Taek Kim

A role for graphene in silicon-based semiconductor devices

As silicon-based electronics approach the limit of improvements to performance and capacity through dimensional scaling, attention in the semiconductor field has turned to graphene, a single layer of carbon atoms arranged in a honeycomb lattice. Its high mobility of charge carriers (electrons and holes) could lead to its use in the next generation of high-performance devices. Graphene is unlikely to replace silicon completely, however, because of the poor on/off current ratio resulting from its zero bandgap. But it could be used to improve silicon-based devices, in particular in high-speed electronics and optical modulators.

7W high-efficiency continuous-wave green light generation by intracavity frequency doubling of an end-pumped vertical external-cavity surface emitting semiconductor laser

The authors report the demonstration of highly efficient continuous-wave green light at 535nm generation by intracavity frequency doubling of an end-pumped vertical external-cavity surface emitting semiconductor laser (VECSEL). The beta-barium borate (β-BaB2O4) crystals, cut for critical type I phase matching at room temperature, are used for second harmonic generation of the laser. In combination with an end-pumped VECSEL and efficient intracavity frequency doubling with a folded three-mirror cavity, more than 7W green light output power is obtained at pump power of 26W. The optical conversion efficiency of 27% is achieved from pump power to second harmonic power.

Electrically Driven Quantum Dot/Wire/Well Hybrid Light‐Emitting Diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.

Three-dimensional ZnO hybrid nanostructures for oxygen sensing application

Three-dimensional (3D) ZnO hybrid nanostructures, composed of a bottom ZnO film, ZnO nanowire arrays, and a top ZnO film, are continuously fabricated by adjusting the supersaturation conditions, using metal–organic chemical vapour deposition, to utilize the vertically aligned ZnO nanowires as an active element in oxygen sensing applications. The oxygen sensing characteristics of the different 3D hybrid nanostructures, fabricated to have different average values of radius and length of the ZnO nanowire arrays, are studied comparatively without complicated nanowire manipulation processes. The decrease of the current flow through the ZnO nanowire arrays with increasing ambient oxygen concentration indicates that the 3D hybrid nanostructures can be applied in oxygen sensors. In addition, the 3D hybrid nanostructure having thinner radius of the nanowire arrays exhibits higher oxygen sensitivity.

Cr/Ni/Au ohmic contacts to the moderately doped p- and n-GaN

We report new Cr/Ni/Au and Ni/Cr/Au tri-layer metallization schemes for achieving low resistance ohmic contacts to moderately doped p- (∼1 × 10 17 /cm 3 ), and n-GaN (∼1 × 10 18 /cm 3 ) respectively. The metallizations were thermally evaporated on 2 μm-thick GaN layers grown on c-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD). Comparisons with bi-layer metallizations such as Ni/Au and Cr/Au were also made. The Cr/Ni/Au contacts showed a low specific contact resistivity of 9.1 × 10 −5 Ω⋅cm 2 to n-GaN while that of Ni/Cr/Au to p-GaN was 8.3 × 10 −2 Ω⋅cm 2 . The Ni/Cr/Au contacts also showed a low specific contact resistivity of 2.6 × 10 −4 Ω⋅cm 2 to n-GaN. The Ni/Cr/Au metallization could made reasonable ohmic contacts to p- and n-GaN simultaneously

Highly efficient yellow photoluminescence from {11–22} InGaN multiquantum-well grown on nanoscale pyramid structure

InGaN/GaN multiquantum wells (MQWs) with a peak wavelength of 570 nm are grown on nanosize GaN hexagonal pyramid structures. Temperature dependent photoluminescence (PL) measurements from 10 to 300 K show a high integrated intensity ratio of 0.45. The emission energy of the MQW monotonically decreases with temperature increase, showing the absence of localized potential. Power dependent PL shows no noticeable blueshift caused by piezoelectric field screening effect. Comparative study of the PL results with those of the InGaN MQW on microsize pyramid show that nanosize pyramids play an important role in suppressing piezoelectric field in addition to the semipolar growth direction. We attribute the high luminescence efficiency of the MQW on nanosize pyramid structures to effectively suppressed piezoelectric field and potential localization.

End-pumped green and blue vertical external cavity surface emitting laser devices

The authors report on the development and demonstration of the high power operation of optically end-pumped vertical external cavity surface emitting laser devices emitting at 532 and 460nm. 2.7W green and 1.4W blue output powers were achieved with a good beam quality by intracavity frequency doubling with second harmonic generation crystal. High efficiency and good beam quality are attributed to the enhanced thermal management by the diamond heat spreader directly bonded to the gain region and effective optical pumping by placing the pump laser diode behind the laser structure.

Efficient blue lasers based on gain structure optimizing of vertical-external-cavity surface-emitting laser with second harmonic generation

We report on the demonstration of highly efficient blue lasers based on intracavity frequency doubling vertical-external-cavity surface-emitting lasers (VECSELs). By optimizing the number of InGaAs quantum wells and employing Al0.3Ga0.7As carrier blocking layers in resonant periodic gain structures, we observed the pump-power-limited output power of 4.5W at 920nm for an InGaAs∕GaAs quantum well VECSEL. With a frequency doubling LiB3O5 crystal inside the cavity, 1.9W continuous-wave 460nm blue output was demonstrated. Power conversion efficiencies (=output power∕pump input power) of 22.5% and 9.5% are realized for λ∼920nm and λ∼460nm, respectively.

920-nm Vertical-External-Cavity Surface-Emitting Lasers With a Slope Efficiency of 58% at Room Temperature

We report a high-power fundamental transverse mode operation of an optically pumped 920-nm vertical-external- cavity surface-emitting laser. The effects of the gain related structural parameters on the laser performance are investigated. Based on the optimization of the gain structure and the thermal management, a maximum pump-limited output power of 12 W in a TEM00 mode with a slope efficiency of 58% (input power of 24 W) was achieved at room temperature.

Fin-channel-array transistor (FCAT) featuring sub-70nm low power and high performance DRAM

For the first time, a highly manufacturable fin-channel array transistor (FCAT) on a bulk Si substrate has been successfully integrated in a 512 M density DRAM with sub-70nm technology. The FCAT shows an excellent short channel behavior, such as extremely low subthreshold swing (SS) (/spl sim/75mV/dec) and DIBL (/spl sim/13mV/V), and a high cell transistor drive current with remarkably low subthreshold leakage current (/spl sim/0.2fA/cell).