Добірка наукової літератури з теми "Active semiconductors"

This thesis presents the electrical characterisation of defects in novel group IV semiconducting materials: semiconducting diamond and silicon germanium (SiGe) virtual substrates. Several methods to clean diamond surfaces are introduced, which lead to the fabrication of a diamond Schottky diode with acceptable characteristics. Current-Voltage (I-V) and Capacitance-Voltage (C-V) measurements were carried out to study the electrical properties of both the diamond and SiGe Schottky diodes. Deep level transient spectroscopy (DLTS) and Laplace DLTS were then carried out to investigate the deep electronic states in these devices. Scanning Electron Microscopy (SEM) was also used to investigate defects in the diamond samples. For the diamond Schottky diodes, I-V and C-V measurements confirmed the quality of the fabricated Schottky diode; the measured phase angle between capacitance and voltage was close to 90° for temperatures greater than 300K and frequencies above 200 kHz and the device clearly exhibited rectification. DLTS and LDLTS measurements of the diamond did not show any signatures that could be attributed to isolated point defects. This could be due to the fact that it was necessary to take the samples to higher temperatures in order to fully ionize the boron in the sample. The boron acceptor is at 0.37 eV above the valence band and therefore only about 5% is ionised at room temperature. During the major part of the study at Manchester, there was no access to a high temperature cryostat. However, a clear capacitance transient was observed at lower temperatures and it is proposed that this is due to emission of holes from boron. Deep traps will be located deeper in the band gap than the boron. An additional problem was that the sample was of polycrystalline structure and is full of grain boundaries, which appear to be implicated in the leakage currents present in our devices. I-V, C-V, DLTS and LDLTS were also used to investigate the deep states in the SiGe virtual substrate. I-V and C-V measurements showed that the SiGe Schottky diode showed some leakage (reported by the suppliers) but nevertheless the diode exhibited rectification. Analysis of the DLTS data showed the presence of a defect in the SiGe samples which could be a structural defect, probably dislocation-related. However, the low background doping meant that a considerable depth below the surface was being measured in DLTS and depth profiling was not possible.

Le contexte de ma thèse se situe dans le cadre de l’imagerie multispectrale infrarouge (IR) et traite notamment de la plasmonique, domaine de l’optique électromagnétique dont le but est d’étudier et d’exploiter des ondes de surface existant à l’interface entre un métal et un diélectrique. On cherche à miniaturiser des fonctions optiques grâce aux nanotechnologies et plus précisément à réaliser du filtrage spectrale IR au niveau du pixel de détection en intégrant un nano-résonateur. Usuellement, on utilise des diélectriques et des métaux mais l’intégration est complexe. J’explore le potentiel offert par les semi-conducteurs fortement dopés pour remplacer les métaux, ce qui pourrait permettre une meilleure intégration aux procédés technologiques de fabrication de photodétecteur ou d’émetteur. J’utilise des semi-conducteurs III-V, compatibles avec la croissance épitaxiale du super-réseau de type 2 (T2SL) des photodétecteurs infrarouge à ondes longues (LWIR). De plus, travailler avec un semi-conducteur fortement dopé offre la possibilité de modifier la fréquence de résonance en ajustant la densité de porteurs libres par action d’une différence de potentiel. J’étudie des architectures de composants "GMR" (Guided-mode resonance), usuellement formées d’un guide d’onde en diélectrique, siège de la résonance, et d’un réseau en diélectrique ou en métal permettant le couplage entre l’onde incidente ou transmise et le mode guidé grâce aux ordres ±1 diffractés par le réseau dans la couche mince. La tendance actuelle est d’intégrer ces composants directement au niveau du pixel de détection mais au prix de nombreuses étapes de fabrication. J’étudie la possibilité d’utiliser exclusivement des semi-conducteurs pour simplifier le procédé de fabrication et permettre une intégration monolithique du filtre au détecteur. Le guide d’onde est constitué d’un semi-conducteur intrinsèque et le réseau en semi-conducteur fortement dopé. La gamme spectrale d’intérêt se situe dans l’infrarouge lointain (8 μm - 14 μm).Dans un premier temps, les démonstrations théorique et