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Thermal stability has been evaluated for ALE-grown Si/Ge interfaces by co-axial impact collision ion scattering spectroscopy. The IML-thick Si layer on Ge was stable only at less than 360°C. The 2ML-thick Si layer on Ge, however, was stable up to 550°C, and Si layers could be also ALE-grown successively on the 2ML-thick Si layer on Ge, while keeping the interface abrupt, since the Si-ALE growth temperature was about 530°C.

A low heat-capacitance substrate has been introduced to lateral grain-growth for Si thin-films. Grains as large as 13μm could be fabricated by irradiation of a single shot of an excimer-laser light pulse with intensity gradient. The film has an orientation in the (110) direction, different from (111) for the conventional excimer-laser-annealed film, and is applicable to a seed layer for solid-phase epitaxial-growth aiming at solar cell application.

We have proposed a new excimer-laser crystallization system based on an optical projection concept. In the proposed system, a collimated excimer-laser light pulse is irradiated to Si thin-films on a glassy substrate, through a phase-shift mask and an optical lens system. Using oneand two-dimensional phase-shift masks, we have examined feasibility of the proposed method.

The application of chemical-vapor-deposited (CVD) amorphous-silicon and silicon-nitride films to active layers of thin-film transistors on a glass substrate is discussed. The maximum process temperature was 485 degrees C. The maximum field-effect mobility and the typical on-off current ratio were more than 0.9 cm/sup 2//V-s and 10/sup 6/, respectively. Advantages of applying the fully plasma-free CVD method in the amorphous-silicon thin-film transistor process are discussed. >

Low-K silica films were grown from the liquid phase using methyl-triethoxy-silane. Physical and chemical properties were investigated together with electronic properties. The film had dense methyl groups and showed insulating characteristics even under as-grown conditions. The dielectric constant, low-field resistivity and breakdown field-strength were 3.6, 1013 Ωcm and IMV/cm, respectively, for the as-grown film. They were improved to 2.6, more than 1015 Ωcm and more than 3MV/cm, respectively, after 300°C vacuum annealing

Layer-by-layer hetero-epitaxy of Ge has been successfully demonstrated on the Si(100) surface by combining the initial IML-Ge film growth on the Si surface and the successive Ge atomic-layer-epitxy (ALE), for the first time. The former was achieved using the substrate temperature modulation with alternate exposures of GeCL4 and atomic H, and the later was established by cyclic exposures of (CH 3)2GeH2 and atomic H under isothermal conditions. XPS measurements confirmed a discrete and uniform increase in the grown Ge film thickness with one monolayer/cycle step up to the critical Ge thickness, and no C contamination at the Ge/Si interface. Critical exposure for the saturated Ge adsorption was different from that for the homo-ALE on the bulk Ge surface.

Poly-Si films were deposited at temperatures as low as 325°C by a newly proposed method. In the proposed method, both Ni-enhanced crystallization effects and vapor-phase deposition were utilized to reduce the deposition temperature. The X-ray diffraction measurement shows that the films were (110)-oriented at temperatures among 325°C to 419°C. When the partial pressure of hydrogen gas in the deposition chamber exceeds 0.1%, crystallization was disrupted. The characteristics of TFTs and pn junction diodes showed its potential applicability to AMLCDs and sollar cells.

Time-delay problems will become extremely severe in active-matrix-addressed ultra-large liquid-crystal display-panels. The problem caused by large crossover capacitance can be solved by moving the signal bus-line to the facing glass plate. The problem caused by large gate capacitance can be solved only by reducing the TFT channel much shorter than the conventional value. This requirement can be satisfied by a novel vertical TFT fabricated by using a selective excimer-laser crystallization method. Switching characteristics have been evaluated for the prototype devices, and good on-characteristics have been confirmed for the O.5μm-long gate device. Improved device structures are also presented.

A new source gas mixture of Si(NCO)4 and N(CH3)3 has been investigated for low-temperature chemical vapor deposition of silicon dioxide. Hydrogen-free films have been successfully deposited at 200° C for the first time. Evaluated results are also presented for deposition characteristics and deposited film properties. The breakdown-field strength and low-field resistivity were as high as 5.9 MV/cm and 6×1016 Ωcm, respectively, for the as-deposited film.

