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ABSTRACT: Resistivity, Hall‐effect, and magnetoresistance measurements have been performed in the temperature range 4.2–300 K on thin films of the ϵ 1 ‐Cu 3 Ge phase that has a long‐range ordered monoclinic crystal structure. The results show that ϵ 1 ‐Cu 3 Ge 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 ∼8×10<sup>22</sup>/cm<sup>3</sup> and is independent of temperature and film thickness. The Hall mobility at 4.2 K is ∼ 132 cm<sup>2</sup>/V s. The elastic mean free path is found to be ∼1200 Å, 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.
Journal of Applied Physics 03/1994; · 2.17 Impact Factor
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ABSTRACT: The thermal stability of coevaporated amorphous WSi 2±x(x≂±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 WSi 2 . The second which occurs at ∼620 °C is the transformation of the hexagonal phase to the thermodynamically stable, metallic, tetragonal phase of WSi 2 . 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.
Journal of Applied Physics 08/1988; · 2.17 Impact Factor
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ABSTRACT: Electrical resistivity in the temperature range of 2–1100 K and Hall‐effect measurements from 10 to 300 K of CoSi 2 , MoSi 2 , TaSi 2 , TiSi 2 , and WSi 2 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 CoSi 2 , MoSi 2 , and TiSi 2 , 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 TaSi 2 , the resistivity data of WSi 2 showed a positive deviation from linearity. It is unique that the residual resistivity, ρ(2 K), of the WSi 2 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 WSi 2 can be greatly reduced by improving the quality of the film, and we have achieved this by using rapid thermal annealing.
Journal of Applied Physics 03/1987; · 2.17 Impact Factor
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ABSTRACT: Electrical and microstructural changes of coevaporated V 7 5 Si 2 5 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 V 3 Si. 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.
Journal of Applied Physics 11/1986; · 2.17 Impact Factor
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ABSTRACT: Resistivity measurements in a wide temperature range (2–1100 K) have been performed on thin films of V3Si, V5Si3, and VSi2 formed on an inert substrate. An anomalous resistivity behavior has been observed in these metallic compounds: The resistivity deviates from linearity and approaches a saturation value at the higher temperatures. The resistivity data can be fitted quite well to a phenomenological expression based on the idea that a limiting resistivity is reached when the electron mean free path is of the order of the interatomic spacing. The electron mean free paths, which have been computed from the experimental data, lend support to the above idea. The saturation phenomenon in V3Si and V5Si3 compounds is characterized by a limiting resistivity of the same magnitude as observed in several A15 materials and in the Mooij correlation, yet in VSi2 the resistivity saturates to a much higher value. The V3Si is a superconductor with a transition temperature around 15 K and a residual resistivity ratio of 10.6. On the other hand, V5Si3 and VSi2 thin compound films do not show superconductivity state down to 2 K. The temperature dependence of the Hall coefficient gives evidence of a complex and different electronic structure of the three compounds.
Phys. Rev. B. 10/1986; 34(9).
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ABSTRACT: The Schottky-barrier heights of Ti and TiSi2 on both n-type and p-type Si(100) have been measured in the temperature range 175–295 K with use of a current-voltage technique. Auger-electron spectroscopy, Rutherford backscattering spectroscopy, and glancing-angle x-ray diffraction were used to monitor the silicide-formation reaction. The results showed that silicide formation has only a small effect on barrier height. The n-type and p-type barrier heights for both the metal and the reacted silicide phase were found to decrease with increasing temperature and with the same coefficient within the experimental accuracy. This coefficient was found to be approximately equal to one-half the temperature coefficient of the indirect energy gap in Si. These results are consistent with the predictions of recent models of barrier formation based on Fermi-level pinning in the center of the indirect band gap.
Phys. Rev. B. 08/1986; 34(4).
