Antilocalization of hole carriers in Pb(1-x)Eu(x)Te alloys in the metallic regime

Magnetoresistance measurements in p-type Pb(1-x)Eu(x)Te alloys, for x varying from 0% up to 5%, have been used to investigate localization and antilocalization effects. These are attributed to both the spin-orbit scattering and to the large Zeeman splitting present in these alloys due to the large values of the effective Lande g factor. The magnetoresistance curves are analyzed using the model of Fukuyama and Hoshino, which takes into account the spin-orbit and Zeeman scattering mechanisms. The spin-orbit scattering time is found to be independent of the temperature, while the inelastic-scattering time increases with decreasing temperature suggesting the electron-phonon interaction as the main scattering mechanism.

Structure of PbTe(SiO(2))/SiO(2) multilayers deposited on Si(111)

The structure of thin films composed of a multilayer of PbTe nanocrystals embedded in SiO(2), named as PbTe(SiO(2)), between homogeneous layers of amorphous SiO(2) deposited on a single-crystal Si( 111) substrate was studied by grazing-incidence small-angle X-ray scattering (GISAXS) as a function of PbTe content. PbTe(SiO(2))/SiO(2) multilayers were produced by alternately applying plasma-enhanced chemical vapour deposition and pulsed laser deposition techniques. From the analysis of the experimental GISAXS patterns, the average radius and radius dispersion of PbTe nanocrystals were determined. With increasing deposition dose the size of the PbTe nanocrystals progressively increases while their number density decreases. Analysis of the GISAXS intensity profiles along the normal to the sample surface allowed the determination of the period parameter of the layers and a structure parameter that characterizes the disorder in the distances between PbTe layers. (C) 2010 International Union of Crystallography Printed in Singapore - all rights reserved

Spin-Orbit Coupling in n-Type PbTe/PbEuTe Quantum Wells

Magnetoresistance measurements were performed on an n-type PbTe/PbEuTe quantum well and weak antilocalization effects were observed. This indicates the presence of spin orbit coupling phenomena and we showed that the Rashba effect is the main mechanism responsible for this spin orbit coupling. Using the model developed by Iordanskii et al., we fitted the experimental curves and obtained the inelastic and spin orbit scattering times. Thus we could compare the zero field energy spin-splitting predicted by the Rashba theory with the energy spin-splitting obtained from the analysis of the experimental curves. The final result confirms the theoretical prediction of strong Rashba effect on IV-VI based quantum wells.

Magnetic ordering of EuTe/PbTe multilayers determined by x-ray resonant diffraction

In this work we use resonant x-ray diffraction combined with polarization analysis of the diffracted beam to study the magnetic ordering in EuTe/PbTe multilayers. The presence of satellites at the (1/2 1/2 1/2) magnetic reflection of a 50 /repetition EuTe/PbTe superlattice demonstrated the existence of magnetic correlations among the alternated EuTe layers. The behavior of the satellites intensity as T increases toward the Neel temperature T(N) indicates that these correlations persist nearly up to T(N) and suggests the preferential decrease of the magnetic order parameter of external monolayers of each EuTe layer within the superlattice. (C) 2008 American Institute of Physics.

Contrasting LH-HH subband splitting of strained quantum wells grown along [001] and [113] directions

Contrasting responses for the temperature tuning of the electronic structure in semiconductor quantum wells are discussed for heterolayered structures grown along (001) and (113) directions. The temperature affects the strain modulation of the deformation potentials and the effective optical gap is tuned along with the intersub-band splitting in the valence band. A multiband theoretical model accounts for the characterization of the electronic structure, highlighting the main qualitative and quantitative differences between the two systems under study. The microscopic source of strain fields and the detailed mapping of their distribution are provided by a simulation using classical molecular-dynamics technics.

Harmonic Distortion of Unstrained and Strained FinFETs Operating in Saturation

The harmonic distortion (HD) exhibited by un-strained and biaxially strained fin-shaped field-effect transistors operating in saturation as single-transistor amplifiers has been investigated for devices with different channel lengths L and fin widths W(fin). The study has been performed through device characterization, 3-D device simulations, and modeling. Nonlinearity has been evaluated in terms of second- and third-order HDs (HD2 and HD3, respectively), and a discussion on its physical sources has been carried out. Also, the influence of the open-loop voltage gain AV in HD has been observed.

Analog performance of standard and strained triple-gate silicon-on-insulator nFinFETs

This work shows a comparison between the analog performance of standard and strained Si n-type triple-gate FinFETs with high-K dielectrics and TiN gate material. Different channel lengths and fin widths are studied. It is demonstrated that both standard and strained FinFETs with short channel length and narrow fins have similar analog properties, whereas the increase of the channel length degrades the early voltage of the strained devices, consequently decreasing the device intrinsic voltage gain with respect to standard ones. Narrow strained FinFETs with long channel show a degradation of the Early voltage if compared to standard ones suggesting that strained devices are more subjected to the channel length modulation effect. (C) 2008 Elsevier Ltd. All rights reserved.

