Dynamical, Vibrational, Electronic and Optical Properties in c-Si:H with Bond-Centered-Hydrogen, H Dimers and Other H Complexes

TL;DR

This study uses ab-initio molecular dynamics to analyze how various hydrogen complexes affect the vibrational, electronic, and optical properties of crystalline silicon, providing insights relevant for photovoltaic applications.

Contribution

It offers a detailed computational investigation of hydrogen complexes in silicon, including newly discovered dimers, and links their signatures to experimental optical spectra.

Findings

Hydrogen complexes modify silicon's electronic states near the energy gap.

Characteristic spectral signatures are identified for different hydrogen complexes.

Results aid in interpreting in-situ optical measurements during silicon film growth.

Abstract

Hydrogen, introduced into crystalline (c-) or amorphous (a-) silicon (Si), plays an important role in improving Si properties for photovoltaics application. Low temperature proton implantation in c-Si and a-Si or H-doping of Si films introduces metastable hydrogen in the bond-centered-position (BCH), which dissociates with increasing temperatures into new metastable complexes. Using ab-initio molecular dynamics we report on the stability of BCH, a convenient hydrogen model system, in crystalline Si and its temperature-induced dissociation products in a wide temperature range, which includes temperatures close to solar cell operating conditions. Particular attention was paid to the newly experimentally discovered H2** dimer, and the H2* dimer, as well as isolated interstitial hydrogen and monohydrides. Each complex leaves a characteristic signature in the frequency spectrum, the density of states (DOS) and in the imaginary part of the dielectric constant that agrees well with experiments. All complexes modify the vicinity of the energy gap of pure c-Si. BCH introduces characteristic donor levels, causing a strong peak in the DOS just below the intrinsic conduction band. The Fermi energy is raised, filling these donor states with two electrons and causing a strong peak in the imaginary part of the dielectric constant in the infrared. The H2** and H2* dimers introduce a low energy tail in the imaginary part of the dielectric constant. The results are important for experimental in-situ optical characterization of Si film growth that often involves hydrogen.