Estudio de semiconductores y dispositivos optoelectrónicos mediante catodoluminiscencia

A lo largo de esta tesis se han explicado los principios básicos de funcionamiento de la técnica de caracterización óptica SEM-CL. Esta técnica se ha empleado para el análisis de semiconductores de gap directo, utilizados en el diseño de dispositivos optoelectrónicos. También se ha descrito el equipo utilizado, mostrando las posibilidades que ofrece para el estudio de estos semiconductores. La posibilidad de excitar semiconductores de gap ancho, debido a la alta energía de los electrones en un SEM, ha permitido analizar cristales de ZnO crecidos por un método hidrotérmico y cristales de GaN crecidos por un método amonotérmico. El objetivo principal era obtener sustratos de alta calidad cristalina para emplearlos en el crecimiento homoepitaxial. Se ha demostrado que se puede emplear la CL para analizar la calidad de la superficie de los cristales y hacer un análisis cualitativo y comparativo de la calidad cristalina en volumen de diferentes cristales. También se han identificados defectos e impurezas que se incorporan durante el crecimiento. La alta resolución de un SEM ha permitido estudiar con detalle la luminiscencia de nanoestructuras (nanohilos:NWs y nanocolumnas:NRs) de ZnO y GaN. En primer lugar se han analizado NWs de diferentes dimensiones de ZnO crecidos sobre sustratos de Si por VLS con Au como catalizador. Seguidamente se analizaron NWs de ZnO crecidos mediante la oxidación de una capa metálica de Zn depositada sobre un sustrato de CdTe. En este punto se analizó la morfología de los NWs mediante SEM y la calidad cristalina con CL para diferentes parámetros de crecimiento de los NWs, como el tiempo de oxidación y el espesor de la capa metálica de Zn depositada. Usando esta misma técnica de oxidación de una capa de Zn, se crecieron NWs de ZnO sobre un sustrato de LiNbO3. Posteriormente se irradiaron con iones de Ar+ con baja energía para entender el efecto de la irradiación sobre ellos. Asimismo se han investigado NRs de ZnO crecidos por MOCVD sin catalizador, observando la influencia de los diferentes parámetros de crecimiento en las propiedades ópticas de los NRs, como la variación de las tasas de flujo de los precursores individuales, el tiempo de crecimiento y la utilización de un capa buffer. Finalmente, se han caracterizado NRs de GaN crecidas por MOVPE. Se ha estudiado la distribución del dopado con Mg en las facetas polares y semipolares. Se ha utilizado las imágenes espectrales de CL para la caracterización de estructuras fotónicas de una forma más detallada, más concretamente se han estudiado con detalles la fabricación de cristales de OP- GaAs y de guías de onda de InP. La posibilidad de variar la energía del haz de electrones, ha sido posible realizar un análisis en profundidad de heteroestrosturas empleadas en el diseño de diodos láseres de alta potencia. Más concretamente se han analizado láseres tipo ridge de InGaAs/AlGaAs y diodos laser multiemisor de GaAs Se ha puesto a punto un sistema EBIC acoplado al SEM, con el que se ha caracterizado los láseres monomodo de InGaAs/AlGaAs. Se ha observado una distribución de defectos en la zona activa que ayudó a entender la cinética de la generación de defectos durante su funcionamiento.Departamento de Física de la Materia Condensada, Cristalografía y Mineralogí

Growth and characterisation of site-controlled InAs/InP quantum dots by droplet epitaxy in MOVPE

InAs/InP quantum dots were grown by droplet epitaxy via metal-organic vapor phase epitaxy (MOVPE). Their structural and optical properties on a planar substrate were extensively investigated. Indium droplets were formed through pyrolysis of trimethylindium and by controlling parameters such as deposition temperature, flow rate and growth interrupt, the density and size of droplets could be manipulated to form optically active quantum dots (QDs) in the 1550nm telecom emission band. After optimising the conditions for QD formation on planar substrates, site-control of the QDs was investigated. The major emphasis of this work and its findings is the deterministic positioning of QDs through the precursor process of positioning of indium droplets within ex-situ fabricated nanohole arrays. Using electron beam lithography and dry etching, nanohole arrays were fabricated, which served as favourable nucleation sites for droplets and subsequently QDs through the process of arsenic crystallisation. Following nanohole fabrication, extensive optimisation of the surface cleaning procedure and buffer growth were developed to reduce impurity levels and obtain high-quality QDs. The spatial arrangement of droplets deviates from randomness when the patterned substrate is employed as a template for growth. This is a new method to site-control QDs by deterministic positioning of droplets in droplet epitaxy

High-Performance III-V Quantum-Dot Lasers Monolithically Grown on Si and Ge Substrates for Si Photonics

Self-assembled III-V quantum dots (QDs) attract intense research interest and effort due to their unique physical properties arising from the three-dimensional confinement of carriers and discrete density of states. Semiconductor III-V QD laser structures exhibit dramatically improved device performance in comparison with their quantum well (QW) counterparts, notably their ultra low threshold current density, less sensitivity to defects and outstanding thermal stability. Therefore, integrating a high-quality QD laser structure onto silicon-based platform could potentially constitute a hybrid technology for the realization of optical inter-chip communications. This thesis is devoted to the development of high-performance InAs/GaAs QD lasers directly grown on silicon substrates and germanium substrates for silicon photonics. In the integration of III-V on silicon, direct GaAs heteroepitaxy on silicon is extremely challenging due to the substantial lattice and thermal expansion mismatch between GaAs and Si. The inherent high-density propagating dislocations can degrade the performance of III-V based lasers on silicon substrates. To enhance the device performance, QW dislocation filters are used here to create a strain field, which bends the propagating dislocations back towards the substrate. Here, we report the first operation of an electrically-pumped 1.3-\mu m InAs/GaAs QD laser epitaxially grown on Si (100) substrate. A threshold current density of 725 A/cm2 and an output power of 26 mW has been achieved for broad-area lasers with as-cleaved facets at room temperature. To avoid the formation of high-density threading dislocations (TDs), an alternative to direct growth of GaAs on silicon substrate is to use an intermediate material, which has a similar lattice constant to GaAs with fewer defects. Germanium appears to be the ideal candidate for a virtual substrate for GaAs growth, because germanium is almost lattice-matched to GaAs (only 0.08% mismatch). In the last 20 years, the fabrication of germanium-on-silicon (Ge/Si) virtual substrates has been intensely investigated with the demonstration of high-quality Ge/Si virtual substrates. The main challenge for the growth of GaAs on Ge/Si virtual substrate is to avoid the formation of anti-phase domains due to the polar/non-polar interface between GaAs and germanium. A new growth technique was invented for suppressing the formation of anti-phase domains for the growth of GaAs on germanium substrates at UCL. Based on this technique, lasing at a wavelength of 1305nm with a threshold current density of 55.2A/cm2 was observed for InAs/GaAs QD laser grown on germanium substrate under continuous-wave current drive at room temperature. The results suggest that long-wavelength InAs/GaAs QD lasers on silicon substrates can be realized by epitaxial growth on Ge/Si substrates. Studies in this thesis are an essential step towards the monolithic integration of long-wavelength InAs/GaAs QD lasers on a silicon substrate, as well as the integration of other III-V devices through fabricating III-V devices on silicon substrates

Inelastic Light Scattering in Low Dimensional Semiconductors

Raman scattering is a powerful technique with which to study the lattice vibrations of semiconductors. Investigations of the phonons of GaInAs-InP heterostructures have shown that although the phonons in GalnAs quantum wells resembled those of bulk GaInAs, they were screened by free carriers. Raman scattering and photoluminescence techniques were employed to estimate the plasma density at which plasmon-phonon coupling became significant. Triple crystal x-ray diffraction measurements complemented the Raman scattering data and provided information on the GaInAs alloy composition and state of strain. It was found that although nominally lattice-matched to the underlying InP, the epitaxially-grown layers were tetragonally distorted in the direction of growth. Assessment of sample damage produced by reactive-ion-etching (RIE) was undertaken as a prerequisite to the study of phonons in fabricated nanostructures. Structural damage to the crystal showed up as a relaxation of the crystalline selection rules allowing the observation of a symmetry-forbidden phonon. The intensity of this phonon correlated well with depth profiling of the damage. Optimised RIE conditions were found to produce negligible crystalline damage. The study of GaAs cylinders (with diameters of less than 100 nanometers) revealed an additional feature in the optical phonon region of their Raman spectra. This feature was identified as a surface phonon of the quantum cylinders. The experimentally-observed frequencies of the surface phonon peaks showed good agreement with calculated frequencies based on vibrations in small, geometrically-regular crystals. The main contribution of this work is the study of the surface phonons of the GaAs quantum cylinders. This is the first time that surface phonons have been observed in small fabricated samples: all previous work has involved specially-prepared crystalline powders or else comparatively large slab geometries. The conclusion that can be drawn from this work is that the cylinders are not only well-defined (as observed from the SEM micrographs) but they are also crystalline. The implication is that such structures can now be fabricated at a sufficiently high level to allow progress in prototype devices such as the quantum dot laser

Bibliography of Lewis Research Center technical publications announced in 1992

This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1992. All the publications were announced in the 1992 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses

Evidence of chlorine ion penetration in InP/InAsP quantum well structures during dry etching processes and effects of induced-defects on the electronic and structural behaviour

International audienceIn this work, the overall point of interest is the occurrence of artefacts associated with dry etching processes on InP-based structures. By artefacts we mean creation of defects in the remaining material after etching, defects which might be deleterious to both performance of the photonic devices being fabricated, and reliability/lifetime of these devices. A specific sample structure was defined on InP with InAsxP1 − x quantum wells (QWs). These QWs are buried within 1 μm from the surface, for maximum sensitivity to reactive species produced in the etch plasma, and are designed with a gradual As/P composition, such that the luminescence peak produced by each QW is clearly identified. These samples thus possess a “built-in” marker including its own scale. We focused on chemistries with chlorine (SiCl4/H2/Ar and Cl2/N2), implemented in an inductively coupled plasma reactor. With such chemistries, etch rates of 0.5 μm/min can be reached. The samples are not really etched, but just exposed shortly to the plasma for the interaction to take place. Actually, we just etch at most a few tens of nanometers. Characterisation was carried out by spectrally-resolved cathodo-luminescence and photo-luminescence. We also measured secondary ion mass spectrometry profiles, which revealed the penetration of chlorine into the samples. High resolution transmission electron microscopy was used, to probe the crystal quality. By comparing doped and un-doped samples, we show that the chlorine observed after exposure consists at least partly in Cl− ions. The other important observation is some mechanical compressive stress, which is also a consequence of the local concentration of Cl impurities after exposure to the plasm