Bugeun Ki

Lateral photovoltaic effect in flexible free-standing reduced graphene oxide film for self-powered position-sensitive detection.

Lightweight, simple and flexible self-powered photodetectors are urgently required for the development and application of advanced optical systems for the future of wearable electronic technology. Here, using a low-temperature reduction process, we report a chemical approach for producing freestanding monolithic reduced graphene oxide papers with different gradients of the carbon/oxygen concentration ratio. We also demonstrate a novel type of freestanding monolithic reduced graphene oxide self-powered photodetector based on a symmetrical metal–semiconductor–metal structure. Upon illumination by a 633-nm continuous wave laser, the lateral photovoltage is observed to vary linfearly with the laser position between two electrodes on the reduced graphene oxide surface. This result may suggest that the lateral photovoltaic effect in the reduced graphene oxide film originates from the built-in electric field by the combination of both the photothermal electric effect and the gradient of the oxygen-to-carbon composition. These results represent substantial progress toward novel, chemically synthesized graphene-based photosensors and suggest one-step integration of graphene-based optoelectronics in the future.

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.

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.

Phosphorus implantation into in situ doped Ge-on-Si for high light-emitting efficiency

We investigated the optical, electrical, and structural properties of epitaxially grown Ge-on-Si substrates after phosphorous implantation. Ion implantation increases n-type doping in Ge for an on-chip light source. However, its effects on Ge should be carefully studied as implantation may increase the recombination sites, and possibly reduce light-emitting efficiency. We studied the light-emitting efficiency of implanted Ge using various material characterizations. We found that phosphorous implantation increased the doping concentration of in situ doped Ge-on-Si, which boosted the photoluminescence by 12–30%. It is therefore critical to optimize the post-annealing and implantation doses to increase light-emitting efficiency of Ge.

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.

Controlling optical properties of Ge-on-Si by thermal annealing and etching process

We studied optical properties of thermally annealed Ge-on-Si. From Raman experiments, tensile strain as well as Si-Ge intermixing were investigated. Significant Γ-band transition peak-shift was confirmed by photoluminescence depending on the thermal annealing conditions.

Raman analysis of in-plane biaxial strain for Ge-on-Si lasers

Tensile strain of Ge-on-Si with post-growth annealing was analyzed using micro-Raman for optical sources in interconnection system. Tensile Stain in epi-Ge distributed non-linearly with SiGe alloy formation at the interface after annealing.