Les nitrures des éléments III-V, ont été largement étudiés en raison de leurs applications dans les diodes électroluminescentes DEL, les diodes laser et les photodétecteurs. L’énergie de la bande interdite « Gap » de ces alliages ternaires ou quaternaires peut être ajustée en fonction de la composition, à des énergies de photons allant de l'infrarouge à l'ultraviolet. Ce gap direct ajustable sur une large gamme, rend ces matériaux très utiles pour les applications photovoltaïques en raison de la possibilité d'inventer non seulement des cellules solaires multi-jonction à haut rendement, mais également les cellules solaires de troisième génération comme les cellules à bandes intermédiaires, reposant uniquement sur les alliages nitrures. En plus de leur grand gap ajustable, les nitrures montrent également d’autres propriétés photovoltaïques intéressantes, comme de faibles masses effectives des porteurs de charge, de fortes mobilités, des coefficients d'absorption élevés ainsi qu’une tolérance aux radiations. La technologie des nitrures III-V a démontré sa capacité à croître des structures cristallines de haute qualité et à fabriquer des dispositifs optoélectroniques, ce qui confirme son potentiel pour le solaire photovoltaïque à très haut rendement. En intégrant ce matériau avec une jonction de silicium cristallin, nous pourrons avoir une cellule multijonction à très haut rendement avec un coût compétitive. / Nitrides of III-V elements, have been widely studied because of their interessting applications in the LED light-emitting diodes, laser diodes and photodetectors. The bandgap of such ternary or quaternary alloys can be adjusted depending on the composition, at photon energies ranging from infrared to ultraviolet. This adjustable direct bandgap over a wide range, making these materials valuable for photovoltaic applications due to the possibility of inventing not only multi-junction solar cells at high efficiency, but also third generation solar cells such as cells with intermediate bandgap, based solely on nitrides alloys. In addition to their large adjustable bandgap, nitrides also show other interesting photovoltaic properties, such as low effective masses of the charge carriers, high mobility, high absorption coefficient and a radiation tolerance. The technology of III-V nitrides has demonstrated its ability to grow high quality crystal structures and to manufacture optoelectronic devices, which confirm its potential for photovoltaic solar energy with very high efficiency. By incorporating this material with crystalline silicon junction, we can have a multijunction cell with very high efficiency with a competitive cost.

Experimental investigations of the optical properties of InGaN/GaN quantum dots are presented. A pulsed laser is used to perform time-integrated and time-resolved microphotoluminescence, photoluminescence excitation, and polarisation-resolved spectroscopy of single InGaN quantum dots under a non-linear excitation regime. The first micro-photoluminescence results from InGaN/GaN quantum dots grown on a non-polar crystal plane (11^-<sub style='position: relative;left: -.4em;'>2</sub>0) are presented. Time-resolved studies reveal an order of magnitude increase in the oscillator strength of the exciton transition when compared to InGaN quantum dots grown on the polar (0001) plane, suggesting a significantly reduced internal electric field in non-polar InGaN quantum dots. Polarisation resolved spectroscopy of non-polar InGaN quantum dots reveals 100% linearly polarised emission for many quantum dots. For quantum dot emissions with a polarisation degree less than unity, a fine structure splitting between two orthogonal polarisation axes can be resolved in an optical setup with a simple top-down excitation geometry. A statistical investigation into the origins of spectral diffusion in polar InGaN quantum dots is presented, and spectral diffusion is attributed to charge carriers trapped at threading dislocations, and itinerant and trapped carriers in the underlying quantum well layer which forms during the growth procedure. Incorporating quantum dots into the intrinsic region of a p-i-n diode structure and applying a reverse bias is suggested as a method to reduce spectral diffusion. Coherent control of the excited state exciton in a non-polar InGaN quantum dot is experimentally demonstrated by observation of Rabi rotation between the excited state exciton and the crystal ground state. The exciton ground state photoluminescence is used as an indirect measurement of the excited state population.

