Research On Preparation And Properties Of Delafossite Based Oxide Infrared Transparent Conductive Thin Film

Infrared transparent conductive film belongs to a cross subject: transparent electronics. Transparent electronics is mainly based on transparent conductive film devices, circuits and systems. It relates to the field of materials, device physics, electrical systems, etc.Infrared transparent conductive film is a new type of electronic thin film- photovoltaic film, which is also a kind of infrared transparent conductive filter. It is not only conductive but also transmitted infrared light. The infrared transparent conductive film can be widely used in sensing technology, optoelectronic technology, electronic and energy industry, military and other fields. Such as infrared gas detector, infrared laser, infrared light-emitting diodes, solar cells, infrared imaging, infrared missiles, spacecraft window, etc.The current transparent conductive film is transparent only in the visible and near-infrared region, and opaque in mid-far infrared region. The main reason is that the plasma oscillation frequency（ωp） of the thin film is high. Therefore, it is very necessary to study the infrared transparent conductive film, which has both scientific significance and practical application value. It relates to the basic properties of the materials, such as electrical conductivity, infrared transmittance and so on.Delafossite materials is a p-type semiconductor oxide with wide band gap（>3.3 e V）. Its plasma resonance frequency is located in the far infrared region. Thus the delafossite thin films show good transmittance in the mid-infrared region. However, low intrinsic conductivity is the key factor to restrict the practical application of delafossite materials. Therefore, it is of great practical significance to prepare delafossite materials with high infrared transmittance and conductivity.Though delafossite based infrared transmittance conductive thin films can be fabricated by many methods, the chemical solution method attracts more attention for its simple preparation process, low cost, etc. Therefore, in this paper, we prepared the delafossite oxides and films by the sol-gel technique and polymer assisted deposition method. We also studied the structure and photoelectric properties in detail. The main work are listed as following:1. The Cu Cr1-xZnxO₂（x = 0, 0.03, 0.05, 0.07, 0.1） polycrystalline material were prepared by the sol-gel method, the structure, morphology and charge transport mechanism of the samples were studied in detail. It was found that the sintering temperature for obtaining stable phase should be higher than 850 ℃. Based on X-ray diffraction（XRD） and Raman spectrum, the crystalline quality of the oxides is improved by the suitable substitution of Cr by Zn. The X-ray photoelectron spectroscopy（XPS） spectra reveal the chemical state of Zn is +2. The Hall and Seebeck coefficients of the pellet samples display a positive sign, indicating p-type conductive characteristics of the obtained oxides. The temperature-dependent resistivity of the oxides is proven to be consistent with small polaron hopping. For the three oxide samples with x = 0, 0.05, and 0.1, the activation energies for the polaron hopping between Zn2+ and Cr3+ sites are 54, 41.5, 32 me V, respectively, which is found to decrease with the increase of Zn content. The electrical conductivity can be remarkably improved by Zn-doping due to the small polaron hopping activation energy.2. The Cu Fe O₂ thin films were prepared on sapphire（001） substrate by sol-gel method with different annealing temperature. The effect of temperature on the structure and morphology of the films was studied, we obtained the best preparation temperature being 900 ℃.Single phase Cu Fe1-xSnxO₂（x = 0, 0.01, 0.03, 0.05, 0.07） films with high c-axis oriented were obtained after annealed at 900℃. The films show high transmittance around 45–78% in the visible band（400-780 nm） and 78–95% in the near infrared spectral region（0.78-3 μm）. The direct band gap of Cu Fe1-xSnxO₂ films showed an increase at the beginning and decreased with the increase of Sn component in thin film samples, this results from the Burstein-Moss effect caused by the tail states and the Sn doping of the films. The p-type characteristics of Cu Fe1-xSnxO₂ thin films were determined by positive Hall coefficients. The resistivity of the film was 36.83 Ωcm for Cu Fe O₂ film at room temperature, significantly decreased to 1.043 Ωcm after substituting Sn2+ for Fe3+. The resistivity showed a decrease at the beginning and increased with the growing of Sn ion doping ratio, this is mainly due to the high concentration of doping leads to the increase of impurity scattering, which reduces the carrier mobility in the thin films.3. The Cu Sc O₂ single crystal thin film was prepared on sappire （ ）1120 by using polymer-assisted-deposition（PAD） method for the first time. Due to the uniaxial locked epitaxy mechanism, the orientation relationship of the film with respect to the substrate are confirmed to be Cu Sc O₂ [3R]（0001）//a-Al2O3（1120）. As a key point in the PAD process, the used polymer materials（PEI and EDTA） not only control the desired viscosity for the process, but also bind the metal ions to prevent premature precipitation and formation of metal oxide oligomers. The technique results in a homogeneous distribution of the metal precursors in the solution as well as the formation of uniform metal organic film. The obtained Cu Sc O₂ thin film from the PAD technique exhibits a low electrical resistivity of 1.047 Ω·cm at room temperature. The Cu Sc O₂ film has a direct band gap and the optical direct bandgap is estimated to be 3.6 e V, film shows high transmittance of 65–90% in the near-IR range（0.78-3 μm）and more than 90% in the mid-IR range（3-5 μm）.4. Highly c-axis oriented Cu Sc1-xSnxO₂（x = 0, 0.03, 0.06） thin films were prepared on sappire （ ）1120 substrate using polymer-assisted deposition（PAD） method. The prepared films retain a transparency of >65% in the near-IR region（0.78-3 μm） and > 90% in the mid-IR region（3-5 μm）. The films exhibited good p-type conductive characteristics, and a low resistivity of 5.59×10-2Ω·cm was achieved for Cu Sc0.94Sn0.06O₂ at room temperature, which is two orders of magnitude lower than the un-doped film at room temperature. It was found that the films showed two charge transport modes from the temperature-dependent electrical conductivity. Thermal activation mode of semiconductor in high temperature range and three dimensional variable range hopping model in low temperature zone. The former, the hopping of holes between the nearest neighbor Cu sites in the same layer determines the electrical transport properties; the latter, the thermal energy weakening may depress the hopping between the nearest neighbor Cu sites, and the contribution of Sc/Sn site hopping becomes more dominant. Hence, the hopping to the Cu site in other Cu layers becomes relatively dominant. The crossover temperature between the two models decreases with increasing Sn-doping composition In addition, as an application of such films, p-Cu Sc O₂:Sn/n-Zn O heterojunction diodes were fabricated using PAD technique. The measured ² V threshold voltage of p-Cu Sc0.94Sn0.06O₂/n-Zn O diode is in reasonable agreement with the bandgap energy of Cu Sc0.94Sn0.06O₂.