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Via temperature-dependent time-resolved photoluminescence spectroscopy, we investigate the radiative and nonradiative recombination processes in thin (quantum well widths of about 1.5 nm) a-plane, m-plane, () and () GaInN/GaN fivefold quantum well structures of varying indium content grown on low defect density GaN substrates and GaN templates. At room temperature, we observe surprisingly short radiative lifetimes in the range from 100 ps to 1 ns for these structures, being about one to two orders of magnitude shorter than for similar c-plane quantum wells. This large difference cannot solely be explained by the larger overlap matrix element in non- and semipolar wells. Higher exciton binding energies and lower effective density of states masses may contribute to an enhanced radiative probability. The nonradiative recombination exhibits a thermally activated behavior with activation energies of about 10 meV for () and around 25 meV for nonpolar quantum wells. These values are lower than the quantum well barrier height and the exciton binding energy, but in a similar range as the localization energies estimated from the radiative recombination.

In this contribution we report on the growth of coherently strained, pseudomorphic InGaN layers on GaN templates by RF-MBE. Structural properties characterized by HR-XRD exhibit good crystalline quality and homogeneity of the grown layers. The incorporation efficiency of In into the InGaN layer was estimated and compared to the literature. Characterization by AFM and in situ RHEED observation showed differences in surface morphology between nitrogen rich and metal rich growth conditions. Further investigations of the optical properties were carried out by PL and spectroscopic ellipsometry measurements.

In contrast to blue and violet emitting GaInN quantum well structures grown on sapphire, which show a high internal quantum efficiency (IQE), green emitting structures suffer from relatively low efficiencies. Usually, the large internal polarization fields are made responsible for such reduced efficiencies. For quantum wells with high In content, however, the efficiencies are even lower than expected. In this work we present the results of our near-field optical investigations of the photoluminescence of green emitting GaInN quantum well structures grown on c-plane sapphire and on bulk GaN substrates. Our aim is to clarify the influence of defects on the spatial structure of the luminescence of green emitting quantum wells. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a UHV cathodoluminescence study of the spontaneous polarization field in polar [0001] and nonpolar [1100] GaN/GaInN single quantum well structures. Employing electron-stimulated desorption a descreening of the spontaneous field in the polar c-plane samples was observed, which counteracts the piezoelectric field in the quantum well. While in c-plane samples a strong blueshift of the emission energy takes place and a drastic increase of emission intensity occurs, no change of the QW emission energy and intensity from m-plane samples was detected. The different behaviour under the same experimental conditions can only be explained with the absence of the spontaneous polarization field in the nonpolar [1100] m-direction.

In x Ga 1―x N/GaN quantum well (QW) structures grown on c-plane surfaces for long wavelength light emitters have been investigated intended. We reached indium concentrations of x In ≥ 0.35 with good optical and structural quality. For QW thicknesses d QW ≤ 2 nm a fully strained layer structure is observed. QWs of such high indium concentrations, however, are very sensitive to the growth conditions of the subsequent layers and thermal stability/degradation becomes an important issue. We modified the growth of the QWs to avoid or minimize V-pit formation without temperature ramping in the barriers and showed that their properties were unchanged when used in the active zone of a laser structure.

Determination of compositions in multiple quantum well structures is a critical issue. Until now only few experimental description of determination of concentrations in the nonpolar m- or a-direction in III-nitrides by means of X-ray diffraction has been published. Using the example of GaInN/GaN multiple quantum well structures we present different methods to determine the indium concentration. After presenting the relevant elasticity relations we adopt a step model introduced by Segmuller and Blakeslee to nonpolar directions. This model allows a simulation of symmetrical 2θ―ω scans. We will discuss the possibilities and also the limitations of this model. Additionally, we present a method to determine concentrations by using the peak position of the zero order superlattice peak. Finally, we compare the methods using samples grown on m-plane SiC and bulk m - plane GaN.

GaInN/GaN multiple quantum well structures grown on polar and nonpolar surfaces have been compared with respect to the indium incorporation efficiency in the quantum wells (QWs). Under the same growth conditions X-ray diffraction measurements reveal similar growth rates and In concentrations for c-plane, a-plane, and m-plane with In contents up to 40%. These results are in good agreement with optical experiments, in particular for homoepitaxial growth. However, there is strong evidence that the optical properties of the nonpolar heteroepitaxial GaInN QWs are dominated by the high density of stacking faults in those samples.

InxGa1� xN/GaN quantum well (QW) structures grown on c-plane and m-plane surfaces have been investigated intended for long wavelength light emitters. On c-plane InxGa1� xN QWs reached indium concentrations of xInX35% with good optical and structural quality. For QW thicknesses dQWp 2n m a fully strained layer structure is observed and the indium concentration is quite homogenous. Under the same growth conditions of the QW region we find similar or even slightly larger indium concentrations on m-plane surfaces. QWs of such high indium concentrations, however, are very sensitive to the growth conditions of the subsequent layers and we observe degradation such as indium outdiffusion or partial relaxation for high growth temperatures.

