The optical properties of AlxGa1−xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al content, the low-temperature PL decay lifetime increases with Al content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the AlxGa1−xN alloys. The Al-content dependence of the energy-tail-state distribution parameter E0, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and, consequently, increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.

Abstract:Piezoelectric effects on the optical properties of GaN/AlGaN multiple quantum wells (MQWs) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. For MQWs with well thickness 30 A and 40 A, the excitonic transition peak positions at 10 K in continuous wave (cw) spectra are redshifted with respect to the GaN epilayer by 17 meV and 57 meV, respectively. The time-resolved PL spectra of the 30 A and 40 A well MQWs reveal that the excitonic transition is in fact blueshifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. We have demonstrated that the results described above is due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWs subject to elastic strain together with screening of the photoexcited carriers. By comparing experimental and calculation results, we conclude that the piezoelectric field strength in GaN/Alo.isGao.8sN MQWs has a lower limit value of about 560 kV/cm. The electron and hole wave function distributions have also been obtained. The implication of our findings on the practical applications of GaN based optoelectronic devices is also discussed.

Piezoelectric effects in GaN/AlGaN multiple quantum wells (MQWs) have been directly probed by picosecond time-resolved photoluminescence (PL) spectroscopy. The time-resolved PL spectra of the 40 Å well MQWs reveal that the PL transition peak position is in fact blueshifted at early delay times due to the collective effects of quantum confinement of carriers, piezoelectric field, and Coulomb screening. However, the spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. By comparing experimental and calculation results, we have obtained a low limit of the piezoelectric field strength to be about 560 kV/cm in the 40 Å well GaN/Al 0.15 Ga 0.85 N MQWs.

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