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Honghwi Park; Jaedong Jung; Mingyuan Liu; Sunghwan Lee; Hongsik Park. Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films. ECS Meeting Abstracts 2021, MA2021-01, 1019 -1019.
AMA StyleHonghwi Park, Jaedong Jung, Mingyuan Liu, Sunghwan Lee, Hongsik Park. Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films. ECS Meeting Abstracts. 2021; MA2021-01 (30):1019-1019.
Chicago/Turabian StyleHonghwi Park; Jaedong Jung; Mingyuan Liu; Sunghwan Lee; Hongsik Park. 2021. "Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films." ECS Meeting Abstracts MA2021-01, no. 30: 1019-1019.
Honghwi Park; Jaedong Jung; Mingyuan Liu; Sunghwan Lee; Hongsik Park. Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films. ECS Transactions 2021, 102, 151 -158.
AMA StyleHonghwi Park, Jaedong Jung, Mingyuan Liu, Sunghwan Lee, Hongsik Park. Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films. ECS Transactions. 2021; 102 (2):151-158.
Chicago/Turabian StyleHonghwi Park; Jaedong Jung; Mingyuan Liu; Sunghwan Lee; Hongsik Park. 2021. "Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films." ECS Transactions 102, no. 2: 151-158.
Minsu Park; Hyowoong Noh; Jaewoon Kang; Junyeong Lee; Hongsik Park. Development of dielectrophoresis chips and an electrode passivation technique for isolation/separation of nanoparticles. JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY 2021, 30, 119 -124.
AMA StyleMinsu Park, Hyowoong Noh, Jaewoon Kang, Junyeong Lee, Hongsik Park. Development of dielectrophoresis chips and an electrode passivation technique for isolation/separation of nanoparticles. JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY. 2021; 30 (2):119-124.
Chicago/Turabian StyleMinsu Park; Hyowoong Noh; Jaewoon Kang; Junyeong Lee; Hongsik Park. 2021. "Development of dielectrophoresis chips and an electrode passivation technique for isolation/separation of nanoparticles." JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY 30, no. 2: 119-124.
Graphene-metal contact is crucial to fabricate high-performance graphene photodetectors since the external quantum efficiency (EQE) of the photodetector depends on the contact properties, and the influence of the contact properties is particularly dominant in short channel devices for high-speed applications. Moreover, junction properties between the channel graphene and graphene near the contact are also important to analyze the photoresponse because the built-in electric field in the junction determines the EQE of the photodetector. In this study, we investigated a relation between the photoresponse and the built-in electric field induced from the doping level difference in the junction between the channel graphene and graphene near the contact. The photoresponse could be enhanced with a high junction barrier height that is tuned by the doping level difference. In addition, we observed that the improved electrical characteristics of channel graphene do not guarantee the enhancement of the photoresponse characteristics of graphene photodetectors.
Jaedong Jung; Honghwi Park; Heungsup Won; Muhan Choi; Chang-Ju Lee; Hongsik Park. Effect of Graphene Doping Level near the Metal Contact Region on Electrical and Photoresponse Characteristics of Graphene Photodetector. Sensors 2020, 20, 4661 .
AMA StyleJaedong Jung, Honghwi Park, Heungsup Won, Muhan Choi, Chang-Ju Lee, Hongsik Park. Effect of Graphene Doping Level near the Metal Contact Region on Electrical and Photoresponse Characteristics of Graphene Photodetector. Sensors. 2020; 20 (17):4661.
Chicago/Turabian StyleJaedong Jung; Honghwi Park; Heungsup Won; Muhan Choi; Chang-Ju Lee; Hongsik Park. 2020. "Effect of Graphene Doping Level near the Metal Contact Region on Electrical and Photoresponse Characteristics of Graphene Photodetector." Sensors 20, no. 17: 4661.
