Search

In the present study, the sol-gel process was used to deposited thin films of the p-type spinel ZnCo2O4 onto glass substrate. Spinel ZnCo2O4 thin films were obtained by annealing the sol-gel derived specimens at 300–400 °C in oxygen for 2 h, and were confirmed by four Raman active modes at approximately 470 cm−1, 514 cm−1, 600 cm−1, and 670 cm−1. The nanostructured and flat surface morphologies of the obtained ZnCo2O4 thin films were observed by field-emission scanning electron microscopy and atomic force microscopy. Uniform film thickness and good adhesion with the substrate were also observed. The binding energy of Zn-2p at 1020.7 ± 0.2 eV (2p3/2) and 1043.8 ± 0.2 eV (2p1/2) revealed the presence of Zn2+. Additionally, the presence of both divalent and trivalent Co cation was indicated by deconvolution of the binding energies at 779.5 ± 0.2 eV (2p3/2) and 795.1 ± 0.2 eV (2p1/2) for Co3+, and at 781.5 ± 0.2 eV (2p3/2) and 796.6 ± 0.2 eV (2p1/2) for Co2+. Two optical bandgaps of the obtained spinel ZnCo2O4 thin films were deduced at 2.38–2.39 eV and 3.95–3.99 eV. The p-type characteristic was confirmed using the hot-probe test. Furthermore, the resistivities of the spinel ZnCo2O4 thin films were 1.58 × 10 2 –2.45 × 103 Ω-cm with corresponding carrier concentrations of 5.57 × 1012-4.29 × 1014 cm−3. Hence, the sol-gel process provides a feasible method for the preparation of p-type spinel ZnCo2O4 thin films.

This study reports the preparation of crednerite-CuMnO 2 thin films using atmospheric pressure plasma annealing. The pristine thin films were deposited onto a quartz substrate using the sol–gel process. The specimens were then annealed using atmospheric pressure plasma at N 2 –(0–20%)O 2 for 20 min. Crednerite-CuMnO 2 thin films were obtained using atmospheric pressure plasma annealing at N 2 –5%O 2 and N 2 –10%O 2 . The lattice parameters of the thin films were a = 0.5574–0.5580 nm, b = 0.2874–0.2879 nm, c = 0.5878–0.5881 nm, and β = 104.15–104.25°, which agree well with previous reports. The Raman shifts of the crednerite-CuMnO 2 thin films were 688 ± 2 cm −1 , 381 ± 2 cm −1 , and 314 ± 2 cm −1 . The binding energy of Cu-2p spectrum of the crednerite-CuMnO 2 thin films was 932.3 ± 0.2 eV representing the Cu + in the thin films. The binding energies of Mn-3p spectrum were 48.1 ± 0.2 eV (Mn 3+ ) and 50.0 ± 0.2 eV (Mn 4+ ). Furthermore, the cation distribution in the thin films was Cu + 1.0 (Mn 3+ 0.6 Mn 4+ 0.4 )O 2 from the X-ray photoelectron spectroscopy measurement. When the crednerite-CuMnO 2 phase was formed, the surface morphology exhibited a compact/dense granular morphology. The optical bandgap of the crednerite-CuMnO 2 thin films was 2.6 eV, 2.8 eV, and 3.5 eV. In addition, the resistivity of the crednerite-CuMnO 2 thin films was (2.61–2.65) × 10 4 Ω cm, which is consistent with previous literature reports. Moreover, the activation energies for the carrier conduction in the crednerite-CuMnO 2 thin films were 0.10–0.12 eV. Hence, an atmospheric pressure plasma annealing offers a green, economic, effective and feasible method for preparing crednerite-CuMnO 2 thin films.

