Preparation Of Two-Dimensional Material Thin Films By Pulsed Laser Deposition And Study Of Optical And Electrical Properties

In the past few decades,the information technology has developed rapidly.The development of information technology is inseparable from the progress of the integrated circuit industry.Semiconductor materials are the foundation of the integrated circuit industry and have attracted much attention from researchers.With the discovery of graphene,two-dimensional semiconductor materials have been extensively studied.Since graphene lacks the intrinsic band gap required by semiconductor devices,it has not been widely used in devices.In contrast,two-dimensional materials such as transition metal dichalcogenides（TMD）and III-VI binary chalcogenides with intrinsic band gaps have been extensively studied.However,how to obtain films with stable yield and large area has always been an important factor restricting the application of two-dimensional materials.Traditionally,two-dimensional material nanosheets are obtained by peeling off the bulk or synthesised through chemical vapor deposition（CVD）method,but they can not fully meet the requirements of industrial production.Pulsed laser deposition（PLD）is a technology traditionally used to deposit oxide films.If used to deposit two-dimensional material films,it can prepare films completely covering the substrates,and it can solve the requirements of large-area and stable-output necessary for industrial production.In view of this,this article mainly studies how to use the PLD method to prepare high-quality MoS₂ and In Se thin films with potential applications,and explores its optical and electrical properties.The main work of this paper is as follows:（1）MoS₂ films were prepared on fluorophlogopite substrates using PLD technology.By annealing in a sulfur atmosphere,the defects and oxidation of the as-grown MoS₂ film are reduced.By comparing the optical and electrical properties before and after annealing,the effectiveness of annealing is proved.First,large-area MoS₂ films have been prepared on the fluorophlogopite substrates using PLD technology.Due to the deposition process in a sulfur-free atmosphere,a large number of defects and oxidation are inevitable in the native films.Therefore,annealing under a sulfur atmosphere at different temperatures has been used to reduce the defects and oxidation.According to the peak area of X-ray photoelectron spectroscopy（XPS）,the proportion of molybdenum oxide after annealing at different temperatures was calculated.The optimal annealing temperature was determined to be about 500℃.The S/Mo ratio of MoS₂ film after annealing at the optimal temperature is 1.98:1,which is very close to the theoretical value.The changes of morphology before and after annealing were observed through scanning electron microscope（SEM）images.At the same time,Raman spectroscopy,photoluminescence（PL）spectroscopy and X-ray diffraction（XRD）spectroscopy provide further evidence of improved crystallinity,which all prove the effectiveness of annealing.In terms of electrical properties,by comparing the Kelvin probe force microscope（KPFM）images before and after annealing with gold as a reference work function,it is found that the Fermi level of MoS₂ has moved 0.175 e V after annealing because molybdenum oxide has been nearly eliminated.（2）The MoS₂ films grown on fluorophlogopite were used to study the thickness-dependent characteristics of MoS₂.Compared with MoS₂ prepared on sapphire substrates,the influence of different substrates on the optical properties of MoS₂ was studied.The photoresponse characteristics of MoS₂ on the sapphire substrates and the influence of temperature on the responsivity and speed of MoS₂are emphatically studied.First,a batch of MoS₂ films with different layers were prepared on fluorophlogopite by controlling the number of laser pulses.Raman spectroscopy is used to study the thickness dependence of phonon frequency and line width.Using Raman mapping and AFM images,the uniformity and morphology of MoS₂ on fluorophlogopite substrates were studied as a function of thickness.The three-layer MoS₂ has the best uniformity.The annealing process has a similar effect on MoS₂prepared on sapphire substrates.However,due to the difference in the degree of surface oxidation and the difference in the concentration of sulfur vacancies,the electron cloud density is different.The inner electron binding energy of MoS₂ on sapphire substrates is slightly larger than that on fluorophlogopite substrates.The phonon redshift and the band gap of MoS₂ become smaller,because the sapphire substrates have different van der Waals force and stress on MoS₂ with these on the fluorophlogopite substrates.At the same time,MoS₂ on the sapphire substrates also exhibits similar thickness-dependent characteristics.Through Raman mapping and photoluminescence mapping images,it is proved that the single-layer MoS₂ film prepared on the sapphire substrate has better uniformity.The temperature has non-negligible effects on the lattice vibration and energy band structure.If MoS₂ is to be used in optoelectronic devices,the temperature effects must be considered.Among the MoS₂ samples with different layers,the single-layer MoS₂ sample has the strongest photoresponse,which has a photoresponsivity as 3 m AW^-1 in a 50μm×150μm area.The temperature-dependent photoresponse experiments of a single layer MoS₂film shows that,near room temperature,the response intensity and response time of MoS₂ generally increase with the temperature.（3）A batch ofε-In Se thin films were prepared on SiO₂/Si substrates using PLD technology.The effects of temperature on the phonon modes and interband electronic transitions of In Se thin film was systematically studied.A centimeter-sizedε-In Se thin film was prepared using the PLD method.The optical images,Raman mapping,and atomic force microscope（AFM）images proved that the thin film has good uniformity and a pureεphase.In order to explore the influence of temperature on phonon modes,a temperature-dependent Raman spectra study,in the temperature range of 123 K to 423 K,was applied to study the variation of lattice vibration with temperature.Due to lattice thermal expansion and anharmonic effects,the frequency of phonons red-shifts as the temperature increases.The frequency and full width at half maximum（FWHM）of²₁₂),¹₁₂)and¹₂₂)modes all show a strong linear relationship with temperature.However,the frequency and FWHM changes of¹₂₂)（）mode show a strong non-linear relationship with temperature,which may be because the resonance effect between the laser and the crystal is weakened by the increasing temperature.In order to explore the influence of temperature on its electronic transitions,the energy band structure was studied by temperature-dependent spectroscopic ellipsometry with the Tauc-Lorentz model.The fitting results show that there are five electronic transitions around 1.33,1.61,2.53,3.73,and 4.64 e V,which generally red-shift with the increasing temperature.Through the Bose-Einstein model,we use the electron-phonon interaction to explain the variation of the optical band gap of In Se thin film with the temperature.