Low-temperature Impurity Breakdown Research Articles

Magnetic-field-induced lateral displacements of current filaments in n-GaAs

The formation of current filaments in n-type GaAs due to the low temperature impurity breakdown has been investigated in crossed electric and magnetic fields with a laser scanning microscope. In a highly compensated, low mobility, epitaxial layer the lateral displacement and stretching of a filament due to the Lorentz force has been observed at currents slightly larger than those required to form a filament between point contacts. The transverse elongation of the filament increases linearly with rising magnetic field strength until the filament is disrupted. In this case periodic current oscillations are generated by a repetitive filament ignition and extinction. The elongation decreases with rising current and finally vanishes even at high magnetic fields. This stiffening of a current filament is attributed to an increased electron temperature and mobility in the filament, which cause the charges of the Hall field to be accumulated at the filament borders rather than at the edges of the sample.

Complex dynamics of current filaments in the low-temperature impurity breakdown regime of semiconductors.

The formation of spatial and spatiotemporal patterns (current filaments) in semiconductors in the regime of low-temperature impurity impact ionization is investigated theoretically. Our model yields stable filaments, breathing filaments, and traveling filaments which show an intermittent spatiotemporal instability, resulting in chaotic oscillations of the average carrier density and of the current. The crucial role of the dielectric relaxation of the transverse electric field is singled out. Different strategies for the numerical treatment are discussed.

Globally coupled dynamics of breathing current filaments in semiconductors

Starting from a system of nonlinear transport equations describing the dynamics of a single breathing current filament, e.g. in the regime of low-temperature impurity breakdown in semiconductors, we introduce a simple nonlocal coupling mechanism for the description of two breathing filaments. It is based on the interaction of the two filaments via the external load circuit. We find that the model describes a selection mechanism allowing the survival of the originally larger filament only. Therefore, it can be considered a generalization of Ostwald ripening in thermodynamic equilibrium. Although the breathing instability is suppressed asymptotically, transient symmetric breathing of the filaments is possible due to a periodic repeller.

NONLINEAR AND CHAOTIC OSCILLATIONS IN SEMICONDUCTORS UNDER THE INFLUENCE OF A TRANSVERSE MAGNETIC FIELD: THE DYNAMIC HALL EFFECT

The dynamic Hall effect is presented as a novel model for transverse magnetic field induced oscillatory instabilities in semiconductors. Comprehensive account is given of the theoretical derivation of nonlinear dynamic magnetotransport properties of hot electrons, a linear stability analysis, and nonlinear simulations for a variety of materials with positive or negative differential conductivity in the regime of low-temperature impurity breakdown. Periodic, chaotic, and intermittent voltage oscillations and period-doubling scenarios as a function of current or magnetic field are found under dc bias. The physical mechanism rests upon the dielectric relaxation of both drift and Hall field, coupled with impurity impact ionization.

Bit-number cumulants as a correlation measure of self-generated oscillations in a semiconductor model for low-temperature impurity breakdown

A semiconductor model for self-generated, coupled oscillations in the regime of lowtemperature impurity breakdown is used to show that the bit-number cumulants of the invariant density of a dynamic system represent a measure for internal correlations between certain subsystems of this system. The bit-number cumulants appear to be an appropriate measure for linear as well as for nonlinear correlations.

Mode-locking of self-generated oscillations in a semiconductor model for low-temperature impurity breakdown

The nonlinear dynamic behaviour of a model for self-generated, spatially coupled current oscillations in two separated parts of a semiconductor is analysed. The model involves impurity impact ionization, nonlinear energy relaxation of hot carriers, and energy exchange between the two subsystems. Quasiperiodicity and mode-locking are obtained, and characterized by a suitably defined rotation number and a spectral bifurcation diagram. The mode-locking structure is found to obey the Farey tree ordering, and can be understood on the basis of the circle map theory, assuming a particular path in the two-dimensional phase diagram of the circle map.

Nonlinear energy relaxation oscillations and chaotic dynamics of hot carriers

It is shown that recently observed complex chaotic behavior associated with low temperature impurity breakdown can be understood by introducing the mean energy per carrier E as an additional dynamic variable besides the carrier densities. A set of coupled nonlinear evolution equations for the carrier density and mean energy is derived by a moment expansion of the Boltzmann equation including, in particular, impact ionization. This process leads to a novel, strongly nonlinear relaxation term in the dynamic energy balance equation. The simplest, spatially homogeneous version of the model yields hysteretic or non-hysteretic static current-voltage characteristics, and spontaneous self-sustained current oscillations. It can also be used to explain the coupling of the oscillations in two spatially separated hot carrier subsystems via energy exchange.

Change in the magnetoresistance of n-GaAs in an electric field

The effect of an electric field on the magnitude of the magnetoresistance in epitaxial n-GaAs films with free carrier concentrations ne=4.0.1015−1.12. 1016 cm−3 at T=4.2‡K was investigated. It was found that in weak electric and magnetic fields for E < Eb (Eb is the intensity of the low-temperature impurity breakdown field) the magnetoresistance (MR) is negative, and for E≥Eb only a positive magnetoresistance is observed. The experimental results are explained by a change in the concentration of centers with a magnetic moment and of electrons in the impurity band in prebreakdown electric fields.

Large Strain Dependence of the Impurity Breakdown in Compensated P-Type Germanium

To investigate the negative resistance mechanism of low temperature impurity breakdown in compensated P-type germanium, the variation of the critical field, E c , and of the sustaining field, E s , under strong uniaxial compressive stresses was studied. The ratio E c / E s in compensated samples was between 3 and 5 when no strain was produced. However, the ratio decreased with the stress down to values between 1.5 and 2 when a large strain was produced. The changes of E c and of E s as a function of the acceptor ionization energy, ε 1 , were found to be E c ∝( ε 1 ) 3/4 and E s ∝( ε 1 ) 5/4 , respectively, at large strain region. It was suggested that the energy loss mechanism by emission of the optical phonon may be responsible for the negative resistance of Cryosar phenomena.