Transport mechanism in amorphous molybdenum silicide thin films

Highlights

• Mott–variable range hopping mechanism in amorphous molybdenum silicide thin films.

• Crossover observed from negative to positive magnetoresistance.

• Temperature dependence of localization length demonstrated.

Abstract

Amorphous molybdenum silicide compounds have attracted significant interest because of their potential applications in devices, particularly single-photon detectors. In this study, we measured the temperature-dependent resistance and magnetoresistance behavior of amorphous molybdenum silicide compounds to understand the charge transport mechanism, which is of great importance for applications but still unclear. We found that Mott variable hopping conductivity dominated the charge transport in sputtered amorphous molybdenum silicide thin films. In addition, the observed magnetoresistance crossover from negative to positive was ascribed to enhanced interference and shrinkage of the electron wave function, where both varied the probability of hopping between localized sites.

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.