Cu 2 O epitaxial films were grown for high mobility p-channel oxide thin-film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase epitaxial films with hole Hall mobilities ∼90 cm2 V−1 s−1 comparable to those of single crystals (∼100 cm2 V−1 s−1). TFTs using the epitaxial film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory (∼0.26 cm2 V−1 s−1 and ∼6, respectively).

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Epitaxial films of a deep-ultraviolet transparent oxide semiconductor, Ga2O3, were fabricated on α-Al2O3 (0001) substrates by pulsed laser deposition. Four-axes x-ray diffraction measurements revealed that the tin-doped Ga2O3 films have a crystal structure different from any known polymorphs of Ga2O3. Its crystal lattice was determined to be an orthorhombic. Top gate field-effect transistor structures were fabricated using the Ga2O3 epitaxial films for n-channels. The channel conductance was modulated by an order of magnitude by gate voltage at room temperature with an estimated field-effect mobility of 5×10−2cm2(Vs)−1.

We grew epitaxial (110) Cu 2 O films on (110) MgO substrates toward high-mobility p-channel oxide thin-film transistors (TFTs). The (110) Cu 2 O films exhibited high Hall mobilities ∼90 cm 2 (Vs) −1 comparable to those of high-quality singlecrystals, which were obtained in a narrow growth condition for 650 nm-thick films. TFTs using the epitaxial (110) Cu 2 O channels exhibited p-channel operation, but the field-effect mobilities and the on-to-off drain current ratio were far from satisfaction (∼0.04 cm 2 (Vs) −1 and ∼2, respectively).In order to investigate the origin of the poor mobility, the films were subjected to post-deposition annealing under various oxygen partial pressures (P O2−A = 0.65-10 −3 Pa). Optical measurements revealed that subgap states exist in all the films and their amounts were increased by postdeposition annealing irrespective of P O2−A . The subgap density of states estimated by the optical analyses are consistent roughly with that estimated from the TFT mobility.

Thin-film photovoltaics (PV) have emerged as a technology that can meet the growing demands for efficient and low-cost large-scale cells. However, the photoabsorbers currently in use contain expensive or toxic elements, and the difficulty in bipolar doping, particularly in a device structure, requires elaborate optimization of the heterostructures for improving the efficiency. This study shows that bipolar doping with high hole and electron mobilities in copper nitride (Cu N), composed solely of earth-abundant and environmentally benign elements, is readily available through a novel gaseous direct nitriding reaction applicable to uniform and large-area deposition. A high-quality undoped Cu N film is essentially an n-type semiconductor, while p-type conductivity is realized by interstitial fluorine doping, as predicted using density functional theory calculations and directly proven by atomically resolved imaging. The synthetic methodology for high-quality p-type and n-type films paves the way for the application of Cu N as an alternative absorber in thin-film PV.