Photoelectrochemical Fabrication And Mechanical Properties Of GaN Surface

With the continuous improvement of the performance of semiconductor devices,the demand for advanced semiconductor materials is becoming more and more robust.As the leading of the third-generation semiconductor materials,Gallium nitride(GaN)is widely used in optoelectronic,RF and power devices due to its wide band gap,high electron mobility,high breakdown electric field and stable chemical properties.Constructing surface micro-and nanostructures is an important technical means to attain the functionalization of semiconductor materials and enhance device performance,and has important applications in the related fields of GaN.In this thesis,the effects of electrochemical etching and nano indentation techniques on the surface properties of GaN are investigated and the related physical mechanisms are explored.The main work of this thesis is focused on the following points:(1)A novel porous GaN composite photoanode is fabricated based on the PEC etching combined with the plasmonic structure of gold nanoparticles.The water-splitting performance and the efficiency enhancement mechanism of this composite photoanode were investigated.Compared with the planar GaN photoanode,the water-splitting efficiency of the composite photoanode is significantly improved,with the photocurrent density,ABPE and IPCE being 3.7,8.3 and 3 times of the planar GaN photoanode,respectively.Due to the chemical inertness of GaN and AuNPs,the photoanode also exhibits good stability and maintains a high current density even after 1800 seconds of operation.It is demonstrated that the enhanced watersplitting efficiency of this composite photoanode is a synergistic effect of the abundant porous structure and the LSPR effect.The porous GaN provides additional specific surface area and reduces the defect density,thus facilitating the generation,separation and transport of the photogenerated carriers.The introduction of AuNPs increases the optical absorption in the UV and visible regions,which can also promote carrier separation.(2)The differences in surface morphology,surface roughness,surface composition,surface wettability,and surface carrier concentration of u-GaN and SI-GaN polar surfaces before and after PEC etching were compared.And by this way,the effect of the doped Fe on the PEC etching characteristics of GaN polar surfaces was investigated.Under the same PEC etching conditions,the surface morphology and surface roughness of the two polar surfaces after etching show significant differences due to the selective etching of local defects formed by Fe doping process.In contrast to u-GaN,Fe doping significantly changes the wettability characteristics of GaN.The carrier concentration on the Ga-surface of Fe-doped GaN increases after etching,while that on both Ga-surface and N-surface of unintentionally doped GaN decreases,which may be attributed to the weakening of the compensation effect caused by the decrease of Fe concentration on the Ga surface during PEC.(3)The elastic modulus and hardness of Si-and Mg-doped GaN were investigated using the nanoindentation method combined with the first-principles calculation,and it was found that the average elastic modulus of Si-doped GaN is 266.50 GPa smaller than 276.53 GPa of Mgdoped GaN,while the average hardness of Si-doped GaN is 18.14 GPa larger than 17.56 GPa of Mg-doped GaN with the analysis of the displacement-load curve plots and the first-principles calculation.