Interferometry and holography are two well-known methods for measuring distances, positions, vibrations, index of refraction etc. In these methods a coherent light source is used to create interference between different parts of the light. Since the wavelength of the light is used as reference it is possible to achieve very good accuracy in the measurements. The need of small and cheap light sources for these applications is large and an interesting alternative would be to use ordinary semiconductor diode lasers. These are unfortunately not designed to give sufficiently good coherence. In this Master Thesis work investigations of how the coherence of semiconductor diode lasers is affected by changes in temperature, injection current and between different individuals are performed. A Michelson interferometer is used to create an interference pattern where the contrast then can be analyzed. The contrast is related to the coherence of the laser, i.e., good coherence will give high contrast. The results show that in order to drive the laser in a stable way it is better to hold the temperature constant and varying the injection current until the wanted output power is achieved instead of doing the opposite. The results also indicate that the best coherence is achieved for low temperatures (around 10 OC) and high injection currents (around 80 mA). During these conditions a contrast of 70 % -80 % is achieved. The result of this Master Thesis work gives a hint on how to run a semiconductor diode laser in a stable way.

No description available.

This thesis is concerned with the preparation of organic light-emitting diodes (LEDs) by using different thin film technologies: the Langmuir-Blodgett (LB) technique; spin-coating and thermal evaporation. The π-conjugated polymer, poly(2-methoxy-5-(5'-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), was used as the emissive layer and was deposited onto patterned indium-tin oxide (ITO) glass using the LB technique or spin-coating. Y-type LB films of MEH-PPV were deposited at a surface pressure of 17 mN m(^-1) with a transfer ratio of 0.95 ± 0.03. Many efforts were made to improve the LB film device performance parameters, such as external quantum efficiency and operating lifetime, by inserting an electron transporting or insulating layer between the emissive layer and top cathode. Annealing the LB films was found to result in an improved operating lifetime. LEDs based on spun films possessed higher external quantum efficiencies than devices made from LB films. The more ordered LB films had a higher probability of intra- and intermolecular interactions and formed more excimer states within the structure. This led to a lower quantum efficiency compared to devices incorporating spun films. The operating lifetime of the LEDs was highly dependent on the morphology of the film surface. A smoother film surface is required for a longer device operating lifetime. A new electron transporting material, 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]pyridine (PDPyDP), was deposited on top of the MEH-PPV spun film. Despite a high external quantum efficiency of 0.7 %, this device suffered from dark regions in the electroluminescence output resulting in degradation of the device. The dark area formation was attributed to delamination of the aluminium electrode from the PDPyDP layer, which was lessened by: (a) annealing the degraded devices; (b) evaporating a thicker aluminium layer at a high rate and (c) inserting a buffer layer (Alq(_3)) between the PDPyDP and the Al top electrode.

No description available.

The charge storage in the a-Si:H PIN photodiode under different light wavelength illuminations has been studied. The leakage current-voltage and capacitance-voltage curves under three fundamental visible light wavelengths, i.e., red, green and blue light were measured. The apparent charge storage density in the negative voltage range was quantified from the capacitance-voltage curve; charges in the positive voltage range were estimated from the leakage current-voltage curve. The measurement was verified with a pre-fabricated circuit which is a charge storage readout device. The diode under the long wavelength light illumination condition stored more charges than that under the short wavelength light illumination condition because the former could penetrate the intrinsic a-Si:H layer deeper than the latter could. The leakage current and charge storage capacity of the diode are determined by the generation of electron-hole pairs and the depletion of charges in the intrinsic layer as well as the supply of charges from the electrodes. A comparison of charge storage capacities of diodes with different intrinsic layer thicknesses is also presented. The number of photogenerated carriers increases with the thickness of the i-layer due to the long penetration depth, but the junction capacitance decreases which results in the decrease of the charge storage capacity. The tradeoff between the photogenerated carriers and the capacitance, combined with thickness-dependent recombination mechanisms increases the complexity of the PIN diode charge storage capacity. The n+- and p+-contact region should be heavily doped so that the storage charge can be confined in the i-layer without diffusing and recombining in the contact region. The n+ and p+ films, prepared by plasma enhanced chemical vapor deposition (PECVD) of a wide range of doping concentration, were fabricated to achieve low bulk resistivity and ohmic contacts with the metal electrodes. Charge storage density was improved after the optimization doped layers in both positive gate voltage and negative gate voltage. The low resistivity contact layers, reduced density of state in the intrinsic layer, and graded p+/i layer account for the enhancement of the charge storage density in the optimized diode.

Chemnitz, Techn. Univ., Diss., 2006.

