Study Of Low Temperature Thermal Annealing Gold-free Ohmic Contact Process

Gallium nitride high electron mobility transistors have excellent characteristics such as high breakdown voltage,high electron saturation drift speed,and low on-resistance,and have broad application prospects in the field of semiconductor power devices.Choosing a Si substrate for epitaxial growth to manufacture GaN HEMT devices is also compatible with CMOS processes,which greatly reduces production costs and improves the competitiveness of the market.At present,a large number of experimental studies have been carried out on GaN HEMT devices at home and abroad.Combined with the current research status of ohmic contact technology,this thesis proposes a low-temperature thermal annealing gold-free ohmic contact technology.The preparation of Si-based GaN HEMT devices based on this process can increase the breakdown voltage of the device and improve the reliability of the device.The main research contents of this thesis are as follows:(1)The effects of different annealing processes and Ti/Al ratio on the ohmic contact characteristics of the device were studied.By optimizing the annealing process,a relatively good ohmic contact characteristic can be obtained under the condition of 550°C and 120 s.A proper amount of Ti can promote the reaction of Al and N to form a low work function Al N compound,which plays a catalytic role and is conducive to the formation of ohmic contacts.Excessive Ti will affect the solid-phase reaction between Ti N and Al,forming a discontinuous Al N layer,leading to higher ohmic contact resistance.(2)The influence of the etching depth of the ohmic front groove on the ohmic contact characteristics of the device is studied.Analyze the current-voltage curve changes of the samples after annealing under different annealing processes.The experimental results show that the ohmic front groove etching treatment can improve the ohmic contact characteristics of the device.When the etching depth is 15 nm,better ohmic contact characteristics can be formed.(3)The difference between the gold-free ohmic contact process and the gold ohmic contact process to prepare ohmic contact electrodes is studied,and the electrical characteristics and the surface morphology of the materials are analyzed and compared.The ohmic contact resistance(R_C=0.87?.mm)and specific contact resistivity(?_C=2.55×10^-5?.cm^-2)of the gold-free device are slightly higher than the ohmic contact resistance(R_C=0.69?.mm)and specific contact resistivity(?_C=1.69×10^-5?.cm^-2)of the gold sample.However,the surface of the gold-free ohmic contact electrode after low-temperature thermal annealing is smoother,and the alloy reaction degree of each layer of metal is also high,which can form good ohmic contact characteristics.(4)The fabrication of Si-based GaN HEMT devices based on the gold-free ohmic contact process and the gold ohmic contact process were carried out respectively.The research results show that the device prepared by the gold-free ohmic contact process has a source-drain saturation current density of 587 m A/mm,which is reduced by 4.7%compared with the gold-based ohmic contact process.The threshold voltage changes insignificantly.But the breakdown voltage of the device is 532 V,an increase of 41.5%.It shows that the low-temperature thermal annealing process of gold-free ohmic contact can improve the leakage of the device and increase the breakdown voltage of the device.The research results of this thesis can provide important support for the process development of high-performance Si-based GaN HEMT devices.