The coupling between longitudinal-optical (LO) phonons and neutral excitons in two different kinds of InGaAs pyramidal quantum dots embedded in either AlGaAs or GaAs barriers is experimentally examined. We find a slightly weaker exciton-LO-phonon coupling and increased linewidth of the phonon replicas for the quantum dots with GaAs barriers compared to the ones with AlGaAs barriers. These results, combined with the fact that the LO-phonon energy of the exciton is the same for both kinds of dots, are taken as evidence that the excitons mainly couple to LO-phonons within the QDs. / Original Publication:Daniel Dufåker, L. O. Mereni, Fredrik K. Karlsson, V. Dimastrodonato, G. Juska, Per-Olof Holtz and E. Pelucchi, Exciton-phonon coupling in single quantum dots with different barriers, 2011, Applied Physics Letters, (98), 25, 251911.http://dx.doi.org/10.1063/1.3600781Copyright: American Institute of Physicshttp://www.aip.org/

Issues of major relevance to the n-type conductivity of Al0.77Ga0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements. The Al0.77Ga0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of AlxGa1−xN is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of AlxGa1−xN has not been widely discussed.

In this research the growth of InN epilayers by high-pressure chemical vapor deposition (HPCVD) and structural, optical properties of HPCVD grown InN layers has been studied. We demonstrated that the HPCVD approach suppresses the thermal decomposition of InN, and therefore extends the processing parameters towards the higher growth temperatures (up to 1100K for reactor pressures of 15 bar, molar ammonia and TMI ratios around 800, and a carrier gas flow of 12 slm). Structural and surface morphology studies of InN thin layers have been performed by X-ray diffraction, low energy electron diffraction (LEED), auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS) and atomic force microscopy (AFM). Raman spectroscopy, infrared reflection, transmission, photoluminescence spectroscopy studies have been carried out to investigate the structural and optical properties of InN films grown on sapphire and GaN/sapphire templates. InN layers grown on a GaN (0002) epilayer exhibit single-phase InN (0002) X-ray diffraction peaks with a full width at half maximum (FWHM) around 200 arcsec. Auger electron spectroscopy confirmed the cleanliness of the surface, and low energy electron diffraction yielded a 1×1 hexagonal pattern indicating a well-ordered surface. The plasmon excitations are shifted to lower energies in HREEL spectra due to the higher carrier concentration at the surface than in the bulk, suggesting a surface electron accumulation. The surface roughness of samples grown on GaN templates is found to be smoother (roughness of 9 nm) compared to the samples grown on sapphire. We found that the deposition sometimes led to the growth of 3 dimensional hexagonal InN pyramids. Results obtained from Raman and IR reflectance measurements are used to estimate the free carrier concentrations, which were found in the range from mid 10^18 cm-3 to low 10^20 cm-3. The optical absorption edge energy calculated from the transmission spectra is 1.2 eV for samples of lower electron concentration. The Raman analysis revealed a high-quality crystalline layer with a FWHM for the E2(high) peak around 6.9 cm^-1. The results presented in our study suggest that the optimum molar ratio might be below 800, which is due to the efficient cracking of the ammonia precursor at the high reactor pressure and high growth temperature.

This thesis presents the development of III-nitride materials for deep-ultraviolet (DUV) light emitting devices. The goal of this research is to develop a DUV laser diode (LD) operating at room temperature. Epitaxial structures for these devices are grown by metalorganic chemical vapor deposition (MOCVD) and several material analysis techniques were employed to characterize these structures such as atomic force microscopy, electroluminescence, Hall-effect measurement, photoluminescence, secondary ion mass spectrometry, transmission electron microscopy, transmission line measurement, and X-ray diffraction. Each of these will be discussed in detail. The active regions of III-nitride based UV emitters are composed of AlxGa1-xN alloys, the bandgap of which can be tuned from 3.4 eV to 6.2 eV, which allows us to attain the desired wavelength in the DUV by engineering the molar fraction of aluminum and gallium. In order to emit photons in the DUV wavelength range (> 4.1 eV), high aluminum molar fraction AlxGa1-xN alloys are required. Since aluminum has very low ad-atom mobility on the growth surface, a very low group V to group III precursor ratio (known as V/III ratio), high growth temperature, and low growth pressure is required to form a smooth surface and subsequently abrupt heterointerfaces. The first part of this work focuses on developing high-quality multi-quantum well structures using high aluminum molar fraction ([Al] > 60%) AlxGa1-xN alloys. Optically pumped DUV lasers were demonstrated with threshold power density as low as 250 kW/cm² for the emission wavelength as short as 248.3 nm. Transverse electric (TE) -like emission dominates when the lasers were operating above threshold power density, which suggests the diode design requires the active region to be fully strained to promote better confinement of the optical mode in transverse direction. The second phase of this project is to achieve an electrically driven injection diode laser. Owing to their large bandgap, low intrinsic carrier concentration, and relatively high dopant activation energy, the nature of these high aluminum molar fraction materials are highly insulating; therefore, efficiently transport carriers into active region is one of the main challenges. Highly conducting p-type material is especially difficult to achieve because the activation energy for magnesium, a typical dopant, is relatively large and some of the acceptors are compensated by the hydrogen during the growth. Furthermore, due to the lack of a large work function material to form a p-type ohmic contact, the p-contact layer design is limited to low aluminum molar fraction material or gallium nitride. Besides the fabrication challenges, these low aluminum molar fraction materials are not transparent to the laser wavelength causing relatively high internal loss (αi). In this work, an inverse tapered p-waveguide design is employed to transport holes to active region efficiently while the graded-index separate-confinement heterostructure (GRINSCH) is employed for the active region design. Together, a multi-quantum well (MQW) ultraviolet emitter was demonstrated.

