Ga-Polar (In,Ga)N/GaN Quantum Wells Versus N-Polar (In,Ga)N Quantum Disks in GaN Nanowires: A Comparative Analysis of Carrier Recombination, Diffusion, and Radiative Efficiency

Abstract

We investigate the radiative and nonradiative recombination processes in planar (In,Ga)N/GaN(0001) quantum wells and (In,Ga)N quantum disks embedded in GaN(0001) nanowires using photoluminescence spectroscopy under both continuous-wave and pulsed excitation. The photoluminescence intensities of these two samples quench only slightly between 10 and 300 K, which is commonly taken as evidence for high internal quantum efficiencies. However, a side-by-side comparison shows that the absolute intensity of the Ga-polar quantum wells is two orders of magnitude higher than that of the N-polar quantum disks. A similar difference is observed for the initial decay time of photoluminescence transients obtained by time-resolved measurements, indicating the presence of a highly efficient nonradiative decay channel for the quantum disks. In apparent contradiction to this conjecture, the decay of both samples is observed to slow down dramatically after the initial rapid decay. Independent of temperature, the transients approach a power law for longer decay times, reflecting that recombination occurs between individual electrons and holes with varying spatial separation. Employing a coupled system of stochastic integro-differential equations taking into account both radiative and nonradiative Shockley-Read-Hall recombination of spatially separate electrons and holes as well as their diffusion, we obtain simulated transients matching the experimentally obtained ones. The results reveal that even dominant nonradiative recombination conserves the power law decay for (In,Ga)N/GaN{0001} quantum wells and disks.

Polarization control in nitride quantum well light emitters enabled by bottom tunnel-junctions

“…It is likely related to the difference in defect formation mechanics, e.g. higher layer contamination [5,6] for growths in the N-polar orientation by metalorganic vapor phase epitaxy (MOVPE) and by molecular beam epitaxy (MBE) [4,[7][8][9][10], due to the drastically different growth dynamics and chemistry of the N-polar and Ga-polar structures. If the reason for the low IQE of N-polar quantum wells is discovered and solved, and the IQE is brought up to the level of Ga-polar heterostructures, constructing LEDs (or laser diodes) requires placing the quantum wells inside the depletion region of a p-n junction diode.…”

Polarity conversion of GaN nanowires grown by plasma-assisted molecular beam epitaxy

“…Whatever the substrate, it has been found that these NWs mostly exhibited N-polarity, even if a careful chemical pretreatement may in some cases lead to a significant fraction of metal-polar NWs [17 bis]. However, N-polar NWs are intrinsically improper to the realization of efficient devices because of the easy incorporation of impurities on the N-polar surface [18], which motivates the efforts to develop a strategy to convert their polarity from N-polar to metal-polar.…”

Luminescent N-polar (In,Ga)N/GaN quantum wells achieved by plasma-assisted molecular beam epitaxy at temperatures exceeding 700 °C

“…Since we know that the internal quantum efficiency (η = τ eff /τ r ) of this reference sample is close to 1 at 10 K, 23 the comparison of the integrated intensities yields η ≈ 0.03 for the N-polar sample at 10 K.…”

“…Clearly, the decay asymptotically obeys a power law independent of temperature, revealing that recombination occurs between spatially separated, individually localized electrons 6 and holes. 23 In this case, recombination can occur by tunneling and/or diffusion (with a subsequent radiative annihilation of electrons and holes at the same spatial location). With increasing temperature, the decay accelerates (but still follows a power law) and the temporally integrated PL intensity decreases strongly, reflecting the presence of nonradiative recombination.…”

“…The temperature dependence of τ r is given by the inverse PL peak intensity of the transient just after the laser pulse. 22 To obtain absolute values for τ r and τ nr , we compare the temporally integrated PL intensity at 10 K to that of a Ga-polar reference QW emitting in the blue spectral range 23 mounted side-by-side.…”