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ABSTRACT: Phase transformations in coevaporated amorphous vanadium‐silicon thin alloy films and bilayer vandium/silicon films have been studied as a function of heat treatment by in situ electrical resistivity measurement together with Rutherford backscattering spectrometry, Seeman–Bohlin glancing angle incidence x‐ray diffraction, and scanning and transmission electron microscopy. In the as‐deposited state the amorphous alloy films were silicon rich, having an atomic ratio of 1:3 for vanadium and silicon, respectively. Upon heat treatment a sharp decrease in resistivity occurs at approximately 250 °C, which has been determined to be a transformation from the amorphous to crystalline VSi 2 phase. The kinetics of the transformation have been obtained by isothermal treatment over the temperature range of 184–220 °C. The transformation is described by a Johnson–Mehl–Avrami‐type equation with an apparent activation energy of 1.30±0.06 eV. Subsequent heat treatment causes a gradual decrease in resistivity up to 850 °C. Upon cooling, a monotonic decrease in resistivity was observed. Heat treatment at high temperatures (900 °C) promotes the growth of nonuniformly distributed silicon grains. For the bilayer vanadium/silicon films, the sheet resistance increases gradually upon heat treatment up to 500 °C, then a sharp decrease is observed, which is due to the formation of VSi 2 . Further heat treatment at higher temperatures (850 °C) promotes a monotonical decrease in the resistance. The cooling behavior is similar to that of the crystallized alloy specimens except for having a slightly lower resistivity value. In a model for the two thin films connected electrically in parallel, the growth kinetics of VSi 2 in the bilayer films has been found to be linear in time over the temperature range of 500–-
;535 °C with an activation energy of 2.23±0.09 eV. The microstructure of films at various stages of annealing have been studied by x‐ray diffraction and transmission electron microscopy. Correlation between the resistivity and microstructure is given and discussed. In situ resistivity of annealed films below room temperature has been measured. Crystalline VSi 2 thin films do not become superconductive down to 2 K.
Journal of Applied Physics 05/1986; · 2.17 Impact Factor
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ABSTRACT: The structural and electrical properties of a Nb-Si thin alloy film as a function of temperature have been studied by Auger electron spectrometry, Rutherford backscattering spectroscopy, transmission electron microscopies, and in situ electrical resistivity and Hall coefficient measurements. The NbSi/sub 2.8/ films were deposited by double electron-gun coevaporation onto oxidized silicon. For electrical measurements samples of a van der Pauw pattern were made through metallic masks. In the as-deposited state the coevaporated alloy film was amorphous. Upon annealing a precipitous drop in resistivity near 270 /sup 0/C has been determined to be the amorphous to crystalline phase transformation. The kinetics of the transformation has been determined by isothermal heat treatment over the temperature range of 224/sup 0/ to 252 /sup 0/C. An apparent activation energy of 1.90 eV has been measured. The nucleation and growth kinetics in the crystallization process show a change in the power of time dependence from 5.5 to 2.4. The microstructures of films at various states of annealing have been correlated to the resistivity change. The crystalline NbSi/sub 2/ shows an anomalous metallic behavior. The resistivity (rho) versus temperature curve has a large negative deviation from linearity (dfl) and it approaches a saturation value (rho/sub sat/) as temperature increases. The resistivity data are fitted by two empirical expressions put forth to explain the resistivity behavior in A15 superconductors at low and high temperatures. One is based on the idea that ideal resistivity must approach some limiting value in the regime where the mean free path becomes comparable to the interatomic spacing and the other is based on a selective electron--phonon assisted scattering.
J. Mat. Res.; (United States). 02/1986; 1:2.
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ABSTRACT: 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 <sup>4</sup>He<sup>+</sup> 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 CrSi 2 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 CrSi 2 . The crystalline CrSi 2 has been determined to be a semiconductor with an energy gap of 0.27 eV. It is p‐type, having a hole concentration of 4×10<sup>1</sup><sup>9</sup> cm<sup>-</sup><sup>3</sup> at room temperature and a hole mobility of 7.2×10<sup>4</sup>×T (temp)<sup>-</sup><sup>3</sup><sup>/</sup><sup>2</sup> cm<sup>2</sup>/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 t<sup>7</sup> (time) dependence with an apparent activation energy of 1.13 eV.
Journal of Applied Physics 04/1985; · 2.17 Impact Factor
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ABSTRACT: We present a theoretical investigation of the reaction occurring at the interfaces between silicon and transition metals. Using the same approach successfully applied to the study of bulk suicides, the electronic properties of different models of silicon-nickel and silicon-palladium interfaces have been studied. The models investigated include: (a) epitaxial silicon-silicide interfaces; (b) isolated transition metal interstitials near the silicon surfaces; (c) adamantane geometry structures as metastable diffusion layer compounds. The theoretical results are used as a guide in order to interpret the available experimental photoemission data of these complex interfaces.
Surface Science.
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ABSTRACT: Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ(T) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Krönig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the free-carrier parameters obtained from infrared spectra and the density of electronic states at the Fermi surface determined from specific-heat measurements were used in such evaluations.
Materials Science Reports.
