| As a representative of the third-generation semiconductor,gallium nitride has the advantages of wide band gap,high electron mobility and high breakdown voltage.It plays an important role in high-temperature and high-frequency electronic devices.In the past decades,numerous effort has been done on gallium nitride materials.However,due to the difficulty of the sample preparation,the experimental study of ultra-thin GaN has only recently emerged.Ultra-thin GaN can be regarded as a two-dimensional form of gallium nitride to a certain extent.It is reported in theoretical works that ultra-thin GaN would transform into a two-dimensional layered material at a certain thickness.Compared with the bulk crystal form,two-dimensional gallium nitride has a wider band gap and a higher internal quantum efficiency.In this work,we anneal gallium selenide in an ammonia environment,with the purpose of directly obtaining two-dimensional gallium nitride.The specific work involves the preparation and characterization of ultra-thin gallium selenide,the amination of ultra-thin gallium selenide,and the characterization of ultra-thin gallium nitride.In addition,we also studied the optical contrast of the gallium selenide flakes,the influence of ammonia flow rate and temperature of the environment,and the micro-area photo luminance properties of obtained ultra-thin gallium nitride.I present my work in four chapters as following:The first chapter is the introduction of this thesis.First,I will introduce the structures,the properties and the application prospects of ultra-thin two-dimensional nanomaterials,gallium selenide and gallium nitride are introduced.Then I will briefly introduce the recent experimental works and the development trend of ultra-thin GaN.In the end,I will outline the background and scientific significance of the research topic.The second chapter discusses the preparation and optical contrast of gallium selenide sheets.We prepared the gallium selenide sheet by micro-mechanical stripping method.The relation between the thickness of the gallium selenide sheet,the material of the substrate and the illumination wavelength was studied by a theoretical calculation.The results show that under a yellow or orange light,the single-layered gallium selenide on the 300 nm SiO2/Si substrate has the largest optical contrast,which is the easiest to find.Under the 550 nm yellow light,the thickness of the gallium selenide sheet is less than 10 nm.The same thickness of the gallium selenide sheet is more optically contrasted on the substrate of the 300 nm thick SiO2 layer.The third chapter discusses the ammoniation quality of gallium selenide flakes under different conditions.The ammonia flow rate,reaction temperature and ammoniation time all have important influences on the quality of the obtained products.Excessive ammonia flow has an etching effect on the surface of the sample,makes it difficult to obtain a flat ultra-thin GaN.On the other hand,if the ammonia flow is too small,the ammoniation may not take place.If the temperature is too high,the sample will evaporate more.If the temperature is too low,the ammoniation reaction will not proceed.Experimental results show that the ammoniation is successful at a temperature of 950 ?,ammonia flow of 200 sccm and a ammoniation time of 3 hours.We characterized the ammoniated gallium selenide flakes by using XRD,Raman,PL,SEM,TEM,etc..The results prove that we have successfully obtained gallium nitride flakes.Compared with bulk crystal form of gallium nitride,the longitudinal optical peak of ultra-thin gallium nitride in the Raman spectrum has a significant blue shift.Also,the PL spectrum has a blue shift because of the quantum confinement effect.The fourth chapter has some prospects for some challenges and development trends in ultra-thin GaN research. |
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