Chanhyoung Park

Bio: Chanhyoung Park is an academic researcher from Yonsei University. The author has contributed to research in topics: Spin coating & Non-volatile memory. The author has an hindex of 5, co-authored 8 publications receiving 123 citations.

Pentacene-based photodiode with Schottky junction

Abstract: We have fabricated a metal/organic semiconductor Schottky photodiode based on Al/pentacene junction. Since the energy band gap of thin solid pentacene was determined to be 1.82 eV, as characterized by direct absorption spectroscopy, we measured spectral photoresponses on our Schottky photodiode in the monochromatic light illumination range of 325–650 nm applying a reverse bias of −2 V. The main features of photo-response spectra were found to shift from those of direct absorption spectra toward higher photon energies. It is because the direct absorption spectra mainly show exciton level peaks rather than the true highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps while the photo-response spectra clearly represents the true HOMO–LUMO gap. Our photo-response spectra reveal 1.97 eV as the HOMO–LUMO gap.

Enhancing the retention properties of ZnO memory transistor by modifying the channel/ferroelectric polymer interface

Abstract: We report on the fabrication of ZnO nonvolatile memory thin-film transistors (NVM-TFTs) with thin poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] ferroelectric layer. Our NVM-TFT operates on glass substrates under low voltage write-erase (WR-ER) pulse of ±20 V with a maximum field effect mobility of ∼1 cm2/V s, maximum memory window of ∼20 V, and WR-ER current ratio of 4×102. When the NVM-TFT has a modified channel/ferroelectric interface with an inserted thin Al2O3 buffer layer, our device shows long retention time of more than 104 s, which is much enhanced compared to that of the other device without the buffer. The dynamic response of our devices with or without the buffer was clear enough to distinguish the WR and ER states as performed with 300 ms pulse.

Transparent photostable ZnO nonvolatile memory transistor with ferroelectric polymer and sputter-deposited oxide gate

Abstract: We report on the fabrication of transparent top-gate ZnO nonvolatile memory thin-film transistors (NVM-TFTs) with 200 nm thick poly(vinylidene fluoride/trifluoroethylene) ferroelectric layer; semitransparent 10 nm thin AgOx and transparent 130 nm thick indium-zinc oxide (IZO) were deposited on the ferroelectric polymer as gate electrode by rf sputtering. Our semitransparent NVM-TFT with AgOx gate operates under low voltage write-erase (WR-ER) pulse of ±20 V, but shows some degradation in retention property. In contrast, our transparent IZO-gated device displays very good retention properties but requires anomalously higher pulse of ±70 V for WR and ER states. Both devices stably operated under visible illuminations.

Synthesis of arsenic-doped p-type ZnO films by addition of As2O3 to the ZnO spin coating solution

Abstract: Arsenic-doped ZnO films were prepared by adding As 2 O 3 to sol–gel ZnO spin coating synthesis. The preferential (002) orientation improved, and the grain size of ZnO lattice increased with As-doping in ZnO films. UV transmittance over 70% with a blue shift was observed after As-doping. In particular, ZnO film with an As concentration of 6.7 at% showed a carrier concentration of 1.62×10 19 /cm 3 , mobility of 94.0 cm 2 /Vs, and resistivity of 3.99×10 −3 Ω cm. It was suggested that formation of As–2V Zn increased the carrier concentration and induced the blue shift, and the increase in grain size increased the mobility of p-type ZnO films.

Existence and classification of Hadamard matrices

Abstract: Hadamard matrices have been widely used in image processing. There are various constructions for Hadamard matrices: Kronecker product, Paley type I and II, Williamson type, etc. We investigate those of the Williamson type, and suggest a search algorithm for Hadamard matrices of the Williamson type. Using this we searched the Hadamard matrices of order 92. It took about 6.8 hours on a DEC Alpha PC. Based on the data for the order 92, we give an estimate for the exhaustive computer search for Hadamard matrices of Williamson type of order 428.

Polymer and Organic Nonvolatile Memory Devices

Abstract: Organic molecules and semiconductors have been proposed as active part of a large variety of nonvolatile memory devices, including resistors, diodes and transistors. In this review, we focus on electrically reprogrammable nonvolatile memories. We classify several possible devices according to their operation principle and critically review the role of the π-conjugated materials in the device operation. We propose specifications for applications for organic nonvolatile memory and review the state of the art with respect to these target specifications. Conclusions are drawn regarding further work on materials and device architectures.

PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics

Abstract: Ferroelectric polymers are the most promising electroactive materials with outstanding properties that can be integrated into a variety of flexible electronic devices. Their multifunctional capabilities, ability to bend and stretch, ease of processing, chemical stability, and the high biocompatibility of polyvinylidene fluoride (PVDF)-based polymers make them attractive for applications in flexible memories, energy transducers, and electronic skins. Here, recent advance in the research of PVDF-based flexible electronic devices is reviewed, including nonvolatile memories, energy-harvesting devices, and multifunctional portable sensors.

Status and Prospects of ZnO-Based Resistive Switching Memory Devices

Abstract: In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges.

Postfabrication annealing of pentacene-based photovoltaic cells

Abstract: We studied the effects of postfabrication annealing on heterojunction photovoltaic cells made from vacuum deposited pentacene and C60. The maximum power conversion efficiency under 115mW∕cm2 illumination increases from 0.45% to 1.07% after annealing the cells at 200°C. The increased performance is a result of better molecular ordering, which leads to an increased shunt resistance and built-in potential.

Electronic Properties of Pentacene versus Triisopropylsilylethynyl-Substituted Pentacene: Environment-Dependent Effects of the Silyl Substituent

Abstract: Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies bu...