koichiro saiki - Profile on Academia.edu (original) (raw)
Papers by koichiro saiki
Physical Chemistry Chemical Physics, 2015
Doping of heteroatoms such as nitrogen into the lattice structure of graphene can tune and tailor... more Doping of heteroatoms such as nitrogen into the lattice structure of graphene can tune and tailor the overall electronic properties.
Nature communications, 2015
Chemical vapour deposition (CVD) growth of graphene has attracted much attention, aiming at the m... more Chemical vapour deposition (CVD) growth of graphene has attracted much attention, aiming at the mass production of large-area and high-quality specimens. To optimize the growth condition, CVD grown graphene is conventionally characterized after synthesis. Real-time observation during graphene growth enables us to understand the growth mechanism and control the growth more easily. Here we report the optical microscope observation of the CVD growth of graphene in real time by focusing the radiation emitted from the growing graphene, which we call 'radiation-mode optical microscopy'. We observe the growth and shrinkage of graphene in response to the switching on and off of the methane supply. Analysis of the growth feature reveals that the attachment and detachment of carbon precursors are the rate-determining factor in the CVD growth of graphene. We expect radiation-mode optical microscopy to be applicable to the other crystal growth at high temperatures in various atmospheres.
Atomic and electronic structure of an unreconstructed polar MgO(111) thin film on Ag(111)
Physical Review B, 2003
Physical Review B, 2005
We have shown, both experimentally and theoretically, that the metal-induced gap states ͑MIGS͒ ca... more We have shown, both experimentally and theoretically, that the metal-induced gap states ͑MIGS͒ can exist in epitaxially grown organic insulator/metal interfaces. The experiment is done for alkane/Cu͑001͒ with an element-selective near edge x-ray absorption fine structure ͑NEXAFS͒, which exhibits a prepeak indicative of MIGS. An ab initio electronic structure calculation supports the existence of the MIGS. When the Cu substrate is replaced with Ni, an interface magnetism may be possible with a carrier doping.
Physical Review B, 2001
Thin film growth of LiCl on a Cu͑001͒ substrate is studied using reflection high-energy electron ... more Thin film growth of LiCl on a Cu͑001͒ substrate is studied using reflection high-energy electron diffraction and Auger electron spectroscopy. A LiCl film was found to grow heteroepitaxially on a Cu͑001͒ substrate. At a substrate temperature of 300 K, a single-domain LiCl film (͓100͔ f ilm //͓110͔ substrate) grows in a layer-bylayer fashion, while a double-domain LiCl film (͓100͔ f ilm //͓100͔ substrate and ͓100͔ f ilm //͓110͔ substrate) grows at substrate temperatures higher than 470 K. The density-functional calculations are performed for model clusters in order to investigate the initial adsorption structure. It is found in the calculation that the LiCl island is adsorbed more stably on the fourfold hollow site of the first Cu layer than on the atop site. The effective charge of Li and Cl of the LiCl molecule decreases with adsorption on Cu͑001͒, due to the shielding by Cu 4s and 4p free electrons.
Thin Solid Films, 2006
As a potential candidate for inorganic FETs in various applications, organic FETs have received m... more As a potential candidate for inorganic FETs in various applications, organic FETs have received much attention during recent years. In order to remove surface contamination and to improve the device performance, surface treatment is generally adopted before deposition of organic films. However, the microstructure of the organic films is very sensitive to growth conditions. Surface treatment and heating can influence the film morphology and the device performance greatly. In order to study the effect of surface treatment on the growth of pentacene film, we deposited and contrasted pentacene films at room temperature and elevated temperature on SiO 2 substrate treated by several typical cleaning procedures. The results indicated that surface treatment could influence the film morphology greatly even for a thickness far beyond the first monolayer. We also found that there was a low critical temperature of about 55-C at a deposition rate of 0.5 nm/min, above which substantial desorption of pentacene molecule occurred regardless of the cleaning procedure. Deposition on heated substrates is widely reported for increased grain size and improved device performance, while the possible desorption seems to be seldom noticed. Therefore, an implication of the low desorption temperature is that the film thickness may be frequently overestimated for deposition on heated substrates. Consequently, errors for any data that are dependent on film thickness may be caused without taking into account the possible desorption.
