An optically enhanced crystalline silicon allotrope: hydrogen passivated type II silicon clathrate

TL;DR

This study demonstrates that hydrogen (deuterium) can be incorporated into low-Na type II silicon clathrates via remote RF plasma, forming NaD and SiD complexes that passivate Na donor states and Si dangling bonds. The passivation significantly reduces carrier density to $\sim 4 \times 10^{17}$ cm$^{-3}$ and enhances photoluminescence by about a factor of $\sim 40$, while shifting emission toward the intrinsic band edge and revealing a new hydrogen-related center near $930$ nm. The clathrate framework remains structurally stable under optimized plasma conditions, with passivation enduring up to ~$400^\circ$C before partial deactivation occurs. Together, these results establish hydrogen passivation as a viable strategy to unlock the optoelectronic potential of Si clathrates and enable Si-based light-emitting devices, offering a general approach for defect engineering in direct-bandgap Si allotropes.

Abstract

While Si clathrates have been explored as promising direct bandgap semiconductors, their practical optoelectronic performance has been limited by high doping levels and structural defects. Hydrogen has long been used to improve the optoelectronic quality of conventional Si, yet its role in clathrate structures remains unexplored. In this study, we demonstrate that hydrogen (deuterium) can be incorporated into type II Si clathrate framework using remote plasma treatment. This process leads to the formation of NaD and SiD complexes, which significantly reduce both the Na donor density and dangling bond defects. Electron paramagnetic resonance confirms nearly a tenfold decrease in Na-related donor states, resulting in the lowest doping level reported in Si clathrates to date. Following passivation, the integrated photoluminescence intensity increases by a factor of 40, accompanied by a blue shift of the main emission peak, consistent with a transition closer to the intrinsic band edge. A new emission peak at 930 nm, attributed to hydrogen-related recombination centers, also appears. These improvements remain stable up to 400 oC. Altogether, this work establishes hydrogen passivation as a viable strategy for enhancing light emission in Si clathrates and opens a new pathway toward their application in Si-based light-emitting diodes and other direct-bandgap optoelectronic devices.

Figures (9)

• Figure 1: Normalized TOF-SIMS ion intensities in low-Na type II Si clathrate after deuterium plasma treatment.
• Figure 2: Normalized TOF-SIMS deuterium ion intensities in low-Na type II Si clathrate films after plasma treatment under two different conditions.
• Figure 3: XRD patterns of type II Si clathrate films before and after D$_2$ plasma treatment. Orange vertical lines indicate the reference pattern for type II Si clathrate (ICDD 98-024-8181). The purple dotted line marks the characteristic peak of poly-Si. All samples were treated for 2 hours under different plasma conditions.
• Figure 4: (a) EPR spectra of low-Na type II Si clathrate at 4.6 K before and after deuterium plasma treatment; the inset shows room-temperature spectra. (b) Integral of the low-temperature spectra from (a). Both spectra were averaged over five scans, using a gain of $10^4$ and a 20 dB attenuation. The spectra before and after passivation were normalized by sample weight.
• Figure 5: (a) Room-temperature and (b) low-temperature EPR spectra of passivated Si clathrate films before and after a series of annealing steps in a sealed EPR tube. The films were sequentially annealed at $280\,^\circ\mathrm{C}$ for 7 hours, $350\,^\circ\mathrm{C}$ for 12 hours, $400\,^\circ\mathrm{C}$ for 12 hours, and $450\,^\circ\mathrm{C}$ for 12 hours. The low-temperature spectra were averaged over five scans using a gain of $10^4$ and 20 dB attenuation. The room-temperature spectra were averaged over ten scans using the same parameters.