Thermal Conductivity And Heterojunction Properties Of Large Area Polycrystalline Molybdenum Selenide Thin Films

Large area two-dimensional materials have great potential for designing and integrating electronic devices with complex components,but how to prepare large area two-dimensional semiconductor materials has been a huge challenge for researchers.After analyzing the advantages and shortcomings of the large area of the preparation of two-dimensional materials,we choose the chemical gas phase deposition method commonly used in industry to prepare the molybdenum selenide thin films.With the miniaturization of electronic components,the thermal dissipation of two-dimensional materials devices becomes a crucial problem impeding the performance.The thermal parameters of most of the two dimensional materials currently in the field are derived from its bulk material,so we need to further analyze the thermal characteristics of two-dimensional film materials.In this paper,a large scale polycrystalline Mo Se₂ thin film was prepared by two-step CVD method,and the thermal conductivity characteristics were studied by non-contact Raman spectroscopy.The results can provide effective guidance for the effective heat control of electronic and optical electronic devices based on selenium.Finally,the analysis of the prepared heterojunction device was carried out,and the further application of selenium molybdenum in photoelectric devices was of reference value.The main findings include the followings:(1)Two-step CVD was used to prepare large-area polycrystalline molybdenum selenide thin films.Large-area Mo Se₂ thin films were prepared on Si O₂/Si substrates by controlling different selenization temperatures.We use optical microscope(OM),XRD and Raman spectroscopy to analysis the construction and electrical characteristic of Mo Se₂ films.The results show that the optimum selenization temperature is 500?.(2)AFM,XPS,SE,XRD and Hall test were used to systematically characterize Mo Se₂ films.These tests indicate the Mo Se₂ films are p-type semiconductor materials with a 2H phase structure,a Mo/Se chemical atom ratio of 1:2.2,and a band gap of1.23 e V.In the experiment of ellipsometry with varying temperature,it is observed that the position of characteristic peaks of A and B excitons shifts red with the increase of temperature,which is mainly due to lattice expansion.The gap width of molybdenum selenide thin films decreases as the increasing of temperature,which accords with Varshni model and is also related to lattice expansion.(3)After the film was transferred to the porous substrate by wet transfer method,Raman spectroscopy was used to study the temperature and power dependece of the suspended film.The first-order temperature coefficient,power shift rate and thermal conductivity of molybdenum selenide films were obtained as-0.0153 cm^-1/K,-2.85cm^-1/m W and 28.48 W/(m·K),respectively.The corresponding experimental model was established in COMSOL software,and the temperature field distribution of the thermal conductivity model and the temperature variation diagram along the X-axis were obtained through finite element analysis.The temperature at the center of the film reached 380 K at 2 s.It means the heat is concentrated in the film.It also shows that the thermal conductivity of the film is low.(4)After the MoSe₂/Si heterojunction is prepared,the temperature dependence of the basic parameters of pn junction are investigated in the temperature-changing experiment.When the temperature increasing,the switching voltage,rectification ratio and ideal factor of the heterojunction decrease,while the reverse saturated current and barrier height increase.The band diagram of Mo Se₂/Si heterojunction is drawn,and the band bending of Mo Se₂/Si heterojunction is 0.56 e V,conduction band discontinuity 0.15 e V and valence band discontinuity 0.04 e V.The valence band barrier height is smaller than conduction band barrier height,and the hole is more easily transmitted from Mo Se₂ to Si side.Finally,the current transfer mechanism of the heterojunction is briefly analyzed.The current transfer mechanism is mainly direct tunneling in the low voltage region,and the current transfer mechanism is mainly F-N tunneling in the high voltage region.