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.
Chiou et al. (Thu,) studied this question.