Brian G. Alberding, W. Robert Thurber, and Edwin J. Heilweil
https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-7-1392
Charge carrier conductivity and mobility for various semiconductor wafers and crystals were measured by ultrafast above bandgap, optically excited time-resolved terahertz spectroscopy (TRTS) and Hall Van der Pauw contact methods to directly compare these approaches and validate the use of the non-contact optical approach for future materials and in situ device analyses. Undoped and doped silicon (Si) wafers with resistances varying over 6 orders of magnitude were selected as model systems because contact Hall measurements are reliably made on this material. Conductivity and mobility obtained at room temperature by terahertz transmission and TRTS methods yield the sum of electron and hole mobility which agree very well with either directly measured or literature values for corresponding atomic and photodoping densities. Careful evaluation of the optically generated TRTS frequency-dependent conductivity also shows it is dominated by induced free carrier absorption rather than small probe pulse phase shifts, which is commonly ascribed to changes in the complex conductivity from sample morphology and evaluation of carrier mobility by applying Drude scattering models. Thus, in this work, the real-valued, frequency-averaged conductivity was used to extract sample mobility without application of models. Examinations of germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP), and zinc telluride (ZnTe) samples were also made to demonstrate the general applicability of the TRTS method, even for materials that do not reliably make good contacts (e.g., GaAs, GaP, ZnTe). For these cases, values for the sum of the electron and hole mobility also compare very favorably to measured or available published data.
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