Evolution of the m-Plane Quantum Well Morphology and Composition within a GaN/InGaN Core–Shell Structure

Pierre-Marie Coulon,Emmanuel D Le Boulbar,Rachel A Oliver,Philip A Shields,Duncan W E Allsopp,Paul R Edwards,Shahrzad Hosseini Vajargah,Ionut Girgel,An Bao,Colin J Humphreys,Robert W Martin

doi:10.1021/acs.cgd.6b01281

Pierre-Marie Coulon, Emmanuel D Le Boulbar + Show 9 more

Open Access

https://doi.org/10.1021/acs.cgd.6b01281

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Abstract

GaN/InGaN core–shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume, and strain relaxation at the free surfaces. The core–shell geometry allows the growth of thicker InGaN shell layers, which would improve the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth, and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two-dimensional (2D) growth mode to three-dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN cor...

Highlights

I nGaN alloys and more generally III-nitride materials have been widely studied since their successful application in blue light-emitting diodes (LEDs) and laser diodes (LDs) in the early 90s.1 With the possibility to tune the wavelength emission from 3.4 eV for GaN, to 0.7 eV for InN, InGaN quantum wells have been employed to achieve UV, blue, green, and white LEDs2,3 and used for solar cells[4,5] or for water splitting.[6,7]
The result is the generation of new dislocations and three-dimensional (3D) growth that directly impact the InGaN emission properties.[8,9,16−18] thick InGaN layers grown with the conventional c-plane orientation are seriously impacted by the quantum-confined Stark effect (QCSE), which results in poor emission efficiency due to the large spatial separation between electron and hole wave functions
Combined atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX) evidence shows that the presence of quasi-periodic atomic steps along the c-axis of the m-plane GaN facets of the NRs induce preferential incorporation of species at or near the step edges, leading to lateral fluctuations in InN content along the c-axis

Summary

■ RESULTS AND DISCUSSION

The InGaN shell layers used for this study were all grown on etched GaN NR arrays with a pitch of 2 μm, obtained by etching a GaN 2D layer down to the silicon substrate.[34]. The initial quasi-periodic spacing of 20−40 nm between the step-like features observed for 0 min remains the same for longer InGaN growths up to the 18 min sample where merging/coalescence and local misalignment of the striation line features leads to an increase in the spacing, especially at the intersections of the m-planes where the spacing can reach up to 100 nm as the striations merge into larger structures (Figure 2f and Figure S2d of the Supporting Information) This is due to the faster growth rate and higher indium incorporation at these positions. The CL spectroscopy data confirm the compositional differences between the initial 2D layer and the relaxed 3D striations found by EDX and further reveal a tendency for the volume-averaged indium content of the 3D striated features to increase with growth time

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES