Abstract
Investigating the impact-ionization-induced avalanche breakdown in homogeneously doped p-germanium samples cooled to liquid-helium temperatures, we observed the spontaneous formation of current oscillations and current filaments in a highly nonlinear regime of the current-voltage characteris-tic. The spontaneous current oscillations exhibit typical nonlinear dynamics as different routes to chaos under small variation of a control parameter. Most strikingly, self-organized emergence of quasiperiodic and mode-locked states can be ascribed to the simultaneous presence of two and more competing fundamental oscillatory modes intrinsic to our semiconductor system. Due to the coupling of the corresponding localized oscillation centers, typical nonlinear phenomena known from the circle-map formalism can be observed. In addition, we report on the spatially resolved observation of current filament patterns developing during avalanche breakdown. Two-dimensional imaging of the nucleation and the dynamics of the current filaments has been obtained by means of low-temperature scanning electron microscopy. These self-generated spatial structures are closely linked to the nonlinear shape of the current-voltage characteristic. Finally, combining spatially and time-resolved measurements enables the localization of the temporal current instabi-lities in the boundary region of the current filaments.
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