Hakyung Jeong

Fabrication and characterization of graphene/AlGaN/GaN ultraviolet Schottky photodetector

We report on the fabrication and characterization of a Schottky ultraviolet graphene/AlGaN/GaN photodetector (PD). The fabricated device clearly exhibits rectification behaviour, indicating that the Schottky barrier is formed between the AlGaN and the mechanically transferred graphene. The Schottky parameters are evaluated using an equivalent circuit with two diodes connected back-to-back in series. The PD shows a low dark current of 4.77 × 10−12 A at a bias voltage of −2.5 V. The room temperature current–voltage (I–V) measurements of the graphene/AlGaN/GaN Schottky PD exhibit a large photo-to-dark contrast ratio of more than four orders of magnitude. Furthermore, the device shows peak responsivity at a wavelength of 350 nm, corresponding to GaN band edge and a small hump at 300 nm associated to the AlGaN band edge. In addition, we examine the behaviour of Schottky PDs with responsivities of 0.56 and 0.079 A W−1 at 300 and 350 nm, respectively, at room temperature.

Register mark measurement errors in high-precision roll-to-roll continuous systems: The effect of register mark geometry on measurement error

It is important to achieve high-precision register control in roll-to-roll continuous printing systems. Thus far, many studies on the dynamics of registers and tension and on register control techniques have identified register control as a problem of controlling and minimizing the disturbance of strain of the substrate. However, register control using printed register marks is necessary, and printing defects in creating these marks cause measurement errors. This study demonstrates by experimental verification that the measurement error is generated by the widening and agglomeration of the register mark. Furthermore, the error is shown to differ with the size and shape of the mark under identical printing conditions. The results illustrate the importance of improving the printing quality of the register mark, selecting the desired geometry for register marks with regard to printability, and utilizing an edge-detection algorithm in the control program for high-precision register control.

Nonvolatile memory devices based on undoped and Hf- and NaF-doped ZnO thin film transistors with Ag nanowires inserted between ZnO and gate insulator interface

Nonvolatile memory devices based on solution-processed thin film transistors (TFTs) of sol–gel-derived undoped ZnO and ZnO doped with Hf and NaF incorporating Ag nanowires (AgNWs) as charge trapping media between the ZnO and insulator interface are demonstrated. The transfer characteristics of the devices exhibited a large clockwise hysteresis due to the carrier trapping at the AgNWs between the ZnO and dielectric interface. The memory devices showed a remarkable, wide, programmable memory window during program/erase operations, which is attributed to the charge carrier trapping and de-trapping in the ZnO/AgNW/SiO2 structure. The device based on Hf-doped ZnO exhibited a larger memory window compared to the device based on NaF-doped ZnO. The threshold voltage shifts of these devices measured over various durations exhibited a large memory window. The fabricated transistor memory devices showed long term retention time characteristics, indicating excellent electrical reliability and stability.

Growth Mechanism and Luminescent Properties of Amorphous SiOx Structures via Phase Equilibrium in Binary System.

Balloon whisk-like and flower-like SiOx tubes with well-dispersed Sn and joining countless SiOx loops together induce intense luminescence characteristics in substrate materials. Our synthetic technique called “direct substrate growth” is based on pre-contamination of the surroundings without the intended catalyst and source powders. The kind of supporting material and pressure of the inlet gases determine a series of differently functionalized tube loops, i.e., the number, length, thickness, and cylindrical profile. SiOx tube loops commonly twist and split to best suppress the total energy. Photoluminescence and confocal laser measurements based on quantum confinement effect of the embedded Sn nanoparticles in the SiOx tube found substantially intense emissions throughout the visible range. These new concepts related to the synthetic approach, pre-pollution, transitional morphology, and permeable nanoparticles should facilitate progress in nanoscience with regard to tuning the dimensions of micro-/nanostructure preparations and the functionalization of customized applications.

Design and synthesis of amorphous SiOx structures generated by Sn quantum dots: growth mechanism and luminescent origin.

SiOx structures with different diameters of a few hundreds of nanometers and/or a few micrometers are prepared using applied thermal evaporation. Subsequently, Sn quantum dot-based SiOx architectures are synthesized via the continuous steps of the carbothermal reduction of SnO2, substitution of Sn(4+) for In(3+), thermal oxidation of Si, Sn sublimation, interfacial reaction, and diffusion reaction consistent with corresponding phase equilibriums. Several crystalline and spherical-shaped Sn quantum dots with diameters between 2 and 7 nm are observed in the amorphous SiOx structures. The morphological evolution, including hollow Sn (or SnOx) sphere and wire-like, worm-like, tube-like, and flower-like SiOx, occurs stepwise on the Si substrate upon increasing the given process energies. The optical characteristics based on confocal measurements reveal the as-synthesized SiOx structures, irrespective of whether crystallinity is formed, which all have visible-range emissions originating from the numerous different-sized and -shaped Sn quantum dots permeating into the SiOx matrix. In addition, photoluminescence emissions ranging between ultraviolet and red regions are in agreement with confocal measurements. The origins of the morphology- and luminescence-controlled amorphous SiOx with Sn quantum dots are also discussed.