Enhancing Broadband Photodetection with Self‑Assembled Dielectric Microcavity Arrays

Introduction

Photodetectors (PDs) underpin a range of commercial applications—optical communication, sensing, and imaging—where high responsivity is essential. Efficient light absorption in the active region drives photocarrier generation, making advanced light‑trapping techniques critical for next‑generation PDs. Emerging demands for tunable, spectra‑selective responsivity further motivate the development of novel light‑manipulation strategies.

Traditional approaches such as random texturing and three‑dimensional nanostructures enhance absorption by exploiting large surface‑to‑volume ratios and extended optical paths. Among these, low‑Q resonant optical cavities offer a compelling route to broadband light trapping via multiple WGM resonances. WGM confinement in dielectric spheres or hollow structures can dramatically amplify light‑matter interactions or funnel light into underlying substrates through leaky modes, leading to improved photocarrier generation and device performance. Prior work has shown that wavelength‑scale dielectric nanospheres enhance absorption and photocurrent in thin‑film solar cells, but their application to photodetectors remains largely unexplored.

In this work, we introduce self‑assembled ZnO microcavity arrays (MCAs) on silicon PIN PDs to engineer light trapping over a wide spectral range. ZnO, a wide‑bandgap semiconductor, can be fabricated via various scalable methods. By employing polystyrene (PS) nanospheres as templates, we produce hollow ZnO microcavities that support multiple low‑Q WGMs. The resulting broadband light trapping translates into measurable responsivity gains across the visible and near‑infrared (NIR) spectrum. Additionally, we demonstrate that by adjusting the cavity shell thickness, we can selectively enhance absorption in the 800–980 nm band, achieving up to a 25 % improvement at 820 nm. This study establishes a practical, low‑cost approach to augment PD performance through resonant light manipulation.

Results and Discussion

The device architecture comprises ZnO MCAs fabricated atop a silicon PIN PD, as illustrated in Fig. 1a and b. The MCAs, derived from 530‑nm PS nanospheres, exhibit an actual core diameter of 470 nm and a shell thickness of ~40 nm (Fig. 1c–d). SEM images confirm the monolayer, hexagonal close‑packed arrangement and the smooth inner surfaces that favor efficient WGM confinement (Fig. 1d, S2a‑b). Diffraction patterns observed across the large‑area MCA array (Fig. S3a) arise from Bragg scattering, reinforcing the periodicity and optical quality of the structure.

Schematic of a ZnO MCA‑decorated PIN PD (a), top view of the device (b), and SEM images of the fabricated MCAs (c, d).

Finite‑difference time‑domain (FDTD) simulations of MCAs on a sapphire substrate reveal transmission valleys at 415, 495, 547, and 650 nm—clear signatures of WGM resonances. The simulated near‑field patterns (Fig. S4) illustrate strong field confinement and leaky‑mode radiation around the cavities, which can couple into the underlying silicon. Experimental transmission spectra (Fig. 2b) align closely with the simulations, confirming the presence of WGMs. The leaky‑mode scattering also manifests in the reflection spectra: theoretical calculations (Fig. 2c) and experimental measurements (Fig. 2d) both show broadband anti‑reflection and distinct resonance peaks. Notably, off‑resonance wavelengths (e.g., 840 nm) exhibit enhanced absorption in the silicon substrate compared to on‑resonance wavelengths (e.g., 660 nm) due to reduced back‑scattering (Fig. 2e). This behavior underscores the importance of resonance quality: high‑Q WGMs can scatter light back, diminishing absorption, whereas lower‑Q leaky modes promote light trapping.

a Theoretical and b experimental transmission spectra of MCAs on sapphire. c, d Corresponding reflection spectra on silicon and bare silicon. e Absorption profiles in silicon with and without MCAs under on‑ (660 nm) and off‑resonance (840 nm) excitation.

