Preparation,Microstructure,electrical And Optoelectrical Properties Of The MoS₂/Si Heterojunctions

MoS₂ has attracted much interest in the area of new-type electronic devices due to its good electrical,optical,electrochemical and mechanical properties.It is an effective route to develop the high-efficiency Si-based MoS₂ electronic devices through the integration between MoS₂ and Si.Importantly,new physics effects and novel properties of materials could be caused by the MoS₂/Si interface.Based on the above background,MoS₂/Si heterostructures were fabricated through the deposition of MoS₂ thin films on Si substrates and the novel electrical and photoelectrical characteristics,including mainly gas sensing and electrical properties caused by the hetero-interface were investigated in detail in this work.Furthermore,doping and interface modification were employed to improve the photoelectrical properties of the fabricated heterostructures and the enhancement mechanisms of the photoelectrical properties were clarified by the energy-band diagrams.The MoS₂/Si heterostructures were fabricated through the deposition of MoS₂ thin films on the Si surface using the DC magnetron sputtering technique.Raman spectra of the materials revealed that the grown MoS₂ thin films were bulk-like.The current curves showed that the heterostructures had obvious rectifying characteristics with the rectification ratio as large as 10² orders of magnitude.The gas sensing properties were studied for the fabricated MoS₂/Si heterostructures under different gas conditions,and the heterostructures showed obvious and different sensing properties to several different kinds of gas molecules,including H₂,NH₃ and H₂O.Compared with the situation in the air,the reverse currents of the heterostructures in the H₂ condition decreased largely and the forward currents were almost invariable,whereas,in the NH₃ and H₂O condition,the heterostructures showed the slight changes and obvious increases for the reverse and forward currents and the sensing response to NH₃ was larger than that to H₂O.The sensing mechanisms to different gases were clarified by the microstructural characterization and the energy-band diagram for the MoS₂/Si interface.The different sensing characteristics to different gases are very important for the MoS₂/Si heterostructures to be applied in the area of developing high-selective gas sensors.The photovoltaic properties of the fabricated Mo S₂/Si solar devices were studied.The devices showed obvious photovoltaic characteristics due to the separation of the photoexcited carriers caused by the built-in electrical field.Under the illumination of 30 mWcm^-2,the open-circuit voltage(V_oc),short-circuit current density(J_sc)and power conversion efficient???were 0.15 V,5.0 mAcm^-2 and 0.89%,respectively.Furthermore,the photovoltaic properties of the solar devices could be improved by the interface modification of the SiO₂ buffer layer insertion into the MoS₂/Si interface,and the device performance was enhanced largely by the SiO₂ buffer,i.e.,V_oc=0.30 V,J_sc=6.5 mAcm^-2 and?=2.6%.Simultaneously,the photovoltaic properties showed large dependence on the thickness of the SiO₂ buffers,and the optimal device performance existed at the buffer thickness of³.0 nm.The modulation effects of the Pd doping on the microstructures and properties of MoS₂thin films were studied.The X-ray photoelectron spectra?XPS?revealed that the substitution of Mo atoms with Pd dopants was realized successfully and the Pd atoms were stabilized by covalent bonding inside the lattice.The results from the energy-band structures of the films demonstrated that the Pd doping could modulate the semiconductor characteristics of the MoS₂ films from n-type to p-type.Owing to the incorporation of the Pd atoms in the MoS₂films,the photovoltaic characteristics of the solar cell were enhanced significantly and an over 350%enhancement of the power conversion efficiency was obtained.The photovoltaic performance of the Pd-doped solar devices was closely related to the concentration of the Pd doping,and the best performance occurred at the molar doping concentration of 1%,giving V_oc=0.39 V,J_sc=9.3 mAcm^-2 and?=4.6%under 30 mWcm^-2 illumination.In summary,in this thesis,we supplied new routes for the integration between MoS₂-based materials and Si semiconductors.Especially,we clarified the design of energy-band structures at the interface,controlling of carrier transport properties,modulation of semiconductor characteristics and coupling between the two materials.These aspects show the scientific significance and application value of this work.