Preparation And Characterization Of GaN Epitaxial Layer Fabricated On Multi-layer Graphene And SiO₂

In recent years,gallium nitride（GaN）has been widely used in lighting,laser and HEMT due to its excellent photoelectric properties.However,its application in the field of flexible display has not made progress,which is mainly due to the extremely complex transfer process between GaN and traditional sapphire substrates.Therefore,the fabrication of GaN on transferable substrates such as multilayer graphene or silicon dioxide（SiO₂）has become a breakthrough for the application of GaN in the field of flexible display.The van der Waals force on the graphene makes GaN grown on it easy to separate,and this force can also effectively reduce the poor crystal quality and high dislocation density caused by lattice mismatch and thermal mismatch.The excellent properties of graphene make GaN/graphene heterostructure a hot research topic in recent years.However,the lack of dangling bonds on the surface of two-dimensional graphene makes it difficult for GaN to nucleate on it.Therefore,the nucleation mechanism of GaN on graphene substrates has become the focus of growth of high quality GaN.At the same time,SiO₂substrates can be removed by hydrofluoric acid solution（HF:NH₄F=1:7）,and GaN grown on this substrate can also be transferred.However,due to the structural disorder of SiO₂ substrates,the key to grow high quality GaN epitaxial layer is to solve the problem of ordered GaN nuclei grown on amorphous substrates.This paper aims at solving these two problems by discussing the key process parameters of nucleation layer on these two substrates.The influence mechanism is analyzed,and then the growth of GaN epitaxial layer is regulated.Specific results are as follows:（1）Firstly,in order to increase the nucleation density of GaN on the surface of graphene,oxygen plasma treatment was carried out on the multi-layer graphene,then the nucleation comparison experiment between the treated and untreated graphene was carried out.It was found that untreated graphene was difficult to nucleate due to the lack of hanging bonds on its surface,and only a small number of GaN nucleation islands were found at the folds and steps.After treatment,oxygen functional groups are formed on the surface of graphene,the defects of graphene increased,which promoting the nucleation density of GaN on multi-layer graphene.The nucleation experiments at high temperature（1000℃）and low temperature（550℃）were compared to study the effect of nucleation temperature on nucleation rate.The results show that high temperature decreases the nucleation rate of GaN on graphene surface,and the adhesion coefficient between Ga and graphene also decreases at high temperature,resulting in the failure of nucleation of GaN on multi-layer graphene.Finally,the effect of Al element on nucleation rate was investigated by using AlGaN as nucleation layer.The results show that Al plays an important role in increasing nucleation rate.The experiment indicated that a dense AlGaN nucleation layer was formed on the multi-layer graphene,then the GaN epitaxial layer were successfully grown.The physical properties of this epitaxial layer show that it is hexagonal wurtzite structure,growing along the c axis<0001>and containing a weak tensile stress ranging from 0.244 to 0.282 GPa which is less than the internal stress of the GaN films grown on traditional sapphire substrates.（2）In order to study the effect of annealing time on the morphology of GaN nucleation layer grown on SiO₂,the comparison experiments of annealing for 2min,10 min and 20 min were carried out.The results show that the prolongation of annealing time will intensify the competitive growth between AlGaN nucleation islands,and eventually lead to larger surface fluctuations of GaN epitaxial layer.It is concluded that the GaN epitaxial layer with better uniformity is obtained when the annealing time is 2 min.In addition,the annealing experiments of nucleation layer at 500 sccm and 1000 sccm NH₃ flow were carried out to analyze the effect of annealing NH₃ flow on the quality of GaN epitaxial layer.The results show that annealing at low NH₃ flow rate can accelerate the decomposition of unstable crystal planes in nucleation layer,and re-crystallize to form stable crystal planes,thus improving the crystal quality.It is concluded that the crystal quality of GaN is the best when the annealed NH₃flow rate is 500 sccm.