Electrical conductivity in extremely disordered molybdenum oxynitrides thin films

TL;DR

This study investigates how chemical composition affects the electronic and structural properties of molybdenum oxynitride thin films, revealing a transition from superconducting to disordered semiconducting behavior with increasing oxygen content.

Contribution

It provides new insights into the relationship between composition, disorder, and electronic properties in molybdenum oxynitride thin films grown by reactive sputtering.

Findings

Films with up to 2% oxygen are superconducting with T_c around 6.8 K.

Higher oxygen content leads to amorphous, semiconducting films with variable-range hopping conduction.

Optical properties indicate a lack of a well-defined band gap due to structural disorder.

Abstract

We report on the influence of the chemical composition on the electronic properties of molybdenum oxynitrides thin films grown by reactive sputtering on Si (100) substrates at room temperature. The partial pressure of Ar was fixed at 90 %, and the remaining 10 % was adjusted with mixtures N$_2$:O$_2$ (varying from pure N$_2$ to pure O$_2$). The crystalline and electronic structures and the electrical transport of the films depend on the chemical composition. Thin films grown using oxygen mixtures up 2 % have gamma-Mo$_2$N phase and display superconductivity. The superconducting critical temperature T$_c$ reduces from ~ 6.8 K to below 3.0 K as the oxygen increases. On the other hand, films grown using oxygen mixtures richer than 2 % are mostly amorphous. The electrical transport shows a semiconductor-like behavior with variable-range hopping conduction at low temperatures. The analysis of the optical properties reveals that the samples have not a defined semiconductor band gap, which can be related to the high structural disorder and the excitation of electrons in a wide range of energies