Transport mechanism in amorphous molybdenum silicide thin films
Introduction
Electronic transport in solids is a well-established interesting subject, and it is generally determined by external and internal factors associated with materials. In particular, as an internal factor, the existence of lattice periodicity determines the state of the electrons inside a material. For example, the electron wave function exhibits an extended state when lattice periodicity exists, whereas a localized state is observed in the absence of periodicity [1]. In addition, external stimuli such as temperature, illumination, and magnetic fields can affect the movement of electrons [[2], [3], [4], [5]]. Due to the absence of lattice periodicity, amorphous materials are naturally selected as prototypical platforms for studying how an external stimulus might affect electronic transport in a localized state [[6], [7], [8]]. Amorphous molybdenum silicide (a-MoSi) is a typical transition-metal-based amorphous semiconductor and recent studies have shown that it has great potential for use in optoelectronic devices, especially single-photon detectors [[9], [10], [11], [12]]. Due to the unique and excellent properties of a-MoSi, including a tunable superconducting transition temperature [10], intrinsically low flux pinning, and homogeneity [11,13], a-MoSi-based devices perform better in terms of their detection efficiency, response time, and timing jitter than other crystalline and conventional amorphous materials [[14], [15], [16]]. In addition, the vortex dynamics of a-MoSi [17] are attracting the interest of researchers.
Considerable efforts have been made to determine the properties of a-MoSi that might make it suitable for applications in detectors. In particular, studies have mainly focused on modifying the superconducting transition temperature to improve the performance of devices [9,10,12,14,15]. However, the electronic transport process under an applied external field remains largely unclear, but it is important for obtaining comprehensive physical insights into the transport behavior and exploring further potential applications [9]. Thus, in the present study, we investigated the electronic transport mechanism in sputtered a-MoSi thin films based on temperature-dependent resistance and magnetoresistance (MR) measurements. MR measurements are generally considered to be powerful for elucidating the underlying transport mechanisms because an applied magnetic field can alter the electron wave function, especially the localized electron wave function [5]. MR investigations of amorphous compounds have been conducted since the 1970s [[18], [19], [20]], including many aspects such as conventional MR, colossal MR, and giant MR, but there have been no reports of the MR behavior of a-MoSi, which also motivated this study.
Section snippets
Experimental
a-MoSi films with thicknesses of ~30 nm were deposited on commercial silicon (>105 Ω cm) substrates at room temperature and without extra substrate heating using the radio-frequency (RF) magnetron sputtering technique. The base pressure of the chamber was less than 1 × 10−4 Pa. The sputtering pressure was 20 mTorr in pure ambient argon. Prior to deposition, the target was pre-sputtered for 30 min to eliminate the influence of the target's surface. In order to study the transport mechanism in
Results and discussion
Fig. 1(a) shows the surface morphology and corresponding cross-section profile for a-MoSi, where the crack-free microstructure was uniform with root mean squared surface roughness of 4.2 ± 0.1 nm. No typical diffraction peaks were observed in the GIXRD pattern obtained for the a-MoSi thin film, as shown in Fig. 1(b), thereby demonstrating that this sample was in the amorphous phase. Fig. 1(c) depicts the XPS full spectrum obtained for the a-MoSi thin film, which demonstrates the presence of Mo
Conclusions
In this study, a-MoSi thin films were grown on silicon substrates at room temperature using the RF magnetron sputtering technique and their charge transport behaviors were investigated by measuring the temperature-dependent resistance and MR. The temperature-dependent resistance was analyzed using a self-consistent method and the Mott–VRH transport mechanism caused by the disorder-induced electronic wave function localization was confirmed. In addition, crossover from nMR to pMR was observed in
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the National Key Research and Development Program of China (2017YFB0503300) and the Fundamental Research Funds for the Central Universities (Nos. 310201911cx024 and 310201911fz048).
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