Sunghoon Lee

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

A transparent bending-insensitive pressure sensor

Measuring small normal pressures is essential to accurately evaluate external stimuli in curvilinear and dynamic surfaces such as natural tissues. Usually, sensitive and spatially accurate pressure sensors are achieved through conformal contact with the surface; however, this also makes them sensitive to mechanical deformation (bending). Indeed, when a soft object is pressed by another soft object, the normal pressure cannot be measured independently from the mechanical stress. Here, we show a pressure sensor that measures only the normal pressure, even under extreme bending conditions. To reduce the bending sensitivity, we use composite nanofibres of carbon nanotubes and graphene. Our simulations show that these fibres change their relative alignment to accommodate bending deformation, thus reducing the strain in individual fibres. Pressure sensitivity is maintained down to a bending radius of 80 μm. To test the suitability of our sensor for soft robotics and medical applications, we fabricated an integrated sensor matrix that is only 2 μm thick. We show real-time (response time of ∼20 ms), large-area, normal pressure monitoring under different, complex bending conditions.

1-nm-capacitance-equivalent-thickness HfO2/Al2O3/InGaAs metal-oxide-semiconductor structure with low interface trap density and low gate leakage current density

We have studied the impact of the Al2O3 inter-layer on interface properties of HfO2/InGaAs metal-oxide-semiconductor (MOS) interfaces. We have found that the insertion of the ultrathin Al2O3 inter-layer (2 cycle: 0.2 nm) can effectively improve the HfO2/InGaAs interface properties. The frequency dispersion and the stretch-out of C-V characteristics are improved, and the interface trap density (Dit) value is significantly decreased by the 2 cycle Al2O3 inter-layer. Finally, we have demonstrated the 1-nm-thick capacitance equivalent thickness in the HfO2/Al2O3/InGaAs MOS capacitors with good interface properties and low gate leakage of 2.4 × 10−2 A/cm2.

Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes

Thin-film electronic devices can be integrated with skin for health monitoring and/or for interfacing with machines. Minimal invasiveness is highly desirable when applying wearable electronics directly onto human skin. However, manufacturing such on-skin electronics on planar substrates results in limited gas permeability. Therefore, it is necessary to systematically investigate their long-term physiological and psychological effects. As a demonstration of substrate-free electronics, here we show the successful fabrication of inflammation-free, highly gas-permeable, ultrathin, lightweight and stretchable sensors that can be directly laminated onto human skin for long periods of time, realized with a conductive nanomesh structure. A one-week skin patch test revealed that the risk of inflammation caused by on-skin sensors can be significantly suppressed by using the nanomesh sensors. Furthermore, a wireless system that can detect touch, temperature and pressure is successfully demonstrated using a nanomesh with excellent mechanical durability. In addition, electromyogram recordings were successfully taken with minimal discomfort to the user.

Self-Aligned Metal Source/Drain InxGa1-xAs n-Metal–Oxide–Semiconductor Field-Effect Transistors Using Ni–InGaAs Alloy

We report that a Ni–InGaAs alloy can be used as a source/drain (S/D) metal for InGaAs metal–oxide–semiconductor field-effect transistors (MOSFETs), allowing us to employ the salicide-like self-align S/D formation. We also introduce Schottky barrier height (SBH) engineering process by increasing the indium content of InxGa1-xAs channels, which successfully reduces SBH down to zero. We propose a fabrication process for self-aligned metal S/D MOSFETs using Ni–InGaAs and demonstrate successful operation of the metal S/D InxGa1-xAs MOSFETs. The In0.7Ga0.3As MOSFETs exhibit an S/D resistance (RSD) that is 1/5 lower than that in P–N junction devices and a high peak mobility of 2000 cm2 V-1 s-1.

Impact of Fermi level pinning inside conduction band on electron mobility of In x Ga 1−x As MOSFETs and mobility enhancement by pinning modulation

We clarified that Fermi levels at InGaAs MOS interfaces are pinned inside conduction band (CB) and that this pinning severely degrades the effective mobility. Also, the energy position of the Fermi level pinning (FLP) is found to be tunable. It is experimentally shown that the increase in the difference between the FLP position and the CB minimum (CBM) leads to high mobility at high Ns region. Also, possible physical origin for this FLP is proposed.

High Performance Extremely Thin Body InGaAs-on-Insulator Metal?Oxide?Semiconductor Field-Effect Transistors on Si Substrates with Ni?InGaAs Metal Source/Drain

The extremely thin body (ETB) InGaAs-on-insulator (-OI) metal–oxide–semiconductor field-effect transistors (MOSFETs) on Si substrates were demonstrated by using Ni–InGaAs alloy metal source/drain (S/D). It has been found that a light doping concentration of ~1016 cm-3 and indium-rich InGaAs channels (In0.7Ga0.3As) provide a high mobility of 1700 cm2 V-1 s-1 even in the channel thickness of 10 nm. This is the first demonstration of ETB III–V-OI MOSFETs combined with the metal S/D technology. We have also achieved excellent ID–VG characteristics with an Ion/Ioff ratio of over 105 and low SS of 120 mV/dec in 5-nm-thick In0.7Ga0.3As-OI MOSFETs.

High‐Frequency, Conformable Organic Amplifiers

Large-bandwidth, low-operation-voltage, and uniform organic amplifiers are fabricated on ultrathin foils. By the integration of short-channel OTFTs and AlOx capacitors, organic amplifiers with a bandwidth of 25 kHz are realized, demonstrating the highest gain-bandwidth product (GBWP) reported to date. Owing to material and process advancements, closed-loop architectures operate at frequencies of several kilohertz with an area smaller than 30 mm(2) .

Reduction in interface state density of Al2O3/InGaAs metal-oxide-semiconductor interfaces by InGaAs surface nitridation

We report the decrease in interface trap density (Dit) in Al2O3/InGaAs metal-oxide-semiconductor (MOS) capacitors by using electron cyclotron resonance plasma nitridation of the InGaAs surfaces. The impact of the nitridation process on the MOS interface properties is quantitatively examined. The plasma nitridation process is observed to form a nitrided layer at the InGaAs surface. The nitridation using microwave power (Pmicrowave) of 250 W and nitridation time (tnitridation) of 420 s can form Al2O3/InGaAs MOS interfaces with a minimum Dit value of 2.0 × 1011 cm−2 eV−1. On the other hand, the nitridation process parameters such as Pmicrowave and tnitridation are found to strongly alter Dit (both decrease and increase are observed) and capacitance equivalent thickness (CET). It is found that the nitridation with higher Pmicrowave and shorter tnitridation can reduce Dit with less CET increase. Also, it is observed that as tnitridation increases, Dit decreases first and increases later. It is revealed from XP...

Electron Mobility Enhancement of Extremely Thin Body In0.7Ga0.3As-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistors on Si Substrates by Metal–Oxide–Semiconductor Interface Buffer Layers

The electron mobility enhancement of extremely thin body In0.7Ga0.3As-on-insulator (-OI) metal–oxide–semiconductor field-effect transistors (MOSFETs) on Si substrates by using In0.3Ga0.7As MOS interface buffer layers was demonstrated. The MOSFETs with the InGaAs thickness of 2/5/3 nm have exhibited the electron mobility of 2810 cm2 V-1 s-1 with an enhancement factor of 4.2 against that of Si MOSFET. We have examined the body thickness (Tbody) dependence of the electron mobility. It was found that a channel thickness fluctuation scattering mechanism strongly affects the mobility in Tbody of around 10 nm and thinner. The formation of a uniform and flat InGaAs-OI wafer is required for further improvements.