Surface Science, 2007
The electronic structure of an octane film grown on Cu(111) and Ni(111) was studied using C K-edg... more The electronic structure of an octane film grown on Cu(111) and Ni(111) was studied using C K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the 1 ML thick octane films on the metal substrates. The pre-peak originated from metal-induced gap states (MIGS) in the band gap of octane. The intensity of the pre-peak for octane/Ni(111) was the same as that of octane/Cu(111), suggesting that there was little difference in the density of unoccupied MIGS between the octane film on Ni(111) and Cu(111). We discuss the metal dependence of the density of unoccupied MIGS on the band structure of the metals.
Review of Scientific Instruments, 2000
Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection... more Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection high-energy electron diffraction. The primary electron current could be reduced by four orders of magnitude to get the image of the same intensity as in the measurement without MCP. The reduction of electron beam enabled observation of ionic crystal surfaces without causing electrolysis and charging effect.
Physical Review Letters, 2008
We report the epitaxial growth of thin films of a small organic molecule (pentacene) on polymer s... more We report the epitaxial growth of thin films of a small organic molecule (pentacene) on polymer substrates with controllable photoalignment over a wide range. The pentacene molecular plane exhibited a distinct orientational change from parallel to perpendicular relative to the polymer chain with increasing substrate polymer alignment. Each orientation consists of twinlike domains. Such characteristics reveal a unique alignment-induced epitaxial transition controlled by the subtle balance of weak interactions, showing a promising approach for tuning the characteristics of organic semiconductor based electronic devices.
Electric-field-induced charge injection or exhaustion in organic thin film transistor
Physical Review B, 2005
Physical Review B, 2005
The spatial distribution and site distribution of metal-induced gap states ͑MIGS͒ are studied by ... more The spatial distribution and site distribution of metal-induced gap states ͑MIGS͒ are studied by thicknessdependent near-edge x-ray absorption fine structure ͑NEXAFS͒ and by comparing the cation and anion-edge NEXAFS. The thickness-dependent NEXAFS shows that the decay length of MIGS depends on an alkalihalide rather than a metal, and it is larger for alkali-halides with smaller band gap energies. By comparing the Cl-edge and K-edge NEXAFS for KCl/ Cu͑001͒, MIGS are found to be states localizing at anion sites.
Journal of Crystal Growth, 2000
Sub-monolayer films of layered semiconductor InSe were grown on MoS 2 substrates by molecular bea... more Sub-monolayer films of layered semiconductor InSe were grown on MoS 2 substrates by molecular beam epitaxy, and the change in their growth features with Se/In flux ratio was investigated using scanning tunneling microscope in vacuum. It was found that InSe domains grown at 340 °C have a hexagonal shape when the Se/In ratio is about 17. Detailed images of the hexagonal InSe domains have revealed that adjacent sides of the hexagon have different structures; one is a straight edge and the other is a disordered edge. When the Se/In ratio was decreased, the disordered edges became predominant, the straight edges disappeared, and the InSe domain became triangular. Contrary, when the Se/In ratio was increased, InSe domains became triangular ones consisting of only straight edges. The growth mechanism of InSe domains is discussed by considering the crystal structure of InSe and the reactivity of each side with incoming atoms. It is suggested that the balance of incorporation rate of In and Se atoms determines the structure of InSe domains.
Journal of Applied Physics, 2006
Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition m... more Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition method. The TFTs were characterized in order to study the effect of thermal annealing on the morphology and carrier mobility of the transistors. For all the TFT samples the mobility exhibited an Arrhenius relationship with temperature, indicating a thermally activated transport that could be explained by the carrier trap and thermal release transport mechanism. Therefore, in order to investigate the annealing effect, we tested the data for a significant period of time after annealing until the temperature recovered to room temperature, so that the thermal activation effect was screened and possible effects of thermal expansion and stress were also ruled out. As a result, we found that only with a temperature below a critical temperature of approximately 45°C could annealing improve the mobility, while annealing with T>50°C would decrease the mobility compared to the value before anneal...
Journal of Applied Physics, 2005
Electrolytically polished surfaces of Ta sheets were anodized in an aqueous KI solution to form T... more Electrolytically polished surfaces of Ta sheets were anodized in an aqueous KI solution to form Ta2O5 films with a relatively high dielectric constant and a high breakdown electric field. Organic field-effect transistors (OFETs) were fabricated on these Ta2O5∕Ta substrates with a bottom gate configuration. The Ta2O5 films anodized at 100V were about 200nm thick, and the dielectric strength was as high as 5MV∕cm. A p-type pentacene OFET fabricated on this bottom gate showed the good field-effect mobility of 0.35cm2∕Vs. By using this high-k gate substrate, it becomes possible to inject a higher amount of charge carriers into organic active layers than on the SiO2∕Si gate conventionally used in most OFETs.
