Cette thèse concerne l’étude des photodétecteurs à base de matériauxsemi-conducteurs organiques (OPDs) sensibles à la lumière verte. Ces travaux sedivisent en cinq parties distinctes. Tout d’abord, une recherche bibliographique suiviedes notions fondamentales sur les matériaux et le fonctionnement des dispositifsorganiques est présentée. Ensuite, un travail sur la méthodologie concernant lafabrication de bancs expérimentaux et sur la métrologie garantissant l’exactitude desdonnées expérimentales sont effectués. Les méthodes de fabrication etd’optimisation, notamment le dépôt par sérigraphie, sont également présentées. Lapartie expérimentale concerne l’étude de l’origine du courant d’obscurité dans lesstructures organiques à base des matériaux donneur et accepteur d’électrons, laconception et la caractérisation d’un photodétecteur organique à l’état de l’art dontles performances optoélectroniques sont proches des dispositifs inorganiques baséssur la technologie silicium. Enfin, l’étude des mécanismes de dégradation d’uncapteur organique est présentée mettant en avant le rôle de l’oxygène et l’humidité. / This thesis deals with the study of photodetectors based on organicsemiconductor materials (OPDs) that are sensitive to green light. There are five partsto this study. First, a bibliographic study following the fundamentals of organicmaterials and the working principle of organic photodetectors is presented. Then, anextended study on the methodology and metrology is detailed, which was carried outin order to design and fabricate new optoelectronic instruments that are able tocharacterize organic devices accurately. Fabrication and optimization steps oforganic photodetectors are detailed. The experimental section concerns the study onthe origin of the dark current in organic devices based on electron donor/acceptorsystems. A choice of materials is discussed and a full characterisation of state-of-theartorganic photodetectors is presented in detail. The measured performances wereclose to the those of inorganic sensors based on silicon technology. Finally, a studyof degradation mechanisms is presented which highlights the role of oxygen andmoisture.

<p>This master’s thesis deals with studies of lateral and vertical carrier transport Dot-in- </p><p>a-Well (DWELL) Quantum Dot Infrared Photodetectors (QDIPs). During the pro ject, </p><p>devices have been developed and tested using a Fourier Transform Infrared (FTIR) spec- </p><p>trometer with the purpose to find the processes governing the flow of photocurrent in </p><p>the different kinds of detectors, the dark current magnitude in the vertical Quantum Dot </p><p>Infrared Photodetector (QDIP) and the Quantum Well Infrared Photodetector (QWIP) </p><p>and the light polarization dependences for the vertical QDIP and the QWIP. </p><p>The lateral carrier transport DWELL QDIP was found to have poor conduction </p><p>in the well mainly due to re-trapping of electrons in this region. The main process gov- </p><p>erning the flow of photocurrent for this type of device at 77K is photo-excitation from </p><p>the Quantum Dot (QD)s to the excited state in the Quantum Well (QW) and further </p><p>thermal excitation. If the electrons are mainly transported in the matrix or the well at </p><p>77K is presently not clear. </p><p>For the vertical carrier transport DWELL QDIP at 77K, the wavelength response </p><p>could be tuned by altering the applied voltage. At higher voltages, the dominant process </p><p>was found to be photo-excitation from the QDs to the excited state in the QW followed </p><p>by thermal assisted tunneling into the GaAs-matrix. At lower voltages, photo-excitation </p><p>from the QDs directly into the the GaAs-matrix was the predominant process. The dark </p><p>current level in the vertical QDIPs was found to be 1.5 to 5 orders of magnitude smaller </p><p>than for the QWIP measured at 77K. Furthermore, the QDIP was found to be close to </p><p>polarization independent. As expected the QWIP had a reduced sensitivity to normal </p><p>incident light. The existence of this signal was attributed to interface scattering of light </p><p>inside the device.</p>

