Some physical properties of ReSi1.75 single crystals

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applied surface science ELSEVIER

Applied Surface Science 91 (1995) 82-86

Some physical properties of ReSil.75 single crystals U. Gottlieb a,b,*, M. Affronte c, F. Nava c, O. Laborde b,d, A. Sulpice b, R Madar a a Laboratoire des Mat£riaux et du G#nie Physique, Ecole Nationale Sup£rieure de Physique de Grenoble, lnstitut National Polytechnique de Grenoble, BP 46, Domaine Universitaire, 38402 St. Martin d'Hkres, France Centre de Recherches sur les Trbs Basses Temperatures, Laboratoire associ£ h l'Universit£ J. Fourrier, CNRS, BP 166, 38042 Grenoble Cedex 9, France c Dipartimento di Fisica, Universitgt degli Studi di Modena, via G. Campi 2 1 3 / A , 41100 Modena, Italy d Laboratoire des Champs Magn£tiques lntenses, CNRS, BP 166, 38042 Grenoble Cedex 9, France

Received 19 March 1995; accepted for publication 12 April 1995

Abstract We investigated the electronic transport properties and the magnetic susceptibility of the semiconducting silicide ReSi 1.75. This compound crystallises in a monoclinic structure (space group P1). The resistivity of this silicide is anisotropic depending on the direction of the current flow. At high temperatures we observe thermally activated behaviour for the resistivity with one (or two) energy gap(s) Eg = 0.16 eV (0.30 eV). Hall effect measurements yield a positive Hall coefficient in the temperature range between 30 and 660 K. At room temperature we found a Hall carrier concentration of 3.7 × 10 ~s c m - 3 and a quite high Hall mobility of 370 cmZ/V • s. As the resistivity, the magnetic susceptibility of ReSij.75 is anisotropic depending on the orientation of the magnetic field relative to the crystallographic axes. At room temperature X is strongly diamagnetic. Below about 50 K, X increases with decreasing temperature.

1. Introduction Transition metal disilicides have retained some attention for their application in silicon-based microelectronics. M o s t o f them exhibit metallic behaviour as MoSi 2 [1] others are known to be semiconductors as /3FeSi 2 [2]. W e report here transport and magnetic measurements performed on single crystals o f the semiconducting silicide ReSil.75. For the metallic silicides VSi 2, NbSi 2 and TaSi 2 we have already done a similar investigation, i.e. transport [3] com-

bined with magnetic measurements [4] and the results obtained were easily reliable with each other. In the framework of the wide study that we are carrying out on the silicides of transition metals [5], we have shown that ReSil.75 is an off-stoichiometric compound and it crystallises in a monoclinic structure (space group P1), which is characterised by the occurrence o f disordered Si vacancies [6]. The lattice parameters are a = 3.138 A, b = 3.120 A, c = 7.670 A and a = 89.90 °.

2. Experimental * Corresponding author. Tel.: + 33 7682 6364; Fax: + 33 7682 6394.

Single crystals o f ReSil.75 were produced by a modified Czochralski pulling technique from an R F

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U. Gottlieb et aL /Applied Surface Science 91 (1995) 82-86

levitated melt in a cold copper crucible. Single crystals obtained with such a method are of cylindrical shape with 2 to 4 cm in length and nearly 1 cm in diameter. Details of the preparation technique are reported elsewhere [1]. For transport measurements we used two samples in the shape of parallelepipeds (typical dimensions 0.5 × 1 × 7 m m 3) which were cut from one initial crystal after being oriented by means of Laue diffraction patterns. Because of the slight difference between the lattice parameters a and b (a = 3.138 A and b = 3.120 ,~) we are not able to distinguish the two different directions [100] and [010] o n the Laue pattern used for orientation. Therefore we will further refer arbitrarily t o these directions. The longest sample dimension was aligned along one of the main crystallographic axis [100] and [001]. We will call "sample A (C)" the sample with its longest dimension along [100] ([001]). We also report Hall~ effect measurements on a non-oriented single crystal which we label "sample X " . For the magnetic measurements we used an oriented piece of single crystalline ReSin:75 (m = 636.7 mg and V = 0.03546 cm3). This holds also for the resistivity samples as w e do not know whether sample A is aligned along [100] rather than along [010]. Resistivity and Hall e f f e c t were measured between 4.2 K and about 800 K. Resistivity below room temperature w a s measured by a 4-point ac method with a typical resolution A p / p = 10-5, Contacts were taken by fine aluminium wires directly thermosonically soldered onto the :sample. Above room temperature resistivity was measured by a d c method and contacts were taken b y tungsten springs. Hall effect was measured using the Van der Pauw method [7] in a magnetic field up to 0.9 T. Further details of the experimental methods used for the transport measurements can be found elsewhere [6], Magnetic measurements were made with a SQUID magnetometer. Magnetic field is given by a superconducting coil, it can be varied up to 7.5 T. The field is aligned along a given crystallographic direction with a precision of about 5 °. The resolution of the' magnetisation is better than one part in 103. Magnetisation was measured at 4 K and at 300 K. It exhibits a strictly linear variation up to the highest field. X = M / H was measured at 0.1 and 6 T from 2 K up to room temperature.

83

3. R e s u l t s a n d d i s c u s s i o n . . . . . . .

In Fig. 1, we present the resistivity of ReSil.75 measured on the samples A and C in the temperature range between 4.2 and 800 K. Above about 300 K, p decreases with increasing temperature showing a n activated behaviour typical for a semiconductor in t h e intrinsic region. Below room temperature, the resistivity of the two samples varies only slightly. Here our samples are in the extrinsic region. For both samples, the resistivity variation with temperature is quite similar, although the resistivity along the a-axis, Pa, is about a factor two higher than the resistivity along c, pc. However, we Cannot tell whether this resistivity anisotropy is an intrinsic property of ReSil.75 or it comes from a different doping in the two samples. In Fig. 2, we present the Hall coefficSent measured on samples C and X between 30 and 700 K. For both samples, the Hall coefficient is positive in the whole temperature range investigated indicating that holes are the dominant carrier type. At room temperature R H = 1.7 cm3//C corresponding to a Hall carrier concentration n H = 1/eR H = 3.7 × 10 j8 cm -3. Above room temperature R H decreases with increasing temperature as for a semiconductor in the intrinsic region. At low temperature ~(T < 50 K), R H tends to a constant value equal for the two samples R H = 5 c m 3 / C corresponding to n H = 1~2× 1018 cm - 3 In order to evaluate the gap of ReSil.75, we suppose that its transport properties are dominated

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U. Gottlieb et al. / Applied Surface Science 91 (1995) 82-86

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slopes in different temperature ranges. Close to room temperature, the observed slope is almost identical to that one observed for sample A while above about 400 K we can deduce Eg = 0.30 eV. These values are quite close to those reported by Long et al. [8], who evaluated by optical measurements on polycrystalline films a direct gap with Eg = 0.36 eV and an indirect gap with Eg = 0.12 eV. The results from the resistivity are confirmed by the Hall effect measurements as can be seen in Fig. 4 where we report the Hall number as a function o f 1 / T where T is the temperature. The experimental data above room temperature can be fitted with an activation law with Eg = 0.32. Combining resistivity and Hall measurements one can deduce the Hall mobility /x H = RH/p. Fig. 5 shows the temperature dependence of the mobility.

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Fig. 5. Hall mobility of ReSil.75 as a function of temperature.

U. Gottlieb et al./ Applied Surface Science 91 (1995) 82-86 -60 -65

ReSil.75 Single Crystal

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Above 300 K the experimental data can be fitted with the simple expression /x n = 2 . 3 9 × 106 cm 2 K 3 / 2 / V • S T - 3 / 2 , which is also indicated in the Fig. 5. The simple T - 3 / 2 dependence of the mobility is generally found in semiconductors and it can be derived by considering the electron-phonon scattering mechanism [9]. At room temperature the Hall mobility [z n 370 cmE/V • s is quite high and this indicates good sample quality. Between 100 and 200 K, the mobility seems to follow a T 3/2 law. This temperature dependence is normally caused by the scattering of the charge carriers by ionised impurities [9]. In Fig. 6, we have plotted the magnetic susceptibility measured at H = 0.1 T as a function of temperature for H parallel tO the three main crystallographic axes of ReSil.75. The measurements with a magnetic field of 6 T yield similar results. For all field orientations X is strongly diamagnetic. A small but significant anisotropy of X can be observed. As the measurements were all performed on the same sample, we can conclude that this anisotropy is an intrinsic feature for ReSil.75. In Fig. 6, one can see that the thermal variation of X is quite similar for the three curves. At low temperatures, X increases with decreasing temperature. A flat minimum can be observed around 50 to 100 K and near room temperature X slightly increases with temperature. The magnetic susceptibility of a semiconductor is =

