Impact of surface roughness on tracer depth profiling and its implications for Cd109 and Zn65 diffusion experiments in solar-grade Cu(In,Ga)Se2 layers

It is shown by numerical simulation that surface roughness affects the measurement of diffusion profiles by means of serial sectioning techniques. This problem arises in radiotracer diffusion studies of Cd and Zn in thin-film solar-grade Cu(In,Ga)Se2, which exhibits an appreciable surface roughness due to the special manufacturing process of the polycrystalline layer structure. We find that in unfavorable cases the experimentally determined diffusivity can be significantly higher than the true diffusion coefficient D. This discrepancy appears to increase with the ratio of the surface roughness Rrms to the average penetration depth 2Dt attained after a diffusion time t. It can be concluded, however, that the employed ion-beam sputtering technique, which involves rotation of the Cu(In,Ga)Se2 diffusion sample, usually leads to experimental errors of ∼10% or less. The results of this study may be also relevant to other depth profiling techniques such as secondary ion mass spectrometry.