Toward Crack-Free Core–Shell GaN/AlGaN Quantum Wells

Abstract

Strain relaxation of nonpolar GaN/Al0.6Ga0.4N multiple quantum wells grown in core–shell geometry by metal–organic vapor-phase epitaxy on GaN wires is investigated. Cracking along the a-direction is observed on the sidewalls of c̅-oriented hexagonal GaN wires. To overcome this issue, an undershell including AlGaN gradient and cladding layers is grown before the active region. While a decrease of the crack density is observed with the undershell, the increase of GaN QW thickness acts as a key parameter to limit the crack formation. In agreement with previous studies performed on AlGaN planar layers, a relaxation criterion is found for a threshold strain energy density of ∼4 J/m2. Considering the quantum well structure as a single AlGaN layer with an average composition, a solution to keep the strain energy density below this relaxation limit is identified by reducing the AlGaN barrier thickness from 5 to 3 nm. Combining the undershell and reduced barrier thickness, a crack-free core–shell AlGaN-based structure is demonstrated with an emission at 280 nm corresponding to the UV-B/C limit.

AlGaN Microfins as Nonpolar UV Emitters Probed by Time-Resolved Cathodoluminescence

“…Compared to microrods, microfins offer a much larger homogeneous area of nonpolar planes (tens of microns in height by up to centimeters in length), resulting in reduced edge effects at intersecting planes. , The properties of GaN microfins have been discussed by Hartmann et al, and dislocation densities on the nonpolar a -plane sidewalls as low as (3 ± 2) × 10 5 cm –2 have been demonstrated . A universal challenge for the approach of GaN/AlGaN core–shell microstructures that other researchers and our group have struggled with is the mitigation of cracking of the AlGaN shell layers which grow under increasingly tensile strain as the aluminum content is raised. , In our case, the use of a graded short-period superlattice (SPSL) allows for the growth of a mostly crack-free AlGaN shell that presents itself as an appropriate template for the growth of nonpolar a -plane AlGaN QWs emitting at the intersection of the UV-A and UV-B spectral regions (further information can be found in Supporting Information S1). As is demonstrated in this study, it is indeed possible to largely translate the superior structural quality of selectively grown GaN microstructures into the AlGaN shell layers.…”

LaueNN: neural-network-based hkl recognition of Laue spots and its application to polycrystalline materials

“…The wires have been designed for UV emission and are grown by metal-organic vapour phase epitaxy on c-sapphire substrate using a silane-assisted method (Koester et al, 2010(Koester et al, , 2011. The experimental details related to growth conditions and structural characterization are given by Grenier et al (2021). The diameter of the wires is about 1 mm and the length is 13 mm.…”

Etching of the SiGaxNy Passivation Layer for Full Emissive Lateral Facet Coverage in InGaN/GaN Core–Shell Nanowires by MOVPE

“…In order to significantly increase the vertical growth rate and reach NWs >10 μm in height, large quantities of SiH 4 must be introduced into the reactor. ,− The introduction of SiH 4 during MOVPE growth leads to the formation of a thin passivation layer of SiGa x N y on the lateral facets of the wires, as reported by Tessarek et al This layer acts as a dielectric mask and promotes the diffusion of the reacting species along the sidewalls up to the top of the wire, thus highly enhancing the vertical growth; , the second effect is the enhanced doping of the GaN core, which is beneficial for subsequent core–shell LED fabrication . The presence of this SiGa x N y layer has since then been evidenced in several other reports. − Unfortunately, this passivation layer impacts the structural and optical quality of the InGaN shell grown on it by degrading the sticking of species and the resulting morphology. , To avoid this issue, often the core growth is finished by a “non-intentionally doped” (nid) GaN portion during which SiH 4 is no longer introduced in the reactor. ,, The surface of the nid GaN is thus not covered by the SiGa x N y passivation layer. When the active InGaN shell region is subsequently grown, the shell covers only the top nid part of the wire, which limits the overall light-emitting area. , In addition, the active region grown on the top part can be strongly degraded by a defective ultra-thin SiGa x N y underlayer (UL) due to the residual SiH 4 in the reactor chamber …”

“…Another method to circumvent the negative effect of the SiGa x N y layer consists in burying it. One possibility is to use AlGaN as a UL before the growth of the LED active region, given its ability to nucleate on SiGa x N y . , However, while AlGaN ULs have been used previously for reducing leakage current , and for burying defects and optimizing crystalline quality, , only few reports have shown a successful increase in the InGaN QW intensity at the NW base . Besides, AlGaN can also lead to cracks in the shell, , as reported in the case of N-polar GaN NWs.…”

“…One possibility is to use AlGaN as a UL before the growth of the LED active region, given its ability to nucleate on SiGa x N y . , However, while AlGaN ULs have been used previously for reducing leakage current , and for burying defects and optimizing crystalline quality, , only few reports have shown a successful increase in the InGaN QW intensity at the NW base . Besides, AlGaN can also lead to cracks in the shell, , as reported in the case of N-polar GaN NWs. Similarly, a GaN layer grown at a low temperature reduces the Ga atom mobility and therefore similarly enhances the nucleation on the top of the SiGa x N y layer, thus burying it.…”