Space charge effects on dopant diffusion coefficient measurements in semiconductors

Systematic errors are likely to affect the results of indirect methods used for measuring dopant diffusion in semiconductors, which, for this purpose should be considered as mixed electronic-ionic conductors. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider here three methods in detail, viz. measurement of p-n junction motion, of current or potential decay, and of the time dependence of capacitance (transient ion drift). We show that space charge effects can lead to overestimating diffusion coefficients by a few orders of magnitude. We use the results of our analyses to review and compare the experimental data obtained by different direct and indirect methods, for Cu diffusion in CuInSe₂, an issue of considerable current interest for solar cells.