| Electronic devices that are operable under extreme temperatures have great potentials in aerospace,down-hole,power generation,nuclear energy and other industries.However,the temperature limit of the current silicon electronics is only 200oC,far below the desired operating temperature of modern devices.Owing to its attractive material properties,such as the wide bandgap,great thermal conductivity,high electric field strength,silicon carbide?SiC?outperforms silicon in many ways and can easily reach the physical limits of silicon devices.SiC has always been recognized as a good candidate for the high temperature,high frequency,high power anti-radiation semiconductor devices and other harsh environment applications to replace Si.As a matter of fact,SiC material alone possess a high temperature working capacity of 1000oC,a major challenge is to develop thermal stabile ohmic contacts for SiC device operating at 500oC or higher temperature.Applications such as SiC-based sensors or probe for extreme harsh environments,it is essential to achieve excellent thermal,electrical,and mechanical stability of ohmic contacts in order to ensure these devices be durable and operate reliably in air or corrosive atmosphere.Compared to the efforts focused on the low-resistivity ohmic contacts,investigation of the thermal stability and electrical characteristics at elevated operating temperatures under harsh environment is lacking.Especially,the high temperature?>450oC?thermal stability study of ohmic contacts simultaneous on n-and p-type 4H-SiC operated in air have not been reported.This dissertation mainly discuss the thermal stability and electrical characteristics study of 4H-SiC ohmic contacts at 500oC.1.High-concentration doping is achieved by ion implantation of the epitaxial layer,the standard Pt/Ti/SiC ohmic contacts were simultaneous formed on n-and p-type 4H-SiC.The results show that Pt/Ti/SiC ohmic contacts exhibits good thermal stability after being aged at 500oC for 300 h in N2.However,the electrical characteristics of the Pt/Ti/SiC samples degrade dramatically when the aging experiment conducted at 500oC in air.The n-and p-type Pt/Ti/SiC samples failed after 100 h and after 200 h,respectively.Auger electron spectroscopy analysis?AES?indicates that almost all Ti atoms near the SiC have been oxidized and cause the ultimate ohmic failure of the Pt/Ti/SiC ohmic contacts.2.In order to avoid the Ti oxidation induced thermal stability problems,the Pt/TaSi2/Ni/Ti/Ni/SiC ohmic contact was presented,which can realize simultaneous ohmic contact to n-and p-type SiC.The specific contact resistance?SCR?of 3.7×10-4?·cm2 and 2.9×10-3?·cm2 have been obtained for the n-and p-type Pt/TaSi2/Ni/Ti/Ni/SiC samples,respectively.Even after 300 h aging at 500oC in air,the contact still remains an ohmic behavior,with the SCR value of 6.5×10-4?·cm2 and 8.3×10-3?·cm2 for the aged n-and p-type Pt/TaSi2/Ni/Ti/Ni/SiC samples.The Pt/TaSi2/Ni/Ti/Ni/SiC samples exhibits a good thermal stability,with a relatively small increasing of the SCR value in the initial 50 h and then remaining stable in the rest of the thermal aging duration.In addition,I-V curves measured at 500oC also remain ohmic behavior.Scanning electron microscopy?SEM?analysis reveals that there is a lot of defects with numerous distinct nano-sized pits on the contact surface.Auger electron spectroscopy?AES?analysis reveals that the oxidation of these areas with pits affects the metal/4H-SiC interface quality,which explains the SCR value of Pt/TaSi2/Ni/Ti/Ni/SiC samples increases with the initial 50 h thermal aging duration in air.In the rest of the aging time,at the expense of the Ta Si2 layer being oxidized to slow down the diffusion rate of oxygen atoms towards the SiC,thereby contributing to the metal/4H-SiC interface quality was not affected by oxidation problems and extending the life of ohmic contact survived in air at 500oC.3.Based on the degradation mechanism of Pt/TaSi2/Ni/Ti/Ni/SiC ohmic contact,an optimized structure of Pt/TaSi2/W/Ni/SiC ohmic contacts was presented,which can realize thermal stable ohmic contact simultaneous to n-and p-type SiC.The SCR of 4.0×10-4?·cm2 and 1.3×10-3?·cm2 have been obtained for the n-and p-type Pt/TaSi2/W/Ni/SiC samples,respectively.Also,SCR value of 1.2×10-4?·cm2 and 6.5×10-5?·cm2 are achieved for aged n-and p-type Pt/TaSi2/W/Ni/SiC samples when electrically probed at 500oC,respectively.SEM analysis reveals that the surface morphologies of Pt/TaSi2/W/Ni/SiC samples keep smooth and stable during the aging process.AES analysis reveals the binary compound Pt2Si formed in the RTA process could inhibit uncontrolled migration of oxygen to the SiC interface,which could contribute to the metal/4H-SiC interface quality was not affected by oxidation problems during the aging process,ensuring the long-term reliability of ohmic contacts extremely stable operating at high temperature in air.The Pt/TaSi2/W/Ni/SiC ohmic contacts presented in this paper realize ohmic contact simultaneous to n-and p-type SiC,which shows excellent thermal stability operating at500oC in air for 200 h.The surface morphologies of Pt/TaSi2/W/Ni/SiC samples keep smooth and stable during the aging process.Also,SCR value of 1.2×10-4?·cm2 and6.5×10-5?·cm2 are achieved for aged n-and p-type Pt/TaSi2/W/Ni/SiC samples when electrically probed at 500oC,respectively.This ohmic metal scheme possess the advantages of excellent thermal stability,good high temperature electrical characteristics and smooth surface morphologies.All of the results indicate that the Pt/TaSi2/W/Ni/SiC simultaneous ohmic contacts to both n-and p-type SiC are promising to be used for high temperature and harsh environment SiC-based devices.These results are of great significance and practical value for take full advantage of high temperature characteristics of SiC devices. |
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