Toward the Super Temporal Resolution Image Sensor with a Germanium Photodiode for Visible Light.

The theoretical temporal resolution limit tT of a silicon photodiode (Si PD) is 11.1 ps. We call "super temporal resolution" the temporal resolution that is shorter than that limit. To achieve this resolution, Germanium is selected as a candidate material for the photodiode (Ge PD) for visible light since the absorption coefficient of Ge for the wavelength is several tens of times higher than that of Si, allowing a very thin PD. On the other hand, the saturation drift velocity of electrons in Ge is about 2/3 of that in Si. The ratio suggests an ultra-short propagation time of electrons in the Ge PD. However, the diffusion coefficient of electrons in Ge is four times higher than that of Si. Therefore, Monte Carlo simulations were applied to analyze the temporal resolution of the Ge PD. The estimated theoretical temporal resolution limit is 0.26 ps, while the practical limit is 1.41 ps. To achieve a super temporal resolution better than 11.1 ps, the driver circuit must operate at least 100 GHz. It is thus proposed to develop, at first, a short-wavelength infrared (SWIR) ultra-high-speed image sensor with a thicker and wider Ge PD, and then gradually decrease the size along with the progress of the driver circuits.

Light-In-Flight Imaging by a Silicon Image Sensor: Toward the Theoretical Highest Frame Rate.

Light in flight was captured by a single shot of a newly developed backside-illuminated multi-collection-gate image sensor at a frame interval of 10 ns without high-speed gating devices such as a streak camera or post data processes. This paper reports the achievement and further evolution of the image sensor toward the theoretical temporal resolution limit of 11.1 ps derived by the authors. The theoretical analysis revealed the conditions to minimize the temporal resolution. Simulations show that the image sensor designed following the specified conditions and fabricated by existing technology will achieve a frame interval of 50 ps. The sensor, 200 times faster than our latest sensor will innovate advanced analytical apparatuses using time-of-flight or lifetime measurements, such as imaging TOF-MS, FLIM, pulse neutron tomography, PET, LIDAR, and more, beyond these known applications.

Methodology of Thermoelectric Power Factor Enhancement by Controlling Nanowire Interface.

The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

Miniaturized planar Si-nanowire micro-thermoelectric generator using exuded thermal field for power generation.

For harvesting energy from waste heat, the power generation densities and fabrication costs of thermoelectric generators (TEGs) are considered more important than their conversion efficiency because waste heat energy is essentially obtained free of charge. In this study, we propose a miniaturized planar Si-nanowire micro-thermoelectric generator (SiNW-µTEG) architecture, which could be simply fabricated using the complementary metal-oxide-semiconductor-compatible process. Compared with the conventional nanowire µTEGs, this SiNW-µTEG features the use of an exuded thermal field for power generation. Thus, there is no need to etch away the substrate to form suspended SiNWs, which leads to a low fabrication cost and well-protected SiNWs. We experimentally demonstrate that the power generation density of the SiNW-µTEGs was enhanced by four orders of magnitude when the SiNWs were shortened from 280 to 8 µm. Furthermore, we reduced the parasitic thermal resistance, which becomes significant in the shortened SiNW-µTEGs, by optimizing the fabrication process of AlN films as a thermally conductive layer. As a result, the power generation density of the SiNW-µTEGs was enhanced by an order of magnitude for reactive sputtering as compared to non-reactive sputtering process. A power density of 27.9 nW/cm2 has been achieved. By measuring the thermal conductivities of the two AlN films, we found that the reduction in the parasitic thermal resistance was caused by an increase in the thermal conductivity of the AlN film and a decrease in the thermal boundary resistance.

The Theoretical Highest Frame Rate of Silicon Image Sensors.

The frame rate of the digital high-speed video camera was 2000 frames per second (fps) in 1989, and has been exponentially increasing. A simulation study showed that a silicon image sensor made with a 130 nm process technology can achieve about 1010 fps. The frame rate seems to approach the upper bound. Rayleigh proposed an expression on the theoretical spatial resolution limit when the resolution of lenses approached the limit. In this paper, the temporal resolution limit of silicon image sensors was theoretically analyzed. It is revealed that the limit is mainly governed by mixing of charges with different travel times caused by the distribution of penetration depth of light. The derived expression of the limit is extremely simple, yet accurate. For example, the limit for green light of 550 nm incident to silicon image sensors at 300 K is 11.1 picoseconds. Therefore, the theoretical highest frame rate is 90.1 Gfps (about 1011 fps).

Dopant-induced intrinsic bistability in a biased nanowire.

Self-consistent calculations of quantum transport through a single donor atom in the semiconductor nanowire show intrinsic bistability of the nonequilibrium electronic state. We attribute this effect to two distinct ion screening mechanisms.