Jae Cheol Shin

Multidimensional Conduction-Band Engineering for Maximizing the Continuous-Wave (CW) Wallplug Efficiencies of Mid-Infrared Quantum Cascade Lasers

By tailoring the active-region quantum wells and barriers of 4.5-5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature <i>T</i>₀ (253 K) and the slope-efficiency characteristic temperature <i>T</i>₁ (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ_4g decreases, resulting in basically the same room-temperature (RT) threshold-current density <i>J</i>_th as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the τ_4g value while further suppressing carrier leakage. As a result, experimental RT <i>J</i>_th values from moderate-taper TA 4.8-μm emitting QCLs are ~14% less than for conventional QCLs and <i>T</i>₁ reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the τ_4g value, due to Stark-effect reduction and strong asymmetry. Then, the RT <i>J</i>_th value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.

High internal differential efficiency mid-infrared quantum cascade lasers

Implementation of the step-taper active-region (STA) design to 8-9 μm-emitting quantum cascade lasers (QCLs) has resulted in both high T0 and T1 values: 220 K and 665 K, and short lower-level lifetimes: 0.12 ps. In turn, the internal differential efficiency ηid, which is the product of the injection efficiency and the differential laser-transition efficiency, reaches values as high as 86 % for both 8.4 μm- and 8.8 μm-emitting QCLs. Such ηid values are 30-50% higher than those obtained from conventional QCLs emitting in the 7-11 μm wavelength range. Achieving both carrier-leakage suppression and miniband-like carrier extraction in mid-infrared (IR) QCLs leads to ηid values close to the fundamental limit of ~ 90 %. In turn, the currently employed fundamental wallplug-efficiency limits over the mid-IR wavelength range have to be increased by ~ 34 % (e.g., the wallplug-efficiency limit at λ= 4.6 μm increases from 29 % to 39 %). Preliminary results from STA-type 4.8-5.0 μm-emitting QCLs include 1.5 W CW operation, and 77 % internal differential efficiency; that is, 30-50% higher than the ηid values obtained from conventional 4.0-6.5μm-emitting QCLs.

Structural defects in a nanomesh of bulk MoS2 using an anodic aluminum oxide template for photoluminescence efficiency enhancement

Two-dimensional (2D) materials beyond graphene have attracted considerable interest because of the zero bandgap drawbacks of graphene. Transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, are the potential candidates for next 2D materials because atomically thin layers of TMDs exhibit unique and versatile electrical and optical properties. Although bulk TMDs materials have an indirect bandgap, an indirect-to-direct bandgap transition is observed in monolayers of TMDs (MoS2, WSe2, and MoSe2). Optical properties of TMD films can be improved by the introduction of structural defects. For example, large-area spatial tuning of the optical transition of bulk MoS2 films is achieved by using an anodic aluminum oxide (AAO) template to induce structural defects such as edge- and terrace-terminated defects in a nanomesh structure. Strong photoluminescence emission peaks with a band gap of 1.81 eV are observed, possibly because of radiative transition at the defect sites. This work shows that the AAO template lithography method has potential for the production of homogenous large-scale nanomesh structures for practical semiconductor processing applications in future MoS2-based electronic and optical devices.

Photoresponse and Field Effect Transport Studies in InAsP–InP Core–Shell Nanowires

A ternary InAsyP1−y alloy is suitable for an application to near-infrared (NIR) optical devices as their direct bandgap energy covers the entire NIR band. A nanowire (NW) system allows an epitaxial integration of InAsyP1−y alloy on any type of substrate since the lattice mismatch strain can be relieved through the NW sidewall. Nevertheless, the very large surface to volume ratio feature of the NWs leads to enormous surface states which are susceptible to surface recombination of free carriers. Here, ternary InAs0.75P0.25 NWs are grown with InP passivation layer (i.e., core–shell structure) to minimize the influence of the surface states, thus increasing their optical and electrical properties. A photoresponse study was achieved through the modeled band structure of the grown NWs. The model and experimental results suggest that 5-nm-thick InP shell efficiently passivates the surface states of the InAs0.75P0.25 NWs. The fabricated core–shell photodetectors and field-effect transistors exhibit improved photoresponse and transport properties compared to its counterpart core-only structure.

Biosensing characteristics of InAs nanowire transistors grown by MOCVD

We demonstrated the ion-sensitive field-effect transistors (IS-FETs) based on nanowires (NWs) with different diameters and doping concentrations to obtain the high sensitivity and various applications. The growth of the catalyst-free InAs NWs was carried out using a horizontal reactor MOCVD system (AIXTRON Inc.). A p-type Si (111) wafer (ρ = 1 -10 Ω-cm) was prepared for the NW growth. Here, NWs with diameters of around 50 ~ 150 nm were grown and the doping concentration also was changed around x±1016~18 /cm2. IS-FETs with the grown InAs NWs were fabricated using the photolithography and the lift-off process. The gas sensing characteristics have been investigated through studying the gate response of the NW conductance in different ambient conditions.

Metalorganic chemical vapor deposition-regrown Ga-rich InGaP films on SiGe virtual substrates for Si-based III-V optoelectronic device applications

Ga-rich InGaP materials are attractive applications for yellow-green spectral range optoelectronics such as light-emitting diodes and solar cells on silicon substrate. Bulk, Ga-rich InGaP films grown by metalorganic chemical vapor deposition on SiGe virtual substrates were investigated in the V/III compositional ratio range of 44.3–402 using chamber pressures from 100 to 200 mbar. These films were nominally lattice matched to the SiGe virtual substrate with a bandgap energy of 2.07–2.09 eV at low temperature (10 K). The authors show that the surface morphology of the Ga-rich InGaP films was dependent on the growth conditions, including the V/III gas phase ratio, pressure, and growth rate. By optimizing the growth conditions, the authors achieved improved surface morphologies of the Ga-rich InGaP films. The hillock density of the films produced using a V/III gas phase ratio of 44.3 and 75.4, a growth pressure of 100 mbar, and a growth rate of 0.9 μm/h was about an order of magnitude lower (30.3–50 × 104 cm...

Sensing characteristics of plasmonic structure based on transferring process of polystyrene nano-beads

We analyzed and demonstrated the double layered metallic nano-structures using polystyrene lift-off process on the conventional surface plasmon resonance (SPR) sensor to enhance the sensitivity of an SPR surface. The double layered plasmonic structures are optimized using the three-dimensional finite-difference time-domain method for the width, thickness, and period of the polystyrene beads. The thickness of the metal film and the metallic nano-hole is 20 and 20 nm in the 305 nm wide nano-hole size, respectively. The double layered metallic nano-structures are fabricated with monolayer polystyrene beads of chloromethyl latex 4% w/v 0.4 μm. The sensitivities of the conventional SPR sensor and the double layered plasmonic sensor are obtained to 42.2 and 60 degree/RIU, respectively. The SPR devices are also applied to the lead ion sensor. The resonance shifts of SPR sensors with and without a poly(vinyl chloride) membrane are 1328 RU and 788 RU from 10-5 M to 10-2 M concentration, respectively.

Asbestos concentration measurement using Differential Interference Contrast microscopy

We designed a new method for imaging and counting the concentration of asbestos fibers. In current research, we combined the principle of Differential Interference Contrast (DIC) microscopy with imaging program for counting their concentration automatically.