negative hydro-static stress (negative pressure) of 6.9 GPa forwurtzite, and higher for NaCl.

However, using the elastic constants given inTable 2, we estimate that this requires a volumestrain of approximately 25%—almost 50 timeslarger than the biaxial strain in the epitaxial layer.Additionally, thick layers of ZB MgS and MnS arenearly completely relaxed. This is shown in Fig.3b, a 400 DCRC trace for the thickest MgS layerproduced so far with almost complete relaxation.In MgS, the compound with the smallest latticemismatch with GaAs, relaxation can be observedin layers only 60 nm thick [8] and should occur ineven thinner layers of MnS.

In optimising the growth conditions, we deter-mined certain factors applicable to both MgS andMnS. First, the maximum thickness is a sensitivefunction of the growth temperature, decreasingrapidly over the temperature range 240–300 1C.However, subsequent in vacuo annealing of asample above the growth temperature has no effecton the stability of the layer. We also found that themaximum thickness under optimum conditionscan be significantly increased by the use of bufferlayers—either MgS/ZnSe or MnS/ZnSe.

We conclude that conversion of the epitaxiallayer from ZB to NaCl (or any other intermediatestructure) does not nucleate within the bulk of theepitaxial layer, but instead originates at thesurface. As described in Section 3, the surface

ell, with a0 ¼ 1=2½1 1 0�; b0 ¼ 1=2½1 1 0� and c0 ¼ c: Mg and S are

sublattices are displaced in the direction of the arrows. (b) Now

along this direction. (c) The unit cell is orthorhombic. Now the

bond angle at the Mg atom marked in the diagram is only 701.

, using the same tetragonal unit cell as before. Relative to the

cR) are [1 1 0], ½1 1 1� and ½111�: The latter two directions are not

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before transformation is covered in ridges approxi-mately 2 ML high. High local stresses associatedwith the ridges could act as nucleation points onthe surface for the transformation. Unfortunately,as the transformation is so fast, we have beenunable to study it in detail.

Under similar conditions, the InGaAs andInAlAs layers grown under tensile strain alsodevelop a variety of structural defects. In parti-cular, cracks approximately 1 mm long are ob-served in InGaAs layers [18]. A crack in a strainedlayer releases some of the strain energy, but alsoincreases the surface free energy, and the resultingcrack size is a balance between the competingeffects. In the case of the metastable II–VIcompounds, a crack, once started will allow thematerial around it to change structure. Theresulting energy release is sufficient to drive thetransformation and the crack would propagateacross the surface of the substrate.

7. Conclusions

Metastable sulphides can be grown on ZBsubstrates using standard MBE growth techni-ques. During growth, the epitaxial layer surfaceevolves ridges. The mechanism causing this is notunderstood, but is presumably related to aniso-tropic relaxation in the layer. Subsequently, thecrystal structure changes to NaCl with the devel-opment of deep cracks. The mechanism of thetransformation is diffusionless, and the transfor-mation starts at the sample surface.

Acknowledgements

We are grateful to EPSRC for continuedfunding.

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

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