Ultrahigh-Performance Polycrystalline Silicon Thin-Film Transistors on Excimer-Laser-Processed Pseudo-Single-Crystal Films

Abstract

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.

Polycrystalline Silicon Thin Film Transistors (Poly-Si TFTs)

“…In contrast to the carrier mobility in a-Si:H, which has remained within the range 0.5-1.0 cm 2 V À1 s À1 over the last 20 years or so, the electron mobility in poly-Si has increased from <5 cm 2 V À1 s À1 to values as high as $900 cm 2 V À1 s À1 over the same period, and this is comparable to the mobility in single crystal SOI MOSFETs (Mitani et al 2008).…”

“…These areas were typically 5 μm square, and small-geometry TFTs (W = 2 μm, L = 1 μm) were used so that they were entirely contained within the grains. In $10 % of the grains, highperformance n-and p-channel TFTs were obtained, with electron and hole mobility values up to $900 and $250 cm 2 V À1 s À1 , respectively, which matched the performance of identically processed single crystal silicon-on-insulator (SOI) MOSFETs (Mitani et al 2008). These high-performance TFTs were located in regions of {100} pseudo-single-crystal (PSX) material, but, as the PSX regions accounted for only 10 % of the crystallized areas, techniques still need to be established which will yield this orientation in all crystallized regions (Mitani et al 2008).…”

“…Various techniques have been explored to address these problems, with one aim being larger grain poly-Si to promote higher carrier mobility. These excimer laser techniques have included sequential lateral solidification (SLS) (Im et al 1998;Sposili and Im 1996;Voutsas 2003;Park et al 2004a), phase-modulated excimer laser annealing (PMELA) (Mitani et al 2008;Oh and Matsumura 1998), and a micro-Czochralski process, μ-Cz, for location-controlled single grain growth (Van der Wilt et al 2001;Rana et al 2005). In all these cases, a technique has been developed to control and extend the lateral grain growth during the SLG process, rather than relying upon the random process, which occurs during conventional ELA.…”

“…With PMELA (Mitani et al 2008;Oh and Matsumura 1998), the laser beam is passed through a phase-shifting mask, so that optical interference in the emergent beam produces the required intensity fluctuation and the preferred implementation uses a single-shot procedure to produce an array of locationcontrolled large grain (or single-grain) areas. These areas were typically 5 μm square, and small-geometry TFTs (W = 2 μm, L = 1 μm) were used so that they were entirely contained within the grains.…”

Oriented Lateral Growth and Defects in Polycrystalline-Silicon Thin Films on Glass Substrates

“…The μ FEn value of an excimer-laser crystallized poly-Si TFT increases with increasing grain size and reaches 320 cm 2 /Vs at an average grain size of 700 nm, at which the dominant factor determining μ FEn varies from grain boundary scattering to lattice scattering (Hara et al, 2002a). Furthermore, a μ FEn value of 914 cm 2 /Vs was reportedly obtained by positioncontrolled large grain growth (Mitani et al, 2008).…”