Transition Metal Oxides for Infrared Optoelectronics

Plasmonics, a field of study in optoelectronics, describes the interaction of free charge carriers in conductors with electromagnetic radiation. Under certain illumination conditions, free charge carriers can become resonant with the electric field component of the light used. This resonance results in a bound electromagnetic wave with unique optical properties that can be utilized for a variety of sensing, spectroscopy and optoelectronic applications.;Historically, plasmonic phenomena were restricted to metallic material systems such as gold and silver, and the light used to excite plasmonic phenomena was part of the UV-VIS energy range of the electromagnetic spectrum. In recent years however, a growing interest in lower energy plasmonics, particularly at infrared energies, emerged from the plasmonics community. Migrating plasmonic phenomena to the infrared regime will enable new technologies such as novel infrared emitters and detectors, advanced IR-spectroscopy techniques, heat assisted data storage or heat scavenging.;Facilitating this transition however will require advances in materials development. The plasmonic host materials employed for UV-VIS applications, in particular the noble metals, are inept to support low loss plasmonics at infrared energies. Novel plasmonic host materials, preferably with tunable properties to cover a wide range of resonance energies, are needed.;This dissertation explores the use of transition metal oxides as plasmonic materials for the mid-infrared (2-10 mum wavelength) energy range. In particular, two material systems, ZnO and CdO, are studied and their applicability towards mid-IR plasmonics is assessed. Additionally, a simulation method to predict plasmonic properties of arbitrary materials has been developed in combination with an infrared spectroscopy technique that allows to experimentally interrogate the plasmonic properties of thin film samples. These techniques are complimentary and yield directly comparable results allowing for an efficient testing of theory against experiment.;The study of heavily doped zinc oxide led to an important proof of concept, demonstrating that wide band gap conducting oxides are indeed applicable for plasmonics in the mid-infrared. However, the ZnO material system revealed the limiting factors present in most transition metal oxides. At the high doping levels needed to support mid-infrared plasmons (>1x1020 cm-3 ) the transport properties degrade. Charge carrier mobilities are low which translates into high optical losses for plasmonic applications. A material that combines high charge carrier concentrations with high carrier mobilities is needed for optimal performance in the infrared. Scattered literature reports suggested that doped CdO might exhibit this rare combination of transport properties.;This lead to the development of a MBE based deposition technique for CdO doped with dysprosium (CdO:Dy). Ideal transport properties for mid-IR plasmonics were found and the material was thoroughly characterized in its structural, transport, optical, and thermal properties. CdO:Dy can sustain extremely large room temperature electron mobilities of >300 cm2 V--1s--1 for a free carrier concentration range of 8x1019-5x1020 cm-3. A crystal defect based model was proposed to describe the structure/property relations leading to the unusual transport properties of CdO:Dy, and the model was tested experimentally. Complimentary ab initio calculations using density functional theory (DFT) were performed for additional verification of the proposed mechanism.;The final part of this thesis describes the application of thin films of CdO:Dy to investigate the absorption mode mixing of plasmons with gas absorptions in the infrared. A significantly increased infrared absorption is found when these two resonant phenomena are combined. Based on the modeling technique developed for this work, the enhancement mechanism was identified and theoretically described. The plasmonic mode-mixing enhancement compares favorably to conventional enhanced infrared absorption measurement schemes based on thin noble metal films. The mode-mixing enhancement infrared absorption effect represents the first step towards technological application using the materials and methods developed in this work.