Contacts on Silicon Carbide by Use of Nickel and Tantalum ---Preparation and Characterisation
Doctoral thesis, 2007
Silicon carbide (SiC) is one of the attractive semiconductors due to its good electrical, thermal and mechanical properties. It is a promising material for high temperature, high power and high frequency application. Reliable electrodes are always necessary to utilise SiC for electronic devices. Nickel (Ni) and tantalum (Ta) can be used for both ohmic and Schottky contact on n-type SiC. Since metallisation represents one of the most important steps in the fabrication of electronic devices, the knowledge of the interaction between Ni, Ta and SiC, as well as electrical behaviour, is of primary importance for understanding and optimising the device performance.
After annealing Ni/SiC samples at 800 and 950oC in vacuum for 20 min, Ni reacts with SiC to form textured Ni2Si and C. Its formation may consist of two stages: initial reaction rate and subsequent diffusion controlled stage. For ultra thin initial Ni layer (3 6 nm), islands formation of Ni2Si is observed after heat treatment. Increasing the Ni film thickness prevents this phenomenon. The C released owing to the reaction forms a thin graphite layer on the top of the surface and also tends to form cluster inside the reaction layer. The overall degree of graphitisation increases with increasing temperature from 800oC to 950oC.
This work also deals with the impact of pre-treatment prior the metal deposition on the distribution of reaction product in Ni/SiC system. Argon ion etching before the Ni deposition helps the formation of multi-layer structure during annealing. For the samples without pre-treatment or with chemical cleaning procedure, there is more C agglomerated at the surface and no multi-layer structure. The possible reason is given. The silicides formed at the interface are dependent on the Ni layer thickness and substrate surface condition.
This thesis reports a systematic in-situ XPS study on Ni silicides focusing on both the core level and Auger peaks. The peak position, shape, as well as satellite are compared in different silicides. The effect of argon ion sputtering on surface composition and chemical states has also been investigated. It has been found that low energy argon ions reduce the atomic mixing of the silicide, but increase preferential sputtering. After correcting the influence of the electron attenuation length, the measured depth profile is successfully reconstructed.
In Ta/SiC system, no evident interface reaction occurs at 650°C. However, with the inco-operation of Ni, the formation temperature of Ta carbide and silicide is lowered due to the mediating effect of Ni. With the annealing temperature increasing to 950C, the dominant carbide changes from Ta2C in Ta/SiC to TaC in Ni/Ta/SiC. A layer structure is developed in both Ni/SiC and Ni/Ta/SiC system. A number of C vacancies are preferentially produced in the near interface region of the underlying SiC. Electrical measurements show that ohmic contact is formed after annealing at or above 800C in Ta/SiC and Ni/Ta/SiC. The alteration of the SiC subsurface by the reaction is important to the formation of the ohmic contact. High enough temperature is important to provide for sufficient interface change to obtain ohmic behaviour.
Keywords: Silicon Carbide