Structure of sputter-deposited V-Si amorphous alloys

ELSEVIER

Journal of Non-Crystalline

Solids 205-207

(1996)

660-664

Structure of sputter-deposited V-Si amorphous alloys T. Fukunaga *, T. Ishizuka, H. Ishihara, T. Koyano, U. Mizutani Department

of Crystalline

Materials

Science, Nagoya

University,

Fwo-cho,

Chikusa-ku,

Nagoya

464-01,

Japan

Abstract The metal-insulator transition of k’Ji,,,-,x amorphous alloys were investigated from a structural point of view. The partial Si-Si and V-Si pair correlations for the V-Si amorphous alloys below 37 at.% V are derived from aOcombination of S(Q)s obtained by X-ray and neutron diffractions. New Si-Si neighbour distribution peak at about 2.85 A, which is not existent in pure amorphous Si, is observed to grow up by alloying V with Si. The coordination numbers of Si atoms and V atoms around a Si atom vary as a function of the V concentration. Especially, it is noteworthy that the rate in the variation abruptly changes at 18-19 at.% V. By comparing the dependence of the electrical conductivity with the V concentration, it can be concluded that the sudden change of the atomic rearrangement is responsible for triggering the metal-insulator transition.

1. Introduction A number of transition-metal phous alloys have been studied

metalloid amorin regard to a

metal-insulator transition, which has been reported to occur at around lo-20 at.% metal [l-4]. Most of the studieshave been carried out from a viewpoint of the electronic properties and electronic structure. The observation of the rearrangementof metalloid atoms induced

when

metal

atoms are added to the amor-

phous metalloid matrix has been so far very limited that no detailed discussionhas been carried out on the change of the atomic structure inducing the metal-insulator transition. Therefore, the detailed observation of the local rearrangementaround a metalloid and a metal atom is necessary to understand the origin of the metal-insulator transition.

* Corresponding 789 3724; e-mail:

author. Tel.: +81-52 789 4462; fax: [email protected].

0022-3093/96/$15.00 Copyright PII SOO22-3093(96)00466-S

0 1996 Elsevier

+81-52

In this paper, we focus on understandinga change in the atomic structure associatedwith the metal-insulator transition. In order to extract information on the local rearrangement of constituent atoms in the metal-insulator transition region, V-Si amorphous alloys were chosenas a sample, in which the transition hasbeen observedin the range 13-15 at.% V by measuringthe electrical conductivity [Il. A negligibly small coherent scattering length of V atom compared to that of Si atom for neutron diffraction enablesus to derive the partial V-Si and Si-Si pair correlation functions from total structure factors observed by X-ray and neutron diffractions.

2. Experimental procedure An alloy target was made from 99.9999% Si and 99.95% V by arc-melting. V,Si,OO-x (x = 12, 14, 15, 19, 21, 24, 29, 37 at.%) amorphousalloys were prepared by DC-sputtering technique. The sample

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was accumulated up to 0.3-0.6 mm in thickness onto a water-cooled copper substrate. The composition was determined by X-ray scanning microanalyser (SEA2010, SEIKO). Measurements of the atomic structure were made by X-ray diffraction with MoKCY radiation and neutron diffraction using the HIT spectrometer installed in BSF of KEK in Japan.

3. Results Fig. 1 and Fig. 2 show structure factors S(Q) observed by X-ray and neutron diffractions for V,xSi, --x amorphous alloys as a function of V content. The total S(Q) of the V-Si amorphous alloy can be described by a weighted sum of three partial structure factors (S,(Q), S,,,(Q) and ssisi(Q)) following the Faber-Ziman definition IS]

s(Q)

C2 b2 = x&,(Q) cb)2

+ 2Cv~;~vbsiSvsi(Q)

C2.b2. =ssisi( + (bj2

Q) 7

(1)

where Ci and bi are the concentration and the coherent scattering length of component i atom (i = V or Si), respectively, and (b) = C,bv + C,,b,,. The coherent scattering length of V atom for neutron diffraction is very small (b, = -0.0382 X lo-l2 cm) compared with that of Si atom (bsi = 0.4149 X

0” 0

I 15 Q (A-‘)

, 20

I 25

Fig. 2. Neutron structure factors SN(Q) for V,SI,,,-, 14, 15, 19, 21, 24, 29, 37 at.%) amorphous alloys.

