Photoluminescence studies of InGaN/GaN multi-quantum wells

Abstract

We report measurements of photoluminescence, photoluminescence excitation spectroscopy and photoluminescence time decay on three MOVPE-grown InGaN/GaN multiple quantum well structures with 13% In in the wells and well widths L z = 1.25, 2.5 and 5.0 nm. The PL spectra are dominated by single emission peaks, together with phonon sidebands spaced by a GaN LO phonon energy (92 meV). The peak energies are red-shifted with respect to energies calculated for exciton recombination in square quantum wells and the wide well sample also shows a significant Stokes shift between emission and absorption. Recombination lifetimes measured at 6 K are energy dependent, increasing as the photon energy is scanned downwards through the emission line. They also depend strongly on well width. On the low energy side of the 5 nm well emission line we measure lifetimes as long as 100 ns. Raising the temperature from 6 to 300 K results in a strong reduction of emission intensity for all samples and reduction of the lifetimes, though by a much smaller factor. The peak positions shift slightly to lower energy but by far less than the shift in the band edge. We consider three different theoretical models in an attempt to interpret this data, an exponential tail state model, a model of localization due to In/Ga segregation within the wells and the quantum confined Stark effect model. The QCSE model appears able to explain most of the data reasonably well, though there is evidence to suggest that, in addition, some degree of localization occurs.

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

“…The samples were excited using the frequency tripled output of a 100 fs mode locked Ti:sapphire laser, with a photon energy of 4.881 eV. The use of a pulsed laser, with a pulse duration much shorter than the carrier recombination lifetimes for both samples, 2,8,34 ensures that the peak carrier concentration in the QWs is determined by the power density of the excitation source. This assumes that the light is mainly absorbed due to the excitation of carriers in the GaN and that these carriers are captured by the QWs.…”

“…1 Initially, it was suggested that the main advantage of non-polar InGaN/GaN QWs over their polar counterparts would be enhanced recombination efficiency due to the much shorter radiative lifetimes 2,3 compared to equivalent c-plane structures. [4][5][6][7][8] Recently, it has also been suggested that the phenomena of efficiency droop could also be influenced by the polarity of the QW structure. Efficiency droop, the reduction in the efficiency of light emitting diodes at high drive currents, is one of the most serious problems in the use of nitride Light emitting diodes (LEDs) for high brightness applications.…”

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

“…We therefore use the time (s e ) required for the PL intensity to decay by a factor of 1=e from the maximum intensity to characterise each decay transient. The low temperature PL decay times of InGaN/GaN QWs are also reported 28,30,[41][42][43][44] to vary at different detection energies across the zero phonon line, with an increase in decay time as the detection energy is reduced. This effect has been explained as follows: as the detection energy is reduced, we detect carriers that are localised in regions of progressively increasing In fraction where the local built-in electric field is stronger, thus leading to a reduction in electron-hole wavefunction overlap.…”

“…This effect has been explained as follows: as the detection energy is reduced, we detect carriers that are localised in regions of progressively increasing In fraction where the local built-in electric field is stronger, thus leading to a reduction in electron-hole wavefunction overlap. 30,41,44 Time-integrated PL spectra and extracted PL decay times, s e , measured at different energies across the spectra as a function of temperature are shown in Fig. 5 for sample A and Fig.…”

“…As discussed above, a strong reduction in PL decay time occurs at 10 K towards the high energy side of the spectrum, with a measured decay time of 16(62) ns detected at 2.744 eV to 10(61) ns at 2.802 eV (PL peak) to 2(60.2) ns detected at 2.897 eV, reflecting the inherent variation in radiative recombination rate amongst the localised states. 28,30,41,44 The difference in PL peak energy at 10 K, in each sample, when measured using the lamp or laser sources occurs because the region excited by the two sources contains QWs with different In contents. This is due to the variation in In fraction in the QWs across the wafers as described earlier.…”