Electronic Structure And Phase Control Of New Chalcogenide Semiconductors

Photoelectric semiconductor materials use the separation of photogenerated carriers and the recombination of injected carriers to realize the mutual conversion of light and electricity.Chalcogenide semiconductors(such as Bi/Sb-based ternary sulfide,GaTe),as a new type of optoelectronic semiconductor,can be very promising due to their proper bandgap,high absorption/luminescence intensity,and good transport characteristics.These materials are very suitable for the manufacture of optoelectronic devices,such as solar cells,photodetectors,photodiodes,etc.In the actual application scenarios of semiconductor materials,different phase conditions(such as disordered phases and phase transitions)will affect the electronic structure of semiconductor materials and their optoelectronic properties.Disordered phases and phase transitions may lead to changes in the bonding mode of atoms in the semiconductor material,which in turn affects its electronic structure so that the bandgap of the material has a larger range of variation.This thesis focuses on the above two phase conditions,using first-principles calculation methods(based on density functional theory),taking two new types of chalcogenide semiconductor materials(AgBiS₂ and GaTe)as the research objects,focusing on intrinsic physical properties such as electronic structure the study.With the goal of exploring the relationship between the structure and properties of semiconductor materials with different phase conditions,we have obtained a series of phase control principles applicable to new chalcogenide semiconductor materials.These results can provide valuable references for experimental research.The main content includes the following two aspects:1.AgBiS₂ cationic disordered phase electronic structure researchThe new ternary sulfide semiconductor AgBiS₂ has two phase structures(normal temperature hexagonal phase and high-temperature cubic phase),of which the cubic phase has a cationic disorderly occupied rock salt structure.There are experimental reports that cubic AgBiS₂ has been successfully synthesized stably at room temperature in recent years.The solar cell prepared with AgBiS₂ cationic disordered rock salt phase as the light absorber has an efficiency of 6.4%.In addition,AgBiS₂ has the objective properties of being non-toxic,environmentally friendly,and easy to prepare,and has good application prospects in the photovoltaic field.However,due to the disordered nature of the cations in the current experimental and theoretical studies,there are certain difficulties in the related research of its electronic structure,which is mainly reflected in the uncertainty of the bandgap.The special quasi-random structure(SQS)method is a structure generation method that can simulate the atomic environment of a disordered structure as much as possible with a finite size unit cell.Therefore,we combined the SQS method with first-principles calculations to simulate the possible disordered structures of AgBiS₂.By analyzing the relationship between the disordered structure and the bandgap,we found the-Bi-S-Bi-S-chain in the disordered phase and-Ag-S-Ag-S-chain structure will cause the band edge of the conduction band to move down and the band edge of the valence band to move up,reducing the bandgap.Through this rule,we have determined the accurate bulk electronic structure of the cationic disordered phase AgBiS₂,with a bandgap value of 0.65 e V.The research results provide references for theory and experiment.2.Research on the phase change behavior of a two-dimensional semiconductor material GaTeGaTe is a material with promising applications in the field of optoelectronic devices in the research of two-dimensional semiconductors in recent years.It has a suitable bandgap value(1.65 e V)and high light responsivity(10⁴ A/W).GaTe has a relatively complex crystal structure,and it is experimentally determined that it has two stable phases(monoclinic phase and hexagonal phase),but the current research on its phase transition process and inducing factors is not clear enough.Under the framework of quasi-harmonic approximation,we predict that when the temperature drops to 216 K,GaTe will undergo a phase transition from a hexagonal phase to a monoclinic phase.We use the nudged elastic band method to calculate the phase transition barriers and the highest potential barrier of the phase transition process is 288 me V/formula.The higher phase transition barrier indicates that the phase transition process is stable,reliable and irreversible.Through the study of the electronic structure properties and phonon properties of the two phases,we verified the stability of the two phase structures and determined that the monoclinic phase of GaTe has a better prospect for photovoltaic applications.