Amorphous selenium based digital radiography has attracted much attention because of selenium's high X-ray absorption and excellent charge transport properties, and the ability to be created thick (typically 100 to 1000 micron) uniform layers over a large area (typically 30 cm X 30 cm) at low processing temperatures (typically at around 50 degree C substrate temperature). In this work, the excess noise in amorphous selenium has been studied. A number of device parameters were altered to study the noise characteristics, such as the metal of the electrodes, bulk material composition, device volume, surface conditions and substrate temperature. All the samples had a transverse geometry with 20 to 200- micron thick layers of amorphous selenium electroded with metal at the top and at the bottom. Sample devices were fabricated by conventional vacuum deposition.<p> Noise power was measured over a limited bandwidth of 1 kHz. The fluctuations for one sample amounted to 1% of the bias current. The excess noise was mainly 1/<i>f</i> noise with the slope ranging from -0.77 to -1.4. Interpretation of the noise spectra was complicated due to the samples' highly non-linear I-V relation and long time transients.<p> The metals of the electrode clearly showed a large effect on both the magnitude and shape of the noise spectrum. Of the metals studied, aluminum produced the least normalized noise and platinum the most. The addition of arsenic caused a decrease in the normalized noise. An additional 0.2% (% wt.) arsenic decreased the 1/<i>f</i> noise magnitude by more than a decade, but did not change the slope. The addition of chlorine did not affect the noise magnitude. Amorphous selenium is quite vulnerable to stress and in particular, external mechanical stress causes crystallization. The surface of the sample was gently abraded, applying the least possible amount of stress to the selenium layer. A change in the surface condition before the top electrode was deposited showed that a roughened surface decreased the noise magnitude substantially. These results strongly indicate that the noise is controlled by the metal-semiconductor interface.<p> Noise characteristics in multilayered samples were examined. The p-i-n and n-i-p structures consisted of 200 micron i-layer with 2 to 6 micron p- and n-like layers. The noise fluctuation in the current are typical of 1/<i>f</i> noise showing a power-law spectrum with slopes between -0.9 to -1.1. These samples showed a substantial decrease in the noise power compared to single layer samples; the additional n-like and p-like layers acted as carrier sources so that the current was not controlled by the metal interface. Hence, the measurements are closer to the intrinsic noise of a-Se. After exposure to 14 R (Roentgen) of X-rays, the normalized noise decreased by a factor of 1.6 for the n-i-p structure.

This master’s thesis deals with studies of lateral and vertical carrier transport Dot-in- a-Well (DWELL) Quantum Dot Infrared Photodetectors (QDIPs). During the pro ject, devices have been developed and tested using a Fourier Transform Infrared (FTIR) spec- trometer with the purpose to find the processes governing the flow of photocurrent in the different kinds of detectors, the dark current magnitude in the vertical Quantum Dot Infrared Photodetector (QDIP) and the Quantum Well Infrared Photodetector (QWIP) and the light polarization dependences for the vertical QDIP and the QWIP. The lateral carrier transport DWELL QDIP was found to have poor conduction in the well mainly due to re-trapping of electrons in this region. The main process gov- erning the flow of photocurrent for this type of device at 77K is photo-excitation from the Quantum Dot (QD)s to the excited state in the Quantum Well (QW) and further thermal excitation. If the electrons are mainly transported in the matrix or the well at 77K is presently not clear. For the vertical carrier transport DWELL QDIP at 77K, the wavelength response could be tuned by altering the applied voltage. At higher voltages, the dominant process was found to be photo-excitation from the QDs to the excited state in the QW followed by thermal assisted tunneling into the GaAs-matrix. At lower voltages, photo-excitation from the QDs directly into the the GaAs-matrix was the predominant process. The dark current level in the vertical QDIPs was found to be 1.5 to 5 orders of magnitude smaller than for the QWIP measured at 77K. Furthermore, the QDIP was found to be close to polarization independent. As expected the QWIP had a reduced sensitivity to normal incident light. The existence of this signal was attributed to interface scattering of light inside the device.

Amorphous selenium based digital radiography has attracted much attention because of selenium's high X-ray absorption and excellent charge transport properties, and the ability to be created thick (typically 100 to 1000 micron) uniform layers over a large area (typically 30 cm X 30 cm) at low processing temperatures (typically at around 50 degree C substrate temperature). In this work, the excess noise in amorphous selenium has been studied. A number of device parameters were altered to study the noise characteristics, such as the metal of the electrodes, bulk material composition, device volume, surface conditions and substrate temperature. All the samples had a transverse geometry with 20 to 200- micron thick layers of amorphous selenium electroded with metal at the top and at the bottom. Sample devices were fabricated by conventional vacuum deposition.<p> Noise power was measured over a limited bandwidth of 1 kHz. The fluctuations for one sample amounted to 1% of the bias current. The excess noise was mainly 1/<i>f</i> noise with the slope ranging from -0.77 to -1.4. Interpretation of the noise spectra was complicated due to the samples' highly non-linear I-V relation and long time transients.<p> The metals of the electrode clearly showed a large effect on both the magnitude and shape of the noise spectrum. Of the metals studied, aluminum produced the least normalized noise and platinum the most. The addition of arsenic caused a decrease in the normalized noise. An additional 0.2% (% wt.) arsenic decreased the 1/<i>f</i> noise magnitude by more than a decade, but did not change the slope. The addition of chlorine did not affect the noise magnitude. Amorphous selenium is quite vulnerable to stress and in particular, external mechanical stress causes crystallization. The surface of the sample was gently abraded, applying the least possible amount of stress to the selenium layer. A change in the surface condition before the top electrode was deposited showed that a roughened surface decreased the noise magnitude substantially. These results strongly indicate that the noise is controlled by the metal-semiconductor interface.<p> Noise characteristics in multilayered samples were examined. The p-i-n and n-i-p structures consisted of 200 micron i-layer with 2 to 6 micron p- and n-like layers. The noise fluctuation in the current are typical of 1/<i>f</i> noise showing a power-law spectrum with slopes between -0.9 to -1.1. These samples showed a substantial decrease in the noise power compared to single layer samples; the additional n-like and p-like layers acted as carrier sources so that the current was not controlled by the metal interface. Hence, the measurements are closer to the intrinsic noise of a-Se. After exposure to 14 R (Roentgen) of X-rays, the normalized noise decreased by a factor of 1.6 for the n-i-p structure.

