K. Namba

Bio: K. Namba is an academic researcher from Osaka University. The author has contributed to research in topics: X-ray photoelectron spectroscopy & Band gap. The author has an hindex of 4, co-authored 6 publications receiving 104 citations.

Spectroscopic observation of interface states of ultrathin silicon oxide

Abstract: Interface states in the Si band gap present at oxide/Si(100) interfaces for ∼3‐nm‐thick Pt/2.1∼3.6‐nm‐thick silicon oxide/n‐Si(100) metal–oxide–semiconductor devices are investigated by measurements of x‐ray photoelectron spectra under biases between the Pt layer and the Si substrate, and their energy distribution is obtained by analyzing the amount of the energy shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. All the interface states observed using this new technique have discrete energy levels, showing that they are due to defect states. For the oxide layer formed in H2SO4+H2O2, the interface states have three density maxima at ∼0.3, ∼0.5, and ∼0.7 eV above the valence‐band maximum (VBM). For the oxide layer produced in HNO3, two density maxima appear at ∼0.3 and ∼0.7 eV above the VBM. The energy distribution for the oxide layer grown in HCl+H2O2 has one peak at ∼0.5 eV. The 0.5 eV interface state is attributed to the isolated Si dangling bond defect. The 0.3 and 0.7 eV ...

New method for determination of energy distribution of surface states in the semiconductor band-gap: XPS measurements under biases

Abstract: The energy distribution of surface states in the semiconductor bandgap has been obtained spectroscopically from measurements of X-ray photoelectron spectra (XPS) under biases. The XPS measurements are performed for [ca. 30 A - thick Pt ca. 25 A - thick oxide /p- InP (1 0 0)] MIS diodes. The difference in binding energy between the In(3d) and Pt(4f) peaks, EInEPt, is changes by applying biases, because of accumulation of charges in the surface states. The energy distribution of the surface states is determined by analyzing the amount of the biasinduced shift in EInEPt. The density of the surface states is high near the mid-gap, near the conduction band minimum and near the valence band maximum.

Effect of chemical oxide layers on platinum-enhanced oxidation of silicon

Abstract: Si oxidation promoted by a platinum (Pt) overlayer has been investigated using x-ray photoelectron spectroscopy and synchrotron radiation ultraviolet photoelectron spectroscopy. Heat treatments of the specimens with 〈∼5-nm-Pt/0.5–1-nm-chemical oxide/Si(100)〉 structure at 300–400 °C increase the oxide thickness to 4–5 nm. The amounts of the suboxide species, a(Si+), a(Si2+), and a(Si3+), in the chemical oxide layers formed in hydrochloric acid (HCl) plus hydrogen peroxide (H2O2) are in the order of a(Si+)>a(Si2+)>a(Si3+), while those for the oxide layers formed in nitric acid (HNO3) have an order of a(Si3+)>a(Si2+)≈a(Si+). The amounts of the suboxide species in the former oxide layers are much higher than those in the latter oxide layers. These results indicate that the HNO3 oxide layers are more highly oxidized, probably resulting in a higher atomic density and a lower defect density. Although the initial chemical oxide layers formed in HCl+H2O2 are thinner than those grown in HNO3, the former oxide layer...

Interface state‐induced shift of the oxide and semiconductor core levels for metal–oxide–semiconductor devices

Abstract: Measurements of x‐ray photoelectron spectra are performed for ∼3‐nm‐thick Pt/∼3.6‐nm‐thick silicon oxide/n‐Si(100) devices under biases between the Pt layer and the Si substrate. It is observed that the oxide Si 2p peak as well as the substrate peaks is shifted upon applying biases. These shifts are caused by a bias‐induced change of the potential drop across the oxide layer due to the change in the amount of the interface state charge. The amount of the shift of the oxide Si 2p peak is well correlated to that of the substrate Si 2p3/2 peak. The energy distribution of the interface states is obtained by analyzing the amount of the shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. The interface state spectrum has one peak near the midgap, and the peak is attributed to isolated Si dangling bond states.

