Study On Preparation And Optoelectronic Properties Of TMDCs/SnS₂ Heterostructures

Two-dimensional materials hold great promise for electronic and optoelectronic devices such as transistors and photodetectors due to their atomically thin thickness,continuously tunable band gap,strong light-matter interactions and excellent electrical properties.Heterostructure photodetectors have been extensively studied as they can effectively promote both efficiency and speed of photoelectric conversion compared to single-material devices.The components of van der Waals heterostructures composed of two-dimensional materials are stacked by weak van der Waals forces,and this can avoid the limitations of factors such as lattice matching in conventional heterostructures.Transition metal dichalcogenides（TMDCs）,as one of the two-dimensional layered van der Waals materials,have many unique properties.The two-dimensional material SnS₂ has a similar structure to TMDCs and has a high light absorption and can form type II（MoS₂/SnS₂）and type III（WSe₂/SnS₂）heterostructures with TMDCs,which are valuable in improving photodetection performance.More research is still needed on the preparation of TMDCs/SnS₂ heterostructures by chemical vapour deposition（CVD）.Therefore,in this thesis,MoS₂/SnS₂ n-n heterostructures and WSe₂/SnS₂ p-n heterostructures were prepared by one-step growth method and two-step growth method,respectively,mainly using halogen salt-assisted confined-space CVD.The effects of experimental parameters on the growth of heterostructures were systematically investigated,the corresponding physical mechanisms were analyzed,and the preparation of samples with controllable morphology,size,and thickness was achieved.Finally,the corresponding heterostructure photodetectors were constructed,targeted tests and analyses were performed according to the heterostructure types,and photodetectors with high responsivity,high detectivity and high response speed were realized.The main research contents of the thesis are as follows:1.MoS₂/SnS₂ heterostructures were grown by a one-step confined-space CVD method to achieve high responsivity detection.By taking advantage of the difference of evaporation temperature between Mo and Sn sources,the sequential growth of MoS₂ and SnS₂ was achieved in the same CVD tube.A series of optoelectronic characterizations including Raman,fluorescence,output,transfer and photoelectric properties confirmed the high quality of the heterostructures.Meanwhile,it can be found that the heterostructure photodetectors composed of thick-layer SnS₂ and MoS₂ have the unique advantage of high responsitivity.The typical MoS₂/SnS₂（thick-layer SnS₂）heterostructure device exhibits an electron mobility of approximately 0.5 cm²V^-1s^-1 and an on/off current ratio of up to 10³.The heterostructure device displays a continuous,stable and fast optical response to visible and near-infrared light,with a maximum responsivity of 9611 A/W under 520 nm laser irradiation and a minimum response time of approximately 513μs under 637 nm laser illumination.2.The effect of SnS₂ thickness on the optoelectronic properties of MoS₂/SnS₂heterostructures was compared and analyzed.Two more types of heterostructure photodetectors were constructed:MoS₂/SnS₂（thin-layer SnS₂）heterostructure and MoS₂/SnS₂（single-layer SnS₂）heterostructure.All the three types of heterostructures show relatively fast response under visible and near-infrared light irradiation.However,the responsivities are quite different.The maximum responsivity of the MoS₂/SnS₂（thin-layer SnS₂）heterostructure is 3485 A/W,while that of the MoS₂/SnS₂（single-layer SnS₂）heterostructure is only 294 A/W.In order to explore the mechanism,MoS₂/SnS₂ heterostructure samples with different thicknesses of SnS₂ were tested and analyzed.The strong interlayer coupling between MoS₂ and SnS₂ is conducive to the separation and transport of photogenerated carriers,which enables the heterostructure devices to obtain fast response.At the same time,thick-layer SnS₂ helps the MoS₂/SnS₂ heterostructure to enhance light absorption and obtain high responsivity.3.The p-type WSe₂ and WSe₂/SnS₂ p-n heterostructures were prepared using a two-step CVD method,and high detectivity was realized in self-driven optical response mode.WSe₂/SnS₂ heterostructures were successfully prepared by using WSe₂ as the substrate for SnS₂growth,and WSe₂-SnS₂ horizontal heterostructures were synthesized by reducing the Sn/S ratio.WSe₂ and WSe₂/SnS₂ heterostructure devices were then produced and their electrical and optoelectronic properties were systematically investigated.The WSe₂ one exhibits p-type transport behaviour and good ohmic contact with the metal electrode,showing a maximum responsivity of 20.2 A/W under 940 nm laser illumination.The WSe₂/SnS₂ heterostructure shows significant rectification behaviour with a rectification ratio of up to 10³.The maximum responsivity of the heterostructure is 227.3 A/W at 0 V bias,and the corresponding highest detectivity is 7.85×10¹² Jones.At the chopping frequency of 300 Hz,the heterostructure displays a fast optical response with rise and fall times of 166μs and 440μs,respectively.