Altering the Fluorination Anodization Pathway of Silicon Near an Electrolyte Freezing Point Promotes the Formation of Blue-Photoluminescent Microstructures

Abstract

A fluoride-anion (F^-) governed electrochemical etching process, i.e., fluorination anodization, is used to construct unique silicon nanostructures owing to its reaction pathway, presenting a powerful and versatile strategy for fabricating advanced microsystems and quantum-based photonics devices. This fluorination anodization approach, which produces nanocrystals in anodized silicon, takes full advantage of the inherent crystalline quality of the prime wafers to achieve well-recognized photoluminescence via the quantum confinement effect. However, for heavily boron-doped (p⁺-type) silicon, fluorination anodization fails to produce a photoluminescent layer because the high doping level results in a coarse etching morphology. In contrast, performing anodization with the electrolyte precooled near its freezing point rather than at room temperature changes the outcome of UV irradiation on the anodized surface─from forming an optically absorbing black layer to producing a bright blue layer composed of nanocrystals (1.8-2.2 nm). Moreover, the cryogenically treated etching behavior appears to shift from anisotropic to isotropic as a result of the altered interfacial reactions. This transition may be attributed to the suppression of crystallographic etching by oxidation-etching control rather than Gösele-Lehmann-model-etching control. The cryogenically designed fluorination pathway near the electrolyte freezing point significantly influences the anodization process of heavily boron-doped silicon, thereby enabling the formation of surface microstructures suitable for low-resistivity silicon photonic and quantum devices. Overall, we report a physical cryogenic treatment that alters the interfacial reactions during fluorination anodization by operating the electrolyte near its freezing point. Under these cryogenic conditions, the anodization behavior is substantially altered, thereby facilitating the formation of hydrogenated or fluorinated surface nanostructures on silicon and promoting advanced semiconductor and photonics manufacturing.

Keywords: cryogenic treatment; fluorination; nanostructure; photoluminescence (PL); silicon photonics.

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Schematic illustration of the electrochemical fluorination anodization setup and temperature-dependent PL characteristics of heavily boron-doped p⁺-type silicon. Anodization at room temperature (RT, left side) produces a black surface without detectable PL emission. Anodization at −70 °C (right side) results in a surface emitting bright blue light, accompanied by multiple PL peaks at 415 and 500 nm, demonstrating enhanced emission intensity and a shift toward shorter wavelengths. The resulting light-emitting nanostructures are CMOS-compatible and stable at room temperature, offering a promising platform for on-chip integration in silicon photonics and quantum devices.

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Schematic diagram of the experimental setup for electrochemical anodization at room or cryogenic temperatures. The setup is designed to achieve temperatures of −30 °C to −80 °C via liquid nitrogen. The core setup for anodization consists of a structured arrangement including a PTFE etching bath, an O-ring for sealing, a silicon sample, a copper electrode, a platinum electrode and a bottom support.

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Correlation between optical emission, surface chemical state, and long-term stability. (a) PL spectra comparing the nonemissive RT sample with the blue-emissive −70 °C sample. The −70 °C sample demonstrated near-perfect spectral retention after 15 months of ambient storage (Dec 2023 vs Mar 2025). (b–g) High-resolution XPS spectra for Si 2p, O 1 s, and F 1 s. Mechanism Transition: The dominance of Si ⁰ at RT (c) confirms a standard fluorination-etching path, whereas the shift to Si⁴⁺ at −70 °C (b) indicates an oxidation-governed regime. To demonstrate the effects of surface passivation, the RT sample (e) exhibits a significantly higher O 1 s intensity (2.2 × 10⁵ cps) than the −70 °C sample (d) (8.0 × 10⁴ cps), which is attributed to disordered postetching oxidation occurring on the unpassivated surface. In contrast, the −70 °C sample (f) exhibited a dramatic 10-fold increase in F 1 s intensity (4 × 10⁴ cps) compared with that of the RT sample (g) (5 × 10³ cps) and a binding energy shift to 687.0 eV, indicating that fluorine was integrated into a surface chemical matrix. These data, coupled with the Si⁴⁺ dominance observed in Figure b, strongly suggest the formation of an in situ passivation SiO_xF_y shell. These results suggest that the SiO_xF_y shell formed at −70 °C may act as a kinetic diffusion barrier that stabilizes the blue luminescent nanocrystals.

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Comparative TEM analysis of p⁺⁺ -type silicon anodized under cryogenic and room-temperature conditions. (a) Cryogenic Anodization (−70 °C): Cross-sectional view (scale bar = 10 nm) revealing isolated, ultrasmall nanocrystals. High-resolution insets highlight lattice fringes of individual crystals with diameters of 2.17 nm (upper) and 1.95 nm (lower-right), respectively. (b) Room-Temperature Anodization (25 °C): Characterization of a control sample etched at 300 mA for 60 min. The morphology consists of a continuous mesoporous network devoid of quantum-confined nanocrystals. The high-resolution inset (upper right, scale bar = 5 nm) displays well-defined silicon crystalline domains exceeding 14 nm in width. The corresponding Selected Area Electron Diffraction (SAED) pattern (lower right, scale bar = 5 1/nm) exhibits sharp, discrete spots, verifying the bulk-like single-crystal integrity of the framework remaining after aggressive ambient-temperature etching.

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Top-view SEM image of silicon anodized at −80 °C. (a) The surface has a highly isotropic morphology characterized by uniformly distributed circular pits and rounded pore openings. The smooth and spherical-like profiles indicate an isotropic etching response that differs markedly from the anisotropic, crystal orientation governed dissolution typically observed at room temperature. (b) Schematic cross-section illustrating that the isotropic etching behavior is confined to localized regions where the fluoride-anion concentration remains sufficiently high to sustain the reaction, rather than occurring uniformly across the entire surface.

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SEM images of silicon samples anodized at different temperatures, highlighting the evolution of porosity and etching anisotropy. All the images were acquired via a Schottky field-emission SEM (SU8030, Hitachi) at an accelerating voltage of 10.0 kV with a magnification of 100,000× and an SE (secondary electron) detector. (a, b) Anodization at room temperature (RT): The top view (a) shows scattered surface features, whereas the cross-sectional view (b) reveals highly anisotropic, vertically aligned mesopores. (c, d) Anodization at −70 °C: The top view (c) and cross-sectional view (d) display a nearly isotropic morphology characterized by circular nanopits and interconnected porous domains, indicating a transition to diffusion-limited, radially symmetric dissolution.