Optical properties of WSi2 and MoSi2 single crystals as measured by spectroscopic ellipsometry and reflectometry

Solid State Connnunications, Printed in Great Britain.

OPTICAL

PROPERTIES

Vo1.62,No.7,

pp.455-459,

OF WSi2 AND MoSi2 SINGLE

0038-1098/87 $3.00 Pergamon Journals

1987.

CRYSTALS AS MEASURED BY SPECTROSCOPIC

+ .OO Ltd.

ELLIPSDMETRY

AND REFLECTOMETRY F. Ferrieu

CNET - CNS - BP : 98 - Chemin du Vieux C. Viguier, U.A.

CNRS 783 - Faculte

Ch&ne - 38243 Meylan Cedex - France

A. Cros

and A. Humbert

des Sciences de Luminy - Departement 13288 Marseille Cedex 9 - France 0.

Thomas,

R. Madar and J.P.

UA CNRS 1109 - ENSIEG - Domaine Universitaire Received

17 Novembre 1986,

in revised

de Physique

- Case 901 -

Senateur

- 38402 St Martin

form 21 January

d’Heres

1987 by E.F.

- France Bertaut

The dielectric functions of monocrystalline MoSi2 and WSi2 and their A comparison is established between reflectivity have been investigated. Absorption these two silicides and several differences are underlined. seen from the dielectric function and the reflectivity are suggested to come from sets of d orbitals bands below and above the Fermi level. In the Infra-red, both materials exhibit different transparency windows, at attributed to partly covalent Si-bonding. 0.46 and 0.6 eV respectively, These silicides are potential intrinsic solar absorbers.

Introduction and of their hiah conductivitv. Because thermal stability, transition metal silicidks have raised a growing interest. Many transition metal are candidates for interconnection silicides ;r;lications in the VLSI technology, but tungsten molybdenum chosen silicides are commonly because of high their refractory character, conductivity

and

passivating

properties.

using a modified Hukin-type4 cold copper crucible. WSi2 and MoSi2 both crystallize in a body centered tetragonal structure (Cllb space group 14/mmml. This structure consists of close packed MSi2 pseudo-hexagonal layers of composition stacked alon 1 the [llO] direction with a so called AB sequence (which is not a close packed sequence). Each W or MO atom is su rounded by ten $ Si atoms at a distance of 2.6 A . The lattice parameters are a = 3.206 A, c = 7.846 A, for MoSi2

Most

works report physical properties, as measured on thin films cosputtering pr obtained by coevaporation techniques. Electrical properties , and structure oriented properties2 have been mostly described. For most transition metal silicides, optical properties have not been investigated due to the In these materials, thin lack of single crystals. films optical measurements, by techniques such as and spectroscopic ellipsometry, may reflectometry3 be affected by the grain size of the films, the roughness and also possibly by some interface Cosputtered or coevaporated interference effects. sintered silicides are often silicon rich, i.e., Single crystals of MoSi and WSi, where x = 2.2. WSi2 have become available4 only recently . -+he present work is a direct continuation of a fi st spectroscopic analysis of MoSi2 single crystals 5 . At the present time it seems very interesting The main reason to compare these two materials. is that calculations of the band structure in both WSi and MoSi2 have been published6. The energy ban 2 structures, the binding cohesive energy and total or projected-integrated density of states are now known. II.

Crystal growth properties

and

summary of

and

a = 3.213

A,

c

q

7.830

A in

the

‘_ye

of

WSi2.

kl;;sirsd,d;rs;;,iz;3.

.d = 6.25 gr. cm, for MoSi2 in the case of WS12 agree with those deduced from the lattice parameters given above. The transport properties of MoSi2 single extracted from the same ingots as the crystals, one used here for o t’cal measurements thoroughly studied8 la. The resistiv/F;e p”“:: anisotropic at room temperature. The resistivity single crystal is p [OOl] = 12.64 pncm :,‘d M;svl10 ] : 16.92 @cm. a similar value In WSi is obtained, p [OOl] I 12.86 Mh’ cm as measured but no anisotropy has been

III.

