A time-resolved millimeter wave conductivity (TR-mmWC) apparatus for charge dynamical properties of semiconductors

Abstract

This article demonstrates a contactless, time-resolved, millimeter wave conductivity apparatus capable of measuring photoconductivity of a diverse range of materials. This cavity-less system determines the time-dependent magnitude of a sample’s charge carrier density-mobility product by monitoring the response of a continuous, millimeter-wave probe beam following excitation of the sample by an ultrafast laser pulse. The probe beam is tunable from 110 GHz to 170 GHz and the sample response data can be obtained over the sub-nanosecond to millisecond time interval. This system has been tested on silicon wafers, S-I GaAs, perovskite thin films, SiO2-Ge(nc), and CdSxSe1−x nanowire samples. We demonstrate a minimum detectable photoconductance change of ∼1 µS, an estimated time resolution for conductance decay of ∼100 ps, and a dynamic range greater than 57 dB. The calibration constant of the system, needed for quantitative calculation of photoconductivity from experimental data, has been determined using silicon wafers. This system has several advantages over currently used microwave and terahertz techniques, such as facile tunability of probe frequency and substantially wider time range for study of decay kinetics, while maintaining an open sample environment that enables characterization of a wide range of sample sizes under controlled environmental conditions.

Semi insulating N-gallium nitride (GaN) on sapphire surface reflection dataset obtained at millimeter wave frequencies 107.35–165 GHz

“…We have used a newly developed quasi-optical, free-space, time-resolved millimeter wave conductivity (TR-mmWC) system [6] operated in the D waveguide-band (107.35 GHz to 165 GHz) with 0.1 GHz resolution to acquire surface reflected probe beam voltages from high resistivity (10 5 Ω-cm) 5 µm silicon doped N-GaN on sapphire substrate. The source for millimeter waves is the backward wave oscillator (BWO) with a spot diameter ∼3 mm, and GaN sample is of commercial grade, and is rotated at an angle of 65.4° from the probe beam direction.…”