Cathodoluminescence for studies of low dimensional semiconductor structures

In this chapter we describe some homogenization processes that may model the macroscopic properties of media whose microscopic behaviour takes lower dimensional or multi-dimensional structures into account. We will show that the method for homogenization described in Chapter 14 can be adapted to the asymptotic analysis of these functionals. In addition to this study, we also have to face the problem of the choice of a proper description of the small-scale functionals themselves, whose natural domain is not the usual Sobolev-space setting. In Sections 24.1 and 24.2 we deal with energies defined on lower-dimensional structures whose homogenization gives rise to integral functionals defined on full dimensional domains. We are going to describe two different and, to some extent, complementary approaches to this problem. We remark that while the limit analysis will be performed in detail, the description of the energies at length scale e is only outlined, leaving some details to the reader. In Section 24.3 we consider energies which describe structures as rods, plates or thin films, where one or more dimensions are negligible with respect to the others. These lower-dimensional energies are obtained by a ┌-limit procedure from (normalized) energies defined on full-dimensional domains contracting to lower-dimensional ones. We treat the case when the full dimensional structures possess in addition a (periodic) cellular structure. For the sake of illustration, we only deal with the simplified case when the period and the ‘thickness ‘ of the structure are on a comparable scale.

We present here the results on electron transport and noise (diffusion) in low‐dimensional semiconductor structures obtained self‐consistently by the Monte Carlo technique. Electron scattering by confined and localized optical phonons in quasi‐one‐dimensional quantum wires and quasi‐two‐dimensional quantum wells is taken into account. In spite of the considerable differences in phonon spectra and their coupling with electrons, and in spite of electron quantization, the diffusivity (noise) and transport parameters in low‐dimensional structures only slightly differ from corresponding bulk material characteristics at room temperature. At low lattice temperatures the existence of several optical phonon modes in low‐dimensional structures causes electron streaming at several frequencies.

The given paper studies and analyzes optical properties of semiconductor structures intended for realization of modulators. The paper considers such optical properties as absorption, transmission and refraction. The dependence of these properties on wavelength, temperature and other factors is investigated. The selection of corresponding semiconductor material for realization two versions of optical modulator, namely, for transmission and refraction is carried out.

For local structure analyses of low-dimensional structures in semiconductors, such as defects, interfaces and surfaces, a new site-selective x-ray absorption fine structure (XAFS) measurement, 'capacitance XAFS method', is presented. This method measures the x-ray photon energy dependence of the capacitance involved in diode structures. Since the capacitance is changed by an x-ray induced photoemission of localized electrons in the low dimensional structures, a site-selective analysis can be made. Although macroscopic x-ray absorption can be measured by conventional XAFS analyses, low-dimensional x-ray absorption coefficients are observed by the capacitance XAFS measurements.

: Our research on electron-phonon and phonon-phonon processes in low-dimensional semiconductor nanostructures has been focused on the effects of a surfaces and interfaces on the electron energy momentum relaxation rates due to acoustic phonon emission, new specific mechanism of electron interaction with acoustic phonons in semiconductor heterostructures, and phonon-induced processes of charge can loosing phase coherence in heterostructures. Our research provides the necessary basis for the control of 1 dissipation and for the design of nanodevices with electron-phonon relaxation and dephasing rates required specific applications. .

Low dimensional structures & devices: Part 1

Achieving a high surface to volume ratio is of advantage in many applications of semiconducting oxides, which can be obtained by reducing the size of the material toward the nanoscale. Beside size reduction, morphology variability and doping content could also induce differences in the luminescence of structures in the micro‐ and nano‐scale widening their applicability. Elongated rutile TiO2 nano‐ and micro‐structures are fabricated by an evaporation‐deposition method using TiN as precursor at temperatures ranging from 800 to 900 °C. Aluminum doped TiO2 micro‐ and nano‐structures are also grown at 940 °C. The presence of Al hinders the growth of structures and retards the anatase to rutile transition. The as‐grown low dimensional structures are characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and cathodoluminescence (CL). The luminescence of the samples depends on the size, morphology, and aluminum content and is mainly dominated by an emission at 1.52 eV and weaker emissions at 2.45 and 2.9 eV, the origin of which is discussed in this work.

Recently, low dimensional semiconductor structures such as metallic nanoparticles, have attracted significant attention for photovoltaics. In this work we study the low-temperature formation of metallic nano-dots in an indium tin oxide (ITO) antireflection coating for solar cells after an appropriate hydrogen plasma treatment. Atomic force microscopy (AFM), scanning spreading resistance microscopy (SSRM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analysis of the morphology, electrical properties and composition of ITO layers deposited on Corning glass at different temperatures and subsequently subjected to hydrogen plasma. Formation of highly conductive nano-structures at hydrogenation temperatures below 250degC is observed. It is shown that the nano-structures formation can be controlled by the time and the temperature of the hydrogenation

Many-body Coulomb interactions dominate nonlinear optical properties of semiconductor structures in many respects. Resulting effects show up most distinct in the temporal evolution of transient four-wave mixing signals. The simplest example in systems with homogeneously broadened absorption is the static exchange interaction, which prevents the occurrence of pure free polarization decay and gives rise to an additional interaction induced signal. For inhomogeneous broadening one obtains photon echo-like behaviour. The temporal duration of the signal, however, is not determined by the width of the inhomogeneous broadening, as is predicted by theory if interaction effects are neglected, but is much smaller due to exciton-exciton interaction. Additional to the static interaction effects electron-electron correlations give rise to laser induced renormalization and dephasing processes which depend sensitively on the polarizations of the driving fields. These effects are analyzed on the basis of quantum-kinetic equations, which allow to describe the temporal evolution of the nonlinear polarizations. Numerical results are presented for various excitation conditions and material systems.

