Molecular Beam Epitaxial Growth Of AlGaN/GaN Heteroj Unctions And Characterization

The third-generation semiconductor materials represented by GaN are far superior to the first-generation and second-generation semiconductors in the field of high-frequency and high-voltage.Moreover,due to the spontaneous polarization and piezoelectric polarization effects,the third-generation semiconductor materials will form high-concentration two-dimensional electron gas at the AlGaN/GaN heterojunction interface,which have great application prospects in high electron mobility transistor HEMT devices.Common heterojunction epitaxy methods include MOCVD,MBE and HVPE.Among them,MBE owns higher epitaxial precision and lower temperature than other two epitaxial means,and is suitable for growing ultra-thin epitaxial layers.In the paper,the main research results of MBE epitaxial sapphire-based AlGaN/GaN heterojunction are as follows:1.A sapphire-based AlGaN/GaN heterojunction structure includes a GaN buffer layer,an AIN insertion layer,an AlGaN barrier layer and a GaN cap layer.The Ga source beam is kept unchanged when the barrier layer is extended,the composition of the barrier layer is changed by controlling the beam of the Al source.It is found that the heteroj unction exhibits a two-dimensional electron gas effect at room temperature when the composition is 0.247.When the system is at room temperature(300 K),the highest mobility is 1020 cm2/Vs.The thickness of the intercalation layer is changed by controlling the growth time of the intercalation layer when the layer is epitaxially inserted.When the thickness of the intercalation layer is 1.19 nm,the two-dimensional electron gas transport property is the best.The mobility is up to 3836 cm2/Vs when it is at 5K.2.After optimizing the barrier layer composition and the thickness of the intercalation layer,the sample characterization results are as follows:(1)The exact composition of the heterojunction barrier layer is 0.242.(2)The surface roughness of the material is 0.441nm,and the interface root mean square roughness is 0.335nm.(3)The thickness of the GaN buffer layer is 610nm,the thickness of the AlN insertion layer is 1.19nm,the thickness of the AlGaN barrier layer is 20.2 nm and the thickness of the GaN cap layer is 6.13 nm.(4)The average dislocation density of the heterojunction material is 2.2×109/cm2,the average edge dislocation density is 3.7×108/cm2 and the average screw dislocation density is 1.8×109/cm2.The screw shape in the bulk material Dislocations play a major role.The dislocation density at the interface of the heterojunction is 3.3×109/cm2,the screw dislocation density is 1.1×109/cm2 and the edge dislocation density is 2.2×109/cm2.The edge dislocation at the interface plays a major role.(5)The two-dimensional electron gas mobility from low temperature(5K)to high temperature(865 K)decreased from 3836 cm2/Vs to 73.8 cm2/Vs,and the carrier concentration increased from 1.2×1013/cm2 to 9.2×1013/cm2.3.The influence of the six scattering mechanisms on the heterojunction mobility is analyzed by the low-temperature field Fang-Howard variational wave function model,When the thickness of the intercalated layer was less than 2.5 nm,the experimental data matched the fitting results in the range from low temperature(5K)to high temperature(865K).4.The HEMT device is prepared by standard process.The DC saturation performance of silicon-based HEMT devices was found to be about 0.05A.The corresponding knee voltage is about 13V,and the switch ratio is about 5000.The threshold voltage is about-3V,the maximum transconductance is about 13mS and the off-state leakage is about 1.28mA.The sapphire-based HEMT device only shows good control performance due to leakage.Therefore,it is proposed to improve the leakage method of the sapphire-based HEMT device by epitaxially depositing a high-resistance GaN buffer layer on the substrate and optimizing the mesa isolation process.