Electrolyte electroreflectance spectroscopy studies on the interfacial behavior of the near-surface In0.15Ga0.85As/GaAs quantum well electrode vertical bar non-aqueous electrolyte

Electrolyte electroreflectance spectra of the near-surface strained-layer In0.15Ga0.85As/GaAs double single-quantum-well electrode have been studied at different biases in non-aqueous solutions of ferrocene and acetylferrocene. The optical transitions, the Franz-Keldysh oscillations (FKOs) and the quantum confined Stark effects (QCSE) of In0.15Ga0.85As/GaAs quantum well electrodes are analyzed. Electric field strengths at the In0.15Ga0.85As/GaAs interface are calculated in both solutions by a fast Fourier transform analysis of FKOs. A dip is exhibited in the electric field strength versus bias (from 0 to 1.2 V) curve in ferrocene solution. A model concerning the interfacial tunneling transfer of electrons is used to explain the behavior of the electric field. (C) 2001 Elsevier Science B.V. All rights reserved.

Conduction band offset and electron effective mass in GaInNAs/GaAs quantum-well structures with low nitrogen concentration

We have investigated the optical transitions in Ga1-yInyNxAs1-x/GaAs single and multiple quantum wells using photovoltaic measurements at room temperature. From a theoretical fit to the experimental data, the conduction band offset Q(c), electron effective mass m(e)*, and band gap energy E-g were estimated. It was found that the Q(c) is dependent on the indium concentration, but independent on the nitrogen concentration over the range x=(0-1)%. The m(e)* of GaInNAs is much greater than that of InGaAs with the same concentration of indium, and increases as the nitrogen concentration increases up to 1%. Our experimental results for the m(e)* and E-g of GaInNAs are quantitatively explained by the two-band model based on the strong interaction of the conduction band minimum with the localized N states. (C) 2001 American Institute of Physics.

Photoluminescence study of InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells

Fourier transform photoluminescence measurements were carried out to investigate the optical transitions in InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells. Samples with electron density n(s) between 0.8 and 5.3 x 10(12) cm(-2) rue studied. Strong recombination involving one to three populated electron subbands with the first heavy-hole subband is observed. Fermi edge singularity (FES) clearly can be observed for some samples. The electron subband energies in the InGaAs/InAlAs step quantum wells were calculated by a self-consistent method, taking into account strain and nonparabolicity effects and the comparison with the experimental data shows a good agreement. Our results can help improve understanding for the application of InGaAs/InAlAs step quantum wells in microelectronic and optoelectronic devices. (C) 1998 Elsevier Science Ltd. All rights reserved.

Hydride Vapor Phase Epitaxy Growth of Semipolar, 10(1)over-bar(3)over-barGaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of (10 (1) over bar(3) over bar)-oriented GaN thick films on patterned sapphire substrates (PSSs) (10 (1) over bar0). From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of (10 (1) over bar(3) over bar) GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations (10 (1) over bar(3) over bar) and (11 (2) over bar2) dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these (10 (1) over bar(3) over bar) GaN films are dotted with oblique pyramids, concurrently lining along the < 30 (3) over bar2 > direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the (10 (1) over bar(3) over bar) GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift-redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

EFFECTIVE-MASS THEORY FOR GAAS/GA1-XALXAS QUANTUM WIRES AND CORRUGATED SUPERLATTICES GROWN ON (311)-ORIENTED SUBSTRATES

The electronic structures of GaAs/Ga1-xAlxAs quantum wires (corrugated superlattices) grown on (311)-oriented substrates are studied in the framework of the effective-mass envelope-function method. The electron and hole subband structure and optical transition matrix elements are calculated. When x=1, the results are compared with experiments, and it is found that the direct transition becomes an indirect transition as the widths of well and barrier become smaller.

THEORY OF THE ELECTRONIC-STRUCTURE OF POROUS SI

A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.

