Preparation And Sensing Applications Of Infrared Transparent Conductive Films

Transparent conductive films are functional materials with both high transmittance and excellent conductive,which are widely used in the field of information and energy technology.Transparent conductive films in the visible band have excellent performance and are well established for use.But in the infrared band（2–20μm）,transparent conductive films have not yet been developed for application.The difficulty arises from the conflict between conductivity and transparency.According to the Drude free electron theory,the plasma wavelength（λ_p）determines the upper limit of transmission wavelength of the film.A common design idea for infrared transparent conductive film is to reduce the carrier concentration of the film,but the reduction in carrier concentration reduces the conductivity of the film.In this thesis,it was found that in addition toλ_p,the carrier scattering frequencyγand the film thickness are also important factors affecting the infrared transparency.Based on this theory,hafnium-doped indium oxide（IHf O）films and tin-doped indium oxide（ITO）films were prepared for infrared transparent conductivity.To reduce the square resistance of the films,a layer of metal mesh grids was added to the ITO films to prepare a bilayer structured film,which basically achieves the application index of infrared transparent conductive films.Using the prepared infrared films,application experiments were carried out in the field of heat management and sensing of devices.The main contents of this thesis are as follows:1.IHf O films were prepared by magnetron sputtering,the effect of film thickness on the optoelectronic properties was investigated.The carrier scattering frequencyγof IHf O transparent conductive films is approximately equal to the plasma frequency（γ≈ω_p）,breaking the plasma wavelength limit and achieving a broad band transparency from visible light（400 nm）to far infrared（20μm）.The prepared IHf O film has an average transmittance of 81.7%in the range of 400–1100 nm and an average transmittance of 48.0%in the range of 1.35–20μm,with a square resistance of 141Ω/□.Infrared IHf O films can be used for radiative cooling of equipment.A test system was constructed to test the radiative cooling performance of the IHf O films.Experimental results show that by replacing commercial ITO films with IR IHf O films,the device operating temperature（～60°C）of circuits can be reduced by 2-4°C,which corresponds to 5-10%of the temperature difference between resistance and room temperature.2.Commercial ITO transparent conductive films are only transparent in the visible and near infrared bands（<2μm）.The infrared spectral response of ITO films with different thickness and carrier scattering frequencies was simulated using the transmission matrix method.It was found that the infrared transparency of ITO films could be achieved by reducing the film thickness.The effects of film thickness and annealing temperature on the optoelectronic properties of the ITO films were investigated by preparing 5-30 nm thick ITO films using magnetron sputtering.It was found that a 10 nm thick ITO film has good infrared transmittance（>80%in the range of 1.35–20μm）and low resistivity(5×10^-4Ω·cm).The actual text results are consistent with the spectral simulation results.Data on the application of infrared ITO films for device heat management were tested using a passive radiative cooling test system.The experimental results show that the use of IR ITO transparent conductive films instead of commercial ITO transparent conductive films can reduce the device surface temperature by approximately 2°C.3.In order to reduce the square resistance of far-infrared indium oxide based thin films to below 10Ω/□.A scheme of combining metal and oxide films was proposed to prepare IHf O/Cu/IHf O trilayer structure films,the effects of the thickness of the copper layer and the thickness of IHf O layer on the optoelectronic properties of the films were investigated.By adding the Cu layer,the optoelectronic properties of the trilayer structure films mainly depended on the Cu layer,and the resistivity was reduced to 1/27 of that of the single-layer IHf O films,but the infrared optoelectronic properties of the films became significantly worse.To solve this problem,a composite ITO/metal grid structured film was prepared using cross-interconnected metal grids instead of a completely continuous metal film.The effects of metal type,mesh thickness and structure on the optoelectronic properties of the ITO/metal mesh composite films were investigated.The results show that the addition of metal grids can significantly reduce the cube resistance of the films without sacrificing the optical properties of the ITO films.The ITO/silver grids composite structured films with>80%IR transmittance in the 0.4–20μm band and cube resistance<10Ω/□,achieve the application target.4.In order to solve the problems of opaqueness and poor stability of waveguide core layer materials,infrared niobium oxide（Nb₂O₅）films were used as a core layer material for the preparation of waveguide devices.The Nb₂O₅films were prepared by magnetron sputtering,and the visible and infrared spectra of Nb₂O₅were tested and the refractive index and extinction coefficient of the Nb₂O₅films were obtained by fitting with an ellipsometer.Using COMSOL Multiphysics software,a rectangular optical waveguide sensor was designed and the waveguide size was optimised using the optical parameters of the Nb₂O₅film.The Nb₂O₅optical waveguide sensor was prepared using a combination of lift-off method and magnetron sputtering techniques.The concentration of methane（absorption wavelength of 3.291μm）was measured using a Nb₂O₅waveguide with an external limiting factor of 11.5%.2 cm long waveguide achieved the limit of detection of 348 ppm（parts per million）at an average time of61.2 s.The innovative of this thesis are as follows:（1）By investigating the infrared optoelectronic properties of IHf O films prepared by magnetron sputtering,it was found that the carrier scattering frequency of IHf O films is approximately equal to the plasma frequency（γ≈ω_p）.The IHf O films break through the plasma wavelength limit and extend the transparent wavelength to the infrared band,reaching 20μm with an average infrared transmittance of 66.9%.（2）Commercial ITO films（140–180 nm thick）are transparent only in the visible and near-infrared bands.In this paper,the mid-infrared and far-infrared transparent ITO films were studied and prepared.The average transmittance of the film in the infrared band reached over 80%.（3）Infrared transparent conductive films with composite structure were prepared by combining metal mesh grids with ITO films.It has a transmission over 80%in the0.4–20μm band and a small square resistance less than 10Ω/□.（4）An infrared waveguide sensor was prepared using Nb₂O₅film as the core layer material by the lift-off method.The sensor is transparent and stable in the visible band.compared to the Chalcogenide glasses material,and the limit of detection of the sensor for methane was 348 ppm.