Pit Formation, Patterning and Flattening of Ge Surfaces in O₂-Containing Water by Metal-Assisted Chemical Etching

Recently, metal-assisted chemical etching has attracted a great deal of attention because it is a simple and low-cost process for the fabrication of nanostructures on a semiconductor surface, mainly Si [1, 2]. This electroless etching is made possible by using noble metals, such as Pt and Ag, to catalyze the reduction of a chemical oxidant, such as H2O2, in the presence of HF. Namely, the reduction is accompanied by the selective oxidation around the loaded metals followed by the dissolution of oxide (SiO2) in HF solution. By controlling the etching conditions, a vast variety of nanostructures from porous structures to nanowire arrays can be fabricated in a chemical lab without the need for expensive equipment. Ge is regarded as a promising channel material in future electronics. It is necessary to develop and/or optimize wet processes, such as lithography, planarization and cleaning, for Ge. Ge oxide (GeO2) is soluble in water, unlike SiO2. Also, O2 molecules dissolved in water are oxidants. These chemical properties enable us to develop a novel process for a Ge surface using metal-assisted chemical etching in water. In this study, we immersed a Ge(100) surface loaded with Pt or Ag particles with the size of 10 nm order into O2-containing water and observed the formation of inverted pyramidal etch pits around the particles. This is caused by the catalytic effect of metals whereby O2 molecules in water are reduced, resulting in an enhanced oxidation of a Ge surface to form soluble GeO2. We confirmed that the O2 concentration in water is a key in etch rate [3, 4]. Then, we tested nanoscale patterning of a Ge surface by metal-assisted etching in O2-containing water. This was achieved by scanning a metal-coated cantilever on a Ge surface in the contact mode in an atomic force microscopy setup. We found that the Ge surface scanned by the cantilever was selectively removed, and the etched depths depended on experimental conditions such as the O2 concentration in water, the pressing force of the cantilever to the Ge surface, and the conduction type (n-type or p-type) of Ge. These findings indicate that the mechanism of inducing enhanced etching along the trajectory of the cantilever probe is a catalytic chemical effect rather than a mechanical one [4]. Finally, we used this metal-assisted chemical etching to improve the microroughness of a Ge surface. In this scheme, a thin film of noble metal (Pt) was deposited on a soft elastomer pad. This Pt surface was brought into contact with a Ge wafer surface by applying 0.01 MPa in O2-containing water. Both the pad coated with the metal and the Ge wafer were rotated independently in the same plane. Consequently, the Ge surface was flattened. The flattening mechanism is as follows. First, protrusions on the Ge surface have a higher probability of coming into contact with the metal surface than do grooves. These protrusions were selectively oxidized by the catalytic effect of the metallic film in water. After the oxidized protrusions were dissolved in water promptly, a flattened surface was created [5]. [1] X. Li et al., Appl. Phys. Lett. 77, 2572 (2000). [2] Z.P. Huang et al., Adv. Mater. 23, 285 (2011). [3] T. Kawase et al., J. Appl. Phys., 111, 126102 (2012). [4] T. Kawase et al., Nanoscale Res. Lett., 8, 151 (2013). [5] T. Kawase et al., ChemElectroChem, 2, 1656 (2015).