I 30 (x = 12,

lo-l2 cm). In contrast, the scattering length of V atom (fv(0) = 23) at the scattering vector Q = 0 for X-ray diffraction is larger than that of Si atom (fsi(0) = 14). The weighting factors of the V,,Si,, amorphous alloy for X-ray and neuron diffractions are calculated by using Eq. (1): X-ray: Sx( Q) = O.O33S,( Q) + 0.299Svsi( Q) + 0.667Ssi,,( Q) ;

(2a)

neutron: SN( Q) = O.OOOSvv(Q) - 0.026Svsi( Q) + 1.026Ssisi(Q) .

P4 The weighting factor of the V-V pair correlation for neutron diffraction is negligibly small in comparison with those of the V-Si and Si-Si pair correlations. It is also noted that the weighting factor of the V-Si pair correlation is negative and much smaller than that of the Si-Si pair correlation, as seenin Eq. (2b). The weighting factor indicates us that the total sN(Q) can be easily regarded as the partial S,,,,(Q) within an error less than 3% in the V,,Siss amorphous alloy. 0 0

2

4

6

8

10

12

14

16

4. Discussion

Q(A-‘, Fig. 1. X-ray structure factors S’(Q) for V,Si,,,-, 15, 19, 21, 24, 29, 37 at.%) amorphous alloys.

(x = 12, 14,

The structural variation as a function of V content is clearly visible in S’(Q) and sN(Q) as shown in

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Fig. 1 and Fig. 2. As mentioned above, the SN(Q) of the V,, Si,, amorphous alloy reflects mainly the partial S&Q>, which resemblesthe S(Q) of pure amorphousSi in shape. The atomic structure of an amorphousSi gradually varies asa function of the V content in both ,SN(Q) and S”(Q). The Sx( Q) of the V,,Si,, amorphous alloy shows a sharp first peak characteristics of a metallic glass. In order to extract further information about a structural changeinducing the metal-insulator transition, we derived the partial Si-Si and V-Si pair correlation functions from the combination of the S’(Q) and SN(Q) data. The contribution of either the V-Si or Si-Si pair correlation can be intentionally reduced to zero by choosing appropriate coefficient in the linear combination of Eqs. (2a) and (2b). The linear combination with a vanishing coefficient for Svsi(Q) or S,,,,(Q) is.expressedas AS( (2) = -0.026Sx(

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Fig. 3. Radial distribution functions (ARDF(r) = W,,FlDF,,(r) + W,isi RDFs&r) = RDFsisi(r)) for V,Si,,,-, (X = 12, 14, 15, 19, 21, 24, 29, 37 at%) amorphous alloys, together with NY?(r) of pure amorphous Si.

Q) + 0.299SN( Q)

= 0.003Svv (Q) + 0.997Ssisi(Q) ,

(3a)

AS(Q) = 1.026Sx( Q) - 0.667SN( Q) = O.O96S,, (Q) + 0.904Svsi( Q) .

w It happens that the coefficients of S,,(Q) in Eqs. (3a) and (3b) becomenegligibly small in comparison with that of S,,,,(Q) and about one-tenth of that of Svsi( Q). The radial distribution functions ARDF( r) obtained by the Fourier transformation of the AS(Q) defined in Eqs. (3a) for all V,Si,-, amorphous alloys are shown in Fig. 3, together with that of pure amorphous Si reported by Fortner and Lannin [6]. The ARDF(v)s for V,,Si,, and Vd4Sijg amorphous alloys were calculated as follows: V,, Si,, amorphousalloy ARDF( r) = O.O03RDFv,( r) + 0.997RDFsisi(Y) (ha) ARDF( r) = O.O96RDF,,( r) + 0.904RDF,,,( Y); (9 V,, Si,, amorphousalloy ARDF( Y) = O.O85RDFv,( r) + 0.915RDFsisi(r) (5a) ARDF( r) = 0.313RDF,,( r) + 0.687RDF,,,( r), (5b)

where RDF”v(v), RDF,,,(r) and RDFs,,,(r) are partial radial distributions of the V-V, V-Si and Si-Si pairs, respectively. When the concentration of V atom increasesto 37 at.%, the weighting factor of the V-V pair correlation becomeslarger to be 0.313 as shown in Eq. (5b). It is generally true that the local atomic structure of an amorphous material is chemically similar to that of a corresponding crystalline compound. Hence, the atomic structure of the VSi, crystal [7] was calculated to allow a comparison with that of the V,,Sis3 amorphousalloy. It was found that no V atom around a V atom is located within 3 i in the VSi, crystal. Therefore, the contribution of the V-V pair correlation to the atomic distribution within 3 A in RDF( r) for the V,,Si,, amorphousalloy should be negligibly small. According to th,e supposition, we believe that the peaks below 3 A in each of ARDF(r)s derived by Eq. (4) or (5) reflect only the partial Si-Si and V-Si pair correlations, respectively. The ARDF(r) of the partial Si-Si pair correlation has two peaks below 3 A corresponding to th,e Si-Si pair correlation; on: is located at about 2.4 A and tte other about 2.85 A. The small peak at about 2.85 A does not exist in the RDF(r) for pure amorphous Si observed by Fortner and Lannin [6] and Mosseri et al. [S] and, hence, it can be concluded that it grows up due to the rearrangementof Si atomsby

