Electronic transport and optical properties of proton-implanted amorphous 2CdO·GeO2 films

Films of amorphous 2CdO·GeO2 with the band gap of 3.4 eV were prepared by rf sputtering. Protons were implanted into the films at doses 2×1014–2×1016 cm−2. On going from 2×1014 to 2×1016 cm−2, dc conductivity at 300 K increased from ∼10−9 to ∼101 S cm−1 and its activation energy fell from ∼1 eV to almost zero (degenerate state). This result indicates that the Fermi level of this amorphous material may be controlled by proton implantation. The sign of Hall and Seebeck coefficients were negative, showing n-type electrical conduction and no pn sign anomaly in Hall voltages. The Hall mobility was of the order of 10 cm2 V−1 s−1 (even at carrier concentration of ∼3×1019 cm−3), which is larger by several orders of magnitude than that of existing amorphous semiconductors. No degradation of visible transparency was observed in all implanted samples. The optical conductivity may be described by the classical Drude formula with a single relaxation time, 2.7×10−15 s. X-ray radial distribution function revealed that the local structure around Cd2+ (coordination number; ∼6) and Ge4+ (∼4) in the amorphous state is close to that of crystalline Cd2GeO4 and the amorphous state has a distribution of Cd–O–Cd bond angles. We assume that the electronic transport properties of the present material primarily originate from the extended conduction bands composed of Cd 5s orbitals.