Exciton freeze-out and thermally activated relaxation at local potential fluctuations in thick AlxGa1−xN layers

Abstract

Al x Ga 1−x N layers with 0.05⩽x⩽0.25 were studied using spectrally and time resolved cathodoluminescence (CL). Continuous wave spectra were taken at temperatures ranging from 5 to 300 K. The near-band-edge peak emission energy exhibits an s-shaped temperature dependence characteristic of disordered systems. This effect is quantitatively explained within a model of potential fluctuations caused by alloy disorder. An s-shape temperature dependence has been observed in other alloy systems including InGaN, however, no systematic study exists for AlGaN. In this work, the s-shape temperature dependence is systematically analyzed as a function of aluminum content and quantitatively correlated with a model of alloy disorder. The shift in the luminescence peak position with respect to the usual temperature dependence of the band gap has been quantified by −σE2/kBT, where σE is the standard deviation of the potential fluctuations. Its dependence on aluminum concentration, x, was found to systematically increase from 7 meV at x=0.05 to 21 meV at x=0.25, following the theory for alloy disorder. The recombination and relaxation kinetics investigated using time-resolved CL are fully consistent with our potential fluctuation model. At 5 K, when the excitons are strongly localized, the exciton lifetime increases monotonically with aluminum content. At elevated temperatures, when the excitons are delocalized, the decay is significantly faster and preferentially nonradiative, regardless of the aluminum content.

Substrate orientation effect on potential fluctuations in multiquantum wells of GaAs∕AlGaAs

“…[9][10][11] Several studies have described the blueshift of the E PL ͑T͒ due to potential fluctuations by using the term E 2 / k B T in the fitting models of the band-gap temperature dependence, where E 2 is the Gaussian dispersion of the Gaussian potential fluctuation profile. [12][13][14][15][16][17] It has been argued that under intermediated-excitation intensities, there is competition between the potential fluctuations and the band-gap renormalization effects, which is related to the radiative-recombination energy of the carriers. 18 preted as a value at which the mechanisms of band-gap renormalization and potential fluctuations cancel each other.…”

Excitonic localization in AlN-rich AlxGa1−xN/AlyGa1−yN multi-quantum-well grain boundaries

“…21 In addition, authors of several pertinent studies have attributed such temperature dependence of NBE energy to carrier population of band-tail states and localization due to carrier freeze-out. 22,23 The final decelerated redshift is subtle in sample C (x ¼ 0.45). This observation coincides with the lower roughness of this sample, which was previously discussed in the AFM results.…”

Time-resolved photoluminescence properties of AlGaN/AlN/GaN high electron mobility transistor structures grown on 4H-SiC substrate

“…It is known in AlGaN alloys grown by MOCVD that there are random fluctuations in the potential due to alloy fluctuations [16][17][18] and this gives rise to carrier localization. Thus, taking into account results of the TRPL measurements and the calculated band structure, we suggest that the AlGaN related emission likely occurred between electrons strongly confined in the triangular AlGaN QW and holes weakly localized on potential fluctuations.…”