Study On Growth Of GaN-based Graded Back Barrier Double Heterojunction

Due to the excellent performance of GaN,GaN-based Al GaN/GaN heterojunction high electron mobility transistors?HEMTs?play an important role in high temperature and high power applications.However,traditional single AlGaN/Ga N heterojunction materials still have problems such as poor carrier confinement,excessive decrease in mobility at high temperature and increased current leakage.With the continuous development of GaN-based materials,GaN-based double heterojunction materials have been widely concerned,which can avoid carrier confinement decreasing in the high gate voltage and high temperature.However,the introduction of the back barrier introduces the parasitic channels in the double heterojunction material,degrading the performance of the device.In order to eliminate the adverse effects of the parasitic channel,the double heterojunction materials with graded back barriers were proposed,but the back barrier of the graded Al component decreases the epitaxial quality and thus degrade the electrical performance.Therefore,it is very meaningful to carry out the research on the optimization of double heterojunction with graded back barrier.At present,many researchers have tried to optimize the double heterojunction materials by changing the graded back barrier structure,but there is no complete comparative analysis of the effects of various gradient layer structures on the double heterojunction materials.In this paper,we focus on the study of the double heterojunction of the graded back barrier,and systematically study the effect of different gradient layer structures on the properties of the double heterojunction materials.The work is as follows:The material properties of the single heterojunction,double heterojunction and graded back barrier double heterogeneous materials are studied.The main facrors influencing the material properties and especially the influence of the fabrication limitation parameters such as channel layer thickness and back barrier Al composition are studied.The influence of the graded back barrier Al composition on the properties of the double heterojunction is studied.The results show that high Al component has certain improvement on the carrier confinement of the material,but the crystal quality and electrical properties of the material are significantly decreased.The effects of non-uniform and uniform graded back barrier structures on the properties of double heterojunction materials were studied.The results show that the uniform graded back barrier double heterojunction materials have higher crystallinity than nonuniform graded back barrier double heterojunction materials slightly worse,but the electrical properties have improved.The effect of monolayer thickness of uniform graded back barrier structures on the properties of the double heterojunction is studied.The results show that the thickness of the monolayer,either it is too thick or too thin could adversely affect the crystal quality and electrical properties of the material.AlN was sputtered on sapphire by the inexpensive magnetron sputtering,and then followed by the MOCVD secondary growth of graded back barrier structures double heterojunction.The various pretreatment processes were used to eliminate the adverse effects of secondary epitaxy.The results show that the crystal quality and electrical properties of the double heterojunctions grown by magnetron sputtering AlN are slightly better than those of the MOCVD nucleation.In conclusion,the growth of the different gradient layer structures of back barrier double heterojunction is analyzed and the structure is opimized to reach the optimal material property.In this thesis,the mobility of the optimized barrier double heterojunction material reaches 1695 cm²/?V·s?and the surface density reaches 1.086×10¹³cm^-2.The mobility of the graded back barrier double heterojunction material grown on magnetron sputtering AlN reached 1736 cm²/?V·s?and the surface density was 9.2×10¹² cm^-2.