U. Rau

Determination of electric transport properties in the pre- and post-breakdown regime ofp-germanium

Conductivity and Hall-effect measurements were performed on single-crystallinep-doped germanium, electrically driven into low-temperature avalanche breakdown via impurity impact ionization. The electric transport properties were determined as a function of the applied electric field in the pre- and post-breakdown regime. The characteristic field dependence of the carrier density, mobility, and drift velocity was found to be reflected in smooth variations of the integral current flow. The breakdown mechanism was demonstrated to involve a mobility which sensitively depends upon the density of the mobile charge carriers. Our experimental findings are qualitatively explained by simple model approaches developed from established breakdown theories.

Imaging of spatio-temporal structures in semiconductors

Abstract Starting from a characterization of the spatio-temporal behavior of different semiconductor systems based upon the impact ionization of shallow impurities by hot charge carriers, we present experimental results concerning imaging of both stationary and dynamical structures by means of low-temperature scanning electron microscopy.

Critical Dynamics near the Onset of Spontaneous Oscillations in p-Germanium

The electrical avalanche breakdown of p-doped germanium at low temperatures displays self-generated current oscillations. The investigation of the transition from a stable fixed point to a limit cycle yields a scaling behaviour known from the saddle node bifurcation on a limit cycle.

SPATIO-TEMPORAL INSTABILITIES IN THE ELECTRIC BREAKDOWN OF P-GERMANIUM

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 characteristic. 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 beeb 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 instabilities in the boundary region of the current filaments.

Exemplary locking sequence during self-generated quasiperiodicity of extrinsic germanium

Abstract We report on experimental investigations of self-organized quasiperiodic and mode-locked behavior during low-temperature avalanche breakdown of p-doped germanium. Under variation of the applied longitudinal magnetic field, undriven spontaneous current oscillations are demonstrated to undergo an exemplary sequence of distinct locking states via an apparent self-similar emergence of high-order quasiperiodic mixing frequencies.

Observation of a Large-Scale Sheetlike Current Filament in a Thin n -GaAs Layer

Spatial patterns of a large-scale sheetlike current filament formed during low-temperature avalanche breakdown have been investigated in a thin epitaxial n-GaAs layer using the low-temperature scanning electron microscope. The sheetlike current filament was nucleated between planar-type ohmic contacts due to impact ionization of neutral shallow donors. The width of the current filament (200∼700 µm) was measured as a function of the sample voltage in the postbreakdown regime. A turbulent pattern observed in the beam-induced firing wave instability has also been measured.

Nucleation and growth of current filaments in semiconductors

The nucleation and disappearance of current filaments in homogeneously slightly doped semiconductors during avalanche breakdown at low temperatures is quantitatively evaluated in terms of a minimum and maximum value of the filament current. Both values arise from a model based on the power balance in the system. Experimental results obtained by the two‐dimensional imaging of the filament configurations in n‐GaAs and p‐Ge agree quantitatively with the predictions of our power balance model.

Dynamics of current filaments in p-type germanium under the influence of a transverse magnetic field

An explanation for the self‐generated formation of spontaneous current oscillations developing during low‐temperature impact ionization breakdown of slightly doped p‐type germanium is presented for the first time, taking advantage of a model experiment. Upon applying a relatively small transverse magnetic field, the spatially inhomogeneous current distribution manifest in the form of individual high‐conducting‐current filaments undergoes a distinct traveling dynamics that is oriented perpendicular to the direction of the electric and the magnetic field (i.e., not coincident with the direction of the current flow). The resulting magnetic‐field‐induced oscillatory behavior can be described qualitatively by simple model considerations.

Evidence of Type-III Intermittency in the Electric Breakdown of p-Type Germanium

Electric oscillations arising spontaneously in the low-temperature avalanche breakdown of p-germanium are investigated with respect to the intermittent routes to chaos. The experimental data display several universal features of type-III intermittency.

Electron-beam induced instability during filamentary current transport inn-GaAs

We report experimental investigations on the current transport ofn-GaAs under the conditions of impact ionization avalanche breakdown at low temperatures. The spontaneous formation of a single current filament was observed by means of low-temperature scanning electron microscopy. An electron-beam induced instability occurs at the onset of filamentation. We demonstrate this instability to be due to the local disturbance at the boundaries of the current filament. Our results are compared to the similar behavior ofn-GaAs under IR irradiation (firing wave instability).