Time-Resolved Photoluminescence of GaN / Ga 0.93 Al 0.07 N Quantum Wells
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In the past few years, the rapid development of the growth of group In nitrides has resulted in the realization of heterostructures, yielding high-brightness light emitting devices, for example. However, a complete understanding of the fundamental mechanisms involved in electronic and optical properties of these artificial materials is still lacking. In particular, only few contributions have been devoted, to date, to the dynamics of radiative recombinations in Irn-nitride heterostructures. So far, most studies have been focused on InGaN / GaN multiple quantum wells, [1-4] because these are currently used in UV solid-state lasers. In this system, a very slow decay of luminescence has been observed and has proved to be ruled by a series of nonintentional and hardly controlled phenomena such as alloy inhomogeneities or self-induced condensation of indium dots. On the other hand, GaN-AlGaN quantum wells have a binary compound as confining layers. The physics of excitons in these heterostructures is thus expected to be closer to that of the model system GaAs / GaA1As, thoroughly studied in the past decade [5-7]. To date, only few results have been reported on GaN / A1GaN quantum wells. [8,9] EXPERIMENT In this communication, we study the variation versus lattice temperature of the photoluminescence (PL) decay times in a GaN / Gao.93A10.07N multiple quantum well, grown by 607 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
molecular beam epitaxy on a sapphire substrate. The sample basically consists of multiple quantum well with five periods of 2.5 nm wide GaN confining layers, separated by 10 nm wide Ga0. 93 A10 .07N barriers. For time-resolved photoluminescence (PL), [4,10] the excitation stage produces 2 ps laser pulses with X - 270 nm, obtained by tripling the frequency of infrared pulses from a Ti-sapphire laser, via a nonlinear crystal. The repetition frequency is 82 MHz. The PL signal is analyzed by a spectrometer and detected by a streak camera with a temporal resolution of -5 ps, after correction of the response function of the apparatus. RESULTS Figure 1 displays the continuous PL spectrum of the sample in logarithmic scale (bottom) and the reflectance spectrum (top). The PL was excited here by the 325 nm line of a He-Cd laser. The result of a simple envelope-function calculation of transition energies for the fundamental E 1 -F 9 1 and E-F 7 1 confined excitons is shown by arrows. The high-energy shoulder of the PL spectrum of the quantum well (QW), at 3.550 eV, corresponds to the principal minimum of reflectivity, to the calculated ground exciton energy and also to the peak energy of the PL spectrum measured under strong pulsed UV excitation. These results indicate that the dominant line of the cw PL, at 3.535 eV, corresponds to localized excitons, induced by well width and depth fluctuations, which are saturated under strong excitation. The PL line (B) at 3.70 eV arises from the Gao.93 A10 .07 N barriers, with 0.8 a LO-phonon replica at 3.61 eV. GaN I AI&oGagN The intensity of the B li
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