Device performance measurements confirm the optical insights. The PIN PDs exhibit typical photodiode IV characteristics (Fig. 3a). Upon MCA decoration, the photo‑response at 850 nm increases by ~25 % relative to the control (Fig. 3b). Spectral responsivity curves (Fig. 3c) show broadband enhancement across the visible and NIR ranges, with the largest gains in the 800–980 nm band (~17 % increase). This aligns with the off‑resonance absorption advantage. The 625–695 nm region—centered on the second‑order WGM (~640 nm)—shows no enhancement, reflecting the back‑scattering effect of high‑Q resonances. Short‑wavelength (< 600 nm) performance also improves, driven by the strong anti‑reflection of the MCAs.

a IV curves under dark and 850 nm illumination. b Current response comparison at 850 nm. c Wavelength‑dependent responsivity with and without MCAs (inset: < 380 nm). d Enhancement ratio highlighting on‑ and off‑resonance regions.

To tune the resonance behavior, we increased the ZnO shell thickness to ~60 nm. The transmission spectrum now shows additional resonances and a red‑shift of existing modes (Fig. 4a), corroborated by simulations (Fig. S7). Reflection spectra (Fig. 4b) reveal higher‑quality resonances, indicating stronger back‑scattering at on‑resonance wavelengths. Responsivity maps (Fig. 4d) confirm that the off‑resonance region (800–980 nm) benefits from higher light trapping, yielding up to a 25 % improvement at 820 nm. Conversely, on‑resonance wavelengths suffer reduced responsivity due to increased scattering.

a Transmission of 60‑nm shell MCAs on sapphire. b Reflection on silicon versus bare silicon. c Responsivity with and without MCAs under 850 nm LED. d Enhancement ratio with on‑/off‑resonance backgrounds highlighted.

Long‑term stability tests show negligible degradation after one year in ambient air, underscoring the robustness of the MCA‑decorated devices (Fig. S8).

Conclusions

We have introduced a scalable strategy to enhance broadband and spectra‑selective photodetection by integrating self‑assembled ZnO microcavity arrays onto silicon PIN PDs. The leaky‑mode WGMs in the MCAs effectively trap light, especially in off‑resonance spectral windows, leading to up to a 25 % responsivity increase in the 800–980 nm communication band. Adjusting the cavity shell thickness allows precise tuning of resonance positions and quality factors, enabling selective enhancement across the ultraviolet to near‑infrared spectrum. This low‑cost, high‑compatibility approach offers a versatile pathway to boost light absorption and photo‑responsivity in a range of optoelectronic devices, including solar cells and LEDs.

Methods/Experimental

Fabrication of PIN PD Devices

The PIN photodiodes were fabricated on 200‑µm‑thick p‑type (100) silicon wafers (resistivity 0.001 Ω cm) purchased from WaferHome. A 20‑µm‑thick intrinsic layer was epitaxially grown, followed by n‑type phosphorus implantation (1 × 10¹⁶ cm⁻², 160 keV). After standard RCA cleaning, photolithography defined a 2.8 mm × 2.8 mm active area. 100‑nm Al contacts (160 µm diameter) were sputtered on the n‑side, while a 50‑nm Au film with a 5‑nm Ti adhesion layer was deposited on the back side.

Fabrication of ZnO MCA Layer

ZnO MCAs were fabricated by templating 530‑nm PS nanospheres (Nanomicro) on the PIN wafer, followed by sputter deposition of ZnO. The PS spheres were subsequently removed by thermal annealing, leaving hollow ZnO microcavities. The ZnO shell thickness was tuned to ~40 nm or ~60 nm by adjusting deposition time.

Characterization

SEM imaging was performed on a Hitachi S‑4800 FE‑SEM. Transmission and reflection spectra were recorded with a Varian Cary 5000 UV‑Vis‑NIR spectrophotometer. Device IV and photocurrent measurements used a CHI660D electrochemical workstation with a probe station and LED sources. External quantum efficiency (EQE) at zero bias was measured with a Newport optical power meter and monochromator. FDTD simulations were conducted using Lumerical FDTD Solutions.

Availability of Data and Materials

All data generated or analyzed during this study are included in the published article and its supplementary information files.

Abbreviations

3D

Three dimensional

EQE

External quantum efficiency

IR

Infrared

IV

Current‑voltage

MCAs

Microcavity arrays

PDs

Photodetectors

PIN

Positive‑intrinsic‑negative

PS

Polystyrene

R_off

Off‑resonance

R_on

On‑resonance

T_shell

Shell thickness

WGM

Whispering‑gallery‑mode