U. Rossow

Composition dependence of polarization fields in GaInN/GaN quantum wells

We report an experimental determination of the internal polarization field in GaInN/GaN quantum wells, due to piezoelectric and spontaneous polarization, utilizing the quantum confined Stark effect, with fields as large as 3.1 MV/cm at 22% In. From its dependence on quantum well composition and strain, we find that the total field in GaInN is a linear combination of polarization charges from GaN and InN. The piezoelectric constants d31 for GaN and InN derived from our data are 1.05±0.05 pm/V and 3.7±0.5 pm/V, in fair agreement with theoretical data.

Optimization scheme for the quantum efficiency of GaInN-based green-light-emitting diodes

We have optimized the internal quantum efficiency (IQE) of GaInN∕GaN quantum-well (QW) structures. For an emission wavelength of 460nm, a high IQE of 73% was achieved. For a longer emission wavelength, calculations predict higher oscillator strength for thinner QWs but higher In content. We observe an improvement in IQE of almost 50% when reducing the QW width from 2.7nmto1.8nm, and increasing the In content for the whole blue to green spectral region with IQE=40% at 525nm. The typical saturation of the output power with increasing current that occurs, particularly for green-light-emitting diodes, is extremely weak in our structures.

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.

Measurement of the indium concentration in high indium content InGaN layers by scanning transmission electron microscopy and atom probe tomography

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.

Determination of the polarization discontinuity at the AlGaN∕GaN interface by electroreflectance spectroscopy

The total polarization discontinuity ΔP at an Al0.31Ga0.69N∕GaN heterojunction has been determined by electroreflectance spectroscopy. This technique is based on the analysis of the Franz–Keldysh oscillations observed above the AlGaN band gap, yielding the barrier electric field strength as a function of the applied bias voltage. The threshold field strength, where the two-dimensional electron gas (2DEG) is depleted, corresponds to a ΔP of 1.1×1013e∕cm2 which is only 85% of the theoretical prediction. Applying the same optical method, the 2DEG density at the heterointerface can be accurately determined, as proven by comparison to Shubnikov–de Haas measurements.

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.

Strong enhancement of Eu+3 luminescence in europium-implanted GaN by Si and Mg codoping

A strong enhancement of Eu3+ luminescence in europium-implanted GaN samples is obtained by codoping with silicon (Si) and magnesium (Mg), simultaneously. The Eu3+ intensity in the 5D0 to 7F2 transition region is found to be 30 times higher compared to europium-implanted undoped GaN. The major contribution to this overall enhancement is due a weak peak present only in europium-implanted Mg-doped GaN at 2.0031 eV (618.9 nm) which is strongly enhanced by codoping both Mg and Si. The excitation process of europium ions is proposed to take place through a donor-acceptor pair related energy transfer mechanism.