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Journal Articles

Valence-band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics

Yamaguchi, Hisato*; Ogawa, Shuichi*; Watanabe, Daiki*; Hozumi, Hideaki*; Gao, Y.*; Eda, Goki*; Mattevi, C.*; Fujita, Takeshi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; et al.

Physica Status Solidi (A), 213(9), p.2380 - 2386, 2016/09

https://doi.org/10.1002/pssa.201532855

Times Cited Count：14 Percentile：50.32(Materials Science, Multidisciplinary)

We report valence-band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. The degree of oxygen functionalization was controlled by annealing temperature, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in the density of states around the Fermi level upon thermal annealing at 600 $^{circ}$ C. The result indicates that while there is an apparent bandgap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of bandgap closure was correlated with the electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of 500 $^{circ}$ C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to an as-synthesized counterpart.

Journal Articles

Oxidation-enhanced condensation of Ge atoms on Si $_{1-x}$ Ge $_{x}$ alloy layer studied by real-time photoelectron spectroscopy

Ogawa, Shuichi*; Hozumi, Hideaki*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Kaga, Toshiteru*; Teraoka, Yuden; Takakuwa, Yuji*

Oyo Butsuri Gakkai Hakumaku, Hyomen Butsuri Bunkakai, Shirikon Tekunoroji Bunkakai Kyosai Tokubetsu Kenkyukai Kenkyu Hokoku, p.67 - 70, 2011/01

The oxidation-enhanced Ge atoms condensation kinetics on an Si $_{1-x}$ Ge $_{x}$ alloy layer has been investigated by the real-time photoemission spectroscopy using the synchrotron radiation. The Si $_{1-x}$ Ge $_{x}$ alloy layer was formed with a thermal evaporation method on a p-type Si(001) surface, and this alloy layer was oxidized at Langmuir-type adsorption. During oxidation at 773 KC, it is found that the Ge atoms are not oxidized, only SiO $_{2}$ film is formed on the Si $_{1-x}$ Ge $_{x}$ alloy layer. Furthermore, the desorption of GeO molecules does not occur during the oxidation of alloy layer. On the other hand, not only Si atoms but also Ge atoms are oxidized at room temperature. This difference can be explained using the unified oxidation model mediated by the point defect generation, namely it is suggested that a lot of vacancies are generated during oxidation of the Si $_{1-x}$ Ge $_{x}$ alloy layer at 773K and Ge atoms diffuse through these vacancies.

Journal Articles

Oxidation-enhanced difusion of C atoms on Si $_{1-x}$ C $_{x}$ alloy layer/Si(001) surface under oxide growth and etching conditions

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

Oyo Butsuri Gakkai Hakumaku, Hyomen Butsuri Bunkakai, Shirikon Tekunoroji Bunkakai Kyosai Tokubetsu Kenkyukai Kenkyu Hokoku, p.181 - 184, 2010/01

The oxidation kinetics on the Si $_{1-x}$ C $_{x}$ alloy layer has been investigated using the real-time XPS measurement. Experiments were performed at the BL23SU of SPring-8. The Si $_{1-x}$ C $_{x}$ alloy layer was formed with exposing a p-type Si(001) surface to ethylene, and the Si $_{1-x}$ C $_{x}$ alloy layer was oxidized at Langmuir-type adsorption (773 K) and 2D oxide island growth (933 K), respectively. In case of Langmuir-type adsorption, it is found that no carbon atoms are oxidized and carbon concentration at the SiO $_{2}$ /Si interface increases. These results indicate the carbon atom condensation occurs, leading to the SiO $_{2}$ /Si $_{1-x}$ C $_{x}$ /Si layers formation. On the other hand, the carbon concentration decrease by 20% in spite of the etching of 38 Si layers in the 2D oxide island growth. Based on these results, it is found that the diffusion of carbon atoms is occurred due to not only oxide growth but also Si etching.

Journal Articles

Real-time photoelectron spectroscopy study of 3C-SiC nucleation and growth on Si(001) surface by carbonization with ethylene

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

JSPS 141 Committee Activity Report, p.317 - 322, 2009/12

It is reported that Si $_{1-x}$ C $_{x}$ alloy layer was formed on an Si(001) surface with C $_{2}$ H $_{4}$ exposure at 933 K before nucleation. However information of the chemical bonding state and concentration of adsorbed carbon has not been clear. Therefore the carbonization reaction kinetics on an Si(001) surface reacted with C $_{2}$ H $_{4}$ exposure was observed by real-time XPS to investigate the 3C-SiC nucleation kinetics. The experiments were performed at the BL23SU of SPring-8. It is suggested that the C1s spectra is composed of at least three chemically-shifted components, which are assigned to carbon-poor Si $_{1-x}$ C $_{x}$ alloy, carbon-rich Si $_{1-x}$ C $_{x}$ alloy and 3C-SiC. The 3C-SiC nuclei began to generate at 8000s. Using C1s and Si2p $_{3/2}$ peaks, carbon concentration of the Si $_{1-x}$ C $_{x}$ alloy layer was estimated to be about 0.17 of x. Therefore it is suggested that critical carbon concentration is 17%.