expérimentale d’un filtre spectral nanostructuré tout semi-conducteur pour l’infrarouge fondé sur la résonance de mode guidé ont été effectuées. J’ai dimensionné puis fabriqué un échantillon dont la première étape consiste en le dépôt par épitaxie d’une couche de GaSb et d’une couche de InAsSb fortement dopé sur un substrat en GaAs avant une étape de photolithographie pour définir le masque de la gravure ionique réactive afin d’obtenir le réseau de diffraction. Un travail expérimental a ensuite permis de caractériser le composant (mesure sous incidence normale, étude angulaire, mesure à basse température) avec notamment la réalisation d’un banc de caractérisation angulaire. En parallèle, j’ai étudié un empilement approprié de matériaux dopés permettant, par application d’une tension électrique, de déplacer les électrons libres issus du dopage dans le réseau et le guide, ce qui modifie alors localement l’indice de réfraction et donc directement les conditions de guidage de la lumière par variation de phase. Différentes approches ont été présentées pour tenter d’ajuster la longueur d’onde de résonance du filtre spectral GMR : accumulation et déplétion des charges dans le réseau de diffraction, insertion d’une jonction P-N dans le guide d’onde, ...Enfin, une première brique pour l’intégration d’un T2SL dans un nano-résonateur optique pour réaliser un photodétecteur nanostructuré tout semi-conducteur a été étudiée. J’ai proposé́ la conception théorique de plusieurs nano-résonateurs intégrant un photodétecteur de type T2SL (filière InAs/GaSb). J’ai conçu trois architectures aux propriétés spectrales distinctes et qui diffèrent notamment par l’épaisseur de la couche de T2SL<br>The context of my thesis deals with infrared (IR) multispectral imaging and in particular with plasmonics, a field of electromagnetic optics whose the aim is to study and exploit surface waves existing at the interface between a metal and a dielectric. We seek to miniaturize optical functions thanks to nanotechnologies and more precisely to perform IR spectral filtering at the detection pixel level by integrating a nano-resonator. Usually we use dielectrics and metals, but the integration is complex. I am exploring the potential offered by heavily doped semiconductors to replace metals, which could allow better integration into technological processes for fabricate a photodetector or emitter. I use III-V semiconductors, compatible with the epitaxial growth of type 2 superlattice (T2SL) of long wave infrared photodetectors (LWIR). Furthermore, working with a heavily doped semiconductor offers the possibility of modifying the resonance frequency by adjusting the density of free carriers by the action of a potential difference.I study architectures of "GMR" components (Guided-Mode Resonance), usually formed by a waveguide in dielectric, where occurs the resonance, and a grating in dielectric or metal allowing the coupling between the incident or transmitted wave and the guided mode thanks to the ±1 orders diffracted by the grating in the thin layer. The current trend is to integrate these components directly at the level of the detection pixel but at the cost of numerous fabrication steps. I am studying the possibility of using exclusively semiconductors to simplify the fabrication process and allow monolithic integration of the filter into the detector. The waveguide consists of an intrinsic semiconductor and the grating of heavily doped semiconductor. The spectral range of interest is in the far infrared (8 μm - 14 μm).First, theoretical and experimental demonstrations of an all-semiconductor nano-structured spectral filter for infrared based on guided-mode resonance were carried out. I dimensioned and then fabricated a sample where the first step consists in depositing by epitaxy a layer of GaSb and a layer of highly doped InAsSb on a GaAs substrate before a photolithography step to define the mask of the etching reactive ionic etching in order to obtain the diffraction grating. An experimental work then made it possible to characterize the component (measurement under normal incidence, angular study, measurement at low temperature) with in particular the realization of an angular characterization setup.In parallel, I studied an appropriate stack of doped materials allowing, by applying an electrical voltage, to move the free electrons from doping in the grating and the guide, which then locally modifies the refractive index and therefore directly the conditions for guiding the light by phase variation. Different approaches have been presented in an attempt to adjust the resonance wavelength of the GMR spectral filter: accumulation and depletion of charges in the diffraction grating, insertion of a PN junction in the waveguide, ...Finally, a first brick for the integration of a T2SL in an optical nano-resonator to make an all-semiconductor nano-structured photodetector was studied. I proposed the theoretical design of several nano-resonators integrating a T2SL type photodetector (InAs/GaSb). I designed three architectures with distinct spectral properties, which differ in particular in the thickness of the T2SL layer

