Yuji Takakuwa

Photoemission study of the SiO2 /Si interface structure of thin oxide films on Si(100), (111), and (110) surfaces

The SiO2/Si interface structure of thin oxide films thermally grown on Si(100), (111), and (110) surfaces under device processing conditions has been investigated using high‐resolution photoemission spectroscopy with synchrotron radiation. The intensity distribution of the so‐called suboxides, Si1+, Si2+, Si3+, displays a strong dependence on the crystallographic orientation of the substrate over the oxidation temperature range from 600 to 900 °C; Si1+ is enhanced in intensity on Si(111) and (110), while the Si2+ intensity is larger than the Si1+ one on Si(100). This orientation dependence is explained in terms of the bond topology of the substrate surface. A Si(110) surface exhibits a rather large Si3+ intensity as compared to Si(100), (111) surfaces, suggesting that Si–Si bonds on the outermost layer of a Si(110) surface are easily broken by oxygen atoms to generate the bridge bond Si–O–Si. The presence of an interfacial Si atom to which hydrogen is bonded is clearly observed. The total suboxide intensity, i.e., the sum of the suboxide intensities depends on both oxidation temperature and substrate orientation, which is interpreted by means of the interfacial roughness and the surface Si atom density of the substrate. It is shown that an ordered crystalline phase of SiO2 is present at the interfacial region. The generation of this phase has a Si(111)‐preferred orientation.

Thermal oxidation of outdiffusing SiO with permeating O2 in a SiO2 film studied by angle-resolved X-ray photoelectron spectroscopy

Abstract During annealing in vacuum and dry O 2 , the amount of suboxide, Θ s , in the 1000 A SiO 2 films on Si(111) was measured as a function of the annealing time by angle-resolved X-ray photoelectron spectroscopy (ARXPS). From ARXPS spectra of Si 2p taken at two polar angles of 0° (bulk sensitive) and 75° (surface sensitive), it was clarified that during annealing in vacuum, Θ s increases with time underneath the surface more largely than on the surface, but annealing in O 2 atmosphere at 0.5 Torr reduces Θ s on the surface more significantly than underneath the surface. These indicate that SiO molecules outdiffusing from the SiO 2 /Si interface are oxidized by permeating O 2 in the SiO 2 films.

Two-dimensional growth and decomposition of initial thermal SiO2 layer on Si(100)

The growth and decomposition kinetics of initial thermal oxide layers on Si(100) were investigated by in situ ultraviolet photoelectron spectroscopy (UPS). From a time evolution of the correlation between the amount of oxide and the clean area observed, it was found that the initial thermal oxidation as well as the decomposition proceed two-dimensionally with the formation of oxide islands until the clean surface is fully covered with oxides, after which the oxides grow deeply toward the substrate with a very slow oxidation rate. The oxide island thickness was evaluated from the X-ray photoelectron spectra of Si-2p to be 7.1 A. The two-dimensional oxidation, in which the oxide growth predominantly occurs at the periphery of oxide islands, means that adsorbed oxygen atoms migrate on the clean surface to be captured at the periphery of the oxide islands, resulting in the two-dimensional enlargement of islands, and be desorbed as SiO during the initial thermal oxidation.

n-type high-conductive epitaxial diamond film prepared by gas source molecular beam epitaxy with methane and tri-n-butylphosphine

An n-type phosphorous (P) doped epitaxial diamond film with high conductivity was grown on a C(001) substrate by gas source molecular beam epitaxy using methane and tri-n-butylphosphine. The electrical conductivity of the diamond film was measured to be 0.33 (Ω cm)−1 at 23 °C with its activation energy to be 0.12 eV. The Hall measurements showed n-type conduction and a carrier concentration of 1.6×1018 cm−3 at 400 °C, which is comparable to the P concentration determined by secondary-ion-mass spectroscopy. These indicate the formation of shallow P donors with high electrical activation efficiency. A p-n junction diode was fabricated by growing a P-doped epitaxial film on a boron-doped C(001) substrate, which showed for the first time a rectification ratio of ∼103 at 10 V.

Electron emission mechanism of diamond characterized using combined x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy system

Clarification on electron emission mechanism of diamond is one essential approach to realize the clear vision of vacuum nanoelectronics. Electric field of less than 5V∕μm is enough to extract electrons from diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from conventional metal emitter tips. Diamond has various advantages as an electron emitter in addition to the low-threshold voltage, such as negative electron affinity and high thermal conductivity. The difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in a practical way. In this study, combined spectroscopy of x-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy/field emission spectroscopy was performed to characterize the electron emission mechanism of diamond. The results indicated the first successful observation of applied voltage dependence on the origin of field-emitted electrons.

