Direct heteroepitaxy of III-Vs on silicon (Si) has always been a challenge and there are various strategies to integrate these materials. This thesis deals with one such strategy known as Epitaxial lateral overgrowth (ELOG) which is extensively supported by experiments. For an application such as a multijunction solar cell, with silicon as a bottom cell, the highest efficiency can be achieved with a top cell having a bandgap of 1.7 eV and hence GaAsP as a material suits the profile. The ELOG GaAsP and GaAs samples were grown using the epitaxial growth technique known as hydride vapor phase epitaxy (HVPE). With its near equilibrium operation capacity, high quality layers were grown. To specifically focus on the crystal defects and dislocations of the atoms, GaAsP was grown on GaAs substrate. Samples with varying growth parameters are investigated using several characterization techniques such as scanning electron microscopy (SEM), Photoluminescence (PL) spectroscopy and Raman spectroscopy. Composition variations (group V elemental incorporation in GaAsP) and crystalline quality are the two major factors that are analyzed. Additionally, ELOG GaAs samples grown on GaAs substrate using HVPE are studied as a reference to observe any strain effects due to the ELOG profile and compare with the GaAsP samples. The ideal goal of this thesis is to optimize the crystalline quality of the ELOG GaAsP samples and to verify that GaAsP grown using ELOG technique has a better crystallinity than the planar growth (direct epitaxy of GaAsP on GaAs substrate) using two major optical characterization tools - PL and Raman spectroscopy. This work is a step towards the development of high efficiency multi-junction solar cells with GaAsP and Si as the respective top and bottom cells. / Direkt heteroepitaxi av III-V på kisel (Si) har alltid varit en utmaning och det finns olika strategier för att integrera dessa material. Den här avhandlingen behandlar en sådan strategi som kallas Epitaxial lateral overgrowth (ELOG) som stöds externt av experiment. För en applikation som en multi junction solcell, med kisel som bottencell, kan den högsta effektiviteten uppnås med en toppcell med ett bandgap på 1,7 eV och därmed GaAsP som ett material som passar profilen. ELOG GaAsP- och GaAs-proverna odlades med användning av den epitaxiella tillväxttekniken känd som hydriddampfasepitaxi (HVPE). Med dess nära kapacitet för jämviktsdrift odlades lager av hög kvalitet. För att specifikt fokusera på kristalldefekter och dislokationer av atomerna odlades GaAsP på GaAs substrat. Prover med varierande tillväxtparametrar undersöks med användning av flera karakteriseringstekniker såsom skanningselektronmikroskopi (SEM), Photoluminescence (PL) -spektroskopi och Raman-spektroskopi. Kompositionvariationer (grupp V elemental inkorporering i GaAsP) och kristallin kvalitet är de två huvudfaktorerna som analyseras. Dessutom studeras ELOG GaA-prover odlade på GaAs-substrat med användning av HVPE som en referens för att observera eventuella belastningseffekter på grund av ELOG-profilen och jämföra med GaAsP-proverna. Det ideala målet med denna avhandling är att optimera den kristallina kvaliteten på ELOG GaAsP-proverna och att verifiera att GaAsP som odlas med ELOG-teknik har en bättre kristallinitet än den plana tillväxten (direkt epitaxi av GaAsP på GaAs underlag) med två huvudsakliga optiska karaktäriseringar verktyg - PL- och Raman-spektroskopi. Detta arbete är ett steg mot utvecklingen av högeffektiva multi junction solceller med GaAsP och Si som respektive topp- och bottenceller.

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PROPERTIES OF LIGHT EMITTING DIODES FOLLOWING COBALT-60 IRRADIATION &Ouml / zcan, Safak M.S., Department of Physics Supervisor: Prof. Dr. ibrahim G&uuml / nal September 2004, 71 pages The main purpose of this study is to investigate the effects of gamma radiation on the properties of the light emitting diodes. GaP and GaAsP LEDs are used in the study. It is observed that the exposure of a light emitting diode affects its various properties. A cobalt-60 gamma-cell is used to irradiate the selected light emitting diodes. For the different total doses of gamma pre-irradiation and post-irradiation I-V characteristics and spectral responses are recorded. The capacitance characteristics are measured at 1MHz at room temperature. Gamma ray bombardment of these LEDs results in reduction of electroluminescent intensity and increase in forward current up to levels tested. In GaP diodes dominant current transport mechanism has found to be effected by irradiation. No noticeable change is observed in the series resistances. The impurity density remains same in the green LED and increases in the red one due to the irradiation, which is deduced from the C-V characteristics. Both the circuit designers and the users should be aware of these effects in order to reach a reliable application for these components in a radiation environment.

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Depuis une vingtaine d’année les nanofils des semiconducteurs suscitent un intérêt majeur pour des applications diverses grâce à leurs propriétés optoélectroniques particulières. Dans le domaine du photovoltaïque ils présentent aussi un atout majeur. La combinaison du fort coefficient d’absorption des semiconducteurs III-V et le faible coût des substrats de silicium permettraient la réalisation des cellules photovoltaïques à faible coût et à haut rendement. C’est dans ce contexte que s’est déroulé cette thèse qui visait le développement des dispositifs à base des nanofils III-V pour le photovoltaïque. Dans une première partie, les techniques de nanofabrication pour la réalisation des dispositifs à base d’ensemble de nanofils pour les cellules photovoltaïques sont présentées. Ensuite, la fabrication et la caractérisation de dispositifs à base d’ensembles de nanofils de GaN pour les applications photovoltaïque sont permis d’ouvrir la voie au développement des cellules solaires tandems d’InGaN⁄Si. Dans la suite des travaux on a étudié la croissance des nanofils de GaAs du type cœur-coquille sur Si ainsi que les étapes technologiques pour la fabrication des dispositifs à base d’ensemble de nanofils dans l’optique de préparer le terrain pour la réalisation d’une cellule tandem III-V sur Si. Enfin la croissance et la caractérisation électro-optique des nanofils contenant des jonctions axiales de GaAsP crus par la méthode VLS-EJM a permis de déterminer le type de dopage et l’optimisation de la structure en vue d’obtenir un effet photovoltaïque. / Over the past twenty years, semiconductor nanowires have attracted major interest for various applications thanks to their particular optoelectronic properties. The combination of the high absorption coefficient of the III-V semiconductors and the low cost of the silicon substrates would allow the realization of photovoltaic cells at low cost and high efficiency. It is in this context that this thesis was developed which focused on the development of devices based on III-V nanowires for photovoltaics. In a first part, the nanofabrication techniques for the realization of devices based on set of nanowires for photovoltaic cells are presented. Next, the fabrication and characterization of devices based on GaN nanowire arrays for photovoltaic applications is paving the way for the development of InGaN / Si tandem solar cells. In the following, we studied the growth of core-shell GaAs nanowires on Si as well as the technological steps for the fabrication of nanowire-based devices in order to prepare the ground for the realization of a tandem III-V cell on Si. Finally, the growth and electro-optical characterization of the nanowires containing axial junctions of raw GaAsP by the VLS-EJM method made it possible to determine the type of doping and the optimization of the structure in order to obtain a photovoltaic effect.

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