Sang Woo Pak

Defect states in hybrid solar cells consisting of Sb2S3 quantum dots and TiO2 nanoparticles

We have studied defect states in an organic-inorganic hybrid solar cell containing Sb2S3 quantum dots (QDs) and TiO2 nanoparticles (NPs) by using deep level transient spectroscopy (DLTS). An Au electrode was deposited as a Schottky contact on the sample, where the Sb2S3 QDs were distributed on the surface of TiO2 NPs by chemical synthesis. The activation energy and capture-cross section of an interface state between the Sb2S3 QDs and the TiO2 NPs were found to be about 0.78 eV and 2.21 × 10−9 cm−2, respectively. Also, the densities of this interface trap under a measurement voltage of −1 V were approximately 2.5 × 1017 cm−3. Based on these results, the interface trap was positioned around Ec − 1.03 eV below the conduction band edge of Sb2S3 QD. Thus, the external quantum efficiency of the solar cell was affected because of its role as a recombination center for carriers generated from Sb2S3 QDs.

Toward negligible charge loss in charge injection memories based on vertically integrated 2D heterostructures

Two-dimensional (2D) crystals have a multitude of forms, including semi-metals, semiconductors, and insulators, which are ideal for assembling isolated 2D atomic materials to create van der Waals (vdW) heterostructures. Recently, artificially-stacked materials have been considered promising candidates for nanoelectronic and optoelectronic applications. In this study, we report the vertical integration of layered structures for the fabrication of prototype non-volatile memory devices. A semiconducting-tungsten-disulfide-channel-based memory device is created by sandwiching high-density-of-states multi-layered graphene as a carrier-confining layer between tunnel barriers of hexagonal boron nitride (hBN) and silicon dioxide. The results reveal that a memory window of up to 20 V is opened, leading to a high current ratio (>103) between programming and erasing states. The proposed design combination produced layered materials that allow devices to attain perfect retention at 13% charge loss after 10 years, offering new possibilities for the integration of transparent, flexible electronic systems.

Band gap modulation of ZnTe1-xOx alloy film by control of oxygen gas flow rate during reactive magnetron sputtering

The band gap energy of ZnTe1-xOx alloy films grown on c-plane sapphire substrates was modulated by controlling the argon-oxygen ratio during radio frequency magnetron sputtering. The ZnTe1-xOx samples were deposited at a substrate temperature of 200 °C and with gas mixtures of 2%–8% oxygen in argon. The optical transparency of the ZnTe1-xOx samples was measured in the 1.5–6.0 eV energy range by optical transmission spectra. The optical band gap, obtained from plots of (αhν)2 as a function of hν, increased from 2.2 to 4.9 eV with increasing oxygen ratio, believed to be a result of a change in bonding structure through composition exchange during film deposition by reactive magnetron sputtering. These results show that the band gap energy of ZnTe1-xOx, ZnOTeO, and (ZnO)1-x(TeO2)x alloy films can be modulated, making them more suited for applications as windows and as active layers for ZnTe-based intermediate band solar cells.

Growth of ZnTe:O Thin Films by Oxygen-Plasma-Assisted Pulsed Laser Deposition

We studied oxygen incorporation into ZnTe thin films with oxygen ambient and oxygen plasma during pulsed laser deposition (PLD). The ZnTe layers deposited by oxygen-plasma-assisted PLD under oxygen partial pressures showed the enhancement of visible absorption due to TeOx formation by oxygen incorporation, which was confirmed by X-ray photoelectron spectroscopy measurement. The ZnTe:O thin films grown under oxygen ambient and plasma produced an energy band structure at about 0.5–0.8 eV below the ZnTe band edge, indicating strong radiative properties. The ZnTe:O samples showed the formation of intermediate bands and p-type semiconducting characteristics, which will be useful for intermediate/defect band solar cells.

