Direct Growth Of Two-Dimensional Layered Semiconductor Heterostructures For High-Performance Optoelectronics

In recent years,many innovative technologies come out,such as artificial intelligence?AI?,5G communication,cloud computing,big data and the internet of things?IOT?.Without exception,the future of these technologies cannot be separated from the prosperity and development of the semiconductor integrated circuit industry.Over the past few decades,the development of integrated circuits has been following Moore's law,the feature size of transistors is scaled down into few nanometers,which has nearly approached the physical size limit.In the forthcoming post-moore era,figuring out which new semiconductor materials to develop and how to obtain devices with high performance are becoming more and more important to promote the development of integrated circuit industry,which affects the heartstrings of both academia and industry.Starting from the study of graphene,two-dimensional?2D?semiconductors with similar layered structure have attracted scholars'attention due to their ultra-thin physical size,novel structural properties and excellent optoelectronic performance.2D layered semiconductor materials are considered to be the lifesavers of the integrated circuit industry,and own great research and application value.In particular,when different kinds of 2D semiconductor materials are assembled together to form heterojunctions by van der Waals force,it can not only engage the excellent properties of each constituent materials,but also introduce the heterojunction interface.Such 2D heterostructures can be used for the transistor,photodetectors,memory,lasers,light-emitting diodes and so on,are considered to be indispensable important component units for next-generation integrated optoelectronic applications.Since 2D layered semiconductor heterostructures are of great importance,an important issue in this field is how to obtain high-quality 2D heterostructures for high-performance device applications.With these ideas in mind,in this dissertation,a variety of ultra-thin 2D layered semiconductors and related heterostructures have been grown through improved chemical vapor deposition?CVD?techniques.Highly crystallized large-scale 2D p-n junctions were successfully prepared,then we constructed different kinds of devices through micro-nano processing technologies,and the effect of heterogeneous junction on the device performance was investigated.From the perspective of the device performance,we selected suitable materials and designed the heterostructures.Through introducing perovskite with excellent light absorption properties into the 2D heterostructures,we obtained high-quality ultra-thin perovskite-based heterostructures,and investigated the thickness-dependent optical and optoelectronic properties,further demonstrated high-performance photodetectors.The main achievements are summarized as follows:?1?By using two-step chemical vapor deposition method,I achieved the direct growth of two-dimensional ultra-thin WSe₂/SnS₂ vertical heterojunction.The maximum lateral size of the achieved triangular heterojunction can reach 835?m,representing the maximum size at that time.Transmission electron microscope?TEM?results confirmed the heterojunction samples are highly crystallized,and moire fringe patterns were observed at the heterojunction area.Two sets of hexagonal patterns were found in the selected area electron diffraction?SAED?results,and the lattice mismatch is calculated to be 14.3%,indicating the van der Waals epitaxy growth mechanism.Atomic force microscope?AFM?characterizations show that the thickness of WSe₂ and SnS₂ is 0.8nm and 0.8 nm,respectively,resulting in a bilayer heterostructure.When the 488 nm laser was irradiated on the heterojunction region,we observed a significant fluorescence quenching phenomenon,which was caused by the interlayer charge transfer.?2?Based on the obtained large-scale WSe₂/SnS₂ heterostructures,we designed and constructed multi-electrode back-gate transistor devices,forming three different kinds of devices on the same sample,namely pure WSe₂ devices,parallel-mode devices and parallel-series mode devices.The electric transport characterizations showed that the pure WSe₂ devices presented p-type conductive characteristics,and the carrier mobility was 0.02 cm²V^-1s^-1.The parallel-mode device showed ambipolar characteristics,in which the electron conduction part comes from the n-type SnS₂,which well confirms that WSe₂/SnS₂ is a vertical p-n junction.In addition,due to the existence of heterogeneous junction,the carrier mobility of this device reaches 10.1 cm²V^-1s^-1,three orders of magnitude higher than that of pure WSe₂ device.We also found that the parallel-series mode heterojunction devices have the lowest leakage current(10^-14 A)and obtain a high on-off ratio of 10⁷.The low leakage current also makes the parallel-series mode devices more suitable for high-sensitivity photodetection.Upon the 520nm laser irradiation,the parallel-series mode devices showed obvious photovoltaic effect,and obtained high photoresponsivity?108.7 mA/W?and ultra-fast response speed?500?s?,which represented the highest level acquired in direct-grown p-n junction devices at that time.?3?We used a three-step full-vapor growth method,employed transition metal chalcogenides?TMDC?as growth substrate,and lead iodide?PbI₂?as an intermediate material,successfully prepared the ultra-thin perovskite/transition metal chalcogenide?PVK/TMDC?heterostructures.It was found that the thickness of PbI₂ in the second step was directly related to the deposition temperature,and the thickness could be regulated from several nanometers to dozens of nanometers.After the vapor-phase intercalation reaction in the third step,ultra-thin perovskite heterostructure was obtained.X-Ray Diffraction?XRD?characterization indicates that PbI₂ can be completely converted into PVK after 4 hours intercalation reaction,and the results of TEM show that the crystal quality of perovskite is very good.In the SAED pattern,a set of tetragonal and another set of hexagonal electron diffraction patterns can be seen,which can be assigned to PVK and WS₂,respectively,which well illustrates the formation of heterostructure.We prepared the cross-section sample of the heterostructure with focused ion beam etching?FIB?,and the results also proved the high quality of the interface.The fluorescence emission behavior of perovskite is very sensitive to its thickness,which can be explained by quantum-confinement effect.Combined with the band structure calculation,we found that the band alignment can be tailerd from type-II to type-I as the thickness of perovskite increase from few nanometers to tens of nanometers thick.Transient absorption spectra characterizations also show that the interfacial carrier behavior is consistent with band alignment.?4?Based on the advantage that we can control the thickness of PVK/TMDC heterostructures,we systematically studied the influence of perovskite thickness on the device performance.We used the transfer electrode method to construct the backgate devices,and the characterization results of the transistors showed that the pure WS ₂ was n-type semiconductor,while with the gradual increase of the thickness of perovskite,the transfer characteristics changed from n-type dominated ambiporlar to p-type dominated ambiporlar,and finally to pure p-type.This is the first time that such thickness-dependent carrier transport operation is reported,in ultra-thin heterostructures,charge carriers in the bottom layer can be transported t hrough tunneling mechanism,forming a parallel model route,while in thick heterostructures,the charge transportation is mainly determined by the top perovskite layers.Upon 520nm laser irradiation,we found that the introduction of perovskite can greatl y improve the light absorption ability of the device,and the photoresponsivity of the heterostructure with 5.2 nm PVK can reach 11174.2 A/W,and the photoresponse speed is faster in thinner heterostructures?64?s for heterostructures with 2.7 nm PVK?.Meanwhile,too thick perovskite?36.2 nm?will not only hinder the interfacial charge separation,but also extend the carrier transport time in the vertical direction,leading to weaker photoresponsivity and slower response speed.