ABSTRACT
Conventional lithography using photons and electrons continues to evolve to scale down three-dimensional nanoscale patterns, but the complexity of technology and equipment is increasing due to diffraction and scattering problems. Physical contact lithography methods, such as nanoimprint and soft lithography, have been developed as an alternative technique. These techniques imprint predefined structures on a stamp to the polymer resist and use the polymer resist as a mask to dry etch the nanostructure on the substrate. In this study, we introduce a method of chemically imprinting crystalline silicon (Si) with a catalytic stamp to enable the direct etching of the Si without using a polymer mask. A metal catalyst is deposited on the predefined structure of the stamp. The stamp physically contacts the Si in the etching bath, and metal-assisted chemical etching occurs on the semiconductor surface. Since the metal catalyst is mounted on a stamp, it can be used repeatedly. This is a technology that combines conventional lithography and etching without using a polymer resist. This technology not only produced nano/microscale arrays of circular and square holes and trench structures but also successfully produced complex eagle-shaped structures that contained such structures.
ABSTRACT
GaAs solar cells with nanostructured emitter layers were fabricated via metal-assisted chemical etching. Au nanoparticles produced via thermal treatment of Au thin films were used as etch catalysts to texture an emitter surface with nanohole structures. Epi-wafers with emitter layers 0.5, 1.0, and 1.5 um in thickness were directly textured and a window layer removal process was performed before metal catalyst deposition. A nanohole-textured emitter layer provides effective light trapping capabilities, reducing the surface reflection of a textured solar cell by 11.0%. However, because the nanostructures have high surface area to volume ratios and large numbers of defects, various photovoltaic properties were diminished by high recombination losses. Thus, we have studied the application of nanohole structures to GaAs emitter solar cells and investigated the cells' antireflection and photovoltaic properties as a function of the nanohole structure and emitter thickness. Due to decreased surface reflection and improved shunt resistance, the solar cell efficiency increased from 4.25% for non-textured solar cells to 7.15% for solar cells textured for 5 min.
ABSTRACT
Light trapping by surface texturing is widely used to improve the performance of optoelectronic devices. In this Letter, we demonstrate nano/micro dual-scale textured GaAs by integrating triangular GaAs by orientation-dependent wet etching and subwavelength nanoholes by metal-assisted chemical etching (MacEtch). This is the first report on nano/micro dual-scale textured GaAs. The reflectance was adjusted by controlling the aspect ratio of the nanoholes by varying the MacEtch duration. The combination of the microstructure and subwavelength structures significantly reduced the solar-weighted reflectance of a bare GaAs substrate by 72%.
ABSTRACT
We demonstrated time-dependent mass transport mechanisms of Au-assisted chemical etching of Si substrates. Variations in the etch rate and surface topology were correlated with catalyst features and etching duration. Nonlinear etching characteristics were associated with the formation of pinholes and whiskers. Variable rates of mass transport as a function of whisker density accounted for the nonlinear etch rates of Si. Nanopinholes on Au catalysts facilitated the vertical mass transport of reactants and byproducts, which dramatically changed the etch rate, surface topology, and porosity of Si. The suggested transport models describe the transient mass transport and the corresponding chemical reactions.
