ALBERT

Notes: The electrical resistivity of monocrystalline TiSi2, TaSi2, MoSi2, and WSi2 has been measured from 4.2 to 1100 K. These disilicides are metallic, yet there is a remarkable difference in the temperature dependence of their intrinsic resistivities. TiSi2 and TaSi2 are found to exhibit a T5 dependence in the temperature range of 13〈T〈30 K and 15〈T〈28 K, respectively, while MoSi2 and WSi2 show a T3.8 dependence from 15 to 40 K. For TiSi2, along the three crystallographic directions 〈100〉, 〈010〉, and 〈001〉, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. The same behavior was observed for TaSi2 along the 〈0001〉 axis, while a negative deviation from the linearity followed by a quasisaturation was observed with the current, parallel to the 〈101¯0〉 axis. The resistivity data of WSi2 and MoSi2 with the current parallel to 〈001〉 and 〈110〉 crystallographic directions showed a positive deviation from linearity. The data are fitted to several theoretical expressions at low temperatures and in the full range of temperatures. The results are discussed in light of these theories.

Notes: We investigated some structural and transport properties of semiconducting ReSi2−δ . In the literature this silicides is reported to crystallize in an orthorhombic structure and to be stoichiometric ReSi2. Our investigations clearly show that the stable composition is ReSi1.75 crystallizing in the space group P1. Transport measurements show thermally activated behavior at high temperatures with one (or two) energy gap Eg=0.16 (0.30 eV). We also report Hall-effect measurements on this material: we found that RH is positive between 30 and 660 K and at room temperature the Hall number nH=1/eRH is equal to 3.7×1018 cm−3. The Hall mobility at room temperature is relatively high (μH=370 cm2/V s) for a single crystal. © 1995 American Institute of Physics.

Notes: Resistivity, Hall-effect, and magnetoresistance measurements have been performed in the temperature range 4.2–300 K on thin films of the ε1-Cu3Ge phase that has a long-range ordered monoclinic crystal structure. The results show that ε1-Cu3Ge is a metal with a room-temperature resistivity of ∼6 μΩ cm. The temperature dependence of resistivity follows the Block-Grüneisen model with a Debye temperature of 240±25 K. The density of charge carriers, which are predominantly holes, is ∼8×1022/cm3 and is independent of temperature and film thickness. The Hall mobility at 4.2 K is ∼ 132 cm2/V s. The elastic mean free path is found to be ∼1200 A(ring), which is surprisingly large for a metallic compound film. The results show that the residual resistivity is dominated by surface scattering rather than grain-boundary scattering. An increase in Ge concentration above 25 at. % (but less than 35 at. %) is found to affect the resistivity and Hall mobility, but not the density of charge carriers.

Notes: HfSi2 polycrystalline thin films, grown by coevaporation of Hf and Si and subsequently annealed at 850 °C, were studied by electrical resistivity measurements (from 10 to 900 K), Hall voltage (from 10 to 300 K), and optical reflectance (at room temperature) from 5 meV to 12 eV. Composition and structure of the films were investigated by Rutherford backscattering spectroscopy and x-ray diffraction. HfSi2 is metallic with (i) a high residual resistivity, (ii) a phonon contribution to the resistivity showing a negative deviation from linearity, and (iii) low-energy interband transitions. Transport measurements yielded a Debye temperature of 430 K, a free-carrier concentration of ∼4×1021 cm−3, and a mean free path of 139 A(ring). The reflectivity was Kramers–Kronig transformed to obtain the dielectric functions which, at low energies, are discussed in term of the Drude model. The optical parameters agree quite well with transport results, thus permitting one to obtain a reasonable value for the Fermi velocity.

Notes: GdSi2 and ErSi2 polycrystalline thin films were studied using electrical resistivity in the temperature range 10–900 K, Hall effect from 10–300 K and reflectivity spectra from 0.2–100 μm at room temperature. Composition and structure in these films were investigated by Rutherford backscattering spectroscopy and x-ray diffraction techniques. These silicides are metallic with (i) a remarkable difference in their residual resistivity, (ii) a phonon contribution to the resistivity which showed a negative deviation linearity, and (iii) low energy interband transitions. Resistivity data indicated that GdSi2 and ErSi2 have a Debye temperature of 328 and 300 K respectively and a limiting resistivity value much higher than that observed in other transition metal disilicides. The charge carrier concentration was estimated to be 4×1021 cm−3 at room temperature according to Hall measurements, and the mean free path was 63 A(ring) and 320 A(ring) for GdSi2 and ErSi2, respectively, at 10 K. The parameters obtained by the optical analysis are in good agreement with those extracted from the transport measurements, thus permitting one to obtain a reasonable value for the Fermi velocity.

Notes: Electrical and microstructural changes of coevaporated V75Si25 alloy thin films have been studied as a function of temperature from room temperature to 830 °C. In situ resistivity measurements, hot-stage transmission electron microscopy, Rutherford backscattering spectroscopy and the Seeman–Bohlin glancing angle incidence x-ray diffraction technique were applied. Upon heat treatment at a heating rate of 8 °C/min, a sharp decrease in resistivity occurs at ∼670 °C which results from an amorphous to crystalline phase transformation. The crystallized phase was identified as V3Si. The mechanism of transformation is random nucleation at a rapidly decreasing rate and a fast quasi-isotropic growth. The kinetics of crystallization have been studied by utilizing electrical resistivity measurements during isothermal heat treatment. Six different temperatures between 570 °C and 630 °C were adopted. The apparent activation energy (∼3.6 eV) obtained from isothermal measurements was found to be in agreement with that obtained from nonisothermal treatments at varying rates of heating. The distinct change of the Avrami mode parameter from 4 to 2 at a constant value of t/τ during the process of crystallization is not immediately understood.

