Impurity Breakdown Research Articles

Symmetry-breaking multiple current filamentation in n-GaAs

We report on theoretical investigations of the nasence of current filaments in thin film n-GaAs samples with two concentric circular contacts (Corbino disks). For a constant applied voltage in the regime of low-temperature impurity breakdown, a radially symmetric impact ionization front breaks up into streamers forming pre-filaments, among which a winner-takes-all dynamics sets in due to global coupling. On sweeping the external bias we find the spontaneous appearance of multiple stable filaments and hysteresis. In a perpendicular magnetic field the filaments are bent due to the Lorentz force.

Nonlinear spatio-temporal dynamics in semiconductors

We review recent theoretical advances in the modeling and computer simulation of complex non-linear spatio- temporal dynamics of charge carriers in semiconductors. Among the particularly instructive examples are investigations of current filaments in doped semiconductors in the regime of low temperature impurity breakdown. The nascence of current filaments in thin GaAs films with different contact geometries, including circular samples (Corbino disks) with symmetry breaking instabilities, is investigated, and the role of global couplings induced by the operating load circuit is analysed.

Direct experimental observation of the Hall angle in the low-temperature breakdown regime ofn−GaAs

Using the technique of photoluminescence imaging during electrical transport in crossed electric and magnetic fields, the rotation of the current flow direction has been observed. The rotation angle is identically the Hall angle. As a model system, a thin n-type GaAs layer in the regime of low-temperature impurity breakdown has been used, generating filamentary current patterns. The measurement of the Hall angle allows one to determine the mobility inside current filaments.

Visualization of lateral movement of current-filaments in n-GaAs

In the regime of low-temperature impurity breakdown the influence of a magnetic field on current filaments yields complex voltage oscillations. With a new optical technique the spatio-temporal dynamics of filaments involving nucleation, lateral movements and extinction have been visualized corresponding to a certain type of electrical oscillation.

Undamped self-oscillations and dynamical chaos under conditions of impurity breakdown of a semiconductor in the fixed-current regime

It is shown theoretically that chaotic oscillations of the Hall field, the Hall constant, and the magnetoresistance appear in a compensated semiconductor under impurity breakdown conditions. It is shown that the transition to chaos occurs via the Feigenbaum scenario.

Modeling Nonlinear and Chaotic Dynamicsin Semiconductor Device Structures

We review the modeling and simulation of electrical transport instabilities in semiconductors with a special emphasis on recent progress in the application to semiconductor microstructures. The following models are treated in detail: (i) The dynamics of current filaments in the regime of low‐temperature impurity breakdown is studied. In particular we perform 2D simulations of the nascence of a filament upon application of a bias voltage. (ii) Vertical electrical transport in layered semiconductor structures like the heterostructure hot electron diode is considered. Periodic as well as chaotic spatio‐temporal spiking of the current is obtained. In particular we find long transients of spatio‐temporal chaos preceding regular spiking.

The problem of intermediate temperatures or electric fields in the scattering of hot electrons by acoustic phonons

An approximate expression is derived for the momentum relaxation time in the quasielastic scattering of hot electrons by acoustic phonons as a function of the electron energy and the lattice temperature. The mobility and the dependence of the impurity breakdown electric field on the degree of compensation are calculated in the electron-temperature approximation. The results of the calculations are in good agreement with the experimental for n-type Ge.

Undamped self-oscillations in a compensated semiconductor under conditions of impurity breakdown in the presence of a magnetic field

Conditions for the generation of undamped self-oscillations in a compensated semiconductor in the presence of a magnetic field are studied.

Low-temperature impurity breakdown in semiconductors: an approach towards efficient device simulation : R.E. Kunz, E. Scholl, R. Nurnberg and H. Gajewski. Solid-State Electronics, 1996, 39(8), 1155

Low-temperature impurity breakdown in semiconductors: an approach towards efficient device simulation : R.E. Kunz, E. Scholl, R. Nurnberg and H. Gajewski. Solid-State Electronics, 1996, 39(8), 1155

Influence of external circuits on filamentarycurrent flow during impurity breakdown in n -typeGaAs

Two-dimensional simulations are performed of low-temperature impurity breakdown in an n-type GaAs film connected to a capacitive and resistive external circuit under current controlled conditions. The results confirm that the transition between nearly-insulating and highly-conductive states, where a current filament is formed or decays in the sample, is synchronised with the charging and discharging of a parallel capacitor. Our simulations also indicate a mechanism for self-sustained relaxation oscillations.

Formation of current filaments in n-type GaAs under crossed electric and magnetic fields

We present two-dimensional simulations of current filaments in the regime of low-temperature impurity breakdown in thin n-GaAs films under the influence of a magnetic field perpendicular to the sample plane. Our simulations show that current filaments are bent by the Lorentz force and their width and curvature increase with magnetic-field strength. Significant asymmetry of the electric fields is observed at opposite filament boundaries. These results agree well with experiments. It is also revealed by simulations that impurity breakdown is accelerated by the presence of an external magnetic field. This is attributed to a finite Hall field inside the filaments.

