Yoichiro Aya

Quantitative measurement and design of texture morphology for high-efficiency thin-film silicon solar cells

To decouple the trade-off relationship between short-circuit current ISC and open-circuit voltage VOC of thin-film silicon solar cells, which vary with texture morphology, it is necessary to first clarify the relationship between a solar cell’s properties and its texture morphology. We have developed a method for quantitatively measuring the texture morphology, which has enabled us to identify novel indices on the basis of texture width and angle individually correlated to ISC and VOC. A texturing process based on these indices should allow ISC and VOC to be improved independently.

Electrical properties of polycrystalline Si1-xGex thin-films prepared by a solid-phase crystallization method

We studied the electrical properties of solid-phase-crystallized polycrystalline Si1-xGex films (SPC poly-Si1-xGex films) by combining Hall-effect measurement and a hydrogen plasma treatment method. In as-crystallized SPC poly-Si1-xGex films, the density of p-type states or defects increased with increasing Ge content. When the hydrogen plasma treatment was applied, the conductivity of the SPC poly-Si and -Ge films changed from p-type to n-type. However, in the SPC poly-Si1-xGex films, the conductivity did not change within our experimental hydrogen plasma treatment time. Because the results for phosphorous-doped SPC poly-Si1-xGex films were consistent with those for both SPC poly-Si and -Ge films, it was estimated that the SPC poly-Si1-xGex films contained numerous p-type states or defects that were not sensitively passivated by hydrogen plasma treatment. These results suggest that SPC poly-Si1-xGex films are preferable materials for low-cost and large electronic devices.

Conductive-Mode Atomic Force Microscopy Study of Amorphous Silicon Nitride Thin Films

We have used conductive-mode atomic force microscopy to study the electrical properties of amorphous silicon nitride thin films (a-SiN) in order to reveal their electrical properties on the nanometer scale. The current-voltage characteristics were fitted by the Pool-Frenkel equation, and the physical quantities showed good agreement with previous reports of a-SiN. The density of trapping centers obtained using this equation had a good agreement with the reported density of dangling bonds of silicon in a-SiN, then the conductance was attributed to electron hopping by the dangling bonds. The conductive areas observed in the current image obtained using this method did not depend on the topography. We considered that the current image represented the two-dimensional distribution of the components in the a-SiN films.

Lateral Solid-Phase Recrystallization from the Crystal Seed Selectively Formed by Excimer Laser Annealing in Ge-Ion-Implanted Amorphous Silicon Films

A new recrystallization method improving the electrical properties of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) and reducing the fluctuation of the electrical characteristics among them is proposed. It can be realized by the amorphization of poly-Si films through Ge-ion implantation, the crystallization of the drain region, which functions as a crystal seed in the subsequent process, through excimer laser annealing (ELA), and lateral solid-phase recrystallization (LSPR) from the drain to the source along the channel through furnace annealing. In this study, basic experiments are performed to determine the optimum condition of ELA for the formation of the crystal seed with good crystallinty and to investigate the aspect of LSPR growth from the seed.

Lateral Solid Phase Recrystallization from the Crystal Seed in Ge-Ion-Implanted Amorphous Silicon Films by Repetition Rapid Thermal Annealing

Lateral solid-phase recrystallization (LSPR) from the crystal seed selectively formed by excimer laser annealing in Ge-ion-implanted amorphous silicon (a-Si) is performed by rapid thermal annealing (RTA). The process is carried out as a basic research study to grow grains in the current direction along the channel from the drain to the source in poly-Si thin film transistors. In this letter, it is shown that repetition RTA, in which on/off of the setting temperature is periodically repeated with a certain heat pulse width, suppresses the random nucleation in a-Si films and enlarges LSPR grains compared with single RTA, when the heat pulse width is sufficiently shorter than the incubation time which is the time before the onset of nucleation.

Local Electrical Properties of Non-Doped Polycrystalline Silicon Thin-Films Evaluated Using Conductive Atomic Force Microscopy

We have studied the electrical properties of non-doped ELA polycrysta lline silicon thin-films using conductive AFM and TEM techniques. The result of conduct ive AFM shows that the mounds in the topography are less conductive and the prairies have line-like, less conductive areas. The TEM result reveals that grain boundaries are loca ted in mounds and prairies. These results indicate that the grain boundaries located in mounds play a domi nant role in the local electrical properties of non-doped ELA polycrystalline silicon t hi -films. We believe, accordingly, that the line-like, less conductive areas are related to the grain boundaries loc at d in the prairies.

Topography and Local Electrical Properties of Nondoped Polycrystalline Silicon Thin Films Evaluated Using Conductive-Mode Atomic Force Microscopy

The topography and local electrical properties of nondoped excimer laser-annealed polycrystalline silicon thin films were simultaneously studied using conductive-mode atomic force microscopy and transmission electron microscopy. The results of conductive-mode atomic force microscopy show that the mounds in the topography are less conductive and the prairies have line-shaped, less conductive areas, where as those of transmission electron microscopy reveal that grain boundaries are located in both the mounds and prairies. These results indicate that the grain boundaries located in the mounds play a dominant role in determing the local electrical properties of nondoped excimer laser-annealed polycrystalline silicon thin films. We believe, accordingly, that the line-shaped, less conductive areas are related to the grain boundaries located in the prairies.