K.M. Mayer

Spatially resolved observation of current filament dynamics in semiconductors

Abstract Two-dimensional imaging of the nucleation and the dynamics of current filaments generated in homogeneous p -doped germanium at 4.2 K during impurity impact ionization induced avalanche breakdown has been performed. The images were obtained by scanning the specimen surface with an electron beam and by recording the beam-induced current change in the voltage-biased samples. This new method is expected to identify in particular the filament configurations showing chaotic temporal resistance behavior.

Imaging of self-generated multifilamentary current patterns in GaAs

Two-dimensional imaging of current filament patterns generated in homogeneousn-type GaAs during avalanche breakdown at low temperatures is reported. The self-generated formation and subsequent growth behavior of distinct single- and multifilament configurations could globally be visualized by means of a scanning electron microscope equipped with a liquid-helium stage. From local conductivity measurements in the smallest possible filaments (typical diameter of about 10 μm) carrier mobilities as high as about 4·106 cm2/Vs at 4.2 K were estimated. Such high-mobility filament channels may become interesting for applications in ultrafast electronic circuits.

Resonance imaging of dynamical filamentary current structures in a semiconductor

Abstract The spatially resolved observation of the nonlinear dynamical behavior of spontaneous current oscillations obtained during low-temperature avalanche breakdown of homogeneously p-doped germanium is reported. Stationary current filaments developing in the breakdown regime through impurity impact ionization were observed two-dimensionally by means of a scanning electron microscope equipped with a liquid-helium cryostage. Further, spontaneous current oscillations showing typical transitions to chaos were localized two-dimensionally by means of a novel resonance imaging technique, which provides spatially resolved analysis of the nonlinear dynamical behavior. From these measurements different oscillation frequencies were clearly identified as spatially separated oscillation centers localized along the stationary current filaments. The electron beam was demonstrated to act as an exemplary control parameter, which can be manipulated both spatially and temporally.

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.

Spatial and Temporal Current Instabilities in Germanium

Current filamentation and chaotic temporal resistance behavior generated in homogeneously p-doped germanium during low-temperature avalanche breakdown have been investigated. Spontaneous current oscillations showing typical chaotic scenarios were found under tiny variation of distinct control parameters (d.c. bias voltage, transverse magnetic field, temperature). By means of a scanning electron microscope equipped with a liquidhelium stage, two-dimensional imaging of current filaments has been performed. In particular, the filament nucleation and the subsequent growth processes could be resolved. Furthermore, the origin of the spontaneous oscillations was found to be closely linked to the formation of the current filament pattern.

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.

Classification of current instabilities during low-temperature breakdown in germanium

We present experimental investigations on the spatio-temporal nonlinear current flow in the post-breakdown regime of p-germanium at liquid-helium temperatures. The basic nonlinear effects are characterized in terms of the underlying semiconductor physics, taking into account the influence of different experimental parameters.

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.

Chaos in semiconductors

Abstract Experiments performed with p-doped Germanium at liquid-He temperatures have shown that the electric current flow develops rich temporal and spatial structures. These structures result from the autocatalytic nature of impurity impact-ionization-induced avalanche breakdown generating the moving charge carriers. We have investigated the spatial patterns using the following experimental methods. First, by placing a proper configuration of ohmic contacts on the specimen surface, we have studied correlations between different sample sections. Second, we have two-dimensionally imaged the current flow patterns by means of low-temperature scanning electron microscopy.

Nascent states of current filamentation in semiconductors governed by negative differential resistance

Abstract We report the first two-dimensional imaging of just nucleating filamentary current flow during low-temperature impact ionization breakdown of n-type GaAs. Various nascent stages of transient spatial structures leading to the formation of a stationary minimal-size filament are resolved in the highly nonlinear breakdown regime of current-controlled negative differential resistance.