Piezoelectric effects on the dynamics of optical transitions in GaN/Al_xGa_1-xN multiple quantum wells

Piezoelectric effects on the dynamics of optical transitions in GaN/AlGaN multiple quantum wells (MQWs) have been investigated by picosecond time-resolved photoluminescence (TRPL) measurements. TRPL spectra of the 40 angstrom 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. By comparing experimental and calculation results a low limit of the piezoelectric field strength of about 560 kV/cm in GaN/Al_0.15Ga_0.85N MQWs and the electron and hole wave functions have been obtained. Temporal response of the excitonic transitions of the GaN/AlGaN MQWs depends on the well width. The recombination lifetimes of the 20 angstrom well MQWs decreases monotonously with an increase of emission energy. However, emission energy dependence of the lifetime on 30, 40, 50 angstrom well MQWs which shows a similar behavior as the cw PL spectrum, is quite different from that of 20 angstrom well MQWs. It has been demonstrated that the results described above are 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 and Coulomb interaction.