Ordered Si Micropillar Arrays via Carbon-Nanotube-Assisted Chemical Etching for Applications Requiring Nonreflective Embedded Contacts

Abstract

Metal-assisted chemical etching (MacEtch, MACE) has been heralded as a robust and cost-effective semiconductor fabrication technique that combines many advantages of wet and dry etching, while simultaneously overcoming their accompanying limitations. However, widespread use of MACE has been hindered partly due to the use of metallic catalysts such as Au that potentially introduce deep-level trap defects into Si processing. Moreover, alternative noble metal catalysts (e.g., Ag) embed an optically reflective film within the etched substrate, which can be detrimental to devices that rely on MACE-generated structures for improved light absorption. Here, a versatile process is detailed whereby carbon nanotube (CNT) composite films are used as catalysts for site-selective etching of Si(100) wafers. The so-called carbon-nanotube-assisted chemical etching (CNT-ACE) method enables solution-based and room-temperature fabrication of vertical Si micropillar arrays. Vertical etch rates (VERs) of Si samples etched by using nominally undoped CNTs and potassium tetrabromoaurate (KAuBr4)-doped CNTs are compared. Enhancement of VER from ∼28 to ∼142 nm/min is observed for KAuBr4-doped CNTs compared to undoped films, which is attributed to a shift in the catalytic film's aggregate reduction potential toward that of pure Au. Raman spectroscopy and Auger electron spectroscopy reveal that the catalytic CNT layer is not degraded during etching. A solar-weighted reflectance (SWR) of ∼2% is measured for Si micropillar arrays with embedded CNT membranes. This represents over 94% reduction in SWR compared to bare Si and 33% reduction compared to Si micropillar arrays fabricated via conventional MACE with embedded Au catalysts. A physical model is provided for the CNT-ACE mechanisms, including additional mass transport pathways for dissolution products through the CNT film. The CNT-ACE method enables complementary metal-oxide-semiconductor (CMOS) compatible fabrication of Si micro/nanostructures for device applications including photovoltaic solar cells and photodetectors and is particularly beneficial for applications wherein nonreflective embedded contacts are desired.