The Synthesis Of Two-dimensional Heterostructures Based On Ⅲ-Ⅵ Group Semiconductor For Optoelectronic Detection

With the rapid development of the semiconductor integrated-circuit industry,our society has stepped into the era of mobile Internet.The semiconductor integrated-circuit industry has greatly changed our production methods and can be found in every aspects of daily life,such as 5G communication,artificial intelligence,and computers.However,with the size of transistors approaching the physical limit,the development of the integrated-circuit industry has faced a bottleneck.How to further promote the development of electronic devices towards miniaturization,low-power consumption,and high performance is a great challenge faced by academia and industry.In 2010,the Nobel Prize in Physics was awarded to the inventor of graphene.Since then,two-dimensional（2D）materials have attracted great attention in recent years.2D materials have an atomically thin thickness,large surface volume ratio,continuously adjustable band gap,and excellent electron transport properties.Particularly,various kinds of 2D materials can be stacked together to form heterostructures by using the interlayer weak van der Waals forces,and the heterostructures often exhibit excellent and distinct physical properties from its component.The 2D layered heterostructures have shown great potential in transistors,photodetectors,light-emitting diodes,photovoltaic energy cells,which makes them also indispensable and important members of new optoelectronic devices in the future.Therefore,how to obtain 2D material heterojunction with outstanding performance and application potential remains a major challenge.Based on the research background,this paper is devoted to the preparation of heterostructures based on novel 2D layered material and the study of photodetector applications.Firstly,we need to look for the 2D material with high light absorption coefficient and high carrier mobility.Then,combining the theory with experiment to design the growth strategy of the material.A variety of heterostructures with high-quality interface have been successfully realized by the improved chemical vapor deposition（CVD）method.Through the design and construction of a variety of vertical heterojunctions,we realize the photodetectors with ultra-low dark current,self-driven,and wide-band detection range,which has laid an important foundation for the next generation optoelectronic devices.The main contents of this paper are listed below:1.The controllable synthesis of vertically stacked GaSe/MoS₂ p-n heterojunctions were achieved by developing a liquid-Ga-assisted CVD technique.The nucleation site and density can be effectively adjusted by dynamically tuning the Ga/Se ratio in the gas precursor,thereby realizing the growth direction and sample size in a controlled manner.Burton-Cabrera-Frank theory is introduced to explain the growth mechanism.In different Ga/Se ratios,active adatoms with different diffusion barriers and Ehrlich-Schwoebel barriers are formed,and thus the vertical and lateral growth directions can be precisely regulated.The clear Moiré patterns in high-resolution transmission electron microscopy（HRTEM）images confirm the vertically stacking of the van der Waals heterostructure.The cross-sectional HRTEM image also shows an atomically sharp heterointerface between GaSe and MoS₂,revealing the highly crystalline of the vertical GaSe/MoS₂ heterostructure.2.The photodetector was designed and fabricated based on the vertical GaSe/MoS₂ p-n junction.The device demonstrates distinct rectification behavior and prominent photovoltaic effect in the range of ultraviolet to visible light（375-633 nm）.What is more,the device has a very large open-circuit voltage of 0.61 V,indicating that it can be worked as a self-driven photodetector.Upon 450 nm laser irradiation,when it works at photovoltaic mode,the photodetector exhibits remarkable photoresponsivity（900 mA/W）,excellent detectivity（6.5×10⁹ Jones）,and fast response speed（5 ms）,suggesting excellent photosensing performance.The photocurrent mapping test reveals that the generation of the photocurrent was driven by the built-in electric field of the GaSe/MoS₂ p-n heterojunction.3.The controlled synthesis of ultrathinβ-In₂Se₃/MoS₂ vertically stacked heterostructures were achieved by a typical two-step CVD method.The heterostructure exhibits a temperature-dependent growth behavior,therefore the thickness and morphology of the heterostructure can be successfully realized through selecting an appropriate growth temperature.Transmission electron microscopy measurements show evident Moirépatterns and two sets of hexagonal arrangement diffraction spots,which indicates successful realization of the van der Waals vertical heterostructures with well-aligned lattice orientation.The theoratical calculation demonstrates the type-Ⅱ band alignment of the In₂Se₃/MoS₂heterostructures.The obvious photoluminescence quenching phenomenon was observed in the heterojunction,indicating the efficient charge transfer behavior between the interlayer,which is consistent with the calculated results.4.The optoelectronic devices were constructed based on theβ-In₂Se₃/MoS₂heterojunction with partly-covered and fully-covered morphologies.For the partly-covered heterojunction device,it has ultra-low dark current(10^-12 A)and high current on/off ratio（10⁶）.The obvious rectification behavior of the device indicates that the built-in electric field of the interface can effectively separate the photogenerated carriers.The device exhibits excellent photosensing properties under different wavelengths of laser irradiation.Upon 450 nm laser irradiation,the devices show high responsivity of4.47 A/W and a detectivity of 1.07×10⁹ Jones.In addition,the detector shows obvious photoresponse to 830 nm near-infrared light,and has a fast response speed（51 ms）at room temperature.The photodetector based on fully-covered heterojunction has a high responsivity（980 mA/W）to the laser wavelength of 830 nm because the top few layer ofβ-In₂Se₃ can effectively improve the absorption abilitiy to the near infrared light.