Study On High-speed Two-dimensional Transition Metal Chalcogenide/Si Bipolar Heterojunction Photodetectors

The extensive research on transition metal dichalcogenides?TMDs?has shown that these are suitable for the next-generation photodetectors in view of the unique properties,i.e.wide wavelength absorption,high power conversion efficiencies and easy to create a van der Walls heterojunction?vdWH?without lattice distortion due to the dangling-bond-free surface.Although a high photoresponsivity was achieved,the main challenges for this kind of photodetectors are to controllably prepare fast response speed for practical application.In this thesis,2D TMDs semiconductor materials,molybdenum disulfide?MoS₂?and molybdenum diselenide?MoSe₂?,were choosen as research objects.Through reasonable selection of material synthesis methods and optimization of device structure,bipolar heterojunction photodetectors with ultra-high response speed and stable performance were prepared successfully.The main research results are as follows:?1?High speed 2D-3D Gr/MoS2/SiNWs bipolar Schottky-NP vdWHs heterojunction.A novel 2D-3D graphene?Gr?/MoS₂/silicon nanowire array?SiNWs?Schottky-NP bipolar van der Waals heterojunction was firstly fabricatedby using solution processed layered MoS₂ nanosheets.Heterojunction?vdWHs?The resultant device exhibits a high-speed response speed up to 17 ns with a broad spectral response characteristic,which is much better than the reported 2D-based photodetectors and is comparable to that of commercial Si photodetectors.Significantly,a responsivity of 0.6AW^-1 and a detectivity of 0.8×10¹³ Jones were achieved.And the predicted 3dB bandwidth is close to 11 MHz,which proves that the device can operatein high frequency environment.?2?High-speed Gr/MoSe2/Si bipolar heterojunction deep ultraviolt?UV?photodetector.Due to the relatively narrow bandgap width of TMD family materials,most of the2D TMDs-based photodetectors are currently reported todetect light wavelength in the visible to near-infrared range.Based on two-dimensional layered MoSe₂ and silicon absorption characteristics,a high speed Gr/MoSe₂/Si bipolar heterojunction was constructed by in situ growth of layered MoSe2 on Si substrates via a pulsed laser deposition?PLD?method..The narrow-bandgap UV photodetectors,clear response in UV light than visible-near-infrared light,were successfully constructed from optimaziton of this heterojunction by using various thickness of the layered MoSe₂.The resultant photodetector shows that the response speed of the device is as high as0.913?s,which is much faster than the those of provious wide-bandgap semiconductors and 2D TMD-based UV photodetectors.In addition,the responsitivity and detectivity of the device reached 4.95×10¹³ Jones and 0.45 A W^-1,respectively.The excellent UV response characteristics are mainly attritubted to the depth regulation of the junction region and the high crystal quality.The layered MoSe₂ film also reduces the surface state density of the device and enhances the collection efficiency of ultraviolet generated photocarriers.Our work would open a new avenue for the design and fabrication of UV-based photodetectors based on 2D TMDs.The device structure design based on 2D TMDs material will provide an important reference for the expansion of 2D materials in the construction of high-speed photodetectors,and also provide a pathway for the construction of silicon-based compatible photodetectors.