Investigations On ZnSnN₂ Thin Films And Its Pn Junctions

We are faced with double crisis of energy sources and ecological environment,which forces all the countries to develop the solar cells rapidly.Thin film solar cells are attracting much attention because of low cost,light mass,flexibility,high efficiency.The core problem of thin film solar cells is developing new photovoltaic materials.In recent years,ZnSnN₂,a member of ternary Zn-IV（Si,Ge,Sn）-N₂ semiconductors,has gained considerable attention because of its favorable properties for use in solar cells.（I）Zn,Sn and N elements are non-toxic,earth abundant and recyclable.（II）ZnSnN₂ possesses a sound absorption coefficient.（III）ZnSnN₂ has a tunable direct band gap.However,the high carrier concentration in as-fabricated ZnSnN₂ limits its application in photoelectronic devices.Up to now there are few reports on ZnSnN₂-based devices.Pn junction with a simple structure,is a basic“building block”of solar cells,LED,detectors and bipolar transistors.The study on the physical properties of ZnSnN₂ and its pn junction can pave a way to construct ZnSnN₂ solar cells in the future.In this work,the influence of preparation conditions on the properties of ZnSnN₂ thin films was investigated.Then p-Si/n-ZnSnN₂ heterojunctions were constructed and their electrical properties were analyzed.The main results were summarized as follows.（1）The ZnSnN₂ thin films were fabricated by magnetron sputtering system.The structural,morphological,optical and electrical properties of the ZnSnN₂ films were investigated as a function of the Zn/Sn atomic ratio in the sputtering target.The polycrystalline semiconducting ZnSnN₂ films were synthesized using Zn₃Sn₁ target at room temperature.The structure and morphology is closely related with the Zn:Sn atomic ratio,and the optical and electrical properties can also be regulated.（2）The p-Si/n-ZnSnN₂ heterojunctions were fabricated using sputtering combined with patterned technology and electron beam evaporation system.The preparation conditions were optimized,such as the size of pn junctions,the thickness of ZnSnN₂ layer and sputtering power.And the influence of these preparation conditions on rectification ratio,ideality factor and series resistance was studied.The optimized pn junction was achieved with a rectification ratio of 81 at±1.5 V.（3）The effect of post-annealing temperature on the properties of both ZnSnN₂ thin films and p-Si/n-ZnSnN2 heterojunction was investigated.The reverse and forward I-V characterization was analyzed by the space charge limited conductivity and the thermionic emission models,respectively.The ideality factor was given by theoretical thermionic emission models and the thermionic emission models with series resistance,then the relationship between series resistance and the performance of pn junctions was studied by comparing the different values of iedality factors.In Summary,semiconducting ZnSnN2 films were fabricated by optimizing the target and post-annealing temperature.Then Si/ZnSnN2 pn junctions with a rectification ratio up to 1066（±1.5 V）were constructed.