Carrier mobility is one of the most important parameters to evaluate the quality and uniformity of graphene. The mobility of graphene is typically extracted from the transconductance of a field-effect transistor fabricated with the graphene layer. However, the mobility value evaluated by this method is imprecise when the contact resistance is non-negligible, or the contact resistance is modulated by the gate bias, which is the case for typical graphene field-effect transistors. Here, we suggest a method for extracting the precise intrinsic field-effect mobility by considering the effective bias across the channel and its gate-induced modulation. We show that the contact resistances of typical graphene field-effect transistors are significantly modulated by gate bias and conventional methods can, therefore, cause a considerable error in the evaluation of the mobility. The proposed method in which the contact-induced error is removed gives a channel-length-independent intrinsic field-effect mobility. This m...
Chang-Ju Lee; Honghwi Park; Jaewoon Kang; Junyeong Lee; Muhan Choi; Hongsik Park. Extraction of intrinsic field-effect mobility of graphene considering effects of gate-bias-induced contact modulation. Journal of Applied Physics 2020, 127, 185105 .
AMA StyleChang-Ju Lee, Honghwi Park, Jaewoon Kang, Junyeong Lee, Muhan Choi, Hongsik Park. Extraction of intrinsic field-effect mobility of graphene considering effects of gate-bias-induced contact modulation. Journal of Applied Physics. 2020; 127 (18):185105.
Chicago/Turabian StyleChang-Ju Lee; Honghwi Park; Jaewoon Kang; Junyeong Lee; Muhan Choi; Hongsik Park. 2020. "Extraction of intrinsic field-effect mobility of graphene considering effects of gate-bias-induced contact modulation." Journal of Applied Physics 127, no. 18: 185105.
We report a fabrication method to produce either nanopillar (NP) or nanohole (NH) array structure along with micrometer structure in a single layer. Upon 200 mm silicon wafer, we produced 200~400 nm NP or NH array by electron beam lithography (EBL). The EBL patterns on positive tone EB resist such as poly(methyl methacrylate) (PMMA) or chemically semi-amplified resist (CSAR) were transferred to hard mask oxide (HMO) layer by the first step reactive ion etching (RIE). We used the HMO as an intermediate layer to connect the EB patterns to photolithography patterns. Upon the EB-patterned HMO layer, large scale photolithography patterns were produced on photoresist (PR), which are further transferred to the HMO layer by the second step RIE. After removing the PR, the mixed EB and photo patterns on the HMO were transferred to the target layer by the third step RIE. Our method offers an efficient way to combine the nanometer-sized EBL patterns with the high-throughput photolithography patterns in a single layer.
Seung Hee Baek; Sunwoong Lee; Ju-Hyun Bae; Chang-Won Hong; Mae-Ja Park; Hongsik Park; Moon-Chang Baek; Sung-Wook Nam. Nanopillar and nanohole fabrication via mixed lithography. Materials Research Express 2020, 7, 035008 .
AMA StyleSeung Hee Baek, Sunwoong Lee, Ju-Hyun Bae, Chang-Won Hong, Mae-Ja Park, Hongsik Park, Moon-Chang Baek, Sung-Wook Nam. Nanopillar and nanohole fabrication via mixed lithography. Materials Research Express. 2020; 7 (3):035008.
Chicago/Turabian StyleSeung Hee Baek; Sunwoong Lee; Ju-Hyun Bae; Chang-Won Hong; Mae-Ja Park; Hongsik Park; Moon-Chang Baek; Sung-Wook Nam. 2020. "Nanopillar and nanohole fabrication via mixed lithography." Materials Research Express 7, no. 3: 035008.
Quantum dots have diverse chemical properties with different ligands attached on the surface. The cysteamine has been used as a ligand for various quantum dots because it has high solubility in water, and it facilitates binding of quantum dot and gold surface. However, the hydrogen bonds in cysteamine cause aggregation of the cysteamine capped quantum dots. In this study, we suggested a simple synthesis method of aggregation-free PbS quantum dot and analyzed the electric and optical properties of the synthesized quantum dot. This study on aggregation-free cysteamine capped quantum dots has the potential to develop advanced quantum dot-based sensor technologies, including biomedical imaging and environmental sensors.
Daekyung Kang; Mareddi Bharath Kumar; Changhee Son; Hongsik Park; Jonghoo Park. Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot. Applied Sciences 2019, 9, 4661 .