Journal of Applied Physics, 2007
The gate bias and temperature dependent field-effect mobility and conductance of a polycrystallin... more The gate bias and temperature dependent field-effect mobility and conductance of a polycrystalline pentacene thin film transistor (TFT) were analyzed to study the charge transport in the material. Since both heating and cooling can obviously change the film morphology, a relatively narrow temperature range was adopted to rule out the possible influence of structure variation on the device characterization. Both mobility and conductance values increased with the gate bias and showed a thermally activated Arrhenius-like behavior, while the threshold voltage deceased with temperature. Several models were compared, and it was found that the observations could only be well interpreted by a multiple trapping model, which suggests that the temperature and gate bias dependences should be attributed to the increased free charge carrier density. The density of trap states in the band gap was evaluated by the field-effect mobility as well as the field-effect conductance data. The results discl...
Japanese Journal of Applied Physics, 2010
Recently it has been shown that the “graphoepitaxy” of organic semiconductor α-sexithiophene (6T)... more Recently it has been shown that the “graphoepitaxy” of organic semiconductor α-sexithiophene (6T) occurs on artificial periodic grooves fabricated on thermally-oxidized Si substrates. In the present study, similarly oriented growth of 6T thin films was found around the edge of metal electrodes. Using this phenomenon to realize single crystalline organic transistors, substrates with short channel Au electrodes were fabricated by photolithography, and 6T films were grown on these substrates by molecular beam deposition. The {010} planes of 6T crystals tended to face the wall of the channel (electrode edge), probably due to the chemical affinity of Au for S atoms in 6T molecules. The transistor characteristics of the test device were measured, and the results suggested that this orientation control technique using the shape of electrodes would lead to improvements in device performance.
Japanese Journal of Applied Physics, 2007
The mobility of pentacene-based organic field-effect transistors (OFETs) fabricated on natural mi... more The mobility of pentacene-based organic field-effect transistors (OFETs) fabricated on natural mica substrates has been highly improved using polymethylmethacrylate (PMMA) or octadecyltrimethoxysillane (OTMS) as a buffer layer between the mica dielectric and the pentacene active layer. The field-effect mobility increased from 2.4×10-3 to 0.14 or 0.31 cm 2 ⋅V-1 ⋅s-1 by inserting the PMMA or OTMS buffer layer, respectively. The suppression of the unfavorable effect of potassium ions using the OTMS buffer layer confirmed the feasibility of using natural mica as a transparent gate dielectric of organic field-effect transistors.
Japanese Journal of Applied Physics, 2003
We present a simple but powerful method to determine the thicknesses of the accumulation and depl... more We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in-situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (VG=-15 V) and 5 nm (VG=15 V).
Chemical Physics Letters, 2006
The effect of UV/ozone treatment of the SiO 2 dielectric layer on the film morphology and the ele... more The effect of UV/ozone treatment of the SiO 2 dielectric layer on the film morphology and the electrical performance of the pentacene thin film transistors (TFTs) was investigated. Decreased threshold voltage and improved mobility were obtained after UV/ozone treatment. The effect of UV/ozone treatment of the substrate in modifying the film growth was distinguished from its effect in modifying the Au electrode by using a series of samples with different architectures. Electron spectroscopic measurement and film morphology observation indicated that the improved TFT performance was achieved mainly by the reduced contamination at the pentacene/SiO 2 interface and the increased grain size.
Applied Surface Science, 2001
We have investigated the electronic structure of K x C 60 (x = 0 ~ 6) using low-energy electron e... more We have investigated the electronic structure of K x C 60 (x = 0 ~ 6) using low-energy electron energy loss spectroscopy (LEELS), especially focusing on the K 3p core-electron excitation spectra. It is found that the structure of the K 3p-excitation spectrum of K x C 60 quite differs from that of KCl. Furthermore, the K 3p-excitation LEELS of K 3 C 60 has been revealed to be different from that of K 6 C 60. K 3p electrons are excited into K 4s-and K 3d-derived empty states in both K x C 60 and KCl, but in the case of K x C 60 the K 3d-derived empty states have a rather complicated structure where several levels are not well separated. Consequently K 3p-excitation LEEL spectra of K x C 60 show widespread plateau-like structures. The difference in the K 3p-excitation spectra of K 3 C 60 and K 6 C 60 is considered to originate from the different crystal field around K + cations in the C 60 molecular crystal.
