Atomic scale origins of sub-band gap optical absorption in gold-hyperdoped silicon

Abstract

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.

Atomistic Mechanisms for the Thermal Relaxation of Au -hyperdoped Si

“…Because the substitutional Au dimer configuration is found to be the most energetically favourable defect reported in Ref. [26], the Au that is released from isolated substitutional sites will strongly favour the formation of a dimer, after they become locally trapped (at isolated vacancies, for example) and relax in adjacent sites. Other migrating Au atoms in proximity to the dimer complex are expected to be attracted to the dimer in a similar fashion to lower the free energy [45], evolving into trimers, and so on, ultimately leading to the nucleation of observable precipitates.…”

“…We examine the role of Au diffusion and trapping in the thermal relaxation process and explore the possibility of Au dimerization during the thermal relaxation by comparing experimental results with first-principles calculations by density functional theory (DFT) published previously in Ref. [26], to which the reader is referred for a more comprehensive analysis of the DFT data. In addition, we explore whether the relaxation behaviour depends on the Au-concentration.…”

“…Although previous high-resolution TEM indicated the formation of spherical precipitates as the final stage of annealing [25], experimental observation of the atomistic evolution of this process is difficult even with highresolution TEM, considering that the transient configurations of the trapped Au may consist of only up to a few Au atoms. However, a first-principles analysis by density functional theory (DFT) has been recently employed to simulate the defect formation energetics for different atomic configurations of supersaturated Au in Si [26]. Since DFT gives the equilibrium configuration when all the atoms are relaxed, the simulated system can give insight into the relaxation process of the Au-hyperdoped Si system following subsequent thermal annealing.…”

“…Finally, DFT results indicate that different Au configurations in the Si lattice are associated with different defect levels within the Si band gap. In particular, isolated substitutional Au, which is experimentally found to be the primary Au configuration in an unrelaxed system, is expected to give rise to optically active mid-gap states that are responsible for the observed sub-band gap optical absorption [26]. Au dimers, on the other hand, are not expected to introduce such mid-gap states based on DFT calculations.…”

A Systematic Study on Au-Capped Si Nanowhiskers for Size-Dependent Improved Biosensing Applications

“…In simulations, each sample with its own geometrical parameters is tested separately. b Results of numerical simulations performed for the electric field y-components of the single samples of S-1, S-2, and S-3 and c the same samples in series when exposed to the wavelengths of 532 nm and 552 nm Au NPs, and the optical band gaps from Au-doped Si are 552 nm (≅ 2.25 eV) [27] and around 605-688 nm (≅ 1.8-2.05 eV) [28,29]. The numerical simulation results given in this study also revealed to be consistent with the previous reports both for the plasmonic extinction wavelengths of Au NPs and sub-bandgap optical extinction of Au/Si (Au-doped Si) nanostructures (Fig.…”