AMA StyleDaekyung Kang, Mareddi Bharath Kumar, Changhee Son, Hongsik Park, Jonghoo Park. Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot. Applied Sciences. 2019; 9 (21):4661.
Chicago/Turabian StyleDaekyung Kang; Mareddi Bharath Kumar; Changhee Son; Hongsik Park; Jonghoo Park. 2019. "Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot." Applied Sciences 9, no. 21: 4661.
Hyowoong Noh; Junyeong Lee; Chang-Ju Lee; Jaedong Jung; Jaewoon Kang; Muhan Choi; Moon-Chang Baek; Jae Hoon Shim; Hongsik Park. Precise evaluation of liquid conductivity using a multi-channel microfluidic chip and direct-current resistance measurements. Sensors and Actuators B: Chemical 2019, 297, 1 .
AMA StyleHyowoong Noh, Junyeong Lee, Chang-Ju Lee, Jaedong Jung, Jaewoon Kang, Muhan Choi, Moon-Chang Baek, Jae Hoon Shim, Hongsik Park. Precise evaluation of liquid conductivity using a multi-channel microfluidic chip and direct-current resistance measurements. Sensors and Actuators B: Chemical. 2019; 297 ():1.
Chicago/Turabian StyleHyowoong Noh; Junyeong Lee; Chang-Ju Lee; Jaedong Jung; Jaewoon Kang; Muhan Choi; Moon-Chang Baek; Jae Hoon Shim; Hongsik Park. 2019. "Precise evaluation of liquid conductivity using a multi-channel microfluidic chip and direct-current resistance measurements." Sensors and Actuators B: Chemical 297, no. : 1.
The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH4 flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.
Jaewoon Kang; Chang-Ju Lee; Jaeeuk Kim; Honghwi Park; Changhee Lim; Junyeong Lee; Muhan Choi; Hongsik Park. Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition. Current Applied Physics 2019, 19, 1414 -1420.
AMA StyleJaewoon Kang, Chang-Ju Lee, Jaeeuk Kim, Honghwi Park, Changhee Lim, Junyeong Lee, Muhan Choi, Hongsik Park. Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition. Current Applied Physics. 2019; 19 (12):1414-1420.
Chicago/Turabian StyleJaewoon Kang; Chang-Ju Lee; Jaeeuk Kim; Honghwi Park; Changhee Lim; Junyeong Lee; Muhan Choi; Hongsik Park. 2019. "Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition." Current Applied Physics 19, no. 12: 1414-1420.
The fabrication of a single pixel sensor, which is a fundamental element device for the fabrication of an array-type pixel sensor, requires an integration technique of a photodetector and transistor on a wafer. In conventional GaN-based ultraviolet (UV) imaging devices, a hybrid-type integration process is typically utilized, which involves a backside substrate etching and a wafer-to-wafer bonding process. In this work, we developed a GaN-based UV passive pixel sensor (PPS) by integrating a GaN metal-semiconductor-metal (MSM) UV photodetector and a Schottky-barrier (SB) metal-oxide-semiconductor field-effect transistor (MOSFET) on an epitaxially grown GaN layer on silicon substrate. An MSM-type UV sensor had a low dark current density of 3.3 × 10−7 A/cm2 and a high UV/visible rejection ratio of 103. The GaN SB-MOSFET showed a normally-off operation and exhibited a maximum drain current of 0.5 mA/mm and a maximum transconductance of 30 μS/mm with a threshold voltage of 4.5 V. The UV PPS showed good UV response and a high dark-to-photo contrast ratio of 103 under irradiation of 365-nm UV. This integration technique will provide one possible way for a monolithic integration of the GaN-based optoelectronic devices.
Chang-Ju Lee; Chul-Ho Won; Jung-Hee Lee; Sung-Ho Hahm; Hongsik Park. GaN-Based Ultraviolet Passive Pixel Sensor on Silicon (111) Substrate. Sensors 2019, 19, 1051 .
AMA StyleChang-Ju Lee, Chul-Ho Won, Jung-Hee Lee, Sung-Ho Hahm, Hongsik Park. GaN-Based Ultraviolet Passive Pixel Sensor on Silicon (111) Substrate. Sensors. 2019; 19 (5):1051.
