Dielectric spectroscopy of semi-insulating gallium arsenide

Dielectric spectroscopy of semiconductors (DSS) employs dielectric measuring techniques to study delayed electronic transitions in and out of localised energy levels in the forbidden gap. 'Horizontal' transitions between levels normally take place in the volume of the material and involve relatively small changes of energy. 'Vertical' transitions between deep levels and the free bands involve energy changes of the order of half the band gap and take place mainly in interfacial space-charge regions and at semiconductor - metal interfaces. DSS is uniquely able to resolve the spectra of these delayed transitions and measurements on semi-insulating ( SI) GaAs in the frequency range 10(-2)-10(4) Hz and between 90 and 380 K show that none of them conform to the expected exponential time dependence. The various transitions are found to have well recognisable spectral 'signatures' involving fractional power-law dependences on frequency and hence on time. The exponents are sensitive to the nature of the transitions and to the types of impurities, and may also depend on the crystalline order. We report for the first time in the context of single-crystal SI GaAs the phenomena of low-frequency dispersion both in the volume and in interfacial transitions, and also of negative capacitance, all of which involve some structural instabilities and the likelihood of electrochemical energy storage. We also show the existence of 'electronic phase transitions` involving subtle reversible changes of electron-lattice interactions at well defined temperatures. We associate the power laws with many-body interactions affecting the transitions in localised levels, which are not well understood probably on account hitherto of lack of experimental evidence. This shows the usefulness of DSS as a unique technique for studies of semiconductor transitions.