A. N. Ionov

Electronic Correlation Effects and the Coulomb Gap at Finite Temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the 29Si and 13C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the 29Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T1. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T1 and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.

Peculiarities of neutron-transmutation phosphorous doping of Si30 enriched SiC crystals: Electron paramagnetic resonance study

We have obtained a high concentration of P donor dopants in 6H‐SiC enriched with Si30 and irradiated with thermal neutrons. It was established that annealing at a relatively low temperature of 1300°C, i.e., 500–600°C lower than that used for annealing SiC with the natural isotope composition after neutron-transmutation doping, gives rise to an electron paramagnetic resonance (EPR) signal corresponding to three different shallow P (sP) donors with large hyperfine interactions. The correlated changes of these sP centers in all the annealing experiments and the similarities to the spectra of shallow N donors demonstrate that these sites have shallow donor levels and a similar electronic structure and that they belong to different lattice sites: two quasicubic and hexagonal. The phosphorus at these three sites is suggested to occupy the C position. Simultaneously the low-temperature EPR signal from another set of P-related donor centers having a small, strongly anisotropic hyperfine interaction is observed. I...

Local distribution of high-conductivity regions in polyamide thin films

Conducting points with ohmic conduction are observed in polyamide thin films by the atomic force microscopy method. The correlation between the distribution of highly conducting points and the roughness of the polymerfilm relief is revealed. The conducting-channel density inside the polymer is shown to depend on the substrate material.

Electron correlation effects at variable-range hopping in doped GaAs, CdTe, Ge and Si

Abstract We report low-temperature electrical resistivity and magnetoresistivity measurements of doped semiconductors below the critical concentration of the metal-insulator transition. The series of samples were chemically doped (CdTe and Si) or were obtained by the neutron transmutation doping technique (GaAs and Ge). The resistivity was measured in the temperature range T = 0.03-4.2 K. All samples show, at the lowest temperatures, variable-range hopping resistivity p(T) = p o exp [(T o/T) s ] with s = ½. Comparison of the experimentally determined T o with the heoretical single-electron value of Efros and Shklovskii, T o = T ESO = 2.86e 2/k B a B k O, shows that multiple-electron transitions are dominant at all compensations except K ≥ 0.7, where a large scale potential relief exists. In this case the single-electron approach seems to be a good approximation.

Josephson current-voltage characteristic of a composite based on polystyrene and graphene oxide

Current-voltage characteristics of the Josephson type have been found for a composite based on polystyrene and graphene dioxide of a mesoscopic size incorporated into it.

Role of electron “lakes” in the negative magnetoresistance effect in the region of Mott hopping conductivity

It is shown for doped and compensated germanium that the appearance of negative magnetoresistance under the conditions of Mott hopping conductivity may be due to the presence of a nonuniform spatial distribution of the electron density, the temperature at which the effect appears apparently being determined by the temperature at which the electron gas condenses into electron “lakes.” A “dead zone” effect was also observed in weak magnetic fields, the threshold field increasing with the nonuniformity of the electron distribution.

Low-temperature conductivity and magnetoconductivity of neutron-transmutation-doped barely metallic GaAs in the vicinity of the metal-insulator transition

Measurements of the temperature dependence of conductivity and magnetoconductivity are presented of barely metallic n-GaAs bulk crystals doped by the neutron-transmutation doping technique resulting in medium-compensated samples of fixed compensation degree in the immediate vicinity of the metal-insulator transition at low temperatures. At n to nc the temperature dependence in tau phi (the phase-breaking time) disappears and therefore at low temperatures the temperature dependence of the conductivity is determined by the electron-electron interaction and necessarily Lint is the smallest relevant length leading to a dependence sigma (T)=bT13/. The experimentally determined density of states at the Fermi level is quite different whether n=nc is reached by an exactly controlled impurity concentration or by magnetic tuning with n=nc(B)>nc(0). This is interpreted by the influence of the magnetic field on the weak-localisation contribution and a change in the density of states at the Fermi level. When n=nc is reached by a controlled impurity concentration, the experimentally obtained density of states at the Fermi level is in good agreement with the value estimated from the impurity concentration and the energy spread of donor states in the gap.

Neutron transmutation doping of silicon 30Si monoisotope with phosphorus

Phosphorus-doped silicon 30Si monoisotope samples with a highly homogeneous impurity distribution at a concentration of 5 × 1016 cm−3 were obtained for the first time by means of neutron transmutation doping.

Electron paramagnetic resonance in neutron-transmutation-doped semiconductors with a changed isotopic composition

Potential applications of electron paramagnetic resonance (EPR) for investigating and controlling the process of neutron transmutation doping (NTD) of semiconducting germanium, silicon, and silicon carbide are discussed. It is shown that EPR enables one to control the process of annealing of radiation-induced defects in semiconductors subject to neutron irradiation and to detect the shallow donors restored in the process of annealing of donor-compensating defects by observing EPR signals from these donors. EPR can be used to separately detect isolated donors and clusters of two, three, and more exchange-bound donor atoms and thereby determine the degree of nonuniformity of the impurity distribution over the crystal. Neutron transmutation doping is demonstrated to produce a fairly uniform arsenic-donor distribution in a germanium crystal. It is argued that semiconductors enriched in the selected isotopes should be used for NTD. The results of an investigation of phosphorus donors in silicon carbide are presented.