This doctorate thesis focuses on investigating the parameter space involved in numerically modeling the bandgap engineering of a GaInP/InGaAs/Ge lattice matched multi-junction solar cell (MJSC) using InAs/InGaAs quantum dots (QDs) in the middle sub-cell. The simulation environment – TCAD Sentaurus – solves the semiconductor equations using finite element and finite difference methods throughout well-defined meshes in the device to simulate the optoelectronic behavior first for single junction solar cells and subsequently for MJSCs with and without quantum dots under concentrated illumination of up to 1000 suns’ equivalent intensity. The MJSC device models include appropriate quantum tunneling effects arising in the tunnel junctions which serve as transparent sub-cell interconnects. These tunneling models are calibrated to measurements of AlGaAs/GaAs and AlGaAs/AlGaAs tunnel junctions reaching tunneling peak current densities above 1000 A/cm^2. Self-assembled InAs/GaAs quantum dots (QDs) are treated as an effective medium through a description of appropriate generation and recombination processes. The former includes analytical expressions for the absorption coefficient that amalgamates the contributions from the quantum dot, the InAs wetting layer (WL) and the bulk states. The latter includes radiative and non-radiative lifetimes with carrier capture and escape considerations from the confinement potentials of the QDs. The simulated external quantum efficiency was calibrated to a commercial device from Cyrium Technologies Inc., and required 130 layers of the QD effective medium to match the contribution from the QD ground state. The current – voltage simulations under standard testing conditions (1 kW/cm^2, T=298 K) demonstrated an efficiency of 29.1%, an absolute drop of 1.5% over a control structure. Although a 5% relative increase in photocurrent was observed, a 5% relative drop in open circuit voltage and an absolute drop of 3.4% in fill factor resulted from integrating lower bandgap nanostructures with shorter minority carrier lifetimes. However, these results are considered a worst case scenario since maximum capture and minimum escape rates are assumed for the effective medium model. Decreasing the band offsets demonstrated an absolute boost in efficiency of 0.5% over a control structure, thus outlining the potential benefits of using nanostructures in bandgap engineering MJSCs.