Notes: Four different Co-silicide compounds were obtained by solid-state reaction at 800 °C in thin bilayers of amorphous silicon and cobalt evaporated on SiO2 substrates. Rutherford backscattering spectroscopy (2 MeV 4He+), x-ray diffraction, and Auger electron spectroscopy were used to obtain information about the chemical and crystallographic characteristics of the samples. Results indicate that in each sample only one of the following phases is present: CoSi2, CoSi, Co2Si, and Co4Si, the latter identified on the basis of the stoichiometric ratio only. Electrical resistivity and Hall effect measurements on van der Pauw structures were carried out as a function of the temperature in the intervals: 10–1000 and 10–300 K, respectively. At room temperature the resistivity ranges from the value 19 μΩ cm for CoSi2 to the value 142 μΩ cm for CoSi. There are some analogies with the case of a classical metal, but remarkable differences are also detectable in the resistivity versus temperature behavior and in the order of magnitude of the resistivity and of the Hall coefficient. In particular, at T〉300 K the resistivity of the CoSi2 samples linearly depends on temperature and is well fitted by the classical Bloch–Grüneisen expression. The other silicides show, in the same temperature range, a deviation from linearity (d2ρ/dT2〈0), while a quasi saturation of the resistivity can be extrapolated at higher temperatures. This saturation phenomenon can be described by the parallel of an ideal conductivity and of a saturation conductivity, and associated with the electron mean free path approaching interatomic distances. A similar model already has been put forth to describe the saturation of the resistivity in systems, such as A-15 superconducting compounds, characterized by a high value of the room-temperature resistivity. The transport parameters, deduced in a free electron framework from the resistivity curves of the Co silicides, show values which are consistent with the proposed model. Hall coefficient versus temperature behavior indicates that between 10 and 300 K a multicarrier effect is present. Conduction is predominantly n type in CoSi and p type in the other silicides.

Notes: Electrical and structural properties of coevaporated Cr-Si thin alloy films and bilayer Cr/Si films as a function of annealing temperature from 10 to 1000 °K have been studied by in situ electrical resistivity and Hall measurements, and structural analysis including MeV 4He+ ion backscattering, x-ray diffraction, Auger electron spectroscopy combined with Ar sputtering, electron microprobe, and scanning and transmission electron microscopy. In the as-deposited state, the coevaporated alloy film was amorphous. Upon annealing, a sharp increase in resistivity occurred near 270 °C and the increase has been determined to be amorphous to crystalline CrSi2 phase transformation. The resistivity increased further with annealing up to 550 °C then a gradual decrease took place beyond 600 °C. In cooling, the resistivity increased monotonically with decreasing temperature. For the bilayer Cr/Si films, the annealing behavior is similar except the sharp increase in resistivity occurred around 450 °C due to the formation of CrSi2. The crystalline CrSi2 has been determined to be a semiconductor with an energy gap of 0.27 eV. It is p-type, having a hole concentration of 4×1019 cm−3 at room temperature and a hole mobility of 7.2×104×T (temp)−3/2 cm2/V sec in the acoustic scattering region. The kinetics of amorphous-to-crystalline transformation of Cr-Si alloy film in the temperature range of 225–25 °C has been determined to follow a t7 (time) dependence with an apparent activation energy of 1.13 eV.

Notes: Electrical resistivity in the temperature range of 2–1100 K and Hall-effect measurements from 10 to 300 K of CoSi2, MoSi2, TaSi2, TiSi2, and WSi2 polycrystalline thin films were studied. Structure, composition, and impurities in these films were investigated by a combination of techniques of Rutherford backscattering spectroscopy, x-ray diffraction, transmission electron microscopy, and Auger electron spectroscopy. These silicides are metallic, yet there is a remarkable difference in their residual resistivity values and in their temperature dependence of the intrinsic resistivities. For CoSi2, MoSi2, and TiSi2, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. At high temperatures, while a negative deviation from the linearity followed by a quasisaturation was observed for TaSi2, the resistivity data of WSi2 showed a positive deviation from linearity. It is unique that the residual resistivity, ρ(2 K), of the WSi2 films is quite high, yet the temperature dependent part, i.e., ρ(293 K) − ρ(2 K), is the smallest among the five silicides investigated. This suggests that the room-temperature resistivity of WSi2 can be greatly reduced by improving the quality of the film, and we have achieved this by using rapid thermal annealing.

Notes: The thermal stability of coevaporated amorphous WSi2±x(x(approximately-equal-to)±0.2) thin films from room temperature to 1000 °C has been studied by in situ resistivity measurements and hot-stage transmission-electron microscopy. During continuous heating two consecutive phase transformations were observed to occur via nucleation and growth processes. The first which occurs at ∼420 °C is the crystallization of the amorphous film to a metastable, semiconducting hexagonal phase WSi2. The second which occurs at ∼620 °C is the transformation of the hexagonal phase to the thermodynamically stable, metallic, tetragonal phase of WSi2. The hexagonal phase is characterized by an acicular morphology and its formation is associated with a drastic increase in resistivity. The crystallites (grains) of the stable tetragonal phase are equiaxed and their formation is associated with a rapid decrease in resistivity. In order to achieve a low value of resistivity, ∼70 μΩ cm at room temperature, the tetragonal phase must be annealed to the neighborhood of 1000 °C. The activation energy for the hexagonal to tetragonal transformation (∼3 eV) was found to be higher than that for the crystallization (∼2 eV). The mode parameters for both transformations were found to be almost the same, n∼2. The characteristics of both transformations were not greatly influenced by the compositional changes.