Masahiro Okabe

POLYCRYSTALLINE SILICON FORMED BY ULTRAHIGH-VACUUM SPUTTERING SYSTEM

Using an ultrahigh‐vacuum (UHV) sputtering system, we could grow two new methods of polycrystalline silicon films. The one is as‐deposited polycrystalline silicon on glass at substrate temperatures under 500 °C. The other is solid‐phase‐crystallization by thermal annealing of as‐deposited amorphous silicon films in a UHV. As‐deposited polycrystalline silicon films were oriented to (220) and grain sizes were determined from half‐width of x‐ray diffraction to be about 40 nm. From the deposition temperature dependence of the x‐ray diffraction peak intensity, the activation energy of the crystalline growth was calculated to be about 0.6 eV. Hydrogen atoms in the sputtering gas lower the reproducibility of as‐deposited poly‐Si. Polycrystalline silicon films produced by thermal annealing of as‐deposited amorphous silicon films at 550 °C in UHV have a (111) orientation. Field‐effect mobilities of the as‐deposited polycrystalline silicon film and the polycrystalline silicon film by UHV thermal annealing were 5 an...

Micro-Scale Characterization of Crystalline Phase and Stress in Laser-Crystallized Poly-Si Thin Films by Raman Spectroscopy

Laser crystallized poly-Si thin films were characterized using micro-Raman spectroscopy. Mapping measurements of the Si optical-phonon line were carried out for large size grains generated by the super-lateral growth, where the mapping interval was as small as 0.2 µm. It was found that the spectral intensity is extremely large at ridges located at grain boundaries. The map of peak-frequency shift indicated that the tensile stress tends to accumulate in grains and relax at grain boundaries. The map of spectral width revealed the abrupt variation of the crystalline phase from large-size grains to microcrystals.

Wide‐viewing‐angle full‐color TFT‐LCDs

— Conventional TFT TN-LCDs display full-color images only within a narrow vertical viewing angle. Viewed obliquely, the display reverses or brightens. We studied these problems by calculating the light propagation through TN cells and concluded that these reversal phenomena are unavoidable in TN-LCDs. To widen the viewing angle, we propose a film-laid double-twisted-nematic display (FDTN), consisting of a conventional TN panel, a light-compensation panel, and two retardation films stacked together. The problem of black images appearing purple is solved using a clockwise-twisted nematic panel as an optical compensator. The viewing angle of the FDTN-LCD is about 1.5 times that of the conventional positive TN-LCD. The contrast ratio normal to the panel exceeds 100.

Silicon-Hydrogen Bonds in Laser-Crystallized Polysilicon Thin Films and Their Effects on Electron Mobility

In this paper, we describe the behavior of H atoms in laser-crystallized poly-Si for thin-film transistors on liquid crystal display panels, where H atoms were introduced by plasma hydrogenation in order to improve mobility. Mobility was determined by the Hall effect measurement. Si-hydrogen bonds were analyzed by Raman scattering. By short-time hydrogenation, the introduced H atoms terminate the dangling bonds in the Si–H configuration mainly at the grain boundaries, which results in the improvement of mobility. With excessive hydrogenation, Si–H2 bonds are generated simultaneously with the degradation of mobility. Si–H2 bonds are mainly formed at the in-grain defects. Hydrogenation using the hot-wire method was also carried out and it was shown that plasma damage does not influence the hydrogenation effects. The relationship among Si–H2, mobility and the amount of in-grain defects was discussed, based on impurity scattering and weak-bond models.

Crystallization at initial stage of low‐temperature polycrystalline silicon growth using ZnS buffer layer with 〈111〉 preferred orientation

We developed a low‐temperature growth technique for polycrystalline silicon (poly‐Si). When Si is deposited on glass substrates at 450 °C, it crystallizes as thickness increases, but 10‐nm‐thick layers of Si are mainly amorphous. Use of a ZnS buffer layer with 〈111〉 preferred orientation facilities crystallization of Si during the initial growth stages. The preferred orientation of poly‐Si on glass substrates is 〈110〉, while that of poly‐Si on the ZnS buffer layer is 〈111〉. This is probably due to local epitaxial growth on polycrystalline ZnS grains with 〈111〉 preferred orientation. Raman spectroscopy showed that the ZnS buffer layer significantly improved the crystallinity of 25‐nm‐thick Si layers.

Effect of hydrogen ion shower doping in polycrystalline silicon thin‐film transistors

We investigated the effect of hydrogen ion shower doping on polycrystallinesilicon thin‐film transistors (p‐Si TFTs). Hydrogen atoms were introduced to the channel region of p‐Si TFTs by PH3/H2 ion shower doping of the source/drain contact. Hydrogen concentration in the channel region can be controlled by altering the gate metal thickness. Hydrogen atoms affect the TFT’s threshold voltage shifts until it becomes negative, in n‐type TFTs. The threshold voltage shift depends on the hydrogen content of the channel region in p‐Si TFTs. This is explained by the existence of Si− 3 trap states in the grain boundaries.

Direct-view polymer-dispersed LCD with crossed Nicols and uniaxial film

— By adding a uniaxial retardation film and using a chlorinated LC mixture, the performance of polymer-dispersed LCDs with crossed Nicols (PDN-LCDs) was improved. A wide viewing angle of more than 60° (contrast ratio >5) for all azimuth directions was obtained. In the horizontal and vertical viewing azimuth, the contrast ratio was over 30 even at an inclination viewing angle of 60°.

Noncrossing TFT matrix with reduced dc level shift and crosstalk

— A noncrossing TFT matrix and drive scheme that eliminates dc level-shift differences among data and reduces crosstalk without a storage capacitor is proposed. To compensate for the dc level shift, an extra TFT is added to each pixel. By applying a compensation pulse to the TFT, the dc level-shift differences among data are reduced to less than 0.01 V. The compensating TFT also provides redundancy. The peak-to-peak data voltage amplitude is lowered by changing the reference voltage according to the LC cell voltage polarity. By using the lowered data voltage and the shielded reference bus structure, crosstalk is reduced sufficiently to allow for 64 gray levels.