gold is diffused into a silicon wafer using a constant-source diffusion with a surface concentration of 1018 cm-3 . how long does it take the gold to diffuse completely through a silicon wafer 400 µm thick with a background concentration of 1016 m-3 at a temperature of 1000 °c? (diffusion coefficient of gold is 4*10-7 cm2 /s at 1000°c)