Imaging Nonradiative Point Defects Buried in Quantum Wells Using Cathodoluminescence

In situ observation of thermal-driven structural transitions of a β-NaYF4 single nanoparticle aided with correlative cathodoluminescence electron microscopy

NaYF4 systems have been widely studied as up-conversion host matrices, and their phase transitions are flexible and worth investigating in great detail. Herein, the evolution of morphology and crystal structure of a Eu3+-doped β-NaYF4 single nanoparticle heated in an air atmosphere was investigated using in situ transmission electron microscopy (TEM). The annealing process revealed that the hexagonal β-NaYF4 phase undergoes sequential transformations into high-temperature cubic phases at both 350 °C and 500 °C. The emission characteristics of Eu3+ in the single nanoparticle after heating treatment were also analyzed using Correlative Cathodoluminescence Electron Microscopy (CCLEM). The results of CCLEM suggest a gradual decrease followed by a subsequent increase in structural symmetry. A comprehensive spectroscopic and structural analysis encapsulates the entire transformation process as NaYF4 → YOF → Y2O3. In situ energy dispersive spectroscopy analyses (EDS) support this reaction process. The aforementioned technique yields correlative lattice-resolved TEM images and nanoscale spectroscopic information, which can be employed to assess the structure-function relationships on the nanoscale.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

Cathodoluminescence (CL) nanoscopy is proven to be a powerful tool to explore nanoscale optical properties, whereby free electron beams achieve a spatial resolution far beyond the diffraction limit of light. With developed methods for the control of electron beams and the collection of light, the dimension of information that CL can access has been expanded to include polarization, momentum, and time, holding promise to provide invaluable insights into the study of materials and optical near-field dynamics. With a focus on the burgeoning field of CL nanoscopy, this perspective outlines the recent advancements and applications of this technique, as illustrated by the salient experimental works. In addition, as an outlook for future research, several appealing directions that may bring about developments and discoveries are highlighted.

Inhomogeneous spatial distribution of non radiative recombination centers in GaN/InGaN nanowire heterostructures studied by cathodoluminescence

In order to elucidate the mechanisms responsible for cathodoluminescence intensity variations at the scale of single InGaN/GaN nanowire heterostructures, a methodology is proposed based on a statistical analysis on ensembles of several hundreds of nanowires exhibiting a diameter of 180, 240 and 280 nm. For 180 nm diameter, we find that intensitiy variations are consistent with incorporation of point defects obeying Poisson's statistics. For wider diameters, intensity variations at the scale of single NWs are observed and assigned to local growth conditions fluctuations. Finally, for the less luminescent nanowires, a departure from Poisson's statistics is observed suggesting the possible clustering of non independent point defects.

Investigation of the Impact of Point Defects in InGaN/GaN Quantum Wells with High Dislocation Densities

In this work, we report on the efficiency of single InGaN/GaN quantum wells (QWs) grown on thin (<1 µm) GaN buffer layers on silicon (111) substrates exhibiting very high threading dislocation (TD) densities. Despite this high defect density, we show that QW emission efficiency significantly increases upon the insertion of an In-containing underlayer, whose role is to prevent the introduction of point defects during the growth of InGaN QWs. Hence, we demonstrate that point defects play a key role in limiting InGaN QW efficiency, even in samples where their density (2-3 × 109 cm-2) is much lower than that of TD (2-3 × 1010 cm-2). Time-resolved photoluminescence and cathodoluminescence studies confirm the prevalence of point defects over TDs in QW efficiency. Interestingly, TD terminations lead to the formation of independent domains for carriers, thanks to V-pits and step bunching phenomena.

Nitrogen vacancies in GaN templates and their critical role on the luminescence efficiency of blue quantum wells

In InGaN-based LEDs, an InGaN layer underlying active region has been widely used to improve the luminescence efficiency of the quantum wells (QWs). It has been reported recently that the role of InGaN underlayer (UL) is to block the diffusion of point defects or surface defects in n-GaN into QWs. The type and the source of the point defects need further investigations. In this paper, using temperature-dependent photoluminescence (PL) measurements, we observe emission peak related to nitrogen vacancies (V_N) in n-GaN. In combination with secondary ion mass spectroscopy (SIMS) measurement and theoretical calculation, it is found that V_N concentration in n-GaN is as high as about 3 × 10¹⁸cm^-3 in n-GaN grown with low growth V/III ratio and can be suppressed to about 1.5 × 10¹⁶cm^-3 by increasing growth V/III ratio. Luminescence efficiency of QWs grown on n-GaN under high V/III ratio is greatly improved. These results indicate high density of nitrogen vacancies are formed in n-GaN layer grown under low V/III ratio and diffuse into quantum wells during epitaxial growth and reduce the luminescence efficiency of the QWs.

Yellow-Green Luminescence Due to Polarity-Dependent Incorporation of Carbon Impurities in Self-Assembled GaN Microdisk

Yellow-green luminescence (YGL) competes with near-bandgap emission (NBE) for carrier recombination channels, thereby reducing device efficiency; yet uncovering the origin of YGL remains a major challenge. In this paper, nearly stress-free and low dislocation density self-assembled GaN microdisks were synthesized by Na-flux method. The YGL of GaN microdisks highly depend on their polar facets. Variable accelerating voltage/power CL, variable temperature PL, and Raman spectroscopy are further performed to clarify the origin of polarity dependence of GaN microdisk YGL behavior, which indicates its independence of dislocations, surface effects, stress, crystalline quality, and gallium vacancies. It was found that the incorporation ability of carbon impurities in the polar (0001) facet is greater than that in the semipolar (101̅1) facets, producing higher content of C_N or C_NO_N defects, resulting in a more pronounced YGL in the polar (0001) facet of GaN.

The role of surface states and point defects on optical properties of InGaN/GaN multi-quantum wells in nanowires grown by molecular beam epitaxy

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO_xdeposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range.