Harmonic distortion of 2-MOS structures for MOSFET-C filters implemented with n-type unstrained and strained FINFETS

This work investigates the harmonic distortion (HD) in 2-MOS balanced structures composed of triple gate FinFETs. HD has been evaluated through the determination of the third-order harmonic distortion (HD3), since this represents the major non-linearity source in balanced structures. The 2-MOS structures with devices of different channel lengths (L) and fin widths (W(fin)) have been studied operating in the linear region as tunable resistors. The analysis was performed as a function of the gate voltage, aiming to verify the correlation between operation bias and HD3. The physical origins of the non-linearities have been investigated and are pointed out. Being a resistive circuit, the 2-MOS structure is generally projected for a targeted on-resistance, which has also been evaluated in terms of HD3. The impact of the application of biaxial strain has been studied for FinFETs of different dimensions. It has been noted that HD3 reduces with the increase of the gate bias for all the devices and this reduction is more pronounced both in narrower and in longer devices. Also, the presence of strain slightly diminishes the non-linearity at a similar bias. However, a drawback associated with the use of strain engineering consists in a significant reduction of the on-resistance with respect to unstrained devices. (C) 2011 Elsevier Ltd. All rights reserved.

Impact of SEG on uniaxially strained MuGFET performance

This work focuses on the impact of the source and drain Selective Epitaxial Growth (SEG) on the performance of uniaxially strained MuGFETs. With the channel length reduction, the normalized transconductance (gm.L./W) of unstressed MuGFETs decreases due to the series resistance and short channel effects (SCE), while the presence of uniaxial strain improves the gm. The competition between the series resistance (R(s)) and the uniaxial strain results in a normalized gm maximum point for a specific channel length. Since the SEG structure influences both R(s) and the strain in the channel, this work studies from room down to low temperature how these effects influence the performance of the triple-gate FETs. For lower temperatures, the strain-induced mobility enhancement increases and leads to a shift in the maximum point towards shorter channel lengths for devices without SEG. This shift is not observed for devices with SEG where the strain level is much lower. At 150 K the gm behavior of short channel strained devices with SEG is similar to the non SEC ones due to the better gm temperature enhancement for devices without SEG caused by the strain. For lower temperatures SEG structure is not useful anymore. (C) 2011 Elsevier Ltd. All rights reserved.

Quantum field theory approach to the optical conductivity of strained and deformed graphene

The computation of the optical conductivity of strained and deformed graphene is discussed within the framework of quantum field theory in curved spaces. The analytical solutions of the Dirac equation in an arbitrary static background geometry for one dimensional periodic deformations are computed, together with the corresponding Dirac propagator. Analytical expressions are given for the optical conductivity of strained and deformed graphene associated with both intra and interbrand transitions. The special case of small deformations is discussed and the result compared to the prediction of the tight-binding model.

Analysis by optical absorption and transmission electron microscopy of the strain inhomogeneities in InGaAs/InP strained layers

Optical absorption spectra and transmission electron microscopy (TEM) observations on InGaAs/InP layers under compressive strain are reported. From the band¿gap energy dispersion, the magnitude of the strain inhomogeneities. Is quantified and its microscopic origin is analyzed in view of the layer microstructure. TEM observations reveal a dislocation network at the layer interface the density of which correlates with ¿¿. It is concluded that local variations of dislocation density are responsible for the inhomogeneous strain field together with another mechanism that dominates when the dislocation density is very low.

Alloy inhomogeneities in InAlAs strained layers grown by MBE

Transmission electron microscopy studies have been performed to characterize InxAl1−xAs layers grown by molecular beam epitaxy on (100) InP substrates. The first observations of compositional nonuniformities in strained InAlAs layers are reported. The coarse quasiperiodic structure present in each sample has been found to be dependent upon the growth parameters and the sample characteristics.

Síntesis y caracterización de nanopartículas semiconductoras de PbTe.

Tesis (Maestría en Ciencias de la Ingeniería Mecánica con Especialidad en Materiales) UANL, 2012.

Low-Cost Micromechanically Tunable Optical Devices: Strained Resonator Engineering, Technological Implementation and Characterization

The rapid growth of the optical communication branches and the enormous demand for more bandwidth require novel networks such as dense wavelength division multiplexing (DWDM). These networks enable higher bitrate transmission using the existing optical fibers. Micromechanically tunable optical microcavity devices like VCSELs, Fabry-Pérot filters and photodetectors are core components of these novel DWDM systems. Several air-gap based tunable devices were successfully implemented in the last years. Even though these concepts are very promising, two main disadvantages are still remaining. On the one hand, the high fabrication and integration cost and on the other hand the undesired adverse buckling of the suspended membranes. This thesis addresses these two problems and consists of two main parts: • PECVD dielectric material investigation and stress control resulting in membranes shape engineering. • Implementation and characterization of novel tunable optical devices with tailored shapes of the suspended membranes. For this purposes, low-cost PECVD technology is investigated and developed in detail. The macro- and microstress of silicon nitride and silicon dioxide are controlled over a wide range. Furthermore, the effect of stress on the optical and mechanical properties of the suspended membranes and on the microcavities is evaluated. Various membrane shapes (concave, convex and planar) with several radii of curvature are fabricated. Using this resonator shape engineering, microcavity devices such as non tunable and tunable Fabry-Pérot filters, VCSELs and PIN photodetectors are succesfully implemented. The fabricated Fabry-Pérot filters cover a spectral range of over 200nm and show resonance linewidths down to 1.5nm. By varying the stress distribution across the vertical direction within a DBR, the shape and the radius of curvature of the top membrane are explicitely tailored. By adjusting the incoming light beam waist to the curvature, the fundamental resonant mode is supported and the higher order ones are suppressed. For instance, a tunable VCSEL with 26 nm tuning range, 400µW maximal output power, 47nm free spectral range and over 57dB side mode suppresion ratio (SMSR) is demonstrated. Other technologies, such as introducing light emitting organic materials in microcavities are also investigated.