Effective-mass theory for InAs/GaAs strained superlattices

By using the recently developed exact effective-mass envelope-function theory, the electronic structures of InAs/GaAs strained superlattices grown on GaAs (100) oriented substrates are studied. The electron and hole subband structures, distribution of electrons and holes along the growth direction, optical transition matrix elements, exciton states, and absorption spectra are calculated. In our calculations, the effects due to the different effective masses of electrons and holes in different materials and the strain are included. Our theoretical results are in agreement with the available experimental data.

Ab initio calculations for the neutral and charged O vacancy in sapphire

The energetics, lattice relaxation, and the defect-induced states of st single O vacancy in alpha-Al2O3 are studied by means of supercell total-energy calculations using a first-principles method based on density-functional theory. The supercell model with 120 atoms in a hexagonal lattice is sufficiently large to give realistic results for an isolated single vacancy (square). Self-consistent calculations are performed for each assumed configuration of lattice relaxation involving the nearest-neighbor Al atoms and the next-nearest-neighbor O atoms of the vacancy site. Total-energy data thus accumulated are used to construct an energy hypersurface. A theoretical zero-temperature vacancy formation energy of 5.83 eV is obtained. Our results show a large relaxation of Al (O) atoms away from the vacancy site by about 16% (8%) of the original Al-square (O-square) distances. The relaxation of the neighboring Al atoms has a much weaker energy dependence than the O atoms. The O vacancy introduces a deep and doubly occupied defect level, or an F center in the gap, and three unoccupied defect levels near the conduction band edge, the positions of the latter are sensitive to the degree of relaxation. The defect state wave functions are found to be not so localized, but extend up to the boundary of the supercell. Defect-induced levels are also found in the valence-band region below the O 2s and the O 2p bands. Also investigated is the case of a singly occupied defect level (an F+ center). This is done by reducing both the total number of electrons in the supercell and the background positive charge by one electron in the self-consistent electronic structure calculations. The optical transitions between the occupied and excited states of the: F and F+ centers are also investigated and found to be anisotropic in agreement with optical data.

Pressure dependence of the electronic subband structure of strained In0.2Ga0.8As/GaAs MQWs

We have measured low-temperature photoluminescence (PL) and optical absorption spectra of an In0.2Ga0.8As/GaAs multiple quantum well (MQW) structure at pressures up to 8 GPa. Below 4.9 GPa, PL shows only the emission of the n = 1 heavy-hole (HH) exciton. Three new X-related PL bands appear at higher pressures. They are assigned to spatially indirect (type-II) and direct (type-I) transitions from X(Z) states in GaAs and X(XY) valleys of InGaAs, respectively, to the HH subband of the wells. From the PL data we obtain a valence band offset of 80 meV for the strained In0.2Ga0.8As/GaAs MQW system. Absorption spectra show three features corresponding to direct exciton transitions in the quantum wells. In the pressure range of 4.5 to 5.5 GPa an additional pronounced feature is apparent in absorption, which is attributed to the pseudo-direct transition between a HH subband and the folded X(Z) states of the wells. This gives the first clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance at the heterointerfaces in MQWs.

Effective-mass theory for InAs/GaAs strained coupled quantum dots

In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Electronic structures of the zinc-blende GaN/Ga1-xAlxN compressively strained superlattices and quantum wells

The electronic structures of the zinc-blende GaN/Ga0.85Al0.15N compressively strained superlattices and quantum wells are investigated using a 6 x 6 Hamiltonian model (including the heavy hole, light hole and spin-orbit splitting band). The energy bands, wavefunctions and optical transition matrix elements are calculated. It is found that the light hole couples with the spin-orbit splitting state even at the k=0 point, resulting in the hybrid states. The heavy hole remains a pure heavy hole state at k=0. The optical transitions from the hybrid valence states to the conduction states are determined by the transitions of the light hole and spin-orbit splitting states to the conduction states. The transitions from the heavy hole, light hole and spin-orbit splitting states to the conduction states obey the selection rule Delta n=0. The band structures obtained in this work will be valuable in designing GaN/GaAlN based optoelectronic devices. (C) 1996 Academic Press Limited

Defect states in cubic GaN epilayer grown on GaAs by metalorganic vapor phase epitaxy

Defect states in cubic GaN epilayers grown on GaAs were investigated with the photoluminescence technique. One shallow donor and two acceptors were identified to be involved in relevant optical transitions. The binding energies of the free excitons, the bound excitons. the donor and the acceptors were determined. These values are in good agreement with recent theoretical results.

