Why do LEDs use multi-quantum wells

DC FieldValueLanguagedc.contributor.advisorHoffmann, Axel-dc.contributor.authorNippert, Felix-dc.date.accessioned2017-05-24T08: 33: 50Z-dc.date.available2017-05-24T08: 33: 50Z-dc.date.issued2017-dc.identifier.urihttp://depositonce.tu-berlin.de/handle/11303/6366-dc.identifier.urihttp://dx.doi.org/10.14279/depositonce-5916-dc.description.abstractInGaN / GaN quantum wells (QWs) are commonly used for blue and green light-emitting diodes (LEDs), for example in solid-state lighting (SSL). While such structures are already in mass-production, there are still several open questions. They show drastic reductions in internal quantum efficiency (IQE) towards longer wavelengths (“green gap”), as well as with increased drive current (“droop”). In addition, advanced high-power applications, such as green laser diodes (LDs), are still far from commercial use. In this work, the principal physical limitations of the InGaN / GaN technology are investigated. To this end, state-of-the-art InGaN / GaN multiple quantum wells (MQWs) structures are investigated in two power regimes. Under intense quasi-resonant, optical excitation a novel high excitation luminescence in the blue spectral region is observed for green-emitting QWs. This, broad (several hundred meV), featureless, fast-decaying (down to 30 ps lifetime) luminescence cannot be attributed to excited electron states, but rather is attributed to the confined hole continuum (CHC). The CHC is formed by excited quasi-continuous hole states confined by the triangular potential created by a QW-barrier pair due to the quantum-confined Stark effect (QCSE). Such states are energetically close to the GaN bulk states, implying that they could contribute to carrier leakage in high-power applications such as laser structures. State-of-the-art LED structures, operating in conventional power regimes and suffering from the green gap and droop phenomena, are investigated by a novel technique, which is established in this work. It relies on differential lifetime (DLT) measurements, performed in an electro-optical pump-probe setup: small-signal time-resolved photoluminescence (SSTRPL). The technique improves on previous approaches by combining steady-state electrical pumping identical to operating conditions, with time-dependent quasi-resonant optical probe excitation. In contrast to current modulation methods, this guarantees carrier insertion into the active layer only. SSTRPL therefore allows detailed insight into the recombination pathways, as conventionally modeled with the ABC model. In a temperature-dependent analysis of blue- and green-emitting MQW LEDs, the origin of the green gap phenomenon is elucidated. First, the Shockley-Read-Hall (SRH) recombination coefficient is shown to have the same magnitude and activation energy in blue and green-emitting InGaN layers, implying very similar defect densities. This allows to withdraw growth issues related to the higher incorporation of In from the list of potential causes of the green gap. Second, radiative and Auger recombination are drastically reduced in green-emitting structures. This effect can partly be blamed on the increasing QCSE, but is also caused by increasing hole localization. The latter is confirmed by considering the abnormal temperature dependence of the radiative recombination coefficient, which is increasing, rather than decreasing, with temperature. Finally, the green gap efficiency reduction is shown to originate primarily from a shift in the balance between radiative and Auger recombination, intricately connected to the increasing localization of holes, as it occurs even in the random alloy. The results obtained suggest that any further efficiency increase in conventional c-plane technology may only be reached by improving the uniformity of the random alloy. This parameter is very difficult to access directly with structural investigations. Therefore, (temperature-dependent) SSTRPL will be an important tool to track progress in the future, because it allows to access the atomic-scale localization properties in a non-destructive electro-optical measurement.endc.description.abstractInGaN / GaN quantum wells (QW) are now used as standard for blue and green emitting light-emitting diodes (LEDs), for example in room lighting. Although such structures are manufactured on a large industrial scale, there are still some unanswered questions. On the one hand, they show a drastic reduction in internal quantum efficiency (IQE) at longer wavelengths (“green gap”), and on the other hand, the efficiency also decreases significantly at high operating currents (“droop”). Furthermore, high-performance applications, such as green laser diodes, are still a long way from being used on an industrial scale. In this work, therefore, the fundamental physical limitations of InGaN / GaN technology are examined. For this purpose, state-of-the-art InGaN / GaN