Atomistic Mechanisms for the Thermal Relaxation of Au -hyperdoped Si

Au-hyperdoped Si produced by ion implantation and pulsed laser melting exhibits sub-band-gap absorption in the near infrared, a property that is interesting for Si photonics. However, the sub-band-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 characterized 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, density-functional theory 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 750C for 3 min. In contrast, spherical Au precipitates are found in samples with higher Au concentrations (>0.14at.%), where the Au atoms and the associated lattice defect distributions are laterally inhomogeneous.