Since the first discovery of the photoelectric effect in anthracene, organic compounds have been studied as multi-functional materials because of their capability of showing a variety of properties such as charge transport, light absorption/emission, photoconductivity, electroluminescence and superconductivity. The work presented in this Ph.D. thesis aims at studying different classes of ?-conjugated organic materials that present functional properties suitable for the realization of opto-electronic devices. In particular we focus our attention on the two specific properties that are deeply correlated to the molecular arrangement in the realization of nano-scale multifunctional devices: charge transport and light emission. In the technologically appealing thin-films, the molecular arrangement is extremely sensitive to the deposition procedures and to the nature of the substrate. Thus, of great interest is the understanding at the micro- and nano-scale of the molecular architecture and morphological features which favour charge transport and/or energy transfer, in order to enhance performances of opto-electronic devices based on thin films. We show that in general linear ?-oligothiophenes can organize advantageously in thin-films in so as to guarantee the proper overlap between molecular orbitals which enables efficient field-effect charge transport. Introducing a new class of branched oligothiophenes (namely spider-like oligothiophenes) we aim at studying how complex 3D molecular architecture can affect optical emission, supermolecular organization and charge transport properties. Investigation on the enhancement of light emission efficiency of organic molecular systems is then carried out presenting a new host-guest lasing system obtained by co-evaporation of an oligo(9,9-diarylfluorene) derivative (T3, donor) with a well-known red-emitter dye (DCM, acceptor). In this binary blend, an efficient Förster energy transfer takes place from the T3 matrix to the dye molecules when the dye concentration is properly optimized. Moreover the mirrorless lasing measurements reveal that amplified spontaneous emission threshold of the 2% DCM:T3 sample is almost an order of magnitude lower than the 2% DCM:Alq3 model system measured in the same experimental conditions. The possibility of combining different functionalities in a single device is of great relevance for the further development of organic electronics in integrated components and circuitry. Organic light-emitting transistors (OLETs) have been demonstrated to be able to combine in a single device the electrical switching functionality of a field-effect transistor and the capability of light generation. When organic materials are implemented as active layers in device realization, interfaces formed by different materials are intrinsically important. The comprehension of the physics behind each interface is a crucial point to design new materials for device applications or to improve the performances of the existing ones Here we present a new approach for realizing ambipolar light-emitting transistor. In the heterostructure we propose the first layer and third layers are optimized for field-effect charge (electrons and holes) transport. The second layer is formed by a host-guest matrix with high optical performance and showing amplified spontaneous emission under optical pumping. The specificity of the presented tri-layer based OLET is the intrinsic separation of the charge transport region from the exciton formation region thus preventing completely the exciton-carrier quenching. The optimization of the charge transport and light emission properties allows the realization of a tri-layer heterojunction presenting balanced electron and hole mobility (~10-1-10-2 cm2/Vs), high charge carrier density in correspondence of the maximum electroluminescence emission (~ 1 KA/cm2) and intense light generation.