Search Results (11 total)

We demonstrate the injection and transport of spin-polarized electrons through n-type doped silicon with in-plane spin valve and perpendicular magnetic-field spin precession and dephasing (“Hanle effect”) measurements. A voltage applied across the transport layer is used to vary the confinement potential caused by conduction-band bending and to control the dominant transport mechanism between drift and diffusion. By modeling the transport in this device with a Monte Carlo scheme, we simulate the observed spin polarization and Hanle features, showing that the average transit time across the short Si transport layer can be controlled over four orders of magnitude with applied voltage. As a result, this modeling allows inference of a long electron-spin lifetime despite the short transit length.

A spin-transport model is employed to study the effects of spin dephasing induced by diffusion-driven transit-time uncertainty through semiconductor spintronic devices where drift is the dominant transport mechanism. It is found that in the ohmic regime, dephasing is independent of transit length and determined primarily by voltage drop across the spin-transport region. The effects of voltage and temperature predicted by the model are compared to experimental results from a 350-μm-thick silicon spin-transport device using derived mathematical expressions of spin dephasing.

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13π precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T₁) lower bound in silicon of over 500 ns at 60 K.

We present an experimental investigation of the hot-electron mean free path in ErAs thin films grown on GaAs. Using an Al∕Al₂O₃∕Al tunnel junction as a hot-electron source for ballistic electron emission spectroscopy, we investigate ErAs films of thicknesses ∼100–∼300 Å. Our results indicate a mean free path of order 100 Å for electrons 1–2 eV above the Fermi level at 80 K.

Because ErAs, a semimetal, grows epitaxially on GaAs(100), ErAs-base/ GaAs-collector metal-base transistors provide a uniquely simple system in which to study the interfacial transverse momentum conservation of hot electrons. This system is also of interest for metal-semiconductor superlattice thermal energy conversion devices that utilize ErAs as the interbarrier material. A key requirement for such devices to outperform bulk thermal energy converters is the nonconservation of transverse momentum. Our results, indicating total nonconservation of transverse momentum, could therefore lead to significantly more efficient thermal energy conversion devices.

We present a detailed description of a scheme to calculate the injection current for metal-semiconductor systems using tunnel-junction electron emission. We employ a Monte Carlo framework for integrating over initial free-electron states in a metallic emitter and use interfacial scattering at the metal-semiconductor interface as an independent parameter. These results have implications for modeling metal-base transistors and ballistic electron emission microscopy and spectroscopy.

As the length scale for semiconductor heterostructures approaches the regime of the lattice constant, our current theory for calculating ballistic hot-electron transport becomes inapplicable. In this case, a method such as the Green’s function formalism should be used to calculate ballistic electron transmission functions from the exact, periodic lattice potential. We present a method for directly calculating the exact surface Green’s function for three-dimensional periodic leads which is necessary for such a scheme. Except in cases of high crystal symmetry, the method is limited by the difficulty to solve a nonsymmetric matrix Riccati equation.

Using planar theory of ballistic electron emission spectroscopy with the addition of scattering at the metal-semiconductor interface, we calculate an expected change in the ratio of the collector current (I_c) to the tunnel current (I_t) as I_t is varied in the well-known system Au/GaAs(100). This alternative spectroscopy is performed experimentally and is shown to differ drastically from the theory, which nevertheless agrees well with standard voltage spectroscopy. From this discrepancy, we question the applicability of one-dimensional (1D) planar theory to an inherently 3D system.

We identify a different class of physical systems that are able to form universal logic gates. By analogy with Si(100) surface dimers, we present a model to analyze the trajectories of the fixed points (interpreted as logic states) under variation of the basic parameters. Using the perspective of catastrophe theory, we show that information processing is the result of cycling the parameters of such systems through a path containing a cusp-type catastrophe. We apply this analysis to the construction of an example based on magnetic memory.

Using the process of spontaneous parametric down-conversion in a two-crystal geometry, we have generated a source of polarization-entangled photon pairs that is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-σ violation of Bell’s inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable.