Tin monoxide (SnO) is a stable p-type oxide semiconductor. This paper reports electrical properties, electronic structures, and thin-film transistors (TFTs) of SnO. Epitaxial films were fabricated by pulsed laser deposition. The Hall mobility and the hole density of the epitaxial films were 2.4 cm 2 V −1 s −1 and 2.5 × 10 17 , respectively. X-ray photoelectron spectroscopy (PES) indicated that the closed-shell 5s 2 orbitals of Sn 2+ ions heavily contribute to the hole conduction path in SnO. Top gate type TFTs (W/L = 300/50 m) employing 20 nm thick SnO channels exhibited field-effect mobilities µ sat = 0.7 cm 2 V −1 s −1 and µ lin = 1.3 cm 2 V −1 s −1 , which are larger by two orders of magnitude than those reported for pchannel oxide TFTs to date. On/off current ratios were ∼10 2 and subthreshold voltage swings (S) ∼7 V/decade. The parameters required for TFT simulations were estimated by ultraviolet PES and first-principles calculations. The TFT simulations indicated that subgap hole trap density in the SnO channel is >10 19 cm −3 , which limits the TFT mobilities and the S values.

Trap densities in amorphous-InGaZnO4 (α-IGZO) are extracted directly from the capacitance-voltage characteristics of thin-film transistors at low frequencies. It is found that the trap densities are flat in the energy gap, and are 1.7×1016cm−3eV−1 in the deep energy far from the conduction band edge (Ec), but become larger near Ec. Moreover, postannealing reduces the trap density near Ec, which is associated with the reduction of the hysteresis in the current-voltage characteristics. The annealed α-IGZO does not have a Gaussian-type state and has fewer tail states than amorphous Si.

Operation characteristics of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) were improved to a subthreshold voltage swing (S) of 217 mV (decade)−1, a mobility of ∼11.4 cm2 (Vs)−1, and a threshold voltage (Vth) of 0.1 V by O3 annealing at a temperature as low as 150 °C. However, the O3 annealing at 300 °C caused serious deterioration and exhibited a bistable transition between a large S state and a large Vth state. This transition is attributed to incorporation of excess oxygen and associated subgap defects with a negative-U characteristic. It also explains why a-IGZO channels deposited at high oxygen pressures do not produce operating TFTs.

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