Neste trabalho, estudamos as propriedades eletrônicas e estruturais de super-redes InP/In IND. 0.53 Ga IND. 0.47As dopadas fortemente com Si (densidade equivalente no bulk superior a 4.4 x 10 POT. 18cm POT. -3). O espectro de Fourier das oscilações de Shubnikov-de Haas apresenta um dubleto característico de elétrons que populam uma minibanda de energia, assim como uma freqüência de oscilação associada a elétrons confinados em uma camada superficial bidimensional (elétrons de Tamm). Verificamos que, para descrever o espectro de energia dos elétrons da minibanda, o modelo de Kronig-Penney é em geral suficiente porém, para descrever adequadamente os estados de Tamm é necessário recorrer a um cálculo auto-consistente completo. A boa resolução do dubleto associado aos elétrons que populam a minibanda de energia permitiu extrair as mobilidades quânticas dos elétrons associados aos hodógrafos extremais (\"cintura\" e \"pescoço\") da mini-superfície de Fermi. O tratamento de dados foi efetuado com a utilização de procedimentos especialmente desenvolvidos, que apresentam a vantagem de não necessitar da utilização de filtros de Fourier sofisticados. A detecção do estado de Tamm nas oscilações de Shubnikov-de Haas é inétida por se tratar de estados de Tamm degenerados. Por ser a mobilidade quântica dos elétrons de Tamm quase duas vezes maior do que a mesma mobilidade para os elétrons da minibanda, em campos magnéticos fracos as oscilações de Shubnikov-de Haas são dominadas pelos elétrons de Tamm, apesar da quantidade de elétrons de Tamm corresponder a apenas em torno de 10% do total de portadores livres em nossas amostras. As super-redes InP/In IND. 0.53 Ga IND. 0.47As:Si apresentam, também, grande redução de portadores livres com a diminuição do período das super-redes. A perda de portadores livres é de 60% quando o período é diminuído em 20%. Esta redução está correlacionada com a quantidade de átomos de ) dopantes que recai na camada interfacial de InAs IND. X P IND. 1-X que se forma quando InP é depositado sobre In IND. 0.53 Ga IND. 0.47As. Um estudo de um conjunto de 8 amostras nos permitiu estimar que a espessura da camada interfacial é de aproximadamente 20 ANGSTRONS. Os dados experimentais indicam que os átomos de Si que recaem nas camadas interfaciais, ao invés de formarem doadores rasos, formam centros profundos com energia de ativação superior a 50 meV. / In this work the electronic and structural properties of InP/In IND. 0.53 Ga IND. 0.47As superlattices heavily doped with Si (bulk equivalent density greater than 4.4 x 10 POT. 18cm POT. -3) were studied. The Fourier spectrum of the Shubnikov-de Haas oscillations presents a double peak characteristic of electrons which populate the first miniband of the energy spectrum, and an additional peak associated to electrons confined in a two-dimensional surface layer (Tamm electrons). We verified that the Kronig-Penney model is, in general, a good approximation to describe the energy spectrum of electrons in the miniband. However, to describe adequately the Tamm states, it is necessary to resort to a full self-consistent calculation of the energy levels in the effective mass approximation. The well-resolved doublet associated to the electrons in the miniband allowed us to extract the quantum mobilities associated to both extremal orbits of the Fermi mini-surface (belly and neck). The data analysis was done by using specially developed procedures, which have the advantage of not requiring the use of sophisticated Fourier filters. The detection of Tamm states throught Shubnikov-de Haas oscillations was done for the first time in superlattices in which the Tammm states are degenerate. On account of the fact that the quantum mobility of the Tamm electrons is about a factor of two greater than the quantum mobility of the miniband electrons, the Shubnikov-de Haas oscillations are dominated by electrons from the Tamm states, especially at weak magnetic fields, despite of the fact that the amount of Tamm electrons is only about 10% of the total amount of free carriers in our sample. The InP/In IND. 0.53 Ga IND. 0.47As:Si superlattices also display a strong reduction in the amount of free carriers when the period of the superlattice decreases. This reduction reaches 60% when the superlatticess period is decreased by only 20%. This reduction correlates with the amount of doping atoms that fall into the interfacial layer InAs IND. X P IND. 1-X which is formed when InP is grown on In IND. 0.53 Ga IND. 0.47As. A study of a set of 8 samples allowed us to estimate that the interfacial layer is approximately 20 ANGSTRONS thick. The experimental data indicate that the Si atoms which fall into the interfacial form deep levels with an activation energy larger than 50 meV.

É feito um resumo dos principais modelos criados para se explicar as intrigantes propriedades do centro DX e atualizar o problema. O decaimento da fotocondutividade persistente (PPC) é medido em AlxGa1-x As dopado com Si e se discute a validade dos modelos em função da cinética de captura dos elétrons pelos centros DX. Boa concordância com o modelo de Chadi e Chang é encontrada desde que se postule a existência de um nível doador mais raso. O crescimento por MBE assim como todo o processamento de amostras para os experimentos realizados é descrito sinteticamente. É discutido também o problema dos contatos a baixa temperatura e a possível influência dos centros DX nos desvios do comportamento ôhmico observados. Inclui-se também a descoberta da. fotocondutividade persistente em AlxGa1-xAs dopado com Pb, que também é relacionado à existência dos centros DX. / A short discussion about the main models created to explain the striking properties of the DX center is done in order to bring the problem up-to-date. The decay of persistent photoconductivity is measured and it is analyzed as a function of the kinetics of electron trapping by DX centers in Si-doped AlxGa1-xAs, according to these models. Good agreement with Chadi and Chang\'s model is found as long as we postulate the existence of a shallower donor. The M.B.E. growth as well as the whole sample processing is shortly described. It in siso diacussed the problem of low temperature contacts and the possible influence of DX centers in the deviation from ohmic behavior. Persistent Photoconductivity has been found in Pb-doped AlxGa1-xAs and it is also related to the DX center existence.

