균일하고 0 V에 가까운 Dirac 전압을 갖는 그래핀 전계효과 트랜지스터 제작 공정

박홍휘 ( Honghwi Park ),최무한 ( Muhan Choi ),박홍식 ( Hongsik Park ) 한국센서학회 2018 센서학회지 Vol.27 No.3

Monolayer graphene grown via chemical vapor deposition (CVD) is recognized as a promising material for sensor applications owing to its extremely large surface-to-volume ratio and outstanding electrical properties, as well as the fact that it can be easily transferred onto arbitrary substrates on a large-scale. However, the Dirac voltage of CVD-graphene devices fabricated with transferred graphene layers typically exhibit positive shifts arising from transfer and photolithography residues on the graphene surface. Furthermore, the Dirac voltage is dependent on the channel lengths because of the effect of metal-graphene contacts. Thus, large and nonuniform Dirac voltage of the transferred graphene is a critical issue in the fabrication of graphene-based sensor devices. In this work, we propose a fabrication process for graphene field-effect transistors with Dirac voltages close to zero. A vacuum annealing process at 300 °C was performed to eliminate the positive shift and channel-length-dependence of the Dirac voltage. In addition, the annealing process improved the carrier mobility of electrons and holes significantly by removing the residues on the graphene layer and reducing the effect of metal-graphene contacts. Uniform and close to zero Dirac voltage is crucial for the uniformity and low-power/voltage operation for sensor applications. Thus, the current study is expected to contribute significantly to the development of graphene-based practical sensor devices.

Frictional Anisotropy of CVD Bi-Layer Graphene Correlated with Surface Corrugated Structures

박선하,최민기,김석준,김송길 한국트라이볼로지학회 2022 한국트라이볼로지학회지 (Tribol. Lubr.) Vol.38 No.6

Atomically-thin 2D nanomaterials can be easily deformed and have surface corrugations which can influence the frictional characteristics of the 2D nanomaterials. Chemical vapor deposition (CVD) graphene can be grown in a wafer scale, which is suitable as a large-area surface coating film. The CVD growth involves cooling process to room temperature, and the thermal expansion coefficients mismatch between graphene and the metallic substrate induces a compressive strain in graphene, resulting in the surface corrugations such as wrinkles and atomic ripples. Such corrugations can induce the friction anisotropy of graphene, and therefore, accurate imaging of the surface corrugation is significant for better understanding about the friction anisotropy of CVD graphene. In this work, the combinatorial analysis using friction force microscopy (FFM) and transverse shear microscopy (TSM) was implemented to unveil the friction anisotropy of CVD bi-layer graphene. The periodic friction anisotropy of the wrinkles was measured following a sinusoidal curve depending on the angles between the wrinkles and the scanning tip, and the two domains were observed to have the different friction signals due to the different directions of the atomic ripples, which was confirmed by the high-resolution FFM and TSM imaging. In addition, we revealed that the atomic ripples can be easily suppressed by ironing the surface during AFM scans with an appropriate normal force. This work demonstrates that the friction anisotropy of CVD bi-layer graphene is well-correlated with the corrugated structures and the local friction anisotropy induced by the atomic ripples can be controllably removed by simple AFM scans.

Synthesis and characterization of grain boundaries-free CVD graphene

노지수,박호범,이민용,유명진,윤희욱 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.0

Large-area monolayer graphene can be synthesized by chemical vapor deposition (CVD) on catalytic copper metal surface. However, the graphene from CVD method usually shows inferior properties as compared to theoretical value of perfect graphene without any structural defects including point defects and grain boundaries. It is widely accepted that the presence of grain boundaries in graphene can lower its outstanding physical properties. In this regard, the synthesis of high-quality graphene (close to single crystal graphene monolayer) has been extensively studied. Likewise, we have prepared a high-quality, large-area graphene with a high carrier mobility of ~12,000 cm2/Vs by using well-oriented copper foil and improved CVD method. Such graphene has still lots of point defects, but no grain boundaries which help improve the quality CVD graphene.

