Study On The Material Growth By Molecular Beam Epitaxy And Device Fabrication Of Ultra-thin Barrier Al(Ga)N/GaN HEMT

The third-generation wide-bandgap Ga N semiconductor material shows the potential for the fabrication of high-frequency and high-power microwave devices and high-efficiency power switching devices owing to its remarkable properties,such as wide bandgap,high electron saturation velocity,high breakdown electric field,and high thermal conductivity.Limited by the manufacture difficulty and high cost of large size,low defect,and semi-insulating Ga N single crystal substrates,Ga N HEMTs are usually heteroepitaxially grown on foreign substrates including sapphire,Si,and Si C.The heteroepitaxial Ga N material features a high density of dislocation defects,which leads to large gate reverse leakage and low breakdown voltage.Simultaneously,the nucleation layer in the heteroepitaxial structure possesses high defect density,which results in poor heat dissipation and high junction temperature,and thus deteriorating the device performance.Homoepitaxial Ga N on free-standing gallium nitride bulk substrate exhibits smooth material surfaces,steep heterojunction interfaces and extremely high crystalline quality,so homoepitaxial Ga N devices enable to achieve higher output power density and higher reliability.As a commonly used growth technique for Ga N material,metal-organic chemical vapor deposition（MOCVD）has some disadvantages like high growth temperature,high unintentional doping concentration,and difficulty in atomic-level thickness controllability,limiting its application in ultrathin barrier Ga N heterostructures with strong polarization.Molecular beam epitaxy（MBE）characterizes low growth temperature,atomic-level thickness control,sharp heterointerface,and low impurity concentration,and usually be exploited to fabricate novel Ga N devices,especially for the ultra-thin barriers Al N/Ga N heterostructures in need of precise thickness controllability.Therefore,this work studies the homoepitaxy of ultra-thin barrier Al（Ga）N/Ga N heterostrcutures with high Al composition and their verification by fabricating HEMT device,including the modulation of material surface morphology and composition,improvement of interface quality and transport characteristics,along with device process and characterization of HEMT.Below are the detailed work and research results:1.Al Ga N/Ga N material with high aluminum composition is homoepitaxially grown by digital-alloy monolayer growth method,together with analyzing the growth kinetics of the composition modulation for the MBE growth of Al Ga N material,and investigating the effect of gallium flux on the composition and morphology of digital-alloy Al Ga N material.By adjusting the number of Al N and Ga N monolayers in each cycle,an Al Ga N material with an Al composition up to 83.19%and a surface root mean square roughness of 0.24 nm corresponding to 2×2μm² scan area is realized.The Al_0.83Ga_0.17N/Ga N heterostructure exhibits an electron mobility of 986 cm²/Vs and carrier concentration of 4.32×10¹³ cm^-2 at room temperature.2.High electron mobility Al N/Ga N heterostructure is obtained by MBE with improved heterointerface quality by adopting indium-assisted surfactant enhanced surface mobility method.During the growth of Al N/Ga N heterostructure,the introduction of indium atoms allows to significantly improve the smooth and uniformity of the heterointerface,and indium atoms are hardly incorporated into the Al N barrier as a surfactant.Single-channel and double-channel Al N/Ga N heterostructures with electron mobilities of 1144 cm²/Vs and 1308cm²/Vs,respectively,are homoepitaxially realized by MBE at room temperature.At 77K,the electron mobilities of single-channel and double-channel Al N/Ga N heterojunctions reach to 4803 cm²/Vs and 6383 cm²/Vs,respectively.3.Ultra-thin barrier Al N/Ga N HEMT devices are demonstrated to verify the transport property of MBE-grown Al N barrier heterostructures.Relying on the conventional Ga N HEMT fabrication process,a single-channel Al N/Ga N HEMT delivers a drain saturated current density of 1 A/mm and a peak transconductance of 264 m S/mm,and a double-channel Al N/Ga N HEMT evidently manifests two peak transconductance of 75.5 m S/mm and 276.4 m S/mm.