Real-time imaging of semiconductor space-charge regions using high-spatial resolution evanescent microwave microscope

Abstract

A high-resolution evanescent microwave probe (EMP) was used to detect and image depletion regions in solar cell p-n junctions in real time. The EMP uses a microwave resonator operating around 10 GHz that is coupled to a thin wire probe. Unable to travel beyond the waveguide discontinuity, the microwave fields set up evanescent fields in the tip’s vicinity. When coupled to an object nearby, these evanescent fields are modified and change the resonant characteristics of the resonator. The microwave conductivity of the nearby object affects the extent of the modification of the probe’s output which is monitored as the probe is scanned over the object. Using these EMP scans, steady-state and transient expansions/contractions of the p-n junction’s depletion region under dc and pulsed reverse/forward biases are mapped. These experimental data along with the conductivity calibration of the EMP were then used to quantitatively calculate doping concentrations, diffusion lengths, and carrier recombination lifetimes in the junction. Junctions are one of the most crucial building blocks of semiconductor devices and these studies clearly show the ability of the EMP in quantitative and nondestructive evaluations of electronic devices and circuits.