In this contribution we address the problems in characterizing typical group-III-nitride QW structures used in LED or laser applications by spectroscopic ellipsometry. Since the buffer layers are transparent in the visible and near UV and the whole layer stack is a few microns thick, the contributions of the nanometer thick QWs to the signal is very small and with ellipsometers with typical resolution in the polarization states hardly to detect. We discuss this in detail for two typical structures – Al-GaN/GaN and InGaN/GaN – and quantify how the resolution must be improved to overcome this experimental limitation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we critically review the usual explanation of the blue shift in the temperature dependence of the light emission, which is commonly observed for polar GaInN/GaN quantum wells at intermediate temperatures. We demonstrate that this blue shift is not necessarily caused by a thermally induced change of occupation in an inhomogeneously broadened density of states. Instead, different energy dependencies of radiative and nonradiative lifetimes may lead to an energy dependence of the internal quantum efficiency strongly influencing the peak position of the luminescence: the piezoelectric fields within the quantum well induce an approximately exponential relationship between the electron-hole overlap matrix element for the radiative transition between the lowest quantized states and the respective transition energy. Via time-resolved photoluminescence spectroscopy, we observe a corresponding exponential energy dependence of radiative lifetimes at low temperatures and almost energy independent nonradiative lifetimes toward room temperature throughout the emission of polar single quantum wells. An analytical model is demonstrated, predicting a significant blue shift of several 10 meV for polar and a negligible shift for nonpolar GaInN/GaN quantum wells at the transition from high to low internal quantum efficiency due to the energy-dependent competition between radiative and nonradiative recombination processes. This model consistently explains the lack of a blue shift in nonpolar quantum wells grown on $m$-plane bulk GaN as well as a reduced characteristic temperature for the onset of the blue shift after artificial defect generation via argon ion implantation.

Using time-resolved photoluminescence spectroscopy on GaInN/GaN multiple quantum well structures, we analyze the radiative and nonradiative processes contributing to the “green gap” in GaN-based light emitting devices. We observe that it is only partly caused by a reduced oscillator strength due to the Quantum Confined Stark Effect (QCSE) which becomes stronger with increasing indium concentration and well width. As the dominant effect we observe a reduction of nonradiative lifetimes when the indium concentration is increased. For higher indium concentrations, we find an additional nonradiative recombination path that might be attributed to an increased generation of defects like misfit dislocations, nitrogen vacancies and/or indium clusters within the optically active region. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InxGa1-xN/GaN quantum well (QW) structures with Indium concentrations above 30% suited for light emitters in the green and beyond have been investigated. The structures were optimized for homogeneous Indium distributions and abrupt interfaces. We obtained very high internal quantum efficiencies (IQE) of 80% and 70% for 460 nm and 510 nm emission wavelength, respectively. However, for high In concentrations the heterostructures are thermally less stable. This is evident from systematic studies including varied GaN cap temperatures and different post annealing procedures. For elevated temperatures we observe a reduction of the PL intensity, a broadening and a shift to higher energies of the PL lines without indication of phase separation. The reason is the soft indium-nitrogen bond, the degradation likely occurs by In interdiffusion or outdiffusion via defects in the structures. The critical temperatures are well below those typical for p-GaN contact layer growth and thus need to be considered in device applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the optical properties of m-plane GaInN/GaN quantum wells (QWs). We found that the emission energy of GaInN QWs grown on m-plane SiC is significantly lower than on non-polar bulk GaN, which we attribute to the high density of stacking faults. Temperature and power dependent photoluminescence reveals that the GaInN QWs on SiC have almost as large internal quantum efficiencies as on bulk GaN despite the much higher defect density. Our results indicate that quantum-wire-like features formed by stacking faults intersecting the quantum wells provide a highly efficient light emission completely dominating the optical properties of the structures.

Laser structures based on GaInN quantum wells were investigated using the variable stripe length technique. Two major problems in designing a green emitting laser diode were identified. The first one is the decrease of the optical confinement for increasing wavelength. To avoid this limitation and to simultaneously maintain good optical confinement we grew samples with lattice-matched AlInN lower cladding layers. The second major issue is to push the emission of the structures to longer wavelength by increasing the indium concentration in the quantum well. We demonstrate optical gain up to a wavelength of 480 nm (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this contribution we focus on degradation processes of GaInN multiple quantum wells (MQWs) used in laser structures grown by metallorganic vapor pressure epitaxy (MOVPE). The influence of ramp-up time as well as the maximum temperature during growth of the barrier on the quantum well (QW) is investigated. Comparison of X-ray and photoluminescence (PL) measurements implies that a sufficient thickness of a cover layer grown at low temperature is required to prevent degradation of the QW. In a next step samples were grown using an optimal thermal budget during growth of the active region to investigate the influence of capping layers. To improve crystalline quality of these layers high temperatures are required. Our measurements show that these high temperatures lead to a decrease in average indium concentration of the QWs. This decrease is temperature as well as time dependent (∝ √t) and can only explained by diffusion processes. The activation energies are in the range 0.7-0.9 eV (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this contribution, the indium concentration profile of an InxGa1−xN/GaN five-fold multi quantum well structure is measured from high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. The results are compared with an atom probe tomography study. Indium concentrations in the range of 26 at.% to 33 at.% are measured in the centre of the quantum wells. An additional indium layer of 14 at.% has been found on top of the quantum wells. In the second part, the temperature dependence of measured intensities in GaN is investigated. Here, multislice calculations in the frozen lattice approximation are carried out in dependence of specimen thickness and compared to experimental data. An increase of intensity with specimen temperature is found.

