Study On The Deposition And Sensitization Of PbS Optical Films

With the continuous advancement of smart cities,there exist an increasing demand for imaging and observation means.As an important extension of the visible light spectrum,short-wave infrared(1-3?m)has received more and more attention due to its good complementarity with the current visible light detection.PbS optical films are often used to make short-wave infrared photoconductive detectors because of their narrow direct band gap(0.41 e V),large exciton wave radius(18 nm),and good photosensitivity to the entire short-wave infrared light.Chemical bath deposition(CBD)technique is the most commonly used method to deposition PbS infrared optical thin films.It has the advantages of low cost,easy processing,and the ability to prepare larger area films.However,the preparation process is hard to control,reactants cannot be added continuously,thin single film deposition thickness(300-500 nm)and poor uniformity of the films limit its development.Focusing on the above-mentioned problems of PbS film deposition,this thesis systematically studied the effects of substrate cleaning,reactant concentration,deposition temperature and solution flow field on the deposition of PbS films by CBD method.The thermodynamic and kinetic mechanisms of the PbS film deposition process was discussed,and the sensitization process of PbS films initially explored.Through the design and optimization of the process conditions,high-quality PbS thin films with a single deposition thickness of 1.6?m,uniform macroscopic morphology without holes on the surface,square crystalline grains and forbidden bandgap width of 0.46 e V were obtained.The main research results are as follows.The effects of substrate cleaning process,deposition temperature,reactant concentration and substrate placement on the deposition of PbS films were investigated.The results showed that:the adhesion on the substrate surface would adversely affect the surface morphology of the films,and the removal of the adhesion on the substrate surface by appropriate cleaning methods could yield PbS films without macroscopic holes;High deposition temperature will make the reaction speed too fast and affect the thermal and flow fields in the reaction vessel,resulting in the flaking of the film;Low reactant concentration will reduc the thickness of the PbS film,which cannot reach the thickness required for detector manufacturing,and the thickness of PbS film can be greatly improved by increasing the reactant concentration;Through the comparison of finite element simulation and experiment,it is found that 40?is the most suitable temperature for PbS film deposition.When the substrate is placed avoiding the heat flow area in the center,the PbS optical film is more uniform macroscopically,and the thickness of the film deposited on the back of the substrate is better than that on the front side.The effects of KOH concentration and deposition time on the grain morphology and size of PbS thin film were systematically investigated,and the results showed that the KOH concentration had a decisive effect on the transition of PbS crystal shape from round to square crystals under the conditions of lead acetate concentration of 0.1 mol/L,thiourea concentration of 0.3 mol/L,trisodium citrate concentration of 0.2 mol/L and deposition temperature of 40?.The kinetic process of PbS thin film deposition is characterized by four stages:small-size grain attachment to the substrate,accumulation of small-size grains,secondary nucleation of clusters of small-size grains to large-size grains,and secondary growth of large-size grains.After understanding the thermodynamic and kinetic mechanism of PbS film deposition,under the process conditions of KOH concentration of 0.71mol/L and deposition time of 2h,high quality PbS films with uniform macroscopic morphology,without holes on the surface square grains and 1.6?m film thickness were obtained.The changes of PbS film crystalline shape and band gap with the changes of KOH concentration and deposition time were investigated.The results showed that under the conditions of 0.1 mol/L of lead acetate,0.3 mol/L of thiourea,0.2 mol/L of trisodium citrate and 40?of deposition temperature,the PbS grain orientation tended to(2 0 0)with the increase of KOH concentration and deposition time.At the same time,the grain size gradually became larger and the band gap gradually became smaller.Under the process conditions of KOH concentration of0.71 mol/L and deposition time of 2 h,the obtained PbS films had a grain size of about 600 nm and a forbidden bandgap width of 0.46 e V.The thermal oxygen sensitization process of PbS thin films was initially explored,and the generation of Pb O_xS_1-x mixed phase and the crystallographic transformation phenomenon were observed from the X-ray diffraction results after annealing the PbS thin films in a pure oxygen atmosphere at 500?for 1 h.The annealing caused the rearrangement of PbS grains,resulting in the shift of the selective orientation from(2 0 0)to(2 2 0)crystallographic planes.The sensitized PbS photosensitive films showed a blue-shift phenomenon in light absorption,the band gap increased to 0.56 e V,and a significant photoresponse occurred under 1550 nm incident light irradiation,which proved that the 500?pure oxygen annealing conditions had a sensitizing effect on the PbS films and laid a good foundation for the subsequent preparation of the infrared detectors.