| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 96 === This dissertation explores and demonstrates enhanced electrical characteristics and reliability of continuous-wave green laser fabricated epi-like silicon transistors on non-silicon substrates, which greatly impacts on active-matrix no/off circuits, nonvolatile memories, linear image sensors, and photo-detector amplifiers for various panels and photonic circuits. The main focus of this dissertation can be divided into four parts.
First, electrical characteristics of continuous-wave (CW) green laser-annealed single-grainlike silicon thin-film transistors in relation to trap-state densities were characterized. As laser power increases, highly crystalline channels form, reducing tail-state densities to as low as 3x1019 eV-1cm-3. This occurrence is responsible for high field-effect electron mobility of 284 cm2/Vs. In contrast, increasing laser power initially reduces the deep-state density and then increases it to 3x1016 eV-1cm-3. This reversal in deep-state density, and thus in the subthreshold slope, as well as a saturating reduction in threshold voltage are associated with the formation of extra interface defects caused by laser-crystallization-enhanced surface roughness.
Next, stability of high hole-mobility thin-film transistors (TFTs) on single-grainlike silicon channels formed by CW laser-crystallization (CLC) during hot-carrier stressing (HCS) was studied. As channel layers become thicker, laser-mediated channel crystallinity increases, increasing channel roughness. On such epi-like polycrystalline silicon (poly-Si) substrates, the poorer interface quality for thicker channels, even those with lower tail-state densities of grain traps, is responsible for the extensive charge trapping and creation of deep-state densities in the fabricated TFTs due to HCS. Hence, on a thin channel with a thickness of 50 nm and ultra-smooth surfaces, HCS hardly degrades the electrical parameters of the devices.
Third, the hole-mobility and reliability of green CW laser-crystallized epi-like Si transistors on glass panel substrates were enhanced by source/drain activation by back-side green laser-irradiation. Green laser-energy was scanned uniformly across junctions, since the gate structures included no interrupt, in an attempt to conduct super visible-laser lateral-activation. The enhancement was thus explained by the formation of continuous improved epi-like Si microstructures with reduced grain defects and with a barely increased number of interface defects over the entire channel/junction. The hole-mobility in such laser-activated devices was as high as 403 cm2/V.s – doubles that of thermally activated devices.
Fourth, panel transistors were activated by front-side CW green laser irradiation. In self-aligned poly-Si TFTs, significant laser-energy penetrates through poly-Si gates owing to the considerably long penetration depth of green light in poly-Si. Green laser-energy was thus uniformly scanned laterally from channels to source/drain regions and, vice versa, in under a millisecond, hardly affected by gate structures. Such spike green-laser annealing yields low parasitic source/drain resistance and quasi-continuous improved poly-Si microstructures in green laser-activated TFTs, with reduced grain defects over the entire channel/junction. Electron-mobility and sub-threshold slope for such transistors that were fabricated on CLC channels of 100 nm, were remarkable values of 530 cm2/V.s and 120 mV/dec, respectively. In gate structures of TiN/SiO2, laser-activated panel transistors that were fabricated on CLC channels of 100 nm, also revealed electron-mobility as high as 230 cm2/V.s. In metal gated panel transistors, front-side CW green laser irradiation intrinsically activates source/drain regions selectively, because of light reflection by metal gates, causes little thermal damage on materials underneath metal gates, which endorses advanced panel or photonic transistors with compound, nanostructured, and functionalized gate dielectrics or polycrystalline materials.
In future, those CW green laser-fabricated transistors will be routines for the development of panel photo-sensors and memories, as well as nano-photonic circuits.
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