Dae-Seon Kim

Surface relief structures for a flexible broadband terahertz absorber

Terahertz (THz) absorbers with surface relief structures (SRSs) were designed and fabricated on a flexible polydimethylsiloxane(PDMS) substrate by using a stamping method. The silicon mold used for the stamping process was prepared by using a crystallographic wet etching method with 45% KOH solution at 80°C. The flexible THz absorber, consisting of micropyramids with a base width of 240 μm, demonstrated nearly perfect absorbance higher than 99% owing to the dramatically reduced surface reflectance of the SRS. The reflectance of the PDMS with the SRS was less than 1%, which is only 1/100th of that measured from a bare PDMS at frequency higher than 1 THz.

Broadband terahertz absorber realized by self-assembled multilayer glass spheres

A broadband terahertz (THz) absorber consisting of multilayer glass spheres and polydimethylsiloxane (PDMS) was realized. The multilayer glass spheres were deposited by repeating a self-assembly method used to form monolayer glass spheres and by the spin-coating of PDMS to fill the gaps between the glass spheres. The average reflection at the surface of the absorber was 0.8% and the absorbance was higher than 98% in the frequency range between 0.7 to 2.0 THz.

Fabrication of cone-shaped subwavelength structures by utilizing a confined convective self-assembly technique and inductively coupled-plasma reactive-ion etching

Cone-shaped subwavelength structures (SWSs) were fabricated on a GaAs substrate by utilizing a confined convective self-assembly process followed by inductively coupled-plasma reactive-ion etching. A self-assembled polystyrene monolayer was used as an etch mask for pattern transfer onto the GaAs substrate. The fabricated SWS, having a cone profile with an aspect ratio of 1.5 and a 300 nm pitch, exhibited very low reflectance throughout the solar spectrum range and exhibited wide tolerance to different optical incidence angles. Reflectance of the cone-shaped SWS on the GaAs surface was less than 4% in a spectral range of 300–1000 nm under a normal incidence condition.

Simple fabrication of an antireflective hemispherical surface structure using a self-assembly method for the terahertz frequency range.

A hemispherical surface structure was fabricated on a sapphire substrate by utilizing a self-assembly and spin-coating process for a terahertz (THz) antireflection coating. The self-assembled glass spheres and spin-coated material led to a gradual change in the effective refractive index. The aspect ratio of the hemispherical surface structure was controlled easily by adjusting the thickness of the B-staged bisbenzocyclobutene used as a coating. The reflectance of the fabricated hemispherical surface structure, having a period of 140 μm, exhibited low reflectance and low Fabry-Perot resonance in a THz spectral range from 0.1 to 1.9 THz.

A nanoscale conical polymethyl methacrylate (PMMA) sub-wavelength structure with a high aspect ratio realized by a stamping method

A high aspect ratio conical sub-wavelength structure (SWS) was designed by using rigorous coupled-wave analysis (RCWA) method and was realized on polymethyl methacrylate (PMMA) film using a stamping technique. The silicon template containing a hexagonal array of conical holes with a period of 350 nm and an aspect ratio of 2.8 was fabricated by electron-beam (e-beam) lithography followed by a two-step etching process. The SWS with a high aspect ratio was easily transferred from the fabricated silicon template to PMMA film using the stamping method. The replicated PMMA SWS has an array of cones with nanoscale tips and an aspect ratio higher than 2.8. The average reflectance and transmittance of the PMMA film with the conical SWS in the wavelength ranging from 500 and 1500 nm was improved from 7.1 and 91.1% to 4.3 and 94.2%, respectively, as compared to flat PMMA film.

Direct integration of subwavelength structure on a GaAs solar cell by using colloidal lithography and dry etching process

An antireflective subwavelength structure (SWS) was realized on the InGaP layer of a GaAs solar cell by using colloidal lithography followed by dry etching process. The fabricated SWS with an aspect ratio of 1.33 and a period of 300 nm showed enhanced optical properties and device characteristics. The average reflectance of the SWS surface of the GaAs solar cell was 7.1% in the wavelength range between 300 and 1000 nm. More solar energy was absorbed by the GaAs solar cell due to the lowered surface reflection by the SWS. It is the dominant factor in the 28.2% improvement of the power-conversion efficiency (η) of the SWS-integrated GaAs solar cell as compared to that of the GaAs solar cells without SWS.

Fluxless eutectic bonding of GaAs-on-Si by using Ag/Sn solder

Fluxless GaAs-on-Si wafer bonding using Ag/Sn solder was investigated to realize uniform and void-free heterogeneous material integration. The effects of the diffusion barrier, Ag/Sn thickness, and Ar plasma treatment were studied to achieve the optimal fluxless bonding process. Pt on a GaAs wafer and Mo on a Si wafer act as diffusion barriers by preventing the flow of Ag/Sn solder into both the wafers. The bonding strength is closely related to the Ag/Sn thickness and Ar plasma treatment. A shear strength test was carried out to investigate the bonding strength. Under identical bonding conditions, the Ag/Sn thickness was optimized to achieve higher bonding strength and to avoid the formation of voids due to thermal stress. An Ar plasma pretreatment process improved the bonding strength because the Ar plasma removed carbon contaminants and metal-oxide bonds from the metal surface.

Terahertz filter integrated with a subwavelength structured antireflection coating

Micro-pyramid shaped subwavelength structures (SWSs) were integrated on both sides of a terahertz (THz) filter by means of stamping methods. Two silicon-based stamping molds fabricated via crystallographic wet etching were utilized to replicate SWSs onto cyclo-olefin copolymer (COC) films coated onto both sides of a THz filter at the same time. The SWSs act as an broadband antireflection coating to reduce the surface reflection loss in a frequency range of 0.2 THz to 1.4 THz. Compared to a THz filter without SWSs, the filter integrated with double-sided SWSs exhibits a low standing wave ratio inside the substrate and THz signal transmission enhancement of up to 10.8%.

Floating-base InGaP/GaAs heterojunction phototransistors with low doped extrincsic base

Floating-base InGaP/GaAs heterojunction phototransistors (HPTs) with low doped extrinsic base region are demonstrated. Electrical and optical characteristics of the fabricated HPTs with and without SiNx passivation are investigated. Moreover, the optical gain of these HPTs is compared in terms of variation of the emitter size. The SiNx passivated HPTs with 50×50 μm2 of emitter size showed the superior high optical gain. The devices exhibited very high optical gain of 159 at optical power of 100 nW under 635 nm illumination.