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.

Ultralow Optical and Electrical Losses via Metal‐Assisted Chemical Etching of Antireflective Nanograss in Conductive Mesh Electrodes

“…MacEtch was used to form embedded electrodes for various Si photodetectors using a catalyst/semiconductor Schottky barrier. [ 43–45 ] In addition, unique optical characteristics were shown in well‐aligned semiconductors nanostructures with metal mesh. A novel front‐electrode structure that avoids the shading loss has been proposed.…”

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing

“…As detailed previously, the masks were prepared by block-copolymer micellar lithography in the presence of IrCl 3 . After deposition a thermal treatment at 375 °C was applied since the formation of iridium oxide occurs at higher temperatures with respect to ruthenium oxide . We further demonstrate that the size of the nanowires can be tuned by applying the same method but with PB-PEO block-copolymers with higher molecular length (75 500 g mol –1 PB 640 PEO 989 ), resulting in perforations and nanowire’s size of 60 nm (Figure S11c,d in Supporting Information).…”

“…However, this alternative has not been widely considered because the general view is that, in MACE, the catalyst is a metal. Recently, alternative catalytic materials have been proposed including graphene, 20 graphene oxide, 21 carbon nanotubes, 22 or titanium nitride. 23 However, these alternative processes require multistep pattern transfer as in the case of metals and are generally less effective than MACE for the fabrication of high aspect ratio structures.…”

Structuring of Si into Multiple Scales by Metal‐Assisted Chemical Etching

“…Similarly, various carbon-based catalyst has been applied for the MaCE of Si. [98][99][100][101] These carbonaceous catalysts can be easily and cleanly removed from the Si by oxygen plasma. Another beneficial feature of carbon-based catalysts is the low light reflectance after the etching of Si, thanks to the lower reflectance of carbon materials than that of typical noble metal catalysts.…”

“…Another beneficial feature of carbon-based catalysts is the low light reflectance after the etching of Si, thanks to the lower reflectance of carbon materials than that of typical noble metal catalysts. [99] Although these catalysts have shown lower etch rate compared to noble metal ones, their compatibility with the present semiconductor processing may widen the applicability of the MaCE.…”