Quantum well infrared photodetectors (QWIP) utilize quantum wells of large bandgap materials to detect infrared radiation. When compared to conventional low bandgap LWIR photodetectors, the QWIP technology offers largest format thermal imagers with much better uniformity. The theoretical part of this study includes the development of a QWIP ensemble Monte Carlo simulator. Capture paths of electrons to quantum wells are simulated in detail. For standard AlGaAs/GaAs QWIPs, at medium and high E-fields L valley quantum well (QW) is a trap for electrons which causes higher capture probability when compared with InP/InGaAs and GaAs/InGaAs QWIPs. The results suggest that high photoconductive gain observed in InP/InGaAs and GaAs/InGaAs QWIPs is not due to good transport properties of binary barrier material but due to higher &amp / #61511 / -L valley energy separation. The experimental part of the study includes the fabrication and characterization of InP/InGaAs and InP/InGaAsP QWIPs and 640x512 FPAs with the main objective of investigating the feasibility of these material systems for QWIPs. The InP/InGaAs and InP/InGaAsP QWIP detectors showed specific detectivity values above 1x1010 cm.Hz1/2/W (70K, f/2, background limited). The devices offer higher allowable system noise floor when compared with the standard AlGaAs/GaAs QWIP technology. It is also experimentally shown that for strategic applications LWIR InP based QWIPs have advantages over the standard QWIP technology. The InP/InGaAs 640x512 QWIP FPA reached 36 mK average NETD value at 70 K with f/1.5 optics and 10 ms integration time. The InP/InGaAsP QWIP on the other hand yielded 38 mK NETD histogram peak at 70 K with f/1.5 optics and 5 ms integration time on 320x256 window of the 640x512 FPA.

Quantum Well Infrared Photodetectors (QWIPs) are strong competitors to other detector technologies for future third generation thermal imagers. QWIPs have inherent advantages of mature III-V material system and well settled fabrication technology, as well as narrow band photo-response which is an important property facilitating the development of dual-band imagers with low crosstalk. This thesis focuses on the development of long/mid wavelength dual band QWIP focal plane arrays (FPAs) based on the AlGaAs/GaAs material system. Apart from traditional single band QWIPs, the dual-band operation is achieved by proper design of a bias tunable quantum well structure which has two responsivity peaks at 4.8 and 8.4 um for midwave infrared (MWIR) and longwave infrared (LWIR) atmospheric windows, respectively. The fabricated large format (640x512) FPA has MWIR and LWIR cut-off wavelengths of 5.1 and 8.9 um, and it provides noise equivalent temperature differences (NETDs) of ~ 20 and 32 mK (f/1.5 at 65 K) in these bands, respectively. The employed bias tuning approach for the dual-band operation requires the same fabrication steps established for single band QWIP FPAs, which is an important advantage of the selected method resulting in high-yield, high-uniformity and low-cost. Results are encouraging for fabrication of low cost, large format, and high performance dual band FPAs, making QWIP a stronger candidate in the competition for third generation thermal imagers