Nanostructured CaWO4, CaWO4 : Eu3+ and CaWO4 : Tb3+ particles: Sonochemical synthesis and luminescent properties

Nanostructured CaWO4, CaWO4:Eu3+, and CaWO4:Tb3+ phosphor particles were synthesized via a facile sonochemical route. X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence, low voltage cathodoluminescence spectra, and photoluminescence lifetimes were used to characterize the as-obtained samples. The X-ray diffraction results indicate that the samples are well crystallized with the scheelite structure of CaWO4.

Estudo óptico de cristais e heterostruturas de CdTe

Neste trabalho privilegiam-se as técnicas ópticas, nomeadamente a fotoluminescência a transmissão, a reflexão e a difusão de Raman na caracterização de defeitos e impurezas em cristais e heterostruturas de CdTe/GaAs. No primeiro capítulo efectua-se uma revisão sucinta das propriedades dos compostos II-VI e são colocados os problemas a investigar. O segundo capítulo é dedicado a alguns aspectos teóricos relevantes para a análise dos resultados obtidos nos capítulos quatro a nove. O equipamento utilizado e as experiências realizadas são descritas no terceiro capítulo. A caracterização por espectroscopia de luminescência do tipo de transições e dos defeitos envolvidos nas amostras de fábrica é efectuada com detalhe no quarto capítulo. Neste capítulo são analisadas ainda as emissões devidas a desvios estequiométricos causados pelo recozimento com excesso e defeito de Cd. No capítulo cinco são estudadas por fotoluminescência amostras dopadas intencionalmente com oxigénio por difusão e mostra-se que este elemento se comporta como trapa isoelectrónica no CdTe tal como acontece no ZnTe. Neste capítulo são estudadas também amostras dopadas com ferro pelo mesmo método e são apresentadas as dificuldades em colocar este dopante em sítios substitucionais, nomeadamente no sítio do Cd. No sexto capítulo é estudada a região 1.4 eV, evidenciando o seu comportamento em função da temperatura e da potência de excitação, o seu perfil e a interacção electrãorede. No sétimo capítulo mostra-se que a técnica micro-Raman com luz visível coerente não permite extrair conclusões fiáveis acerca das inclusões de Te na superfície do CdTe, uma vez que a radiação ao ser focada nas amostras, induz a formação de aglomerados nos quais o Te é o elemento dominante. Neste capítulo calcula-se também a concentração de portadores livres através da interacção do plasmão com o fonão óptico longitudinal. O oitavo capítulo é dedicado ao estudo de camadas de CdTe/GaAs com diferentes espessuras nomeadamente na análise da distribuição das deslocações e da deformação na superfície em função da espessura. Os resultados obtidos são comparados através da largura a meia altura das curvas de DCXRD (“Double Crystal X Ray Diffraction”) e dos espectros de reflectância. Por fotoluminescência, são caracterizados os defeitos introduzidos durante o crescimento, são utilizados e desenvolvidos modelos complementares na distinção do tipo de transições ópticas obtidas. No nono capítulo, por espectroscopia de absorção e de reflexão em cristais de CdTe e em camadas de CdTe/GaAs na região reststrahlen são determinadas as frequências dos modos ópticos longitudinal e transversal com bastante precisão. Os resultados obtidos — pelas relações de dispersão de Kramers- Kronig — são simulados pelo modelo do oscilador harmónico classico, mostrando que ambos os métodos descrevem de forma semelhante o comportamento do CdTe nessa região sendo possível determinar as frequências ópticas transversal e longitudinal, as constantes dieléctrica óptica e estática e os coeficientes de amortecimento e de Szigeti. Nas heterostruturas e nas camadas mais espessas determina-se também a concentração de portadores de carga n. No décimo capítulo resumem-se as conclusões do trabalho e são abordados aspectos relacionados com desenvolvimentos possíveis de futuros trabalhos.