OPTICAL CHARACTERIZATION OF INGAN HETEROSTRUCTURES FOR BLUE LIGHT EMITTERS AND VERTICAL CAVITY LASERS: EFFICIENCY AND RECOMBINATION DYNAMICS By Serdal Okur, Ph.D. A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2014. Major Director: Ümit Özgür, Associate Professor, Electrical and Computer Engineering This thesis explores radiative efficiencies and recombination dynamics in InGaN-based heterostructures and their applications as active regions in blue light emitters and particularly vertical cavities. The investigations focus on understanding the mechanism of efficiency loss at high injection as well as developing designs to mitigate it, exploring nonpolar and semipolar crystal orientations to improve radiative efficiency, integration of optimized active regions with high reflectivity dielectric mirrors in vertical cavity structures, and achieving strong exciton-photon coupling regime in these microcavities for potential polariton lasing. In regard to active regions, multiple double heterostructure (DH) designs with sufficiently thick staircase electron injection (SEI) layers, which act as electron coolers to reduce the overflow of hot electrons injected into the active region, were found to be more viable to achieve high efficiencies and to mitigate the efficiency loss at high injection. Such active regions were embedded in novel vertical cavity structure designs with full dielectric distributed Bragg reflectors (DBRs) through epitaxial lateral overgrowth (ELO), eliminating the problems associated with semiconductor bottom DBRs having narrow stopbands and the cumbersome substrate removal process. Moreover, the ELO technique allowed the injection of carriers only through the high quality regions with substantially reduced threading dislocation densities compared to regular GaN templates grown on sapphire. Reduced electron-hole wavefunction overlap in polar heterostructures was shown to hamper the efficiency of particularly thick active regions (thicker than 3 nm) possessing three-dimensional density of states needed for higher optical output. In addition, excitation density-dependent photoluminescence (PL) measurements showed superior optical quality of double heterostructure (3 nm InGaN wells) active regions compared to quantum wells (2 nm InGaN wells) suggesting a minimum limit for the active region thickness. Therefore, multiple relatively thin but still three dimensional InGaN active regions separated by thin and low barriers were found to be more efficient for InGaN light emitters. Investigations of electroluminescence from light emitting diodes (LEDs) incorporating multi DH InGaN active regions (e.g. quad 3 nm DH) and thick SEIs (two 20 nm-thick InGaN layers with step increase in In content) revealed higher emission intensities compared to LEDs with thinner or no SEI. This indicated that injected electrons were cooled sufficiently with thicker SEI layers and their overflow was greatly reduced resulting in efficient recombination in the active region. Among the structures considered to enhance the quantum efficiency, the multi-DH design with a sufficiently thick SEI layer constitutes a viable approach to achieve high efficiency also in blue lasers. Owing to its high exciton binding energy, GaN is one of the ideal candidates for microcavities exploiting the strong exciton-photon coupling to realize the mixed quasiparticles called polaritons and achieve ideally thresholdless polariton lasing at room temperature. Angle-resolved PL and cathodoluminescence measurements revealed large Rabi splitting values up to 75 meV indicative of the strong exciton-photon coupling regime in InGaN-based microcavities with bottom semiconductor AlN/GaN and a top dielectric SiO2/SiNxDBRs, which exhibited quality factors as high as 1300. Vertical cavity structures with all dielectric DBRs were also achieved by employing a novel ELO method that allowed integration of a high quality InGaN cavity active region with a dielectric bottom DBR without removal of the substrate while forming a current aperture through the ideally defect-free active region. The full-cavity structures formed as such were shown to exhibit clear cavity modes near 400 and 412 nm in the reflectivity spectrum and quality factors of 500. Although the polar c-plane orientation has been the main platform for the development of nitride optoelectronics, significant improvement of the electron and hole wavefunction overlap in nonpolar and semipolar InGaN heterostructures makes them highly promising candidates for light emitting devices provided that they can be produced with good crystal quality. To evaluate their true potential and shed light on the limitations put forth by the structural defects, optical processes in several nonpolar and semipolar orientations of GaN and InGaN heterostructures were investigated. Particularly, stacking faults were found to affect significantly the optical properties, substantially influencing the carrier dynamics in nonpolar (1-100), and semipolar (1-101) and (11-22)GaN layers. Carrier trapping/detrapping by stacking faults and carrier transfer between stacking faults and donors were revealed by monitoring the carrier recombination dynamics at different temperatures, while nonradiative recombination was the dominant process at room temperature. Although it is evident that nonpolar (1-100)GaN and semipolar (11-22)GaN require further improvement of material quality, steady-state and time-resolved PL measurements support that (1-101)-oriented GaN templates and InGaN active regions exhibit optical performance comparable to their highly optimized polar c-plane counterparts, and therefore, are promising for vertical cavities and light emitting device applications.

