Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QWs) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, even when using processing techniques such as epitaxial lateral overgrowth (ELO), non-polar GaN grown on sapphire presents high densities of extended defects. While dislocations are considered as non-radiative recombination centers, basal plane stacking faults (BSFs) are optically active and give rise to an emission centered at 3.42 eV. We present here a low-temperature time-resolved (TR) cathodoluminescence (CL) study of exciton dynamics as a function of the local BSF density in a-plane (Al,Ga)N/GaN single QWs grown by molecular beam epitaxy on a ELO-GaN template. Our TR-CL set-up has spatial and temporal resolutions of 50 nm and 50 ps, respectively, and experiments can be carried out between 25 and 300 K. We first demonstrate through CL experiments the existence of regions nearly free of any BSF as well as of regions with bundles of BSFs, indicating that the BSF distribution in the QW plane reproduces well the one of the underlying a-plane GaN template. We confirm the results obtained by Badcock et al., who demonstrated that the intersection of BSFs with the QW leads to the formation of QWRs, and we then study the local relaxation-recombination dynamics of excitons in both QW and QWRs. We show, in particular, that the dynamics of QW excitons is dominated by their capture by the BSFs and therefore exhibits a strong position dependency, explaining the large range of values reported so far for exciton radiative lifetimes in non-polar (Al,Ga)N/GaN QWs. We finally demonstrate that below 60 K, QWR excitons exhibit a zero-dimensional behavior, which we relate to their binding on localization centers such as QW-width fluctuations.

We present a low-temperature time-resolved cathodoluminescence study on (In,Ga)N/GaN quantum wells grown on the (11-22) facets of non-coalesced ELO-GaN. Taking advantage of the quantum confined Stark effect, such structures have been proposed as promising monolithic white light emitters. Here, we show that in order to achieve solar-like white light emission from this kind of structure, one has to pay attention on the respective evolutions of radiative and non-radiative QW exciton decay times along the (11-22) facets.

Low-temperature photoluminescence is studied in detail in GaN nanocolumns NCs grown by plasma-assisted molecular beam epitaxy under various conditions substrate temperature and impinging Ga/N flux ratio . The relative intensities of the different emission lines, in particular those related to structural defects, appear to be correlated with the growth conditions, and clearly linked to the NC sample morphology. We demonstrate, in particular, that all lines comprised between 3.10 and 3.42 eV rapidly lose intensity when the growth conditions are such that the NC coalescence is reduced. The well-known line around 3.45 eV, characteristic of GaN NC samples, shows, however, a behavior that is exactly the opposite of the other lines, namely, for growth conditions leading to reduced NC coalescence, this line tends to become more prominent, thus proving to be intrinsic to individual GaN NCs.

Time-resolved cathodoluminescence at 27 K has been performed on a-plane GaN grown by epitaxial lateral overgrowth. We detail the relaxation and recombination mechanisms of excitons [free or bound to neutral donors, or bound to I1-type basal stacking faults (BSFs)] in relation to the local density in BSFs. We describe the slow exciton capture rate on isolated BSFs by a diffusion model involving donors via a hopping process. Where BSFs are organized into bundles, we relate the shorter rise time to intra-BSF localization processes and the multiexponential decay to the type-II band alignment of BSFs in wurtzite GaN.

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I1 stacking faults in the overall recombination mechanism of excitons.

A detailed study of excitons in unstrained GaN nanocolumns grown by plasma assisted molecular beam epitaxy on silicon substrates is presented. The time-integrated and time-resolved photoluminescence spectra do not depend significantly on the (111) or (001) Si surface used. However, an unusually high relative intensity of the two-electron satellite peak of the dominant donor-bound exciton line is systematically observed. We correlate this observation with the nanocolumn morphology determined by scanning electron microscopy, and therefore propose an interpretation based on the alteration of wave functions of excitonic complexes and of donor states by the proximity of the semiconductor surface. This explanation is supported by a model that qualitatively accounts for both relative intensities and time decays of the photoluminescence lines.

Picosecond time-resolved cathodoluminescence to study GaN based materials