Source/Drain Engineering for Parasitic Resistance Reduction for Germanium p-MOSFETs

A reduction of parasitic resistance is presented with incorporation of preamorphization implantation (PAI) and self-aligned Cu₃Ge in the source/drain region for germanium p-MOSFETs. Full activation of boron in the amorphous layer can be obtained during solid-phase epitaxial growth, and a concentration as high as 4 x 10²⁰/cm³ is achieved. This nonthermal equilibrium concentration is maintained during the subsequent Cu₃Ge formation. Cu₃Ge is adopted as a contact metal in germanium p-MOSFETs for the first time, due to its superior electrical properties (6.8 muOmegaldrcm for resistivity and ~1 x 10^-7 Omega cm² on p-type germanium for specific contact resistance). The fabricated p+/n diode yields a five order of magnitude between forward and reverse currents, which can be attributed to the reduction in parasitic resistance. The low reverse current mitigates concerns of possible deep-level traps introduced by copper. It also confirms the nonexistence of extended defects created by PAI as a result of the unique role of vacancies in germanium. With high dopant concentrations achieved by PAI and low resistance of Cu³Ge, excellent MOSFET characteristics are demonstrated in self-aligned Cu³Ge p-MOSFETs. A 15% mobility enhancement over Si universal mobility and a 60% parasitic resistance reduction are achieved.