Cuprous oxide (Cu₂O) is a p-type oxide semiconductor expected as active layers of optoelectronic devices such as solar cells and thin film transistors (TFTs) because it has a large hole mobility ~100 cm² V^-1s^-1 for single crystal [1]. However, it is in general difficult to grow high quality Cu₂O thin films exhibiting large mobility because the chemical stoichiometry is easily changed by growth conditions. Here, we report the epitaxial film growth of high mobility Cu₂O films on MgO (110) single crystal substrates. We also succeeded in p-channel operation of Cu₂O channel TFTs.

The Cu₂O films were prepared by a pulsed laser deposition technique using a Cu metal target. The substrate temperature (T_sub= 650 – 800 ^oC) and the oxygen partial pressure (Po₂= 10^-3 – 10⁰ Pa) were varied. Hole mobility (m_h) and carrier concentration (N_h) were measured by Hall effect using the van der Paw configuration. TFTs with amorphous Al₂O₃ gate insulators were fabricated by a standard photolithography and lift-off process.

It was found that pure semiconducting Cu₂O films were obtained in growth conditions of T_sub= 650 – 700 ^oC and Po₂= 0.30 – 1.0 Pa, while Cu impurity was observed at Po₂ <0.30 Pa and Cu₂O films were polycrystalline insulating at T_sub≥ 730 ^oC. The high-resolution X-ray diffraction measurements revealed that the pure semiconducting Cu₂O films were grown epitaxially with the relationship of [110] Cu₂O // [110] MgO in out-of-plane and [1-10] Cu₂O // [1-10] MgO in in-plane. The tilting and twisting angles of the crystallites were 0.25 and 0.20 degrees, respectively, indicating that the films have high crystalline quality. 650-nm-thick Cu₂O films showed large hole mobilities up to m_h ~ 90 cm² V^-1s^-1 (N_h ~ 1.6×10¹⁴ cm^-3), which is comparable to that of a single crystal.

The TFTs using Cu₂O channels exhibited typical p-channel behavior in a depilation mode. The field-effect mobility was estimated to be ~0.25 cm² V^-1s^-1.

[1] E. Fortin et al.,Can. J. Phys., 44, 1551 (1966).

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