In this study, pure crednerite-Cu 1.1 Mn 0.9 O 2 films were deposited onto quartz substrates using a sol–gel processing and a two-step annealing process. The sol–gel-derived films were annealed at 500 °C for 1 h in air and post-annealed at 600–700 °C for 2 h in N 2 . X-ray diffraction patterns showed that the films were CuO and Cu x Mn 3 − x O 4 phases in air annealing. When the films were post-annealed above 600 °C in N 2 , a pure CuMnO 2 phase with the monoclinic crednerite structure (space group: C2/m) was obtained. The lattice parameters of the crednerite-Cu 1.1 Mn 0.9 O 2 films where a = 0.5579–0.5587 nm, b = 0.2878–0.2881 nm, c = 0.5880–0.5891 nm, and β = 104.16–104.34° and were agreement with the literature reports. The binding energies of Cu-2p of the crednerite-Cu 1.1 Mn 0.9 O 2 films were 932.3 ± 0.2 eV and 952.3 ± 0.2 eV to represent the monovalent Cu in the films. Additionally, the binding energies of Mn-3p of the crednerite-Cu 1.1 Mn 0.9 O 2 films were 47.5 ± 0.2 eV, 48.2 ± 0.2 eV, and 50.0 ± 0.2 eV and revealed coexistence of +2, +3, and +4 valences in the films. The cation distributions of the crednerite-Cu 1.1 Mn 0.9 O 2 films prepared using post-annealing at 650 °C and 700 °C were Cu + 1.1 [Mn 4+ 0.25 Mn 3+ 0.51 Mn 2+ 0.24 ] 0.9 O 2 and Cu + 1.1 [Mn 4+ 0.24 Mn 3+ 0.52 Mn 2+ 0.24 ] 0.9 O 2 , respectively. Two optical bandgaps of the crednerite-Cu 1.1 Mn 0.9 O 2 films at 4.5–4.0 eV and 3.5–3.0 eV were observed. The electrical conductivities of the crednerite-Cu 1.1 Mn 0.9 O 2 films were 1.20 × 10 −5 –2.50 × 10 −5 S cm −1 . Moreover, the activation energies for the carrier conduction were 0.20–0.30 eV. Hence, our results demonstrate that sol–gel processing is a feasible preparation method for crednerite-CuMnO 2 films.

In this study, post-annealing conditions, the partial oxygen pressures (pO2) and temperatures, influence on the formation of delafossite–CuFeO2 thin films is studied. The sol–gel derived films were annealed at 500 °C in air and post-annealed at 500–850 °C in pO2 = 10−2 to pO2 = 5 × 10−5 atm. The CuO and CuFe2O4 phases appeared when the sol–gel derived films were post-annealed below 800 °C for 2 h in pO2 = 10−2 atm, 650 °C for 2 h in pO2 = 10−3 atm, and 550 °C for 2 h in pO2 = 5 × 10−5 atm. Pure delafossite–CuFeO2 phase was detected as specimens were post-annealed above 800 °C for 12 h in pO2 = 10−2 atm, 650 °C for 12 h in pO2 = 10−3 atm, and 550 °C for 12 h in pO2 = 5 × 10−5 atm. The surface of post-annealed thin films exhibited a nanoparticle-like morphology when the specimens exhibited CuO and CuFe2O4 phases. However, the surface revealed granular features caused by the formation of the delafossite–CuFeO2 phase. The formation of the delafossite–CuFeO2 phase, which resulted from the chemical reaction of the CuO and CuFe2O4 phases in the post-annealing process, is consistent with thermodynamics. The optical bandgaps of delafossite–CuFeO2 thin films prepared using post-annealing ranged between 3.1 and 3.2 eV. The electrical conductivities of delafossite–CuFeO2 thin films were (1.62–6.37) × 10−1 S cm−1 and the carrier concentrations were (1.52–8.84) × 1017 cm−3. The pO2 and temperatures in the post-annealing process played primary roles in the formation of delafossite–CuFeO2 thin films in this study.

In this study, the CuCr 1− x O 2 films with x = 0.00–0.25 were prepared on a quartz substrate by sol–gel processing. The films were first deposited onto a quartz substrate by spin-coating. The specimens were annealed at 500 °C in air for 1 h and post-annealed in N 2 at 700 °C for 2 h. As the films were post-annealed in N 2 , a pure delafossite-CuCrO 2 phase appeared in the CuCr 1− x O 2 films below x = 0.20. However, an additional CuO phase appeared at x = 0.25. The pure delafossite-CuCrO 2 phase can exist within x ≤ 0.20 in CuCr 1− x O 2 films. The binding energies of Cu-2p 3/2 and Cr-2p 3/2 in the CuCr 1− x O 2 films with the pure delafossite-CuCrO 2 phase were 932.1 ± 0.2 eV and 576.0 ± 0.2 eV, respectively. The surface exhibited elongated grain features when the pure delafossite-CuCrO 2 phase was present in the CuCr 1− x O 2 films. The maximum transmittance of the CuCr 1− x O 2 films with the pure delafossite-CuCrO 2 phase was approximately 80%, which moved toward the visible region with the increasing x -value. The film absorption edges were observed at 400 nm, which were sharper with the increasing x -value. The optical bandgaps of CuCr 1− x O 2 films with the pure delafossite-CuCrO 2 phase were approximately 3.0 eV. The electrical conductivity of CuCr 1− x O 2 films with the pure delafossite-CuCrO 2 phase was 1.1 × 10 −3 S cm −1 ( x = 0.00), and increased to 0.16 S cm −1 ( x = 0.20). The corresponding carrier concentration of CuCr 1− x O 2 films with the pure delafossite-CuCrO 2 phase was 2.8 × 10 14 cm −3 ( x = 0.00), and markedly increased to 1.8 × 10 16 cm −3 ( x = 0.20). The Cr-deficient condition in delafossite-CuCrO 2 films enhances film electrical conductivity and carrier concentration, but retains the film's high-visible transparency.

