Detrapping and retrapping of free carriers in nominally pure single crystal GaP, GaAs and 4H-SiC semiconductors under light illumination at cryogenic temperatures

TL;DR

This study investigates how light affects microwave properties of high-purity GaP, GaAs, and 4H-SiC semiconductors at cryogenic temperatures, revealing carrier trapping dynamics through sensitive whispering gallery mode measurements.

Contribution

It introduces a phenomenological model explaining microwave property changes due to carrier detrapping and retrapping, not direct band transitions, in high-purity semiconductors at low temperatures.

Findings

Detected permittivity and loss changes of a few ppm under illumination.

Estimated detrapping and retrapping relaxation times.

Demonstrated sensitivity of whispering gallery modes to impurity-related carrier dynamics.

Abstract

We report on extremely sensitive measurements of changes in the microwave properties of high purity non-intentionally-doped single-crystal semiconductor samples of gallium phosphide, gallium arsenide and 4H-silicon carbide when illuminated with light of different wavelengths at cryogenic temperatures. Whispering gallery modes were excited in the semiconductors whilst they were cooled on the coldfinger of a single-stage cryocooler and their frequencies and Q-factors measured under light and dark conditions. With these materials, the whispering gallery mode technique is able to resolve changes of a few parts per million in the permittivity and the microwave losses as compared with those measured in darkness. A phenomenological model is proposed to explain the observed changes, which result not from direct valence to conduction band transitions but from detrapping and retrapping of carriers from impurity/defect sites with ionization energies that lay in the semiconductor band gap. Detrapping and retrapping relaxation times have been evaluated from comparison with measured data.