Tailoring the surface density of silicon nanocrystals embedded in SiOx single layers

In this article, we explore the possibility of modifying the silicon nanocrystal areal density in SiOx single layers, while keeping constant their size. For this purpose, a set of SiOx monolayers with controlled thickness between two thick SiO2 layers has been fabricated, for four different compositions (x=1, 1.25, 1.5, or 1.75). The structural properties of the SiO x single layers have been analyzed by transmission electron microscopy (TEM) in planar view geometry. Energy-filtered TEM images revealed an almost constant Si-cluster size and a slight increase in the cluster areal density as the silicon content increases in the layers, while high resolution TEM images show that the size of the Si crystalline precipitates largely decreases as the SiO x stoichiometry approaches that of SiO2. The crystalline fraction was evaluated by combining the results from both techniques, finding a crystallinity reduction from 75% to 40%, for x = 1 and 1.75, respectively. Complementary photoluminescence measurements corroborate the precipitation of Si-nanocrystals with excellent emission properties for layers with the largest amount of excess silicon. The integrated emission from the nanoaggregates perfectly scales with their crystalline state, with no detectable emission for crystalline fractions below 40%. The combination of the structural and luminescence observations suggests that small Si precipitates are submitted to a higher compressive local stress applied by the SiO2 matrix that could inhibit the phase separation and, in turn, promotes the creation of nonradiative paths.

Antireflection coatings from analogy between electron scattering and spin precession

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band.

Local noise analysis of a Schottky contact: combined thermionic-emissiondiffusion theory

A theoretical model for the noise properties of Schottky barrier diodes in the framework of the thermionic-emission¿diffusion theory is presented. The theory incorporates both the noise inducedby the diffusion of carriers through the semiconductor and the noise induced by the thermionicemission of carriers across the metal¿semiconductor interface. Closed analytical formulas arederived for the junction resistance, series resistance, and contributions to the net noise localized indifferent space regions of the diode, all valid in the whole range of applied biases. An additionalcontribution to the voltage-noise spectral density is identified, whose origin may be traced back tothe cross correlation between the voltage-noise sources associated with the junction resistance andthose for the series resistance. It is argued that an inclusion of the cross-correlation term as a newelement in the existing equivalent circuit models of Schottky diodes could explain the discrepanciesbetween these models and experimental measurements or Monte Carlo simulations.

Effects of the epitaxial layer thickness on the noise properties of Schottky barrier diodes

An analytical theory to describe the combined effects of the epitaxial layer thickness and the ohmic contact on the noise properties of Schottky barrier diodes is presented. The theory, which provides information on both the local and the global noise properties, takes into account the finite size of the epitaxial layer and the effects of the back ohmic contact, and applies to the whole range of applied bias. It is shown that by scaling down the epitaxial layer thickness, the current regime in which the noise temperature displays a shot-noise-like behavior increases at the cost of reducing the current range in which the thermal-noise-like behavior dominates. This improvement in noise temperature is limited by the effects of the ohmic contact, which appear for large currents. The theory is formulated on general trends, allowing its application to the noise analysis of other semiconductor devices operating under strongly inhomogeneous distributions of the electric field and charge concentrations.

Effect of long-range Coulomb interaction on shot-noise suppression in ballistic transport

We present a microscopic analysis of shot-noise suppression due to long-range Coulomb interaction in semiconductor devices under ballistic transport conditions. An ensemble Monte Carlo simulator self-consistently coupled with a Poisson solver is used for the calculations. A wide range of injection-rate densities leading to different degrees of suppression is investigated. A sharp tendency of noise suppression at increasing injection densities is found to scale with a dimensionless Debye length related to the importance of space-charge effects in the structure.

Chaotic dynamics of electric-field domains in periodically driven superlattices

Self-sustained time-dependent current oscillations under dc voltage bias have been observed in recent experiments on n-doped semiconductor superlattices with sequential resonant tunneling. The current oscillations are caused by the motion and recycling of the domain wall separating low- and high-electric-field regions of the superlattice, as the analysis of a discrete drift model shows and experimental evidence supports. Numerical simulation shows that different nonlinear dynamical regimes of the domain wall appear when an external microwave signal is superimposed on the dc bias and its driving frequency and driving amplitude vary. On the frequency-amplitude parameter plane, there are regions of entrainment and quasiperiodicity forming Arnold tongues. Chaos is demonstrated to appear at the boundaries of the tongues and in the regions where they overlap. Coexistence of up to four electric-field domains randomly nucleated in space is detected under ac+dc driving.

Chaos in resonant-tunneling superlattices

Spatiotemporal chaos is predicted to occur in n-doped semiconductor superlattices with sequential resonant tunneling as their main charge transport mechanism. Under dc voltage bias, undamped time-dependent oscillations of the current (due to the motion and recycling of electric field domain walls) have been observed in recent experiments. Chaos is the result of forcing this natural oscillation by means of an appropriate external microwave signal.

Local noise analysis of a Schottky contact: combined thermionic-emissiondiffusion theory

A theoretical model for the noise properties of Schottky barrier diodes in the framework of the thermionic-emission¿diffusion theory is presented. The theory incorporates both the noise inducedby the diffusion of carriers through the semiconductor and the noise induced by the thermionicemission of carriers across the metal¿semiconductor interface. Closed analytical formulas arederived for the junction resistance, series resistance, and contributions to the net noise localized indifferent space regions of the diode, all valid in the whole range of applied biases. An additionalcontribution to the voltage-noise spectral density is identified, whose origin may be traced back tothe cross correlation between the voltage-noise sources associated with the junction resistance andthose for the series resistance. It is argued that an inclusion of the cross-correlation term as a newelement in the existing equivalent circuit models of Schottky diodes could explain the discrepanciesbetween these models and experimental measurements or Monte Carlo simulations.

Effects of the epitaxial layer thickness on the noise properties of Schottky barrier diodes

An analytical theory to describe the combined effects of the epitaxial layer thickness and the ohmic contact on the noise properties of Schottky barrier diodes is presented. The theory, which provides information on both the local and the global noise properties, takes into account the finite size of the epitaxial layer and the effects of the back ohmic contact, and applies to the whole range of applied bias. It is shown that by scaling down the epitaxial layer thickness, the current regime in which the noise temperature displays a shot-noise-like behavior increases at the cost of reducing the current range in which the thermal-noise-like behavior dominates. This improvement in noise temperature is limited by the effects of the ohmic contact, which appear for large currents. The theory is formulated on general trends, allowing its application to the noise analysis of other semiconductor devices operating under strongly inhomogeneous distributions of the electric field and charge concentrations.

Obtención de láminas delgadas de Y-Ba-Cu-O sobre Si (100)

Se ha utilizado la evaporación secuencial de Cu, Y2O3 y BaF2 para la obtención de láminas delgadas superconductoras de YBa-CuO sobre substratos de Si monocristalino con orientación [100], recubiertos con una lámina barrera de Zr02. Se han estudiado los efectos de la variación de los espesores relativos de las láminas constituyentes y del espesor total de la lámina resultante. Las láminas se han caracterizado mediante medidas de la variación de la resistencia con la temperatura, microscopía electrónoca de barrido, difractometría de rayos X, microson-da electrónica y espectometría de masas de iones secundarios. Las láminas presentan un ligero carácter semiconductor en el estado normal, con temperaturas de inicio de la transición su-perconductora alrededor de 90 K, y resistencia nula, en el mejor de los casos, a 45 K.