85

the sum of the temperature independent diamagnetic lattice susceptibility X1 and the paramagnetic susceptibility of the charge carriers Xc- Here the number of charge carriers is low, so we may suppose that the lattice contribution accounts for the main part of the susceptibility of ReSil.75. It results from the diamagnetic susceptibility of the core electrons of the atoms. It increases strongly with Z where Z is the atomic number. Note that Z = 75 for Re. The observed susceptibility in ReSiL75 is in a good agreement with the diamagnetic contribution observed in the metallic silicides VSi 2, NbSi 2 and TaSi 2 [4]. The anisotropy of the magnetic susceptibility is of the same order of magnitude as for the metallic silicides [4]. The slight increase of X around room temperature must be associated to the beginning of the intrinsic regime and the increasing number of charge carriers. The increase of the susceptibility with decreasing temperature below around 50 K, follows something like a Curie law i.e. Xc ~ 1 / T . It could result from the magnetic moments coming from the progressive local•sat•on of the impurity charge carriers.

4. Conclusions We have measured the resistivity, the Hall effect and the magnetic susceptibility of ReSil.75 single crystals for different crystal orientations. The transport measurements show thermally activated b e haviour at high temperatures while they tend to assume temperature independent values at low temperatures. The anisotropy observed in the resistivity was confirmed by magnetic susceptibility measurements of being an intrinsic feature of ReSil.75. From resistivity measurements one can deduce an energy gap of 0.16 eV when the current is parallel to the a axis. For IIIc, however, we can deduce two possible gaps: Eg = 0.16 eV below 400 K and Eg = 0.30 eV for higher temperatures. Hall effect measurements indicate that ReSiL75 is p-doped with a charge carrier concentration of 3.7 × 1018 cm -3 and a room temperature mobility of 370 c m 2 / V . s . The magnetic susceptibility increases with decreasing temperature below around 40 K and this can be interpreted by means of localised magnetic moments.

86

u. Gottlieb et al. / Applied Surface Science 91 (1995) 82-86

Acknowledgements T h c a u t h o r s w o u l d like to t h a n k Dr. A. R o u a u l t , M. K u h n a n d C. G a r c o n for t h e i r help in s a m p l e p r e p a r a t i o n . T h i s w o r k w a s f i n a n c i a l l y s u p p o r t e d by the E u r o p e a n C o m m u n i t y in the f r a m e w o r k o f the program Human Capital and Mobility, contract number E R B C H R X T C T 9 3 0 3 1 8 .

References [I] c. Thomas, J.P. Senateur, R. Madar, O. Laborde and E. Rosencher, Solid Stale Commun. 55 (1985) 629.

[2] U. Birkholz and J. Schehn, Phys. Status Solidi 27 (1968) 413. [3] U. Gottlieb, O. Laborde, O. Thomas, A. Rouauh, J.P. Senateur and R. Madar, Appl. Surf. Sci. 53 (1991) 247. [4] U. Gonlieb, A. Sulpice, I~,. Madar and O. Laborde. J. Phys.: Condens. Matter 5 (1993) 8755. [5] F. Nava, K.N. Tu, O. Thomas, J.P. Senateur, R. Madar, A. Borghesi, G. Guizzetti, U. Gottlieb, O. Laborde and O. Bisi, Mater. Sci. Rep. 9 (1993) 141. [6] U. Gottlieb, B. Lamberl-Andron, F. Nava, M. Affronte, O. Laborde, A. Rouault and R. Madar, to bc published. [7] L.J. van der Pauw, Philips Res. Rep. 13 (1958) I. [8] RG. Long, M.C. Bost and J.E Mahan, Thin Solid Fihns 162 (1988) 29. [9] H.M. Rosenberg, Low Tcmperature Solid State Physics (Clarendon, Oxford, 1975) p. 224.