New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

Abstract

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)