This doctorate thesis focuses on investigating the parameter space involved in numerically modeling the bandgap engineering of a GaInP/InGaAs/Ge lattice matched multi-junction solar cell (MJSC) using InAs/InGaAs quantum dots (QDs) in the middle sub-cell. The simulation environment – TCAD Sentaurus – solves the semiconductor equations using finite element and finite difference methods throughout well-defined meshes in the device to simulate the optoelectronic behavior first for single junction solar cells and subsequently for MJSCs with and without quantum dots under concentrated illumination of up to 1000 suns’ equivalent intensity. The MJSC device models include appropriate quantum tunneling effects arising in the tunnel junctions which serve as transparent sub-cell interconnects. These tunneling models are calibrated to measurements of AlGaAs/GaAs and AlGaAs/AlGaAs tunnel junctions reaching tunneling peak current densities above 1000 A/cm^2. Self-assembled InAs/GaAs quantum dots (QDs) are treated as an effective medium through a description of appropriate generation and recombination processes. The former includes analytical expressions for the absorption coefficient that amalgamates the contributions from the quantum dot, the InAs wetting layer (WL) and the bulk states. The latter includes radiative and non-radiative lifetimes with carrier capture and escape considerations from the confinement potentials of the QDs. The simulated external quantum efficiency was calibrated to a commercial device from Cyrium Technologies Inc., and required 130 layers of the QD effective medium to match the contribution from the QD ground state. The current – voltage simulations under standard testing conditions (1 kW/cm^2, T=298 K) demonstrated an efficiency of 29.1%, an absolute drop of 1.5% over a control structure. Although a 5% relative increase in photocurrent was observed, a 5% relative drop in open circuit voltage and an absolute drop of 3.4% in fill factor resulted from integrating lower bandgap nanostructures with shorter minority carrier lifetimes. However, these results are considered a worst case scenario since maximum capture and minimum escape rates are assumed for the effective medium model. Decreasing the band offsets demonstrated an absolute boost in efficiency of 0.5% over a control structure, thus outlining the potential benefits of using nanostructures in bandgap engineering MJSCs.

abstract: III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown lattice-matched to commercially available GaSb substrates, and the adjustment of the Bi and Sb mole fractions enables both lattice constant and bandgap to be tuned independently. This dissertation provides a comprehensive study of the surface morphology and the structural and chemical properties of InAsSbBi alloys grown by molecular beam epitaxy. 210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions. Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant. Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Les systèmes optomécaniques, dans lesquels les vibrations d'un résonateur mécanique sont couplées à un rayonnement électromagnétique, ont permis l'examen de multiples nouveaux effets physiques. Afin d'exploiter pleinement ces phénomènes dans des circuits réalistes et d'obtenir différentes fonctionnalités sur une seule puce, l'intégration des résonateurs optomécaniques est obligatoire. Ici nous proposons une nouvelle approche pour la réalisation de systèmes intégrés et hétérogènes comportant des cavités à cristaux photoniques bidimensionnels au-dessus de guides d'ondes en silicium-sur-isolant. La réponse optomécanique de ces dispositifs est étudiée et atteste d'un couplage optomécanique impliquant à la fois les mécanismes dispersifs et dissipatifs. En contrôlant le couplage optique entre le guide d'onde intégré et le cristal photonique, nous avons pu varier et comprendre la contribution relative de ces couplages. Cette plateforme évolutive permet un contrôle sans précédent sur les mécanismes de couplage optomécanique, avec un avantage potentiel dans des expériences de refroidissement et pour le développement de circuits optomécaniques multi-éléments pour des applications tels que le traitement du signal par effets optomécaniques. / Optomechanical systems, in which the vibrations of a mechanical resonator are coupled to an electromagnetic radiation, have permitted the investigation of a wealth of novel physical effects. To fully exploit these phenomena in realistic circuits and to achieve different functionalities on a single chip, the integration of optomechanical resonators is mandatory. Here, we propose a novel approach to heterogeneously integrated arrays of two-dimensional photonic crystal defect cavities on top of silicon-on-insulator waveguides. The optomechanical response of these devices is investigated and evidences an optomechanical coupling involving both dispersive and dissipative mechanisms. By controlling optical coupling between the waveguide and the photonic crystal, we were able to vary and understand the relative strength of these couplings. This scalable platform allows for unprecedented control on the optomechanical coupling mechanisms, with a potential benefit in cooling experiments, and for the development of multi-element optomechanical circuits in the frame of optomechanically-driven signal-processing applications.