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We report on extremely sensitive measurements of changes in the microwaveproperties of high purity non-intentionally-doped single-crystal semiconductorsamples of gallium phosphide, gallium arsenide and 4H-silicon carbide whenilluminated with light of different wavelengths at cryogenic temperatures.Whispering gallery modes were excited in the semiconductors whilst they werecooled on the coldfinger of a single-stage cryocooler and their frequencies andQ-factors measured under light and dark conditions. With these materials, thewhispering gallery mode technique is able to resolve changes of a few parts permillion in the permittivity and the microwave losses as compared with thosemeasured in darkness. A phenomenological model is proposed to explain theobserved changes, which result not from direct valence to conduction bandtransitions but from detrapping and retrapping of carriers from impurity/defectsites with ionization energies that lay in the semiconductor band gap.Detrapping and retrapping relaxation times have been evaluated from comparisonwith measured data.

journal of applied physics

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