Optical and transport properties of GaN/(Al,Ga)N heterostructures in prospect of infrared unipolar devices

Over the past decade, a growing interest appeared for III-nitride semiconductors, in view of their potential applications in intersubband (ISB) devices. If these materials are nowadays famous, particularly for having revolutionized domestic lighting thanks to the GaN-based white light emitting diodes, they also feature interesting properties that make them good candidates for infrared (IR) devices. Indeed, their large conduction band offsets (» 1.8 eV for the AlN/GaN system) enable near-IR ISB applications, at telecommunication wavelengths. Moreover, their large LO-phonon energy, makes them good candidates for devices operating at room temperature in the THz range. They also exhibit ultrafast ISB recombination dynamics, and therefore are promising for high frequency optoelectronic devices. In the first part, this work presents results obtained on near and mid IR quantum cascade detectors (QCDs). Those devices are a first good step towards quantum cascade lasers (QCLs), as they possess similar multi quantum well (MQW) structures, and similar transport mechanisms. GaN-based QCDs, grown by plasma-assisted molecular beam epitaxy (MBE) have already been demonstrated in the near-IR. In this work, the feasibility of such structures using an NH3-source MBE system is demonstrated using optimized growth conditions. In addition, two QCDs presenting novel features are described. The first one, for near-IR applications, detects two different wavelengths, notably at ¿ 1 micrometer which corresponds to the shortest wavelength ever reported for an ISB device. The second one proposes a new design, relying on a thick alloy extractor instead of a MQW cascade structure, which can be scaled for long wavelengths operations. The second part presents an in-deepth study on resonant tunneling diodes (RTDs). Those devices rely on the non-linear coherent transport of electrons through two potential barriers sandwiching a quantum well (QW). Resonant tunneling is characterized by the appearance of a resonance peak followed by a negative differential resistance in the current-voltage transport measurements. GaN-based RTDs have been extensively studied, since they exhibit a twofold interest: (i) they could be used as oscillators emitting in the THz range, and (ii) they forma key element to understand the mechanism of vertical transport across superlattices, a necessary milestone for the realization of QCLs. However, the reported results are controversial i and present discrepancies. In order to understand these discrepancies, a comprehensive study, together with an original approach are proposed: using weakly strained quantum heterostructures, namely low Al content double barrier GaN/AlGaN QW achieved after MOVPE growth condition optimization. A self-align process is also developed to downsize the device, in order to study the impact of dislocations. The excellent quality of the grown layers is demonstrated by means of photoluminescence spectroscopy, photo-reflectivity and X-ray diffraction. If the samples reveal similar discrepancies as reported in the literature (hysteresis and rapid ageing), this study points out their potential origins, following a deep investigation of the processing issues. Finally, a simple theoretical model is proposed to explain the difficulties encountered to observe quantum coherent electronic transport in GaN-based RTDs.

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