Annealing effect on the structural and optoelectronic properties of Cu-Cr-O thin films deposited by reactive magnetron sputtering using a single CuCr target

The variation in the chemical composition of Cu-Cr-O films with annealing temperature is revealed in Table 1, which shows two significant findings. First, the ratio of Cu to Cr in these films was measured to be about 2, although an equimolar CuCr target was used in this study. The higher Cu content in the films may be due to the difference in sputtering yields between Cu and Cr [15]. Second, only marginal variation was observed in the chemical composition with an increment in annealing temperature. The evaporation of these constituent elements did not occur in the films even when annealed at temperatures up to 800°C.

Figure 1 shows the XRD patterns of the Cu-Cr-O films with different post-annealing temperatures. No visible diffraction peaks appeared in the as-deposited film, indicating that it was amorphous. As the annealing temperature was heightened to 500°C, the film began to crystallize, mainly forming spinel CuCr₂O₄ and monoclinic CuO phases. From the perspective of elemental composition, the higher the copper content, the more preferentially the copper-rich phase is formed. However, in terms of thermodynamics, the Gibbs free energy of the CuCr₂O₄ phase is relatively negative at lower temperatures, indicating that the generation of CuCr₂O₄ phase is thermodynamically favorable [16]. When the annealing temperature attained 600°C, the spinel CuCr₂O₄ phase disappeared, and it was replaced by the delafossite CuCrO₂ phase, because increasing the annealing temperature rapidly reduces the Gibbs free energy of the CuCrO₂ phase even lower than that of the CuCr₂O₄ phase, thereby promoting the reaction of CuCr₂O₄ with CuO to form CuCrO₂. These relevant reaction equations and thermodynamic data can be expressed as follows: 1 1/2Cu2O(s)+1/4O2(g)→CuO(s) \[\begin{align}1/2C{{u}_{2}}{{\text{O}}_{(\text{s})}}+1/4{{\text{O}}_{2(\text{g})}}\to \text{Cu}{{\text{O}}_{(\text{s})}} \end{align}\] 2 ΔG1=ΔG1∘+RTln(pO2)=−65222.25+46.955T−RTln(pO2) 3 CuO(s)+Cr2O3(s)→CuCr2O4(s) \[\begin{align}Cu{{O}_{(\text{s})}}+C{{r}_{2}}{{\text{O}}_{3(\text{s})}}\to CuC{{r}_{2}}{{\text{O}}_{4(\text{s})}} \end{align}\] 4 ΔG2=ΔG2∘+RTln(pO2)=3667−15.85T \[\begin{align}\Delta\!\!\text{ }{{\text{G}}_{2}}=\text{ }\!\!\Delta\!\!\text{ G}_{2}^{\circ }+RT\ln (\text{p}{{\text{O}}_{2}})=3667-15.85\text{T} \end{align}\] 5 CuCr2O4(s)+CuO(s)→2CuCrO2(s)+1/2O2(g) \[\begin{align} CuC{{r}_{2}}{{\text{O}}_{4(\text{s})}}+Cu{{\text{O}}_{(\text{s})}}\to 2CuCr{{\text{O}}_{2(\text{s})}} \\ +1/2{{\text{O}}_{2(\text{g})}} \\ \end{align}\] 6 ΔG3=ΔG3∘+RTln(pO2)=79512.5−68.15T−RTln(pO2) \[\begin{align} \text{ }\!\!\Delta\!\!\text{ }{{\text{G}}_{3}} & =\text{ }\!\!\Delta\!\!\text{ G}_{3}^{\circ }+RT\ln (\text{p}{{\text{O}}_{2}})=79512.5 \\ {} & -68.15\text{T}-RT\ln (\text{p}{{\text{O}}_{2}}) \\ \end{align}\] where ΔG and ΔG° are the variations of the Gibbs free energy and the standard Gibbs free energy, respectively. R is the gas constant, T is the absolute temperature, and pO₂ is the oxygen partial pressure. In these equations, the subscript “(s)” is utilized to indicate substances in the solid state, while the subscript “(g)” represents substances in the gaseous state. From the above equations, when the pO₂ is 7.0 × 10⁻⁵ atm, the ΔG can be calculated, and the variation of ΔG with temperature can be drawn, as shown in Figure 2. When the annealing temperature exceeds 600°C, it is found that ΔG₃<ΔG₂<ΔG₁, which means that the CuCrO₂ phase becomes a thermodynamically favorable phase. As the annealing temperature was heightened to 800°C, the diffraction peaks became stronger and sharper, implying grain growth in the CuCrO₂ and CuO phases.

Fig. 1.

Fig. 2.

Figure 3 shows the SEM image of the Cu-Cr-O films with different post-annealing temperatures. The as-deposited film was composed of nanoparticles with size ranging from approximately 4 nm to 10 nm (Figure 3a). An annealing procedure at 500°C generated larger particles, about 30–40 nm in size, with an agglomerate surface characteristic that left voids inside the film (Figure 3b). With the further increase in annealing temperature to 800°C, the film grains grew significantly, and the particles size increased to hundreds of nanometers (Figures 3c–3e). While higher temperature increases particle size by enhancing atomic diffusion, it also enlarges the void by causing atoms to coalesce near the void edges due to capillary forces.

Fig. 3.

