We have investigated the influence of post-deposition annealing on the optical and electrical properties of c-axis oriented zinc oxide films prepared on sapphire substrates by electron beam evaporation. The ZnO films as-deposited and annealed in air were colourless and transparent in visible range and had sharp ultraviolet absorption edges. It is found that the optical bandgap energy of the films lies in the range of ∼3.27 to ∼3.30 eV depending on the annealing regime. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states extending into the bandgap was obtained and a value of 44 meV can be achieved by annealing in air.
Transparent conductive films of 2 at% Ga-doped ZnO films were prepared on C-plane sapphire substrates by e-beam evaporation in vacuum. The optical absorption, reflectance, structural and electrical properties of 2 at% Ga-doped ZnO films were investigated. The films are highly transparent (>80%) in visible-NIR ranges, and the optical bandgap exhibits a blue shift for the as-deposited films from 3.30 eV to 3.83 eV and for heat treatment from 3.27 eV to 3.60 eV for 2 at% Ga-doped ZnO films with respect to pure ZnO films. Through resistivity, optical constants (ε, σ , -Im ε −1 and ω p ) and carrier concentration obtained from reflectivity and transmittance spectra for 2 at% Ga-doped ZnO films, we found that these films behave as n-type semiconductors exhibiting high carrier concentration N ∼ 10 21 cm −3 . This also gives an opportunity to predict electrical behaviour of transparent conductive films on the basis of the analysis of absorption and reflection measurements.
Lithium-doped zinc oxide (ZnO) films were prepared on C-plane sapphire substrates by the e-beam deposition technique in vacuum. The dependences of the optical absorption edge characteristics, structural, and photoelectrical properties on Li content (from 0 to 10 at.%) were investigated. It is found that the variations of the energy bandgap E g , the spread in the tail of the band edge (parameter E 0 ), the electrical resistivity ρ, and the lattice constant c with impurity concentration do not have monotonic character. Abrupt jumps of the optical and electrical characteristics were observed as the impurity concentration changed near 0.8 at.% Li. The significant enhancement of photoconductivity response for 0.8 at.% Li-doped ZnO films also correlates with the enhanced electrical resistivity up to 2 × 10 6 Ω cm and the appearance of a photoinjection current.
Resistance switching (RS) characteristics of Al/ZnO:Li/LaB 6 and Al/ZnO:Li/La 2 O 3 /LaB 6 devices in which LaB 6 and lithium-doped ZnO (ZnO:Li) films are regarded as shallow work function metal and p-type semiconductor, respectively, are studied. The alternation from bistable unipolar memory switching (URS) to monostable threshold switching (MTS) in the Al/ZnO:Li/LaB 6 device is observed. These two switching behaviors can be activated separately depending on the polarity of applied dc voltage: with a positive polarity the URS behavior is measured, while the MTS behavior is observed with a negative polarity. With increase in the number of switching cycles, the MTS and URS behaviors irreversibly transform to bipolar resistance switching (BRS) behavior. The Al/ZnO:Li/La 2 O 3 /LaB 6 device shows only the BRS behavior, but after certain number of cycles device serves as a rectifying diode. On the basis of the I-V and C-V characteristics, it is concluded that RS properties depend on the barrier height and reactivity between LaB 6 metal and ZnO:Li oxide.
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