Optoelectronic Performance Of MoS₂/BaTiO₃ Field Effect Transistor

Field-effect transistors(FET)show widely promising applications in photodetectors.Improvement in the charge carrier mobility and density for a FET device is intrinsically important in the high-performance photodetection field,where one of the most common and effective approaches is to use the gate dielectric with high dielectric constant.After a long-term exploration,it comes to a consensus that the ferroelectric with high dielectric constant effectively elevates the charge carrier mobility and optoelectronic performances of a FET.This optimization mostly benefits from ferroelectrics coupled residual polarization-field which,however,seems futile regarding increasing the charge carrier density.Therefore,it has not been thoroughly studied whether a similar high-dielectricconstant paraelectric gate dielectric can effectively optimize the optoelectronic performances through lattice regulation to increase the carrier mobility and density of FET.Obviously,it is highly desirable to carry out the corresponding systematic studies to establish the correlation between the properties of paraelectric gate dielectric and the device's performance.In this thesis,paraelectric cubic barium titanate(BaTiO3)with high dielectric constant and MoS2 with high mobility and optoelectronic properties are used as gate dielectrics and channel materials respectively to fabricate the MoS2/B aTiO3 photodetector.Through deliberately introducing lattice defects in paraelectric cubic BaTiO3,we systematically investigate the MoS2/BaTiO3 photodetectors with different thickness of defective BaTiO3 as the gate dielectric and light-absorbing layer of the device to synergistically enhance the charge carrier mobility and density.The main research contents and results are obtained below:(1)Optimization of the synthesis and dielectric properties of the paraelectric cubic BaTiO3 films by a solution-based spin-coating method.By adjusting the preparation conditions,such as precursors ratio,precursors solution,and annealing temperatures,etc.,we find the optimized conditions to prepare BaTiO3 film in high crystallinity without any impurities is that the annealing temperature is 650?,and the Ba/Ti molar ratio in solution is 0.9976.Because the mixed solvent of acetic acid and 2-methoxyethanol can interact with Ti source to form a stable metal complex and improve the contact between the precursor solution and the substrate,the prepared 300 nm BaTiO3 film shows very high dielectric constant and low dielectric loss(??60,tan??0.07 at 1 kHz),while the 100 nm film shows high dielectric properties(??11,tan??0.01 at 1 kHz)as well.UV-Vis spectrum clearly exhibits the light absorption peaks in the region of UV and non-zero absorption in whole visible light region.Elevated light absorption ability is associated with the lattice defects in spin-coated and subsequently annealed BaTiO3 film,which is confirmed by XPS results of defect Ti3+and oxygen vacancies.As lattice defect accumulates,defect states induced sub-bandgap absorption can facilitate the photoabsorption.These results lay the foundation of that the paraelectric BaTiO3 film can improve the optoelectronic performance of the device.(2)Synthesis and optoelectronic performance of MoS2 film by chemical vapor deposition(CVD)and bulk exfoliation methods.By adjusting the preparation conditions of CVD,such as substrate placement,reaction temperature and introduction time of sulfur powder.We finally synthesize the MoS2 flakes at 774? via implemented the traditional upside-substrate-CVD,and find their characteristic peak separation(?K)at 24.1 cm-1.Because the vacuum degree of tubular furnace is not enough,MoS2 intends to form separate clusters rather than continuous film.As a consequence,the single layer,two layers,three layers and several layers 2D MoS2 are obtained by the bulk exfoliation method and subsequent transferred to Si substrate.Raman peak separation can be used as an indicator of the film thickness,e.g.,peak separation of single layer film is only 18.1 cm-1 while that of bulk is up to 25 cm-1.Because single layer MoS2 is a direct band gap semiconductor,while as the thickness increases,it becomes an indirect band gap material and PL signal slightly decreases.This result proves that single layer MoS2 has the highest PL quantum yield.(3)Construction and performance of MoS2/BaTiO3 FET.The device with 300 nm BaTiO3 exhibits high field-effect properties(mobility:1.95 cn2V-1s-1;on/off ratio:107)and excellent optoelectronic properties(responsivity:598.47 A/W;detectivity:1.25×1012 Jones).The optoelectronic performances of the device with 100 nm BaTiO3(responsivity:1.2 A/W;detectivity:4×1010 Jones)and 300 nm SiO2(responsivity:0.2 A/W;detectivity:2×1010 Jones)are worse than that of 300 nm BaTiO3.In this work,lattice defects and interface energy band engineering were introduced to describe the mechanism:while the light transmittance of few-layer MoS2 exceeds 80%at 405 nm,the BaTiO3 below absorbs the 405 nm light passing through the MoS2 channel owing to the sub-bandgap absorption induced by lattice defect,and transmits the photogenerated charge carriers to the MoS2,increasing the total photocurrent.Compared with the thicker BaTiO3 film,devices with other dielectric show poor optoelectronic performance owing to the decrease(or lack)of light absorption of BaTiO3 and charge carriers transmitted to MoS2.Compared with other existing MoS2 photodetector with difference gate dielectric,the MoS2/BaTiO3 device in this work shows remarkable optoelectronic performance.These results strongly support that BaTiO3 can be used not only as gate dielectric for FET to improve charge screening,but also as photoabsorption layer in photodetector to improve its light detection performances.