The quantum efficiency of micro-light emitting diodes (micro-LEDs) is lower than that of large area LEDs. This efficiency reduction is typically attributed to the nonradiative Shockley-Read-Hall recombination at the surface defects and current leakage through the sidewall region without a clear distinction between these effects. In this work, we attempt to find out which of these phenomena is most critical for the reduced efficiency of micro-LEDs. This has been done by mapping electroluminescence (EL) and photoluminescence (PL) and measuring PL dynamics in blue GaN micro-LEDs fabricated by dry etching. It has been found that in the as-etched device, the EL intensity is much lower than in devices with KOH etching and atomic layer deposition of SiO2. This effect is especially pronounced close to the sidewalls. On the other hand, PL decay times are similar in as-etched and passivated devices, both in their center and at the sidewalls. This allows concluding that the main mechanism of the reduced efficiency of micro-LEDs fabricated by dry etching is the current leakage in the sidewall region and not the nonradiative recombination. The KOH etching has been found to be the most efficient means to eliminate the current leakage.

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6-1 μm and the hole diffusion coefficient ∼0.6 cm2/s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.

Long wavelength InGaN/GaN quantum well (QW) light emitting diodes (LEDs) are essential components of solid-state lighting and displays. However, efficiency of these devices is inferior to that of blue LEDs. To a large degree, this occurs because equilibration of injected holes between multiple QWs of the active region is hindered by the high GaN quantum confinement and polarization barriers. This drawback could be overcome by lateral hole injection via semipolar QWs present on facets of V-defects that form at threading dislocations in polar GaN-based structures. In this work we have tested the viability of this injection mechanism and studied its properties by time-resolved and near-field spectroscopy techniques on multiple QW devices. We have found that indeed the hole injection via the V-defects does take place, the mechanism is fast, and the hole spread from the V-defect is substantial making this type of injection feasible for efficient long wavelength GaN LEDs.

The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs.

The efficiency of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on uniformity of hole distribution between the QWs. Typically, transport between the QWs takes place via carrier capture into and thermionic emission out of the QWs. In InGaN/GaN QWs, the thermionic hole transport is hindered by the high quantum confinement and polarization barriers. To overcome this drawback, hole injection through semipolar QWs located at sidewalls of V-defects had been proposed. However, in the case of the V-defect injection, strong lateral emission variations take place. In this work, we explore the nature of these variations and the impact of the V-defects on the emission spectra and carrier dynamics. The study was performed by mapping electroluminescence (EL) and photoluminescence (PL) with a scanning near-field optical microscope in LEDs that contain a deeper well that can only be populated by holes through the V-defects. Applying different excitation schemes (electrical injection and optical excitation in the far- and near-field), we have shown that the EL intensity variations are caused by the lateral nonuniformity of the hole injection. We have also found that, in biased structures, the PL intensity and decay time in the V-defect regions are only moderately lower that in the V-defect-free regions thus showing no evidence of an efficient Shockley--Read-Hall recombination. In the V-defect regions, the emission spectra experience a red shift and increased broadening, which suggests an increase of the In content and well width in the polar QWs close to the V-defects.

InGaN/GaN quantum well (QW) light emitting diodes (LEDs) are essential components of solid-state lighting and displays. However, the efficiency of long wavelength (green to red) devices is inferior to that of blue LEDs. To a large degree, this occurs because the equilibration of injected holes between multiple QWs of the active region is hindered by GaN quantum confinement and polarization barriers. This drawback could be overcome by volumetric hole injection into all QWs through semipolar QWs present on the facets of V-defects that form at threading dislocations in polar GaN-based structures. In this work, we have tested the viability of this injection mechanism and studied its properties by time-resolved and near-field spectroscopy techniques. We have found that indeed the hole injection via the V-defects does take place, the mechanism is fast, and the hole spread from the V-defect is substantial, making this type of injection feasible for efficient long wavelength GaN LEDs.

Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron-hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 10 1 ¯ 1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs.

This thesis focuses on effects that are critical to achieving high internal quantum efficiency (IQE) in GaN-based light-emitting diodes (LEDs) that emit in a broad spectral range, from violet to green-yellow. These effects include interwell carrier transport in multiple quantum well (QW) structures, lateral transport in the QW plane, and radiative and nonradiative recombination. 

The investigation is conducted with the time-integrated and time-resolved near- and far-field photoluminescence (PL) spectroscopy. Measurements are performed on polar single and multiple In_xGa_1-xN QW structures of different alloy compositions, which are supplemented with a self-consistent solution of one-dimensional Schrödinger and Poisson equations and an evaluation of the carrier density dynamics. 

Interwell carrier transport is studied to determine the conditions required for a uniform interwell carrier distribution in an LED active region. Such a distribution would decrease the detrimental impact of the nonradiative Auger recombination and increase the IQE. Since the hole transport is the bottleneck for this process, ambipolar interwell transport, determined by the slower holes, is studied. Standard time-resolved PL measurements are performed on multiple QW structures with a different number of In_0.12Ga_0.88N QWs and different barrier parameters in terms of thickness and material. Photoexcited carrier transport over the multiple QW structure is monitored by measuring PL rise times from a deeper detector QW. Such measurements make it possible to distinguish the interwell transport mechanism at high temperatures (e.g., thermionic emission - ns range) and low temperatures (e.g., ballistic - sub-ps range). In standard InGaN/GaN structures, the interwell hole transport is found to be inefficient. Studies of transport and IQE in structures with InGaN barriers of different compositions, as well as thin GaN or AlGaN interlayers between the QWs and barriers, allowed the design of structures with fast, efficient interwell transport and high IQE. These measurements are performed for blue LED structures; however, the conclusions could be extended to QWs emitting at longer wavelengths, for which the issue of the nonuniform interwell carrier distribution is even more severe. 

