Contactless mobility measurements using two-photon excitation and a terahertz probe

Time-dependent THz spectroscopy is widely used for measuring mobility in novel electronic materials, in which the mobility is often adversely affected by defects and unintentional dopants [1]. One of its great advantages over other techniques is that it does not require ohmic contacts. Recently, it was shown that the mobility values extracted from optical-pump, THz-probe measurements are consistent with contact-based Hall Van der Pauw measurements [2]. However, because of the sub-micron penetration depth of above band gap light in direct bandgap semiconductors, the carrier densities required to produce a measurable change in THz transmission are high enough that the mobility can be reduced by electron-electron scattering. Below gap two-photon excitation offers an alternative since it is able to excite carriers throughout the entire semiconductor thickness at once, resulting in a significantly lower carrier density for the same amount of THz absorption. We present results for ZnSe and GaP and compare the THz measurements to conventional z-scan and pump absorption on the same samples.

[1] M. C. Beard, G. M. Turner, and C. A. Schmuttenmaer, J. Phys. Chem. B 106, 7146 (2002).
[2] B. G. Alberding, W. R. Thurber, and E. J. Heilweil, J. Opt. Soc. Am. B 34, 1392 (2017).