Physical Chemistry Chemical Physics, 2015
Doping of heteroatoms such as nitrogen into the lattice structure of graphene can tune and tailor... more Doping of heteroatoms such as nitrogen into the lattice structure of graphene can tune and tailor the overall electronic properties.
Nature communications, 2015
Chemical vapour deposition (CVD) growth of graphene has attracted much attention, aiming at the m... more Chemical vapour deposition (CVD) growth of graphene has attracted much attention, aiming at the mass production of large-area and high-quality specimens. To optimize the growth condition, CVD grown graphene is conventionally characterized after synthesis. Real-time observation during graphene growth enables us to understand the growth mechanism and control the growth more easily. Here we report the optical microscope observation of the CVD growth of graphene in real time by focusing the radiation emitted from the growing graphene, which we call 'radiation-mode optical microscopy'. We observe the growth and shrinkage of graphene in response to the switching on and off of the methane supply. Analysis of the growth feature reveals that the attachment and detachment of carbon precursors are the rate-determining factor in the CVD growth of graphene. We expect radiation-mode optical microscopy to be applicable to the other crystal growth at high temperatures in various atmospheres.
Atomic and electronic structure of an unreconstructed polar MgO(111) thin film on Ag(111)
Physical Review B, 2003
Physical Review B, 2005
We have shown, both experimentally and theoretically, that the metal-induced gap states ͑MIGS͒ ca... more We have shown, both experimentally and theoretically, that the metal-induced gap states ͑MIGS͒ can exist in epitaxially grown organic insulator/metal interfaces. The experiment is done for alkane/Cu͑001͒ with an element-selective near edge x-ray absorption fine structure ͑NEXAFS͒, which exhibits a prepeak indicative of MIGS. An ab initio electronic structure calculation supports the existence of the MIGS. When the Cu substrate is replaced with Ni, an interface magnetism may be possible with a carrier doping.
Physical Review B, 2001
Thin film growth of LiCl on a Cu͑001͒ substrate is studied using reflection high-energy electron ... more Thin film growth of LiCl on a Cu͑001͒ substrate is studied using reflection high-energy electron diffraction and Auger electron spectroscopy. A LiCl film was found to grow heteroepitaxially on a Cu͑001͒ substrate. At a substrate temperature of 300 K, a single-domain LiCl film (͓100͔ f ilm //͓110͔ substrate) grows in a layer-bylayer fashion, while a double-domain LiCl film (͓100͔ f ilm //͓100͔ substrate and ͓100͔ f ilm //͓110͔ substrate) grows at substrate temperatures higher than 470 K. The density-functional calculations are performed for model clusters in order to investigate the initial adsorption structure. It is found in the calculation that the LiCl island is adsorbed more stably on the fourfold hollow site of the first Cu layer than on the atop site. The effective charge of Li and Cl of the LiCl molecule decreases with adsorption on Cu͑001͒, due to the shielding by Cu 4s and 4p free electrons.
Thin Solid Films, 2006
As a potential candidate for inorganic FETs in various applications, organic FETs have received m... more As a potential candidate for inorganic FETs in various applications, organic FETs have received much attention during recent years. In order to remove surface contamination and to improve the device performance, surface treatment is generally adopted before deposition of organic films. However, the microstructure of the organic films is very sensitive to growth conditions. Surface treatment and heating can influence the film morphology and the device performance greatly. In order to study the effect of surface treatment on the growth of pentacene film, we deposited and contrasted pentacene films at room temperature and elevated temperature on SiO 2 substrate treated by several typical cleaning procedures. The results indicated that surface treatment could influence the film morphology greatly even for a thickness far beyond the first monolayer. We also found that there was a low critical temperature of about 55-C at a deposition rate of 0.5 nm/min, above which substantial desorption of pentacene molecule occurred regardless of the cleaning procedure. Deposition on heated substrates is widely reported for increased grain size and improved device performance, while the possible desorption seems to be seldom noticed. Therefore, an implication of the low desorption temperature is that the film thickness may be frequently overestimated for deposition on heated substrates. Consequently, errors for any data that are dependent on film thickness may be caused without taking into account the possible desorption.
Surface Science, 2007
The electronic structure of an octane film grown on Cu(111) and Ni(111) was studied using C K-edg... more The electronic structure of an octane film grown on Cu(111) and Ni(111) was studied using C K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the 1 ML thick octane films on the metal substrates. The pre-peak originated from metal-induced gap states (MIGS) in the band gap of octane. The intensity of the pre-peak for octane/Ni(111) was the same as that of octane/Cu(111), suggesting that there was little difference in the density of unoccupied MIGS between the octane film on Ni(111) and Cu(111). We discuss the metal dependence of the density of unoccupied MIGS on the band structure of the metals.