Darmstadt, Techn. Univ., Diss., 2004.

Ces dernières années, la consommation énergétique n’a cessé de s’accroitre. Pour contrer cela, les systèmes de conversion électrique se doivent d’être plus performants, d’améliorer les rendements tout en limitant l’encombrement des systèmes, réduisant ainsi les matières premières utilisées. Parmi les systèmes de conversion efficaces, le correcteur du facteur de puissance et sa diode "boost" assurent un rendement de conversion d’environ 95 %. Ces travaux de thèse s’intéressent à la réalisation de ce type de diode en utilisant un matériau différent du traditionnel silicium, le nitrure de gallium (GaN), ce qui pourrait permettre d’atteindre des rendements de conversion supérieurs à 98 %. Le GaN est épitaxié sur Si et des diodes Schottky sont réalisées à sa surface. Des protocoles expérimentaux ont été mis en place en vue d’optimiser leurs performances, notamment la réduction des densités de courant de fuite. / These past few years, energy consumption has continued to increase. To counter this, the power conversion systems need to be more efficient, to improve yields while limiting systems volume, which reduce raw materials. Among efficient conversion systems, the power factor corrector and its "boost diode" provide a conversion efficiency of about 95%. This work concerned with the realization of this type of diode using a different material from the traditional silicon, the gallium nitride (GaN), which could allow higher conversion efficiencies of 98% to be achieved. GaN is grown up on Si and Schottky diodes are made on its surface. Experimental protocols were implemented in order to optimize their performance, especially the reduction of leakage current densities. Two research areas were defined: surface treatments and the realization of the guard ring. Our results showed that the different surface preparations either by wet or plasma can reduce leakage current.

No description available.

Le carbure de silicium (SiC) possède plusieurs propriétés exceptionnelles comme une large bande interdite, un champ électrique critique et une vitesse de saturation des porteurs élevée pour remplacer le silicium (Si) dans des domaines de fonctionnement jusque-là inaccessibles avec le Si. Un nombre important de démonstrateurs des composants de puissance en SiC faisant état de performances remarquables ainsi que la disponibilité commerciale des composants en SiC confirment la maturité de la filière SiC et montrent les progrès technologiques réalisés au cours des dernières années. Cependant, il existe peu d’études sur les coefficients d’ionisation du SiC, lesquels sont pourtant indispensables pour prévoir précisément la tenue en tension des composants de puissance en SiC. Ce travail contribue donc à mieux déterminer ces coefficients. Pour cela, un bon nombre de diodes spécialement conçues pour la détermination des coefficients d’ionisation du SiC par la technique OBIC (Optical Beam Induced Current) ont été réalisées sur différents wafers de SiC-4H et de SiC-6H, deux polytypes courant du SiC. Cette technique repose sur un faisceau de laser ultraviolet qui génère des paires électrons-trous dans la zone de charge d’espace d’une diode sous test. La mesure du courant résultant permet d’accéder aux coefficients d’ionisation. A partir des mesures OBIC sur les diodes réalisées, nous avons pu déduire les coefficients pour ces deux polytypes du SiC. Plus particulièrement, les coefficients d’ionisation du SiC-4H sont déterminés dans une large gamme de champ électrique grâce aux mesures sur les différents dopages. Les paramètres des coefficients déterminés dans ce travail peuvent être utilisés en conception de dispositifs haute tension pour prédire plus précisément l’efficacité de leur protection périphérique. / Silicon carbide (SiC) has several exceptional properties as a wide band-gap, a high critical electric field and a high saturation velocity of carriers to replace silicon (Si) in the applications previously inaccessible with Si. A significant number of SiC power devices showing outstanding performances and the commercial availability of SiC devices confirm the maturity of SiC industry and show the SiC technological advances in recent years. However, there are few studies on the ionization coefficients in SiC, which nevertheless essential to accurately predict the breakdown voltage of SiC power devices. This work contributes to better determine these coefficients. For this, numerous diodes which are specifically designed for the determination of ionization coefficients in SiC by using OBIC (Optical Beam Induced Current) technique were realized on different wafers of 4H-SiC and 6H-SiC, two usual polytypes of SiC. This technique relies on an ultraviolet laser beam which generates electron-hole pairs in the space charge region of a diode under test. The resulting current measurement provides access to the ionization coefficients. From OBIC measurements performed on the diodes, we were able to deduce the ionization coefficients for the both polytypes of SiC. In particular, the ionization coefficients for 4H-SiC are determined in a wide range of electric field through measurements on devices with different doping level. The parameters of ionization coefficients determined in this work can be used in design of high voltage devices to predict more accurately the efficiency of periphery protections.