Research On Low Temperature Ohmic Contact And GaN-based HEMT

With the continuous upgrading and development of the semiconductor industry,traditional Si-based power semiconductor devices have entered a performance bottleneck period.The third-generation wide-bandgap semiconductor materials represented by gallium nitride(GaN)have excellent characteristics such as large forbidden band width,high electron saturation velocity,high breakdown field strength,and high thermal conductivity,which make GaN-based materials and HEMT devices have outstanding advantages in high-power and high-frequency applications,and have quickly become a hot spot in international research.In 2021,the"14th Five-Year Plan"lists gallium nitride(GaN)and silicon carbide(Si C)wide-gap semiconductors as development goals.In the next five years,electronic devices based on third-generation semiconductor materials will be widely used in lidar,5G communications,new energy vehicles,data centers and other scenarios.Making high-quality,low-temperature and low-resistivity ohmic contacts is one of the key technologies for GaN-based devices.This article focuses on reducing the annealing temperature of ohmic contacts,and proposes two solutions under the premise of ensuring high-performance ohmic contacts:one is to make non-annealed ohmic contacts on heavily doped semiconductors;the other is to thin the barrier layer in the ohmic region by etching,Adjust the metal structure of the ohmic electrode to realize a new low-temperature ohmic contact.Facing one of the biggest challenges in the future application of GaN HEMT devices-low-temperature ohmic contact,this article starts from the carrier transport mechanism and device physics such as gold semi-contact,and focuses on the key process of GaN power devices and the realization of low-temperature ohmic contacts.Systematic research and technological development have been carried out on the height of the gold half-contact barrier.The specific experiment content is as follows:First,the basic physical characteristics of metal-semiconductor contacts are explored,and the necessary conditions for achieving high-quality ohmic contacts are revealed.Through theoretical formula derivation,the three main transport mechanisms of ohmic contact carriers on semiconductors with different doping concentrations are compared and analyzed.The method for measuring ohmic contact resistance is systematically explained,which provides a theoretical basis and evaluation method for the follow-up research on GaN-based devices and low-temperature ohmic manufacturing processes.Second,the contact performance of high-temperature annealing and non-annealing Ti/Al/Ni/Au ohmic electrodes on GaN and In0.03GaN heavily doped semiconductors was studied,and atomic force microscopy(AFM)and high-resolution transmission microscopy(HR-TEM)were used.The surface morphology of the metal on the heavily doped semiconductor material and the microstructure change of the gold half-contact interface were analyzed by other characterization methods.After comparing and analyzing the results of annealing temperature,we proposed a non-annealing heavily doped semiconductor ohmic contact scheme.At the same time,after conducting TLM variable temperature test and tunneling mechanism fitting analysis on ohmic contacts at different annealing temperatures on heavily doped semiconductors,we found that the field emission mechanism is the main mechanism of current transport in ohmic contacts on heavily doped semiconductors.Finally,we conducted ohmic contact electrical stress reliability experiments on heavily doped samples at different annealing temperatures,and found that too high a temperature would damage the electrical reliability of the ohmic contact performance of the device.Third,the Ta/Al/Ti/TiN and Ta/Ti/Al/Ni/Au new Ta-based ohmic contacts are studied.The relationship between the etching depth of the barrier layer and the performance of the ohmic contact is analyzed,and the structure model is established to reveal that the tunneling mechanism is the main carrier transport mechanism of the Ta-based ohmic contact on the semiconductor after the barrier layer in the ohmic region is etched..The low-temperature ohmic process developed in this paper reduces the annealing temperature from above 850°C of the traditional ohmic process to 650°C,and the low-temperature annealed Ta-based ohmic contact electrode has good contact performance and electrical reliability.Fourth,this article integrates the Ta/Al metal thickness ratio,ohmic barrier layer etch thickness,Ta-based low-temperature ohmic contact and gate electrode technology studied in the full text,using Ta/Ti/Al/Ni/Au low-temperature ohmic technology Al GaN/GaN HEMT devices were fabricated on Al GaN/GaN heterojunction epitaxial materials on sapphire(Al₂O₃)substrates,and electrical characteristics were performed on Al GaN/GaN HEMT devices made by traditional high-temperature annealing Ti/Al/Ni/Au ohmic electrodes In comparison,the influence of the difference in ohmic contact on the electrical characteristics and reliability of the device is analyzed.