Ce travail est porté sur les propriétés structurales et optiques de structures à boîtes quantiques III-nitrures dopées avec des terres rares réalisées par épitaxie à jets moléculaires.<br />Pendant la croissance, les terres rares ont une influence drastique sur les boîtes, expliquée par les propriétés surfactantes des atomes de terres rares.<br />La caractérisation optique et structurale montre que les boîtes sont dopées efficacement avec les ions de Eu, Tm et Tb. D'autres localisations des terres rares ont été trouvées, par exemple, pour le Tm, à l'interface du GaN des boîtes et de l'AlN.<br />Ce travail s'intéresse aussi à la dynamique d'excitation de boîtes quantiques dopées aux terres rares. La photoluminescence de couleurs intéressantes est stable de la température de l'hélium liquide jusque la température ambiante.<br />Des structures à boîtes quantiques plus complexes sont aussi abordées : des boîtes InGaN:Eu QDs et des boîtes GaN co-dopées, importante pour la réalisation de composants.<br />Une attention particulière a été mise sur les couches de GaN dopé Eu, où différents sites pour l'Eu ont été mis en évidence près de surface et à l'intérieur de matériau.

No description available.

The experiment divided into two parts. One is silicon solar cell process. The other is InGaN solar cell process. Borosilicafilm solution spin onto the n-type silicon (111) substrate and spread through the high-temperature furnace tube to form a p-n junction silicon solar cell. Then, evaporate top and rear contact by electron beam evaporation system. InGaN p-i-n structure solar cell grows on sapphire substrate by plasma-assisted molecular beam epitaxy system (PA-MBE) and its process is by repeated photolithography, inductive coupled plasma etching and wet etching. In the device fabrication process, the first is defining the sample size(mesa). Second, etched to the n-type GaN layer, and then coated metal as electrode. Finally, we get the device. In the measurement, the measurement of I-V curve of samples in the light by solar simulator of AM1.5 G light source observe open circuit voltage, short circuit current, fill factor, and efficiency. In addition, we measure the external quantum efficiency of the samples by IPCE and observe the photoelectric conversion efficiency of samples at different wavelength. Observed the sample quality and the indium composition of InGaN layer by XRD. We observe the InGaN band gap shift by variable-temperature photoluminescence spectra.

In this thesis the methods of optical beam induced current (OBIC), multi-photon excitation, and confocal microscopy were employed to study InGaN LED¡¦s. Recently, important breakthrough and achievement have been made in the developments of InGaN based opto-electronic components. As a result, it is important to characterize the properties and the performance of InGaN based devices with various techniques. In this thesis, we have used 2-photon OBIC microscopy to observe various such LED¡¦s. We found that the LED¡¦s exhibit dotted pattern which can not be seen under 1-photon excitation. In addition, we have employed micro-spectroscopy to characterize the active layer of these LED¡¦s. These results will be discussed in this thesis in detail.

We study the optical properties of InGaN/GaN multiple quantum wells grown on silicon (111) substrate with different buffer layers. Because of the lattice mismatch and mismatch in thermal expansion coefficient, there exists stresses in the nitride sample grown on silicon substrates, which influence the growth properties and optical properties. A set of buffer layers was proposed in order to reduce the stress in our samples. The influence on optical properties is investigated in our work. In Raman spectra, we observed the characteristic phonon mode of GaN. According to the variation of E2 mode, the stress can be estimated. From our results, growing buffer layers can effectively reduce the stress in the sample. From temperature dependent and power dependent photoluminescence¡]PL) measurement, we found that appropriate buffer layers bring about less stress and better efficiency of luminescence. There are absorption of GaN and some vibrational behaviors in PLE spectra. According to the stokes shift calculated from temperature dependence PL and PLE spectra, we infer that the mechanism of recombination is not only carrier localization. The recombination is involved with the interaction of carriers and longitudinal optical phonons, and the stokes shift is independence on temperature.

