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The carrier dynamics of Al‐rich AlGaN/AlN quantum well (QW) structures in the presence of strong carrier localization is reported. Excitation density‐dependent photoluminescence (PL) measurements at low temperatures reveal a clear correlation between the onset of efficiency droop and the broadening of the time‐integrated PL spectra. While the droop onset is heavily impacted by the localization strength, the PL emission broadening is observed almost exclusively on the high energy side of the emission spectrum. Spectrally resolved PL decay transient measurements reveal a strong dependency of the carrier lifetimes on the emission photon energy across the spectrum, consistent with a distribution of localized states, as well as on the temperature, depending on the localization strength of the investigated structure. The characteristic “S”‐shaped temperature dependence of the PL emission energy is shown to be directly correlated to the thermal redistribution of carriers between localized states. Based on these findings, the role of carrier localization in the recombination processes in AlGaN QW structures is underlined and its implications for efficiency droop are discussed.

Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV-C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm−1. The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary-ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV-C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV-C devices by MOCVD on sapphire substrates are outlined.

AlGaN basierte Laterale Polare Strukturen (LPS) können mit Hilfe der metallorganischen Gasphasenepitaxie gewachsen werden, indem man die c-Achse periodisch invertiert. Derartige Strukturen haben das Potential, um für Frequenzverdopplung von kohärentem Licht in das ultraviolette (UV) Spektralgebiet verwendet zu werden. Weitere Anwendungen liegen im Bereich lateraler p/n-Übergänge. Bezüglich ihrer Herstellung gibt es derzeit noch zwei wesentliche Herausforderungen, die gelöst werden müssen, um das volle Potential des Materialsystems für optoelektronische Bauelemente im UV ausnutzen zu können: (1) In AlGaN kann eine hohe Konzentration von intrinsischen und extrinsischen Störstellen zu einer reduzierten Quanteneffizienz von optischen Bauelementen führen. Dies ist besonders bei höheren Dotierungen der Fall, da es dort zur Selbstko mpensation des Dotanden kommen kann. (2) Werden III- und N-polare Domänen nebeneinanderliegend in einer LPS gewachsen, so wird in der Regel ein Schichtdickenunterschied zwischen den Domänen beobachtet. In der vorliegenden Arbeit wurden beide genannten Herausforderungen detailliert bearbeitet und gelöst. Im Fall von mit Si oder Mg dotiertem AlGaN führen hohe Störstellen-konzentrationen wie Stickstoffvakanzen, H oder O zu hohen Widerständen und einer niedrigen Beweglichkeit der freien Ladungsträger. Diese hohen Konzentrationen werden dadurch erklärt, dass im Bereich hoher Dotierungen die Formierungsenergien dieser S törstellen verringert werden, was zu einem erhöhten Einbau von Kompensatoren führt. Bestrahlt man AlGaN Filme während des Wachstums mit UV-Licht (entsprechend einer Anregungsenergie größer als die Bandlücke), führt dies zu einem verringertem Einbau von Störstellen. So kann z.B. in GaN:Mg eine stark reduzierte blaue Lumineszenz (2.8 eV) gemessen werden, was auf einen verringerten Einbau von Stickstoffvakanzen schließen lässt und zu einem geringerem elektrischen Widerstand führt. Des Weiteren wird im Rahmen dieser Arbeit gezeigt, dass das hier vorgestellte Modell zur Kontrolle des Einbaus von Defekten sowohl für n- als auch p-leitende Halbleiter verwendet werden kann. Die zweite Herausforderung des Schichtdickenunterschiedes der N- und III-polaren Domänen in LPS (2) wird in dieser Arbeit durch einen Oberflächenmassentransport zwischen den polaren GaN Domänen erklärt. Abhängig von der Wachstumsbedingung führt dieser Massentransport zu einem bevorzugten Wachstum jeweils einer polaren Domäne. Folglich konnten aber auch Bedingungen gefunden werden, in denen die Wachstumsrate der Domänen identisch ist und hoch qualitative AlGaN LPS mit Mikrometer großen Domäne n gewachsen werden. Abschließend wurden die Erkenntnisse aus (1) und (2) kombiniert und verbesserte elektrische und optische Eigenschaften lateraler p/n Übergänge nachgewiesen, wenn diese mit UV-Licht während des Wachstums bestrahlt wurden. Beides, sowohl die Störstellenkontrolle mittels UV-Bestrahlung als auch die Kontrolle der N- und III-polaren Domänen in LPS, wurde nie zuvor demonstriert und eröffnen einzigartige neue Möglichkeiten., AlGaN can be used for the fabrication of lateral polar structures (LPS) by a periodic inversion of the c-axis as achieved by a polarity control scheme during its growth by metal organic chemical vapor deposition (MOCVD). These structures can be used for second harmonic generation in the ultraviolet spectral region, as well as for lateral p/n-junctions. The two major challenges addressed in this work exist in the general implementation of the AlGaN technology and in the fabrication of AlGaN LPS, and both prevent the realization of AlGaN UV-emitters. These challenges are: (1) the presence of a high concentrations of native defects and extrinsic impurities in AlGaN that can reduce the efficiency of optoelectronic devices, especially in the case of high doping with Mg or Si, and (2) as typically observed, a growth rate difference that exists during the simultaneous growth of III- and N-polar domains adjacent to each other in a LPS. In this dissertation, solutions to these two challenges are based on two novel and original approaches involving (1) Fermi-level point defect control schemes, and (2) mass transport control between polar domains. In approach (1), the control of point defects and reduction of compensations, VN, H or O are identified as the cause of high resistivity and low mobility in AlGaN due to their low formation energies at high Mg or Si doping concentrations. To confirm and quantify the Fermi-level point defect control scheme, above bandgap illumination during the growth of GaN was demonstrated to control the incorporation of these defects. Significant reductions in the point defect were achieved as evidenced by changes in their corresponding luminescence, for example, the blue luminescence at 2.8 eV (VN-related). Additionally, reduced resistivity and atomic concentrations, such as an order of magnitude lower H concentration, were observed. These studies confirm that the point defect management scheme developed in this work can control compensation in n-type as well as in p-type semiconductors. In approach (2), mass transport control between polar domains, the surface mass transport between differently oriented domains was determined to be a function of the Ga supersaturation. Using Ga supersaturation as a control, a condition where identical growth rates for both polarities was obtained and high quality AlGaN LPS with domains in the micrometer scale could be fabricated. Furthermore, the first AlN LPS was grown and studied. Finally, the utility of these two new approaches was demonstrated by fabricating an AlGaN lateral p/n-junction that exhibited a marked decrease in compensation effects within each particular domain. Both point defect control via above bandgap illumination, as well as the control of the growth rate difference of LPS, will have considerable impact on the field of III-V-nitrides as these novel approaches will potentially facilitate a new class of devices in the future.