Abstract
We report the critical factors that control the geometry of silicon nanostructures produced by metal-assisted chemical etching (MacEtch) using self-assembled islands from an ultrathin film of gold. We have conducted a systematic study of the process parameters that control the geometry of the metal structures and the resulting etched nanostructures. Compared to prior reports, which have focused on the crystal orientation and solution stoichiometry, our study finds that the anisotropy of the etched nanostructures is primarily controlled by the deposited metal geometry, while solution stoichiometry and crystal orientation play relatively minor roles. Using an optimized self-assembled geometry and etch process, we demonstrate what we believe is the highest aspect ratio to date (greater than 5000∶1) for high density top-down etched silicon nanostructures. These structures, which we refer to as silicon nanowalls, are in the size regime where quantum confinement effects could potentially be exploited for next-generation optoelectronic components and devices.
Highlights
Nanostructured semiconductors have received significant attention over the past decade because of their potential to alter the optical and electrical properties from bulk crystalline values
Steps consisted of an initial oxide etch/clean in a buffered oxide etch (BOE) solution, controlled reoxidation in an oxygen plasma, physical vapor deposition (PVD) metal deposition, metal-assisted chemical etching (MacEtch) in a HF þ H2O2 bath, critical point drying, and inspection
While MacEtch has been shown to work with a variety of metals,[3,14] our work focuses on gold for several reasons: silver has been studied most extensively, but its percolation threshold is relatively thick and the resulting self-assembled structures subsequently form features much larger than 10 nm
Summary
Nanostructured semiconductors have received significant attention over the past decade because of their potential to alter the optical and electrical properties from bulk crystalline values. It is well known that metal films deposited onto (semi)insulating substrates evolve to a thin-film state via a morphological sequence: it initially forms compact islands, elongated islands, percolation, hole filling, and the thin-film state.[20] By precise control of film thickness, we have been able to utilize the metal patterns of this morphological sequence as the catalyst for the MacEtch process. This is more attractive compared to metal salt baths because PVD deposition is widely available with greater uniformity, low cost, and ultrahigh purity. The focus of this paper is the fabrication process for MacEtch of silicon nanostructures with lateral features smaller than 10 nm using PVD patterned gold
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