Study Of MOCVD Growth Of AlN And AlGaN Films Based On PVD-AlN Buffer

AlGaN material is a kind of direct wide bandgap semiconductor materials whose band gap energy can be continuously adjusted from 6.2 eV to 3.4 eV.Besides,ultraviolet photoelectric devices based on AlGaN materials have the advantages of continuous adjustable wavelength,all-solid-state,small size,high temperature resistance,radiation resistance,low energy consumption,long-lifetime,energy saving and environmental protection.Therefore,they have wide application prospects and huge potential in civil,industrial,agricultural and even military fields,and is heavily demanded.Deposited by magnetron sputtering of physical vapor deposition?PVD?technology,PVD-AlN buffer has the unique advantages of good uniformity,convenience of epitaxy growth,lower cost,lower lattice mismatch,which has been widely used for epitaxial growth of GaN materials.On the other hand,compared with the dependence of AlN nucleation layer grown by conventional MOCVD on substrate materials,the dependence of PVD-AlN buffer on substrate materials is very weak.If the epitaxy growth of high-quality AlN and AlGaN materials on PVD-AlN buffer realizes,it can be theoretically extended to any high temperature resistant substrate materials.However,there are few systematic studies on the MOCVD regrowth of AlN and AlGaN materials on PVD-AlN buffer.Therefore,the regrowth of AlN and AlGaN films was investigated by metal organic chemical deposition?MOCVD?based on PVD-AlN buffer.The main contents of this paper are summarized as follows:Firstly,PVD-AlN buffers with different thickness were prepared by magnetron sputtering deposition.Their single crystal characteristics and uniform c-axis orientation of the surface AlN islands were verified.The crystalline quality and surface morphology of the buffer layers with different thickness were studied.With the increase of sputtering deposition thickness,the overall dislocation densities of the AlN buffer layer decreases,and the size of the surface AlN islands increases gradually,but the orientation to c-axis deteriorates.Secondly,the effect of the thickness of PVD-AlN buffer on the MOCVD regrowth of AlN films was studied.With the increase of buffer layer thickness,the overall dislocation density of AlN films decreased.On the other hand,due to the size and c-axis orientation of buffer layer AlN islands,more internal stress were accumulated during the MOCVD regrowth of AlN,resulting in more surface cracks.Besides,the effect of the MOCVD regrowth thickness of AlN on the material quality was studied.It's concluded that cracks will occur on the surface of AlN films when it exceeds a certain critical thickness?about800 nm?.Moever,the effect of cut-off direction and angle of sapphire substrates on the quality of AlN films is studied.With the increase of cut-off angle,the overall dislocation density of AlN films decreases.The transformation from two-dimensional growth mode to step flow growth mode and the difference of in-plane stress state during the AlN regrowth process were verified by means of AFM and XRD characterizations.A reasonable explanation is given based on the corresponding theoretical model.Thirdly,based on the PVD-AlN buffer method,AlN gradient interlayer technology was introduced into MOCVD regrowth of AlN films.The size and orientation of AlN islands on the buffer layer were effectively controlled.1.5?m thick AlN films with high quality,crack-free and atomic-level surface were successfully prepared.The regulation mechanism of AlN islands and the growth mechanism of AlN on the gradient interlayer were described in detail.Finally,The effect of the thickness of PVD-AlN buffer layer on the direct MOCVD regrowth of n-type Al_0.6Ga_0.4N material was studied,and the optimal thickness was obtained.On this basis,in order to further improve the material quality,the composition-graded Al_xGa_1-xN interlayer technology was introduced into MOCVD regrowth of AlGaN based on the PVD-AlN buffer method.The n-type Al_0.6Ga_0.4N film with high quality was prepared by optimizing the thickness of the composition-graded Al_xGa_1-xN interlayer.The effect of interlayer thickness on the crystallization quality,surface morphology,in-plane stress state and electrical properties of AlGaN film were studied.And finally,we proposed a theoretical growth model,which reasonably explained the experimental phenomena.