Spectroscopic ellipsometry measurements Ellipsometry measurements have been carried out on a rotating polarizer spectroscopic ellipsometer. The already system has been described elsewhere in the literature. BY assuming an optically semi infinite medium, the dielectric function can then be extracted from :

physical

Because of the high melting points of refractory silicides such as MoSi2, the Czo”cdhri?frZI respectively 1980°C and 2165”C7, growth of these materials can only be achieved by

= Sin2

@ (1 + tan2 0

1-R ( ---12) l+R

(1)

where Q is the angle of incidence and r(= r /rs is the ratio of the reflectivity of light p Prallel (p) or perpendicular (s) to the incidence plane. 455

456

OPTICAL PROPERTIES OF WSi2 AND MoSi2 SINGLE CRYSTALS

We carried out the measurement for two angles of incidence 70' and 75' and observed a larger anisotropy for a 75" incidence. Our results correspond to this latter case. The investigated energy ranges here from 1.3 to 5 ev and the background noise on the measured values is within 1 percent which unfortunately does not enable us directly to get numerical1 the third derivative K2 of the dielectric function . Another source of inaccuracies is the assumption of an abrupt interface which is certainly not the case as the two crystals were only mechanically polished. Typical size of the samples are 4 x 6 mm. Mechanical grinding and felt polishing were used to obtain a mirrorlike surface in order to minimize the diffusion scattering of light after reflection. Surface roughness as well as the native oxide surface layer are also expected to modify the effective dielectric function mostly in the U.V. range, but not around the singularities, such as interband optical transitions, which occur here5 in the 1-3 ev spectral range. For both crystals, spectroscopic ellipsometry was carried out with the incidence plane of light respectively parallel and perpendicular to the uniaxis, i.e. the c tetragonal axis. The silicides were cut with the c crystalline axis parallel to the sample surface, (110) planes as determined by X-ray Laue diffraction patterns. The real and imaginary part of the dielectric fonction are Kramers-KrBnig transformed each other, therefore only , the imaginary part of the effective dielectric function , is reported. Measurements were done with the plane of incidence of light being the (110) and (001) crystal planes. The results for both MoSi2 and WSi2 crystals are shown in figure 1.

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.*.*,\

/I F,

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2 ‘\,

:..... \, ..%......\:, ‘l.. 1. ‘..“:.+ _. ‘,.. \., L_... ‘\ ‘....., .................b.: ._... -~-I

Vol. 62, No. 7

first study peaks in respectively at 2.f~8, 2.47 ev, (El) and at 3. i 2 ev, (E2), were found for MoSi2. These values are similar to those found here, i.e., 2.60 ev for the c axis, (110), and 2.45, 3.75 ev in the (a, b) plane. We have found a broad shoulder, in both case around 4.15 ev which arise from surface effects. The may effective dielectric function of WSi2, (see the corresponding dotted lines in Figure l), behaves very closely to that of MoSi2. However, the amplitude of , has a lower value for WSi2, than for MoSi2. Note also that the anisotropy between the (a, c) plane and the (a, b) plane in the case of MoSi and the singularities El, E2, are slightly shif e ed for WSi . A detailed list of these values is given in tab 1e 1. Some insight is obtained from the sum which gives the effective number of electrons per atom contributing to the absorption, optiyi2 "e.,f. wi hln a given photon energy range J :

neff(w)

(m/2 n2Nae2)

q

y'

w

c2

(w)

dw

(2)

Wl ~~~o~s~si'~',e~"~'~o~O~~s~~,~~~

.NaC~~~y~~~sg~:

the numerical integration of equation (21, we found an effective number of electrons varying between 2.40 and 3.0, for WSi2 and MoSi2 respectively. The total number of electrons in that range of energy, which corresponds to the densities of states involved in the optical transitions, is very much the same between MoSi2 and WSi2. The dielectric function ci (w),represents the absorption and thus involves t e Joint density of states. At this level, the information should be found in the theoretical band st ucture calculation published for WSi2 and MoSi2 6 . From these diagrams, see Figure 3 in ref. (6), two sets of energy bands are found, above and below Ef, separated by about 2.5 ev and parallel to each other in a large region of the Brillouin zone. The strong absorption peaks El and E2 in Figure 1 could be identified as interband transitions involving W atoms hybridized d states, coming from these energy positions of the band structure. However if interband transitions between d bands through the Fermi level can account for the main absorption feature observed at 2.5 ev, this is not Table 1 effective and E ,

Respective positions of maxima in the dielectric function , i.e., El following the crys $ al orientation. corresponds to the effective number of n ff(U If of equation (2) between 1.3 ev and 4.8 e f ectrons ev.

-

Xtal

directions

As it was the case for MoSi25, the aim of the experiment is to compare the energy position of MoSi2 and WSi2 maxima. In the case of MoSi2, we found, agreement with the see figure 1, a go0 results previously published 9 . As shown in this

I

Orientation

I

El

I

E2

I

"eff(U)

I

Vol.

62,

No.

