Study On The Nucleation Process And Selective Growth Of Non-Polar A-plane GaN Films

Non-polar gallium nitride(GaN)materials have shown great potential in optoelectronics and electronic devices because they can completely avoid the influence of polarized electric fields.Due to the great difficulty in the preparation of large-sized GaN substrates,the main research work has been focused on epitaxial GaN films on other substrates such as sapphire and silicon carbide.However,heteroepitaxial nonpolar GaN generally suffers from poor surface morphology,high defect density,and anisotropy of crystal quality,which severely limit the application of non-polar GaN materials.In view of the above problems,this paper proposes a new lateral epitaxial growth method,namely directly growing a-plane GaN on patterned r-plane sapphire substrate by metal organic chemical vapor deposition(MOCVD)technology.Besides,the influence of the growth conditions of aluminum nitride(AlN)nucleation layer on a-plane GaN has been systematically studied,and the mechanism behind these effects has been explored.The specific research content and experimental results are as follows:1.Aiming at the shortcomings of the traditional lateral epitaxial growth process,the method of direct epitaxy of non-polar a-plane GaN on titanium patterned r-plane sapphire substrate is proposed.First,hole patterned titanium mask is fabricated on the r-plane sapphire substrate,and then GaN is selectively epitaxially grown in the hole to finally obtain a flat a-plane GaN film.Compared with the sample grown on flat sapphire substrate,GaN grown on the titanium patterned substrate not only has a large improvement in surface morphologyand crystal quality,but also its anisotropy is significantly reduced.Through scanning electron microscope(SEM),asymmetric Xray diffraction reciprocal space maps(RSMs)and Raman spectra,it is found that the lateral dimension of the mosaic along the m-axis after using the titanium patterned substrate is significantly increased,thereby reducing the anisotropy of surface morphology and crystal quality of the a-plane GaN.The advantage of this method is that the coalescence thickness is thin,the cost is low,and it is easier to implement.So the use of titanium patterned substrate epitaxy is an effective and practical method for obtaining high quality a-plane GaN.2.Compared with GaN,AlN has higher stacking fault formation energy and lower adatom mobility.Therefore,we use AlN as a nucleation layer to grow a-plane GaN on r-plane sapphire substrates.The crystalline and surface qualities of a-plane GaN are found to closely depend on the growth conditions of AlN nucleation layer.As the growth temperature of AlN decreases,the AlN grains become larger and sparser,delaying the merging of GaN islands and increasing the lateral growth distance,thus effectively reducing the defect density in the GaN film.The growth time of the low temperature AlN layer is further optimized,and a-plane GaN films with reduced anisotropy in the crystalline quality,surface morphology and in-plane strains are achieved.It is found that the lateral growth lengths along different directions of GaN can be modulated by the growth time of AlN nucleation layer,thereby changing the anisotropy of the GaN film.This study shows that the AlN nucleation layer with suitable growth conditions can significantly improve the surface morphology and crystal quality of a-plane GaN.