Silicon‑Based Photodetectors with Resonant Cavities for Near‑Infrared Polarimetric Sensing

Background

The rapid evolution of optical communications demands inexpensive polarimetric photodetectors (PDs) operating in the NIR. Traditional III‑V and II‑VI compounds, such as GaAs/InGaAs and TeCdHg, offer high absorption coefficients but suffer from costly growth processes and limited CMOS compatibility. Silicon, the backbone of the semiconductor industry, has emerged as a viable platform for optoelectronics thanks to its mature processing, compatibility with CMOS, and recent advances in silicon photonics. By integrating silicon nanowire detectors with photonic structures, researchers have begun to explore new functionalities, including polarimetric detection.

Building on prior work with silicon nanowire PDs, this paper presents an all‑silicon detector that embeds a sub‑wavelength metallic grating to serve simultaneously as a polarizer and a resonant cavity. Three challenges drive the design: (1) extending silicon nanowire sensitivity into the NIR, (2) integrating a compact polarizer, and (3) enhancing responsivity through light‑harvesting structures. The proposed architecture combines a sub‑wavelength gold grating, a silicon nanowire array of controlled height, and a plasmonic resonator that facilitates hot‑electron generation and internal photoemission (IPE) at the Schottky interface, thereby enabling broadband, polarization‑sensitive NIR detection.

Methods/Experimental

Design of the All‑Si Polarization Detectors

Figure 1a illustrates the device geometry. Silicon nanowire arrays were fabricated on lightly n‑type silicon (500 µm thick, 1–10 Ω cm) with a pitch of 400 nm and heights (H) ranging from 100 nm to 300 nm, using a standard dry‑etch process. A Schottky barrier forms at the gold–silicon interface. Figure 1b shows the surface‑plasmonic resonator, consisting of gold layers above and below the nanowire array.

Schematic diagram of the resonator‑tuned metal‑semiconductor photodetector in silicon and its photo‑electronic principle. a, b The device layout. c, d Energy band diagrams for the Schottky junction under illumination with and without a DC bias. e Internal photoemission of hot electrons generated by surface plasmons.

Figures 1c and 1d depict band bending at the Si–Au interface under different bias conditions. Photon energies exceeding the silicon band‑gap (hν > E_g) normally generate electron–hole pairs, which are absorbed only in the visible range. However, hot electrons produced by the internal photoemission effect (IPE) in the gold layer can traverse the Schottky barrier, enabling NIR detection. The sub‑wavelength gold grating on the nanowire top also functions as a polarizer, tuning the resonance wavelength via its geometric parameters.

FDTD Simulations

To optimize the structure for high‑efficiency NIR polarimetric detection, we performed 3D FDTD simulations using Lumerical. Periodic boundary conditions were applied in the x and y directions, with perfectly matched layers along z. A TM‑polarized plane wave propagating along z served as the excitation source. The gold grating was set to a thickness of 85 nm, a width of 200 nm, and a pitch of 400 nm. Reflection and transmission monitors at the top and bottom of the silicon substrate yielded absorption spectra via A = 1 − R − T.

Device Fabrication

The device was fabricated using electron‑beam lithography. A 300‑nm PMMA layer was spin‑coated and baked at 180 °C for 12 min. After JEOL 6300FS exposure, the resist was developed in MIBK/IPA (1:3) at 23 °C for 60 s. Native oxide was removed with 2 % buffered HF, followed by thermal evaporation of 2 nm Cr/70 nm Au. Lift‑off in acetone at 60 °C completed the patterning. The gold grating was subsequently etched into the silicon via reactive ion etching (RIE) in a fluorine‑based plasma, forming the nanowire array. Finally, a 15‑nm Au film was deposited over the entire device to create the resonant cavity.

Photo‑Electric Characterization

Electrical measurements were carried out in the 0.7–1.1 µm wavelength range using a calibrated light source (OPM 35S). Current–voltage (I–V) characteristics were recorded under illumination and in the dark.

Results and Discussion

Figure 2a–d display the cross‑sectional geometry of the four device variants: (1) planar Si with bonding pad, (2) Au grating on planar Si, (3) Au grating atop Si nanowires, and (4) the full resonant‑cavity detector. Simulated transmission, reflection, and absorption spectra are shown in Figures 2e–g. For a 210 nm‑high nanowire array, the absorption at 860 nm peaks with a full width at half maximum (FWHM) of ~300 nm, attributable to Fabry–Pérot resonances between the top and bottom gold layers. The high electric field intensity between the metal layers, illustrated in Figure 2h(iii), indicates efficient plasmon‑to‑hot‑electron conversion.