Chicago/Turabian StyleChang-Ju Lee; Chul-Ho Won; Jung-Hee Lee; Sung-Ho Hahm; Hongsik Park. 2019. "GaN-Based Ultraviolet Passive Pixel Sensor on Silicon (111) Substrate." Sensors 19, no. 5: 1051.
Graphene grown on a copper (Cu) substrate by chemical vapor deposition (CVD) is typically required to be transferred to another substrate for the fabrication of various electrical devices. PMMA-mediated wet process is the most widely used method for CVD-graphene transfer. However, PMMA residue and wrinkles that inevitably remain on the graphene surface during the transfer process are critical issues degrading the electrical properties of graphene. In this paper, we report on a PMMA-mediated graphene transfer method that can effectively reduce the density and size of the PMMA residue and the height of wrinkles on the transferred graphene layer. We found out that acetic acid is the most effective PMMA stripper among the typically used solutions to remove the PMMA residue. In addition, we observed that an optimized annealing process can reduce the height of the wrinkles on the transferred graphene layer without degrading the graphene quality. The effects of the suggested wet transfer process were also investigated by evaluating the electrical properties of field-effect transistors (FETs) fabricated on the transferred graphene layer. The results of this work will contribute to the development of fabrication processes for high-quality graphene devices, given that the transfer of graphene from the Cu substrate is essential process to the application of CVD-graphene.
Honghwi Park; Changhee Lim; Chang-Ju Lee; Jaewoon Kang; Jaeeuk Kim; Muhan Choi; Hongsik Park. Optimized poly(methyl methacrylate)-mediated graphene-transfer process for fabrication of high-quality graphene layer. Nanotechnology 2018, 29, 415303 .
AMA StyleHonghwi Park, Changhee Lim, Chang-Ju Lee, Jaewoon Kang, Jaeeuk Kim, Muhan Choi, Hongsik Park. Optimized poly(methyl methacrylate)-mediated graphene-transfer process for fabrication of high-quality graphene layer. Nanotechnology. 2018; 29 (41):415303.
Chicago/Turabian StyleHonghwi Park; Changhee Lim; Chang-Ju Lee; Jaewoon Kang; Jaeeuk Kim; Muhan Choi; Hongsik Park. 2018. "Optimized poly(methyl methacrylate)-mediated graphene-transfer process for fabrication of high-quality graphene layer." Nanotechnology 29, no. 41: 415303.
The UV-to-visible rejection ratio is one of the important figure of merits of GaN-based UV photodetectors. For cost-effectiveness and large-scale fabrication of GaN devices, we tried to grow a GaN epitaxial layer on silicon substrate with complicated buffer layers for a stress-release. It is known that the structure of the buffer layers affects the performance of devices fabricated on the GaN epitaxial layers. In this study, we show that the design of a buffer layer structure can make effect on the UV-to-visible rejection ratio of GaN UV photodetectors. The GaN photodetector fabricated on GaN-on-silicon substrate with a step-graded AlxGa−xN buffer layer has a highly-selective photoresponse at 365-nm wavelength. The UV-to-visible rejection ratio of the GaN UV photodetector with the step-graded AlxGa1−xN buffer layer was an order-of-magnitude higher than that of a photodetector with a conventional GaN/AlN multi buffer layer. The maximum photoresponsivity was as high as 5 × 10−2 A/W. This result implies that the design of buffer layer is important for photoresponse characteristics of GaN UV photodetectors as well as the crystal quality of the GaN epitaxial layers.
Chang-Ju Lee; Chul-Ho Won; Jung-Hee Lee; Sung-Ho Hahm; Hongsik Park. Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1−xN Buffer Layer. Sensors 2017, 17, 1684 .
AMA StyleChang-Ju Lee, Chul-Ho Won, Jung-Hee Lee, Sung-Ho Hahm, Hongsik Park. Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1−xN Buffer Layer. Sensors. 2017; 17 (7):1684.
Chicago/Turabian StyleChang-Ju Lee; Chul-Ho Won; Jung-Hee Lee; Sung-Ho Hahm; Hongsik Park. 2017. "Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1−xN Buffer Layer." Sensors 17, no. 7: 1684.