Conducting Electrons in Amorphous Si Nanostructures: Coherent Interference and Metal-Insulator Transitions Mediated by Local Structures

TL;DR

This paper explores how local structures in amorphous silicon influence electron conduction, revealing quantum interference effects and mechanisms for metal-insulator transitions that could inform new electronic applications.

Contribution

It demonstrates that nanoscale electrons in amorphous Si exhibit quantum interference and that local structures like ultrasoft bonds significantly affect conduction and phase transitions.

Findings

Nanoscale electrons in amorphous Si show quantum interference effects.

Local structures can be manipulated to control metal-insulator transitions.

Electron-phonon interactions in ultrasoft bonds influence charge mobility.

Abstract

Without a periodic reference framework, local structures in noncrystalline solids are difficult to specify, but they still exert an enormous influence on materials properties. For example, thermomechanical responses of organic and inorganic glasses sensitively depend on the distribution of free volume or soft spots$^{1,2}$. Meanwhile, strong electron localization$^{3}$ that endows unparalleled electrical breakdown strengths to amorphous insulators is easily compromised by local defects that promote inelastic tunneling over a variable range$^{4,5}$. Here we report how metallic conduction can overcome strong localization in amorphous insulators of small dimensions, and how local structures can manifest their spectacular influence on such conduction. In amorphous Si, nanoscale electrons are so coherent that they exhibit robust quantum interferences reminiscent of the mesoscopic phenomena seen in weakly localized metal crystals$^{6}$. Yet ultrasoft Si bonds emerge as the key local structures whose extraordinarily strong electron-phonon interaction coerces itinerant electrons into moving slowly at low temperature, even becoming trapped at all temperature when Si-O/N sites are provided. The local structures can be manipulated by a voltage or pressure to regulate charge storage, charge flow and metal-insulator transition. Also made of Ge and oxides and nitrides, nanostructured amorphous conductors could offer opportunities for new applications.