We report on frequency-dependent contrast in d(S11)/dV measurements of a variably doped p-type silicon sample in the frequency range from 2 GHz to 18 GHz. The measurements were conducted with a scanning microwave microscope. The measurements were done at selected frequencies while varying the DC tip voltage. The measured d(S11)/dV signal shows a maximum for doping concentrations (NA) of 1015 cm−3−1016 cm−3 at 2.3 GHz. As the microscope operating frequency is increased, this maximum sequentially “switches” through the regions of increasing dopant concentration, displaying a maximum for NA of 1017 cm−3−1018 cm−3 at 17.9 GHz. The frequency dependent “switching” is attributed to the physics of tip-to-sample interaction, particularly as related to the frequency-dependent local surface resistance and the depletion capacitance that control the RC time constant of tip-to-sample interaction. This provides a unique platform for local, frequency-selective, spatially resolved microwave spectroscopy of semiconducting materials.

Amorphous gallium/titanium (Ga,Ti) co-doped indium oxide (In2O3) (GTiIO) films present significant potential as transparent conducting electrodes for use in flexible electronic devices. Amorphous GTiIO films were grown on silicon and glass substrates using linear-facing target sputtering and various oxygen flow rates. Transmittance was as high as 80% in the visible range for O2 flow rates of 0.3, 0.7, and 1.0 SCCM. The lowest resistivity was obtained at f(O2) = 0.3 SCCM with 0.47 mΩ cm. Dielectric functions were determined, and optical gap energies were estimated as close to 3.4 eV using a Tauc formula. We show that Drude tail amplitude is proportional to the carrier density. An electronic transition was found at 2.7 eV from the optical spectra of the thin film at f(O2) = 0 SCCM; this transition was designated to be an inter-conduction band transition from the first to the second conduction band. Using Hall parameters and Drude tail amplitudes in dielectric functions, the effective mass of GTiIO films was determined as 0.398 m0 at f(O2) = 0.3 SCCM.