Characterising and understanding Au-hyperdoped Si for sub-band gap optical absorption

Abstract

The unparalleled technological maturity of silicon (Si) can be exploited to develop CMOS-compatible optoelectronics such as photodetectors and imaging arrays. However, the low-attenuation wavelengths commonly used in fibre-optics (up to 1650 nm) fall below the 1.12 eV band gap of Si (efficient absorption only occurs at wavelengths less than 1100 nm), thus requiring the realisation of sub-band gap photoresponse. A promising method to achieve this is to add an intermediate band within the band gap by incorporating appropriate impurities into the Si lattice at high concentrations (often beyond the thermodynamic solubility limit), or hyperdoping. Indeed, Au-hyperdoped Si made by ion implantation and pulsed laser melting (PLM) has been shown to exhibit strong sub-band gap optical absorption in the near-infrared and has led to the demonstration of a Si-based near-IR photodetector. The Au sub-band gap absorption has been shown to increase with the Au dose, and significant room for further improvement of the device performance has been predicted. While these results illustrate the potential of hyperdoped Si for photodetection in the near-infrared, the material properties of Au- and other transition-metal-hyperdoped Si remain elusive. With this as a premise, this PhD work has focused on characterising and understanding the properties of Au-hyperdoped Si. In this thesis, detailed Rutherford backscattering spectrometry and channeling measurements are undertaken to examine the lattice position of the Au atoms. It is shown that the Au occupies mostly substitutional lattice positions within the hyperdoped Si lattice. In addition, by varying the iplant energy and the implanted Au dose, thicker layers of Au-hyperdoped Si with higher Au concentrations are demonstrated. However, although the Au atoms remain significantly substitutional (more than 50% substitutional in most cases) at high Au concentrations, the Au distribution is found to be non-uniform. Further structural characterisation by transmission electron microscopy and energy dispersive spectroscopy reveals a new observation where filaments of single crystalline, Au-rich Si regions emerge after PLM. The local concentration within such filaments is estimated to be at least 3 at. %, and the proximate Si lattice is found to be slightly skewed. These features suggest a novel segregation regime in Au-hyperdoped Si that is distinctly different to conventional 'cellular breakdown' in hyperdoped Si, in which impurity precipitation might be observed at cell-walls. In spite of the inhomogeneous behaviour of Au-hyperdoped Si at high Au concentrations, the sub-band gap optical absorption is found to continue to increase with increasing substitutional Au dose. This is consistent with density functional theory calculations in which the isolated substitutional Au configuration is found to give rise to significant sub-band gap optical absorption. This enhancement in sub-band gap optical absorption is found to deactivate after subsequent thermal annealing. We show that this behaviour can be correlated with a loss of Au substitutionality. Furthermore, the detailed atomistic mechanism for the thermal relaxation of Au-hyperdoped Si is investigated by correlating the experimental observations with density functional theory results. We show that the thermal relaxation of Au-hyperdoped Si is a multi-step process which involves (1) the exchange of substitutional Au with interstitial Au, (2) the trapping of Au to local sinks, (3) the formation of Au dimers, and (4) the clustering and nucleation of Au precipitates. Furthermore, the activation energy associated with loss of Au substitutionality is found to be 1.6 eV, a similar value to the diffusion of Au in defective Si. In the final chapter of this work, experimental evidence for a vacancy trapping model is presented. We propose that vacancies are introduced during the resolidification process to minimise local strain around Au-rich regions. Furthermore, the trapped vacancies are shown to be decorated with Au after subsequent thermal annealing. We show that such a behaviour is not limited to Au-hyperdoped Si and may be a universal phenomenon that occurs during the incorporation of large size impurities into Si as the Si is rapidly resolidified from a laser-induced melt. Show more