H. Bremers

Auger recombination in GaInN/GaN quantum well laser structures

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.

Strain-induced defects as nonradiative recombination centers in green-emitting GaInN/GaN quantum well structures

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.

Magnetic properties of FeMnAl alloys

Abstract The magnetical properties of Fe89−xMn11Alx alloys have been studied by Mossbauer spectroscopy and magnetization measurements. The presented examples show both ferromagnetic (x = 14) and re-entrant ferromagnetic (x = 40) behaviour.

Highly efficient light emission from stacking faults intersecting nonpolar GaInN quantum wells

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.

Atomic scale investigations of ultra-thin GaInN/GaN quantum wells with high indium content

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.

Room temperature excitonic recombination in GaInN/GaN quantum wells

The dependence of radiative and nonradiative lifetimes on the excess carrier density in GaInN/GaN quantum wells is studied via time-resolved photoluminescence spectroscopy over a wide range of excitation densities. Our results differ from the predictions of simple free-carrier models: density independent radiative lifetimes clearly evidence the excitonic nature even at room temperature. At high densities, nonradiative lifetimes are weakly temperature dependent and proportional to the inverse of the density, implying an excitonic, threshold-less Auger process. Furthermore, in the intermediate density regime between low and high injection, an increase of the nonradiative lifetimes is observed, which is typical for Shockley-Read-Hall-type recombination.

On the influence of Al on the magnetic ground state of 3d ferromagnetic alloys

Abstract This paper describes variations in the mean atomic magnetic moment with Al admixtures in transition element alloys, as functions of the number of 3d + 4s electrons and of the atomic concentration of Al. From the experimental results presented, it seems that the presence of the admixtures results in the completion of the 3d zone of the transition elements. The population increase of the 3d band is interpreted as a consequence of its narrowing and its displacement in the energy spectrum.

Anti-localization suppresses non-radiative recombination in GaInN/GaN quantum wells

The light emission efficiency of (AlGaIn)N heterostructures and light-emitting diodes is exceptionally high, despite the high density of threading dislocations generally found in such structures. It has become common to attribute the high efficiency to compositional fluctuations or even phase separation in the active GaInN quantum well region. The resulting localization of charge carriers is thought to keep them from recombining non-radiatively at the defects. Here, we show that carriers are mobile at room temperature rather than localized and that under suitable growth conditions hexagonal V-shaped pits decorating the defects exhibit narrow sidewall quantum wells with an effective bandgap significantly larger than that of the regular c-plane quantum wells. Thereby nature provides a unique, hitherto unrecognized mechanism generating a potential landscape which effectively screens the defects themselves by providing an energy barrier around every defect. Thus even for mobile charge carriers non-radiative recombination is effectively suppressed leading to the unexpectedly high emission efficiency.

Large internal quantum efficiency of In-free UV-emitting GaN∕AlGaN quantum-well structures

We have achieved dramatic improvement of the internal quantum efficiency (IQE) for ultraviolet-emitting GaN∕AlGaN quantum-well (QW) structures. Despite a defect density of a few 109cm−2 and the use of an In-free QW we achieve best values for the IQE at room temperature of 26%. Under strong nonresonant excitation, the IQE even increases to 38%. We observe a weak dependence of the IQE on excitation power for our structures. This indicates that similar mechanisms as for GaInN-based light emitters are present.

Analysis of indium incorporation in non- and semipolar GaInN QW structures: comparing x-ray diffraction and optical properties

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.