Tin dioxide SnO₂ is an eminent n-type conductive oxide with a wide bandgap and a high conductivity, and therefore extensive investigations have been carried out to apply them for transparent electrodes, gas sensors, and thin film transistors (TFTs). TFTs using oxide channels have been limited to n-channel type because there are a few materials system known as p-type semiconductors. Tin monoxide, SnO, is a unique and interesting material in this aspect; SnO films show p-type conduction ⁽¹⁾, which is thought to originate from the fact that the hole transport paths are made from the closed-shell 5s² orbitals of Sn²⁺ ions. Nevertheless, there have been few reports even on transport and optical properties of SnO. Here, we report epitaxial film growth of SnO and its electronic and optical properties. We have succeeded in operating p-channel TFTs with reasonable mobilities as well.

The epitaxial SnO films were deposited by PLD on YSZ(001) substrates. The substrate temperature and the oxygen partial pressure were varied from 400-700℃ and 10^-4 ~10^-1 Pa, respectively. The epitaxial relationships were (001)SnO//(001)YSZ in out-of-plane and (100)SnO//(1-10)YSZ in in-plane. Hall effect measurements indicated that the film had a hole density of 2.5 x 10¹⁷ cm^-3 and a hole mobility of 2.4 cm²(Vs)^-1 at RT. 20 nm thick SnO layers were employed as channels of top gate type TFTs (W/L = 300/50 μm). 210nm thick amorphous almina layers were deposited by PLD as gate insulators. Finally, the TFTs were annealed by RTA in vacuum at 200 ^oC for 5 min to reduce the gate leakage current. The TFTs operated in a p-type depletion mode with a threshold voltage of ~5V and an on/off ratio of ~10². Field effect mobility and saturation mobility were 1.2 cm²(Vs)^-1 and 0.7 cm²(Vs)^-1, respectively.

⁽¹⁾X. Q. Pan et. al., J. Electroceram. 7, 35 (2001).

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