The depth below the surface at which the concentration is 1017 atoms/cm³ if the surface concentration is 1018 atoms/cm³ is 0.32 cm.
The expression is given as :
( Cs - Cx ) / ( Cs - Co ) = erf ( x / 2 √D × t )
where, Cs = concentration of surface = 1018 atoms/cm³
Cx = concentration at depth = 1017 atoms/cm³
Co - concentration of solute in solid
erf = error function
( Cs - Cx ) / ( Cs - Co ) = (1018 - 1017) / 1018
= 0.00098
0.00098 = erf ( x / 2 √D × t )
0.000868 = x / 2 √ 2 × 10⁻² × 5 (3600)
x = 0.32 cm
The depth is 0.32 cm
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