Oral presentation

Oxidation kinetics of SiGe alloy layer studied by real-time XPS

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

The oxidation of SiGe alloy layer is used for the concentration process of Ge in the CMOS devices. In this study, the oxidation reaction kinetics on SiGe alloy layers was investigated by real-time XPS to reveal the oxidation and Ge concentration kinetics. Experiments were performed at the BL23SU of SPring-8. Ge was deposited on the Si(001) surface at room temperature. After deposition, the sample was annealed at 933 K for 20 min. and then oxidized using oxygen gas at 773 K. The O 1s, Si 2p and Ge 3d photoelectron spectra were measured during oxidation. O 1s photoelectron intensity increased rapidly and then saturate after 2000 s. The oxidation rate on the SiGe alloy surface was slower than that of the Si clean surface. However, SiO $_{2}$ component on the SiGe alloy surface increased. It was found that Ge was not oxidized. These results indicate that Ge atoms diffuse into Si substrate. This Ge diffusion may be caused by the oxidation-induced strain.

Oral presentation

Temperature dependence of exclusive SiO $_{2}$ formation during thermal oxidation of Si $_{1-x}$ Ge $_{x}$ alloy layer on Si(001) surfaces

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

In order to clarify the concentration mechanism of Ge atoms for strained-Si CMOS devices, the oxidation reaction kinetics of Si $_{1-x}$ Ge $_{x}$ alloy layer was observed by real-time XPS. By comparison of the oxidation reaction kinetics on the Si $_{1-x}$ Ge $_{x}$ alloy layer among two oxidation temperature (room temperature and 773 K), it was found that Ge was significantly oxidized at room temperature, although SiO $_{2}$ was exclusively formed at 773 K. This result indicates that the oxidation rate of Ge atoms depends of the diffusion of Ge atoms due to oxidation-induced strain.

Oral presentation

Real-time XPS observation of oxidation processes on Si alloy surfaces; Comparison of mixing effects of C and Ge

Hozumi, Hideaki*; Kaga, Toshihide*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

In order to clarify the difference of the oxidation reaction mechanism between an Si $_{1-x}$ Ge $_{x}$ and Si $_{1-x}$ C $_{x}$ alloy layer, the real time photoelectron spectroscopy was employed for observing the oxidation rate and behavior of Ge and C atoms during oxidation. The experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. By comparing between the both alloy surfaces, it is found that initial oxidation rate on the SiGe alloy was slower than that on the SiC alloy. Here, only Si atoms were oxidized on the both alloy surfaces. C atoms were condensed at SiO $_{2}$ /Si interface and 3C-SiC alloy were formed. Ge atoms were diffused into Si substrate. These facts are resulted in the oxidation-strain-induced point-defect generation, in which C and Ge atoms are exchanged for Si atoms through the vacancies.

Oral presentation

Real-time synchrotron radiation photoelectron spectroscopic observation of high temperature thermal anneal processes of oxidized graphene

Hozumi, Hideaki*; Yamaguchi, Hisato*; Kaga, Toshihide*; Eda, Goki*; Mattevi, C.*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.

no journal, ,

In order to clarify the time evolution of the chemical bonding state during thermal reduction of graphene oxide (GO), real-time photoelectron spectroscopy was employed for observing the thermal reduction kinetics of GO. The GO was prepared by the modified Hummer method. The experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. The XPS measurements were performed simultaneously during the annealing at 473 K, 673 K, 873 K, and 1073 K. The C1s photoelectron spectra are decomposed by 8 components. The - transition loss peak intensity is propotional to the intensity of sp $^{2}$ graphene components with temperature elevation. In addition, defect intensity increased in proportion with the sp $^{2}$ graphene intensity. These facts indicate that defects were formed on the graphene during reduction and these defects cause the recovery of electric conductivity, that is, the appearance of Fermi edge.