Traveling carrier-density waves in n-type GaAs at low-temperature impurity breakdown.

We present a model for traveling charge-density wave instabilities in a semiconductor revealing S-shaped negative differential conductivity. It is based on microscopic generation-recombination rates including impurity impact ionization, which are obtained from Monte Carlo simulations. By a linear stability analysis and by numerical solution of the full nonlinear system we show that traveling-wave instabilities occur in the regime of both negative and positive differential conductivity. Our simulations give detailed insight into the spatiotemporal dynamics of the instabilities, and explain self-generated small-amplitude oscillations observed at the onset of breakdown. \textcopyright{} 1996 The American Physical Society.

Spatial structure of impact-ionization-induced current filaments in n-GaAs films

We present theoretical and experimental investigations of current filaments in thin n-GaAs films in the regime of low-temperature impurity breakdown. Simulations combining a Monte Carlo approach for the scattering and generation - recombination processes with a drift-diffusion model on a two-dimensional spatial domain yield detailed information about the distribution of the electron densities, electron temperature, current density and electric field. The theory is confirmed by spatially resolved measurements using a novel technique of quenched photoluminescence.

Dynamics of nascent current filaments in low-temperature impurity breakdown.

We present two-dimensional simulations showing the spatiotemporal dynamics of the formation of current filaments in n-type GaAs films in the regime of impurity impact ionization breakdown. From the spatial distribution of carrier densities, electron temperature, current density, and electric field for nascent and for fully developed filaments, we find a three-stage scenario for breakdown: (i) front creation and propagation from the cathode, (ii) stagnation in the phase of a rudimentary filament, (iii) filament growth. Our model combines semiclassical rate equations with microscopic transport parameters, which are obtained from Monte Carlo simulations. \textcopyright{} 1996 The American Physical Society.

Visualization of current filaments in n-GaAs by photoluminescence quenching

Hot electrons in semiconductors are known to quench radiative impurity and exciton recombination photoluminescence. This effect has been applied in a low invasive technique to determine the spatial form of current filaments generated by impurity breakdown in high purity n-GaAs epitaxial layers at low temperatures. Observations on samples with Corbino disc contacts clearly demonstrate symmetry breaking and self-organization by the current filamentation.

Shallow donor impact ionization in n-InP and n-GaAs: influence of doping and compensation

The donor impact-ionization coefficient is calculated using the Boltzmann transport equation. It is shown that impact-ionizing collisions of free electrons with shallow donors induce strong cooling of the hot-electron distribution function. From the obtained results it is found that in the analysis of impurity breakdown or related processes the approach based on phenomenological equations is valid only for small changes of electron concentration. Corresponding graphs for the impact-ionization coefficient are presented for n-InP and n-GaAs.

The hot electron distribution function under impurity breakdown conditions

AbstractThe Boltzmann transport equation is solved by sparse‐matrix method under conditions that are typical for experiments of neutral impurity breakdown at low lattice temperatures. A strong cooling effect due to impact‐ionizing collisions is found on the hot carrier distribution function in moderately doped semiconductors. Implications of the cooling effect on the impact ionization coefficient are discussed. The results of simulation are presented for parameter values that are typical for n‐type GaAs doped with shallow donors and compensating acceptors, and compared with experimental data.

Self-oscillations at photoinduced impurity breakdown in GaAs.

Self-oscillations at photoinduced impurity breakdown in GaAs.

Monte Carlo simulation of impact-ionization-induced breakdown and current filamentation in δ-doped GaAs

We present Monte Carlo simulations of impact-ionization-induced impurity breakdown in n-GaAs at 4.2 K with dopants arranged in parallel monoatomic planes (\ensuremath{\delta} doping). ssS-shaped current-field characteristics are obtained by a direct single-particle Monte Carlo simulation for homogeneously doped material. For \ensuremath{\delta}-doped GaAs we calculate the spatial distribution of the carrier density, mean electron energy, and space-charge field self-consistently as a function of time by a weighted ensemble Monte Carlo method. For initial values on the low-conductivity branch of the ssS-shaped characteristics we find that current filaments centered around the \ensuremath{\delta}-doped layers are formed. Our simulations explain the experimentally observed shrinking of the bistable regime as well as its shift to lower voltages as compared to homogeneous material.

Dynamics of stochastically induced and spatially inhomogeneous impurity breakdown in semiconductors

Impact ionization of trapped carriers in high electric fields at Helium temperatures can lead to threshold switching, i.e., switching of a semiconductor sample from a nearly insulating state to a high conducting steady state due to an applied voltage that exceeds the threshold value for breakdown. This phenomenon is investigated theoretically based on a set of partial differential equations containing the relevant carrier transport mechanisms. In a first analysis the impact of fluctuations on the breakdown process within the framework of a master equation description of spatially homogeneous generation-recombination kinetics is computed. Then, taking into account spatially inhomogeneous longitudinal modes, we find that threshold switching constitutes a three-stage process, consisting of a stage of front creation and propagation, a stage dominated by a seesawlike mode, and one in which impact ionization occurs nearly homogeneously throughout the sample.