Yoshimi Ohta

Improvement of carrier diffusion length in silicon nanowire arrays using atomic layer deposition

To achieve a high-efficiency silicon nanowire (SiNW) solar cell, surface passivation technique is very important because a SiNW array has a large surface area. We successfully prepared by atomic layer deposition (ALD) high-quality aluminum oxide (Al2O3) film for passivation on the whole surface of the SiNW arrays. The minority carrier lifetime of the Al2O3-depositedSiNW arrays with bulk silicon substrate was improved to 27 μs at the optimum annealing condition. To remove the effect of bulk silicon, the effective diffusion length of minority carriers in the SiNW array was estimated by simple equations and a device simulator. As a result, it was revealed that the effective diffusion length in the SiNW arrays increased from 3.25 to 13.5 μm by depositing Al2O3 and post-annealing at 400°C. This improvement of the diffusion length is very important for application to solar cells, and Al2O3 deposited by ALD is a promising passivation material for a structure with high aspect ratio such as SiNW arrays.

Optical assessment of silicon nanowire arrays fabricated by metal-assisted chemical etching.

Silicon nanowire (SiNW) arrays were prepared on silicon substrates by metal-assisted chemical etching and peeled from the substrates, and their optical properties were measured. The absorption coefficient of the SiNW arrays was higher than that for the bulk silicon over the entire region. The absorption coefficient of a SiNW array composed of 10-μm-long nanowires was much higher than the theoretical absorptance of a 10-μm-thick flat Si wafer, suggesting that SiNW arrays exhibit strong optical confinement. To reveal the reason for this strong optical confinement demonstrated by SiNW arrays, angular distribution functions of their transmittance were experimentally determined. The results suggest that Mie-related scattering plays a significant role in the strong optical confinement of SiNW arrays.

Metal-Assisted Chemical Etching Using Silica Nanoparticle for the Fabrication of a Silicon Nanowire Array

30-nm-diameter silica nanoparticles with a carboxyl radical (COO-) were successfully dispersed on an amino-treated silicon wafer at about 20 nm intervals owing to the repulsion among nanoparticles with negative charges. The dispersed silica nanoparticles were used as the mask for the preparation of silicon nanowire (SiNW) arrays by metal-assisted chemical etching (MAE). The diameter of the prepared SiNWs was approximately 30 nm from their transmission electron microscope image.

Numerical approach to the investigation of performance of silicon nanowire solar cells embedded in a SiO2 matrix

The electrical characteristics of silicon nanowire (SiNW) solar cells with p-type hydrogenated amorphous silicon oxide (Eg = 1.9 eV)/n-type SiNWs embedded in a SiO2/n-type hydrogenated amorphous silicon oxide (Eg = 1.9 eV) structure have been investigated using two- and three-dimensional device simulators, taking into account the quantum size effect. The average bandgap of a SiNW embedded in SiO2 increased from 1.15 to 2.68 eV with decreasing diameter from 10 to 2 nm, owing to the quantum size effect. Note that under sunlight of AM1.5G, the open-circuit voltage (Voc) of SiNW solar cells also increased to 1.46 V with decreasing diameter of the SiNWs to 2 nm. This result suggests that it is possible to enhance Voc by applying the quantum size effect, and a SiNW is a promising material for all-silicon tandem solar cells.

Low-cost thermo-electric infrared FPAs and their automotive applications

This paper describes three low-cost infrared focal plane arrays (FPAs) having a 1,536, 2,304, and 10,800 elements and experimental vehicle systems. They have a low-cost potential because each element consists of p-n polysilicon thermocouples, which allows the use of low-cost ultra-fine microfabrication technology commonly employed in the conventional semiconductor manufacturing processes. To increase the responsivity of FPA, we have developed a precisely patterned Au-black absorber that has high infrared absorptivity of more than 90%. The FPA having a 2,304 elements achieved high resposivity of 4,300 V/W. In order to reduce package cost, we developed a vacuum-sealed package integrated with a molded ZnS lens. The camera aiming the temperature measurement of a passenger cabin is compact and light weight devices that measures 45 x 45 x 30 mm and weighs 190 g. The camera achieves a noise equivalent temperature deviation (NETD) of less than 0.7°C from 0 to 40°C. In this paper, we also present a several experimental systems that use infrared cameras. One experimental system is a blind spot pedestrian warning system that employs four infrared cameras. It can detect the infrared radiation emitted from a human body and alerts the driver when a pedestrian is in a blind spot. The system can also prevent the vehicle from moving in the direction of the pedestrian. Another system uses a visible-light camera and infrared sensors to detect the presence of a pedestrian in a rear blind spot and alerts the driver. The third system is a new type of human-machine interface system that enables the driver to control the cars audio system without letting go of the steering wheel. Uncooled infrared cameras are still costly, which limits their automotive use to high-end luxury cars at present. To promote widespread use of IR imaging sensors on vehicles, we need to reduce their cost further.

Wireless power transmission between a NIR VCSEL array and silicon solar cells

Aiming widespread use of wireless power transmission, optical wireless power transmission at wavelength of 975 nm between vertical cavity surface emitting laser (VCSEL) arrays and a single crystalline Si solar cell was studied. The power generation efficiency of the cell was measured as high as 33% which was higher than the efficiency of 18% at the time of irradiation with sunlight. This is because the transmission loss is mainly eliminated. However, further reduction of the electrical resistance of the solar cell is necessary because the efficiency decreases with increasing incident energy.

Observation of light scattering properties of silicon nanowire arrays

Silicon nanowire (SiNW) arrays were pre metal assisted chemical etching (MAE) method for the ap to solar cells. The SiNW arrays were mechanically peeled the Si substrate to obtain the optical properties of SiNW themselves. The absorptance of the SiNW array with the 10 μm is much higher than theoretical absorptance of 10 flat Si wafer. The angular distribution function (transmittance of SiNW arrays was also measured. It was that the Mie-related scattering plays an important ro strong optical confinement of the SiNW arrays.