Phase transition and conductivity modification of porous silicon via CO2 laser processing

Abstract

This study investigates how laser irradiation modifies the optical, structural and electrical properties of porous silicon (PS). PS samples were fabricated by electrochemically etching p-type crystalline silicon wafers at a current density of 63 mA/cm2 for 20 minutes. A continuous wave CO2 laser (10.6 µm, 5–40 W) was then applied. A comprehensive suite of characterisation techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM), Raman spectroscopy and current-voltage (I-V) measurements was used to analyse the samples before and after laser irradiation. The unique interaction between the 10.6 µm CO2 laser and PS is attributed to the significant IR absorption of PS at this wavelength, unlike bulk silicon. Upon laser irradiation, vibrational excitation triggers localised heating and the formation of heat zones, which induce two distinct stages. During the pre-melting consolidation stage, localised heating leads to partial melting of pore walls, causing adjacent pores to merge. As pores coalesce, the number of individual pores decreases, and the material between them consolidates, resulting in thicker pore walls, wider remaining pores, and an overall reduction in porosity. SEM and AFM analyses reveal a decrease in PS layer thickness and a smoother surface, with a roughness decreasing from 1.48 to 0.82 nm, approaching that of the underlying crystalline silicon substrate. In the melting stage, where the temperature exceeds the melting point of the silicon, the AFM images show a complete pore collapse, indicating that the silicon has fully melted. Raman spectroscopy transitions from a broadened, redshifted spectrum with an additional lower-frequency peak to a sharp crystalline peak at 520 cm-1 (with a 2.8 cm-1 full width at half maximum) in the laser-modified regions. Increasing laser intensity and exposure further broadens the peak and induces a redshift, accompanied by the emergence of a secondary peak, suggesting stress effects. XRD confirms the reformation of a single-crystalline structure, while XPS analysis reveals significant oxidation and the formation of a silicon oxide layer in the laser-irradiated areas, a layer that when removed by HF etching restores the Raman spectral profile. Electrical characterisation shows that the laser-processed areas exhibit metallic-like behaviour, as evidenced by I-V measurements and KPFM data indicating a high-work function network. This behaviour is attributed to the uneven distribution of boron during electrochemical etching, leading to the formation of heavily doped silicon nanowires. Upon laser irradiation, these regions melt and recrystallise, resulting in localised metallic-like conduction because of the high density of charge carriers and a reduced bandgap. Overall, the controlled laser irradiation process offers a precise means of tailoring PS properties, with significant implications for advanced applications in optical waveguides, biosensors, photovoltaics, microelectronics, energy storage, and plasmonic devices