Defects in transition-metal-hyperdoped Si and their role in near-infrared light detection
Creators: name: Lim, Qi
Publisher: name: The Australian National University
Year: 2021

description: The bandgap of Si makes it transparent to light with wavelengths longer than 1.1 um. This includes most of the technologically useful telecommunication bands in the near-infrared (NIR). Conversely, InGaAs, HgCdTe, or Ge-based detectors are ideal in this spectral range but integration onto a Si platform for efficient signal processing can be complicated and expensive. Thus, there is a strong incentive to develop a Si-based NIR detection capability to reduce these costs, enhance scalability, and leverage the mature manufacturing infrastructure of the Si industry. This is expected to have a significant impact on the fields of photovoltaics, optical computing, and infrared imaging to name a few. Hyperdoped Si has been identified as a promising material for Si-based NIR detection. It involves high dose implantation of deep level impurities such as Au, Ti, and Ag followed by nanosecond pulsed laser melting (PLM). Above a critical concentration (N_C = 6e19 per cubic cm), an intermediate impurity band forms in the Si bandgap, enabling efficient sub-bandgap light absorption in the NIR. However, despite the high NIR absorption levels, NIR photodetectors do not show a correspondingly high performance and suffer from extremely low external quantum efficiencies (less than 0.01%). This thesis explores the role of process-induced defects in first generation hyperdoped Si NIR detectors. A focus is placed on Au-hyperdoped Si. A combination of electrical and optical techniques is first used to characterise the diode properties and then gauge the possible impact of defects on the NIR response. Deep level transient spectroscopy (DLTS) reveals a range of defects within the active device region with appreciable defect densities (~1e15 per cubic cm). These defects exist in the Si substrate well beyond the implanted and PLM region, and are generated from the implantation and PLM processes. The most prominent defect is a deep electron trap located at E_C-0.35 eV in the Si bandgap. Collectively referred to as the process-induced defects, they are also found to contribute to the NIR photocurrent together with photo-generated carriers from the Au-hyperdoped regions. Based on these results, routes to an optimised device design are explored with a number of factors identified for areas of improvement such as the carrier lifetime, device geometry, surface properties, and metal-hyperdoped Si electrical contacts. In particular, further experiments aimed at improving contact quality uncovered some surprising results. The Au hyperdoped layer was not heavily p-type as previously assumed in the literature but semi-insulating and n-type. Furthermore, a thick surface oxide layer and segregated Au gave rise to an effective p+ surface layer, presumably arising from acceptor states at the surface. Finally, the ability to hyperdope silicon with Ag and Ti is explored. Structural characterisation indicates that, unlike Au, Si cannot be hyperdoped with these impurities. Surface segregation and filamentary breakdown were identified as competing processes which inhibit reaching a hyperdoped regime. This study highlights the limitations but also the opportunities in utilising hyperdoped Si as a material for NIR detection.