Hiroshi Nagayoshi

Amorphous Silicon Thin-Film Transistors Employing Photoprocessed Tantalum Oxide Films as Gate Insulators

Amorphous silicon thin-film transistors (a-Si:H TFTs) with photoprocessed TaOx/photo-CVD SiNx double layer gate insulator have been fabricated. The usefulness of photoprocessed TaOx film as a gate insulator and the reason for improvement in TFT characteristics thereby are discussed. There is a correlation among TFT characteristics, a-Si:H/SiNx interface properties and stress in the gate insulator. Better TFT characteristics are obtained for higher compressive stress in the gate insulator due to better interface properties. The photoprocessed TaOx film plays a role in changing the stress and consequently improves TFT characteristics. Annealing of TaOx film in the presence of both UV irradiation and an oxygen ambient after photo-CVD deposition also improves the stability of TFT characteristics.

Residual Stress of a-Si1-xNx:H Films Prepared by Afterglow Plasma Chemical Vapor Deposition Technique

Relationships between residual stress and other film properties of amorphous silicon nitride films deposited by microwave afterglow plasma of NH3 and by photochemical vapor deposition have been studied. There are differences in the dependence of N-H and Si-H bond density and residual stress on the film composition between the films deposited by the two methods. The residual tensile stresses in both films, however, reach a maximum value of 7-9×109 dyn/cm2 when the N-H bond density is nearly equal to the Si-H bond density. The residual stress can be reduced by adjusting the N-H and Si-H bond density while maintaining the high breakdown field strength and high resistivity.

Surface Texturing of Silicon by Hydrogen Radicals

The surface texturing method for crystalline Si using hydrogen radicals generated by a tungsten hot filament was developed. We found that tungsten particles supplied from a tungsten filament work as an etching mask against hydrogen radicals. The surface morphology and feature size of the texture structure could be controlled by the particle deposition condition on the Si(100) surface. An inverted pyramid structure was obtained when the particle density was high, suggesting that the etching reaction induced by hydrogen radicals is anisotropic. The reflectance spectra of hydrogen-treated Si surface using this method showed a very low surface reflectance of less than 1% in the range from 200 to 900 nm without any antireflection coatings. The particles on the silicon surface can easily be removed using HF+HNO3 solution. This method is also effective for the texturing of Si(111) wafer, having a potential for the texturing of multicrystalline silicon.

High-Rate Selective Etching of a-Si:H Using Hydrogen Radicals

A high-rate selective etching method of hydrogenated amorphous silicon (a-Si:H) using hydrogen radicals is presented. A very high etch rate of 2.7 µm/min was obtained at 50°C using a microwave hydrogen afterglow method. However, amorphous silicon nitride ( a-SiN1.2:H), silicon oxide ( SiO2), silicon carbide ( a-SiC0.5:H), and Al films were not etched under the same conditions. These results suggest that high-rate selective etching of a-Si:H can be achieved using hydrogen radicals. This method is suitable for the fabrication of large-area devices.

Novel PV array/module I-V curve simulator circuit

This paper describes the basic characteristics of the PV module/array simulator circuit suitable for a PV system study, especially for the system using multi inverters like an AC module system. This circuit magnifies an I-V output of a small pn photo-sensor using analog technology, enabling the independent magnification adjustment of the photo-current and the output voltage of the photo-sensor. A fill factor is also easily changeable. Hence, this simulator can simulate the I-V characteristics of many kinds of PV modules or arrays. The output power of the simulator circuit can be controlled by a light intensity supplied from LED.

SiNx :H/SiO2 Double-Layer Passivation With Hydrogen-Radical Annealing For Solar Cells

A SiNx :H/SiO2 double-layer structure as the passivation layer and hydrogen-radical post annealing were introduced to decrease the surface recombination velocity on crystalline silicon. Effective lifetime was much increased by introducing a SiNx :H/SiO2 double-layer passivation structure instead of a SiNx :H or SiO2 single-layer, while the hydrogen radical post annealing improves the interface characteristics more effectively than N2 annealing. The results of C–V measurement suggest that many positive charges exist in the SiNx :H layer. The combination of field effect by these charges and the decrease of SiO2/c-Si interface defects by hydrogen-radical annealing effectively decrease the surface recombination velocity.

Peak-power reduction with 100 kW PV and battery combined system at Shonan Institute of Technology

The grid connected 100 kW PV and 345 kWh battery combined system introduced in 1998 at Shonan Institute of Technology has been investigated. The system, equipped with PV and large battery, has a peak cut function against the big surge in electric power consumption in summer as well as working as an emergency electric power source. The daily peak of power consumption during summer occurs at 2:00 p.m. This time does not coincide with the time of peak power generation. The use of the PV/battery combined peak cut function effectively cuts consumption by 7% during the peak consumption period.

Etching and Surface Modification of GaAs by Hydrogen Radicals Generated by Hydrogen Microwave Afterglow Method

We etched crystalline GaAs using hydrogen radicals generated by the hydrogen microwave afterglow method and determined the dependence of etching rate on substrate temperature, microwave power, and sample distance from the quartz tube supplying hydrogen radicals. From the Arrhenius plot, the activation energy was found to be 0.43 eV. The surface morphology could be varied from flat to textured by changing the etching conditions.

Silicon Whisker Growth Using Hot Filament Reactor with Hydrogen as Source Gas

Si whisker growth on a silicon substrate, using only pure hydrogen gas flow in a hot filament chemical vapor deposition reactor, has been studied by scanning and transmission electron microscopy. At the initial stage of the growth, tungsten–silicide particles are formed due to hydrogen radical etching by the filament. Simultanously the Si substrate exhibits a surface texturing. After long residence times of hydrogen gas in the reactor, silicon whiskers, with diameters between 10 and 50 nm, were observed on the textured silicon surface. Each whisker has a silicide particle at its tip. A mechanism of silicon whiskers growth using this method with hydrogen gas is proposed.

Effect of hydrogen radical annealing on SiN passivated solar cells

Abstract Remote plasma was used for PE-CVD of SiN films and it was found that hydrogen radical (H * ) annealing of c-Si cells with SiN films improved the efficiency of the cells. Cell efficiency of 21.8% was obtained by applying a SiN/SiO 2 double-layer structure on the emitter of a PERL-type solar cell. It was found that the H * annealing has two effects: it reduces surface recombination velocity (SRV); and it degrades bulk-lifetime of p-type c-Si. To apply SiN practically, it is effective to use a rear n-floating or a triode structure. Reducing the exposed area of the p-type substrate by using n-type diffused layer increases the efficiency of solar cells.