Hydrogen diffusion and electronic metastability in amorphous silicon

Hydrogen diffusion and its role in the many electronic metastability phenomena in hydrogenated amorphous silicon (a-Si:H) is reviewed. A-Si:H contains about 10 at% hydrogen, most of which is bonded to silicon. The hydrogen diffuses at relatively low temperatures by releasing hydrogen from the Si-H bonds into interstitial sites. The reactions of hydrogen with the silicon dangling bonds and the weak bonds provide a hydrogen-mediated mechanism for electron-structural interactions, which are manifested as electronic metastability. The annealing of light-induced defects, the equilibration of defects and dopants, the stretched exponential relaxation kinetics, and the atomic structure formed during growth, are all attributed to hydrogen diffusion.     

Theory of metastability in simple metal nanowires

Thermally induced conductance jumps of metal nanowires are modeled using stochastic Ginzburg-Landau field theories. Changes in radius are predicted to occur via the nucleation of surface kinks at the wire ends, consistent with recent electron microscopy studies. The activation rate displays nontrivial dependence on nanowire length, and undergoes first- or second-order-like transitions as a function of length. The activation barriers of the most stable structures are predicted to be universal, i.e., independent of the radius of the wire, and proportional to the square root of the surface tension. The reduction of the activation barrier under strain is also determined.     

Hydrogen above saturation at silicon vacancies: H-pair reservoirs and metastability sites

We propose that hydrogen-passivated multivacancies which appear to be fully saturated with H can actually capture additional H in electrically inactive sites. In silicon, first-principles total energy calculations show that splitting an (m>or=2) multivacancy into a mono- and an (m-1) vacancy provides a low-strain pairing site for H, 0.4 eV per H lower than any known bulk pairing site. This monovacancy ejection mechanism is an excellent candidate for the H reservoir found both in crystalline and amorphous Si. A distinct H pairing on the fully saturated m vacancies, by forming an internal surface Si-Si dimer, provides the final state of light-induced metastable degradation of hydrogenated amorphous silicon.     

Lattice vibration of amorphous and disordered crystalline silicon

The disappearance of low-energy tunneling states in an amorphous solid discovered recently has shown that the amorphous structure is not their primary cause, as had previously been widely accepted. We have also found that the same tunneling states can be generated in crystalline silicon as the crystal structure is gradually disordered by Si⁺ and B⁺ ion implantation. Most strikingly, their number densities saturate at their characteristic level regardless whether the crystal amorphizes or not. It is concluded that the nature of the tunneling states, and the cause for their saturation density, should be explored in disordered crystals.     

Theory of hopping conductivity in disordered semiconductors

The bond percolation method is used to investigate the conditions under which a linear (and not exponential) temperature dependence of hopping conductivity can be observed.Translated from Izvestiya Vysshikh Uchbenykh Zavedenii, Fizika, No.2, pp. 52–62, February, 1976.     

Theory of hopping transfer in disordered systems

The problem of hopping conduction in a system with randomly distributed impurity centers at low temperatures has been considered in terms of the linearized balance equation. The Gochanour-Andersen-Fayer diagram technique has been used to obtain a self-consistent expression for the configuration-averaged Green’s function, which describes the charge transfer in a disordered system with inclusion of Fermi correlations. In the framework of the developed formalism, a relationship has been derived for hopping conductivity as a function of the temperature with a power law of the density of states. The results obtained agree well with the percolation approach and, in the static limit, lead to the Mott’s law.     

Theory of phonon linewidth in orientationally disordered crystals

The dynamics of coupled translational (T) and rotational (R) modes is studied with special reference to the experimental situation in the alkali cyanides. The dynamic equations are extended and, in addition to the bilinear T-R coupling, anharmonic interactions of the form T-T-R are considered. These interactions lead to additional friction coefficients for translational and rotational motion. Transport coefficients are evaluated with mode-mode coupling theory. The validity of orientational relaxation models coupled to lattice vibration is confirmed as a particular case of the present theory at low frequencies. An extension to the high frequency regime is proposed.Dedicated to Prof. Dr. H.E. Müser ob the occasion of his 60th birthday     

Theory of exchange coupling in disordered magnetic multilayers

We consider a mechamism of exchange coupling based on interaction between electrons in a nonmagnetic layer. Depending on the ratio of inverse time of diffusion of electrons between ferromagnetic layers and ferromagnetic splitting of conducting electrons, this mechanism describes the transition from ferromagnetic to noncollinear ordering of magnetizations of ferromagnetic layers.     

Electronic properties of hydrogen-related complexes in pure semiconductors

Hydrogen has been shown to activate the neutral impurities carbon, silicon and oxygen in ultra-pure germanium and form shallow level complexes. The double acceptors beryllium and zinc in silicon and germanium, as well as the triple acceptor copper in germanium, can be partially passivated, leading to single hole acceptors. The study of the electronic level spectrum of the single carrier bound to these centers at low temperatures has provided much information on symmetry and composition. Most centers reveal a symmetry axis along [1 1 1] and are static. In some cases hydrogen has been found to tunnel between equivalent real space positions. hoto hermal onization pectroscopy (PTIS) has been the most important tool for the study of the optical transitions of the hole (electron) in these hydrogen containing complexes. This photoconductivity technique combines high sensitivity with high resolution and permits the study of shallow acceptors or donors present at concentrations as low as 10⁸ cm^-3. Even lower limits may be attained under favorable circumstances.     

Theory of hopping and multiple-trapping transport in disordered systems

We present a general theory to describe equilibrium as well as nonequilibrium transport properties of systems in which the carriers perform an incoherent motion that can be described by means of a set of master equations. This includes hopping as well as trapping in the localized energy region of amorphous or perturbed crystalline semiconductors. Employing the mathematical analogy between the master equations and the tight binding problem we develop approximation schemes using methods of many-particle physics to derive expressions for the averaged propagator of the carriers and the conductivity tensor. The calculated conductivity and Hall conductivity of hopping systems compare extremely well to computer simulations over the whole range of frequency, density, and temperature. We are able to derive expressions for dispersive transport in hopping as well as trapping systems that contain the results of earlier theories of Scher, Montroll and Noolandi, Schmidlin as special cases and establish criteria for the occurrence of dispersive transport in such systems. We find that in principle hopping can lead to dispersive transport if the times and densities are very low, but actual experimental data are more easily explained in terms of multiple trapping.     

Theory of thermal conductivity in the disordered electron liquid

We study thermal conductivity in the disordered two-dimensional electron liquid in the presence of long-range Coulomb interactions. We describe a microscopic analysis of the problem using the partition function defined on the Keldysh contour as a starting point. We extend the renormalization group (RG) analysis developed for thermal transport in the disordered Fermi liquid and include scattering processes induced by the long-range Coulomb interaction in the sub-temperature energy range. For the thermal conductivity, unlike for the electrical conductivity, these scattering processes yield a logarithmic correction that may compete with the RG corrections. The interest in this correction arises from the fact that it violates the Wiedemann–Franz law. We checked that the sub-temperature correction to the thermal conductivity is not modified either by the inclusion of Fermi liquid interaction amplitudes or as a result of the RG flow. We therefore expect that the answer obtained for this correction is final. We use the theory to describe thermal transport on the metallic side of the metal–insulator transition in Si MOSFETs.