How much better are InGaN/GaN nanodisks than quantum wells—Oscillator strength enhancement and changes in optical properties

Abstract

We show over 100-fold enhancement of the exciton oscillator strength as the diameter of an InGaN nanodisk in a GaN nanopillar is reduced from a few micrometers to less than 40 nm, corresponding to the quantum dot limit. The enhancement results from significant strain relaxation in nanodisks less than 100 nm in diameter. Meanwhile, the radiative decay rate is only improved by 10 folds due to strong reduction of the local density of photon states in small nanodisks. Further increase in the radiative decay rate can be achieved by engineering the local density of photon states, such as adding a dielectric coating.InGaN/GaN quantum wells (QWs), with a bandgap tunable over the full visible spectral range, play a vital role in high efficiency visible light emitting diodes (LEDs) and laser diodes 1 . InGaN/GaN quantum dots (QDs) also hold the promise as high temperature quantum photonic devices 2,3 . Unfortunately, a large, strain-induced electric field in III-N heterostructures often severely suppresses the oscillator strength of the exciton and, hence, the radiative decay rate and the internal quantum efficiency (IQE). Since strain is relaxed near free surfaces, nanodisks (NDs) in nanowires, which has a large surfaceto-volume ratio, have been widely considered as a promising solution for improving the IQE of InGaN/GaN photonic devices [4][5][6][7][8][9][10][11] . The accompanying improvement in the radiative decay rate is also important for realizing ultrafast singlephoton sources using InGaN/GaN QDs 3,12 .Despite the observation of enhancements in photoluminescence (PL) intensity and decay rate in InGaN/GaN NDs 4,10,13 , the size-dependent behavior of IQE η int and radiative decay rate γ r , mostly affected by the emitter's oscillator strength f os , remains unclear. Experimental studies using top-down nanopillars reported up to ten-fold enhancement in emission intensity only in nanopillars larger than 100 nm in diameter 10 . However, theoretical studies predicted that strain relaxation is most significant only in the region < 20 nm from the sidewall, suggesting significant improvement of IQE could be possible in small disks ∼ 40 nm in size 9 . Furthermore, no comparisons so far have taken into account the influence of external optical efficiencies on the measured PL intensity, such as input laser absorption efficiency η abs , local density of photon states (LDPS) ρ 14,15 and emission collection efficiency η col 16 , which hinders the extraction of η int , γ r as well as f os .In this work, we systematically compare the PL properties of NDs with diameters ranging from 15 nm to 2 µm. We first present two NDs at the two ends of the diameter range, highlighting their fundamentally different spectral responses to the excitation-induced carrier-screening of piezoelectric fields. We then examine how PL energy, decay time and intensity changes continuously with ND diameter reduction, from which we extract the enhancement of f os , γ r , and η int through careful analysis of various external optical efficiencies.The sample w...

Carrier dynamics in site- and structure-controlled InGaN/GaN quantum dots

“…shows the piezoelectric field is the determining factor of the potential profile, as explained in [11]. The larger strain-relaxation at the sidewall compared to the center of the nanodisk leads to a weaker piezoelectric field and, thus, higher exciton energy.…”

“…The large amount of blueshift (∼ 500 meV) from QW to QD shows that the piezoelectric polarization gives the dominant contribution to the exciton potential profile in our QDs [11]. The piezoelectric polarization lowers the bandgap due to the quantum-confined Stark effect [35].…”

“…The oscillator strength increases due to strain relaxation, whereas the LDPS decreases as the nanodisk diameter is reduced [11]. Meanwhile, since an exciton can be thermally scattered out of the radiation zone in the momentum space, the averaged γ r is expected to decrease as temperature increases [12,13].…”

“…The biexciton emission is different from exciton emission by its binding energy, which is typically negative in InGaN QDs due to the repulsive exciton-exciton Coulomb interaction between the two excitons [11,[19][20][21][22][23][24][25][26]. This effectively leads to a lowering of the potential barrier for a biexciton by its binding energy, which can be extracted from the overall spectral linewidth, as explained in Sec.…”

“…To identify the source of the exciton potential profile we follow the same procedure as in [11]. We measure the PL energy of nine dense arrays of QD-nanodisks with diameters varying from 19 nm to 33 nm as well as a QW-nanodisk with 5 µm diameter on the same sample at a temperature of 10 K. We use a very low excitation intensity P = 1 W/cm 2 to avoid significant screening of the electric field.…”