Metal-Assisted Chemical Etching for Simple, Cost-Effective, and Large-Scale III-V Semiconductor Nanofabrication
Authors: Wilhelm Thomas S. , Kolberg Alex P. , Wang Zihao , Soule Cody W. , Baboli Mohadeseh A. , Yan Jian , Preble Stefan F. , Mohseni Parsian K. 
Published: February 2020

Nanostructured semiconductor architectures have been the topic of extensive research for decades due to their advantages over their bulk or planar counterparts. However, many III-V materials are often plagued by a host of nanofabrication difficulties stemming from incompatible or inefficient top-down etching, or expensive bottom-up growth techniques. Metal-assisted chemical etching (MacEtch) and inverse MacEtch (I-MacEtch) are powerful and solution-based nanofabrication alternatives that combine many attractive processing advantages, including the simplicity and cost-efficiency of conventional wet-chemical etching, and the ability to yield high aspect-ratio structures that are typically generated via reactive-ion etching. Furthermore, MacEtch is not hindered by many common fabrication limitations, such as the isotropic or crystallographic tendency of traditional wet etching, and the detrimental sidewall damage and lattice defects that stem from high energy ion bombardment during dry etching. MacEtch samples consist of a thin metallic catalyst layer (e.g., Au, Ag, or Pt) in intimate contact with a semiconductor. The metal-interfaced semiconductor is then submerged in an etching solution containing an acid (e.g., hydrofluoric acid or sulfuric acid) and an oxidant (e.g., hydrogen peroxide or potassium permanganate). Fundamentally, MacEtch mechanisms are modelled after a galvanic cell; that is, cathodic and anodic half reactions occur at the solution/catalyst and catalyst/semiconductor interfaces, respectively. The metal catalyzes the reduction of oxidant species within the solution, thereby locally injecting holes into the interfaced substrate. Preferential dissolution of the selectively oxidized material realizes high aspect-ratio structures using solution-based processing. While this process is commonly applied to Si and binary III-V systems, MacEtch fabrication of two key ternary compound III-V material systems is demonstrated here for the first time, including Au-enhanced InGaP and AlGaAs. Firstly, I-MacEtch of heteroepitaxial InGaP/GaAs is presented, and differential etch rates between epilayer and substrate are exploited as a viable method to produce suspended III-V nanofoils. Secondly, I-MacEtch of AlGaAs is reported for fabrication of ordered nanopillar arrays, whose aspect-ratio is shown to be tunable with Al fraction (x) and etching temperature (T). Finally, an entirely solution-based and lithography-free MacEtch process is demonstrated for fabrication of black GaAs composed of sub-wavelength dimension nanostructures. The work detailed here provides simple, low-cost, and efficient means to customize nanofabrication processes for specific needs. It is anticipated that these methods can be utilized for adaptable and versatile processing of nanomaterials for LEDs, lasers, HEMTs, and solar cells.

Fig. 1 – (a) false-colored cross-sectional SEM image and (b) schematic depicting overlapping hole distributions for Au-patterned I-MacEtch of suspended InGaP nanofoils [1]. False-colored tilted-view SEM images of Au-patterned I-MacEtch of AlGaAs (c) before and (d) after Au removal via mechanical exfoliation, and (e) a model illustrating the ability to tune the etch anisotropy with increasing Al content for a fixed etch temperature (x|
T
) and increasing temperature for a fixed Al content (T|
x
) [2].

[1] – T. S. Wilhelm et al., ACS Appl. Mater. Interfaces, 10, 2058 (2018)

[2] – T. S. Wilhelm et al., ACS Appl. Mater. Interfaces, 10, 27488 (2018)

Figure 1