7

OPTICAL PROPERTIES OF WSi, AND MoSi2 SINGLE CRYSTALS

In fact, one the only possible interpretation. can also find some regions of the Brillouin zone where transitions involving d bands and Ef can We do not explain the 2.5 ev absorption peak. want to speculate too much on the origin of this peak, but we notice that there is a good agreement between its energy (2.5 ev) and the binding energy of the large and main peak in the total integrated density of states (DOS) and projected W(d) or interband Si(p) DOS of WSi2. This favors transitions involving these W(d) and Si(p) The calculated DOS electron bands ending at Ef. *one are over the Brillouin and integrated interpretation of the anisotropy effect is not maximum quite straightforward. The E2’ nly in undetectable in the (a, c) plane, appears It has been suggested P , from the (a, b) plane. valence band density of states differences, that metal d electrons are more strongly coupled with Si p ones along a and b than along c. If we compare now the two silicides WSi2 and MoSi2 considering the (a, b) plane, it has been seen in Figure 1, that the shoulder (E2 maximum in is slightly shifted, 3.75 to 4.05 ev, and E2 (w)), broadened when going from MoSi2 to WSi2. Such a feature could be associated with the ionicity difference, pointed out by Kleinman6, so that a stronger interaction corresponding to more delocalized states in the case of WSi2, results in a wider gap value. 111.1. Determination of the mean optical index/ application to WSi cosputtered thin films For optical c t aracterization of thin films of WSi and MoSi2, the refractive index values of bot 6 materials are higly desirable. In Figure 2, a

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I

I

4

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>

25.0

5.0 -

0.0 1.0

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MEANVALUE

------COSPUTlEREDW Silieide I

I

I

2.0

3.0

4.0

WV)

5.0

Figure 2 Comparison between i) (a,~) and (a,b) planes in WSi2 single crystal effective dielectric medium functions , approximation, EMA, dielectric function CE~> = l/3 the continuous line, and + 2/3 E i.e., F-? 1119 the effecat’fde dielectric function of a 150 nm thick, cosputtered tungsten polycide film. effect due to the A clear interference multilayer the silicide structure, and transparency appears below 2.6 ev.

457

the dielectric made between comparison is measured in WSix, with x = 2.2 function, as cosputtered tungsten polycide and the mean value , as calculated from the Effective Medium Approximation (EMA) theory, i.e., = l/3 ~~~ + Z/3 cab. The dielectric functions cat and cab correspond to the single crystal measurements respectively in the (a, c) and (a, b) plane. Both results are very close with the only difference that the film 150 nm thick exhibits oscillations in the effective function below 2.6 ev revealing multi-reflection interferences in the The origin of such effect is poly-silicide layer. It could be attributed to several not so obvious. of the factors such as, i) the over-stoichiometry ii) the which is silicon rich and, silicide, interface roughness introduced by the finite grain size of the crystallites in the film, typically Moreover another possible around 50 to 60 nm. reason is that the interferences are associated to a transparency of the material in this energy This will be confirmed as shown below by range. Then the the reflectivity measurements. determination of oxide layers grown on silicide films with a single-wavelength, (i.e. at 2.0 ev, lead to standard HeNe) ellipsometer would erroneous oxide thickness determination, because of the “transparency” of the silicide film. Reflectivity in the visible and the infra-red The reflectivity has been measured between 0.2 ev and 3 ev for the WSi2 crystal. Our results are shown in figure 3a and 3b. Up to 0.6 ev, we used a Fourier Transform Infra-red Briiker IFS85 spectrometer with an Ag mirror as reference. Between 0.6 and 3.0 ev, the reflectivity R was measured using a reflectometer giving an absolute value of A with an accuracy of about 0.2 Z in the Here the main source of inaccuracy visible range. comes from scattering of light due to surface Similar measurements in the case of roughness. above 0.6 ev turned difficult because MoSi2, i.e., of the roughness of the sample. Only reflectivity in the Infra-red is shown in figure 3b. reflectivity The overall shapes of the WSi2 and ;i;;tra (Figure 3) are the same for Concerning the visible range, we note Both shou :’ ders centered around 1.2 and 2.5 ev. with absorption can be associated strong corresponding to interband transitions. This is in agreement with the ~2 (,_..I), spectra presented before, but the 1.2 ev transltlon is only reported here from reflectometry measurements, and corresponds only to the slight shoulders in the spectra as deduced from ellipsometry measurements (See figure 1). The low energy part of the reflectivity spectra is characterized for both silicides by a very sharp cut-off and a region of very low reflectance around 0.6 ev for WSi2 which differs definitely from MoSi2 around 0.45 ev. To explain these low reflectivity windows, we can use the theoretical densities of states. The formation of the silicide WSi2 has been shown to lead to partially covalent bonding between Si and W6. It is this covalent bonding which causes the dip in the WSi2 DOS at the Fermi energy. This quasi-gap in the density of states near Ef is due to the fact that relatively few bands cut Ef and offer a possibility of intraband transitions. The influence of the quasi-gap is to induce interband transition onsets at. 0.5 ev and 1 ev. We can now propose an interpretation of observed the reflectivity spectrum the of WSi2. Due to influence of the strong interband transitions occurring above 0.5 - 1.0 ev, there is a drop of IV.