<br>Fin dalla scoperta dell’effetto fotoelettrico nell’antracene, i composti organici sono stati studiati come materiali multifunzionali data la loro capacità di mostrare una varietà di proprietà differenti, come il trasporto di carica, emissione/assorbimento di luce, fotoconduttività, elettroluminescenza e superconduttività. Il lavoro presentato in questa tesi di dottorato si prefigge lo scopo di studiare differenti classi di materiali organici ? coniugati che presentino le proprietà funzionali adatte per la realizzazione di dispositivi optoelettronici. In particolare viene prestata particolare attenzione allo studio di due specifiche proprietà che sono profondamente connesse con l’organizzazione molecolare nei dispositivi multifunzionali con dimensioni nanometriche: il trasporto di carica e l’emissione di luce. Nei film sottili, univocamente considerati interessanti dal punto di vista tecnologico, l’organizzazione molecolare è fortemente dipendente dai processi di deposizione e dalla natura del substrato. Per aumentare le prestazioni dei dispositivi basati sui film sottili risulta fondamentale comprendere le strutture supermolecolari e le caratteristiche morfologiche su scala micro- e nanometrica che possono favorire il trasporto di carica e/o i processi di trasferimento di energia. Si dimostra che in generale gli oligotiofeni lineari depositati in film sottile possano organizzarsi vantaggiosamente in modo da garantire l’opportuna sovrapposizione tra gli orbitali molecolari che permette un efficiente trasporto di carica. Introducendo una nuova classe di oligotiofeni ramificati, denominati spider-like, ci proponiamo di studiare come una complessa architettura 3D possa modificare le proprietà di emissione, di organizzazione supermolecolare e di trasporto. Si procede quindi ad indagare la possibilità di aumentare l’efficienza di emissione di luce di sistemi organici molecolari mediante l’introduzione di un nuovo sistema host-guest con proprietà di lasing ottenuto sublimando un derivato diarilfluorenico (T3, donore) con una noto colorante emettitore nel rosso (DCM, accettare). In questa soluzione solida binaria, si verifica un efficiente trasferimento di energia alla Förster tra la matrice di T3 e le molecole di colorante quando la concentrazione di colorante viene opportunamente ottimizzata. Inoltre, la soglia di emissione spontanea amplificata del campione avente le molecole di DCM disperse al 2% in peso nel T3 risulta quasi un ordine di grandezza più bassa rispetto a quella del campione modello misurato nelle stesse condizioni sperimentali avente la stessa concentrazione in peso si molecole di DCM disperse in una matrice di Alq3. La possibilità di combinare diverse proprietà funzionali in un unico dispositivo risulta di notevole interesse per un ulteriore sviluppo dell’elettronica organica nei componenti integrati e nei circuiti. Si è dimostrato che i transistor organici ad emissione di luce sono capaci di combinare in un singolo dispositivo le proprietà di switch dei transistor ad effetto di campo con la capacità di generare luce. Quando i materiali organici vengono utilizzati come strati attivi nei dispositivi, le interfacce formate dai diversi materiali assumono un ruolo di primaria importanza. La comprensione dei processi fisici che avvengono ad ogni interfaccia è cruciale per disegnare nuovi materiali per dispositivi o per aumentare le prestazioni quelli già esistenti. In questo lavoro di tesi viene presentato un nuovo approccio per realizzare transistor ambipolari ad emissione di luce. Nell’eterogiunzione che viene proposta il primo e il terzo strato sono dedicati al trasporto di portatori di carica (elettroni e lacune) per effetto di campo mentre il secondo strato è formato da una soluzione solida host-guest che mostra efficiente emissione di luce ed emissione spontanea di luce se pompata otticamente. La specificità dell’approccio che presentiamo è che le regioni di trasporto di carica sono fisicamente separate da quella in cui avviene la formazione dell’eccitone. In questo modo viene ridotta completamente l’interazione tra l’eccitone e il portatore di carica. Dopo aver ottimizzato il trasporto di carica e le proprietà di emissione di luce, si è potuto realizzare un dispositivo basato sull’eterogiunzione a tre strati che presenta valori di mobilità per gli elettroni e le lacune bilanciati (~10-1-10-2 cm2/Vs), alta densità di portatori di carica in corrispondenza del massimo di elettroluminescenza (~ 1 KA/cm2) e intensa emissione di luce.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003.<br>Includes bibliographical references (leaves 67-69). Also available in electronic version. Access restricted to campus users.