Crystallized poly-Si Thin Film Transistors Research Articles

Electrical Stress Effect on On-Current of MILC Poly-Si Thin-Film Transistors

In this study, we studied the effect of the electrical stress on the on-current of metal-induced laterally crystallized poly-Si TFTs. It was found that the electrical performance of polycrystalline silicon thin-film transistors (TFTs) is greatly affected by the electrical stress. Under the electrical stress condition, the drain current increases due to hot-electron trap at the drain junction. The computer simulation revealed the fact that the improvement mechanism can be reproduced by effective channel length shortening. It turns out that analysis of the capacitance and output characteristics supports this model.

Meyer–Neldel Rule for Effective Channel Mobility in the Subthreshold Region of Poly-Si Thin-Film Transistors

It is verified that the entire subthreshold region of poly-Si thin film transistors (TFTs) is a pseudo-subthreshold region dominated by drift current, where the carrier effective channel mobility is found to follow the Meyer–Neldel rule (MNR). Characteristic MN energies extracted from both metal-induced laterally crystallized poly-Si TFTs and excimer laser annealed TFTs are all close to the optical phonon energy of Si, providing strong evidence to the MNR. Carrier thermionic emission over grain boundary barriers activated by multiphonon absorption process is the origin of the MNR.

Channel Width Splitting Effect on Driving Characteristics of Silicide Seed-Induced Laterally Crystallized Poly-Si Thin-Film Transistors

The electrical performance of low-temperature polycrystalline silicon thin-film transistors (TFTs) is greatly affected by structures such as channel geometry, lightly doped drain, film thickness, etc. In this paper, the split channel polycrystalline silicon TFTs are fabricated through the silicide seed induced lateral crystallization method by dividing channel width with fixed numerical channel size. The number of channels splitting from 1 to 10 significantly improves driving characteristics such as driving current, field-effect mobility, subthreshold slope, and threshold voltage. It is found that the two main causes of improvements are the enlargement of effective channel width and a decrease in series resistance due to the crystal filtering effect on narrowly split channels, which lead to trap-state density reduction during lateral crystal growth. The causes of enlarged effective channel width and the decreased series resistance by the crystal filtering effect are demonstrated through capacitance-voltage and trap-state density extraction.

Evaluation of Thermal Annealing Before and After Formation of Gate Insulator Films by Extracting Trap Densities for SPC Poly-Si TFTs

We have evaluated the effects of thermal annealing before formation of gate insulator films (preannealing) and thermal annealing after formation of gate insulator films (postannealing) by extracting trap densities for solid-phase crystallized poly-Si thin-film transistors. It is suggested that the preannealing densifies the poly-Si films and induces additional internal stress after the thermal oxidation of the gate insulator films. On the other hand, the postannealing also relieves the internal stress. These conflicting effects indicate that the order of the thermal annealing is very important to improve device performance.

Comparison of transistor characteristics between excimer-laser and solid-phase crystallized poly-Si thin-film transistors

We have compared transistor characteristics between excimer-laser crystallized (ELC) and solid-phase crystallized (SPC) poly-Si thin-film transistors (TFTs). First, off characteristic and characteristic parameters, such as the subthreshold swing and field-effect mobility, of the ELC TFT are superior to those of the SPC TFT, which indicates that the trap states in the ELC TFT are fewer than those in the SPC TFT. Next, the activation energy (Ea) for the on characteristic of the ELC TFT is larger than that of the SPC TFT, which originates from the difference of the grain structures. Moreover, Ea for the off characteristic of the ELC TFT is much larger than that of the SPC TFT, which is caused by the difference of the trap distribution.

Performance enhancement of excimer laser crystallized poly-Si thin film transistors with fluorine implantation technology

Polycrystalline silicon thin film transistors (poly-Si TFTs) with the ion implantation of fluorine elements were investigated in this study. The electrical performance and reliability were reported comprehensively. Experimental work has shown the electrical characteristics of excimer laser crystallized F-ions-implanted poly-Si TFTs are improved effectively, especially for field effect mobility. It is also found that the fluorine piled up at the poly-Si interface during thermal annealing, for the TFT fabricated without a prior deposition of pad oxide. The stronger Si–F bonds replace the Si–Si/Si–H, leading to the superior electrical reliability. However, the dose of F ions is critical in poly-Si, or the electrical characteristics of TFT devices will be degraded.

Near-infrared femtosecond laser crystallized poly-Si thin film transistors

Polycrystalline silicon (poly-Si) thin film transistors (TFTs) fabricated by near-infrared femtosecond laser annealing (FLA) are demonstrated. The FLA-annealed poly-Si channels exhibit low tail-state, deep-state, and midgap-state densities of grain traps. Characteristics such as field-effect mobility, threshold voltage, and subthreshold slope for FLA-annealed poly-TFTs are comparable to those of conventional approaches. A wide process window for annealing laser fluences was confirmed by examining the changes in electrical parameters for transistors with various channel dimensions.

Effect of the Grain Growth Process on the Characteristics for the Excimer Laser Crystallized Poly-Si Thin Film Transistors

AbstractWe have found, for excimer laser crystallized poly-Si thin films, that there are two different grain growth processes that depend on the energy density. Columnar grains grow laterally at lower energy densities. The other grain growth process at higher energy densities is shown to be secondary grain growth caused by a less oriented structure with fine granular grains. A TFT with the maximum mobility is obtained at the border for the lower energy grain growth. Grain boundary and intragrain defects around grain boundary formed through the secondary grain growth reduce the mobility in spite of considerable grain enlargement.