Organic semiconductors are very attractive for thin film Organic Photodetectors (OPDs) since they possess a number of desirable attributes for optical sensing including high absorption coefficients over visible and ultraviolet wavelengths and compatibility with large-area deposition processes such as ink-jet, screen printing, and solution processing. OPDs, in general, utilize a vertical device architecture where the photoactive organic semiconductor layers are sandwiched between top and bottom electrodes that provide electrical contact. More recently, an interest in utilizing a lateral device architecture instead of the vertical one, has emerged. In this architecture, the two contacts are positioned on the two sides of the photoactive material with respect to the direction of the incoming signal, separated by a small gap. However, the factors governing lateral OPDs’ photo-response are still not well understood. In this thesis, we fabricate top-contact lateral OPDs using a thermal evaporation only fabrication process. We study the factors governing both the dark and photo currents of lateral OPDs. The effect of the wide gap between the two electrodes on the current-voltage characteristics is discussed and the role of space charge limited conduction is investigated. The contributions in the photoresponse of light scattering through the active layers as well as the back reflection of light at the metallic contacts are emphasized. The reproducibility over repeated operation cycles of both dark and photo currents values is explored. Exposure to light of the lateral OPD is found to lead to a significant increase in the dark current. The role of the conductivity enhancement in the channel due to light-induced trap filling is investigated. External quantum efficiency and detectivity estimates are given for deep ultraviolet lateral (DUV) OPDs. A comparison with vertical DUV OPDs performances is provided. Finally, the use of a phosphorescent sensitizer doped in the absorbing bottom layer to improve top-contact lateral OPDs efficiency is discussed.

Organic semiconductors are very attractive for thin film Organic Photodetectors (OPDs) since they possess a number of desirable attributes for optical sensing including high absorption coefficients over visible and ultraviolet wavelengths and compatibility with large-area deposition processes such as ink-jet, screen printing, and solution processing. OPDs, in general, utilize a vertical device architecture where the photoactive organic semiconductor layers are sandwiched between top and bottom electrodes that provide electrical contact. More recently, an interest in utilizing a lateral device architecture instead of the vertical one, has emerged. In this architecture, the two contacts are positioned on the two sides of the photoactive material with respect to the direction of the incoming signal, separated by a small gap. However, the factors governing lateral OPDs’ photo-response are still not well understood. In this thesis, we fabricate top-contact lateral OPDs using a thermal evaporation only fabrication process. We study the factors governing both the dark and photo currents of lateral OPDs. The effect of the wide gap between the two electrodes on the current-voltage characteristics is discussed and the role of space charge limited conduction is investigated. The contributions in the photoresponse of light scattering through the active layers as well as the back reflection of light at the metallic contacts are emphasized. The reproducibility over repeated operation cycles of both dark and photo currents values is explored. Exposure to light of the lateral OPD is found to lead to a significant increase in the dark current. The role of the conductivity enhancement in the channel due to light-induced trap filling is investigated. External quantum efficiency and detectivity estimates are given for deep ultraviolet lateral (DUV) OPDs. A comparison with vertical DUV OPDs performances is provided. Finally, the use of a phosphorescent sensitizer doped in the absorbing bottom layer to improve top-contact lateral OPDs efficiency is discussed.

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Diese Arbeit gibt einen tiefgehenden Einblick in verschiedene Aspekte von auf (In,Ga)N/GaN Heterostrukturen basierenden Leuchtdioden (LEDs), mittels Molekularstrahlepitaxie entlang der Achse von Nanodrähten (NWs) auf Si Substraten gewachsen. Insbesondere wurden die Wachstumsparameter angepasst, um eine Koaleszierung der Nanodrähte zu vermindern. Auf diese Weise konnte die durch die NW-LEDs emittierte Intensität der Photolumineszenz (PL) um einen Faktor zehn erhöht werden. Die opto-elektronischen Eigenschaften von NW-LEDs konnten durch die Verwendung von Indiumzinoxid, anstatt von Ni/Au als Frontkontakt, verbessert werden. Zudem wurde demonstriert, dass auch selektives Wachstum (SAG) von GaN NWs auf AlN gepufferten Si Substraten mit einer guten Leistungsfähigkeit von Geräte vereinbar ist und somit als Wegbereiter für eine neue Generation von NW-LEDs auf Si dienen kann. Weiterhin war es möglich, strukturierte Felder von ultradünnen NWs durch SAG und thermische in situ Dekomposition herzustellen. In den durch die NW-LEDs emittierten Elektrolumineszenzspektren (EL) wurde eine Doppellinenstruktur beobachtet, die höchstwahrscheinlich von den kompressiven Verspannungen im benachbarten Quantentopf, durch die Elektronensperrschicht verursachten, herrührt. Die Analyse von temperaturabhängigen PL- und EL-Messungen zeigt, dass Ladungsträgerlokalisierungen nicht ausschlaggebend für die EL-Emission von NW-LEDs sind. Die Strom-Spannungs-Charakteristiken (I-V) von NW-LEDs unter Vorwärtsspannung wurden mittels eines Modells beschrieben, in das die vielkomponentige Natur der LEDs berücksichtigt wird. Die unter Rückwärtsspannung aktiven Transportmechanismen wurden anhand von Kapazitätstransientenmessungen und temperaturabhänigigen I-V-Messungen untersucht. Dann wurde ein physikalisches Modell zur quantitativen Beschreibung der besonderen I-V-T Charakteristik der untersuchten NW-LEDs entwickelt. / This PhD thesis provides an in-depth insight on various crucial aspects of light-emitting diodes (LEDs) based on (In,Ga)N/GaN heterostructures grown along the axis of nanowires (NWs) by molecular beam epitaxy on Si substrates. In particular, the growth parameters are adjusted so as to suppress the coalescence of NWs; in this way the photoluminescence (PL) intensity emitted from the NW-LEDs can be increased by about ten times. The opto-electronic properties of the NW-LEDs can be further improved by exclusively employing indium tin oxide instead of Ni/Au as top contact. Furthermore, the compatibility of selective-area growth (SAG) of GaN NWs on AlN-buffered Si substrates with device operation is demonstrated, thus paving the way for a new generation of LEDs based on homogeneous NW ensembles on Si. Ordered arrays of ultrathin NWs are also successfully obtained by combining SAG and in situ post-growth thermal decomposition. A double-line structure is observed in the electroluminescence (EL) spectra emitted by the NW-LEDs; it is likely caused by compressive strain introduced by the (Al,Ga)N electron blocking layer in the neighbouring (In,Ga)N quantum well. An in-depth analysis of temperature dependent PL and EL measurements indicates that carrier localization phenomena do not dominate the EL emission properties of the NW-LEDs. The forward bias current-voltage (I-V) characteristics of different NW-LEDs are analysed by means of an original model that takes into account the multi-element nature of LEDs based on NW ensembles by assuming a linear dependence of the ideality factor on applied bias. The transport mechanisms in reverse bias regime are carefully studied by means of deep level transient spectroscopy (DLTS) and temperature dependent I-V measurements. The physical origin of the detected deep states is discussed. Then, a physical model able to describe quantitatively the peculiar I-V-T characteristics of NW-LEDs is developed.