Nanoscale morphology, optical dynamics and gas sensor of porous silicon

Abstract

We investigated the multifaceted gas sensing properties of porous silicon thin films electrodeposited onto (100) oriented P-type silicon wafers substrates. Our investigation delves into morphological, optical properties, and sensing capabilities, aiming to optimize their use as efficient gas sensors. Morphological analysis revealed the development of unique surfaces with distinct characteristics compared to untreated sample, yielding substantially rougher yet flat surfaces, corroborated by Minkowski Functionals analysis. Fractal mathematics exploration emphasized that despite increased roughness, HF/ethanol-treated surfaces exhibit flatter attributes compared to untreated Si sample. Optical approaches established a correlation between increased porosity and elevated localized states and defects, influencing the Urbach energy value. This contributed to a reduction in steepness values, attributed to heightened dislocations and structural disturbances, while the transconductance parameter decreases. Simultaneously, porosity enhances the strength of electron‒phonon interaction. The porous silicon thin films were further tested as effective gas sensors for CO₂ and O₂ vapors at room temperature, displaying notable changes in electrical resistance with varying concentrations. These findings bring a comprehensive exploration of some important characteristics of porous silicon surfaces and established their potential for advanced industrial applications.

Keywords: Gas sensor performance; Morphological properties; Optical properties; Porous silicon; Thin films.

Figure 1

SEM micrographs of the (a–b) Si#1:4, (d–e) Si#1:5, (g–h) Si#1:6, (j–k) Si#1:7, and (m–n) Si#1:8 porous silicon surfaces using magnifications of 15,000 × and 35,000x. On the right, correspondent cross section images (15,000x).

Figure 2

Representative 3-D AFM micrographs of (a) Si#0, (b) Si#1:4, (c) Si#1:5, (d) Si#1:6, (e) Si#1:7, and (f) Si#1:8 porous silicon surfaces.

Figure 3

(a)‒(b) Topographical profile and (c)‒(d) Abbott-Firestone curves of electrodeposited porous Si surfaces.

Figure 4

Minkowski volume, Minkowski boundary, and Minkowski connectivity of electrodeposited porous Si surfaces.

Figure 5

Adsorption coefficient of Si surfaces with different porosity.

Figure 6

Diagram of (αhν)² versus hν and determination E_g from (a) Si#1:4 to Si#1:6, (b) Si#1:7, and (c) Si#1:8.

Figure 7

The relation between Eg and width of E_u of Si porous surfaces.

Figure 8

Experimental value of the resistance-based gas sensors to (a) CO₂ (b) O₂ at 20 °C and different gas concentrations (30, 50, 70 and 90 sccm).

Figure 9

Response (%) of CO₂ and O₂ gas sensors in different gas concentration (a) 30 sccm (b) 40 sccm (c) 70 sccm (d) 90 sccm.