Y. Nakato

Improvement of indium-tin-oxide/silicon oxide/ n -Si junction solar cell characteristics by cyanide treatment

The performance of 〈indium-tin-oxide (ITO)/silicon oxide/n-Si(100)〉 junction solar cells is improved by immersing Si wafers in a potassium cyanide solution before the ITO deposition. It is found from x-ray photoelectron spectroscopy measurements that about 3% monolayer cyanide (CN−) ions are present on the Si surface after the cyanide treatment. The temperature dependence of the current–voltage curves shows that the band bending in n-Si is increased by the cyanide treatment. The increase in the band bending is attributed to an upward Si band edge shift caused by the presence of CN− ions at the oxide/Si interface and/or in the oxide layer. Conductance–voltage measurements show that the density of trap states considerably decreases after the cyanide treatment. The conductance decrease is attributed to the passivation of interface states by the adsorption of CN− ions on Si dangling bonds.

Fermi level pinning on HF etched silicon surfaces investigated by photoelectron spectroscopy

A widely used approach to obtain smooth oxide-free and (partially) H-terminated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CH3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solutions of varying concentrations. It is usually assumed that such treatments result in a dramatic reduction of the surface density of states and that, therefore, no surface band bending can occur. In our experiments we investigated the electronic surface structure of a number of CP4A/HF treated n- and p-Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconductor materials because the positions of the core level peaks directly depend on the position of the Fermi level within the band gap at the surface. Our experiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in th...

Nitridation of silicon oxide layers by nitrogen plasma generated by low energy electron impact

Low temperature nitridation of silicon oxide layers by nitrogen plasma generated by electron impact is investigated using x-ray photoelectron spectroscopy (XPS) and synchrotron radiation ultraviolet photoelectron spectroscopy and it is found that a large amount of nitrogen can be incorporated in the layers. The valence band structure of the oxide surface nitrided at 25 °C is similar to that of Si3N4, while that nitrided at 700 °C resembles the mixture of silicon oxide and silicon oxynitride. Measurements of XPS depth profiles show that the nitrogen concentration is high near the surface and the oxide/Si interface.

LOW TEMPERATURE CATALYTIC FORMATION OF SI-BASED METAL-OXIDE-SEMICONDUCTOR STRUCTURE

Si‐based metal–oxide–semiconductor structure is formed at temperatures as low as 300 °C using the catalytic activity of the platinum (Pt) layer. X‐ray photoelectron spectroscopy and transmission electron micrography measurements show that heat treatments of the ∼5 nm‐Pt/∼1 nm‐chemical oxide/Si(100)〉 devices at 300 °C increase the thickness of the oxide layer to 4–4.5 nm and the oxide layer is present between the Pt layer and the Si substrate, but not on the Pt surface. It is found that the thin chemical oxide layer effectively prevents the Pt diffusion and the silicide formation during the heat treatments. Heat treatments in dry‐ and wet‐oxygen result in nearly the same oxide thickness. Oxygen atoms (or oxygen ions) produced at the Pt surface are suggested to be a diffusing species through the Pt and silicon oxide layers.

Modification of semiconductor surface with ultrafine metal particles for efficient photoelectrochemical reduction of carbon dioxide

A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-type electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency.

Mechanism of carrier transport through a silicon‐oxide layer for 〈indium‐tin‐oxide/silicon‐oxide/silicon〉 solar cells

The mechanism of carrier transport through a thin silicon‐oxide layer for 〈spray‐deposited indium‐tin‐oxide (ITO)/silicon‐oxide/Si〉 solar cells has been studied by measurements of the dark current density as a function of the thickness of the silicon‐oxide layer, together with the observation of transmission electron micrographs. Cross‐sectional transmission electron micrography shows that a uniform silicon‐oxide layer with the thickness of ∼2 nm is present between ITO and Si when the ITO film is deposited on a flat Si(100) surface at 450 °C. The dark current density under a depletion condition strongly depends on the thickness of the silicon‐oxide layer. It is concluded from these results that quantum mechanical tunneling is the dominant mechanism for the charge carrier transport through the silicon‐oxide layer. On the other hand, when the ITO film is deposited on a mat‐textured Si surface at the same temperature, a nonuniform silicon‐oxide layer is formed, with ITO penetrating into the silicon‐oxide lay...

Hole diffusion length and temperature dependence of photovoltages for n-Si electrodes modified with LB layers of ultrafine platinum particles

The mechanism of generation of high open-circuit photovoltages (Vocs) of 0.62–0.63 V for n-Si (~ 1 Ω cm) electrodes modified with colloidal Pt particles (4 nm in diameter) is investigated by measurements of minority-carrier (hole) diffusion length (Lp) and temperature dependence of Voc. Langmuir-Blodgett (LB) layers of colloidal Pt particles are used to control the Pt density on n-Si. The Lp value is determined to be 200 μm, irrespective of whether n-Si is modified with Pt or not. The temperature dependences of Vocs at 203–298 K have been explained well by our previously proposed model. It is shown that heat treatments of the Pt-modified n-Si electrodes increase the area and the width of the direct Pt-Si contacts and thus decrease Voc, but minority-carrier controlled (ideal) solar cells are obtained if the electrodes are prepared under appropriate conditions.

Improvement of Electrical Characteristics of 〈Indium‐Tin Oxide/Silicon Oxide/Polycrystalline n‐Si〉 Solar Cells by a KCN Treatment

The photovoltage and the fill factor of indium-tin oxide (ITO)/silicon oxide/polycrystalline n-Si junction solar cells are increased by immersing Si in a potassium cyanide solution before the deposition of an ITO film. Measurements of the temperature dependence of the dark current-voltage curves show that the mechanism of the current flow through the Si depletion layer is changed from trap-assisted multistep tunneling to thermionic-assisted tunneling by the KCN treatment, indicating a decrease in the density of the trap states in the Si depletion layer. Measurements of the electrode conductance also show that the trap density is greatly reduced by the KCN treatment. The improvement of the electrical characteristics is attributed to the decrease in the trap density and an increase in the barrier height in n-Si caused by the inclusion of cyanide ions at the oxide/Si interface and/or in the silicon oxide layer.

New current and potential oscillations for reduction reactions on platinum electrodes in acid solutions containing high concentration hydrogen peroxide

A new electrochemical oscillation (called oscillation B) appears, in addition to a reported one (called oscillation A), for reduction reactions on platinum electrodes in acid solutions containing hydrogen peroxide in case where the H 2 O 2 concentration is made high. Also, oscillation A becomes more pronounced and more stable as the H 2 O 2 concentration increases except the case of very high concentrations (≥1.1 M). Oscillation A appears in a narrow potential region just before hydrogen evolution, whereas oscillation B appears in a potential region of hydrogen evolution. Both oscillation A and B are strongly affected by the concentrations of H + as well as H 2 O 2 , suggesting that they arise from the competition between the H 2 O 2 and H + reduction. It is concluded that the high and low current states for oscillation A correspond to the H 2 O 2 reduction and almost no H 2 O 2 reduction (nor hydrogen evolution), respectively, whereas the high and low current states for oscillation B correspond to the H 2 O 2 reduction and hydrogen evolution, respectively. Mechanisms for sudden transitions between these states are discussed qualitatively.

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

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...