Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8 K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi-polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295 K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 × 1012 cm−2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers.

Al, Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al, Ga) N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al, Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented "polar" and (11 (2) over bar 2) oriented "semipolar" materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al, Ga) N nanostructure-based LED demonstrators are presented.

We compare the optical properties of GaN/Al0.15Ga0.85N multiple quantum wells grown on an m-plane (10-10) oriented GaN substrate with those of (1-101)-oriented GaN/Al0.15Ga0.85N multiple quantum wells grown by metal-organic chemical vapor deposition on a patterned (001) 7°-off oriented silicon substrate. The optical properties are studied in 8K-300K temperature range. They reveal strong in-plane optical anisotropies as predicted by group theory. Polarized time resolved temperature- dependent photoluminescence experiments are performed providing access to the relative contributions of the non-radiative and radiative recombination processes. We deduce the variation of the radiative decay time with temperature in the two polarizations. A global comparison of the light emission efficiency in both samples is finally addressed and brought in agreement with the predictions of k.p calculations.

We report an unusual temperature dependence of exciton lifetimes in arrays of GaN nanostructures grown onsemipolar (11-22) oriented Al0.5Ga0.5Nalloy by molecular beam epitaxy.Atomic force microscopy measurementsrevealed: (i) a one-dimensional ordering tendency along the [1-100] crystallographic direction together with (ii)an in-plane anisotropy of the nanostructure lateral shape with respect to [1-100] and [11-23] crystallographicaxes. As a consequence, a morphological transition from dot-shaped islands forming an array of nanochains towire-shaped objects elongated along the [1-100] direction was evidenced with the increase of the GaN depositedamount. Nanostructures of different dimensionality were fabricated including quantum dots (QDs), quantumwires (QWRs), and quantum wells (QWs), and the excitonic behavior was investigated as a function of thenanostructure shape. The measured temperature dependencies of the exciton radiative decay revealed its directcorrelation with a spatial confinement, resulting in a temperature-independent exciton lifetime in the case ofQDs, a square root dependence in the case of QWRs, and a linear dependence for QWs. These results, as well asabsolute values of measured lifetimes, are in agreement with theoretical predictions.

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8K–300K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells. VC 2014 AIP Publishing LLC.

AlGaN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlGaN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11-22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaNAl0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated