Tungsten Telluride As Far-infrared Transparent Conductive Films

Infrared transparent conductive film is a functional film that possesses conductivity and high transmittance in the infrared band.As an important part of photoelectric devices such as infrared photodetectors,infrared light-emitting diodes and infrared liquid crystal switches,it has attracted the attention of domestic and foreign researchers.Until now,many kinds of materials have been used as infrared transparent conductive films,but they cannot synergize well with infrared transparent and conductive properties yet,especially in the far infrared band（8～12μm）.As a typical topological semimetal,tungsten telluride has been widely used in the fields of visible light detection and pyroelectricity because of its special transport properties.However,the following problems remain:（1）The reported optoelectronic properties of tungsten telluride indicate that it is a potential transparent conductive material,but no researcher used it as an infrared transparent conductive film;（2）Existing studies have only reported its transmittance in the visible band,whereas the transparent properties in the far infrared band are yet to be evaluated;（3）Previous research have mainly focused on the effects of film thickness,metal doping,and other factors on the electrical properties of the film,while the relationship between microstructure,component ratio and infrared transparent conductive properties remains unclear.In response to the above problems,we tried to introduce tungsten telluride into the field of infrared transparent conductive films,as well as carrying out researches on tungsten telluride as far-infrared transparent conductive films with the combination of experiments and spectral fitting methods.The main conclusions are summarized as follows:（1）By studying the influence of substrate temperature on the growth and properties of tungsten telluride films,we found that increasing the substrate temperature can improve the crystallinity of the film and enhance the infrared transmittance（T）and conductivity（σ）.The tungsten telluride film prepared at a substrate temperature of 700°C has the best far-infrared transparency and conductivity(T_8¹2μm=55.7%,σ=1046.1 S/cm),which is attributable to its combination of low carrier concentration and high mobility.Compared with the traditional transparent conductive film indium tin oxide,the figure of merit of tungsten telluride film in the far-infrared band are nearly two orders of magnitude higher,which greatly facilitates the synergy of far-infrared transparent properties and conductive properties.This study shows that reducing the carrier concentration and increasing the mobility can effectively synergize the far-infrared transparent and conductive properties.（2）We studied the effects of film thickness and the introduction of substrate bias on the structure and properties of tungsten telluride thin films.We found that increasing the film thickness can increase the conductivity of the film at the expense of the infrared transmission.The conductivity boost is a result of the improved crystal quality,which increases mobility.However,a high film thickness will increase the absorption of infrared photons and reduce the infrared transmittance,and the optimal film thickness for synergistic infrared transparency and conductive properties is determined to be 20 nm.The introduction of substrate bias affects the grain size of the film,and proper bias can improve the conductivity of the film but reduce the infrared transmittance.This is due to the increase in carrier concentration.Finally,we improved its figure of merit in the far-infrared band by introducing an anti-reflection layer.It is demonstrated that the introduction of substrate bias and anti-reflection coating is a feasible approach to synergize infrared transparency and conductive properties.（3）By increasing the content of Te and W in tungsten telluride thin films,we found that the Te-rich film has excellent far-infrared transmittance but low electrical conductivity,which is attributable to the low carrier concentration derived from the low Te defect density;W-rich films have higher electrical conductivity but lower far-infrared transmittance,which is attributable to the enhanced carrier concentration owing to the high W interstitial atom content.Among them,Te-rich tungsten telluride thin films are more suitable for optical devices such as infrared dome and infrared windows thanks to their excellent far-infrared transparency.W-rich tungsten telluride thin films are more suitable for the top infrared transparent electrodes of devices such as infrared lasers,light-emitting diodes,and liquid crystal infrared switches due to their high conductivity.Finally,we introduced thin Zn S layer as an anti-reflection coating on the W-rich film,which significantly improved the transmittance in the mid-and far-infrared bands and achieved the best figure of merit.This provides a method for improving the far-infrared transparent and conductive properties of tungsten telluride thin films.