multiple Quantum wells (MQW) examined in two different performance regimes. In green-emitting QWs, a new type of luminescence in the blue spectral range is observed in quasi-resonant, optical high excitation. This is spectrally very broad (several hundred meV), continuous and has a rapid decay time (up to down to 30 ps). This luminescence band cannot be assigned to excited electronic states. Instead, it is described as a recombination of electrons with highly excited holes in confinement (CHC). The confinement results from the tilted band structure in the heterostructure, with a QW and a barrier forming a triangular potential. In this are quasi-continuous hole states. Since these states are energetically close to the GaN valence band edge, they represent a potential loss channel, especially for high excitation applications such as laser diodes. For LED structures that are operated in more moderate power ranges and that are affected by “Green Gap” and “Droop”, a new spectroscopic technique is established in this work. For this purpose, the differential lifetime of the charge carriers (DLT) is determined with an electro-optical pump-probe method, the small-signal time-resolved photoluminescence (SSTRPL). This method improves conventional methods by combining electrical excitation (pump), analogous to normal LED operation, with time-dependent, quasi-resonant optical excitation (sample). In contrast to methods in which the current is modulated, this can guarantee that the additional charge carriers are exclusively are generated in the QWs. With the help of the SSTRPL, a detailed insight into the recombination channels usually modeled with the ABC model can be obtained. In a time-dependent study of blue and green emitting LEDs, the origin of the “green gap” finally becomes clearer. On the one hand, the non-radiating Shockley Read Hall recombination coefficient in blue and green LEDs is roughly the same size and also has a comparable activation energy. This implies very comparable defect densities in the respective InGaN layers, so that growth problems due to the higher indium concentration as a potential cause of the “green gap” can be excluded. On the other hand, there is a drastic reduction in the radiant and Auger recombination in the green spectral range. This reduction can only partially be explained by the increasing Quantum Confined Stark Effect. In addition, an increasing hole localization is also responsible for this phenomenon. This can be done with the help of the abnormal temperature dependence of the radiating recombination coefficient be confirmed, which increases with increasing temperature, instead of sinking. Ultimately, the “green gap” can be attributed primarily to a shift in the recombination equilibrium between radiating and Auger recombination, which is linked to increasing hole localization, which is inevitable even with a completely random alloy of InN and GaN. Finally, it follows that further increases in efficiency in conventional, c-planar technology can only be achieved if the uniformity of the alloy is improved. However, it is very difficult to evaluate this parameter using structural methods, since the deviations from the ideal alloy are only apparent at the atomic level. Therefore, the developed method (SSTRPL) is ideally suited, especially as a function of temperature, to evaluate any progress in this area, as it allows direct access to the localization properties in a non-destructive measuring process.dedc.description.sponsorshipEC / FP7 / 318388 / EU / Nanostructured Efficient White LEDs / NEWLEDendc.description.sponsorshipDFG, SFB 787, Semiconductor - Nanophotonics: Materials, Models, Componentsendc.language.isoenendc.relation.hasparthttp://dx.doi.org/10.14279/depositonce-5926endc.relation.hasparthttp://dx.doi.org/10.14279/depositonce-5927endc.relation.hasparthttp://dx.doi.org/10.14279/depositonce-5928endc.rights.urihttp://rightsstatements.org/vocab/InC/1.0/endc.subject.ddc530 physicsdedc.subject.otherLEDendc.subject.othernitridesendc.subject.otherAugerendc.subject.othergreen gapendc.subject.otherrecombination coefficientsendc.subject.otherexcited statesendc.subject.otherlocalizationendc.subject.otherNitridesdedc.subject.otherRecombination coefficientsdedc.subject.otherexcited statesdedc.subject.otherLocalizationdedc.titleNon-radiative loss mechanisms in InGaN / GaN multiple quantum well light-emitting diodesendc.typeDoctoral thesisentub.accessrights.dnbdomainentub.publisher.universityorinstitutionTechnical University Berlinendc.contributor.grantorTechnical University Berlinendc.contributor.refereeHoffmann, Axel-dc.contributor.refereeMaultzsch, Janina-dc.contributor.refereeWaag, Andreas-dc.date.accepted2017-01-24-dc.title.translatedNon-radiative loss mechanisms in InGaN / GaN multi-quantum well light-emitting diodesdedc.type.versionacceptedVersionenAppears in Collections:Inst. 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