Formation of Etch Pits on Germanium Surfaces Loaded with Reduced Graphene Oxide in Water
Authors: Nakade Kazuki , Hirano Tomoki , Li Shaoxian , Saito Yusuke , Mori Daichi , Morita Mizuho , Kawai Kentaro , Arima Kenta 
Published: February 2020

Metal-assisted chemical etching is a simple and low-cost etching process for semiconductor surfaces. There have been many reports on Si surfaces loaded with noble metals in HF solutions with oxidants such as H2O2.[1,2] Noble metals catalyze the reduction of the oxidant (H2O2), resulting in enhanced oxidation of the Si surface around the loaded metals. Because the oxide is immediately dissolved in the HF solution, selective etching of the Si surface occurs. This etching mode is used to form a variety of three-dimensional nanostructures not only on Si but also on other semiconductor surfaces.
We have applied this etching mode to the machining of a Ge surface in O2-containing water. So far, we revealed fundamental etching properties such as pore formation and patterning in this system with noble metals (Pt and Ag),[3] and recently, we demonstrated Pt-assisted chemical flattening.[4,5] In this scheme, a catalyst plate comprising a soft elastomer coated with a sputtered Pt film, and a Ge wafer were brought into contact and rotated independently in the same plane in water. The processed Ge surface includes few protrusions with a lateral size on the order of 10 nm, which is probably caused by the selective removal of protrusions from the Ge surface by the catalytic activity of Pt.
However, a problem in this system is the use of noble metals as catalysts. After metal-assisted chemical etching, residual metals on a semiconductor surface have to be removed. For example, aqua regia is effective for dissolving Pt. However, such a strong oxidative solution causes severe damage to a Ge surface. To solve this issue, graphene can be used as a substitute for noble metals to achieve catalyst-assisted chemical etching.[6]

In this talk, we discuss the etching properties of a p-type Ge(100) surface loaded with reduced graphene oxide (RGO) in O2-containing water. In order to obtain the RGO, a graphene oxide (GO) ink, used as a starting material, was either heated at 900°C in Ar ambient for 10 min or immersed in a solution comprising the GO ink, hydrazine and N,N-dimethylformamide.[7] Then we deposited the obtained RGO on a Ge surface in the form of aggregated particles or dispersed flakes. After immersing the samples into water exposed to air, we found that the Ge surface was preferentially etched around the loaded RGO. The etching rate as well as the etched morphology was revealed by atomic force microscopy observations, and the etching mechanism is proposed. These findings show the possibility of using RGO as a catalyst to enhance the chemical etching of a Ge surface in water.
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