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taking place when V atoms are introduced into the pure amorphous Si matrix. The coordination numbers of the Si-Si correlations calculated from areas of the 1st and 2nd peaks are plotted in Fig. 4, together with their peak positions. The Si-Si distances of the 1st and 2nd peaks slightly increase with increasing the V content. The coordination numbers also gradually vary as a function of V content in both the 1st and 2nd Si-Si correlations. As is clear from Fig. 4 a decreasing or increasing rate of the coordination number abruptly changes at about 18 at.% V. The result suggests that the short range structure around Si atoms changes over about 18 at.% V. Fig. 5 shows the coordination number and the 1st nearest distance of the V-Si correlation as a function of the V concentration. The coordination number of V atoms around a Si atom was calculated from the area of the 1st peak at about 2.55 A. The number gradually increases with increasing the V concentration but then the increasing ratio suddenly changes at around IS-19 at.‘% V, although the V-Si distance remains constant throughout the V concentration. The coordination number of V atoms around a Si atom approached 4.1 for the V,, Si,, amorphous alloy, the number of which is close to that for the VSi, crystal. The present result suggests an increase of the short range order existing in the VSi, crystal. Evidence for an insulator-metal transition has 3.4 3.2

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at% V Fig. 5. The coordination the 1st nearest distance the V content.

number of V atoms around a Si atom and of the V-S correlation as a function of

been reported at around 13 to 15 at.% V by Boghosian and Howson [l] from the results for the temperature dependence of electrical conductivity for the V-Si amorphous alloys. The insulator-metal transition must be strongly related to the structural changes. Especially the change of the Si-Si correlation at tbout 2.85 A and the V-Si correlation at about 2.55 A should be deeply related to the transition. Therefore, our results allow us to conclude that a structural change responsible for the insulator-metal transition takes place in the vicinity of 18-19 at.% V.

I 1

5. Conclusions

Fig. 4. The coordination numbers of the Si-Si correlations calculated from the areas of the fkst and second peaks at 2.4 w and 2.85 A, together with their distances as a function of the V content.

The structure of the V,Si,,,-, amorphous alloys was studied by using a combination of X-ray and neutron diffractions. Partial Si-Si and V-Si pair correlation functions could be derived by taking an advantage of the small coherent scattering length of V atom in comparison with that of Si atom for neutron diffraction. The accommodation of V atoms in the, amorphous Si yielded a new peak at about 2.85 A in the partial Si-Si distribution function. The atomic neighbour distances concerning the first nearest Si-S$ newly grown up Si-Si and V-Si pairs below 3 A did not change significantly. In contrast, a drastic change of the coordination numbers of the Si-Si and V-Si pair correlations was apparently

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observed at around 18-19 at.% V in the V-Si amorphous alloys, the concentration of which is very close to that of the transition observed by the measurement of the electrical conductivity. Therefore, we believe that the structural changes show the atomic rearrangement inducing the metal-insulator transition.

Acknowledgements One of the authors (T.F.) owes this work to the support of the Kawasaki Steel 21st Century Foundation.

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References [l] H.H. Boghosian and M.A. Howson, Phys. Rev. B41 (1990) 7397. [2] M. Vergnat, G. Marchal, M. Piecuch and M. Gerl, Solid State Commun. 50 (1984) 237. [3j Ph. Mangin and G. Marchal, J. Appl. Phys. 49 (1978) 1709. [4] J.B. Korttight and A. Bienenstock, Phys. Rev. B37 (198% 2979. [5] T.E. Faber and J.M. Ziman, Philos. Mag. 11 (1965) 153. [6] J. Fortner and J.S. Lannin, Phys. Rev. B39 (1989) 5527. [7] P. Villars and L.D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, Vol. 3 (1985) 3197. [S] R. Moss&, C. Sella and J. Dixmier, Phys. Status Solidi (a)52 (1979)

475.