Synthesis of graphene ribbons using selective chemical vapor deposition

An, Hyosub,Lee, Wan-Gyu,Jung, Jongwan Elsevier 2012 Current Applied Physics Vol.12 No.4

<P><B>Abstract</B></P><P>A new method for implementing graphene ribbons using selective graphene growth on metal-sidewall by chemical vapor deposition has been proposed. In this method, Ni catalyst is pre-patterned before chemical vapor deposition, and graphene film is selectively grown on the sidewall of the nickel for graphene ribbons. The graphene ribbons were confirmed by TEM image and Raman spectroscopy, and the fabricated graphene ribbon transistors showed well gate-modulated output characteristics. We believe this sidewall-graphene could be useful for applications such as graphene sensors which require high surface area of graphene.</P> <P><B>Highlights</B></P><P>► We propose a new method for implementing graphene ribbons. ► The method uses selective graphene growth on the sidewall of catalyst. ► The graphene ribbons were confirmed by TEM and Raman spectroscopy. ► Finally, the fabricated graphene transistors showed good gate-modulated output characteristics.</P>

Growth of high-quality graphene monolayer by chemical vapor deposition

노지수,박호범,이민용,윤희욱,유명진 한국공업화학회 2015 한국공업화학회 연구논문 초록집 Vol.2015 No.1

Chemical vapor deposition (CVD) using Cu catalyst is the most possible way for producing large-size graphene monolayer. It has been a great challenge to prepare high quality graphene to improve graphene properties since graphene from CVD method usually has grain boundaries which degrade its electrical properties. Recently, centimeter-size single crystal graphene has been synthesized by reducing nucleation density of graphene. However, more practical synthesis of single crystal graphene is still desirable for large area application. Here we demonstrate large-area, single crystal graphene sheet with a high carrier mobility of ~12,000 ㎠/Vs, using thermal CVD method. We found the structure defects of graphene arestrongly dependent on the catalytic Cu substrate such as grain size, crystal structure and surface condition.

Rapid-Thermal Pulse 화학증착법에 의해 증착된 그래핀 박막에서 촉매금속 Ni의 두께 및 열처리 조건의 영향

나신혜,윤순길,Na, Sin-Hye,Yoon, Soon-Gil 한국재료학회 2011 한국재료학회지 Vol.21 No.2

Mono- and few-layer graphenes were grown on Ni thin films by rapid-thermal pulse chemical vapor deposition technique. In the growth steps, the exposure step for 60 s in $H_2$ (a flow rate of 10 sccm (standard cubic centimeters per minute)) atmosphere after graphene growth was specially established to improve the quality of the graphenes. The graphene films grown by exposure alone without $H_2$ showed an intensity ratio of $I_G/I_{2D}$ = 0.47, compared with a value of 0.38 in the films grown by exposure in H2 ambient. The quality of the graphenes can be improved by exposure for 60 s in $H_2$ ambient after the growth of the graphene films. The physical properties of the graphene films were investigated for the graphene films grown on various Ni film thicknesses and on 260-nm thick Ni films annealed at 500 and $700^{\circ}C$. The graphene films grown on 260-nm thick Ni films at $900^{\circ}C$ showed the lowest $I_G/I_{2D}$ ratio, resulting in the fewest layers. The graphene films grown on Ni films annealed at $700^{\circ}C$ for 2 h showed a decrease of the number of layers. The graphene films were dependent on the thickness and the grain size of the Ni films.

No Tilt Angle Dependence of Grain Boundary on Mechanical Strength of Chemically Deposited Graphene Film

Jong Hun Kim,Sung Joo An,Jong-Young Lee,Eunji Ji,James Hone,Gwan-Hyoung Lee 한국세라믹학회 2019 한국세라믹학회지 Vol.56 No.5

Although graphene has been successfully grown in large scale via chemical vapor deposition (CVD), it is still questionable whether the mechanical properties of CVD graphene are equivalent to those of exfoliated graphene. In addition, there has been an issue regarding how the tilt angle of the grain boundary (GB) affects the strength of graphene. We investigate the mechanical properties of CVD graphene with nanoindentation from atomic force microscopy and transmission electron microscopy. Surprisingly, the samples with GB angles of 10° and 26° yielded similar fracture stresses of ~ 80 and ~ 79 GPa, respectively. Even for samples with GB exhibiting a wider range, from 0° to 30°, only a slightly wider fracture stress range (~ 50 to ~ 90 GPa) was measured, regardless of tilt angle. The results are contrary to previous studies that have reported that GBs with a larger tilt angle yield stronger graphene film. Such a lack of angle dependence of GB can be attributed to irregular and well-stitched GB structures.

