Controlled Synthesis Of 2D Bi₂O₂Se Semiconductor Material And Photodetector Fabrication

In recent years,two-dimensional Bi2O2Se semiconductor material have been reported for ultra-high carrier mobility and the observation of quantum oscillations in unpackaged devices.Lots of attention has been paid to Bi2O2Se due to the extremely small effective electron mass in the material as well as high hall mobility up to 20000cm22V-1s-1 at 1.9K,small band gap about 0.8 eV and excellent stability.Infrared photodetector based on Bi2O2Se was reported to have an ultra-high responsivity and intrinsic photoelectric response speed close to graphene,which once again aroused interest of Bi2O2Se materials in optoelectronic application.Many experimental studies have reported the application of electronic and optoelectronic devices based on Bi2O2Se.Due to the large light absorption coefficient,the responsivity of photodetector is higher than photodetectors made by other two-dimensional semiconductor materials.There are also many reported paper on theoretical calculations;including the excellent thermoelectric properties of Bi2O2Se materials(the calculated ZT is 4.2),ferroelectric and ferroelastic properties by doping Te,S and other elements.It shows that Bi2O2Se has huge potential in many fields such as optoelectronics,thermoelectricity and ferroelectricity,and requires a large number of experiments to verify and achieve theoretical calculation results.However,the problem encountered at present is that the Bi2O2Se crystal grown on the mica is difficult to be separated from the substrate due to strong interaction,which is liminting device fabrication and performance investigations.Electrode evaporation will introduce defect state at the interface between Bi2O2Se and the electrode,result in decrease of response speed of the device.Regarding to above problems,I realized the controlled growth of out-of-plane growth Bi2O2Se crystals by regulating growth temperature and pressure.The device was prepared by the transfer electrode method,in which the whole transfer process is free of adhesive polymers or etching liquid may introduce contaminants and defects,ensuring the intrinsic properties of the two-dimensional Bi2O2Se material not be damaged.Furthermore,fabrication method of van der Waals heterojunction of Bi2O2Se and other two-dimensional materials was also explored and the photosensitized photo detector and self-powered photodetector were built up.The main research contents are as follows:By regulating the pressure and temperature of CVD,I found that out of plane ultra-thin Bi2O2Se crystals can be synthesized at lower pressures,and the interaction between the crystal and the substrate is quite small that can be easily transferred to any other substrate.Moreover,the out-of-plane growth of Bi2O2Se crystal has the same lattice structure as in-plane Bi2O2Se and demonstrating excellent electrical properties that the electron mobility of the FET device on Si/SiO2 is 230cm2V-1s-1 at room temperature and 3194cm2V-1s-1 at 90K.I systematically analyzed the effects of different pressure and temperature on crystal growth,and eventually I can control the growth orientation,crystal size,crystal thickness,and nucleation density of Bi2O2Se.Furthermore,thickness-modulated in-plane homojunctions Bi2O2Se crystals were synthesized by substrate pre-treatment and second growth method.I fabricated FET structure photodetector with out of plane growth Bi2O2Se transferred on Si/SiO2 substrate.By adjusting the back gate,the performance such as responsivity and response speed of photo detector can be increased.After optimizing the electrode structure and the thickness of Bi2O2Se,the photo responsivity of FET device can approach 1×108A/W under a small light intensity.A photocurrent larger than 50?A was generated at a light intensity of 1mW/cm2.At the same time,it is also possible to decrease the gain by negative back gating and obtain a faster photoelectric response speed of 320?s.In order to obtain a faster photodetector device,I introduce electrode transfer method to fabricate MSM device with Bi2O2Se grow on mica substrate,in which defect state was effectively reduced and realized high response speed of 47.6?s.I further improve the photoelectric response speed with the in-plane homojunction device.Built-in electric field between different thicknesses of Bi2O2Se contributes to the separation of photogenerated carriers and improves the quantum efficiency and device response speed of 4.8?s was obtained.I introduced a facile probe tip assisted transfer method to Bi2O2Se transfer,by which I can achieve adhesive polymer free transfer of Bi2O2Se onto other two-dimensional or three-dimensional materials.Two different kinds of photodetector was made by Bi2O2Se/graphene heterostructures.One is sensitized photodetector based on Bi2O2Se as the light absorbing layer and graphene as the electron transport layer,that can achieve high responsivity up to 3.8×106A/W at applied bias voltage of 0.1V.In addition,I fabricated a self-powered photodetector based on,Bi2O2Se/graphene heterojunction.The device can output a large photocurrent without an external bias and is very sensitive to external illumination.