This study reports the preparation of delafossite-CuCrO2 thin films were prepared on quartz substrates using sol–gel processing and post-annealing with an atmospheric pressure plasma torch. The films were first deposited on a quartz substrate by spin coating. The specimens were then annealed at 500 °C in air and post-annealed with an atmospheric pressure plasma torch with N2–5% O2 at 650 °C for 20 min. The specimens annealed in air exhibited CuO and CuCr2O4 phases. Post-annealing using an atmospheric pressure plasma torch obtained the pure CuCrO2 (delafossite, R 3 ¯ m ) phase. The binding energies of the Cu-2p3/2 and Cr-2p3/2 peaks of the CuCrO2 thin films were centered at 932.1 ± 0.2 eV and 576.1 ± 0.2 eV, which revealed the valence state of Cu+ and Cr3+ in the films. The chemical composition of CuCrO2 thin films was close to the stoichiometry. As the CuCrO2 phase formed, the film surface began to exhibit agglomerate features and the cross-sectional morphology showed an equiaxed grain feature. The average transmittance of CuCrO2 thin films was approximately 66% in the visible region. The direct optical bandgap of CuCrO2 thin films was 3.08 eV, which is consistent with reported data in the literature. The positive Seebeck coefficients of the CuCrO2 thin film prepared by post-annealing using an atmospheric pressure plasma torch confirmed the p-type characteristics of the film. The electrical conductivity of CuCrO2 thin films was 2.7 × 10−2 Scm−1 with a carrier concentration of 2.8 × 1013 cm−3. Hence, post-annealing using an atmospheric pressure plasma torch is an effective tool and a feasible method for preparing transparent conductive delafossite thin films.

In this study, transparent conductive CuFeO2 thin films were deposited onto a quartz substrate using a low-cost sol–gel process and sequential annealing in N2. The sol–gel derived films were annealed at 500 °C for 1 h in air and then annealed at 700 °C in N2 for 2 h. The CuO and CuFe2O4 phases appeared as the film annealed in air, and a single CuFeO2 phase (delafossite, R3m) appeared as the film annealed in N2. X-ray photoelectron spectroscopy showed that the chemical composition of the CuFeO2 thin films was similar to the stoichiometry. The optical bandgap of the CuFeO2 thin films was 3.1 eV. The p-type characteristics of the films were verified by Hall-effect measurements. The electrical conductivity and carrier concentration of the CuFeO2 thin films were 0.358 S cm−1 and 5.34 × 1018 cm−3, respectively. These results show that the proposed low-cost sol–gel process provides a feasible method of depositing transparent CuFeO2 thin films.

Aluminum nitride (AlN) films were successfully deposited at room temperature onto p-type (1 0 0) silicon wafers by manipulating argon ion beam voltages in a dual ion beam sputtering (DIBS). X-ray diffraction spectra showed that aluminum nitride films could be synthesized above 800 V. The (0 0 2) orientation was dominant at 800 V, above which the orientation was random. The atomic force microscope (AFM) images displayed a relatively smooth surface with the root-mean-square roughness of 2–3 nm, where this roughness decreased with argon ion beam voltage. The Al 2p3/2 and N 1s spectra indicated that both the aluminum-aluminum bond and aluminumnitrogen bond appeared at 600 V, above which only the aluminumnitrogen bond was detected. Moreover, the atomic concentration in aluminum nitride films was concentrated in aluminum-rich phases in all cases. Nevertheless, the aluminum concentration markedly increased with argon ion beam voltages below 1000 V, above which the concentration decreased slightly. The correlation between the microstructure of aluminum nitride films and argon ion beam voltages is also discussed.