Masaki Hirota

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.

Infrared sensor with precisely patterned Au black absorption layer

Thermoelectric infrared sensors has been fabricated by adding to the CMOS process a surface micromachining technique and a highly accurate process for forming an infrared radiation absorbing layer. The sensor, or thermopile, consists of alternating areas of p-type and n-type polysilicon connected in series on a Si3N4 layer. An anisotropic etching technique using hydrazine is employed to form a thermally isolated membrane. While a Au-black layer for infrared radiation absorption provides the best absorption efficiency over a broad infrared wavelength region, it has been difficult to pattern the layer precisely. Patterning is accomplished by forming the Au-black layer by a low-pressure vapor deposition technique on amorphous Si and a PSG sacrificial layer and then removing it on PSG by the lift-off technique or wet etching PSG. This technique makes it possible to obtain a Au-black pattern with the same degree of accuracy as with the CMOS process. As a result, sensor performance has been improved and a device array has also been achieved. A simple sensor design method has been established by which simulations are easily conducted using a thermal equivalent circuit based on the CMOS process. Prototype sensors, having external dimensions of 160 micrometer X 160 micrometer, achieved responsivity of 300, 149 and 60 V/W and a time constant of 2.0, 0.46 and 0.27 msec in the air, respectively. These performance figures surpass the performance reported to date for thermoelectric infrared sensors.

120×90 element thermopile array fabricated with CMOS technology

This paper presents the first-ever 120×90 element thermoelectric IR focal plane array (FPA) fabricated wiht CMOS technology. The device has a high repsonsivity of 3,900 V/W and a low cost potential. The overall chip size is 14.4 mm × 11.0 mm with a 12.0 mm × 9.0 mm imaging area. The device structure was optimzed for a vacuum-sealed package. Each detector consists of two pairs of p-n polysilicon thermocouples and an NMOS transistor and has external dimensions of 100μm x 100μm and an internal electrical resistance of 90kΩ. The precisely patterned Au-black IR absorbing layer was achieved by both a low-pressure vapor deposition technique and a lift-off technique utilizing a PSG sacrificial layer. These techniques make it possible to obtain a Au-black pattern with the same degree of accuracy as with the CMOS process. The Au-black layer showed high absorpitivty of more than 90 percent to the light source with a wavelength of from 8 to 13μm. This performance is suitable for consumer electronics as well as automotive applications.

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.

Thermoelectric infrared imaging sensors for automotive applications

This paper describes three low-cost thermoelectric infrared imaging sensors having a 1,536, 2,304, and 10,800 element thermoelectric focal plane array (FPA) respectively and two experimental automotive application systems. The FPAs are basically fabricated with a conventional IC process and micromachining technologies and have a low cost potential. Among these sensors, the sensor having 2,304 elements provide high responsivity of 5,500 V/W and a very small size with adopting a vacuum-sealed package integrated with a wide-angle ZnS lens. One experimental system incorporated in the Nissan ASV-2 is a blind spot pedestrian warning system that employs four infrared imaging sensors. This system helps alert the driver to the presence of a pedestrian in a blind spot by detecting the infrared radiation emitted from the person’s body. The system can also prevent the vehicle from moving in the direction of the pedestrian. The other is a rearview camera system with an infrared detection function. This system consists of a visible camera and infrared sensors, and it helps alert the driver to the presence of a pedestrian in a rear blind spot. Various issues that will need to be addressed in order to expand the automotive applications of IR imaging sensors in the future are also summarized. This performance is suitable for consumer electronics as well as automotive applications.

Thermoelectric infrared imager and automotive applications

This paper describes a newly developed thermoelectric infrared imager having a 48 X 32 element thermoelectric focal plane array (FPA) and an experimental vehicle featuring a blind spot pedestrian warning system, which employs four infrared imagers. The imager measures 100 mm in width, 60 mm in height and 80 mm in depth, weighs 400 g, and has an overall field of view (FOV) of 40 deg X 20 deg. The power consumption of the imager is 3 W. The pedestrian detection program is stored in a CPU chip on a printed circuit board (PCB). The FPA provides high responsivity of 2,100 V/W, a time constant of 25 msec, and a low cost potential. Each element has external dimensions of 190 μm x 190 μm, and consists of six pairs of thermocouples and an Au-black absorber that is precisely patterned by low-pressure evaporation and lift-off technologies. The experimental vehicle is called the Nissan ASV-2 (Advanced Safety Vehicle-2), which incorporates a wide range of integrated technologies aimed at reducing traffic accidents. The blind spot pedestrian warning system alerts the driver to the presence of a pedestrian in a blind spot by detecting the infrared radiation emitted from the persons body. This system also prevents the vehicle from moving in the direction of the pedestrian.

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.

Low-Cost Infrared Imaging Sensors for Automotive Applications

This paper describes two newly developed low-cost thermoelectric infrared imaging sensors, having a 48 × 32 element thermoelectric focal plane array (FPA) and a 120 × 90 element thermoelectric FPA, respectively, and two experimental vehicle systems. The FPAs provide high responsivity of 2.100 V/W and 3.900 V/W, respectively. The FPAs are basically fabricated with a conventional IC process and micromachining technologies and have a low cost potential. One experimental system incorporated in the Nissan ASV-2 is a blind spot pedestrian warning system that employs four infrared imaging sensors. This system helps alert the driver to the presence of a pedestrian in a blind spot by detecting the infrared radiation emitted from the person’s body. The system can also prevent the vehicle from moving in the direction of the pedestrian. The other is a rearview camera system with an infrared detection function. This system consists of a visible camera and infrared sensors, and it helps alert the driver to the presence of a pedestrian in a rear blind spot. The sensor performance is suitable for use in various safety and occupant comfort systems.