To prepare antireflection coating (ARC) by wet process is important technology for low cost fabrication of solar cells. In this research, we consider the optical reflectance of a three layer stack structure of ARC films on the pyramidally textured single-crystalline silicon substrates. Each layer of the ARC films is deposited by a spin-coating method. The triple layers consist of SiO2, SiO2-TiO2 mixture, and TiO2 films from air to the silicon substrate in that order, and the refractive index is slightly increased from air to the substrate. Light reflection can be reduced further mainly due to graded index effect. The optimized three layer structure ARC shows that the reflectance is below 0.048 at the wavelength of 600 nm.

We prepared alumina passivation films for p-type silicon substrates by sol-gel wet process mainly using aluminum isopropoxide (Al(O-i-Pr)3) as a precursor material. The precursor solution was spin-coated onto p-type silicon substrates and then calcined for 1 hour in air. Minority carrier lifetime of the passivated wafers was evaluated for different calcination temperature conditions. We also compared the passivation quality of the alumina passivation films using different alumina precursor, aluminum acetylacetonate (Al(acac)3). Obtained effective minority carrier lifetime indicated that the lifetime is strongly depends on the calcination temperature. The substrate calcined below 400°C shows relatively short lifetime below 100 µsec. On the other hand, the substrate calcined around 500°C to 600°C indicates lifetime from 250 to 300 µsec. Calcination temperature dependence of the lifetime for the samples using Al(O-i-Pr)3 precursors shows almost the same as that using Al(acac)3.

We fabricated silicon solar cells with spin-coated sol-gel alumina passivation layers on the rear side. Spin-coated alumina passivation films have moderate passivation quality and are inferior to atomic layer deposited passivation films. However, low-cost and low temperature process of the sol-gel deposition is still beneficial for the cells using commercially available Cz silicon wafers. Thus, we consider an applicability of the spin-coated alumina passivation layer for rear side passivation. Dependence of cell efficiency on contact spacing and contact diameter of a rear electrode was investigated by both experiments and numerical calculation. The experimental results indicated that conversion efficiency of the cell is enhanced from 9.1% to 11.1% by optimizing an aperture ratio and contact spacing of the rear passivation layers. Numerical calculation indicated that small contact diameter with low aperture ratio of a rear passivation layer is preferable to achieve good cell performance in our experimental condition. We confirmed the effectivity of the spin-coated alumina passivation films for rear surface passivation of the low-cost silicon solar cells.

We have fabricated ferroelectric Pb(Zr,Ti)O3 (PZT) thin films using low-pressure consolidation process during the sol-gel method. Drastic improvements of electrical properties have been obtained for the PZT thin films fabricated with low-pressure consolidation process. A remanent polarization (Pr) of 37 µC/cm2 and a coercive field (EC) of 64 kV/cm have been achieved. In addition, the leakage current of the PZT films fabricated using low-pressure consolidation is 102 times smaller than that of the films fabricated with the usual process of sol-gel method. It is also found that the low-pressure consolidation process is effective on improvements of electrical properties of PZT films fabricated at lower crystallization temperatures and with sub-100 nm thickness.

Surface morphology and piezo response on SBT films were simultaneously measured by scanning probe microscopy. In a sample that had many short-circuited capacitor pads, some curious structures were observed on the SBT film surface. The piezo image partially did not correspond with the AFM image. Some specific grains were revealed to be piezo defects. Also observed were some smaller grains with flat surface, which showed good ferroelectricity. Next, we carried out simultaneous measurements of surface morphology and leakage current. The scanning at an intentionally high voltage was repeated until the leakage points were found. We found the leakage points, which were on some large grains, not at grain boundaries or on the flat smaller grains. In another SBT film derived from an unrefined source, many ferroelectric defects were observed despite there being no curious structures on the surface. Purity has an important bearing on the ability to avoid these defects. Thus, these nanoscopic investigations would greatly facilitate understanding of the mechanisms responsible for problems and enable optimization of the process conditions in device fabrication.

Co-ferrite thin films have been fabricated on Corning glass substrates by a chelating sol-gel process. Structural and magnetic properties of the films have been studied as a function of annealing temperature using an X-ray diffraction (XRD) and a vibrating sample magnetometer. XRD results revealed that most of the Co-ferrite grains were randomly oriented. Rapid annealing (RA) and standard annealing (SA) processes were used for the variation of heat treatment and the characteristic comparison. Coercivity was changed with the thermal condition and the magnetization increased with the soaking time. With prolonged soaking time, however, the coercivity decreased due to the diffusion of cations from the glass substrate. RA in the preparation of Co-ferrite thin films was effective for preventing interdiffusion at interfaces and for forming a single phase in the case of reduced soaking time. A yttria stabilized zirconia (YSZ) buffer layer between the Co-ferrite layer and the substrate was effective for improving the magnetic properties of the films at higher temperatures. It was observed that Co-ferrite thin films were composed of grains typically 35 nm in size and their rms roughness was approximately 1.3 nm. The saturation magnetization of the thin films by subjected to rapid annealing at 900C for 150 seconds was 400 emu/cm3 and the coercivity of the films was approximately 3000 Oe.