화학증기증착 그래핀에서 결정 경계의 열전도 측정

이우민(Woomin Lee),김경덕(Kenneth David Kihm),박재성(Jae Sung Park),이준식(Joon Sik Lee) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.2015 No.11

Chemically vapor deposited (CVD) graphene grows a polycrystalline structure that consists of many grain domains. Grain boundaries between grain domains will have negative effect on the in-plane thermal transport in polycrystalline graphene. In this study, we measured thermal conductance of grain boundaries in polycrystalline graphene grown by CVD. Three CVD grown graphene samples with different grain sizes, ranging from 0.5 ㎛ to 4 ㎛, were made by carefully controlling the synthesis conditions and the grain sizes of each graphene were defined by mild dry annealing (MDA) and digital image processing. The thermal conductivities of the graphene were measured by using the optothermal Raman technique that is based on the Raman signal peak dependency on temperature. Finally, the thermal conductance of grain boundaries in graphene was obtained by fitting a theoretical model describing effective thermal conductance of common polycrystalline materials. We find the boundary conductance of graphene to be about 0.9 GW/㎡K and the boundary conductance slightly increases with increasing temperature. This result will help the predictions of thermal transport in polycrystalline graphene for simulation.

Synthesis of Monolayer Graphene Having a Negligible Amount of Wrinkles by Stress Relaxation

Mun, Jeong Hun,Cho, Byung Jin American Chemical Society 2013 NANO LETTERS Vol.13 No.6

<P>For the chemical vapor deposition (CVD) of graphene, the grain growth of the catalyst metal and thereby surface roughening are unavoidable during the high temperature annealing for the graphene synthesis. Considering that nanoscale wrinkles and poor uniformity of synthesized graphene originate from the roughened metal surface, improving surface flatness of metal thin films is one of the key factors to synthesize high quality graphene. Here, we introduce a new method for graphene synthesis for fewer wrinkle formation on a catalyst metal. The method utilizes a reduced graphene oxide (rGO) interfacial layer between the metal film and the wafer substrate. The rGO interlayer releases the residual stress of the metal thin film and thereby suppresses stress-induced metal grain growth. This technique makes it possible to use much thinner nickel films, leading to a dramatic suppression of RMS roughness (∼3 nm) of the metal surface even after high temperature annealing. It also endows excellent control of the graphene thickness due to the reduced amount of total carbon in the thin nickel film. The synthesized graphene layer having negligible amount of wrinkles exhibits excellent thickness uniformity (91% coverage of monolayer) and very high carrier mobility of ∼15 000 cm<SUP>2</SUP>/V·s.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2013/nalefd.2013.13.issue-6/nl4005578/production/images/medium/nl-2013-005578_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl4005578'>ACS Electronic Supporting Info</A></P>

No Tilt Angle Dependence of Grain Boundary on Mechanical Strength of Chemically Deposited Graphene Film

Kim, Jong Hun,An, Sung Joo,Lee, Jong-Young,Ji, Eunji,Hone, James,Lee, Gwan-Hyoung The Korean Ceramic Society 2019 한국세라믹학회지 Vol.56 No.5

Although graphene has been successfully grown in large scale via chemical vapor deposition (CVD), it is still questionable whether the mechanical properties of CVD graphene are equivalent to those of exfoliated graphene. In addition, there has been an issue regarding how the tilt angle of the grain boundary (GB) affects the strength of graphene. We investigate the mechanical properties of CVD graphene with nanoindentation from atomic force microscopy and transmission electron microscopy. Surprisingly, the samples with GB angles of 10° and 26° yielded similar fracture stresses of ~ 80 and ~ 79 GPa, respectively. Even for samples with GB exhibiting a wider range, from 0° to 30°, only a slightly wider fracture stress range (~ 50 to ~ 90 GPa) was measured, regardless of tilt angle. The results are contrary to previous studies that have reported that GBs with a larger tilt angle yield stronger graphene film. Such a lack of angle dependence of GB can be attributed to irregular and well-stitched GB structures.