Electrical and Magnetic Properties of Tunneling Device with FePt Magnetic Quantum Dots

We have studied the electrical and magnetic transport properties of tunneling device with FePt magnetic quantum dots. The FePt nanoparticles with a diameter of 8∼15 ㎚ were embedded in a SiO₂ layer through thermal annealing process at temperature of 800℃ in N₂ gas ambient. The electrical properties of the tunneling device were characterized by current-voltage (Ⅰ-Ⅴ) measurements under the perpendicular magnetic fields at various temperatures. The nonlinear Ⅰ-Ⅴ curves appeared at 20 K, and then it was explained as a conductance blockade by the electron hopping model and tunneling effect through the quantum dots. It was measured also that the negative magneto-resistance ratio increased about 26.2% as increasing external magnetic field up to 9,000 G without regard for an applied electric voltage.

Structural and Magnetic Properties of Fe-Diluted Si Alloy Films by Pulsed-Laser Deposition

Fe-diluted Si alloys grown on p-type Si (100) substrates by pulsed-laser deposition method were studied for tructural, electrical, and magnetic properties. The X-ray diffraction patterns for these alloy samples showed a few of peaks with cubic structures such as FeSi, Fe3Si, and Fe4Si. The Fe-composition in alloys are confirmed as Fe atomic percent about 1.25∼6.49 % from energy dispersive spectroscopy measurement. The resistivity as a function of the reciprocal temperature was indicated an exponential increase with two activation energies of 5.21 and 7.79 meV. The maximum value of the magnetization at 10 K was about 100 emu/cc, and the ferromagnetism was also observed until 350 K from total magnetization as a function of temperature with applied magnetic field of 3,000 Oe.

Locally Gated SnS 2 /hBN Thin Film Transistors with a Broadband Photoresponse

Next-generation flexible and transparent electronics demand newer materials with superior characteristics. Tin dichalcogenides, Sn(S,Se)2, are layered crystal materials that show promise for implementation in flexible electronics and optoelectronics. They have band gap energies that are dependent on their atomic layer number and selenium content. A variety of studies has focused in particular on tin disulfide (SnS2) channel transistors with conventional silicon substrates. However, the effort of interchanging the gate dielectric by utilizing high-quality hexagonal boron nitride (hBN) still remains. In this work, the hBN coupled SnS2 thin film transistors are demonstrated with bottom-gated device configuration. The electrical transport characteristics of the SnS2 channel transistor present a high current on/off ratio, reaching as high as 105 and a ten-fold enhancement in subthreshold swing compared to a high-κ dielectric covered device. We also demonstrate the spectral photoresponsivity from ultraviolet to infrared in a multi-layered SnS2 phototransistor. The device architecture is suitable to promote diverse studied on flexible and transparent thin film transistors for further applications.

Enhancement of near-infrared detectability from InGaZnO thin film transistor with MoS2 light absorbing layer

We report an enhancement of near-infrared (NIR) detectability from amorphous InGaZnO (α-IGZO) thin film transistor in conjunction with randomly distributed molybdenum disulfide (MoS2) flakes. The electrical characteristics of the α-IGZO grown by radio-frequency magnetron sputtering exhibit high effective mobility exceeding 15 cm2 V-1 s-1 and current on/off ratio up to 107. By taking advantages of the high quality α-IGZO and MoS2 light absorbing layer, photodetection spectra are able to extend from ultra-violet to NIR range. The α-IGZO channel detector capped by MoS2 show a photo-responsivity of approximately 14.9 mA W-1 at 1100 nm wavelength, which is five times higher than of the α-IGZO device without MoS2 layer.

High photoresponsivity from multilayer MoS2/Si heterojunction diodes formed by vertically stacking

We investigated a vertically stacked p+-n heterojunction diode consisting of a two-dimensional (2D) molybdenum disulfide (MoS2) crystal and a heavily doped p+-type Si substrate. The MoS2 flakes are transferred onto p+-Si substrates by using a scotch tape-based exfoliation method. The performances of n-MoS2/p+-Si diodes are investigated by I-V measurement under light illumination using light emitting diodes with various wavelengths. It appears that multilayer MoS2 has sufficient thickness to absorb incident light from the visible to near-infrared range with a high sensitivity. With the advantages of a simple device structure as well as improved contact quality between the MoS2 and silicon interface, an ideality factor of 1.09 can be achieved. The diodes reveal an ultra-high photoresponsivity of about 980 A/W at a wavelength of 525 nm with a strong dependence on the light wavelength and intensity, while they show a high specific detectivity on the order of 109 cm·Hz1/2/W from the visible to near infrared sp...