| Summary: | 碩士 === 國立交通大學 === 物理研究所 === 103 === Cobalt disilicide (CoSi_2) is a kind of common silicides in semiconductor industry. The crystal structure of CoSi_2 is very similar to silicon, so it's ready to grow good epitaxial layers on silicon substrates and the resulting CoSi_2 possesses low resistivity and long elastic mean free path (l_e). It also one of the very few silicides which have superconductivity. In this study, we investigate the electronic transport properties in the CoSi_2 layers. One of the most interested issues for us is the quantum interference effects (e.g. weak anti-localization effect), the related phase coherent length and superconducting (SC) fluctuation effect at low temperatures.
The CoSi_2 layers are made by the method of solid phase epitaxy. Briefly speaking, we deposit patterned cobalt films on pure silicon substrates, and then perform a high-temperature annaeling to drive cobalt diffusion and form CoSi_2 layers. To understand the effect of disorderness on the electrical transport in CoSi_2, we have made samples of different thicknesses (53 and 25 nm) on the substrates of different crystal orientations (Si<100> and Si<111>) and measured them with a low-noise AC resistance bridge at variable temperatures and magnetic fields. The resistivities at 4 Kelvin are between 2.2 and 5 μΩ⋅cm. The magneto-resistances (MRs) within +/- 1 Tesla are positive and show parabolic shape in the high-field region (0.2 - 1 Tesla). From these data, we can extract the carrier mobilities and l_e's which range from 50 to 130 nm. Besides, the thinner the CoSi2 layers, the lower the superconducting transition temperatures (Tc). Their Tc's locate between 1.2 and 1.5 K.
As the temperature is between 4 K and 20 K, the low-field (+/- 0.2 Tesla) MRs reveal clear sharp dips at zero field. The dips are just the signature of weak anti-localization (WAL) and indicate strong spin-orbit coupling (SOC) in this material. We utilize the well established theory of two-dimentional (2D) WAL effect and SC fluctuation to analyze the data and hence extract phase coherent lengths L_φ and the strength parameter β of SC fluctuation at different temperatures. After the analysis, we found that the L_φ can be longer than one micron which is much longer than those in normal conductors. The β is smaller than the theoretical value. This result can be attributed to the strong SOC which induces spin mismatch on the virtual Cooper pairs more quickly. After the L_φ is transformed into dephasing rate (1/τ_φ), we found a rough trend that the samples of shorter le's show higher dephasing rates. Furthermore, 1/τ_φ has two distinct temperature dependences in low-T and high-T regimes, that is, 1/τ_φ~T^1 and 1/τ_φ~T^3, respectively. In the low-T regime, the origin of 1/τ_φ~T^1 is mainly due to electron-electron (e-e) scattering (1/τ_ee) in 2D systems. With the theoretical estimation of e-e scattering strength Aee in the dirty limit, we found that the theoretical Aee is close to the experimental one extracted from 1/τ_φ and both lie in the order of magnitude of 1×〖10〗^9 Hz/T. The temperature dependence of 1/τ_φ~T^3 at higher temperatures fits the theoretical description of electron-phonon scattering (e-ph) rate in the clean limit. The theoretical and experimental e-ph scattering strength A_ep's are both in the order of magnitude of 1×〖10〗^7Hz/T^3. This result is reasonable for our samples because the le is much longer than the phonon wavelength such that the e-ph scattering in CoSi_2 actually corresponds to the model of clean limit.
Comparing to analysis method in the literature, we have found that the curve-fitting of our MRs can not be well done if we do not include the SC fluctuation effect. Another unclear point is the strong SOC in this material. From our result, the SOC strength seems no dependence on the disorderness (i.e. l_e). We hope our study can help to develop more future applications with CoSi_2.
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