A study of irradiation-induced defects in silicon using low temperature photoluminescence

Irradiation-induced defects in silicon, using low temperature photoluminescence as a probe of defect properties were investigated. The goal of this research was to gain new understanding of defects which degrade solar cell characteristics in a radiation environment. In this regard, an important aspect of this program was a study of radiation damage and annealing in lithium doped silicon, which is useful in reducing solar cell degradation. Luminescence was used to study defects because this property reveals electron transitions through a number of defect energy levels at any given annealing stage; the luminescence spectra give excellent resolution of many defect energy levels, and these measurements can be used to give defect symmetry in the lattice, impurity dependence, and annealing properties

Effect of electrical bias on spin transport across a magnetic domain wall

We present a theory of the current-voltage characteristics of a magnetic domain wall between two highly spin-polarized materials, which takes into account the effect of the electrical bias on the spin-flip probability of an electron crossing the wall. We show that increasing the voltage reduces the spin-flip rate, and is therefore equivalent to reducing the width of the domain wall. As an application, we show that this effect widens the temperature window in which the operation of a unipolar spin diode is nearly ideal.Comment: 11 pages, 3 figure

Quantum information processing based on P-31 nuclear spin qubits in a quasi-one-dimensional Si-28 nanowire

We suggest a new method of quantum information processing based on the precise placing of P-31 isotope atoms in a quasi-one-dimensional Si-28 nanowire using isotope engineering and neutron-transmutation doping of the grown structures. In our structure, interqubit entanglement is based on the indirect interaction of P-31 nuclear spins with electrons localized in a nanowire. This allows one to control the coupling between distant qubits and between qubits separated by non-qubit neighboring nodes. The suggested method enables one to fabricate structures using present-day nanolithography. Numerical estimates show the feasibility of the proposed device and method of operation.Comment: 7 pages, 4 figure

Nonlinear spin-polarized transport through a ferromagnetic domain wall

A domain wall separating two oppositely magnetized regions in a ferromagnetic semiconductor exhibits, under appropriate conditions, strongly nonlinear I-V characteristics similar to those of a p-n diode. We study these characteristics as functions of wall width and temperature. As the width increases or the temperature decreases, direct tunneling between the majority spin bands decreases the effectiveness of the diode. This has important implications for the zero-field quenched resistance of magnetic semiconductors and for the design of a recently proposed spin transistor.Comment: 5 pages, 3 figure

Limitations on the attainable intensity of high power lasers

It is shown that even a single $e^-e^+$ pair created by a super strong laser field in vacuum would cause development of an avalanche-like QED cascade which rapidly depletes the incoming laser pulse. This confirms the old N. Bohr conjecture that the electric field of the critical QED strength $E_S=m^2c^3/e\hbar$ could never be created.Comment: 4 pages, 3 figure

Spin-orbit interaction from low-symmetry localized defects in semiconductors

The presence of low-symmetry impurities or defect complexes in the zinc-blende direct-gap semiconductors (e.g. interstitials, DX-centers) results in a novel spin-orbit term in the effective Hamiltonian for the conduction band. The new extrinsic spin-orbit interaction is proportional to the matrix element of the defect potential between the conduction and the valence bands. Because this interaction arises already in the first order of the expansion of the effective Hamiltonian in powers of Uext/Eg << 1 (where Uext is the pseudopotential of an interstitial atom, and Eg is the band gap), its contribution to the spin relaxation rate may exceed that of the previously studied extrinsic contributions, even for moderate concentrations of impurities.Comment: extended version, 5+ page

Impact Ionization and Carrier Multiplication in Graphene

We develop a model for carrier generation by impact ionization in graphene, which shows that this effect is non-negligible because of the vanishing energy gap, even for carrier transport in moderate electric fields. Our theory is applied to graphene field effect transistors for which we parametrize the carrier generation rate obtained previously with the Boltzmann formalism [A. Girdhar and J. Leburton, Appl. Phys. Lett. 99, 229903 (2011)] to include it in a self-consistent scheme and compute the transistor I-V characteristics. Our model shows that the drain current exhibits an "up-kick" at high drain biases, which is consistent with recent experimental data. We also show that carrier generation affects the electric field distribution along the transistor channel, which in turn reduces the carrier velocity

Spin diffusion/transport in nn-type GaAs quantum wells

The spin diffusion/transport in $n$-type (001) GaAs quantum well at high temperatures ($\ge120$ K) is studied by setting up and numerically solving the kinetic spin Bloch equations together with the Poisson equation self-consistently. All the scattering, especially the electron-electron Coulomb scattering, is explicitly included and solved in the theory. This enables us to study the system far away from the equilibrium, such as the hot-electron effect induced by the external electric field parallel to the quantum well. We find that the spin polarization/coherence oscillates along the transport direction even when there is no external magnetic field. We show that when the scattering is strong enough, electron spins with different momentums oscillate in the same phase which leads to equal transversal spin injection length and ensemble transversal injection length. It is also shown that the intrinsic scattering is already strong enough for such a phenomena. The oscillation period is almost independent on the external electric field which is in agreement with the latest experiment in bulk system at very low temperature [Europhys. Lett. {\bf 75}, 597 (2006)]. The spin relaxation/dephasing along the diffusion/transport can be well understood by the inhomogeneous broadening, which is caused by the momentum-dependent diffusion and the spin-orbit coupling, and the scattering. The scattering, temperature, quantum well width and external magnetic/electric field dependence of the spin diffusion is studied in detail.Comment: 12 pages, 6 figures, to be published in J Appl. Phy

Nano granular metallic Fe - oxygen deficient TiO2−δ_{2-\delta} composite films: A room temperature, highly carrier polarized magnetic semiconductor

Nano granular metallic iron (Fe) and titanium dioxide (TiO$_{2-\delta}$) were co-deposited on (100) lanthanum aluminate (LaAlO$_3$) substrates in a low oxygen chamber pressure using a pulsed laser ablation deposition (PLD) technique. The co-deposition of Fe and TiO$_2$ resulted in $\approx$ 10 nm metallic Fe spherical grains suspended within a TiO$_{2-\delta}$ matrix. The films show ferromagnetic behavior with a saturation magnetization of 3100 Gauss at room temperature. Our estimate of the saturation magnetization based on the size and distribution of the Fe spheres agreed well with the measured value. The film composite structure was characterized as p-type magnetic semiconductor at 300 K with a carrier density of the order of $10^{22} /{\rm cm^3}$. The hole carriers were excited at the interface between the nano granular Fe and TiO$_{2-\delta}$ matrix similar to holes excited in the metal/n-type semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS) devices. From the large anomalous Hall effect directly observed in these films it follows that the holes at the interface were strongly spin polarized. Structure and magneto transport properties suggested that these PLD films have potential nano spintronics applications.Comment: 6 pages in Latex including 8 figure