Formation of transition-metal-based ohmic contacts to n-Mg2si by plasma activated sintering

Electrode materials consisting of Cu, Ti and Ni were formed on Bi-doped n-type Mg₂Si by means of a monobloc plasma-activated sintering (PAS) technique. Due to the difference in thermal expansion coefficients between Ti and Mg₂Si, rather high residual thermal stresses gave rise to the introduction of cracks, which were mainly located in the Mg₂Si layer, when Ti was used as the electrode material. In the case of the Cu electrodes, monobloc sintering could not be performed in a reproducible manner because Cu melts abruptly and effuses at around 973K, which is 100 K lower than the sintering temperature that is required for Mg₂Si of good crystalline quality. When compared with the results for Cu and Ti, the monobloc PAS process for Ni was both stable and reproducible. The room-temperature I-V characteristics of Ni electrodes were considered to be adequate for practical applications, with durable Mg₂Si-electrode junction properties being realized at a practical operating temperature of 600 K with δT = 500 K. The highest open circuit voltage (V_oc) observed was 41 mV at δT = 500 K (between 873 K and 373 K) for Ni electrodes fabricated using the monobloc PAS process. The voltage (V) and current (I) values with a 10 Ohm load were ̃ 48 mV and ̃ 2 mA at δT = 500 K.