Preparation And Properties Of P-type Delafossite Structure Oxide Semiconductor CuFeO₂

Transparent conductive oxides are widely used in liquid crystal display panels,solar cells,photodetectors,etc.due to their high optical transmission and good conductivity.However,most transparent conductive oxide materials that can be prepared at present are n-type semiconductors.It is difficult to prepare p-type semiconductor materials.In this paper,p-type CuFeO₂ films were prepared by magnetron sputtering based on the valence band modification theory,and the effects of different factors on the structure and optoelectronic properties of the films were studied.Then p-type CuFeO₂ powders were prepared by hydrothermal method.The doping effects of Ca and Ni ions on the structure and photoelectrochemical properties of CuFeO₂ powders were studied.amorphous Cu-Fe-O thin films were prepared on the quartz substrate by RF magnetron sputtering and pure phase 3R-CuFeO₂ thin film was obtained after annealing at 900°C in N₂ atmosphere.The structural,optical and electrical properties of CuFeO₂ thin films prepared by magnetron sputtering under oxygen fluxes of 0%,2%,6%and 9%were investigated.XRD results showed that the Cu₂O impure phase appeared under low oxygen condition,Raman spectroscopy confirmed the presence of oxygen defects in the film.The UV-visible transmission spectra show that the transmittance of the film in the visible light region increases gradually as the oxygen flow rate increases from 0%to 9%.When the deposition duration is 20 min,the light transmission of the films at wavelength of 600 nm were 8%,9%,20%,and 29%under the oxygen flow ratesof 0%,2%,6%and 9%respectively,with direct band gap of the film increaseing from 2.98 eV to 3.09 eV.All the films are in good Ohmic contact with Cu electrodes,with photoelectric responses under light conditions.Hall test shows p-type conductivity in all samples,the prepared thin films with the oxygen flow rate of 9%has the minimum carrier concentration of 2.79×10¹⁷ cm^-3 and the maximum carrier mobility of 9.74 cm²·V^-1·s^-1.and the films with the oxygen flow rate of 0%has the maximum carrier concentration of 9.21×10¹⁹ cm^-3 and the minimum carrier mobility of 0.035 cm²·V^-1·s^-1.When the oxygen flow rates changes from 0%to9%,the resistivities of films are 2.0Ω·cm,0.93Ω·cm,2.6Ω·cm,and 2.3Ω·cm,correspondingly.When N₂ flow rate changes from 0%to 31%during the sputtering process,CuFeO₂ thin films mainly composed by 3R-CuFeO₂ phase are obtained after annealing process.CuO phase appears in the film once N₂ flow rate reaches 21%.When the sputtering duration is 2 h,the transmittance of film at 600 nm increases from 0.8%for undoped film to 20%for film deposited at N₂ flow rate of 31%.All the films deposited with N₂ flowing are in good Ohmic contact with Cu electrodes.All films are in p-type conductive with N₂ flow rate from 0%to 21%,the carrier concentrations are significantly improved while the carrier mobilities are reduced for films deposited with N₂ flowing as compared with the undoped film.The corresponding resistivity gradually decreases from 2.3Ω·cm to 0.382Ω·cm,which suggests that the doping by N₂ flowing can effectively improve the visible light transmittance and electrical conductivity of the CuFeO₂ films.The film deposited at high N₂ flow rate of 31%exhibits n-type conductivity,indicating that the introduction of excessively N₂ can change the conductivity type of CuFeO₂.CuFeO₂ powders with 0%,3%and 6%Ca²⁺doping were successively synthesized by hydrothermal method,the effects of Ca²⁺doping on the structure and photoelectrochemical properties of CuFeO₂ were studied.XRD patterns show that both 2H and 3R crystals exist in the powders,and Fe₃O₄ phase appears in the powder when the Ca²⁺doping concentration reaches 6%.The direct band gaps of CuFeO₂powders are 3.43 eV,3.55 eV,and 3.53 eV in order,and the indirect band gaps are1.03 eV,1.06 eV,and 1.09 eV,respectively with Ca²⁺doping concentration from 0%to 6%.3%Ca²⁺doped CuFeO₂ powder has the maximum carrier concentration of1.15×10¹⁹ cm^-3 and the maximum flat band potential of 0.8 V as compared with the undoped sample,suggesting that 3%Ca²⁺doping can effectively increase the carrier concentration and thus the conductivity of CuFeO₂ powder.CuFeO₂ powders with 0%,3%and 6%Ni²⁺doping were synthesized by hydrothermal method.The effects of Ni²⁺doping on the structure and photoelectrochemical properties of CuFeO₂ were investigated.XRD pattern reveals the appearance of Cu₂O impure phase once the Ni²⁺concentration reaches 3%.The mapping analysis of the sample with 3%Ni²⁺doping shows that Cu,Fe,O,and Ni are homogeneously present in the powder.The direct band gaps are 3.43 eV,3.33 eV,and3.02 eV,while the indirect band gaps are 1.03 eV,0.82 eV,and 0.59 eV,respectively for CuFeO₂ powders with Ni²⁺doping concentration from 0%to 6%.With the increasing of Ni²⁺doping concentration,the doping concentration and thereby flat band potential and gradually increase,leading to the decrease of charge transfer resistance and the increase of photocurrent.The CuFeO₂ powder with 6%Ni²⁺doping has the maximum carrier concentration is 1.14×10¹99 cm^-33 and the maximum flat band potential is 0.72 V.This indicates that Ni²⁺doping can effectively increase the carrier concentration and thus the electrical conductivity of CuFeO₂ powder.