abstract: Nitride semiconductors have wide applications in electronics and optoelectronics technologies. Understanding the nature of the optical recombination process and its effects on luminescence efficiency is important for the development of novel devices. This dissertation deals with the optical properties of nitride semiconductors, including GaN epitaxial layers and more complex heterostructures. The emission characteristics are examined by cathodoluminescence spectroscopy and imaging, and are correlated with the structural and electrical properties studied by transmission electron microscopy and electron holography. Four major areas are covered in this dissertation, which are described next. The effect of strain on the emission characteristics in wurtzite GaN has been studied. The values of the residual strain in GaN epilayers with different dislocation densities are determined by x-ray diffraction, and the relationship between exciton emission energy and the in-plane residual strain is demonstrated. It shows that the emission energy increases withthe magnitude of the in-plane compressive strain. The temperature dependence of the emission characteristics in cubic GaN has been studied. It is observed that the exciton emission and donor-acceptor pair recombination behave differently with temperature. The donor-bound exciton binding energy has been measured to be 13 meV from the temperature dependence of the emission spectrum. It is also found that the ionization energies for both acceptors and donors are smaller in cubic compared with hexagonal structures, which should contribute to higher doping efficiencies. A comprehensive study on the structural and optical properties is presented for InGaN/GaN quantum wells emitting in the blue, green, and yellow regions of the electromagnetic spectrum. Transmission electron microscopy images indicate the presence of indium inhomogeneties which should be responsible for carrier localization. The temperature dependence of emission luminescence shows that the carrier localization effects become more significant with increasing emission wavelength. On the other hand, the effect of non-radiative recombination on luminescence efficiency also varies with the emission wavelength. The fast increase of the non-radiative recombination rate with temperature in the green emitting QWs contributes to the lower efficiency compared with the blue emitting QWs. The possible saturation of non-radiative recombination above 100 K may explain the unexpected high emission efficiency for the yellow emitting QWs Finally, the effects of InGaN underlayers on the electronic and optical properties of InGaN/GaN quantum wells emitting in visible spectral regions have been studied. A significant improvement of the emission efficiency is observed, which is associated with a blue shift in the emission energy, a reduced recombination lifetime, an increased spatial homogeneity in the luminescence, and a weaker internal field across the quantum wells. These are explained by a partial strain relaxation introduced by the InGaN underlayer, which is measured by reciprocal space mapping of the x-ray diffraction intensity. / Dissertation/Thesis / Ph.D. Physics 2012

InGaN is a promising semiconductor for solid state lighting thanks to its bandgap which spans the entire visible regime of the electromagnetic spectrum. InGaN is grown heteroepitaxially due to the absence of a native substrate; however, this results in a strained film and a high dislocation density—two effects that have been associated with efficiency droop, which is the disastrous drop in efficiency of a light-emitting diode (LED) as the input current increases. Heteroepitaxially grown nanowires have recently attracted great interest due to their property of eliminating the detrimental effects of the lattice mismatch and the corollary efficiency droop. In this study, InGaN nanowires were grown on a low-cost Si (111) substrate via molecular beam epitaxy. Unique nanostructures, taking the form of mushrooms, have been observed in localized regions on the samples. These nanomushrooms consist of a nanowire body with a wide cap on top. Photoluminescence characterization revealed that the nanowires emit violet-blue, whilst the nanomushrooms emit a broad yellow-orange-red luminescence. The simultaneous emission from the nanowires and nanomushrooms forms white light. Structural characterization of a single nanomushroom via transmission electron microscopy revealed a simultaneous increase in indium and decrease in gallium at the interface between the body and the cap. Furthermore, the cap itself was found to be indium-rich, confirming it as the source of the longer wavelength yellow-orange-red luminescence. It is believed that the nanomushroom cap formed as a consequence of the saturation of growth on the c-plane of the nanowire. It is proposed that the formation of an indium droplet on the tip of the nanowire saturated growth on the c-plane, forcing the indium and gallium adatoms to incorporate on the sidewall m-planes instead, but only at the nanowire tip. This resulted in the formation of a mushroom-like cap on the tip. How and why the indium droplets formed is not entirely clear, but a localized temperature dip may have been the cause. Ultimately, the simultaneous growth of nanowires and nanomushrooms on the same substrate may pave the way to the development of a phosphor-free, efficient, inherent white LED.