Visible Emission from Oxygen‑Doped Silicon Nanocrystals on Laser‑Engineered Black Silicon

Abstract

We fabricated black silicon (BS) using a nanosecond pulsed laser (ns‑laser) under vacuum or in an oxygen atmosphere. Remarkably, room‑temperature photoluminescence (PL) measurements reveal strong visible emission from the BS surface after annealing. A pronounced lasing peak near 600 nm appears within a Purcell cavity structure. Detailed PL analysis shows that the electronic states of oxygen‑doped silicon nanocrystals are the primary source of the visible emission, with distinct peaks at ~400, 560, and 700 nm. These findings suggest a viable route toward silicon‑based white‑light devices.

Background

Bulk silicon, with its indirect band gap of 1.12 eV, exhibits poor light‑emission efficiency. Yet, integrating efficient silicon light emitters into chips remains a top priority for next‑generation optoelectronics. Recent studies have reported room‑temperature visible emission from low‑dimensional silicon nanostructures, especially black silicon (BS) produced by pulsed‑laser processing [1–6]. Pulsed lasers—both femtosecond and nanosecond—dramatically alter silicon’s optical properties, and the visible emission from BS has attracted considerable scientific interest, although its mechanism is still debated [13–15]. In this work, we generate BS surfaces with a ns‑laser in vacuum and in an 80 Pa oxygen environment, achieving efficient visible emission. Annealing further enhances the emission, and PL spectra reveal that oxygen‑doped silicon nanocrystals (NCs) dominate the visible output near 400, 560, and 700 nm, opening prospects for silicon‑based solid‑state lighting.

Experiments and Results

A pulsed‑laser etching (PLE) system was used to fabricate the BS surface. The ns‑laser, focused to a ~10 µm spot, processed p‑type (10 Ω·cm) silicon wafers in vacuum (sample I) or in an 80 Pa oxygen atmosphere (sample II) (Fig. 1a). A plasmonic lattice spontaneously emerged on the BS during the PLE process (inset of Fig. 1a). SEM imaging (Fig. 1b) shows a highly absorptive surface (<10 % reflectance) with a refractive index of ~1.88 in the visible range, consistent with Kramers–Kronig relations [16, 17]. TEM confirms the presence of silicon NCs in the annealed BS (Fig. 1c).

a Structure depiction of the PLE device. b SEM image of the BS surface after annealing. c TEM image of silicon nanocrystals in the BS.

PL spectra were recorded under 266‑nm excitation at 300 K and 10–200 K in a 1 Pa chamber. Temperature and annealing time critically influence crystallization. A 1000 °C anneal for 15 min optimizes visible PL at 300 K for sample II (oxygen) and yields strong emission at 10 K for sample I (vacuum).

Comparative analysis shows that sample I exhibits a robust 330 nm peak at 10 K (black curve, Fig. 2a), attributable to NC emission, while sample II shows a markedly enhanced 400 nm peak at 300 K (red curve, Fig. 2b), indicating oxygen‑induced impurity states.

a PL spectra from 300 to 500 nm at low temperature: sample I (black) vs. sample II (red). b PL spectra at 300 K: sample I (black) vs. sample II (red), showing broader enhanced peaks in sample II.

At ~560 nm, the PL intensity of sample II (red) outshines sample I (black) at 300 K, again pointing to oxygen‑related impurity states (Fig. 3).

PL spectra near 560 nm at 300 K: sample I (black) vs. sample II (red).

Figure 4a displays PL spectra of sample I (vacuum) at 300 K as a function of excitation power. The broad band reflects NC size dispersion. After annealing at 1000 °C, the broad band disappears, and distinct impurity‑related peaks emerge near 600 nm and 700 nm (Fig. 4b).

a PL spectra vs. excitation power (300 K, sample I, vacuum). b PL spectra after 1000 °C annealing.

Most strikingly, a sharp PL peak with lasing at ~600 nm appears within a micrometer‑scale Purcell cavity on the BS surface under 514‑nm excitation (Fig. 5). The optical gain, extracted using a variable‑strip‑length method, reaches ~130 cm⁻¹.

a Optical image of the Purcell cavity on the BS. b Sharper PL peak with lasing near 600 nm (300 K, 514‑nm excitation).

Discussion

PL decay analysis of Si NCs with varying diameters reveals a clear transition from indirect to direct band‑gap behavior. Smaller NCs (<2 nm) exhibit fast decay (<1 ns) corresponding to direct‑gap emission (~400 nm and ~560 nm). Larger NCs (>2.5 nm) show slow decay (~µs), characteristic of indirect‑gap emission (~700 nm), alongside fast components from impurity states (Fig. 6).

a PL decay at ~400 nm (fast photons). b PL decay at ~560 nm (fast photons, ns). c PL decay at ~700 nm (fast photons from impurity states, ns; slow photons from large NCs, µs).

The emission model (Fig. 7) confirms that direct‑gap transitions dominate in sub‑2 nm NCs, while indirect‑gap transitions and impurity‑state emissions prevail in larger NCs, consistent with quantum confinement theory.

Emission model: direct‑gap (fast, <2 nm NCs) vs. indirect‑gap (slow, >2.5 nm NCs) and impurity‑state contributions.

Conclusion

Our study demonstrates that ns‑laser‑fabricated BS hosts both micro‑ and nanostructures that enable visible‑wavelength PL suitable for LED applications. By comparing samples prepared in vacuum and in 80 Pa oxygen, we confirm that oxygen‑impurity states in silicon NCs are responsible for the robust room‑temperature emission at 400, 560, 600, and 700 nm, while the 330 nm peak at 10 K arises from NC luminescence. These results pave a new path toward silicon‑based visible light sources and white‑light devices.

Methods

Photoluminescence Measurement

PL spectra were recorded under 266 or 488‑nm excitation at 300 K and 17–200 K in a 1 Pa chamber. Sharp peaks with stimulated emission and direct‑gap characteristics were observed. A lasing peak near 600 nm on the BS, achieved after annealing, was quantified by the variable‑strip‑length method, yielding an optical gain of ~130 cm⁻¹. PL decay spectra at 400, 560, and 700 nm were measured using ps‑pulsed 266‑nm excitation.