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Thesis advisor: Pradip Bakshi / This thesis is a theoretical study of the electron transport and response properties of epitaxially grown, low-dimensional semiconductor quantum well heterostructures, under steady-state, current driven (nonequilibrium) conditions. These structures operate in the Terahertz (THz) frequency and submillimeter wavelength range, and are the leading candidates for compact, coherent sources of THz radiation. This work is divided into two parts: Part I consists of an analytical study of the individual quantum well units, and the tunneling transmission characteristics, for which reasonably accurate algebraic expressions are obtained. An underlying philosophy of this work is the desire to describe each of the key components involved, independently, through these simple analytical expressions. In Part II the numerical study of the transport and radiation response of the quantum well structures specially designed to generate THz radiation based on the plasma instability concept is presented. Several models are proposed which describe the overall electron transport and which determine the underlying nonequilibrium steady state. In particular, the key features of the experimental current-voltage (IV) curves for such structures are explained, and the corresponding response properties are determined. The modeling and simulation of these potential optoelectronic devices is a crucial tool for elucidating the precise mechanisms and interplay of the many microscopic processes which give rise to the observed behavior. Key features of the radiation response arise from the intersubband plasma instability which occurs due to the resonant interaction of an emission and an absorption mode, and these features are compared with the experimental observations. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Im Rahmen der Dissertation wurden molekularstrahlepitaktische Verfahren zur Züchtung von Hetero-und Quantenstrukturen auf der Basis der Gruppe II-Oxide entwickelt. Insbesondere wurde ein Wachstumsregime weit entfernt vom thermischen Gleichgewicht etabliert, welches die Mischung von CdO und MgO mit ZnO in phasenreiner Wurtzitstruktur ermöglicht, wobei die Gleichgewichtslöslichkeitsgrenzen dramatisch überschritten werden. In den Mischkristallen kann die Bandlücke kontinuierlich von 2.2 bis 4.4 eV eingestellt werden. Das Wachstum verläuft in einem zweidimensionalen Modus und resultiert in atomar glatten Ober- und Grenzflächen. Ausgeprägte RHEED- Intensitätsoszillationen erlauben die atomlagengenaue Kontrolle der Schichtdicken und somit die Realisierung wohl-defi- nierter Einzel- und Mehrfachquantengrabenstrukturen. Diese zeichnen sich durch eine hohe Photolumineszenzquantenausbeute im gesamten sichtbaren Spektralbereich aus. Laseraktivität kann vom UV bis zum grünen Wellenlängenbereich bei Zimmertemperatur erzielt werden. Das Potenzial dieser Quantenstrukturen in Hinblick auf ihre Anwendung in opto-elektronischen Bauelementen wird diskutiert. / This work focuses on the development of the novel growth approaches for the fabrication of Group II-oxide materials in the form of epitaxial films and heterostructures. It is shown that molecular-beam epitaxial growth far from thermal equilibrium allows one to overcome the standard solubility limit and to alloy ZnO with MgO or CdO in strict wurtzite phase up to mole fractions of several 10 %. In this way, a band-gap range from 2.2 to 4.4 eV can be covered. A clear layerby- layer growth mode controlled by oscillations in reflection high-energy electron diffraction makes it possible to fabricate atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition range. On appropriately designed structures, laser action from the ultraviolet down to green wavelengths and up to room temperature is achieved. The properties and potential of the "state-of-the-art" materials are discussed in relation to the advantages for their applications in various optoelectronic devices.