Diagrams of the four devices investigated and corresponding FDTD results. a Planar Si substrate. b Au grating on Si. c Au grating on Si nanowires. d Full detector with top and bottom Au layers. e–g Transmission, reflection, and absorption spectra. h Electric field distributions at 860 nm for structures (b), (c), and (d).

Polarization analysis (Figure 3a) shows a peak absorption at 860 nm, with a periodic variation of absorption versus polarization angle yielding a peak‑to‑valley ratio of ~17:1. Optimization of the grating profile could further enhance this ratio.

Theoretical polarization properties of the resonant detector. a Absorption spectra versus polarization angle. b Polarization‑dependent absorption at 860 nm.

SEM micrographs (Figure 4) confirm the fabrication fidelity of each structure, with the resonant detector displaying the expected bilayer gold cavity and nanowire array.

SEM images of the four device variants: (a) bonding pad only, (b) Au grating on planar Si, (c) Au grating on Si nanowires, (d) full resonant detector.

Electrical measurements (Figure 5a) reveal that the resonant detector (Str.4) exhibits an order‑of‑magnitude higher photocurrent under a negative bias of −2 V compared to the other structures. Dark current remains low (~27 nA) due to increased nanowire resistance and a depletion layer at the 15‑nm Au/Si interface.

Measured I–V curves and responsivity of the resonant detector. a Logarithmic I–V under 16.6 mW cm⁻². b Dark I–V. c Responsivity spectrum at −2 V. d Bias dependence of responsivity at 860 nm.

Analysis of the dark I–V using the thermionic emission model yields a Schottky barrier height Φ_B ≈ 0.57 eV and an ideality factor n ≈ 1.43, confirming thermionic emission as the dominant transport mechanism. Responsivity peaks at 0.386 A W⁻¹ with a 150 nm FWHM, consistent with the simulated absorption profile. A Lorentzian fit to the responsivity spectrum gives Φ_B ≈ 0.58 eV, further validating the IPE detection mechanism.

Under varying illumination intensities (Figure 6b), the detector’s photocurrent scales linearly (I_ph = A P^θ, θ ≈ 1), confirming that photocurrent is governed by the number of generated carriers. Responsivity reaches 0.386 A W⁻¹ at −2 V and 0.146 A W⁻¹ at 0 V.

Photodetector performance under varying light intensities. a Logarithmic I–V curves in dark and under illumination. b Responsivity versus illumination intensity at −2 V. c Photocurrent response to square‑wave illumination.

Polarization sensitivity (Figure 7) shows peak‑to‑valley ratios of 5.6, 6.4, and 8.3 for the planar Au grating, Au grating‑nanowire, and resonant detector, respectively, demonstrating the enhanced polarimetric capability of the resonant architecture. Fast photocurrent modulation with polarization angle further confirms the device’s suitability for polarimetric sensing.

Polarimetric detection demonstration. a Photocurrent versus polarization angle. b Time‑resolved photocurrent response to polarization changes.

Conclusions

We have demonstrated a fully silicon‑based photodetector that integrates a sub‑wavelength gold grating, a silicon nanowire array, and a bilayer gold resonant cavity to achieve NIR polarimetric detection. The device delivers a peak responsivity of 0.386 A W⁻¹ at 860 nm under a −2 V bias and a polarization peak‑to‑valley ratio of 8.3, outperforming conventional silicon PDs in the NIR regime. FDTD simulations confirm that the detection wavelength can be tuned via structural parameters, offering flexibility for future applications. Further optimization of grating geometry and reduction of the top gold thickness are expected to improve responsivity and polarization contrast, paving the way for cost‑effective, high‑performance all‑silicon NIR sensors.

Abbreviations

3D:

Three‑dimensional

DC:

Direct current

EBL:

Electron beam lithography

FDTD:

Finite‑difference time‑domain

FWHM:

Full width at half maximum

IPE:

Internal photoemission effect

I‑V:

Current‑voltage

MS:

Metal‑semiconductor

NIR:

Near‑infrared

NW:

Nanowire

PDs:

Photodetectors

RIE:

Reactive ion etch

SEM:

Scanning electron microscope