Fabrication and selective wet etching of Si0.2Ge0.8/Ge multilayer for Si0.2Ge0.8 channel gate-all-around MOSFETs

The fabrication and selective wet etching of Si0.2Ge0.8/Ge stacked multilayer for Si0.2Ge0.8 channel gate-all-around metal–oxide–semiconductor field-effect transistors (MOSFETs) has been demonstrated in detail in this research. First, a high crystal quality of Si0.2Ge0.8/Ge multilayer is realized using a Ge strain relaxed buffer. Subsequently, selective wet etching of Ge versus Si0.2Ge0.8 is investigated in detail. It has been observed that common acid or alkali solutions cannot etch Ge. However, the use of strong oxidizing solutions are necessitated to achieve the selective etching of Ge to Si0.2Ge0.8, such as H2O2 or HNO3. Moreover, it is noted that the selectivity of Ge to Si0.2Ge0.8 increases as H2O2 or HNO3 concentrations rise. However, the use of 30% H2O2 solution, which is the highest concentration in our lab, still gets an obvious erosion of Si0.2Ge0.8 with a rounded profile. Therefore, a stronger oxidation solution (HNO3) is employed to check the selective etching of Ge versus Si0.8Ge0.2. Consequently, a rectangular shape of Si0.2Ge0.8 is attained as the HNO3 concentration increases to more than 60% due to its stronger oxidizing capacity. Finally, 65% HNO3 solution is chosen as the optimal condition for the Si0.2Ge0.8 nanowire release process due to its high selectivity to Si0.2Ge0.8 and low surface roughness.