Infrared Transparent Conductive Films Based On Bismuth Selenide

Infrared transparent conductive films refer to a type of functional films with high conductivity and high infrared transmittance,and can be used in military or sensing fields such as infrared optical windows,infrared imaging,infrared gas detectors,and so on.Currently,the most widely used infrared transparent conductive material is tindoped indium oxide?In₂O₃:Sn,ITO?,which has a high conductivity of more than 2000S/cm with a transmittance of about 50%in the mid-infrared band of 3 to 5?m.This can basically meet the needs of transparent conductivity in the mid-infrared band.However,because of the high carrier concentration,the transmittance of ITO in the far-infrared band of 8 to 12?m drops to zero rapidly,and its performance can't meet the requirements of transparent conductors in the far-infrared.Therefore,researchers are committed to developing various new types of infrared transparent conductive materials to meet the needs of transparent conductors in both mid-and far-infrared.However,the following problems still exist:?1?The most widely studied design ideas of the infrared transparent conductive materials can't meet the requirements of practical application.There is a certain contradiction between infrared transparency and electrical conductivity in terms of the physical origin.To balance the trade-off,the design strategy for state-of-the-art transparent conductors is to dope wide-bandgap semiconductors.This method will cause a high carrier concentration of the films,which will decrease the transmittance in the infrared band,and it will be impossible to achieve far-infrared transparency.?2?The three-dimensional topological insulators represented by bismuth selenide have currently attracted a great deal of attention due to their unique surface state.The distinct transport properties make them applicable to spintronic devices,high-frequency devices,quantum computing and many other fields.However,there have been no report on the use of three-dimensional topological insulators as transparent conductive materials.Here,we have used bismuth selenide as an infrared transparent conductive thin-film material,and prepared a series of bismuth selenide films by magnetron sputtering.Moreover,a new design idea for infrared transparent conductive materials has been put forward.The main research content can be summarized as following:?1?Preparation of polycrystalline bismuth selenide films.Since magnetron sputtering is a non-equilibrium process,we must ensure that the prepared bismuth selenide films are crystalline phases.By changing the substrate temperatures and annealing temperatures,we successfully obtained layered bismuth selenide films with good crystallinity.The optimal substrate temperature and annealing temperature are set to both 300?through the tests of the sample performance,where the bismuth selenide film has good conductivity?about 350 S/cm?and high transmittance in the infrared band of 2.5 to 12?m?about 70%?.Through a series of characterizations and transmittance spectral fitting,we found that the improvement of the infrared transparent conductive properties of the bismuth selenide films with various preparing and processing temperatures can be attributed to the decrease of the carrier concentration and the increase of the relaxation time.?2?The influence of thickness on the infrared transparent conductive properties of bismuth selenide films.Under the premise of ensuring the crystallinity s,we are committed to further improve the properties of bismuth selenide films.Due to the layered growth mode of bismuth selenide,the thickness of films inevitably has a great impact on the performance.Therefore,we prepared bismuth selenide films with different thickness by changing the deposition time.Through performance tests,the optimal thickness is set to 45 nm.Through a series of characterization and transmittance spectral fitting,we revealed that the carrier concentration of the films decreases with the increase of film thickness,while the mobility and relaxation time of the films increases.However,the excessive mobility will result in an increase in reflectivity in the infrared band,which leads to the reduction in transmittance.In addition,through comparison with ITO films,it is proved that reducing the carrier concentration and increasing the mobility are feasible design ideas for infrared transparent conductive films.?3?Effect of introducing selenium on infrared transparent conductive properties of bismuth selenide films.After obtaining the bismuth selenide films with the best performance,we began to study the modification of bismuth selenide films through doping.Because bismuth selenide films are prone to generate selenium vacancies,we prepared bismuth selenide films with different concentration of selenium by co-sputtering.Through performance tests,the optimal power of selenium target is set to 20W.The improvement of the performance?the infrared transmittance of 3 to 5?m increase to 73%,and the conductivity to 1193 S/cm?proves that introducing selenium is an effective modification method,which provides a feasible method for the further improvement of the infrared transparent conductive properties of the bismuth selenide films.