Thus, whether the film properties obtained by high-temperature annealing are better is worth exploring. Figure 4 shows the AFM images of the Cu-Cr-O films with different post-annealing temperatures. These results were in agreement with the FESEM observations. The high heat induces coalescence of adjacent grains, resulting in a rapid increase in RMS roughness from 0.75 nm to 20.01 nm with increasing annealing temperature. Figure 5 shows TEM images with the selected area diffraction (SAD) patterns of the Cu-Cr-O films with different post-annealing temperatures. The 500°C-annealed film consisted of equiaxed CuO and CuCr₂O₄ grains (Figure 5a). The grains and void size increased along the film growth direction. The thickness-dependent change in grain size is a common phenomenon for annealed growth. When the annealing temperature was increased to 600°C, the film exhibited increased void density and grain growth (Figure 5b). In this situation, the CuCr₂O₄ reacted with CuO to form CuCrO₂ until CuCr₂O₄ was depleted. However, no distinct two-phase structure in this film could be observed, probably due to insufficient atom diffusion at the present annealing temperature. When the annealing temperature was continued to increase to 800°C, these grains and voids also continued to grow (Figure 5c). Visible phase segregation was present in the microstructure, as confirmed by EDS analysis (not shown). The above findings were in good agreement with the XRD analysis.

Fig. 4.

Fig. 5.

Figure 6a shows the light transmittance of the Cu-Cr-O films with different post-annealing temperatures. The as-deposited film only exhibited a very low transmittance of 7.9% in a wavelength region of 500–900 nm. Increasing the annealing temperature to 500°C resulted in sharper fundamental absorption edges and increased transmittance up to 31.4%, which was caused by improved crystal quality. These CuO and CuCr₂O₄ with narrow band gap and high photo response in the film were responsible for this result [17, 18]. At an annealing temperature of 600°C, the film achieved the highest transmittance of 49.5%. The increase in grain size and the replacement of the CuCr₂O₄ phase by the CuCrO₂ phase with a wide band gap were the major factors. When the annealing temperature was 800 °C, however, the transmission of the film decreased to 36.7%, which was associated with higher roughness, increased void defects, and larger CuO grains. The part of incident light was redirected to diffuse scattering instead of specular scattering because of the increment in surface roughness and void defects [19]. The as-deposited film was characterized by a gentle absorption edge, implying its high defect density in film. As annealing temperature increased, the absorption edge became sharper, which was attributed to crystallization and grain growth. The band gap of the Cu-Cr-O films was measured by a linear extrapolation on the x-axis of plot of the (αhv)² vs. photon energy (hv), as shown in Figure 6b. The band gap energy values of the as-deposited, 500°C-annealed, 600°C-annealed, 700°C-annealed, and 800°C-annealed films were found to be 1.25, 1.78, 1.98, 1.96, and 1.84 eV, respectively. The low band gap of the deposited films was also associated with a high defect density. The initial increase of band gap with was dependent on the formation and growth of these crystal phases. This subsequent decrease of band gap may be related to a reduction in carrier concentration, known as the Burstein–Moss effect [20, 21].

Fig. 6.

Table 2 shows the Hall effect measurement results for the Cu-Cr-O films with different postannealing temperatures. Due to the positive Hall coefficients measured in the Cu-Cr-O films, the holes were confirmed to be the major carriers of these films, indicating a p-type semiconductor behavior. A considerable variation was found in the electrical property of CuO, CuCr₂O₄, and CuCrO₂ films studied by different authors, likely due to differences in the films’ composition and structure that contributed to this inability to fully explain electrical behavior. The as-deposited film had a very high electrical resistivity that could not be measured by a Hall system, hence it was omitted from Table 2. After annealing was conducted at 500°C, the films exhibited measurable electrical resistivity. When the annealing temperature attained 600°C, the carrier concentration and Hall mobility of the film increased. CuCrO₂ is generally known to have lower electrical resistivity than CuCr₂O₄ [22–25]. Coupled with the larger grain size, the substitution of CuCr₂O₄phase with the CuCrO₂ phase was thought to be responsible for the superior electrical performance. As the annealing temperature was further increased, however, the electrical property of the film deteriorated, mainly due to the decrease in carrier concentration. Yoshida et al. have confirmed that the major source of conductive carriers in Cu-Cr-O p-type semiconductors was interstitial O and Cu vacancies [26, 27]. In addition, Banerjee et al. found that electrical resistivity was reduced by extra O atoms present at interstitial sites and grain boundaries [28]. In the present study, while annealing could increase the grain size, it also reduced some point defects that contributed to hole concentration, such as such as interstitial O and Cu vacancies, leading to increased electrical resistivity. The values of Hall mobility showed scattering variations with increasing annealing temperature from 600°C to 800°C. The increase in Hall mobility originated from the decrease in grain boundary scattering and point defect scattering due to the increase in grain size [29]. However, the larger surface roughness and void density at higher annealing temperature facilitated the chemisorption of nitrogen, oxygen, and water vapor into the film, thus increasing barrier height and degrading Hall mobility [30]. Accordingly, the film annealed at 600^°C had the lowest electrical resistivity of 41 Ω-cm.

Based on the above results, although 600°C-annealed film has the optimal optoelectronic performance, its phase composition contains a large amount of CuO phase with narrow energy gap, resulting in poor light transmittance compared with other published papers. The ESCA result shows that the Cu content in film is about twice the Cr content. The composition difference between the film and the target caused by the magnetron sputtering method is too large to be ignored, which will be an unfavorable factor that must be considered and eliminated by adjusting the target composition in the further study of single-target magnetron sputtering in the future.