Study On The Preparation And Growth Process Simulation Of Lead Chalcogenide Films

The lead sulphide films,PbX(X=S,Se),are all group IV-VI compounds with NaCltype crystal structures.Their narrow forbidden band widths,large Bohr exciton radii and high thermoelectric coefficients give them good optical and optoelectronic properties and are widely used in the field of infrared detection.It has been found that the thin film material has superior optical properties compared to the bulk material.Therefore,the preparation of high quality thin films of sulphur compounds of lead has become a hot topic of modern research.In this thesis,a combination of theoretical simulations and experimental validation was used to investigate the growth mechanism of Pb-Si films on glass substrates and singlecrystal silicon substrates.PbX(X=S,Se)thin films were prepared by chemical bath deposition method,and the morphological characterisation and optical properties of the prepared films were analysed by X-ray diffractometer,field emission scanning electron microscope,energy spectrometer and Fourier transform infrared spectroscopy,and the effects of deposition time and deposition temperature on the surface morphology,chemical composition,crystal structure and optical properties of the films were investigated.The main findings are divided into the following aspects.(1)The thermomechanical growth process of lead films of sulphur compounds on Si(100)substrates was simulated using density flooding theory,and the effects of lattice constants and crystal orientation on their energy band structure,density of states and optical properties were investigated.The results show that both PbSe and PbS films grow along the[200] orientation,whether the surface energy is dominant or the interfacial energy is dominant,while the [220] orientation is dominant when the strain energy is dominant,and when the two growth orientations are in dynamic equilibrium,the orientation of the films is influenced by the layer thickness and microstrain.The results of the simulations show that the [220]-oriented films have a narrower forbidden band width,and that there are significant differences in the forbidden band widths for films with different lattice constants in the studied range,with the smallest forbidden band width of about 0.325 e V for PbS films with a lattice constant of about 5.90 ?.(2)The effect of deposition time and temperature on the microstructure and optical properties of PbS films on two substrates showed that the overall crystallinity of the films was high,the thickness was at the nanometre level,the lattice constant was significantly lower than that of the bulk material,and the Pb/S stoichiometry ratio was approximately 1:1.For the glass-based films,the deposition temperature was 47 °C and the deposition time was60 min.The optical band gap was 0.224 e V for PbS/Si films with a grain size of 86.1 nm and low microscopic stresses,and the smallest optical band gap(0.215 e V)for PbS/Si films with a deposition temperature of 37 °C and a deposition time of 60 min.Comparison of the PbS films on different substrate materials revealed that the PbS films prepared on both substrates had a good orientation and all grew along the [200] orientation,which is consistent with the theoretical simulation results.Compared to the PbS/Si(100)films,the PbS films grew on glass substrates with higher orientation,more pronounced weaving and denser film layers,allowing for better epitaxial growth.However,in terms of optical properties,it is clear that the forbidden band width of PbS films on single crystal Si(100)substrates is narrower under the same deposition conditions.(3)Studies on the effects of deposition time and deposition temperature on the microstructure and optical properties of PbSe films on both substrates show that the overall crystallinity of the films is higher under different deposition parameters,the thickness of the film layer is at the nanometre level and the Pb/Se stoichiometry ratio is about 1:1,and the regulation of the film thickness,grain size and optical band gap can be achieved by adjusting the deposition temperature and deposition time.For the glass-based films,a deposition temperature of 80 °C and a time of 180 min resulted in a grain size of 86.1 nm and a film thickness of 121 nm,with minimal microscopic stress,and an optical band gap of 0.220 e V,which was more sensitive in the IR region compared to other samples.For the PbSe/Si films,the optical band gap was smallest(0.220 e V)at a deposition temperature of 80 °C and a deposition time of 150 min.A comparison of the PbSe films on different substrate materials revealed that the PbSe films prepared on both substrates had good orientation and all grew along the [200] orientation.Compared to the PbSe/Si(100)films,the PbSe films on glass substrates grew with higher orientation,more pronounced weaving and denser film layers,allowing for better epitaxial growth.In terms of optical properties,it is clear that PbSe films on single crystal Si(100)substrates are more sensitive in the IR region under the same deposition conditions.(4)Oxygen doping was achieved by oxidative heat treatment of the PbS films.The oxidative heat treatment temperature can significantly affect the crystal structure of the films,with higher crystallinity of the films under different oxidative heat treatment conditions and a significant increase in grain size compared to the untreated samples.For PbS films,the grain size first becomes larger and then decreases as the oxidation heat treatment temperature increases,and the lattice constant varies with the amount of oxygen doping,with the best crystallinity at an oxidation heat treatment temperature of 150 °C and an oxygen doping of5.14 at.%,with a lattice constant of approximately 5.915 ?.For PbSe films,as the oxidation heat treatment temperature increases,the films show The concentration of oxygen in the film increased with the increase of oxidation heat treatment temperature,and the lattice constant increased with the increase of oxygen concentration.(5)By comparing the effects of different doping concentrations on the band structure and optical properties,it was found that the band structure changed significantly with the increase of oxygen doping concentration,the valence band and conduction band density of states increased significantly,and the forbidden band width decreased,and when the doping concentration was higher than 3.125 at.%,the valence band and conduction band energy levels intersected and formed a conductor.The doping of oxygen is beneficial to the absorption of photon energy in the near infrared region,and the absorption coefficient gradually increases.