We prepared alkoxide solutions to fabricate SrBi2Ta2O9 (SBT) ferroelectric capacitors with IrO2 electrodes. In this process, to minimize excess bismuth, the Sr : Bi : Ta mole ratio was kept at 0. 9 : 2. 1 : 2. 0, i. e. , nearly stoichiometric. Three types of solution - mixed-only (MIX), complexed (COMP), and hydrolyzed (HYD) - were used. The HYD capacitor had low absolute leakage current, 10-7 A/cm2 order, and good saturation properties to 6 V. When voltage was applied to each capacitor at 2 to 6 V, MIX and COMP capacitors showed only partial hysteresis loops due to a high leakage current, reflecting the I-V characteristics. These results are probably due to film density caused by metaloxane network bonding. A fatigue endurance test was conducted using cycling of polarization switching at 6 V using the HYD capacitor with IrO2 electrodes. Slight changes were, however, observed in hysteresis loop configuration, but good hysteresis properties were kept up to 1. 04 1012 cycles. We compared SBT ferroelectric thin films fabricated with Pt electrodes and with IrO2 electrodes. Scarcely any difference due to SBT in the XRD pattern was seen, depending on the substrate material. We found that the use of IrO2 electrodes had the effect of decreasing the crystallization temperature. On Pt and IrO2 electrodes, the two films have surface morphology quite different from that of the rod-like structure wellknown for SBT films prepared using a metal 2-ethylhexanate solution. Their surfaces show a similar morphology with relatively large, closely packed grains. A comparison of the I-V characteristics after reannealing showed that the capacitor with IrO2 electrodes had a higher leakage current than that with Pt electrodes. The leakage current was probably due to the density of the film and interface between the SBT film and electrodes.

Pb(ZrxTi1-x)O3 (PZT) thin films were prepared on various electrodes. When Ir system materials were used as electrodes, fatigue properties of PZT thin films were improved. Moreover, in the case of the PZT thin film on an Ir/IrO2 electrode, not only fatigue but imprint properties were clearly improved. We could find these improvements were caused by good barrier effect of IrO2 from secondary ion mass spectroscopy (SIMS) analysis. By applying these Ir system electrodes, we fabricated stacked capacitors on polycrystalline silicon (poly-Si) plugs. In spite of high temperature thermal processing, we found poly-Si plugs were ohmically connected with the bottom electrodes of the capacitors from hysteresis measurements and I-V characteristics, and could greatly expect them for practical use.

In this paper, we discuss ferroelectric materials suitable for a metal ferroelectric metal insulator semiconductor FET (MFMIS FET). It is important for a ferroelectric material to have a low dielectric constant to enable the application of sufficient electric field to a ferroelectric layer. Films of Sr2Nb2O7 (SNO) and Sr2(Ta1-xNbx)2O7 (STNO) were prepared by the sol-gel method on Pt/IrO2 electrodes for an MFMIS FET. The ferroelectricities of STNO films were confirmed in the range of x=0. 1-0. 3. In case of x=0. 3, the largest remanent polarization was obtained in the hysteresis loop. The remanent polarization and the coercive field are 0. 5 µ C/cm2 and 44 kV/cm, respectively. The film had a low dielectric constant (ε=53). It is considered that the characteristics of STNO thin films are suitable for MFMIS FET.

Recently, reliable Pb (Ti, Zr) O3 (PZT) thin-film capacitors with robust switching endurance were fabricated successfully by incorporating oxidizable metal and oxide electrodes. However, the reasons for the drastic improvement in the switching endurance property was not clear. Degradation of polarizations by switching is called "polarization fatigue. " This paper describes the mechanisms of polarization fatigue, and discusses ways for improving of that property from the standpoints of microstructure, and of interactions between the PZT and the electrode materials of the capacitors. It is clearly identified that the causes of the fatigue are the unexpected formation of a surface transition layer of PZT, which is strongly dependent on the crystallization process, and a decrease in the interfacial capacitances due to the accumulation of oxygen vacancies between the PZT and non-oxidizable metal electrodes with high work functions such as Pt. Oxidizable metal and oxide electrodes suppress by oxidation-reduction reactions the accumulation of oxygen vacancies. Fatigue-free PZT thin-film capacitors can be formed if oxidizable metal or conductive oxide electrodes are incorporated in columnar-grain-structured PZT thin-film. In sharp contrast, fatigue and retention properties of PZT thin-film capacitors with Ir electrodes were degraded by modification of the PZT with 1 atm%-Nb and 1 atm%-La even though its grain structures were columnar.

It is well known that PZT material properties are strongly dependent on the Zr/Ti ratio. The reliability characteristics, such as the retention and imprint properties, of PZT thin film correlate with the reliability of FRAM(R). PZT films with various Zr/Ti ratios, 60/40, 52/48, 45/55, 40/60 and 30/70 were prepared on Pt/IrO2 and Pt/Ti electrodes by a sol-gel process. With lower Zr/Ti ratio, the grain size becomes smaller and the film was highly oriented to (111) crystallographic plane. But, pyrochlore phase in the PZT films on Pt/IrO2 electrode was detected by SEM and XRD. Hysteresis and pulse responses were measured on the capacitors. With lower Zr/Ti ratio, Pr and Vc become larger. It was found that the preferential (111) orientation played an important role in determining Pr. Voltage shifts which are related to imprint are dependent on the Zr/Ti compositional ratio. Increasing the Ti concentration causes the voltage shift to increase due to more oxygen vacancies. But, this tendency was not in accordance with the results of Qos which were measured by capacitor test simulation of imprint properties for FRAM operation, because the results of Qos did not change monotonically with Ti concentration. However, the capacitor for 45/55 film grown on Pt/IrO2 had good imprint property, similar to the capacitor for sputtered PLZT. In addition, H2 degradation of PZT capacitor with Pt/IrO2 was studied. It was found that H2 annealing degraded the PZT capacitors even at temperatures as low as 150. Thus, imprint and H2 degradation are serious problems for PZT capacitors and these will be a key issue in the reliability of FRAM.