Structural tunability and origin of two-level systems in amorphous silicon

TL;DR

This study investigates the structural properties of amorphous silicon films, revealing that nanovoids are the primary origin of two-level systems, with properties influenced by growth conditions and associated with atomic disorder and local strain.

Contribution

It provides new insights into the atomic-scale origin of two-level systems in amorphous silicon, linking them to nanovoids and their structural environment, and explores how growth parameters affect these features.

Findings

Nanovoids are responsible for reduced atomic density.

Two-level systems are associated with nanovoids and loosely bonded regions.

Higher growth temperature reduces dangling bonds and atomic disorder.

Abstract

Amorphous silicon films prepared by electron beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si when films of thickness greater than ~300 nm are grown at 425 $^{\circ}$C and at <1 $\r{A}$/sec. A combination of spectroscopic techniques provide insight into atomic disorder, local strains, dangling bonds, and nanovoids. Electron diffraction shows that the short-range order of the amorphous silicon is similar at all growth temperatures, but fluctuation electron microscopy shows that films grown above room-temperature show a form of medium-range order not previously observed in amorphous silicon. Atomic disorder and local strain obtained from Raman spectroscopy reduce with increasing growth temperature and show a non-monotonic dependence on thickness. Dangling bond density decreases with increasing growth temperature and is only mildly dependent on thickness. Positron annihilation Doppler broadening spectroscopy and electron energy loss spectroscopy show that nanovoids, and not density variations within the network, are responsible for reduced atomic density. Specific heat and mechanical loss measurements, which quantify the density of tunneling two-level systems, in combination with the structural data, suggest that two-level systems in amorphous silicon films are associated with nanovoids and their surroundings; which are in essence loosely bonded regions where atoms are less constrained.