Oral presentation

Real time XPS observation of oxidized graphene reduction processes

Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Kaga, Toshihide*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.

no journal, ,

no abstracts in English

Oral presentation

Experimental verification of the unified Si oxidation model, 4; Dependence on surface oxidation reaction manner

Ogawa, Shuichi*; Hozumi, Hideaki*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

In this study, real-time XPS observation of Si(001) surface oxidation is employed to investigate the surface oxidation reaction manner dependence of SiO $_{2}$ /Si interface and growth rate of the second oxide layer. Oxidation experiments were performed at the surface reaction analysis apparatus placed at BL23SU, SPring-8. A p-type Si(001) surface was oxidized by dry oxygen. In the Langmuir-type adsorption, the amount of strained Si atoms at the end of surface oxidation is 0.46 ML, and then significant interface oxidation can be observed. On the other hand, in the 2D oxide island growth, the amount of strained Si atoms is 0.15 ML and progress of interface oxidation hardly observed. These results indicate that when interface strain is large, the interface oxidation rate also large, and then that strongly depends on Si surface oxidation manner.

Oral presentation

Real-time XPS observation of carbonization reactions on Si(001) surface with ethylene

Takakuwa, Yuji*; Ogawa, Shuichi*; Hozumi, Hideaki*; Kawamura, Tomofumi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden

no journal, ,

In order to investigate the surface reaction kinetics on Si $_{1-x}$ C $_{x}$ alloy layer, the carbonization reaction on Si(001) surface by ethylene using the real-time XPS observation by synchrotron radiation. Experiments were performed at the surface reaction analysis apparatus placed at BL23SU, SPring-8. A p-type Si(001) surface heated as 913 K was carbonized using C $_{2}$ H $_{4}$ diluted by He. It is found from Si 2p photoelectron spectra that Si-C bounding component increases in proportion as dosage of C $_{2}$ H $_{4}$ , and same tendency is observed from dosage dependence of C 1s photoelectron intensity. These results indicate that the dissociative adsorption of C $_{2}$ H $_{4}$ on Si(001) surface is zero-order reaction, and the activation energy of diffusion of adsorbed C atoms into Si subsurface is very small.

Oral presentation

Real-time XPS measurements for carbonization and oxidation processes at Si(001) surfaces

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

In this study, real-time XPS was applied to investigate the effect of lattice strain due to the dimerization on an Si(001) surface. The Si $_{1-x}$ C $_{x}$ alloy layer was formed on a p-type Si(001) surface by C $_{2}$ H $_{4}$ exposure. Carbonization and oxidation reactions were observed by XPS to investigate the oxidation rate and behavior of C atom. Experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. The photoelectron spectroscopy measurement of C1s, O1s and Si2p was performed during carbonization and oxidation on an Si(001) surface. Oxidation rate on carbonized Si(001) surface was faster than that on clean Si(001) surface. Since C1s intensity was almost constant during oxidation, it is suggested that C atoms did not exist in SiO $_{2}$ and diffuse into the Si substrate.

Oral presentation

Effects for Si oxidation mechanisms by carbon atoms in the SiO $_{2}$ /Si(001) interface

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

The oxidation kinetics of Si $_{1-x}$ C $_{x}$ layer prepared with C $_{2}$ H $_{4}$ exposure was observed by real-time XPS to investigate effects of substrate stress due to displacement of hetero atom on a p-type Si(001) surface. Experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. Oxidation rate on the Si $_{1-x}$ C $_{x}$ layer was larger than that of a clean surface. It was found that carbon atoms diffused into substrate without CO or CO $_{2}$ desorption during oxidation. Furthermore "an unified Si oxidation reaction model mediated by point defect generation" could explain these results with correlation.

Oral presentation

Real-time observation for carbonation and oxidation processes at Si(001) surface by high briliance synchrotron radiation XPS

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

We investigated effects of lattice strain due to displacement of hetero atoms on a p-type Si(001) surface during oxidation. Experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. In order to change the lattice strain, an Si $_{1-x}$ C $_{x}$ layer was formed by carbonization with C $_{2}$ H $_{4}$ exposure. Oxidation rate on the carbonized surface was faster than that on a clean surface. C1s photoemission intensity normalized by Si bulk intensity was almost constant during oxidation at the surface and gradually decreased at the interface. It was suggested that C atoms did not exist in the SiO $_{2}$ layer and diffused into the Si substrate, and the growth of oxide was related with the normalized C1s intensity.

Oral presentation

Role of strain in SiO $_{2}$ /Si interfacial oxidation, 2; Carbon diffusion during 2D oxide island growth

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

The oxidation reaction of Si $_{1-x}$ C $_{x}$ alloy layer which was formed with C $_{2}$ H $_{4}$ exposure on a p-type Si(001) surface was observed by real-time XPS to investigate the role of lattice strain. Experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. When Si $_{1-x}$ C $_{x}$ alloy layer was oxidized at 660 $^{circ}$ C, oxidation mode was 2D oxide island growth. Etching advanced as well as oxidation films growth. Nearly 45 layers were etched for 7000 s from oxidation start regardless of oxide coverage was nearly 3 percents. This suggested SiO desorption occurred. In this result, Si $_{1-x}$ C $_{x}$ alloy layer that was formed on surface 4th layers should be completely rejected, but C 1s intensity that was normalized by Si 2p bulk intensity was constant. Therefore it was suggested carbon atoms diffused into Si substrate in SiO desorption.