Interface states in the Si band-gap obtained from XPS measurements under biases

Abstract: Measurements of X-ray photoelectron spectra were performed for 〈∼3 nm Pt/2.5–3.6 nm SiO 2 / n -Si(100)〉 biases between the Pt layer and the Si substrate. The Si 2p peaks due to the Si substrate and the oxide layer are both shifted by biasing, the magnitude of the shift of the former peak being much larger than that of the latter. The shifts are well correlated to each other, and are attributed to a change in the potential drop across the oxide layer caused by charges in interface states. Using the method developed by us, the interface states are found to have discrete energy levels with energies depending on the formation method of the silicon oxide layers, and are attributed to Si dangling bonds in various environments.

Surface photovoltage phenomena: theory, experiment, and applications

Abstract: The theoretical concepts, experimental tools, and applications of surface photovoltage (SPV) techniques are reviewed in detail. The theoretical discussion is divided into two sections. The first reviews the electrical properties of semiconductor surfaces and the second discusses SPV phenomena. Next, the most common tools for SPV measurements and their relative advantages and disadvantages are reviewed. These include the Kelvin probe and the use of MIS structures, as well as other less used techniques. Recent novel high-spatial-resolution SPV measurement techniques are also presented. Applications include surface photovoltage spectroscopy (SPS) which is a very effective tool for gap state spectroscopy. An in-depth review of quantitative analyses, which permit the extraction of various important surface and bulk parameters, follows. These analyses include: carrier diffusion length; surface band bending, charge, and dipole; surface and bulk recombination rates; surface state distribution and properties; distinction between surface and bulk states; spectroscopy of thin films, heterostructures and quantum structures; and construction of band diagrams. Finally, concluding remarks are given.

Nitric acid oxidation of Si to form ultrathin silicon dioxide layers with a low leakage current density

Abstract: Ultrathin silicon dioxide (SiO2) layers with excellent electrical characteristics can be formed using the nitric acid oxidation of Si (NAOS) method, i.e., by immersion of Si in nitric acid (HNO3) solutions. The SiO2 layer formed with 61 wt % HNO3 at its boiling temperature of 113 °C has a 1.3 nm thickness with a considerably high density leakage current. When the SiO2 layer is formed in 68 wt % HNO3 (i.e., azeotropic mixture with water), on the other hand, the leakage current density (e.g., 1.5 A/cm2 at the forward gate bias, VG, of 1 V) becomes as low as that of thermally grown SiO2 layers, in spite of the nearly identical SiO2 thickness of 1.4 nm. Due to the relatively low leakage current density of the NAOS oxide layer, capacitance–voltage (C–V) curves can be measured in spite of the ultrathin oxide thickness. However, a hump is present in the C–V curve, indicating the presence of high-density interface states. Fourier transformed infrared absorption measurements show that the atomic density of the SiO...

Measuring the role of surface chemistry in silicon microphotonics

Abstract: Utilizing a high quality factor (Q~1.5×10^6) optical microresonator to provide sensitivity down to a fractional surface optical loss of alpha s [prime] ~10^–7, we show that the optical loss within Si microphotonic components can be dramatically altered by Si surface preparation, with alpha s [prime] ~1×10^–5 measured for chemical oxide surfaces as compared to alpha s [prime] <=1×10^–6 for hydrogen-terminated Si surfaces. These results indicate that the optical properties of Si surfaces can be significantly and reversibly altered by standard microelectronic treatments, and that stable, high optical quality surface passivation layers will be critical in future Si micro- and nanophotonic systems.

Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy

Abstract: Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell.

XPS study of chemically etched GaAs and InP

Abstract: The surface composition of p‐type GaAs etched in HCl or Br2 in methanol, and n‐type InP etched in HCl, H2SO4, HNO3, or Br2 in methanol has been studied by means of x‐ray photoelectron spectroscopy (XPS). The surface compositions of GaAs and the binding energy of the surface As atoms vary with the etching solution and with the extent of oxidation of the surface. The full width at half‐maximum of the Ga(3p) photoelectron peak increases upon exposure of etched GaAs to air. The XPS results are compared with Schottky barrier heights previously measured for similarly prepared surfaces with Pb contacts. The amount of oxidized P on InP surfaces is higher after an HNO3 etch than after HCl, H2SO4, or Br2/methanol treatments. An HCl etch leaves an unoxidized slightly In‐rich surface.