MBE Epitaxial Growth Of Ultra-thin Barrier Layer AlN/GaN Heterostructure

GaN HEMT devices have made many achievements in high-temperature,high-voltage,high-frequency,and high-power applications,due to the excellent properties of GaN and other III-nitride materials.These properties include large bandgaps,high electron saturation velocity,high critical breakdown electric field,and strong piezoelectric and spontaneous polarization,and existence of two-dimensional electron gas.Researchers have made many efforts on material epitaxy,device structure design,and device preparation process,hoping that the material and device performance will be closer to the theoretical value.Compared with traditional Al GaN/GaN HEMT devices,AlN/GaN heterojunctions is very challenging because of large lattice mismatch and chemical-property differences between AlN and GaN.The thesis focuses on epitaxial growth of AlN/GaN heterojunctions,preparation of AlN/GaN HEMT devices,and performance analysis.The main research contents and results are as follows:Influence of different III/V ratio on the quality and surface morphology was studied of GaN.It was found that the GaN materials grown at the boundary of the medium gallium-rich mode and the Ga droplet mode had a flat surface with a small roughness,as low as 0.5 nm,which helps to improve AlN/GaN interface quality and transport properties of the AlN/GaN heterojunctions.This growth model has laid a solid foundation for the growth of high-quality AlN/GaN heterojunction materials.The critical thickness of AlN on the GaN material was calculated.AlN/GaN heterojunctions with different thicknesses of AlN were grown to modulate the strain energy of the barrier layer.The thickness of 3 nm was determined as the optimal thickness.With the optimal thickness,the AlN barrier quality was improved by the invention of Ga assisted growth method.The 2DEG concentration,mobility and square resistance reached 1.99×10¹³ cm^-2,1019 cm²/V·s,and 308?/?,respectively.We have prepared two types of AlN/GaN HEMT devices based on our MBE grown AlN/GaN heterojunctions.It was found that the PECVD passivation process had a severe impact on the heterojunctions,which doubled the square resistance.The dry etching process of the gate trenches damaged the barrier layer under the gate,causing depletion of the 2DEG beneath the gate.Therefore,the threshold voltages became positive,and the saturation output current droped sharply.The AlN/GaN HEMT devices without the surface and dry etching process of gate trenches have a saturated output current I_dsmax was close to 1000 m A/mm,and a peak transconductance G_m higher than 300m S/mm.The current collapse tests were implemented on the AlN/GaN heterojunction devices without the passivation process.It was found that when the bias was(-8V,20V),current collapse increased to 12.21%,which was less than the influence of the self-heating effect on the output current.Therefore,we have proved that the AlN/GaN heterojunctions grown by MBE method had a small current collapse and can effectively improve the device performance.To summarize,the thesis systematically studied the superior property of PAMBE growth of AlN/GaN heterojunctions,preparation and electrical analysis of AlN/GaN HEMTs.High-quality AlN/GaN heterojunctions were obtained using PAMBE technology.AlN/GaN HEMTs with excellent DC output characteristics were demonstrated.