(a) XRD patterns of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films grown on (001) sapphire substrates with 2θ = 10∘ to 80∘. (b) An enlarging region of 2θ = 14∘ to 24∘ to obviously show the minority phase peak. Note that the symbol (♦) indicates the delafossite phase, while the peak indicated by the symbol (⋄) corresponds to the cubic spinel phase.
AFM three-dimensional images of the CuCr1−xMg x O2 films with the Mg compositions of (a) 0%, (b) 2%, (c) 4%, (d) 6%, (e) 8%, (f) 10%, and (g) 12%, respectively. Note that the scale height is 80 nm and the measured area is 2 × 2 μm2.
The surface SEM images of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films, whose magnification ration is 1:40 000. Note that the picture (h) indicates the cross-sectional image for the film with the Mg composition of 4%, and the scale is given in this picture.
Raman spectra of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films under the excitation line of 488 nm. Note that the dashed lines show the positions of Raman-active phonon modes from the cubic spinel phase, indicated by the symbol (⋄). In addition, the symbol indicates the observed trace from the sapphire substrate.
Transmittance spectra of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films. For comparison, the dashed line is the transmittance spectrum of the sapphire substrate. The inset (a) shows the absorption coefficient for the films with the Mg composition below 6%; and the inset (b) shows the absorption coefficient for the films with a higher Mg composition. Note that the arrow in the inset (a) indicates that there is a minimal band gap for the CuCr1−xMg x O2 films.
(a) Plots of the photon energy for the estimation of direct optical band gap energies from the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films. (b) Plots of the photon energy for the estimation of indirect band gap energies from the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films.
Schematic representation of the electronic band structure for the CuCr1−xMg x O2 films with the Mg compositions of (a) 0%, (b) 6%, and (c) 12%, respectively. EF denotes the Fermi energy level, Emin denotes the weak absorption energy level, E 1 and E 2 indicate the splitting energy levels of the conduction band. Note that the symbols and indicate the nature hole and electron of the CuCrO2 material. However, the symbol ( ) indicates the hole induced by Mg-doping.
(a) Plots of of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films, and the linear fit of each curve determines the slope coefficient k. The arrows show the sample labels with the different component of Mg-doping. The positions of the arrows indicate the crossover (Tcross ) at which the electric conductivity starts to deviate from the thermal activation behavior. Note that the temperature range is 150–400 K. (b) Plots of of the CuCr1−xMg x O2 (0 ≤ x ≤ 12%) films, and the temperature range is 80–220 K.
The Mg composition dependence of (a) the slope coefficient k, (b) the activation energy Ea , and (c) Tcross at which the electric conductivity deviates from a thermal activation behavior, respectively. The solid lines and equations represent the linear fitting results to guide the eyes. Note that the shade part indicates different composition dependence behavior.
Process of the preparation for Mg-doped CuCrO2 precursors and films.
The electrical properties of the CuCr1−xMg x O2 (0% ≤ x ≤ 12%) films at room temperature.
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