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The ideal single-photon source displaying high brightness and purity, emission on-demand, mature integration, practical communication wavelength (i.e., in the telecom range), and operating at room temperature does not exist yet. In 2018, a new single-photon source was discovered in gallium nitride (GaN) showing high potential thanks to its telecom wavelength emission, record-high brightness, good purity, and operation at room temperature. Despite all these assets, its coupling to photonic structures has not been achieved so far. In this article, we make a first step in this direction. First, we analyze whether stacking faults are indeed a necessary condition for obtaining such emitters in GaN layers. Then, we discuss the challenges associated to a low spatial density and to a spectrally wide distribution of emitters, which necessitate their location to be determined beforehand and the photonic structure resonance to be tuned to their emission wavelength. The design and fabrication of bullseye antennas are thoroughly described. Finally, we fabricate such bullseyes around telecom emitters and demonstrate that the embedded emitters are able to sustain the necessary clean-room process and still operate as single-photon emitters after the fabrication steps, with room-temperature purities up to 99% combined with repetition rates in the order of hundreds of kHz. The findings in this work demonstrate that telecom single-photon emitters in GaN operating at room temperature are well adapted for single-photon applications where brightness and purity are the required figures of merit, but highlight the numerous difficulties that still need to be overcome before they can be exploited in actual quantum photonic applications.

In this work, the physical and the electrical properties of vertical GaN Schottky diodes were investigated. Cathodo-luminescence (CL), micro-Raman spectroscopy, SIMS, and current-voltage (I-V) measurements were performed to better understand the effects of physical parameters, for example structural defects and doping level inhomogeneity, on the diode electrical performances. Evidence of dislocations in the diode epilayer was spotted thanks to the CL measurements. Then, using 2D mappings of the E 2 h and A1 (LO) Raman modes, dislocations and other peculiar structural defects were observed. The I-V measurements of the diodes revealed a significant increase in the leakage current with applied reverse bias up to 200 V. The combination of physical and electrical characterization methods indicated that the electrical leakage in the reverse biased diodes seems more correlated with short range non-uniformities of the effective doping than with strain fluctuation induced by dislocations. [ABSTRACT FROM AUTHOR]

The variation of the internal quantum efficiency (IQE) of single InGaN quantum well structures emitting from blue to red is studied as a function of the excitation power density and the temperature. By changing the well width, the indium content, and adding a strain compensation AlGaN layer, we could tune the intrinsic radiative recombination rate by changing the quantum confined Stark effect, and we could modify the carrier localization. Strong quantum confined Stark effect and carrier localization induce an increase in the carrier density and then favor Auger non-radiative recombination in the high excitation range. In such high excitation conditions with efficient Auger recombination, the variation of the IQE with the photo-excitation density P is ruled by a universal power law independent of the design: IQE=IQEMAX – a log10P with a close to 1/3. The temperature dependences of the different recombination mechanisms are determined. At low temperature, both quantum confined Stark effect and carrier localization trigger electron-electron repulsions and therefore the onset of the Auger effect. The increase in the value of coefficient C with changing temperature reveals indirect Auger recombination that relates to the interactions of the carriers with other phonons than the longitudinal optical one. [ABSTRACT FROM AUTHOR]

We have grown (11-22)-oriented GaN/Al0.5Ga0.5N quantum wells (QWs) using molecular beam epitaxy on GaN (11-22)-oriented templates grown by metal-organic vapor phase epitaxy on m-plane oriented sapphire substrates. The performance of epitaxial growth of GaN/Al0.5Ga0.5N heterostructures on the semi-polar orientation (11-22) in terms of surface roughness and structural properties, i.e., strain relaxation mechanisms is discussed. In addition, high resolution transmission electron microscopy reveals very smooth QW interfaces. The photoluminescence of such samples are strictly originating from radiative recombination of free excitons for temperatures above 100 K. At high temperature, the population of localized excitons, moderately trapped (5 meV) at low temperature, is negligible. [ABSTRACT FROM AUTHOR]

Securing optical information to avoid counterfeiting and manipulation by unauthorized persons and agencies requires innovation and enhancement of security beyond basic intensity encryption. In this paper, we present a new method for polarization-dependent optical encryption that relies on extremely high-resolution near-field phase encoding at metasurfaces, down to the diffraction limit. Unlike previous intensity or color printing methods, which are detectable by the human eye, analog phase decoding requires specific decryption setup to achieve a higher security level. In this work, quadriwave lateral shearing interferometry is used as a phase decryption method, decrypting binary quick response (QR) phase codes and thus forming phase-contrast images, with phase values as low as 15°. Combining near-field phase imaging and far-field holographic imaging under orthogonal polarization illumination, we enhanced the security level for potential applications in the area of biometric recognition, secure ID cards, secure optical data storage, steganography, and communications. [ABSTRACT FROM AUTHOR]

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition. As the layer growth temperature increases, the cluster density decreases but their size indicted by the number of atoms increases. Moreover, APT reveals that threading dislocations are decorated with Mg atoms. Off-axis electron holography provides complementary information about the electrical activity of the Mg doping. As only a small fraction of dopant atoms are ionised at room temperature, this fraction is increased by annealing the specimen to 400 °C in situ in a transmission electron microscope (TEM). A strong reduction of the dopant electrical activity is observed for increases in the layer growth temperature. The correlation of APT with TEM-based techniques was shown to be a unique approach in order to investigate how the growth temperature affects both the chemical distribution and electrical activity of Mg dopant atoms. [ABSTRACT FROM AUTHOR]

We report on the structural in-plane anisotropy of GaN films grown on A-plane sapphire substrates by metal organic chemical vapor deposition. It is found that GaN:Si grown on A-face sapphire exhibits a strongly anisotropic wafer bending in the two orthogonal in-plane directions, with a ∼24% larger curvature along the c-axis of sapphire than along the m-axis. Using a model developed for an elastically anisotropic bilayer structure and using our curvature data, the anisotropic biaxial stresses in the two in-plane directions have been estimated as σ1x≅-1.3 GPa and σ1y≅-1.1 GPa along parallel and perpendicular to c-axis of sapphire, respectively. This anisotropic stress is also responsible for the distortion of the GaN hexagonal basal plane, as evidenced by x-ray diffraction measurements. The broadening of full width at half maximum of the GaN (0002) x-ray reflections varies with different azimuthal angles, correlated with the tilt of the c-axis of GaN. The in-plane epitaxial relationships between the GaN (0001) and A-face sapphire are found as a-axis of GaN aligned with c-axis of sapphire and m-axis of GaN aligned with m-axis of sapphire. [ABSTRACT FROM AUTHOR]