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.

Incorporating ultrahigh concentrations of deep-level dopants in silicon drastically alters silicon’s optoelectronic properties. Photodiodes built from silicon hyperdoped with gold extend light sensitivity into the shortwave infrared region, far beyond the absorption edge of a pristine silicon sample. Deep-level dopants, however, also enhance carrier recombination; even though hyperdoped silicon has great light absorption properties, short charge carrier lifetime limits its applications. In this work, using terahertz spectroscopy, we investigate the charge carrier lifetime of gold–hyperdoped silicon, where the gold dopants are introduced by either film deposition or ion implantation, followed by pulsed laser melting. Using reactive ion etching, we measure how carrier lifetime changes when dopant concentration profiles are altered. Furthermore, using a 1D diffusion and recombination model, we simulate carrier dynamics when electrons are excited by sub-bandgap light. Our results show that the dopant distribution profile heavily influences excited carrier dynamics. We found that etching improves the half-life by a factor of two. In the short-wave-infrared range, the gold dopants are both light absorption centers and recombination centers. Focusing on optoelectronic properties in the short-wave-infrared region, our results suggest that these samples are over doped—etching much of the gold dopants away has little impact on the number of excited electrons at a later time. Our results suggest that dopant profile engineering is important for building efficient optoelectronic devices using hyperdoped semiconductors.

In recent years, infrared photodetectors using silicon hyperdoped with deep-level dopants started to demonstrate extended light detection beyond the silicon’s absorption edge. The reported responsivities or external quantum efficiencies, however, are typically low. Focusing on gold-hyperdoped silicon and using time-resolved terahertz spectroscopy, a non-contact photoconductivity measurement, we investigated how hyperdoping parameters affect charge carrier lifetimes. Correlating the observed lifetime characteristics with dopant distribution profiles, we identify factors that impact carrier lifetime most significantly. Specifically, the charge carrier lifetime reduces with increasing gold concentrations, increasing ion implantation energies, and increasing pulsed-laser melting fluences. Both ion implantation energy and laser fluence affect the dopant incorporation depths. The total gold dose implanted and laser fluence affect the carrier distribution profile, particularly the concentration spike toward the surface. Oxide passivation and the number of laser pulses do not impact the carrier lifetime significantly. Our findings benefit future device developments.

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Ion implantation of transition metals into Si, followed by pulsed laser melting and rapid solidification, shows promise for making Si devices with sub-band gap optoelectronic response. We study Si implanted with Au at doses ranging from 1015 – 1016 at./cm2, with all but the lowest dose exhibiting interface breakdown during solidification, resulting in heavily defected layers. Terahertz photocarrier lifetime measurements confirm that layers with breakdown show recombination lifetimes of about 100 ps, compared to 800 ps for a layer with no breakdown. Device measurements, however, show more photoresponse at 1550 nm in a layer with breakdown than in a layer without. The results suggest that avoiding breakdown may be desirable but might not necessarily be imperative for making a useful device.