Zinc oxide, with its direct wide bandgap and high exciton binding energy, is a promising material for optoelectronic devices. Quantum confinement effect and high surface to volume ratio of the nanowires imparts unique properties to them and makes them appealing for researchers. So far, zinc oxide nanowires have been used to fabricate various optoelectronic devices such as light emitting diodes, solar cells, sensors and photodetectors. To fabricate those optoelectronic devices, many different synthesis methods such as metal organic chemical vapor deposition, chemical vapor deposition, pulsed laser deposition, electrodeposition and hydrothermal method have been explored. Among them, hydrothermal method is the most feasible one in terms of simplicity and low cost. In this thesis, hydrothermal method was chosen to synthesize zinc oxide nanowires. Synthesized zinc oxide nanowires were then used as electrically active components in light emitting diodes and ultraviolet photodetectors. Hybrid light emitting diodes, composed of inorganic/organic hybrids are appealing due to their flexibility, lightweight nature and low cost production methods. Beside the zinc oxide nanowires, complementary poly [2- methoxy -5- (2- ethylhexyloxy) - 1,4 -phenylenevinylene] MEH-PPV and poly (9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) hole conducting polymers were used to fabricate hybrid light emitting diodes in this work. Optoelectronic properties of the fabricated light emitting diodes were investigated. Zinc oxide emits light within a wide range in the visible region due to its near band edge and deep level emissions. Utilizing this property, violet-white light emitting diodes were fabricated and characterized. Moreover, to take advantage over the responsivity of zinc oxide to ultraviolet light, ultraviolet photodetectors utilizing hydrothermally grown zinc oxide nanowires were fabricated. Single walled carbon nanotube (SWNT) thin films were used as transparent electrodes for the photodetectors. Optoelectronic properties of the transparent and flexible devices were investigated. A high on-off current ratio around 260000 and low decay time about 16 seconds were obtained. Results obtained in this thesis reveal the great potential of the use of solution grown zinc oxide nanowires in various optoelectronic devices that are flexible and transparent.

Nanomaterials are widely investigated by researches and because of their unique properties they have been utilized in many different devices. Nanowires are one of these materials which show deviated mechanical, chemical, physical and optical, properties from their bulk counterparts. These deviations in properties of the nanowires are based on both their high surface to volume ratio and quantum confinement effect. Lately optical properties of nanowires have received great attention as they also exhibit good light sensitivity. Germanium is a semiconductor, which has been used widely as an active material in infrared light detectors. Due to excellent light detection of germanium its nanostructures have also been widely studied in optoelectronic devices. Germanium nanowires have been used in many devices such as field effect transistors, diodes, field emitters and photodetectors. Synthesis of high quality and high aspect ratio germanium nanowires could make important contributions to these devices. There are several synthesis methods for germanium nanowires. These are electrochemical etching, solvothermal, supercritical v fluidic, laser ablation, chemical vapor deposition and vapor transport methods. Among these methods, high quality, single crystalline, defect free germanium nanowires using accessible solid powder precursors could be synthesized with vapor transport method. In the first part of this thesis, germanium nanowire growth with vapor transport method is investigated. One of the most advantageous features of this method is using solid powder precursors instead of toxic gases. Until now, three different kinds of solid germanium precursors have been reported in vapor transport method, all of them are investigated and the resulting nanowires are compared in this thesis. Vapor transport method enables high control over the morphology of the nanowires. The most important parameters which affect the morphology of the nanowires are temperature, pressure and precursor type. Therefore, a detailed parametric study is provided based on these parameters and their effect on the final diameter of the nanowires is determined. The as &ndash / synthesized nanowires contain a very thick oxide layer on their surface. Therefore, oxide removal with acid etching is also investigated in this thesis. In the second part of this thesis, utilization of the germanium nanowire networks in fully transparent, flexible and network enhanced photodetectors is investigated. In order to obtain a germanium nanowire network, the as-synthesized nanowires are transferred from growth substrate to the device substrate by sonication and vacuum filtration. Silver nanowires and single walled carbon nanotubes are used as fully transparent electrodes. Both rigid and flexible photodetectors are fabricated and their current-voltage characteristics and photoresponse behaviors with different germanium nanowire densities are determined.

Visible-blind and solar-blind ultraviolet photodetectors based on GaN/AlGaN were designed, fabricated, and characterized for commercial and military applications. High performance back-illuminated solar-blind MSM achieved external quantum efficiency of ~48%. The dark current of 40x40μm MSM was less than the instrument measurement limitation of 20fA for a bias <100V. No photoconductive gain was observed. With an n-type doped high-Al ratio "window" Al₀.₆Ga₀.₄N layer, back-illuminated solar-blind p-i-n photodiode achieved a quantum efficiency of ~55% at zero-bias. Absorption edge study of both MSM and p-i-n photodetectors, based on device spectral responses, resulted in a performance comparison of MSMs and p-i-ns, as the solar-blind photodetection requires a sharp solar-blind rejection. Photoconductive detectors and avalanche photodetectors, with the internal gain advantage, have been discussed as well. A 30μm diameter GaN avalanche photodiode achieved a gain >23, with a dark current less than 100pA. The breakdown showed a positive temperature coefficient of 0.03 V/K that is characteristic of avalanche breakdown. SiC APDs, as candidates for visible-blind applications, have been designed, fabricated and characterized. An avalanche gain higher than 10⁵, with a dark current less than 1nA, showed the potential of SiC APD replacing PMTs for high sensitivity visible-blind UV detection. A silicon-based optical receiver has been presented in the Appendix. With the photodiode internal avalanche gain ~4, a sensitivity ~-6.9dBm at 10Gbps has been achieved. / text