Gold hyperdoped silicon exhibits room temperature sub band gap optical absorption, with potential applications as infrared absorbers/detectors and impurity band photovoltaics. We use first-principles density functional theory to establish the origins of the sub band gap response. Substitutional gold AuSi and substitutional dimers AuSi − AuSi are found to be the energetically preferred defect configurations, and AuSi gives rise to partially filled mid-gap defect bands well offset from the band edges. AuSi is predicted to offer substantial sub-band gap absorption, exceeding that measured in prior experiments by two orders of magnitude for similar Au concentration. This suggests that in experimentally realized systems, in addition to AuSi, the implanted gold is accommodated by the lattice in other ways, including other defect complexes and gold precipitates. We further identify that it is energetically favorable for isolated AuSi to form AuSi − AuSi, which by contrast do not exhibit mid-gap states. The formation of dimers and other complexes could serve as nuclei in the earliest stages of Au precipitation, which may be responsible for the observed rapid deactivation of sub-band gap response upon annealing.

Nanosecond pulsed laser melting can be used to rapidly recrystallize ion-implanted Si through liquid phase epitaxy. The rapid resolidification that follows the melting results in a supersaturation of impurities and hyperdopes the Si, inducing novel optoelectronic properties with a wide range of applications. In this work, structural changes in the Si lattice in Au-hyperdoped Si are studied in detail. Specifically, we show that the local skewing of the lattice observed previously in regions of extremely high Au concentrations (>1.4 at. %) can be related to the displacement of Au from perfect lattice positions. Surprisingly, although the incorporation of the larger Au atoms into Si is expected to cause swelling of the lattice, reciprocal space mapping shows that a small amount (0.3 at. %) of lattice contraction (decrease in lattice parameter) is present in the hyperdoped layer. Furthermore, positron annihilation spectroscopy shows an elevated concentration of vacancies in the hyperdoped layer. Based on these observations and with the aid of density functional theory, we propose a phenomenological model in which vacancies are kinetically trapped into lattice sites around substitutional Au atoms during resolidification. This vacancy trapping process is hypothesized to occur as a means to minimize lattice strain and may be universal in pulsed laser melted Si systems.

Au-hyperdoped Si produced by ion implantation and pulsed laser melting exhibits sub-band gap absorption in the near infra-red, a property that is interesting for Si-photonics. However, the subband gap absorption has previously been shown to be thermally metastable. In this work, we study the atomistic processes that occur during the thermal relaxation of Au-hyperdoped Si. We show that the first step in thermal relaxation is the release of substitutional Au from lattice sites. This process is characterised by an activation energy of around 1.6 eV, a value similar to that associated with Au diffusion in Si, suggesting that both processes could be rate limited by the exchange of substitutional and interstitial Au atoms. As the system further relaxes, Au is found to locally diffuse and become trapped at nearby lattice defects, notably vacancies and vacancy complexes. In fact, DFT results suggest that the formation of Au dimers is energetically favourable after the Au becomes locally trapped. The dimers could subsequently evolve into trimers, etc., as other diffusing Au atoms become trapped at the dimer. At low Au concentrations, this clustering process does not form visible precipitation after annealing at 750 o C for 3 minutes. In contrast, spherical Au precipitates are found in samples with higher Au concentrations (> 0.14 at. %), where the Au atoms and the associated lattice defect distributions are laterally inhomogeneous.

Sr2Ti7O14, a new phase, is synthesized by leveraging the innate chemical and thermo-dynamic instabilities in the SrTiO3-TiO2 system and non-equilibrium growth techniques. The chemical composition, epitaxial relationships, and orientation play roles in the formation of this novel layered phase, which, in turn, possesses unusual charge ordering, anti-ferromagnetic ordering, and low, glass-like thermal conductivity.

Hyperdoping germanium with gold is a potential method to produce room-temperature shortwavelength-infrared radiation (SWIR; 1.4-3.0 μm) photodetection. We investigate the charge carrier dynamics, light absorption, and structural properties of gold-hyperdoped germanium (Ge:Au) fabricated with varying ion implantation and nanosecond pulsed laser melting conditions. Time-resolved terahertz spectroscopy (TRTS) measurements show that Ge:Au carrier lifetime is significantly higher than that in previously studied hyperdoped silicon systems. Furthermore, we find that lattice composition, sub-bandgap optical absorption, and carrier dynamics depend greatly on hyperdoping conditions. We use density functional theory (DFT) to model dopant distribution, electronic band structure, and optical absorption. These simulations help explain experimentally observed differences in optical and optoelectronic behavior across different samples. DFT modeling reveals that substitutional dopant incorporation has the lowest formation energy and leads to deep energy levels. In contrast, interstitial or dopant-vacancy complex incorporation yields shallower energy levels that do not contribute to sub-band-gap light absorption and have a small effect on charge carrier lifetimes. These results suggest that it is promising to tailor dopant incorporation sites of Ge:Au for SWIR photodetection applications.