Comparison of the luminous efficiency of Ga- and N-polar In${}_x$Ga${}_{1-x}$N/In${}_y$Ga${}_{1-y}$N quantum wells grown by plasma-assisted molecular beam epitaxy

Abstract

We investigate the luminescence of Ga- and N-polar In${}_x$Ga${}_{1-x}$N/In${}_y$Ga${}_{1-y}$N quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6H-SiC substrates. In striking contrast to their Ga-polar counterparts, the N-polar QWs prepared on freestanding GaN do not exhibit any detectable photoluminescence. Theoretical simulations of the band profiles combined with resonant excitation of the QWs allow us to rule out carrier escape and subsequent surface recombination as the reason for the absence of luminescence. To explore the hypothesis of a high concentration of nonradiative defects at the interfaces between wells and barriers, we analyze Ga- and N-polar QWs prepared on 6H-SiC as a function of the well width. Intense luminescence is observed for both Ga- and N polar samples. As expected, the luminescence of the Ga-polar QWs quenches and red-shifts with increasing well width due to the quantum confined Stark effect. In contrast, both the intensity and the energy of the luminescence from the N-polar samples are essentially independent of well width. Transmission electron microscopy reveals that the N-polar QWs exhibit abrupt interfaces and homogeneous composition, excluding emission from In-rich clusters as the reason for this anomalous behavior. The microscopic origin of the luminescence in the N-polar QWs is elucidated using spatially resolved cathodoluminescence spectroscopy. Regardless of well width, the luminescence is found to not originate from the N-polar QWs, but from the semipolar facets of v-pit defects. These results cast serious doubts on the potential of N-polar QWs grown by plasma-assisted molecular beam epitaxy for the development of long-wavelength light emitting diodes. What remains to be seen is whether unconventional growth conditions may enable a significant reduction in the concentration of nonradiative defects.

arXiv comment12 pages, 10 figures

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• Excitonic and deep-level emission from N- and Al-polar homoepitaxial AlN grown by molecular beam epitaxy
• Ga-polar (In,Ga)N/GaN quantum wells vs. N-polar (In,Ga)N quantum disks in GaN nanowires: Comparative analysis of carrier recombination, diffusion, and radiative efficiency
• Blue (In,Ga)N Light-Emitting Diodes with Buried n+-p+ Tunnel Junctions by Plasma-Assisted Molecular Beam Epitaxy
• Interface recombination in Ga- and N-polar GaN/(Al,Ga)N quantum wells grown by molecular beam epitaxy