Mg Diffusion For Preparation Of Normally-off GaN HEMT Devices And Its Mechanism Research

Owing to the excellent material characteristics such as a large band gap and a strong breakdown field,Ga N as the third-generation semiconductor material has been widely used in power electronics fields.Especially,the unqique Al Ga N/Ga N heterostructre with high saturation electron concentration,high electron mobility,low on-channel resistance and high breakdown field strength,has received enormous attention in Al Ga N/Ga N high electron mobility transistors?HEMTs?for power electronics applications.Unfortunatly,the unmodified Al Ga N/Ga N HEMT device is a normally-on device,which is in a conducting state without applying a gate voltage.Thus improving the process cost and complexity of circuit design and increasing the power consumption of device switching.Therefore,developing the normally-off Ga N-based HEMT devices in practical applications are of great significance.Among widely employed methods to prepare normally-off HEMT devices,exploriting p-Ga N material as the gate cap layer to increase the threshold voltage shows good reliability,which is mainly used in industrial production of normally-off HEMT devices.However,some critical issues such as the high etching accuracy,low material damage and low p-type doping concentration still remain.To solve above problems,a novel method for manufacturing p-type gate normally-off HEMT device was for the first time proposed in this work.Based on the theory of Mg atom diffusion,and the local p-type doping of the Al Ga N barrier layer was formed by the penetration and diffusion of metal Mg into the Al Ga N barrier layer at high temperature,thereby achieving conveniently and efficiently Normally closed HEMT device.The effects of material surface and interface states and temperature conditions on Mg atom diffusion mechanism and device performance were discussed in detail,and related process optimization was carried out.The obtained results are as follows:?1?Based on the diffusion equation,the relationship between the diffusion coefficient and the defects was established.The effects of the surface and interface states of Al Ga N and Mg materials on the Mg diffusion mechanism and device performance were studied and analyzed.The effect of the surface and interface state of Al Ga N and Mg materials on the diffusion of Mg was analyzed through the presence or absence of ICP etching,revealing the mechanism of diffusion and the principle of achieving normally-off characteristics.It was found that ICP etching will increase the roughness of Al Ga N surface and Al Ga N/Mg interface and hence introduce a large number of surface state defects,which is facerable for the improvement in diffusion efficiency and diffusion concentration of Mg atoms,thereby achieving the heavy p-type doped Al Ga N barrier layer and the increased threshold voltage of Ga N HEMT devices.Further,the interface state of the material after different etching time was studied,and its correlation with the device performance was obtained.Meantime,the structure of Mg O passivation layer was designed to effectively suppress the gate leakage,and the optimal process route and the optimal performance of the device are summarized accordingly.After ICP gate etching for 5 s,together with Mg diffusion at 600°C for 1 min,a p-type doping concentration of 10²¹ cm^-3,a normally-off threshold voltage of 1.4 V,and excellent leakage performance of 2?10^-7 m A/mm was reached.?2?The effect of annealing temperature on the Mg diffusion mechanism and performance was analyzed.In this chapter,the effect of annealing temperature on the diffusion behavior of magnesium atoms in Al Ga N lattices and the resulting changes in device performance were discussed in detail.The internal influence mechanism of annealing temperature on Mg diffusion was revealed,and the corresponding internal substitution behavior of Mg atoms in the Al Ga N lattice was obtained.Resultly,by replacing Ga atoms and Al atoms in the Al Ga N material to form a single acceptor to release holes,p-type doping of the material was realized.An increase in annealing temperature led to an enhancement in the reaction efficiency of Ga or Al atoms in the Al Ga N lattice replaced by Mg,and more Ga or Al atoms in the material were replaced by Mg atoms out of the original lattice position,which aggregated onto the surface of the material and resulted in a rough surface and interface of the material.Accordingly,the correlation between the annealing temperature and the hole concentration in the material was established.By controlling the annealing temperature,the hole concentration can be futher changed and thus the threshold voltage was tuned.As a result,on a condition of the low-temperature Mg diffusion at 400°C,the normally-off characteristic of the device with a threshold voltage of 0.1 V was achieced.This work provides a novel approach for the preparation of low-temperature and high-performance normally-off HEMT devices.In summary,this paper the effect of Al Ga N surface and Al Ga N/Mg interface state on the Mg diffusion mechanism and device performance by etching was analyzed,futhermore,by studying the effect of annealing temperature on the behavior of Mg diffusion mechanism and device performance,the optimum process route and positive device performance were obtained.This paper will provide an important guidance for the in-depth insight of the development of normally-off Ga N-based HEMT devices.