Nonlinear percolation conductivity and negative differential resistivity in microcrystalline PbTe layers with an adjustable potential well

The effect of electric fields on the electrical conductivity of PbTe films with block sizes smaller than the Debye screening length is studied. As the temperature is varied, a readjustment of the potential well is observed due to thermal spread of barriers with height ϕkT and the expansion of higher barriers. Spatial ensembles, which consist of several blocks that increase rapidly with temperature, are established for each T. This process leads to an increase in the height of the potential barriers as the linear size of these ensembles increases. This determines the potential well in these films and their nonlinear properties, which originate in the nonlinear percolation conductivity of a microscopic crystalline system with intergranular barriers. A comparison with the experimental data of Shklovskii shows that the scale length of the spatial inhomogeneity a=3.7×10⁻⁶ cm at T=4.2 K corresponds to the average block size. The value of a increases with temperature, reaching 5×10⁻⁴ cm at T=240 K. This mechanism for electrical conductivity is compared with the hopping conductivity with a variable hopping length. The negative differential resistance in the structures examined here is found to be electrothermal in nature. Zh. éksp. Teor. Fiz. 116, 276–298 (July 1999)     

On the differential conductivity of semiconductor superlattices

The nature of negative differential conductivity (NDC) of a semiconductor superlattice was studied. It is shown that the presence of regions with a negative effective mass in a Brillouin miniband is not necessary for NDC to set in. NDC exists even in superlattices with parabolic and superquadratic miniband dispersion relations, where the electron effective mass is positive everywhere and, in this case, is fully determined by Bragg reflections of the electron. When the electron Bragg reflections are suppressed by optical phonons, NDC can disappear completely. NDC is retained only if there is a sizable region with a negative effective mass in the miniband.     

Theory of space-charge waves in semiconductors with negative differential conductivity

Phenomena that accompany optical excitation of eigenmodes of electron gas oscillation in semiconductors with an N-shaped current-voltage (I-U) characteristic in a strong electric field are investigated theoretically. The dependence of the current flowing through a sample on the oscillation frequency of the interference pattern of light illuminating the sample is analyzed. Nonsteady-state and nonuniform illumination produces an internal electric field, which interacts resonantly with the eigenmodes when the oscillation frequency of the interference pattern coincides with an eigenfrequency of electronic gas oscillation. As the maximum of the I-U curve is approached, the interaction becomes nonlinear in character.     

Modifying electron-phonon scattering rates in semiconductor quantum wells with thin AlAs layers

We have studied theoretically the electron-phonon scattering rates in GaAs/AlAs quantum wells which have additional thin AlAs layers in them using the dielectric continuum approach for the phonons. The confined and interface phonon modes and the intersubband electron phonon scattering rates of these structures have been calculated. The system with an additional AlAs layer is found to have intersubband electron scattering rates which are increased modestly as compared to those for the corresponding quantum well. These results show that scattering rates in general are expected to depend only weakly on the effects of system structure on the optical phonon spectra.     

Pattern formation in S-shaped negative differential conductivity material

To study the electrical current density and the potential distribution inS-shaped negative differential conductivity materials, a theoretical model is developed which is based upon the introduction of an interface dividing the material into two parts of different electrical behaviour. This interface model which leads to a set of coupled nonlinear evolution equations for two physical quantities at the boundary layer is that of an activator inhibitor type as used to describe self-controlled processes in chemistry and biology. Numerical calculations of inhomogeneous stationary states are presented for various parameters. From the numerical calculations these states are found to be stable with respect to small variations of the parameters (structure-stability) and fluctuations of the dependent variables. The model is confirmed by experiments performed on silicon pin diodes. The measured potential distribution is in a good qualitative agreement with the theoretical calculations.     

Observation of negative differential conductivity in a FET with structured gate

Using electron beam lithography, we have fabricated a novel quantum device in which a lateral surface superlattice (LSSL) replaces the gate of a high electron mobility transistor (HEMT). We have observed strong negative differential conductivity (NDC) which we believe could be due to the onset of Bloch oscillations. Other devices, identical in all ways except that the gates are solid instead of grid-like as in the BlochFET, did not show NDC. Alternative explanations for the NDC are discussed and discounted for various reasons. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

The nature of the high-voltage photo-emf in thin semiconductor layers

We have shown in [1] that in pyramidal crystals of semiconductors with linear dimensions of the order of a micron it is possible for a photodiffusion-emf of 0.025 V to arise.In this paper a mechanism of producing a high-voltage photo-emf in thin layers of semiconductors is proposed and discussed, the value of which considerably exceeds the width of the forbidden zone of the semiconductor in question, starting from the concept of the series connection of a large number of such elements. Despite the fact that this phenomenon was discovered quite long ago [2], its nature has not been finally cleared up.A photo-emf of 10 + 100 V over a 1-cm length of the film at room temperature is found in CdTe [2], Sb₂S₃, Sb₂Se₃ [3], Ge, Si [4, 5], PbS [6], GaAs [7], and ZnS [8].     

A new mechanism for negative differential conductivity in superlattices

Electrons in a superlattice at high fields are suitably described in terms of localized states related to the potential wells. Conduction is dominated by transitions between adjacent wells if the potential drop over a superlattice period exceeds the width of the lowest miniband. It is shown that the probability of these transitions, and consequently the current as well, decrease with increasing field. This effect originates from decreasing overlap between electron states of neighbouring wells, and it is found for the interaction with acoustic phonons and with impurities as well.     

Spiking in an activator-inhibitor model for elements with S-shaped negative differential conductivity

Self-organized spatio-temporal dynamics of electrical transport is described by a simple noncubic activator-inhibitor system derived for layered semiconductor structures. The model exhibits a novel mode of self-sustained oscillations due to current filaments switching on and off (spiking) which may be periodic or chaotic. Additionally, we obtain complex multifilamentary spatio-temporal patterns. As such phenomena have been observed in various devices exhibiting S-shaped negative differential conductivity, our model is suggested to describe a generic mechanism.     

Long-range surface plasmon polaritons at THz frequencies in thin semiconductor layers

We present a theoretical investigation of THz long-range surface plasmon polaritons propagating on thin layers of InSb.The metallic behavior of doped semiconductors at THz frequencies allows the excitation of surface plasmon polaritons with propagation and confinement lengths that can be actively controlled.This control is achieved by acting on the free carrier density,which can be realized by changing the temperature of InSb.     

Millimetre-wave negative differential conductance in GaInAs/AlInAs semiconductor superlattices

We report microwave and temporal measurements on nonlinear transport in GaInAs/AlInAs superlattice n⁺n⁻n⁺diodes. The superlattices have wide minibands, wider than 80 meV. The microwav experiments are performed for 0–65 GHz. The diodes exhibit reflection gain up to 65 GHz over a bias-dependent Hakki resonance. The temporal experiments use electro-optic sampling to push the measurement range beyond 200 GHz. Harmonics of the fundamental Hakki resonance are observed up to order three at 150 GHz. A simple admittance model for negative differential conductance devices accounts for the results in both experimental approaches. They are in agreement with miniband transport and demonstrate superlattice negative differential conductance far in the millimetre-wave domain.     

Multipolaritons in semiconductor thin layers: Interference effects in the reflectance spectra

Summary We consider the case of a plurality of polariton modes due to the valence band structure, and give a one-dimensional model to compute the optical functions in the exciton frequency range. Interference effects of each mode with itself and with the other modes are identified by a fine structure in the reflectivity spectrum. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Four-probe measurements of the electrical conductivity of semiconductor layers containing inhomogeneous regions

The possibility of performing four-probe measurements of conductivity of semi-conductor layers containing inhomogeneous regions is evaluated. Exact formulas are obtained for determination of semiconductor conductivity for arbitrary position of the probe near a circular inhomogeneity, and for two probe positions near two circular inhomogeneities. An approximate formula is presented for use with semiconductors containing several inhomogeneous regions. Theory is compared with experimental results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 30–33, July, 1977.