Design And Simulation Of Epitaxial Structure Of AlGaN Buffer Layer Based On Local P-type Doping

In recent years,GaN materials have been rapidly promoted in the fields of communications,electric vehicles,photovoltaic inverters and other fields.At present,in the field of silicon-based GaN epitaxial growth,the introduction of AlGaN buffer layer can alleviate lattice mismatch and thus alleviate defects such as dislocation,thereby increasing the breakdown voltage of epitaxial structures,but the difficulty of epitaxial growth increases with the increase of buffer layer thickness and number of layers.Moreover,when the AlGaN layer acts as a back barrier to increase the breakdown voltage,it comes at the expense of reducing the current density.The above problems restrict the role of the AlGaN layer in the epitaxial structure,which in turn hinders the further promotion of GaN-based HEMT.Therefore,there is still a lot of room for research on the epitaxial structure of silicon-based GaN based on the AlGaN layer.Based on the role played by the AlGaN layer in the epitaxial structure,a HEMT epitaxial structure containing a gradient AlGaN buffer layer is designed with a total thickness of 3μm,of which the AlGaN buffer layer thickness is 1.3μm,which can provide model guidance for subsequent growth.Further,when the AlGaN layer acts as a back barrier layer,the introduction of a region of different concentrations of local p-doped region structure can increase the breakdown voltage of the device with little impact on the output current density of the device.Using the Silvaco Atlas simulator,the effect of the concentration of p-doping and the geometry of the doped region on breakdown voltage were obtained.Compared with the Al_0.18Ga_0.82N back barrier structure,local p-doped in the back barrier layer of 7×10¹⁶ cm^-3 concentration can reduce the peak electric field inside the device by 3.06×10⁵V/cm,increase the breakdown voltage by 10.4%,and reduce the current density by only 8.4%.The optimal distance between the doped region and the channel corresponding to the three different concentrations(3×10¹⁶cm^-3,5×10¹⁶ cm^-3,7×10¹⁶cm^-3)are 50/100/150 nm,respectively.Then,using metal-organic-chemical-vapor-deposition（MOCVD）on a six-inch silicon substrate to achieve the above 3μm GaN-based epitaxial structure containing a gradient AlGaN buffer layer,when the number of AlGaN layers increased from single layer to 4 layers,the corresponding dislocation density decreased from 2.88×10¹⁰ cm^-2 to 1.77×10¹⁰ cm^-2,and then the total thickness of the epitaxial structure was increased to 5μm,and the dislocation density decreased with the increase of thickness.During epitaxial growth,the Ga-preflowing method between the Al N nucleation layer and the AlGaN buffer layer was used to reduce the cracks on the surface,and the optimal preflowing time was 20 s.Finally,the vertical breakdown voltages of structures of different thicknesses（3/4/5μm）were compared.The results show that as the thickness of the AlGaN buffer layer increases,the vertical breakdown voltage of the epitaxial structure also continues to rise,and when the total thickness is 4μm and 5μm,the breakdown voltage(@10^-5 A)increases by 3.4 times and 5.5 times compared with the 3μm thick structure.