Shoichi Nakano

Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)

A new type of a-Si/c-Si heterojunction solar cell, called the HIT (Heterojunction with Intrinsic Thin-layer) solar cell, has been developed based on ACJ (Artificially Constructed Junction) technology. A conversion efficiency of more than 18% has been achieved, which is the highest ever value for solar cells in which the junction was fabricated at a low temperature (<200°C).

Interference-Free Determination of the Optical Absorption Coefficient and the Optical Gap of Amorphous Silicon Thin Films

A new method to determine the optical absorption coefficient (α) of thin films is presented. α of hydrogenated amorphous silicon (a-Si:H) based alloys can be accurately determined from transmittance (T) and reflectance (R) by using T/(1-R), which almost completely eliminates disturbance from the optical interference effect. The method is applicable to any thin films, as long as the film is a single layer. Based on the interference-free α, various methods to determine the optical gap (EOPT) of a-Si:H, a-SiC:H, and a-SiGe:H films are discussed. The (nαhν)1/3 plot and the (αhν)1/3 plot are most suitable for characterizing these films. The well-known (αhν)1/2 plot is less suited for detailed discussion of the EOPT than the cube root plot, because the plot includes a large ambiguity in the EOPT. The effect of the optical interference effect on the determination of the EOPT is also discussed.

Enlargement of Poly-Si Film Grain Size by Excimer Laser Annealing and Its Application to High-Performance Poly-Si Thin Film Transistor

By both numerical simulation and experimental investigation, we found it possible to enlarge the grain size (?3000 ?) of polycrystalline silicon (poly-Si) films by excimer laser annealing, using a new method to control the solidification process of molten Si - low-temperature (?400?C) substrate heating during laser annealing. Poly-Si thin-film transistors (TFTs) fabricated by this new excimer laser annealing method showed a high field-effect mobility of 230 cm2/V?s, and good uniformity of field-effect mobility (?10%) within the effective laser irradiation area.

Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method

Dramatic lateral grain growth of nondoped poly-Si films (maximum grain size: ~4.5 µm, film thickness: 500 A) with strong crystallographic (111) orientation on glass substrates has been achieved using an excimer laser annealing method, namely at low temperature below 400°C and in a processing time shorter than a second, for the first time. The surface morphology of these poly-Si films was very smooth and the crystallinity was excellent with minimal internal defects. These poly-Si films have monomodally distributed grain sizes, with an average grain size of around 1.5 µm. As a result of experimental study, we speculate that the basic driving force behind this lateral grain growth was surface free energy anisotropy, as the same mechanism was observed in high-temperature furnace annealing of doped poly-Si thin films.

Preparation of c-Axis Oriented AlN Thin Films by Low-Temperature Reactive Sputtering

C-axis oriented aluminum nitride (AlN) thin films on (110) silicon were prepared by reactive RF magnetron sputtering in argon and nitrogen atmosphere without substrate heating. We investigated the dependence of some properties for the AlN thin film on sputtering conditions, especially N2 concentration. It was found that c-axis orientation tended to improve gradually with decreasing N2 concentration. The full width of half the maximum intensity (FWHM) of the rocking curve for a (002) plane of hexagonal AlN was 1~2 degrees at a 10% N2 concentration. This was a suitable value for surface acoustic wave (SAW) device. IDT/AlN/(110)Si structure SAW resonators were fabricated. It was confirmed that the insertion loss was 14 dB and phase velocity was 4800 m/s, respectively.

Dielectric Characteristics of (A1/21+·A1/23+)TiO3 Ceramics at Microwave Frequencies

We have investigated the dielectric characteristics at microwave frequencies of perovskites with the formula (A1/21+A1/23+)TiO3, (where A1+ represents an alkali metal from the lithium to potassium series and A3+ represents a member of the lanthanide series from lanthanum to lutetium). It was found for the first time that in the case of Li1+ substitution at the A1+ site, the larger the ionic radius of the element which substituted at the A3+ site, the higher the dielectric constant was and the greater the negative shift of the temperature coefficient of resonant frequency was. For the (1-x)(Li1/21+Sm1/23+)TiO3-x(Na1/21+Sm1/23+)TiO3 system at 3 GHz, high dielectric properties of er=81, Q=2050 and τf=+17ppm/ °C were obtained when x=0.4.

PREPARATION OF ALUMINUM NITRIDE THIN FILMS BY REACTIVE SPUTTERING AND THEIR APPLICATIONS TO GHZ-BAND SURFACE ACOUSTIC WAVE DEVICES

Single crystal aluminum nitride (AlN) thin films were prepared by a low‐temperature reactive sputtering on basal plane sapphire [(001)Al2O3] at a substrate temperature of less than 315 °C. Surface acoustic wave (SAW) characteristics with an interdigital transducer /(001)AlN/(001)Al2O3 structure were investigated. The phase velocity and temperature coefficient of delay time are 5750–5765 m/s and 55–63 ppm/°C at KH=1.2–1.6, respectively. Resonator‐type 1‐GHz‐band SAW filters with its structure were fabricated. The insertion loss and suppression were 23 dB and more than 20 dB, respectively.

Microwave Dielectric Properties of CaO-Li2O-Ln2O3-TiO2 Ceramics

We have investigated the microwave dielectric properties of the Li2O-Ln2O3-TiO2 system and the CaO-Li2O-Ln2O3-TiO2 system (Ln: Lanthanide), both of which have a perovskite structure. The relationship between the kind of lanthanide element and dielectric properties at microwave frequencies was confirmed. It was also found that the ionic radius of lanthanide ions has a strong influence on the dielectric properties. Excellent dielectric properties of er=110, fQ value=4500 GHz and high temperature stability for the resonant frequency at 3 GHz were obtained with a composition of CaO:SrO:Li2O:Sm2O3:TiO2=15.0:1.0:9.0:12.0:63.0 (molar ratio). Also, band-pass filters made of this material were examined. The filters were greatly miniaturized to the size of about 2×5×4 mm3 (0.04 cm3).

The Influence of the Si-H2 Bond on the Light-Induced Effect in a-Si Films and a-Si Solar Cells

The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.

High mobility poly-Si TFT by a new excimer laser annealing method for large area electronics

A high-mobility (280 cm/sup 2//V-s), high-throughput poly-Si TFT (thin-film transistor) formed using a low-temperature process (<or= 600 degrees C) has been achieved through a novel excimer laser annealing method. The method uses thin, active-layer poly-Si film (500 AA) and involves controlling the solidification process of molten Si by low-temperature (<or= 400 degrees C) substrate heating during laser annealing. Poly-Si film grain formed by this process is radically larger in size, and has minimal internal defects. The maximum grain size is over 5000 AA, and uniformity in field effect mobility was found to be +or-10% within the effective laser irradiation area.<<ETX>>