Oral presentation

Enhancement of carbon diffusion caused by thermal oxidation on Si $_{1-x}$ C $_{x}$ alloy layer/Si(001) surfaces

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

In this study, to clarify the behavior of C on the basis of the oxidation kinetics, the oxidation kinetics of an Si $_{1-x}$ C $_{x}$ alloy layer was investigated by real-time photoelectron spectroscopy. The Si $_{1-x}$ C $_{x}$ alloy layer was formed by exposing an Si(001) surface to ethylene (C $_{2}$ H $_{4}$ ). Although the solubility of C in an Si bulk is as small as about 3.5 10 $^{17}$ cm $^{-3}$ , it is reported that the solubility of C can be increased by about 10 $^{4}$ times due to the dimerization-induced strain. This makes it possible to form an Si $_{1-x}$ C $_{x}$ alloy layer on the Si(001) surface with x up to 0.2 but within only the first several Si layers. From a comparison between the changes in the depth profile of C and oxygen uptake, the enhancement of C diffusion is considered in terms of the oxidation-induced strain.

Oral presentation

Strained Si atoms at SiO $_{2}$ /Si interface during oxidation of Si $_{1-x}$ C $_{x}$ alloy layer on Si(001) surfaces

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

The oxidation of Si $_{1-x}$ C $_{x}$ surface was studied by real-time X-ray photoelectron spectroscopy using synchrotron radiation to clarify the correlation between oxidation-induced strain and oxidation kinetics. Firstly, a p-type Si(001) surface was carbonized by C $_{2}$ H $_{4}$ . Then, the surface was oxidized at 773 K under 5.010 $^{-5}$ Pa of O $_{2}$ . After saturation, the O $_{2}$ pressure was increased to 1.310 $^{-3}$ Pa. Si-2p, O-1s, and C-1s photoelectron spectra were measured alternately during oxidation. From the time evolution of C-1s/Si-2p(Si $^{0}$ ) ratio, it was found that the Si $^{0}$ composes of bulk and strained Si atoms. The increase of C-1s/Si-2p(Si $^{0}$ ) ratio suggests the concentration of carbon at the interface. After O $_{2}$ increase, the interface oxidation proceeds. The rate and the oxidation-induced strain decreased with oxidation. We concluded the interface oxidation rate is enhanced by oxidation-induced strain.

Oral presentation

Time duration of 3C-SiC nucleation in surface carbonization of Si(001); Critical carbon concentration

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

The 3C-SiC formation on the Si(001) surface by ethylene has been studied. The facts that 3C-SiC nucleation takes place after incuvation time and four Si $_{1-x}$ C $_{x}$ alloy layers are formed during the incuvation time. In this study, critical carbon concentration for SiC nucleation has been investigated. Time evolution of Si2p and C1s photoemission peaks were observed by real-time synchrotron radiation photoemission spectroscopy during exposure Si(001) substrate to ethylene gas at 913 K. Consequently, 3C-SiC nucleation takes place at carbon concentration of 15%.

Oral presentation

Real-time XPS study of reduction of graphene oxide; Effect of hydrazine treatment

Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; Chhowalla, M.*; et al.

no journal, ,

Oral presentation

Carbon condensation and 3C-SiC growth caused by oxidizing Si $_{1-x}$ C $_{x}$ alloy layers on Si(001) substrate

Hozumi, Hideaki*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Harries, J.; Teraoka, Yuden; Takakuwa, Yuji*

no journal, ,

Strained-Si channel complementary metal-oxide-semiconductor (CMOS) devices has been developed to improve the carrier mobility. In such CMOS devices, the channel layer is formed on an Si $_{1-x}$ Ge $_{x}$ or by an Si $_{1-x}$ C $_{x}$ alloy layer. In the case of Si $_{1-x}$ Ge $_{x}$ alloy layer, the oxidation of alloy layer is employed for Ge concentration. On the other hand, the detail of oxidation kinetics on the IV group alloy layer has not been cleared yet. In this study, the oxidation reaction kinetics on an Si $_{1-x}$ C $_{x}$ alloy layer was investigated by real-time X-ray photoelectron spectroscopy to reveal the oxidation rate and behavior of C atoms during oxidation.