Transport and infrared properties of titanium chalcogenides with V and Nb substitutions

Physica 105B (1981) 151-155 North-Holland Publishing Company

TRANSPORT AND INFRARED PROPERTIES OF TITANIUM CHALCOGENIDES W I T H V AND Nb SUBSTITUTIONS F. LI~VY, H.P. V A T E R L A U S and H. B E R G E R Laboratoire de Physique appliqu~e, EPF, Lausanne, Switzerland

The electrical and infrared optical properties of titanium chalcogenides with cation substitutions are investigated as a function of impurity concentrations and temperature. Vanadium and niobium substitutions originate a strong increase of resistivity at low temperature related to the localization of carriers. The optical reflectivity measurements confirm qualitatively, on the one hand, the increased carder densities due to additional doping impurities and, on the other, the loss of carders at the structural phase transition.

1. Introduction

2. Single crystals

The transport and optical properties in layered titanium dichalcogenides have been investigated by several scientists to characterize the electronic structures of these layered crystals [1-4]. Recent results show thai TiS2 is a degenerate semiconductor [5] whereas TiSe2 is a semimetal and undergoes a structural phase transition at 200 K which is coupled with a strong resistivity anomaly [1,6]. Cation substitutions have a remarkable influence on the electronic properties of these compounds [7-9]. The most significant effect is a localization of charge carriers at low temperature induced by substitutions like V or Nb which carry a concentration-dependent magnetic moment. In this paper results are reported on measurements of electrical resistivity and Hall coefficient in titanium chalcogenides doped with V, Nb and Ni. They are compared with the free carders parameters obtained from infrared reflectivity measurements at room temperature and below the phase transition in TiSe2 with V substitutions. Conclusions will be drawn about conflicting electrical and optical results for the interpretation of the electronic properties of these crystals.

Single crystalline samples of Til-xMxSe2 (M = V, Nb, Ni) have been grown from the elements by iodine transport reactions with excess selenium. The growth temperature was kept as low as possible (Tg < 700*(2) in order to preserve the best stoichiometry. The composition of the crystals was checked by electron microprobe analysis. The measured concentrations correspond to the nominal, within an error of 510% [10]. In contrast to the best stoichiometric TiSe2 samples, the V-doped crystals show selenium deficiency up to 3%. In the case of Ni-doped samples, preliminary results point out that a limited solubility range exists and the actual concentrations must be lower than the nominal. The influence of a large number of other substitutions has been investigated [9]. For example, Sc and Y impurities cannot be appreciably incorporated so that they could not be detected. However, in TiSe2 crystals grown with additions of Y, Sc and Zr, the temperature dependences of the resistivity and of the Hall coefficient are comparable with those of the most stoichiometric TiSe2 (e.g. Til-xYxSe2: for xno~= 0.01, p ~ / p ( 2 9 5 K) = 3.3 and for xno~ = 0.1, p ~ / p ( 2 9 5 K) = 3.4).

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F. LEvy et al./Transport and 1R properties in doped TiSe2

152

The crystal lattice parameters of several mixed and doped crystals have been measured by powder X-ray techniques. Ti:e 1T structure has been generally found except for Til-xVxSe2 with x > 0.5 which exhibit a complex pattern.

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The electrical resistivity and the Hall coefficient in the plane of the layers between room temperature and liquid helium temperature were measured with the van der Pauw geometry. A few typical results are summarized in fig. 1 where the resistivity is represented as a function of temperature. The strongest localization effect arises for Ti0.99V0.01Se2 and for Ti0.sNb0.2Se2, as can be seen from the value of the resistivity at low temperature represented as a function of the alloy concentration in fig. 2. For Ti0.sNb0.2S2, the resistivity value p(5 K) is smaller than for Ti0.90V0.04S2reported equal to about 2.3 × 10-2 l~cm [8].

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Fig. 1. Electrical resistivity as a function of temperature for typical Ti chalcogenides containing V and Nb substitutions in comparison with stoichiometric compounds.