Experimental works have been reviewed on poly-Si/poly-SiCx hetero TFTs aiming at extremely low off-current even under intense light illumination conditions with reasonable field-effect mobility. The results indicated that the hetero TFT having the stacked poly-Si/poly-SiCx layers is promising as a switching device in matrices

The cycle time for a Si-ALE growth was shortened by confined gas exposures. A Si(111) substrate at 600°C was sequentially exposed to SiH2Cl2 (DCS) as a source gas confined in a chamber and to atomic H as an active reducer gas created by H2 decomposition at a hot W filament in the chamber. An ideal growth rate was achieved with the DCS confinement duration of 3 s. The atomic H irradiation duration was shortened to less than 7 s, by H2 confinement. Various advantages of the confinement are also discussed.

XPS measurement showed that undesirable SiNH component was reduced drastically from the low-temperature deposited SiN surface by intense ArF excimer-laser irradiation. Although the improved layer was as thin as 15nm, it was very effective to stop diffusion of N atoms from the bottom SiN layer to the top Si layer during the excimer-laser recrystallization step. N-diffused Si layer at the Si/SiN interface was less than the XPS resolution limit for the pre-annealed SiN structure, but about 5nm thick. As a result, the field-effect mobility of the poly-Si/SiN TFT was increased drastically by laser-irradiation to SiN film. Annealing characteristics are also presented for the various SiN film thicknesses and for both the ArF and KrF excimer-laser lights.

A novel technique has been proposed for selective and in-situ excimer-laser crystallization and doping to thin poly-Si films. Dopant atoms are supplied, during the Si laser crystallization process, to the Si film on glass from the doped SOG (spin-on-glass) film coated on the top. Conductivity of the processed film was increased to more than 10S/cm from about 10−8S/cm of the starting film. This technique has been applied to the bottom gate Amorphous-Si TFTs with self-aligned poly-Si source and drain. The electron field-effect mobility was 1.0cm2/Vs and the on/off current ratio was more than 106. No parasitic effects were observed, and the hole conduction was effectively suppressed. This in-situ crystallization and doping technique can also be applied to the top gate a-Si TFT process.

It was observed that the bias stressing of low-temperature polycrystalline silicon thin-film transistors (LTPS-TFTs) gave rise to anomalous instabilities in device characteristics, such as negative threshold voltage shifts in both n- and p-channel TFTs, and the recovery of the shift even under stress. From detailed investigations of these phenomena, it was found that the instability is caused by self-heating due to the drain current, and is related to hydrogen and/or water diffusing into the gate oxide from the interlayer dielectric. It is considered that these phenomena will become more significant in future scaled TFTs; therefore, it is important that low-temperature processes taking account of water-related components are carefully introduced.

We have developed a low-temperature (plasma-free) thermal-CVD method for the purpose of eliminating the plasma-CVD method on the amorphous-silicon thin-film transistor (a-Si TFT) process. The motivation comes from the fact that, in the plasma-CVD method, there arc many shortcomings originating from inherent properties of plasma, and that only the CVD method has potentially no drawbacks among various alternative methods. High deposition temperature, the most serious technological barrier in the present a-Si and silicon- nitridc (SiN) CVD methods, has been overcome by the use of high silicon-hydridcs and high nitrogen-hydrides for source gases. Electronic properties of the CVD-produced a-Si film arc improved by post-hydrogenation. Excimer-laser annealing is the next generation technology for improving dramatically the electronic properties of the CVD-produced Si and SiN films. This paper reviews these novel technologies developed in our group aiming at the TFT process, and also the CVD-produced TFT characteristics.

The characteristics of thin-film transistors (TFTs) fabricated on pseudo-single-crystal (PSX)-Si thin films were examined. The variations of mobility were more than the theoretical values derived from the crystallographic orientation dependence of a bulk Si metal–oxide–semiconductor (MOS) transistor. To clarify the origin of this discrepancy, the relationships between the TFT characteristics and the crystallographic orientation of Si films in the channel region were investigated by using an electron backscattering pattern (EBSP) method. It was found that the surface orientation dependence for the PSX-Si TFT was different from that for a bulk Si MOS transistor, especially for the p-channel mode. A group of TFTs having a nearly {100}-oriented nucleus had a mobility close to those of simultaneously processed silicon-on-insulator (SOI) devices in the p-channel mode as well as in the n-channel mode. In contrast, a group of TFTs having a nearly {110}-oriented nucleus had a low and widely scattered mobility. The reason for these results is that twin boundaries with dislocations are easily generated in a grain grown from a {110}-oriented nucleus in order to compensate for the difference of the growth rates in different directions.