Antibonding configurations of hydrogen in silicon-germanium alloys

Abstract

Defects with a hydrogen atom at an antibonding site are identified in germanium-rich SiGe alloys ͑Ge 1−x Si x ͒ implanted with protons at low temperatures. Infrared absorption spectra of proton-implanted Ge 1−x Si x reveal hydrogen-related absorption lines, which are similar to those observed previously in pure germanium, together with three unreported absorption lines. For crystals with a silicon content of 1.2 at. % these three lines are observed at 815.8, 1430.2, and 1630.7 cm −1 . The three lines are ascribed to the wag mode, the stretch mode, and to the first overtone of the wag mode, respectively, of the negatively charged state of a hydrogen atom located at an antibonding site and attached to a substitutional silicon atom. The assignment is based on the comparison between the observed frequencies and those known for similar structures, the variation of line intensities with the alloy composition, the absorption frequency shifts resulting when hydrogen is replaced by deuterium, and the thermal stability of the absorption lines, as well as comparison with the results of ab initio calculations. A conversion process takes place at temperatures in the range 20-125 K between negatively charged hydrogen at a germanium antibonding site to a silicon antibonding site. This provides direct evidence that silicon acts as an effective trap for negatively charged hydrogen in SiGe alloys. Based on the data obtained for samples with different silicon contents we give an estimate of the low temperature diffusion length of hydrogen in Ge-rich SiGe before it becomes trapped at an antibonding site attached to Ge and discuss why local vibrational modes of negatively charged hydrogen remain unobserved for silicon and silicon-rich SiGe alloys.

Correlation of dynamical, vibrational and optical properties in c-Si with bond-centered-hydrogen and other hydrogen complexes

“…We use the notation H 2 ** following the authors in Ref. [16], who associated several peaks in their Fourier Transform Infrared Spectroscopy (FTIR) spectrum to that dimer. In the literature, the same notation has been used to label a different H complex [14].…”

“…There is strong experimental evidence of the dissociation of the BCH complex into both these complexes. In n-type Si crystals, with proton implanted below 40 K, three new peaks in the FTIR absorption spectrum were observed that became stronger upon heat treatment at temperatures in the range 50 -300 K [16]. After annealing at 320 K the three peaks disappeared.…”

“…It shows that H 2 ** is a product of BCH dissociation at 360 K and becomes unstable as the temperature is raised, as the absence of H 2 ** in our calculations at 610 and 650 K suggests. Experimental evidence of the existence of H 2 * comes mainly from FTIR spectra obtained at 77 K of Si samples implanted with protons at room temperature [11] and also from [16]. The VDOS of the AB-like H atoms in H 2 ** at 360 K is displayed in Fig.…”

“…It is the scope of this paper to use ab-initio Born Oppenheimer Molecular Dynamics (BOMD) to study on a microscopic level the structure of the BCH complex, its stability, and dissociation products in a wide temperature range starting from as low as 50 K to standard operational temperatures of semiconductor devices and up to 650 K. Our results show that dissociated BCH forms mainly the newly discovered hydrogen H 2 ** dimer [16] (Fig. 1d), observed for the first time dynamically, the H 2 * dimer ( Fig.…”

“…A new dimer of trigonal symmetry, the H 2 ** dimer ( Fig. 1d), was theoretically investigated [15], and a year later several peaks in the spectra of Fourier Transform Infrared Absorption experiments under uniaxial stress of proton implanted c-Si samples were attributed to that defect [16].…”