Propriétés optiques dans l'infrarouge des nanotubes de carbone et du graphène

Les nanotubes de carbone et le graphène sont des nanostructures de carbone hybridé en sp2 dont les propriétés électriques et optiques soulèvent un intérêt considérable pour la conception d’une nouvelle génération de dispositifs électroniques et de matériaux actifs optiquement. Or, de nombreux défis demeurent avant leur mise en œuvre dans des procédés industriels à grande échelle. La chimie des matériaux, et spécialement la fonctionnalisation covalente, est une avenue privilégiée afin de résoudre les difficultés reliées à la mise en œuvre de ces nanostructures. La fonctionnalisation covalente a néanmoins pour effet de perturber la structure cristalline des nanostructures de carbone sp2 et, par conséquent, d’affecter non seulement lesdites propriétés électriques, mais aussi les propriétés optiques en émanant. Il est donc primordial de caractériser les effets des défauts et du désordre dans le but d’en comprendre les conséquences, mais aussi potentiellement d’en exploiter les retombées. Cette thèse traite des propriétés optiques dans l’infrarouge des nanotubes de carbone et du graphène, avec pour but de comprendre et d’expliquer les mécanismes fondamentaux à l’origine de la réponse optique dans l’infrarouge des nanostructures de carbone sp2. Soumise à des règles de sélection strictes, la spectroscopie infrarouge permet de mesurer la conductivité en courant alternatif à haute fréquence des matériaux, dans une gamme d’énergie correspondant aux vibrations moléculaires, aux modes de phonons et aux excitations électroniques de faible énergie. Notre méthode expérimentale consiste donc à explorer un espace de paramètres défini par les trois axes que sont i. la dimensionnalité du matériau, ii. le potentiel chimique et iii. le niveau de désordre, ce qui nous permet de dégager les diverses contributions aux propriétés optiques dans l’infrarouge des nanostructures de carbone sp2. Dans un premier temps, nous nous intéressons à la spectroscopie infrarouge des nanotubes de carbone monoparois sous l’effet tout d’abord du dopage et ensuite du niveau de désordre. Premièrement, nous amendons l’origine couramment acceptée du spectre vibrationnel des nanotubes de carbone monoparois. Par des expériences de dopage chimique contrôlé, nous démontrons en effet que les anomalies dans lespectre apparaissent grâce à des interactions électron-phonon. Le modèle de la résonance de Fano procure une explication phénoménologique aux observations. Ensuite, nous établissons l’existence d’états localisés induits par la fonctionnalisation covalente, ce qui se traduit optiquement par l’apparition d’une bande de résonance de polaritons plasmons de surface (nanoantenne) participant au pic de conductivité dans le térahertz. Le dosage du désordre dans des films de nanotubes de carbone permet d’observer l’évolution de la résonance des nanoantennes. Nous concluons donc à une segmentation effective des nanotubes par les greffons. Enfin, nous montrons que le désordre active des modes de phonons normalement interdits par les règles de sélection de la spectroscopie infrarouge. Les collisions élastiques sur les défauts donnent ainsi accès à des modes ayant des vecteurs d’onde non nuls. Dans une deuxième partie, nous focalisons sur les propriétés du graphène. Tout d’abord, nous démontrons une méthode d’électrogreffage qui permet de fonctionnaliser rapidement et à haute densité le graphène sans égard au substrat. Par la suite, nous utilisons l’électrogreffage pour faire la preuve que le désordre active aussi des anomalies dépendantes du potentiel chimique dans le spectre vibrationnel du graphène monocouche, des attributs absents du spectre d’un échantillon non fonctionnalisé. Afin d’expliquer le phénomène, nous présentons une théorie basée sur l’interaction de transitions optiques intrabandes, de modes de phonons et de collisions élastiques. Nous terminons par l’étude du spectre infrarouge du graphène comportant des îlots de bicouches, pour lequel nous proposons de revoir la nature du mécanisme de couplage à l’œuvre à la lumière de nos découvertes concernant le graphène monocouche.