Enhanced Near‑Infrared Performance of Sulfur‑Hyperdoped Silicon Photodiodes Fabricated by Femtosecond Laser‑Induced Pulsed Melting

Abstract

Impurity‑mediated near‑infrared (NIR) photoresponse in silicon is a key objective for next‑generation photovoltaics and photodetectors. In this study, we fabricated a series of n⁺/p photodiodes by combining sulfur ion implantation with femtosecond pulsed laser melting (PLM). The resulting hyperdoped silicon devices exhibited a striking increase in NIR absorptance and photocurrent. The device implanted at 1 × 10¹⁴ ions cm^–2 delivered the best performance, demonstrating that a modest dopant dose, when coupled with PLM, can yield low‑cost broadband silicon photodetectors.

Background

Conventional silicon suffers from a 1.12 eV bandgap, which limits its intrinsic NIR sensitivity. Numerous strategies—ranging from surface texturing to sub‑bandgap doping—have been explored to overcome this barrier. The breakthrough of chalcogen‑supersaturated silicon, achieved by laser irradiation in an SF₆ atmosphere, revealed that silicon can be doped beyond its equilibrium solubility, creating impurity bands that absorb below the bandgap. The conical microstructures formed during laser melting further enhance light trapping, boosting absorption efficiency. Building on these concepts, we engineered sulfur‑hyperdoped silicon through ion implantation followed by femtosecond PLM, and evaluated the electrical and optical response of n⁺/p junction photodetectors.

Methods

One‑side polished p‑type (100) silicon wafers (300 µm thick, 8–12 Ω cm resistivity) were implanted with 1.2 keV ³²S⁺ ions to a depth of ~40 nm. Implantation doses of 1 × 10¹⁴, 1 × 10¹⁵ and 1 × 10¹⁶ ions cm^–2 were used. PLM was performed using a 1 kHz train of 100 fs, 800 nm pulses at a fluence of 0.5 J cm^–2; the 200 µm laser spot was scanned to pattern 10 mm × 10 mm squares. Rapid thermal annealing at 600 °C for 30 min in N₂ completed the process.

Absorptance (A) was derived from reflectance (R) and transmittance (T) measured with a UV–Vis–NIR spectrophotometer (Shimadzu UV3600) and integrating sphere (A = 1 – R – T). Carrier concentration and mobility were obtained via room‑temperature Hall effect (van der Pauw method). To probe the impurity‑mediated sub‑bandgap absorption, Fourier‑transform photocurrent spectroscopy was performed using a chopped FTIR globar source and lock‑in detection.

Results and Discussion

Figure 1 shows that PLM dramatically elevates absorptance across the visible and NIR ranges compared with as‑implanted samples. Annealing, however, reduces NIR absorption due to two main mechanisms: (1) the erosion of the conical nanostructures diminishes light‑trapping, and (2) sulfur atoms are optically inactivated through bond rearrangement. The microstructured surface thus plays a pivotal role in the enhanced NIR response.

a–c Absorptance versus fabrication process and implantation dose. d Impurity band within the Si bandgap enabling sub‑bandgap absorption. e SEM image of silicon spikes. f Optical path on the microstructured surface.

Because the surface topology is identical across samples, NIR absorptance primarily depends on the sulfur concentration. Earlier work identified a sulfur‑related level at ~614 meV, which underpins the pronounced NIR absorption. Pre‑annealing, the absorptance is relatively insensitive to dose: 10¹⁶ and 10¹⁵ ions cm^–2 yield similar values, while 10¹⁴ ions cm^–2 shows a modest decrease. Post‑annealing, the absorptance drop is attributed to dopant diffusion to grain boundaries and the formation of sulfur‑defect clusters, reducing the active impurity band density.

a Absorptance versus ion‑implantation dose (PLM‑treated). b Electrical properties of microstructured silicon before and after annealing.

Carrier sheet density rises with dose while mobility falls, consistent with Shockley‑Read‑Hall recombination: higher dopant levels shorten carrier lifetime, raising recombination probability. Annealing reduces sheet density dramatically due to thermal diffusion.

Figure 3 presents the photoresponse spectra. A distinct peak near 960 nm reflects generation of electron‑hole pairs in the Si substrate, extracted by the built‑in field of the n⁺/p junction. In the NIR band (1100–1600 nm), the 10¹⁴ ions cm^–2 device outperforms higher‑dose counterparts, achieving the highest photocurrent. Hall data show a bulk concentration of ~10¹⁹ cm^–3 and the best mobility at this dose, confirming that an optimal trade‑off between absorption and carrier transport yields superior device performance.

Photoresponse of n⁺/p detectors at different doses. Inset: device layout with interdigitated contacts on the microstructured surface.

Conclusions

Our systematic study demonstrates that sulfur hyperdoping via ion implantation followed by femtosecond PLM significantly enhances silicon’s NIR absorptance and photodetector performance. The device implanted at 1 × 10¹⁴ ions cm^–2 exhibited the highest photoresponse, confirming the critical balance between impurity‑induced absorption and carrier mobility. This scalable, low‑cost approach paves the way for broadband silicon photodiodes suitable for integrated photonics and energy harvesting applications.