Review of Nanostructured Black Silicon Applications

Abstract

Silicon is a cornerstone of modern electronics, yet its high surface reflectance and 1.07 eV bandgap limit photon absorption, especially beyond 1.1 µm. Black silicon—silicon engineered with nanostructured spikes—offers broadband absorption from UV to infrared, making it ideal for photodiodes, photodetectors, solar cells, and more. This review surveys the optical and electronic advantages of black silicon, summarizes key applications, and discusses remaining challenges in scalability and material quality.

Background

Traditional silicon exhibits >40 % reflectance, severely curbing the efficiency of optoelectronic devices. The large bandgap restricts sensitivity to wavelengths above 1.1 µm. Black silicon, first fabricated in 1995 via reactive ion etching (RIE) and later refined with femtosecond (fs) laser processing, displays a micro‑nanospike surface that traps light and dramatically lowers reflectance. Sulfur, selenium, and tellurium doping during laser irradiation introduces deep level donors, extending absorption into the near‑infrared (NIR). These properties enable black silicon to outperform conventional silicon in photodetection and photovoltaics while remaining compatible with CMOS fabrication.

Absorptance Enhanced in Black Silicon

The micro‑spike morphology and dopant‑induced sub‑bandgap states together boost absorptance from 1100 to 2500 nm. Laser parameters—spot size, pulse number, fluence, and scanning speed—control spike geometry and dopant incorporation. Post‑irradiation annealing repairs lattice damage; however, excessive heat diffuses chalcogen dopants, reducing NIR absorption. Optimal annealing (≈ 775 K for 10–30 min) balances defect healing with dopant retention, preserving high infrared absorptance. Figures illustrate the dramatic absorption differences between doped and undoped, as well as the influence of annealing time.

a Micro‑structured versus unstructured silicon absorptance. b Black silicon spectra doped with S, Se, Te, and N. c Annealing time dependence of NIR absorptance. d 1550 nm absorptance versus laser pulse count. e Photocurrent of micro‑structured APDs at 1.31 µm. f I–V curves for different annealing temperatures.

Application of Black Silicon

Photodiodes

Black silicon photodiodes exhibit over ten‑fold quantum efficiency compared to bulk silicon, without compromising noise or dark current. Fs‑laser‑processed (111) wafers, doped with sulfur in SF₆, yield APDs that generate three times more carriers at 1.064 and 1.310 µm under ≥900 V bias. Optimized laser fluence and annealing (30 min) further enhance responsivity, as shown in the accompanying figure.

a Responsivity versus annealing temperature. b Responsivity versus laser fluence. c Quantum efficiency spectra of micro‑structured versus flat APDs. d I–V characteristics of a 100‑µm micro‑structured detector. e Noise power density versus photocurrent. f Responsivity at 250‑µm diameter under 0–3 V reverse bias.

Photodetectors

Black silicon extends spectral sensitivity from 250 to 2500 nm, achieving ten‑fold higher responsivity than commercial PIN photodiodes in the visible and 1.6 µm range. Fabrication in SF₆ produces the highest quantum efficiency after annealing, with NIR gains attributable to enhanced absorption rather than new energy bands. Reported devices reach 92 A/W at 850 nm and 119 A/W at 960 nm under 3 V bias, with a generation–recombination gain of ~1200. Hall measurements confirm high surface carrier concentration (~10¹⁸ cm⁻³) and mobility (~100 cm² V⁻¹ s⁻¹).

Solar Cells

Fs‑laser‑textured silicon yields >94 % absorption, surpassing conventional pyramidal or pillar structures. Solar cells fabricated with black silicon demonstrate 25–30 % higher photocurrent and up to 16.8 % conversion efficiency. Key performance drivers include surface passivation to mitigate recombination and optimized etch times to preserve short‑wavelength IQE. Comparative studies show black silicon devices outperform planar cells by >35 % in short‑circuit current density.

Field Emission

Micro‑spike morphology provides low turn‑on fields (~1.3 V/µm) and high current densities (up to 0.5 mA/mm² at 50 V/µm). Field emission displays stable operation across a wide voltage range, making black silicon attractive for displays, ion thrusters, and microwave amplification. Thermal annealing reduces emissivity but preserves emission characteristics.

Luminescence

Laser‑structured black silicon shows strong photoluminescence (PL) when excited with sub‑bandgap wavelengths. PL intensity scales sub‑linearly with excitation power (γ ≈ 0.44), indicating bound‑bound recombination via defect states. Temperature‑dependent studies reveal quenching above 120 K, consistent with thermal activation of non‑radiative pathways.

Surface‑Enhanced Raman Spectroscopy (SERS)

Black silicon integrated with gold nanoparticles yields high‑sensitivity SERS, enabling on‑chip detection of analytes such as Nile Blue and Rhodamine 6G. The nano‑spike surface enhances local electromagnetic fields, producing strong Raman signals at low resolution.

Hydrophobic Surface

By tuning fs‑laser fluence, black silicon surfaces exhibit superhydrophobicity (contact angle > 130°). This property facilitates self‑cleaning solar cells and reduces dust accumulation, preserving optical performance over time.

Conclusions

Black silicon mitigates silicon’s inherent drawbacks—high reflectance, limited bandgap, and indirect transitions—by introducing a nanospike surface that enhances absorption, reduces reflectance, and creates mid‑gap states. These advantages translate into high‑efficiency solar cells, sensitive NIR photodetectors, and robust field‑emission devices. Remaining challenges include scaling laser‑fabrication for industrial throughput and refining annealing protocols to balance defect repair with dopant retention. Continued research into process optimization and device integration will unlock black silicon’s full commercial potential.

Abbreviations

Al‑BSF

Aluminum back surface field

APD

Avalanche photodiode

CCD

Charge‑coupled device

fs

Femtosecond

IQE

Internal quantum efficiency

MFC

Micro fuel cell

near‑IR

Near‑infrared

NIR/SWIR

Near‑/shortwave‑infrared

PL

Photoluminescence

PSi

Porous silicon

QE

Quantum efficiency

RIE

Reactive ion etching

RTA

Rapid thermal annealing

SNSPDs

Superconducting nanowire single‑photon detectors