Characterization of the surface layer on a strained Si wafer by electrochemical methods

To assess the surface reactivity of strained Si wafers, open circuit potential (ocp) measurement and scanning surface potential microscopy (SPoM) were carried out. Surface conditions of the wafers were varied by changing the structure of the wafers, i.e., the Ge content of the SiGe underlayer and the thickness of the strained Si layer. The ocp value of the strained Si was negative compared with that of nonstrained Si, and it shifted in the negative direction with an increase in Ge content of the SiGe layer and/or in the thickness of the strained Si layer, indicating that the degree of strain and/or the density of defects at the Si wafer surface affect its reactivity. In addition, the ocp value shifted in the positive direction to reach the value of the nonstrained Si, possibility due to stress relaxation, when the thickness of strained Si exceeds a certain value, which is expected to be the "critical" thickness. The SPoM analysis confirmed that the negative potential shift takes place locally at the surface, whose contrast was associated with the crosshatchlike surface morphology, suggesting that there is nanoscopic difference in strain at the strained surface.