Theoretical Simulation Study On The Growth Process And Mechanism Of GaN Thin Films On AlN Substrates

Gallium nitride(GaN)is widely used in optoelectronic components.Due to the lack of homogeneous substrate,GaN films are usually prepared by epitaxial growth on heterogeneous substrates.However,due to the high lattice and thermal mismatch between the film and the substrate,the crystallinity of the film is poor.To reduce lattice and thermal mismatch,aluminum nitride(AlN)layers are usually introduced between the substrate and the film.However,the dislocation distribution and stress evolution during GaN film growth on AlN substrate have not been clearly understood,and the microscopic growth mechanism and structure of GaN film have not been deeply understood.In this thesis,molecular dynamics simulation was used to study GaN films grown on AlN substrate,the effects of AlN substrate orientation and growth temperature on the quality of GaN films were investigated,and the surface morphology,roughness,crystal structure,dislocation and stress distribution of gan films at different growth temperatures were analyzed,and the surface morphology,roughness,crystal structure,dislocation and stress distribution of gan films at different growth temperatures were analyzed.The results show that the surface morphology and crystal quality of GaN films grown on(0001)Al-terminated AlN surface are better than those grown on(000)N-terminated AlN surface.The surface morphology and crystallinity of GaN films are improved with increasing growth temperature.Under the same conditions,the dislocation density of GaN film deposited on(0001)Al-terminated AlN surface is lower than that of Gan film deposited on(000)N-terminated AlN surface.It is also found that dislocation usually occurs at the interface of AlN substrate and GaN film,and then expands along the growth direction.Due to the lattice mismatch between the film and the substrate,the maximum fluctuation range of the mean biaxial stress and the mean normal stress in GaN film mainly occurs at the interface.When the stress fluctuation is the lowest,the film with good crystallinity is obtained,and the influence of growth temperature on the mean normal stress is weak.On the other hand,the surface roughness of GaN films deposited on r surface of semi-polar AlN substrate does not change significantly with temperature,but the content of wurtzite structure in the films is relatively high and there are few defects.The GaN film deposited on surface a of non-polar AlN substrate has the lowest surface roughness,the highest wurtzite structure content and the lowest defect concentration.Thin films grown on semi-polar and non-polar AlN substrates eliminate polymorphism.The results show that GaN films grown on non-polar and semi-polar AlN substrates have better crystal quality than those grown on polar AlN substrates.GaN films grown on non-polar AlN substrates have the best crystal quality.In order to further understand the density,stress and dislocation evolution of a specific region during GaN film growth,the Alpole AlN substrate was expanded and GaN film was divided into slices with a thickness of 20(?).The results show that the largest number of dislocations with the Bergdahl vector b=1/3<1-100> are obtained at different temperatures.Type a dislocation can climb during film growth,and the berger vector remains constant.In contrast,(a+c)-type dislocations tend to form a ring of dislocations or react,resulting in a-type dislocations.These dislocations are likely to occur in a spatially random manner,with partial annihilation,and ultimately affect the microstructure of the film.After reaching the peak value near the heterogeneous interface,and decreases with the thickening of the film.In addition,dislocation occurs mainly at grain boundaries,and atomic stress gradients exist in atoms near the dislocation lines,providing driving force for dislocation migration.Finally,the density variation and stress distribution during GaN film deposition and relaxation are studied.The results show that the density of each layer is relatively stable during deposition at different temperatures,but increases during relaxation.In addition,the biaxial stress of each layer oscillates slightly,reaching its maximum near the interface,but decreasing during relaxation.In summary,the surface morphology,roughness,crystal structure,dislocation distribution and internal stress of GaN films at different growth temperatures were obtained.These findings can provide some guidance for the preparation of high quality GaN films on AlN substrates.