In the case of Ni doping, the largest increase of resistivity at low temperature corresponds to p(5 K)/p(295 K) = 1.5. The values of the Hall coefficient at room temperature are reported in fig. 3 for several a l l o y e d Z i S e 2 crystals, as a function of the concentration. For Ti0.sNb0.2S2 the Hall coefficient has been found to be smaller than 10-3 cm 3 A -1 s -1. This value is not much different from the value in TiS2 and obviously depends on the stoichiometric cation/anion ratio. The most impressive effect therefore occurs for Ti0.99V0.01Se2 [11]. Magnetic measurements have suggested that V occupy Ti sites and act as traps for the electrons [12]. The positive Hall coefficient at low temperature in Ti~-xVxSe2 (x > 0.01) [9] tends to confirm this interpretation, in comparison with TiSez where it becomes negative below the structural phase transition [1, 11].

153

F. IMvy et al./Transport and IR properties in doped TiSe2 i

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The lowest value R . = 10 -3 cm 3 A -1 S -1 measured for x = 0.9 cannot be c o m p a r e d with the m e a s u r e m e n t s on 2H-NbSe2 [13] since different polytypes are present. 4. Inh-ared properties The reflectivity spectra ( E _ L c ) of Til-xVxSe2 single crystals (0-> x -->0.05) were measured at nearly normal incidence at 300 and at 77 K. The r o o m t e m p e r a t u r e spectra (fig. 4) are characterized by the Eu transverse optical phonon m o d e at about 143 cm -1 and by a d a m p e d D r u d e edge whose m i n i m u m strongly depends on V concentration. T h e spectra were analyzed by fitting the reflectivity curves with the help of a dielectric function e ( v ) containing a d a m p e d Lorentzian oscillator for the p h o n o n and a limited D r u d e term for the plasmon [14]. The plasma frequency vp is related to the density of the charge carriers N and to the effective mass ratio m*: N / m * - v 2. T h e plasma frequencies and the corresponding values of N / m * are given in table I for several concentrations x. N / m * increases with progressive V doping. It is not clear whether this variation is caused mainly by an increase of the carrier density N or by a decrease of the effective mass. The reflectivity spectra of the same crystals at

100

200 600 600 1000 2000 FREQUENCY (cm "1)

4000

Fig. 4. Reflectivity spectra of Til-xVxSe2 single crystals at room temperature for various concentrations x. The fitted spectra are represented by broken lines. 77 K show the low t e m p e r a t u r e distorted phase (fig. 5). First, as a consequence of the reconstructed Brillouin zone the broad p h o n o n feature at r o o m t e m p e r a t u r e is split into several sharp phonon peaks at low temperature. The presence of some of these phonon modes, even for x = 0.05, suggests that a larger impurity concentration is necessary to suppress the phase transition. Secondly, in the case of TiSe2, the D r u d e edge steepens drastically below Tc. This corresponds with the loss of carriers at the phase transition because of the opening of gaps at the Fermi Table I Free carriers parameters for Tit-xVxSe2 x

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F. Ldvy et al./Transport and IR properties in doped TiSe2

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surface [15]. The corresponding values at 77 K, i.e. ~,p = 1080 cm -~ and N / m * = 1.3 × 1019 c m -3, are much smaller indeed than those at 300 K. The frequency dependence of the reflectivity in the doped samples is less steep. This can be due to a poorer stoichiometric cation/anion ratio. 5. Band structure and charge carriers

These results show much evidence which cannot be interpreted in a simple model for the charge carriers. At room temperature and for Til_xVxSe2 crystals of comparable concentrations, the electrical resistivity remains nearly constant, whereas the optical resistivity strongly decreases. The decrease of the Hall coefficient is consistent with the increase of the negative charge carrier density with progressive V doping, even in the presence of dominating holes in stoichiometric TiSe2. The drastic decrease of the optical resistivity, however, calls Drude's model in question.

