Torsten Langer

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.

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.

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.

Large optical polarization anisotropy due to anisotropic in-plane strain in m-plane GaInN quantum well structures grown on m-plane 6H-SiC

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.

Faraday isolators for high average power fundamental mode radiation

The basic principles of faraday isolation and thermal effects in the rod geometry under high power operation have been reviewed. The temperature dependency of the verdet-constant and the magnetization of NdFeB have been identified as the limiting factor of isolation ratio for an unchilled high power isolator device. The focal shift due to the isolator with rod-geometry is independent from beam diameter and strongly depending on beam quality. A highly compact isolator for unpolarized radiation up to 400 W fundamental mode for industrial application surrounding with minimal thermal lensing has been developed. At 400 W and 10-40 °C a transmission of 95-97 %, an isolation larger 20 dB, a thermal focus shift of less than 2 rayleigh ranges and a M2 smaller than 1.2 have been achieved.

Radiative recombination in polar, non-polar, and semi-polar III-nitride quantum wells

Efficient radiative recombination is one of the key properties enabling high performance light emitting devices. We have performed an in-depth experimental analysis of radiative recombination in polar, nonpolar, and semipolar III-nitride quantum wells (QWs), which allows us to elucidate and quantify its mechanisms. We are able to distinguish between localized and free exciton recombination, we clearly see the effect of polarization fields via the quantum-confined Stark effect, and we observe the effect of the valence band structure associated with crystal orientation and strain.

Nonradiative recombination due to point defects in GaInN/GaN quantum wells induced by Ar implantation

In this contribution, we quantitatively investigate nonradiative recombination due to argon implantation induced point defects in GaInN/GaN quantum wells via time-resolved photoluminescence spectroscopy. A significant reduction of carrier lifetimes in the QW is observed already for implantation doses of 1 × 1011 cm-2 and higher due to nonradiative recombination at implantation defects. These new nonradiative processes exhibit thermal activation energies below 40 meV, therefore being a dominant loss mechanism at room temperature. The thermal stability of the defects has been analyzed using rapid thermal annealing (RTA) at 800°C and 850°C. We find a partial recovery of the nonradiative lifetimes after RTA indicating an elimination of some defects.

Internal quantum efficiency of nitride light emitters: a critical perspective

The internal quantum efficiency (IQE) is a key property of light-emitting semiconductor structures. We critically review the most popular methods for determining the IQE. In particular, we discuss the impact of low- temperature non-radiative recombination on temperature-dependent CW photoluminescence measurements. Using temperature-dependent time-resolved photoluminescence we establish a method to verify 100 % IQE at low temperature and thus to obtain absolute internal quantum efficiencies at all temperatures.

Efficiency droop in nitride LEDs revisited: impact of excitonic recombination processes

The efficiency droop in nitride LEDs is currently attributed to either carrier-density-dependent nonradiative recombination or to carrier leakage, both being discussed in terms of a single-particle picture. Our time-resolved photoluminescence results show that the radiative lifetime is independent of carrier density, while the nonradiative lifetime scales with the inverse of the carrier density. This can not be understood in a single-particle model. By means of a many-particle theory approach we obtain a consistent picture with both radiative and Auger recombination enhanced by excitonic electron-hole correlation. In the high carrier density limit single-particle radiative and Auger recombination are recovered.