Review of Scientific Instruments, 2000
Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection... more Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection high-energy electron diffraction. The primary electron current could be reduced by four orders of magnitude to get the image of the same intensity as in the measurement without MCP. The reduction of electron beam enabled observation of ionic crystal surfaces without causing electrolysis and charging effect.
Physical Review Letters, 2008
We report the epitaxial growth of thin films of a small organic molecule (pentacene) on polymer s... more We report the epitaxial growth of thin films of a small organic molecule (pentacene) on polymer substrates with controllable photoalignment over a wide range. The pentacene molecular plane exhibited a distinct orientational change from parallel to perpendicular relative to the polymer chain with increasing substrate polymer alignment. Each orientation consists of twinlike domains. Such characteristics reveal a unique alignment-induced epitaxial transition controlled by the subtle balance of weak interactions, showing a promising approach for tuning the characteristics of organic semiconductor based electronic devices.
Electric-field-induced charge injection or exhaustion in organic thin film transistor
Physical Review B, 2005
Physical Review B, 2005
The spatial distribution and site distribution of metal-induced gap states ͑MIGS͒ are studied by ... more The spatial distribution and site distribution of metal-induced gap states ͑MIGS͒ are studied by thicknessdependent near-edge x-ray absorption fine structure ͑NEXAFS͒ and by comparing the cation and anion-edge NEXAFS. The thickness-dependent NEXAFS shows that the decay length of MIGS depends on an alkalihalide rather than a metal, and it is larger for alkali-halides with smaller band gap energies. By comparing the Cl-edge and K-edge NEXAFS for KCl/ Cu͑001͒, MIGS are found to be states localizing at anion sites.
Journal of Crystal Growth, 2000
Sub-monolayer films of layered semiconductor InSe were grown on MoS 2 substrates by molecular bea... more Sub-monolayer films of layered semiconductor InSe were grown on MoS 2 substrates by molecular beam epitaxy, and the change in their growth features with Se/In flux ratio was investigated using scanning tunneling microscope in vacuum. It was found that InSe domains grown at 340 °C have a hexagonal shape when the Se/In ratio is about 17. Detailed images of the hexagonal InSe domains have revealed that adjacent sides of the hexagon have different structures; one is a straight edge and the other is a disordered edge. When the Se/In ratio was decreased, the disordered edges became predominant, the straight edges disappeared, and the InSe domain became triangular. Contrary, when the Se/In ratio was increased, InSe domains became triangular ones consisting of only straight edges. The growth mechanism of InSe domains is discussed by considering the crystal structure of InSe and the reactivity of each side with incoming atoms. It is suggested that the balance of incorporation rate of In and Se atoms determines the structure of InSe domains.
Journal of Applied Physics, 2006
Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition m... more Pentacene thin film transistors (TFTs) were fabricated by the organic molecular beam deposition method. The TFTs were characterized in order to study the effect of thermal annealing on the morphology and carrier mobility of the transistors. For all the TFT samples the mobility exhibited an Arrhenius relationship with temperature, indicating a thermally activated transport that could be explained by the carrier trap and thermal release transport mechanism. Therefore, in order to investigate the annealing effect, we tested the data for a significant period of time after annealing until the temperature recovered to room temperature, so that the thermal activation effect was screened and possible effects of thermal expansion and stress were also ruled out. As a result, we found that only with a temperature below a critical temperature of approximately 45°C could annealing improve the mobility, while annealing with T>50°C would decrease the mobility compared to the value before anneal...
Journal of Applied Physics, 2005
Electrolytically polished surfaces of Ta sheets were anodized in an aqueous KI solution to form T... more Electrolytically polished surfaces of Ta sheets were anodized in an aqueous KI solution to form Ta2O5 films with a relatively high dielectric constant and a high breakdown electric field. Organic field-effect transistors (OFETs) were fabricated on these Ta2O5∕Ta substrates with a bottom gate configuration. The Ta2O5 films anodized at 100V were about 200nm thick, and the dielectric strength was as high as 5MV∕cm. A p-type pentacene OFET fabricated on this bottom gate showed the good field-effect mobility of 0.35cm2∕Vs. By using this high-k gate substrate, it becomes possible to inject a higher amount of charge carriers into organic active layers than on the SiO2∕Si gate conventionally used in most OFETs.
