Photoluminescence dynamics of exciton-exciton scattering in a lightly alloyed InGaN thin film

Abstract

We have investigated the photoluminescence ͑PL͒ dynamics of a lightly alloyed In 0.02 Ga 0.98 N thin film under intense excitation conditions at 10 K. In the In 0.02 Ga 0.98 N thin film, a PL band due to exciton-exciton scattering, the so-called P band, appears with a thresholdlike nature in the excitation-power region higher than ϳ3 J / cm 2 . Under the condition that the exciton-exciton scattering occurs, the PL-decay profile consists of a fast decay component of the P band ͑of the order of 10 ps͒ and a slow decay component of a localized exciton band. The decay time of the P band gradually shortens with increasing excitation power. The change in the decay time can be explained qualitatively by the photon-like characteristics of the lower polariton branch that is the final state of the P emission. Furthermore, we have clearly observed the temporal change in the peak energy of the P band, which reflects an effective temperature of the excitonic system. The energy shift of the P band suggests that the P emission process reaches equilibrium with the lattice temperature after ϳ100 ps.Under intense excitation conditions, inelastic scattering of excitons is a key process leading to stimulated emission. 1 In the scattering process of two n = 1 excitons, one is scattered into a high-energy exciton state with n Ն 2, while the other is scattered into a photon-like state ͑lower polariton branch͒ under energy and momentum conservation of the two excitons. This exciton-exciton scattering process causes the so-called P emission that has been mainly investigated in wide gap semiconductors: e.g., CdS, 2 ZnO, 3,4 CuI, 5-7 GaN, 8-10 and lightly alloyed InGaN. 11 Recently, InGaNbased optoelectronic devices have been significantly developed in the wavelength region from near ultraviolet to green light. The luminescence mechanism of InGaN has been discussed from an aspect of exciton localization due to spatial fluctuations of alloy potentials. 12,13 In our previous work, 11 we demonstrated that the exciton-exciton scattering process occurs in a lightly alloyed In 0.02 Ga 0.98 N thin film under a considerably low excitation power in comparison with the case of GaN. A possible explanation for this phenomenon is as follows. The results obtained using confocal microscopy indicate the formation of alloy-potential domains in InGaN epilayers. 14 In the In 0.02 Ga 0.98 N thin film, the potential depth is rather shallow because the alloy composition is very low. Thus, it is expected that the potential domains for the localization of excitons are easily filled under intense excitation conditions, and then those act as collision centers for free excitons, resulting in the enhancement of the efficiency of exciton-exciton scattering.In the present study, we have investigated the photoluminescence ͑PL͒ dynamics of exciton-exciton scattering leading to the P band in a lightly alloyed In 0.02 Ga 0.98 N thin film under intense excitation conditions. Note that there has been no report on the PL dynamics of the P band in InGaN systems until now. We h...

Carrier thermalization under stimulated emission in In0.17Ga0.83N epilayer at room temperature

“…Other possible mechanism of SE, such as exciton-exciton scattering, can be excluded because the SE peak does not show a redshift with increasing excitation density. 14,15 Note that the excitation density dependence of emission at 460 nm is apparently different from that of SE peak. The PL intensity of SPE band increases with increasing excitation density up to 3.68 mJ/cm 2 , and then saturates.…”