Formation of Ohmic Contact With Low Contact Resistance on n-GeSn by Fermi Level Depinning Using Plasma Treatment

Processes for Fermi level (FL) depinning on metal/n-GeSn (the doping concentration of 10(19) cm(-3) with 4.5% Sn content) contact were investigated by plasma treatment on GeSn surface. Among CF4, NH3, and O-2 plasma, O-2 plasma treatment on GeSn surface prior to metal deposition is the most viable avenue to mitigate FL pinning in terms of ohmic-like behavior with improved reverse-biased current by a factor of 14 300 as compared with the untreated samples, a low contact resistance (R-C) of 3 x 10(-7) Omega-cm(2), and the Schottky barrier height (Phi(BN)) of 0.12 eV for Al/n-GeSn, and it is primarily due to the formation of GeSnOx, which possesses the desirable bandgap (E-G), conduction band offset (Delta E-c), and permittivity (kappa) value with the capability to passivate the surface dangling bonds and consequently decreases the number of surface states. R-C and Phi(BN) compare favorably with other schemes for FL depinning for Ge technology. Furthermore, the effectiveness of FL depinning is also evidenced by the S factor as high as 0.58. Besides the promising electrical properties, this plasma-based process can tolerate larger process variation and, therefore, a larger process window due to self-limiting thickness of the GeSnOx. By combining these advantages, surface treatment by O-2 plasma paves the promising path for emerging GeSn technology.