Carrier transport processes in hydrogenated amorphous silicon-based thin-film transistors (a-Si:H TFT's͒ are investigated by spin-dependent transport ͑SDT͒. Spin-dependent photoconductivity ͑SDPC͒ signals arising from less than 10 6 spins in a small transistor are detected with an adequate signal-to-noise ratio. SDPC measurements reveal two different limiting steps for the light-induced leakage current in TFT's depending on the gate voltage: bulk recombination in undoped a-Si:H and recombination near the source junction. Also, the leakage current mechanism under high source-drain fields is identified by SDT measurements in the dark as electron hopping via defect states located at the interface between undoped a-Si:H and the passivation silicon nitride layer. Both silicon dangling bonds and nitrogen dangling bonds seem to be involved in the electron hopping process. At temperatures below 100 K, spin-dependent hopping of electrons in conduction-band tail states is observed. The change of the dominant transport path from extended state conduction to variable range hopping conduction with decreasing temperature is confirmed by SDT measurements.
Low temperature, 600~ annealing of LPCVD films was investigated using x-ray diffraction, ESR, TEM, and carrier mobility measurements. An optimum deposition temperature of about 550~ was found to yield good crystallinity and high electron mobility for annealed films; large grain sizes, a maximum crystallite size, and a maximum electron spin density were also observed for films deposited at the optimum temperature. The crystallite number was shown to be constant if the deposition temperature was below 570~ Electron spring density for the as-deposited films correlated with the crystalline volume by x-ray diffraction measurements on the films after annealing. This implies that only those amorphous components with high electron spin density can be converted into the crystalline phase by 600~ annealing.
Excellent characteristics of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with long and narrow grains aligned one-dimension have been experimentally clarified for the first time. The field effect mobility and on-off transition slope of n-channel and p-channel devices were as high as 685 cm 2 V À1 s À1 and 190 mV/decade and 145 cm 2 V À1 s À1 and 104 mV/ decade, respectively. Fluctuations of characteristics were considerably reduced by widening the channel, and uniform characteristics were observed when there were approximately twenty long grains within the channel. These results were obtained when the TFT channel was formed within a region free from grain boundaries formed by head-on collision of laterally growing grains and seeds used to initiate lateral grain growth. Material properties are discussed from the viewpoint of device characteristics.
Thin-film transistors (TFTs) were fabricated on polycrystalline silicon (poly-Si) films formed by position-controlled largegrain growth technology using an excimer laser. The field-effect mobility, on-off transition slope, and threshold voltage were 914 cm 2 V À1 s À1 , 93 mV/decade, and 0.58 V for the n-channel device, and 254 cm 2 V À1 s À1 , 122 mV/decade, and À0:43 V for the p-channel device, respectively. These values indicate that TFTs had an ultrahigh performance comparable to that of {100}-oriented crystal-silicon metal-oxide-semiconductor (MOS) transistors. Furthermore, their effective mobilities had the same effective field and temperature dependences as those of MOS transistors, indicating that electrons and holes were predominantly scattered not by random grain boundaries or defects in the Si film, but by phonons at the SiO 2 -Si interface, similarly to those of crystal-silicon MOS transistors. These attractive results were obtained as a result of the fact that the TFT channel region was made up of nearly {100}-oriented single grains.
The nonlinear behavior of the modulational instability of nearly constant amplitude, weakly dispersive, weakly nonlinear circularly polarized MHD waves with an initial periodic modulation is studied. The phenomena of recurrence is discussed by results from a multiple time scale development near marginal growth, a numerical study for moderate wavenumbers and finally a study of qualitative dynamics for nearly constant wavetrains. The conclusion is that the phenomena of recurrence and slow randomization of periodic wave motion of nonlinear MHD waves can only occur for large wavenumbers compared to the marginal one. For wavenumbers near the marginal one, it is expected that an initially constant amplitude wave will break irreversibly into a broad spectrum of waves.
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