Elektrische Charakterisierung hybrider PEDOT/ZnO- und PEDOT/GaN-Strukturen hergestellt mittels oxidativer chemischer Gasphasendeposition

Organic poly(3,4-ethylenedioxythiophene) (PEDOT) as well as the inorganic materials gallium nitride (GaN) and zinc oxide (ZnO) are established materials for electrical devices. Ideally, connecting those materials allows for the combination of the advantages whilst circumventing the challenges thus forming a good platform for the realization of technically complex devices like three-dimensional core-shell light-emitting devices (LEDs). So far, established deposition techniques like spincoating lack the ability to deposit homogeneous coatings on complex structures like inorganic nano- or micropillars. A promising deposition technique that fulfills these requirements is oxidative chemical vapor deposition (oCVD). oCVD allows for conformal and homogeneous polymer coatings on all substrates that are stable in vacuum independently of the individual complexity of the surface.
In this work, the electrical characteristics of hybrid PEDOT/GaN and PEDOT/ZnO structures are investigated. The PEDOT layer is deposited using oCVD. The thesis is subdivided into two main topics. Firstly, the parameters of the oCVD process will be examined, secondly, the electrical properties of the interfaces in question will be characterized. Concerning the oCVD process, the influence of parameters like deposition time, substrate temperature, and temperature of the oxidizing agent on reproducible PEDOT film thickness and lateral conductivity will be demonstrated. Longer deposition times, lower substrate temperatures, and higher temperatures of the oxidizing agent lead to thicker PEDOT layers. The largest impact to control the process has the substrate temperature. Higher substrate temperatures result in larger lateral conductivities which are in the order of several 100 S/cm for the deposited PEDOT films presented in this work. Depositing p-doped oCVD-PEDOT on n-doped substrates like n-ZnO or n-GaN results in an electrical diode characteristic in current-voltage measurements. The rectification ratio comprises up to eight orders of magnitude. The electrical characteristics of the hybrid interface show a great thermal (proven up to 150?C) and temporal stability of more than two years upon storage in darkness and at ambient conditions. Applying the Schottky model to the current-voltage characteristics of the hybrid interface allows for the deduction of ideality factors and electronic barrier heights. The ideality factors are below 2.0 and imply a low defect density at the interface. Mean barrier heights are in the order of 1.3 eV and 1.4 eV for PEDOT/ZnO and PEDOT/GaN interfaces, respectively. Ideality factors and mean barrier heights follow the predicted temperature dependency of a lateral inhomogeneous barrier height in agreement with the Schottky model. This supports the claim of metallic behavior of PEDOT and thermionic emission as the dominant conduction mechanism at the interface. The hybrid organic-inorganic interfaces can be applied for applications like hybrid LEDs as is shown in the outlook of this work for a planar device. The results, therefore, highlight a path to the design and construction of complex hybrid organic-inorganic devices like hybrid LEDs based on GaN micropillars.