458

OPTICAL PROPERTIES OF WSi, AND M&i,

1.1

I b . . . . . ..

I

i .. .

. . . . . . . . . . . . . .

I

I

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I

I

I

SINGLE CRYSTALS

Vol. 62, No. 7

I

WSi, IR REFLECTIVITY

n

A VISIBLEREFLECTlVllY

.i

0.6

3.0 E(eV)

WV) b

a

Figure 3a Reflectivity of tungsten silicide crystal under normal incidence. A minimum reflectivity dip occurs at the limit of the infra-red range (- 0.6 ev). 3b A comparison between the reflectivity of MoSi2 and WSi2 single crystals in the Infra-red. The minimum of reflectivity is shifted from 0.45 eV, i.e. for MoSi2, to 0.6 ev for WSi2. A similar effect is observed in thin films.

reflectivity around 0.6 ev (polarization effects). this Near the material is nearly energy, transparent (interband absorption at energies With lower than the quasi gap are not likely). increasing energy, there is a rise in ‘2 because of interband transitions in the visible part of the spectrum. Our leads to a explanation quasi-transparency of WSi2 in the reflectivity window. This has been confirmed experimentally for which using films about 3000 A thick The situation interferences have been observed. described here may be compared to the one observed But we do not believe that for WSi2 in silver. the transparency window is associated with a resonance because the reflectivity dip, plasma although well marked for WSi2, is not as sharp as We are presently trying to determine the in Ag. dielectric function of WSi2 further in the IR range in order to study in greater details the origin of the reflectivity dip. V. Conclusion In conclusion, new and interesting

WSiz and optical

MoSi2 appear properties.

to

have Our

results show strong absorption structures in the visible range. They are not present in the spectra of the pure metals and reflect the Si (s, p) W(d) hybridization associated to the formation of the silicides. We have also evidenced a transparency window in the infra-red around 0.46 eV and 0.6 eV for MoSi2 and WSi2 respectively with very sharp reflectivity decreases. This peculiar and unusual property has been related to the quasi-gap occurring in the density of states of the silicides near Ef and due to partly covalent Si-W or Si-Mo bonds. It is worth emphasizing that these materials have the appropriate spectral reflectivity for the realization of an intrinsic solar absorber, being reflecting in the infra-red and absorbing in the visible part of the spectrum with a very sharp transition region around 2 pm. Acknowledgments One of the authors, f. Ferrieu, is indebted to G. Giiltz, for providing the poly-silicide cosputtered sample and also to Dr. G. Fishman for fruitful discussion during the completion of this work.

Vol. 62, No. 7

OPTICAL PROPERTIES OF WSi2 AND MoSi2 SINGLE CRYSTALS

459

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4 5 6

B.Z. Li and R.C. Aitken, APL 46 (41, 401 (1985). A. Perio and J. Torres, APL 45, 8, 857-859 (1984). W. Henrion, G. Beddies and H. Lange, Phys.

Stat. Soli. (b) 131, K55-58 (1985). 0. Thomas, J.P. Senateur R. Hadar 0. Laborde, and E. Rosencher, Solid State Comm. 55, 7, 629-632 (1985). R. Madar, J.P. Senateur and Ph. Ged, Phys. Rev. B 29, 12, 6981-6984 (1984). B.K. Bhattacharyya, D. Bylander and L. Kleinman, Phys. Rev. B, 32, 12, 7973-7978

(1985).

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S.P. Hurarka, J. Vat. Sci. Technol. 17(4) 775-792. W. Jeitschko, Acta Cryst., 833, 2347 (1977). w. Zachariasen, Z. Physik Chem., 1288, 39 (1927). 0. Laborde, submitted to J. of Physics F (5 dec. 1985). 0. Thomas, Thesis-Grenoble(France) 1986. See for example L. Vi& and H. Cardona, Phys. Rev. 6 29, 12, 6739 (1984).