Real-time monitoring of oxidation on the Ti(0 0 0 1) surface by synchrotron radiation photoelectron spectroscopy and RHEED–AES

The oxidation kinetics on the Ti(0 0 0 1) single crystal surface was investigated by real-time Auger electron spectroscopy combined with reflection high-energy electron diffraction (RHEED-AES) and high-resolution photoelectron spectroscopy using synchrotron radiation. The RHEED specular reflection spot intensity showed an oscillatory behavior, at the maxima and minima of which the oxygen uptake curve obtained by O KLL Auger electron intensity shows breaks. This agreement indicates that growth of oxides progresses with changing the surface morphology and the oxygen uptake rate. The O 2 dose dependence of Ti 2p photoelectron spectrum measured at photon energies of 574.6 eV (surface sensitive) and 1549 eV (bulk sensitive) revealed that (1) oxidation of metallic Ti atoms with growing Ti 2 O and TiO on the surface makes the surface morphology roughened up to ∼17 L; (2) the roughened morphology is recovered by oxidation of Ti 2 O and TiO into Ti 2 O 3 , Ti 3 O 5 and TiO 2 up to ∼45 L; (3) after almost saturation of TiO 2 growth in the surface layer the surface morphology is roughened again by further oxidation at the interface up to ∼140 L; and (4) then the oxygen uptake rate is considerably suppressed due to the passivation effect of grown oxides.

Real-time monitoring of the Si carbonization process by a combined method of reflection high-energy electron diffraction and Auger electron spectroscopy

The carbonization process of a preferential-domain Si(001)2×1 surface with ethylene was investigated by a combined method of reflection high-energy electron diffraction and Auger electron spectroscopy. It is found that the carbonization process during the so-called incubation time is the Si1−xCx alloy formation before the nucleation of 3C–SiC grains. A reaction model for the Si1−xCx alloy formation and for the 3C–SiC grain growth is proposed for substrate temperatures of 600–750 °C. From the model, we postulate that the external supply of Si and C should be started just at the completion of the lateral 3C–SiC grain growth at temperatures of 600–650 °C in order to obtain thick 3C–SiC layers with a flat surface morphology.

Development of an Amorphous Selenium-Based Photodetector Driven by a Diamond Cold Cathode

Amorphous-selenium (a-Se) based photodetectors are promising candidates for imaging devices, due to their high spatial resolution and response speed, as well as extremely high sensitivity enhanced by an internal carrier multiplication. In addition, a-Se is reported to show sensitivity against wide variety of wavelengths, including visible, UV and X-ray, where a-Se based flat-panel X-ray detector was proposed. In order to develop an ultra high-sensitivity photodetector with a wide detectable wavelength range, a photodetector was fabricated using a-Se photoconductor and a nitrogen-doped diamond cold cathode. In the study, a prototype photodetector has been developed, and its response to visible and ultraviolet light are characterized.

Outdiffusion and Subsequent Desorption of Volatile SiO Molecules during Annealing of Thick SiO2 Films in Vacuum

The depth profile of so-called suboxides in the surface region for 1000-A-thick SiO 2 films annealed in vacuum at 800°C was measured by angle-resolved X-ray photoelectron spectroscopy. It was found that the amount of suboxide distributed under the surface increases rapidly and almost saturates with annealing time, while the change in the amount of suboxide present on the surface is very small and increases gradually with annealing time. This indicates that very fast outdiffusion of SiO molecules through thick SiO 2 films takes place at as low as 800 o C, leading to desorption from the SiO 2 surface

Rate-Limiting Reactions of Growth and Decomposition Kinetics of Very Thin Oxides on Si(001) Surfaces Studied by Reflection High-Energy Electron Diffraction Combined with Auger Electron Spectroscopy

The growth and decomposition kinetics of very thin oxides on Si(001) surfaces were investigated by reflection high-energy electron diffraction combined with Auger electron spectroscopy (RHEED–AES) to monitor the reaction rates of oxide growth and decomposition in real time, and changes in surface structure and interface morphology simultaneously. The oxides prepared by stopping the second oxide layer growth at various coverages following the first oxide layer growth by Langmuir-type adsorption at 500 °C under an O2 pressure of 2.6×10-4 Pa were isothermally annealed by increasing temperature from 500 to 666 °C at the same time that O2 gas was quickly evacuated. It was observed that (1) oxide thickness continued to decrease significantly as measured on the basis of changes in O KLL Auger electron intensity ΔIO-KLL before voids appeared at a nucleation time tN as recognized by the appearance of half-order spots in RHEED patterns, (2) the SiO2/Si interface morphology was considerably roughened corresponding to ΔIO-KLL during void nucleation as observed by the evolution of the RHEED intensity of bulk diffraction spots ΔIbulk, (3) the ratio of ΔIbulk to ΔIO-KLL was almost constant, and (4) oxide decomposition rate during void nucleation, which was approximately given by ΔIO-KLL/tN, showed a good linear correlation with the second oxide layer growth rate α immediately before starting the decomposition reaction. The good correlation between ΔIO-KLL/tN and α clearly indicates that the rate-limiting reaction of the second oxide layer growth is closely related to that of the oxide decomposition during void nucleation. All the above-mentioned observations can be comprehensively interpreted using a reaction model proposed for the rate-limiting reactions of oxide growth and decomposition, in which the point defect generation (emitted Si atoms + vacancies) caused by the strain due to the volume expansion of oxidation plays a crucial role because of the high reactivity of the emitted Si atoms and vacancies with dangling bonds. Under an O2 atmosphere, both emitted Si atoms and vacancies are the preferential adsorption sites of O2 molecules in the oxide and at the SiO2/Si interface, respectively. The oxide can be decomposed by the emitted Si atoms to produce SiO molecules, which desorb from the surface, leading to oxide removal in vacuum with SiO2/Si interface morphology roughening, but may be oxidized within the oxide under an O2 atmosphere.