Size-dependent optical properties of sputter-deposited nanocrystalline p-type transparent CuAlO2 thin films -: art. no. 084308

Nanocrystalline, p-type semiconducting, transparent CuAlO2 thin films were deposited by direct current sputtering of a prefabricated polycrystalline CuAlO2 target, with deposition time as a variable parameter. Transmission electron micrographs reveal the formation of CuAlO2 nanoparticles. For the films deposited in 3, 9, and 15 min, the average particle sizes are determined to be around 10, 20, and 30 nm, respectively. The interplaner spacings calculated from selected area electron-diffraction patterns obtained from transmission electron microscopy confirmed the proper phase formation of the material. X-ray diffraction measurements of the films deposited for 15 and 45 min show some diffraction peaks, which depict the rhombohedral crystal structure of the material. The band-gap values obtained from the optical transmission and reflection data, for the films deposited in 3 and 9 min, are 3.94 and 3.84 eV, respectively, whereas for those films deposited in 15 and 45 min, the band-gap values lie in the range of 3.72-3.60 eV. These values are quite larger than that of bulk material, which is around 3.34 eV. The particle sizes in these cases are, respectively, 30 and 60 nm. It is well known that quantum confinement in semiconductor nanocrystals leads to the band-gap widening with respect to the bulk material. In this report we have observed an increase in the band gap of the nanostructured p-type semiconducting CuAlO2 thin film with the decrease in the particle size. Room-temperature photoluminescence measurements of this material are reported, which showed an ultraviolet near-band-edge emission around 3.60 eV, which originates from the generation and recombination of electron-hole pairs, namely, excitons. From the semiquantitative arguments it is supposed that the natural layered structure of delafossite CuAlO2 is responsible for the stability of the excitons. Also a blueshift of the emission peaks is observed with a decrease in the particle size, which may be described as another experimental indication of the quantum confinement effect within our nanocrystalline CuAlO2 thin films. The p-type nature of the films was established by hot-probe and thermopower measurements. (C) 2005 American Institute of Physics.