Nanocrystalline silicon thin-film transistors

High-quality hydrogenated nanociystalline silicon (nc-Si:H), directly-deposited by a standard hydrogenated amorphous silicon (a-Si:H) technology, is considered the most desirable thin-film transistor (TFT) channel material for high-performance TFTs, exhibiting the benefits of both low-temperature polycrystalline silicon (LT poly-Si) TFTs (high mobility, good stability), and a-Si:H TFTs (good uniformity, low thermal budget). It is thus believed that nc-Si:H TFT technology addresses the above benefits and can lead to monolithic total integration of switching devices and peripheral circuit drivers based on complementary metal-oxide-semiconductor (CMOS) logic on large-area glass substrates and even flexible plastics. Hence, this thesis mainly focuses on the nc-Si:H material and device research with the point of view of feasibility of high-performance nc-Si:H technology for high-density TFT active-matrix backplanes.; Undoped and highly phosphorus doped (n+) nc-Si:H films deposited using a high H2 dilution in SiH4 (adding PH 3 for n+ nc-Si:H films) in two different conventional 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) systems (multi-chamber system and single-chamber system) at 75--260°C are presented. The films are systematically evaluated using two development strategies of process parameters (deposition conditions; RF power and, mainly, H2 dilution) and film topology (evolution with film thickness) by means of electrical, structural, and chemical composition characterization measurements. We demonstrate high-crystallinity undoped nc-Si:H and n+ nc-Si:H films with high conductivity for TFT applications. In addition, we study the effects of material stability and post thermal annealing of 75°C undoped and n+ nc-Si:H films for plastic substrate applications. In particular, nc-Si:H films deposited using a single-chamber PECVD system hold very low bulk oxygen concentration (Co 1016-1018 atoms/cm3) in the deposition temperature range of 75--260°C. We propose that atomic hydrogen leads not only to high-crystallinity and high-purity nc-Si:H formation but also to material stability.; In order to examine the impact of highly crystalline undoped nc-Si:H and/or highly conductive n+ nc-Si:H films as active channel and ohmic contact layers, respectively, we fabricate two TFT types of structures, viz. bottom-gate trilayer staggered TFTs and top-gate staggered TFTs. It is expected that the former can use the well-established industrial TFT process, while the latter can deliver high carrier mobility. Basic current-voltage characteristics and electrical stability measurements of fabricated TFTs are carried out. (Abstract shortened by UMI.)...