Studies of the carrier recombination and IQE are performed on single QWs with a focus on long wavelength (green, green-yellow) emitting structures, in which the IQE is much smaller than for the violet and blue-emitting wells. Radiative and nonradiative carrier recombination times are determined at different temperatures, revealing a record-high IQE of ∼60% in the green-yellow QWs. 

Since nonradiative recombination is often assigned to extended defects, near-field spectroscopy is applied to study the impact of V-defects related to dislocations in polar GaN-based structures. The parameters of PL spectra, as well as radiative and nonradiative recombination times, show large spatial variations. The increased nonradiative recombination related to the dislocations is revealed only in their immediate vicinity, suggesting that their impact on the IQE and device performance, contrary to common belief, should be small.

Avhandlingen fokuserar på effekter som är avgörande för att uppnå hög intern kvanteffektivitet (IQE) i GaN-baserade lysdioder (LED) som emitterar inom ett brett spektralområde, från violett till gröngult. Dessa effekter inkluderar laddningsbärartransport mellan brunnar i flera kvantbrunnar (QW)-strukturer, lateral transport i QW-planet och strålnings- och icke-strålningsrekombination. 

Undersökningen gjordes genom tidsintegrerad och tidsupplöst fotoluminescensspektroskopi (PL) i när- och fjärrfält. Mätningar utfördes på polära enkla och multipla In_xGa_1-xN QW-strukturer bestående av olika legeringssammansättningar. De kompletterades med självkonsistent lösning av endimensionella Schrödinger- och Poisson-ekvationer och utvärdering av bärardensitetsdynamiken. 

Laddningsbärartransporten mellan brunnar studerades för att bestämma förhållanden som krävs för en enhetlig bärarfördelning mellan brunnar i en aktiv LED-region. Sådan distribution skulle minska den skadliga effekten av den icke-strålande Auger-rekombinationen och öka IQE. Eftersom håltransporten är flaskhalsen under denna process, studerades ambipolär transport mellan brunnar, utifrån de långsammare hålen. Standard tidsupplösta PL-mätningar utfördes på flera QW-strukturer med olika antal In_0,12Ga_0,88N QW:er och olika barriärparametrar (tjocklek, material). Fotoexciterad bärartransport över den multipla QW-strukturen övervakades genom att mäta PL-stigningstider från en djupare detektor-QW. Sådana mätningar gjorde det möjligt att särskilja transportmekanismen mellan brunnar vid höga temperaturer (termionisk emission, ns-intervall) och låga temperaturer (ballistiskt, sub-ps-intervall). Det konstaterades att håltransporten mellan brunnar i standard InGaN/GaN-strukturer var ineffektiv. Studier av transport och IQE i strukturer med InGaN-barriärer av olika sammansättning, och tunna GaN- eller AlGaN-mellanskikt mellan QW:erna och barriärer möjliggjorde design av strukturer med snabb och effektiv transport mellan brunnar och hög IQE. Dessa mätningar utfördes för blå LED-strukturer; dock skulle slutsatserna kunna utvidgas till QW:er som emitterar vid längre våglängder och för vilka problemet med olikformig bärarfördelningen mellan brunnar är ännu större.

Studier av laddningsbärarrekombinationen och IQE utfördes på enstaka kvantbrunnar med fokus på emitterande strukturer med lång våglängd (grön, grön-gul), där IQE är mycket mindre än för de violett och blåemitterande brunnarna. Strålande och icke-strålande bärarekombinationstider bestämdes vid olika temperaturer, vilket avslöjade en rekordhög IQE på ∼60 % i de gröngula QW:erna. 

Eftersom den icke-strålande rekombinationen ofta tillskrivs utökade defekter, användes närfältsspektroskopi för att studera effekten av V-defekter relaterade till dislokationer i polära GaN-baserade strukturer. Parametrar för PL-spektra, samt strålande och icke-strålande rekombinationstider visade stora rumsliga variationer. Den ökade ickestrålande rekombinationen relaterad till dislokationerna avslöjades endast i deras omedelbara närhet, vilket tyder på att deras inverkan på IQE och enhetens prestanda, i motsats till vad man tror, borde vara liten.

Ways to improve efficiency of high-power LEDs based on InGaN/(In)GaN multiple quantum wells are explored by studying interwell carrier transport and recombination. Best results are achieved for InGaN barriers with thin GaN or AlGaN interlayers.

Efficient high-power operation of light emitting diodes based on InGaN quantum wells (QWs) requires rapid interwell hole transport and low nonradiative recombination. The transport rate can be increased by replacing GaN barriers with that of InGaN. Introduction of InGaN barriers, however, increases the rate of the nonradiative recombination. In this work, we have attempted to reduce the negative impact of the nonradiative recombination by introducing thin GaN or AlGaN interlayers at the QW/barrier interfaces. The interlayers, indeed, reduce the nonradiative recombination rate and increase the internal quantum efficiency by about 10%. Furthermore, the interlayers do not substantially slow down the interwell hole transport; for 0.5 nm Al0.10Ga0.90N interlayers the transport rate has even been found to increase. Another positive feature of the interlayers is narrowing of the QW PL linewidth, which is attributed to smoother QW interfaces and reduced fluctuations of the QW width.