Improved technologies for growing semiconductor thin films of different structural and electronic properties, and even with layer thickness approaching atomic dimensions, have provided new opportunities for basic scientific studies and device applications. New fabrication technologies have also made it possible to reduce device dimensions to the point where quantum size effects must be considered in order to realistically describe the device operation and reliably predict their performance for electronic and optical applications. In this chapter, we shall present a partial summary of advances in low-dimensional semiconductor materials and their device applications.

Widely used MRI methods show brain morphology both in vivo and ex vivo at very high resolution. Many of these methods (e.g., T2*-weighted imaging, phase-sensitive imaging, or susceptibility-weighted imaging) are sensitive to local magnetic susceptibility gradients produced by subtle variations in tissue composition. However, the spectral resolution of commonly used methods is limited to maintain reasonable run-time combined with very high spatial resolution. Here, the authors report on data acquisition at increased spectral resolution, with 3-dimensional high spectral and spatial resolution MRI, in order to analyze subtle variations in water proton resonance frequency and lineshape that reflect local anatomy. The resulting information compliments previous studies based on T2* and resonance frequency. The proton free induction decay was sampled at high resolution and Fourier transformed to produce a high-resolution water spectrum for each image voxel in a 3D volume. Data were acquired using a multigradient echo pulse sequence (i.e., echo-planar spectroscopic imaging) with a spatial resolution of 50 × 50 × 70 μm(3) and spectral resolution of 3.5 Hz. Data were analyzed in the spectral domain, and images were produced from the various Fourier components of the water resonance. This allowed precise measurement of local variations in water resonance frequency and lineshape, at the expense of significantly increased run time (16-24 h). High contrast T2*-weighted images were produced from the peak of the water resonance (peak height image), revealing a high degree of anatomical detail, specifically in the hippocampus and cerebellum. In images produced from Fourier components of the water resonance at -7.0 Hz from the peak, the contrast between deep white matter tracts and the surrounding tissue is the reverse of the contrast in water peak height images. This indicates the presence of a shoulder in the water resonance that is not present at +7.0 Hz and may be specific to white matter anatomy. Moreover, a frequency shift of 6.76 ± 0.55 Hz was measured between the molecular and granular layers of the cerebellum. This shift is demonstrated in corresponding spectra; water peaks from voxels in the molecular and granular layers are consistently 2 bins apart (7.0 Hz, as dictated by the spectral resolution) from one another. High spectral and spatial resolution MR imaging has the potential to accurately measure the changes in the water resonance in small voxels. This information can guide optimization and interpretation of more commonly used, more rapid imaging methods that depend on image contrast produced by local susceptibility gradients. In addition, with improved sampling methods, high spectral and spatial resolution data could be acquired in reasonable run times, and used for in vivo scans to increase sensitivity to variations in local susceptibility.

Photoluminescence (PL) from (Al, Ga)As quantum-wire arrays grown on vicinal (111)B GaAs substrates by molecular beam epitaxy is reported for the first time. A peak at 1.826 eV is attributed to excitonic recombination in the built-in quantum-wire array, and a peak at 1.759 eV and a low PL emission band also originate from the serpentine superlattice structure. For comparison, the epilayers were also deposited on (100) GaAs substrates simultaneously. But the PL results indicate that they are the alloy-well structures. We explain these PL results with a model and draw a conclusion that steps on (111)B surface can play a very important role in crystal growth.

In this serie of lectures we present some of the basic issues in semiconductor spintronics at an introductory level. We start with a short overview of low dimensional semiconductor structures and their applications. We then move on to discuss the notions of spin generation, injection, relaxation and detection in spintronics, with an emphasis in the application of diluted magnetic semiconductors for spin generation and injection. Finally, we discuss the manipulation of spin in semiconductor nanostructures like quantum dots, and its applications in quantum computation.

Nanoscience represents a rapidly expanding area of research that includes aspects of many areas such as physical science, materials science and engineering. The growth and investigation of low dimensional semiconductor structures offer new possibilities for light emitting devices. In a bulk semiconductor electrons and holes can move freely through the crystal. Reducing the semiconductors dimensions may drastically change their recombination properties. Size dependent optical properties, greater electron/hole overlap for enhanced photoluminescence efficiency, and discrete single electron-hole charging are some of the consequences of carrier confinement.

One of possible attempt to get light emission out of silicon is to use low dimensional Si structures. There we report about nanocrystalline composite Si films with visible photoluminescence prepared by reactive pulsed laser deposition. We have examined the effects of nanocrystallite size and matrix composition on the optical properties. As a result, the structure of these composite films and superlattice based thereon have the ability of controlling the quantum size and consequently their optoelectronic properties.