Youngwoon Yoon

Microstructural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices

SnO2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Transmission electron microscopy (TEM) and electron diffraction pattern measurements showed that these SnO2 thin films were nanocrystalline. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.62×1016 cm−3, respectively. Raman scattering measurements showed that the grain sizes of the nanocrystalline films were below 10 nm, which was in reasonable agreement with the result obtained from the high-resolution TEM measurements. Photoluminescence measurements showed a broad peak below the band-to-band emission. These results can help improve the understanding of SnO2 nanocrystalline films grown on p-InP (100) substrates for applications in high-sensitivity gas sensors.

A Highly Planar Fluorinated Benzothiadiazole‐Based Conjugated Polymer for High‐Performance Organic Thin‐Film Transistors

High-mobility and low-voltage-operated organic field-effect transistors (OFETs) are demonstrated by the design of a new fluorinated benzothiadiazole-based conjugated polymer with fluorinated high-k polymer dielectrics. A record-breaking high hole mobility of 9.0 cm(2) V(-1) s(-1) for benzothiadiazole-based semiconducting polymers is achieved by the excellent planarity of the semiconducting polymer.

Structural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on p-InSb (111) substrates

SnO2 thin films were grown on p-InSb (111) substrates by radio-frequency magnetron sputtering at low temperature. Atomic force microscopy images showed that the root mean square of the average surface roughness of the SnO2 films grown on the InSb (111) substrates with an Ar/O2 flow rate of 0.667 and at a temperature of 200 °C had a minimum value of 2.71 nm, and x-ray diffraction and transmission electron microscopy (TEM) measurements showed that these SnO2 thin films were polycrystalline. Auger electron spectroscopy and bright-field TEM measurements showed that the SnO2/p-InSb(111) heterointerface was relatively abrupt. High-resolution TEM measurements revealed that the SnO2 films were nanocrystalline and that the grain sizes of the nanocystalline films were below 6.8 nm. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.67×1016 cm−3, respectively, and the current–voltage curve indi...

H2 gas-sensing characteristics of SnOx sensors fabricated by a reactive ion-assisted deposition with/without an activator layer

Abstract SnOx-based gas-sensor devices have been fabricated by a reactive ion-assisted deposition technique. SnOx films of 400 A thickness are deposited on sputtered amorphous SiO2 substrates and then Pt films (600 A) are sputtered on top of them as upper electrodes. From XRD and TEM studies, as-deposited SnOx films are seen to be amorphous and become polycrystalline after annealing at 500°C in air. In quantitative XPS analyses, the O/Sn atomic ratio in the annealed films shows the nearly stoichiometric value of two and the binding energy of Sn 3d5/2 is 486.43 eV, very close to 486.75 eV of the standard tin oxide powder. When sensitivity is defined as a relative decrement in resistance, these sensor devices show nearly 100% sensitivities to 1000–5000 ppm H2 in air at 200°C and above. By attaching an ultra-thin metal activator, the response time can be reduced to less than one tenth of its previous value.

Optical and electrical properties of titanium dioxide films with a high magnitude dielectric constant grown on p‐Si by metalorganic chemical vapor deposition at low temperature

Metalorganic chemical vapor deposition of titanium dioxide (TiO2) on p‐Si(100) using titanium isopropoxide and nitrous oxide via pyrolysis at relatively low (∼500 °C) temperature was performed to produce high quality TiO2/p‐Si interfaces and to fabricate TiO2 insulator gates with a dielectric constant of high magnitude. Scanning electron microscopy shows that the surfaces of the TiO2 films have very smooth morphologies. From the x‐ray diffraction analysis, the grown layer was found to be a polycrystalline film. Raman spectroscopy showed the optical phonon modes of a TiO2 thin film. The stoichiometry of the TiO2 film was investigated by Auger electron spectroscopy. Room‐temperature current‐voltage and capacitance‐voltage measurements clearly revealed metal‐insulator‐semiconductor behavior for the samples of the Ag/TiO2/p‐Si. The interface state density at the TiO2/p‐Si interface was approximately high 1011 eV−1 cm−2 at the middle of the Si energy gap, and the dielectric constant determined from the capacit...