In the beginning, the reflectivity curves were interpreted in terms of two plasmons [16] and later, with the help of one plasmon only [17, 18]. The similarity and the monotonous variation of the free carrier reflectivity between non-stoichiometric Til-xSe2 (n-type) [16] and best stoichiometric TiSe2 (p-type) [14, 17, 18] suggest that the analysis with the help of a Drude-like model implies an average procedure over the electrons and holes present in Z i S e 2 at room temperature. This restricts the physical meaning of the optical parameters (carrier densities, effective masses and life times) deduced from the optical data. In the single plasmon term used to fit the reflectivity at room temperature, both types of carriers must contribute. The interpretation of the sensitive increase of Vp with concentration x therefore needs a more rigorous model. Qualitatively, this increase is in some way produced by a higher electron density in doped samples. The small values of Vp and of N / m * in TiSe2 at 77 K is compatible with a loss of carriers at the phase transition. The carrier density at low temperature had been valued 5.6 x 1019cm -3 [7]. This would lead to an effective mass ratio parallel with the plane of the layers of 4.3. Such heavy d-like electrons are not surprising in comparison with other group IV transition metal dichalcogenides [19]. The higher level of reflectivity in the low temperature spectra for x > 0 is due to the presence of excess cations. The resulting additional electrons appear to mask the influence of localization revealed by resistivity measurements. 6. Conclusions

The investigation of titanium chalcogenides with various cation substitutions show that a localization of charge carriers occurs in several systems at low temperature. In Zi0.6Hf0.4Se 2 [20] the weak increase of resistivity at low temperature is understandable as a disorder effect, as in distorted 1T-TaS2 below 100 K. In systems containing transition metal substitutions, like V, Nb, Fe, and Ni, the influence of the chemical disorder must be enhanced by preferential elec-

F. IMvy et al./Transport and IR properties in doped T/Se2

tronic states of the substituted cations in connection with the existence of a magnetic moment. The resulting electrical behaviour characterized by a strong increase in resistivity at low temperature is quite general in titanium chalcogenides with transition metal substitutions. References [I] F.J. DiSalvo, D.E. Moncton and J.V. Waszczak, Phys. Rev. B14 (1976) 4321. [2] A.H. Thompson, Phys. Rev. Lett. 35 (1975) 1786. [3] W.Y. Liang, G. Lucovsky, J.C. Mikkelsen and R.H. Friend, Phil. Mag. B39 (1979) 133. [4] J.A. Holy, K.C. Woo, M.V. Klein and F.C. Brown, Phys. Rev. B16 (1977) 3628. [5] R.H. Friend, D. J6rome, W.Y. Liang, J.C. Mikkelsen and A.D. Yoffe, J. Phys. C: Solid State Phys. 10 (1977) L 705. [6] F.C. Brown; Physica 99B (1980) 1409. [7] F.J. DiSalvo and J.V. Waszczak, Phys. Rev. B17 (1978) 3801. [8] C. L6vy-Clement, A. Katty, A.T. Chang and O. Gorochov, J. Phys. C: Solid State Phys. 11 (1978) L 647.

155

[9] F. L6vy, J. Phys. C: Solid State Phys. 12 (1979) 3225, 13 (1980) 2901. [10] F. L~vy and H.P. Vaterlaus, Solid State Commun., to be published. [11] S. Uchida, K. Tanabe, K. Okajiama and S. Tanaka, Solid State Commun. 31 (1979) 517. [12] C. Schlenker, C. Landee, R. Buder and F. L6vy, J. Magn. Magn. Mat. 15-18 (1980) 91. [13] D.J. Huntley and R.F. Frindt, Can. J. Phys. 52 (1974) 861. [14] H.P. Vaterlaus, S. Ansermet, M. Py and F. L~vy, Solid State Commun. 35 (1980) 925. [15] G. Margaritondo, J.H. Weawer, F. L6vy, N.G. Stoffel and A.D. Katnani, to be published. [16] W.Y. Liang, G. Lucovsky, R.M. White, W. Stutius and K.R. Pisharody, Phil. Mag. 33 (1976) 493. [17] G. Lucovsky, R.M. While, W.Y. Liang and J.C. Mikkelsen, Phil. Mag. 34 (1976) 907. [18] J.A. Wilson, A.S. Barker Jr., F.J. DiSalvo Jr. and J.A. Ditzenberger, Phys. Rev. 18 (1978) 2866. [19] H. Isom~iki, J. von Boehm and P. Krusius, J. Phys. C: Solid State Phys. 12 (1979) 3239. [20] I. Taguchi, Solid State Commun. 32 (1979) 679.