Compensation assessment in ‘‘undoped’’ high-resistivity GaAs

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Abstract
Attainment of semi-insulating status when an ‘‘undoped’’ GaAs crystal is grown from the melt requires a delicate balance among concentrations of ‘‘unintentional’’ donor and acceptor impurities, and defects, notably the EL2 midgap donor. In qualifying and improving material for device uses, defect identification and characterization is important. The compensation balance is analyzed in this paper for various ‘‘undoped’’ crystals, relying largely on Hall data over the 290–430 K range, coupled with optical absorption measurements of carbon and EL2. The temperature-dependent data, converted into Fermi energy and into EL2 ionized fraction, provide a clearer picture than just room-temperature measurements as to whether EL2 controls the Fermi energy (giving the desired semi-insulating behavior), or whether a shallower defect species is in control.