Yunwon Song

In-plane and out-of-plane mass transport during metal-assisted chemical etching of GaAs

We have demonstrated the dependence of the metal-assisted chemical etching of GaAs on catalyst thickness. For ultra-thin (3–10 nm) Au catalysts, we found that the etch rate was significantly enhanced, an unexpected phenomenon in light of the conventional mechanism. Numerous pinholes in the metal catalyst are postulated to enable out-of-plane mass transport of reactants and products across the catalyst-covered GaAs. When this process is dominant, the GaAs etch rate is facilitated and an anisotropic profile is formed. With thicker (>15 nm) Au catalysts, the conventionally known in-plane mass transport becomes dominant and lowers the etch rate with an isotropic profile. To our knowledge, this is the first report that experimentally verifies the vertical mass transport during metal-assisted chemical etching of semiconductors. Metal-assisted chemical etching of GaAs with controlled metal catalyst thickness suggests that this technique is more attractive and useful for a wide range of practical applications.

Thermally driven metal-assisted chemical etching of GaAs with in-position and out-of-position catalyst

We have identified the characteristic stages of metal-assisted chemical etching of GaAs. Distinct changes in the surface topologies were classified into etching at incubation, out-of-position, and in-position of the metal catalyst. Thermally activated chemical etching and mass transport influence the catalytic reactions of electronic holes, oxidation of GaAs, and subsequent removal of porous GaAs. At low temperatures with a slow etch rate, only porous regions encircling the metal catalyst were etched on the GaAs. At mid temperatures, chemical etch occurred substantially out-of-position of the Au catalyst, forming arrays of craters on GaAs. At high temperatures, etching occurred in-position of the Au catalyst with an array of bumps on the GaAs. After a prolonged etch, high aspect ratio pillars were fabricated with a high vertical etch rate and the pillar diameters were shrunk to nano-scale with a controlled lateral etch. Three distinct stages that consecutively evolved during metal-assisted chemical etching of GaAs were determined to be caused by thermally driven electronic holes and chemical reactions.

Catalyst feature independent metal-assisted chemical etching of silicon

We demonstrate metal-assisted chemical etching of Si substrates with consistent etching rates for a wide range of metal catalysts of dots and stripes in meshes and solid arrays. The governing mechanism switched from in-plane to out-of-plane mass transport with metal catalysts, which resulted in highly anisotropic chemical etching in-position of micron-scale metal catalyst. Dramatic changes in etch rates and surface topologies were interpreted as resulting from diffusivity of the reactants and byproducts through the nanoholes in the metal catalyst. Experimentally verified out-of-plane mass transport extends the capability of metal-assisted chemical etching to the fabrication of nano- and micron-scale three-dimensional semiconductors.

Nonlinear Etch Rate of Au-Assisted Chemical Etching of Silicon

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.

Localized TiSi and TiN phases in Si/Ti/Al/Cu Ohmic contacts to AlGaN/GaN heterostructures

Microstructural changes in Si/Ti/Al/Cu (10/40/60/50 nm) Ohmic contacts to AlGaN/GaN heterostructure were investigated for complementary metal-oxide semiconductor compatible processes. Si/Ti/Al/Cu metallization exhibited a low specific contact resistance of 3.6 × 10−6 Ω-cm2 and contact resistance of 0.46 Ω-mm when a Si interfacial layer was used. Without a designated barrier metal, TiSix alloys that formed in the metallic region effectively suppressed Cu diffusion. The shallow TiN junction in AlGaN/GaN was attributed to TiSix in the metallic regions. Microstructural changes were detected by systematic physical characterization.

Nano/micro dual-textured antireflective subwavelength structures in anisotropically etched GaAs

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%.

Chemical Imprinting of Crystalline Silicon with Catalytic Metal Stamp in Etch Bath

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.