Journal of Applied Physics, 2007
The gate bias and temperature dependent field-effect mobility and conductance of a polycrystallin... more The gate bias and temperature dependent field-effect mobility and conductance of a polycrystalline pentacene thin film transistor (TFT) were analyzed to study the charge transport in the material. Since both heating and cooling can obviously change the film morphology, a relatively narrow temperature range was adopted to rule out the possible influence of structure variation on the device characterization. Both mobility and conductance values increased with the gate bias and showed a thermally activated Arrhenius-like behavior, while the threshold voltage deceased with temperature. Several models were compared, and it was found that the observations could only be well interpreted by a multiple trapping model, which suggests that the temperature and gate bias dependences should be attributed to the increased free charge carrier density. The density of trap states in the band gap was evaluated by the field-effect mobility as well as the field-effect conductance data. The results discl...
Japanese Journal of Applied Physics, 2010
Recently it has been shown that the “graphoepitaxy” of organic semiconductor α-sexithiophene (6T)... more Recently it has been shown that the “graphoepitaxy” of organic semiconductor α-sexithiophene (6T) occurs on artificial periodic grooves fabricated on thermally-oxidized Si substrates. In the present study, similarly oriented growth of 6T thin films was found around the edge of metal electrodes. Using this phenomenon to realize single crystalline organic transistors, substrates with short channel Au electrodes were fabricated by photolithography, and 6T films were grown on these substrates by molecular beam deposition. The {010} planes of 6T crystals tended to face the wall of the channel (electrode edge), probably due to the chemical affinity of Au for S atoms in 6T molecules. The transistor characteristics of the test device were measured, and the results suggested that this orientation control technique using the shape of electrodes would lead to improvements in device performance.
Japanese Journal of Applied Physics, 2007
The mobility of pentacene-based organic field-effect transistors (OFETs) fabricated on natural mi... more The mobility of pentacene-based organic field-effect transistors (OFETs) fabricated on natural mica substrates has been highly improved using polymethylmethacrylate (PMMA) or octadecyltrimethoxysillane (OTMS) as a buffer layer between the mica dielectric and the pentacene active layer. The field-effect mobility increased from 2.4×10-3 to 0.14 or 0.31 cm 2 ⋅V-1 ⋅s-1 by inserting the PMMA or OTMS buffer layer, respectively. The suppression of the unfavorable effect of potassium ions using the OTMS buffer layer confirmed the feasibility of using natural mica as a transparent gate dielectric of organic field-effect transistors.
Japanese Journal of Applied Physics, 2003
We present a simple but powerful method to determine the thicknesses of the accumulation and depl... more We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in-situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (VG=-15 V) and 5 nm (VG=15 V).
Chemical Physics Letters, 2006
The effect of UV/ozone treatment of the SiO 2 dielectric layer on the film morphology and the ele... more The effect of UV/ozone treatment of the SiO 2 dielectric layer on the film morphology and the electrical performance of the pentacene thin film transistors (TFTs) was investigated. Decreased threshold voltage and improved mobility were obtained after UV/ozone treatment. The effect of UV/ozone treatment of the substrate in modifying the film growth was distinguished from its effect in modifying the Au electrode by using a series of samples with different architectures. Electron spectroscopic measurement and film morphology observation indicated that the improved TFT performance was achieved mainly by the reduced contamination at the pentacene/SiO 2 interface and the increased grain size.
Applied Surface Science, 2001
We have investigated the electronic structure of K x C 60 (x = 0 ~ 6) using low-energy electron e... more We have investigated the electronic structure of K x C 60 (x = 0 ~ 6) using low-energy electron energy loss spectroscopy (LEELS), especially focusing on the K 3p core-electron excitation spectra. It is found that the structure of the K 3p-excitation spectrum of K x C 60 quite differs from that of KCl. Furthermore, the K 3p-excitation LEELS of K 3 C 60 has been revealed to be different from that of K 6 C 60. K 3p electrons are excited into K 4s-and K 3d-derived empty states in both K x C 60 and KCl, but in the case of K x C 60 the K 3d-derived empty states have a rather complicated structure where several levels are not well separated. Consequently K 3p-excitation LEEL spectra of K x C 60 show widespread plateau-like structures. The difference in the K 3p-excitation spectra of K 3 C 60 and K 6 C 60 is considered to originate from the different crystal field around K + cations in the C 60 molecular crystal.