Optical properties of mixed transition metal dichalcogenide crystals

Volume 9, number 1

OPTICAL PROPERTIES

December 1989

MATERIALS LETTERS

OF MIXED TRANSITION

METAL DICHALCOGENIDE

CRYSTALS

G.H. YOUSEFI Department of Physics, Sardar Pate1 University, Vallabh Vidyanagar, Gujarat 388 120, India

Received 18 August 1989; in final form 12 October I989

Mixed compounds of transition metal dichalcogenides with a general formula MO, _,Wse, (O
tical band composition.

1. Introduction

MoSe, and WSe2 are the end compounds of the series of mixed compounds with the general formula Mo,_,W,Se2. These end compounds have been studied extensively for their properties because of their applications as optical windows and other photovoltaic devices. These compounds have also been studied for their interesting optical and excitonic properties [ l-41, because they closely approximate two-dimensional semiconductor systems. However, there has always been an uncertainty about the size of the forbidden band gap and about the origin of certain features in the optical spectra. The optical properties of MoSe2 have been studied above the fundamental absorption edge by optical absorption measurements [ 5-8 1, by reflectivity measurements [ 9,10 ] and by electron energy-loss measurements [ 111. The optical absorption is known to arise through the interaction of the excited electrons with the lattice perturbed by vibrations or imperfections. In fact, the absorption phenomenon can be considered quantum mechanically as a two-step process in which the electron absorbs a photon and is excited to an intermediate state, where it interacts with the lattice vibrations or imperfections and reaches a final state, the net result being the absorption of a photon [ 121. The functional dependence of the absorption coefficient on the incident photon energy will indicate the nature of the associated electronic transition. The present paper reports the variation of op38

gaps

of

Mo,_,W,Se,

crystals

with

2. Experimental The mixed transition

metal dichalcogenide

crys-

200180160-

140IZOf-

80 60 40

‘I/l

i/ 20 I 1.0

I

/

I

1.1

I I

I

I

1.2 hv (eV) -

1.3

Fig. 1. Variation of (ahv)‘12 versus hv for various compositions. The compositions are x=0 ( l), 0.2 (o), 0.4 (X ),0.8 (A) and I.0 (A).

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Volume 9, number

MATERIALS

1

tals used in the present study were grown by the direct vapour transport technique. The details of crystal growth and characterization have been reported elsewhere [ 131. The absorption measurements were made with a UV-VIS-NIR spectrophotometer (Shi-

December

LETTERS

madzu UV-365, Japan) 2000 nm.

1989

in the wavelength range 300-

3. Results and discussion The absorption the relation,

coefficient

Z(X) =Z(O) exp( -ax)

(Ywas calculated

using

,

where Z(X) is the transmitted intensity, Z(0) is the incident intensity and x the thickness of the sample. The electronic transition at the fundamental absorption edge was analysed by plotting (oh v) n versus h v, assuming the relation [ 14,15 ] (ahv)“=A(hv-Eg),

x

, 1.35

;

I

I

I

I I

8

1.45

,I

;

I I

I

1

6

1.5 5

1.6 5

I

1.75

hu(eV) Fig. 2. Variation of (c&u)~ versus hu for various compositions. The compositions are x=0 (.). 0.2 (o ), 0.4 ( x ), 0.8 (A ) and 1.0 (A). Table 1 Variation

of the optical band gaps with composition

Composition

MoSe* MoO.sWO.&Z Mo0.6W0.,Sez Moo.2W0.,Se2 WSez

Band gap (eV ) direct

indirect

I .49

1.03

1.52 1.57 1.59 1.64

1.08 1.14 1.18 1.22

where A is a constant, hv is the energy of the incident photon, Eg is the optical band gap and n assumes values of 2 and l/2 for allowed direct and indirect transitions respectively. It is observed that for the series of compositions under study, the absorption could be explained satisfactorily with n = l/2 in the lowerenergy range (fig. 1) and with n = 2 in the higher-energy range (fig. 2 ). The values of the band gaps calculated from these curves are shown in table 1. The direct and indirect band gaps obtained in the present study for the end compounds, namely MoSe, and WSe,, are in good agreement with the reported values [ 2,16,17]. The band gaps show a gradual increase in their value as the tungsten content in the mixed compound increases. From these studies, it may be concluded that the mixed system MO, _XWXSez (0
Acknowledgement The author expresses his indebtedness to Dr. R. Rousina, National Research Council, Ottawa, Canada for her assistance in the work.

References [ 1] R.A. Hazelwood,

Thin Solid Films 6 ( 1970) 329.

39

Volume 9, number 1

MATERIALS LETTERS

[2] A.M. Goldberg, A.R. Beal, F.A. Levy and E.A. Davis, Phil. Mag. 32 (1975) 367. [3] J.A. Wilson and A.D. Yoffe, Advan. Phys. 18 ( 1969) 193. [4] E. Fortin and F. Raga, Phys. Rev. 11 (1975) 905. [ 51R.F. Frindt and A.D. Yoffe, Proc. Roy. Sot. A 69 ( 1963) 273. [6] B.L. Evans and P.A. Young, Phys. Sat. Sol. 25 (1965) 417. [ 71 B.L. Evans and R.A. Hazelwood, Phys. Stat. Sol. 4a ( 1971) 181. [8] A.R. Beal, J.C. Knightsand W.Y. Liang, J. Phys. C 5 (1972) 3540. [ 91 V.V. Sobolev, V.I. Donetskikh, A.A. Opalovski, V.E. Feddrov, E.V. Iobkov and A.P. Mazhara, Soviet Phys. Semicond. 5 ( 197 1) 909.

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December 1989

[lo] H.P. Hughes and W.Y. Liang, J. Phys. C 7 (1974) 1023. [ll]T.S.Moss, J.Appl. Phys. 32 (1961) 2136. [ 121 H.Y. Fan, Rept. Progr. Phys. 14 (1956) 119. [ 131 M.K. Agarwal and P.A. Wani, Mater. Res. Bull. 14 ( 1979) 825. [ 141 J.P. Pankove, Optical process in semiconductors (Butterworth, London, 1971) ch. 3. [ 15 ] T.S. Moss, Optical properties of semiconductors (Butterworth, London, 1961) ch. 3. [ 161 W. Kautek, H. Tributsch and H. Gerischer, J. Electrochem. Sot. 127 (1980) 2471. [ 171 R.F. Frindt, J. Phys. Chem. Solids 24 (1963) 1107.