The origin of the green gap for GaInN/GaN quantum wells is investigated via temperature-dependent time-resolved photoluminescence spectroscopy. A strong correlation between nonradiative lifetimes and total strain energy is observed, although the wells are almost fully strained. We discuss this observation in terms of nonradiative recombination at defects which contribute to a beginning partial relaxation. The formation energy of a defect is likely reduced by the amount of its released strain energy. We therefore expect an exponential dependence of the defect density on this released strain energy. Our measured nonradiative lifetimes are consistent with a cumulative strain driven generation of defects.

A method for determining concentrations from high-angle annular dark field-scanning transmission electron microscopy images is presented. The method is applied to an InGaN/GaN multi-quantum well structure with high In content, as used for the fabrication of light emitting diodes and laser diodes emitting in the green spectral range. Information on specimen thickness and In concentration is extracted by comparison with multislice calculations. Resulting concentration profiles are in good agreement with a comparative atom probe tomography analysis. Indium concentrations in the quantum wells ranging from 26 at. % to 33 at. % are measured in both cases.

Using scanning transmission electron microscopy (STEM), we have studied ultra-thin ( 25 %) suitable for blue-green light emitting devices. We are able to analyze the QW on an atomic scale with high resolution STEM and derive the indium content quantitatively. In our analysis, we find that indium is not only incorporated into the QW but also into the barriers under certain growth conditions. We observe indium tails or even plateau-like structures in the barriers, caused by excess indium being supplied during quantum well growth.

Homogeneous nitrogen-polar GaN core-shell light emitting diode (LED) arrays were fabricated by selective area growth on patterned substrates. Transmission electron microscopy measurements prove the core-shell structure of the rod LEDs. Depending on the growth facets, the InGaN/GaN multi-quantum wells (MQWs) show different dimensions and morphology. Cathodoluminescence (CL) measurements reveal a MQWs emission centered at about 415 nm on sidewalls and another emission at 460 nm from top surfaces. CL line scans on cleaved rod also indicate the core-shell morphology. Finally, an internal quantum efficiency of about 28% at room temperature was determined by an all-optical method on a LED array.

We investigated the optical polarization anisotropy of m-plane GaInN/GaN quantum well structures on m-plane SiC and bulk GaN substrates. On bulk GaN, the degree of polarization increases with increasing indium content according to the larger strain-induced separation of the topmost valence bands. On m-plane SiC, however, we observe constantly large polarization ratios of around 90% and more. From an x-ray strain state analysis and calculations of the valence band energies, we find that an anisotropic strain of the GaN buffer layer leads to a very strong separation of the topmost valence bands resulting in a large degree of polarization.

We have studied the growth of GaInN/GaN quantum wells on various polar, nonpolar and semipolar planes. From a detailed x-ray diffraction analysis, we derive the strain state and the composition of the quantum wells. The optical emission energy is obtained from photoluminescence spectra and modelled taking into account the deformation potentials and the Stark shifts. Both x-ray and optical data consistently show that indium incorporation is identical on the polar, nonpolar and semipolar planes within the experimental uncertainty.

Nonradiative loss processes are a major concern in nitride-based light emitting devices. Utilizing optical gain measurements on GaInN/GaN/AlGaN laser structures, we have studied the dependence of the total recombination rate on excess carrier density, up to rather high densities. From a detailed quantitative analysis, we find a room-temperature Auger recombination coefficient of 1.8 ± 0.2 × 10−31 cm6/s in the bandgap range 2.5 − 3.1 eV, considerably lower than previous experimental estimates. Thus, Auger recombination is expected to be significant for laser diodes, while it is not likely to be a major factor for the droop observed in light-emitting diodes.

Using electron beam irradiation, cathodoluminescence, and photoluminescence under ultrahigh vacuum conditions, we study the effect of spontaneous polarization on polar (0001) and nonpolar (11−00) GaInN/GaN quantum well structures. We use cathodoluminescence measurements with an electron beam irradiation time of up to several hours. A drastic blueshift of the quantum well emission accompanied by a 100-fold increase of intensity is observed in polar samples. These changes can be described by an activation of the spontaneous polarization field due to the desorption of surface charges, which counteracts the piezoelectric field in the quantum well. Etching or annealing of the surface leads to similar effects. The influence of the sample structure was investigated by varying the cap thickness of the samples. A different time- dependent behavior of changes in the quantum well emission energy and the intensity depending on cap thickness and acceleration voltage was observed. This can be explained by de-screening ...

We compare radiative and nonradiative recombination in polar and nonpolar GaInN/GaN quantum wells and demonstrate that purely radiative recombination can be achieved even at room temperature. The radiative recombination is due to free excitons, which partially dissociate at elevated temperatures. Nonradiative recombination is strongly inhibited in the polar case but not in the nonpolar case.