Light-emitting diodes (LEDs) have been regarded as promising candidates for visible light communication (VLC); however, strong internal polarization fields in common c-plane LEDs, especially green 

High-Bandwidth Green Semipolar (20-21) InGaN/GaN Micro Light-Emitting Diodes for Visible Light Communication

The growth of (11–20) or a-plane quantum dots and quantum wells by plasma-assisted molecular-beam epitaxy has been studied. It is shown that Ga-rich conditions lead to the formation of quantum dots, whereas quantum wells are obtained in N-rich conditions. Combining various experimental techniques, it is furthermore demonstrated that quantum dot nucleation along [1–100] and quantum well morphology in the (1–100) plane are influenced by anisotropic growth of AlN buffer layer. Moreover, it is established that peculiar morphological features of quantum dots and quantum wells, in particular the asymmetric shape of quantum dots, are related to the polar character of the [0001] direction in wurtzite nitride material.

Anisotropic morphology of nonpolar a-plane GaN quantum dots and quantum wells

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should also be transferable to other wurtzite semiconductors.

https://iopscience.iop.org/article/10.1088/0022-3727/47/42/423001/pdf

Luminescence associated with stacking faults in GaN

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should be transferable also to other wurtzite semiconductors.

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.

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

New conditions for one-step ELO were implemented to grow coalesced (1120) non-polar and (1122) semi-polar GaN layers starting, respectively, from R- and M-plane sapphire A great part of the stacking faults (SFs) and dislocations are filtrated resulting in GaN material with better structural and optical properties In the ELO-like (1120) and (1122) films, the near band edge emission dominates photoluminescence spectra and is in the range 345-348 eV depending on lattice deformation The strongest emission is met for the semi-polar (1122) ELO When mask stripes are not normal to the c-axis, a singular ELO is developed with inclined coalescence facets However, in this case, SFs overgrow above the mask and so lead to poor optical properties, dominated by SF and dislocation related peaks In any case, the internal electric field reduction in (Al,Ga)N/GaN non- or semi-polar quantum-wells stacks is better viewed when the heterostructures are grown on ELO with stripes normal to the c-axis

Reduction of stacking faults in (11$ \bar 2 $0) and (11$ \bar 2 $2) GaN films by ELO techniques and benefit on GaN wells emission

Coalesced nonpolar $(11\overline{2}0)$ and semipolar $(11\overline{2}2)$ GaN layers obtained by epitaxial lateral overgrowth on sapphire have been studied by photoluminescence and reflectivity under normal incidence as a function of temperature and light polarization. In nonpolar GaN, two strongly allowed excitonic levels $A$ and $C$ are evidenced by reflectivity and are also observed in the luminescence spectra. The third exciton level $(B)$ is only observed in the luminescence spectra, showing that its oscillator strength is mainly concentrated in transitions involving light polarized along the growth axis. The lower energy $A$ exciton state is allowed for light polarized perpendicularly to the $c$ axis. Therefore, for light polarized along the $c$ axis, the thermal quenching of the luminescence of $A$ results in a strong increase of the higher energy exciton emission, with an activation energy corresponding to their energetic separation. We observe a similar temperature induced increase of luminescence for transitions involving basal stacking faults and donor-acceptor pairs. In the latter case, our observation is evidence of an acceptor excited state due to the valence band splitting. Careful examination of the energy differences between the $A$ exciton luminescence spectra recorded in both polarizations leads to an exchange splitting of $1\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, in good agreement with previous determinations. Our experimental results, exciton energies, and oscillator strengths are in good agreement with those calculated from a $\mathbf{k}=\mathbf{0}$ strained wurtzite valence band Hamiltonian using strain values measured by x-ray diffraction. Finally, the results for semipolar GaN are qualitatively similar to those obtained from nonpolar GaN. However, the optical selection rules are not as severe, owing to the fact that normal incidence light is never fully polarized perpendicular to the wurtzite basal plane.

Band-edge photoluminescence and reflectivity of nonpolar (112̄0) and semipolar (112̄2) GaN formed by epitaxial lateral overgrowth on sapphire

Low temperature spatially resolved cathodoluminescence was carried out on GaN films grown by the epitaxial-lateral-overgrowth (ELO) technique with the nonpolar (11-20) and the semipolar (11-22) orientations on R- and M-sapphires, respectively. Defect related optical transitions were identified and their localization was correlated to different regions of ELO. The sample microstructure was further investigated by plan-view and cross-section transmission electron microscopies. It is shown that the defect related emissions are mainly localized in the seed of the samples, but different defects occur as well in the wings, especially in the case of nonpolar GaN. The structural defect densities are lowest in the overgrown wings of semipolar GaN. In particular, the [0001] wing region of semipolar ELO-GaN is almost defect-free with a cathodoluminescence spectrum dominated by the GaN band-edge emission at 3.476eV.

Cathodoluminescence spectroscopy of epitaxial-lateral-overgrown nonpolar (11-20) and semipolar (11-22) GaN in relation to microstructural characterization

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Reduction of stacking faults in (11$ \bar 2 $0) and (11$ \bar 2 $2) GaN films by ELO techniques and benefit on GaN wells emission

Band-edge photoluminescence and reflectivity of nonpolar (112̄0) and semipolar (112̄2) GaN formed by epitaxial lateral overgrowth on sapphire

Cathodoluminescence spectroscopy of epitaxial-lateral-overgrown nonpolar (11-20) and semipolar (11-22) GaN in relation to microstructural characterization