Magnetic Field Induced Localization in Semiinsulating GaAs

Conductivity experiments were carried out on samples of semiinsulating GaAs at liquid helium temperature in magnetic field up to 6 T. During the measurements the samples were persistently illuminated with infrared light which allowed to populate with electrons a part of shallow donor and conduction band states. Current-voltage characteristics showed an abrupt jump of the current at a threshold electric field which is interpreted as a result of impact ionization of electrons bound on shallow donors and in the tail of the bottom of the conduction band. The jump of the current decreases as the magnetic field increases and disappears for a sufficiently high magnetic field B$\text{}_{0}$. The value of B$\text{}_{0}$ grows with growing light intensity. These results are explained by magnetic-field induced localization of electrons on long-range fluctuations of the electrostatic potential. The localization transition was confirmed by the current dependence on temperature measured at different magnetic fields. A peak on these curves was observed. Its position coincides with the temperature above which impact ionization is not observed. A possible mechanism explaining appearance of the peak is presented.