Research On AlGaN/GaN Heterojunction Based On AlN/sapphire Substrate

The third-generation nitride wide-bandgap semiconductor represented by GaN has high electron saturation speed and strong anti-radiation ability,which has broad application prospects in the fields of electronics and optoelectronic devices.Since both GaN and AlN in the nitride system have a strong polarization effect,the conventional AlGaN/GaN heterojunction does not require doping to generate a two-dimensional electron gas with extremely high concentration and high mobility.We can use this feature to fabricate high electron mobility transistors,which can greatly improve the operating frequency and stability of the device.However,although GaN-based high electron mobility transistors have emerged in high-frequency and high-power application field,they can also realize the normal operation of electronic systems.However,compared with Si-based electronic devices,the growth of nitride heterojunctions is still at an immature stage.High dislocation density,poor surface state,and stress on materials due to heteroepitaxial growth are all problems that need to be solved urgently in the optimization of nitride heterojunction growth.The fundamental solution to these problems lies in optimizing the growth conditions of the heterojunction and finding the optimal growth conditions.At the same time,we also need to understand the physical mechanism of the improvement of the quality of the heterojunction materials by these optimal growth conditions,which can be better for the optimization of nitride heterojunction growth.This paper mainly studies the growth of AlGaN/GaN heterojunctions on thick film AlN sapphire substrates and magnetron sputtering AlN sapphire substrates.The optimal growth conditions are found by changing the substrate type and growth conditions,and through a series of the characterization method of material quality reveals the physical mechanism of substrate type and growth conditions to improve the quality of heterojunction materials.The main research results of this article are as follows:1.By changing the thickness of the magnetron sputtered AlN,the high temperature AlN nucleation layer and the nitridation time,several growth conditions are systematically studied for the effect of these conditions on the AlGaN/GaN heterojunction grown on the magnetron sputtered AlN sapphire substrate.influences.Finally,a high-quality AlGaN/GaN heterojunction with a 2DEG mobility and areal density of 2162 cm²/Vs and9.227×10¹²cm^-2was achieved.Through High resolution X-ray diffraction,Photoluminescence,Raman,Atomic force microscope and Hall tests,we tested samples under different experimental conditions and got several key conclusions on growth optimization:(1)There is an optimal nitriding time for growing heterojunctions on sapphire substrates with magnetron sputtering AlN,which is about 5 minutes,and this nitriding time does not change with the thickness of magnetron sputtering AlN.(2)Regardless of the thickness of the magnetron sputtered AlN on the substrate,the high-temperature AlN nucleation layer has a vital impact on the quality and electrical properties of the heterojunction material.(3)Under the same growth conditions,magnetron sputtering an AlN sapphire substrate with a thickness of about 20 nm can obtain the best quality heterojunction.2.Growing thick film AlN materials on sapphire substrates with 1°and 4°bevel cut sapphire substrates and magnetron sputtering AlN thickness of 10nm,40nm and 75nm,and the material quality of the thick film AlN materials was characterized.The quality of the thick-film AlN material grown on the bevel substrate is lower than that of the thick-film AlN material grown on the magnetron sputtering AlN substrate,and the greater the bevel angle,the worse the material quality.The surface of the thick film AlN grown on the magnetron sputtering AlN substrate has small pits whose number changes with the thickness of the magnetron sputtering.These small pits are formed by the outcropping of dislocations on the surface.AlGaN/GaN heterojunctions were grown on five thick AlN substrates,and then several conclusions were obtained through a series of characterization tests:(1)The quality and electrical properties of the heterojunction material grown on the oblique cut thick film AlN substrate are not as good as the heterojunction obtained by growing thick AlN film after magnetron sputtering AlN,and the larger the oblique angle,the higher the material quality and electrical properties.The deterioration of the characteristics is more obvious.(2)After magnetron sputtering AlN,the substrate on which thick film AlN is grown has an optimal thickness(about 40nm)so that the quality and electrical properties of the heterojunction material grown are the best.Among them,the small pits formed by threading dislocations in the thick film AlN play a key role in stress relief and the improvement of the material quality and electrical properties of the heterojunction.