Organic conjugated material-based broadband terahertz wave modulators

A simple and efficient broadband terahertz (THz) wave modulator based on an organic conjugated material thin film, 200-nm thick organic copper phthalocyanine (CuPc) film, deposited on a Si wafer was proposed. External laser beams significantly decrease the transmittance of THz pulses through the CuPc film over all frequency of the pulse. Modulation efficiency reaches as high as 55% under a cw-laser beam of 80 mW.

Nanoscopic Management of Molecular Packing and Orientation of Small Molecules by a Combination of Linear and Branched Alkyl Side Chains

We synthesized a series of acceptor-donor-acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear n-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22° for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π-π stacking of the molecular backbone, whereas n-octyl groups on the SI core hinder close π-π stacking to some degree. Conversely, the longer linear n-octyl groups on the DPP arms facilitate close intermolecular packing via octyl-octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.

Structural properties and interfacial layer formation mechanisms of PbTiO3 thin films grown on p‐Si substrates

Ferroelectric PbTiO3 thin films were grown on Si(100) substrates by metalorganic chemical vapor deposition via thermal pyrolysis at relatively low temperature (∼500 °C) using Pb(tmhd)2, Ti(OC3H7)4, and N2O. Transmission electron microscopy results suggested that the grown PbTiO3 films were polycrystalline layers. Auger depth profiles indicated that the compositions of the as‐grown films consisted of lead, titanium, and oxygen uniformly distributed throughout the films and that the films exhibited smooth interfaces. These results indicate that the growth of polycrystalline PbTiO3 layers instead of epitaxial films originated from the formation of an interfacial amorphous layer prior to the creation of the films. Further, a mechanism for the formation of an interfacial layer between the PbTiO3 thin films and the p‐Si substrates is presented.

Growth of γ-Al2O3 thin films on silicon by low pressure metal-organic chemical vapour deposition

Abstract Metal-organic chemical vapour deposition of Al2O3 using aluminium isopropoxide (Al(OC3H7)3) and nitrous oxide (N2O) via thermal pyrolysis was investigated with the goal of producing high quality Al2O3p-Si(100) interfaces. From the X-ray diffraction analysis, the film was found to be a γ-Al2O3 heteroepitaxial film. The stoichiometry of the grown Al2O3 film was similar to that of sapphire observed from Auger electron spectroscopy. Room temperature capacitance-voltage measurements clearly reveal metal-insulator-semiconductor behaviour for samples with the Al2O3 insulator gate, and the interface state densities at the Al2O3p-Si heterointerface were approximately 1011 eV−1 cm−2, at levels centred in the silicon energy gap.

High Crystalline Dithienosilole-Cored Small Molecule Semiconductor for Ambipolar Transistor and Nonvolatile Memory

We characterized the electrical properties of a field-effect transistor (FET) and a nonvolatile memory device based on a solution-processable low bandgap small molecule, Si1TDPP-EE-C6. The small molecule consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The as-spun Si1TDPP-EE-C6 FET device exhibited ambipolar transport properties with a hole mobility of 7.3×10(-5) cm2/(Vs) and an electron mobility of 1.6×10(-5) cm2/(Vs). Thermal annealing at 110 °C led to a significant increase in carrier mobility, with hole and electron mobilities of 3.7×10(-3) and 5.1×10(-4) cm2/(Vs), respectively. This improvement is strongly correlated with the increased film crystallinity and reduced π-π intermolecular stacking distance upon thermal annealing, revealed by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) measurements. In addition, nonvolatile memory devices based on Si1TDPP-EE-C6 were successfully fabricated by incorporating Au nanoparticles (AuNPs) as charge trapping sites at the interface between the silicon oxide (SiO2) and cross-linked poly(4-vinylphenol) (cPVP) dielectrics. The device exhibited reliable nonvolatile memory characteristics, including a wide memory window of 98 V, a high on/off-current ratio of 1×10(3), and good electrical reliability. Overall, we demonstrate that donor-acceptor-type small molecules are a potentially important class of materials for ambipolar FETs and nonvolatile memory applications.