Low-temperature microwave response of barely metallic arsenic-doped silicon

Microwave measurements (frequency shifts and Q changes) of Si:As samples in the vicinity of the metal-insulator transition have been made with two different helical resonators and a cylindrical cavity in the frequency range 124 MHz to 13.9 GHz at temperatures to 1.5 K on samples from 0.72N(c) to 2.45N(c) [N-c=8.6x10(18)/cc]. The resonators were calibrated with a constantan [Cu0.55Ni0.45] sample exhibiting close to Drude behavior. The measurements are in the regime h omega < kT <(E-F-E-c)< h omega(p) and are far into the Hagen-Rubens regime (omega tau < 1), but where the skin depth delta is less than the sample thickness t for metallic samples. Any significant "perfect conductor" contribution to Delta omega/omega is ruled out by calculations and the experimental results. The ratio R=(Delta omega/omega)/Delta(1/Q) directly yields the transverse dielectric response epsilon(t)(N,omega,T). The epsilon(t)(T similar to 1.5 K) values are consistent with a large Drude contribution -(omega(p)tau)(2) plus a positive interband contribution primarily from 1s-A(1)-1s-T-2 impurity band transitions. The Drude component is consistent with the scaling behavior of sigma(dc)(N,T -> 0).