Quantum-Confined Stark Effect and Recombination Dynamics of Spatially Indirect Excitons in MBE-Grown GaN-AlGaN Quantum W
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.69 (1999) ABSTRACT We analyze the low-temperature photoluminescence decay times, for a series of MBEgrown samples embedding GaN-AIGaN quantum wells. We investigate a variety of configurations in terms of well widths, barrier widths and overall strain states. We find that not only the wells but also the barriers are submitted to large built-in electric fields. In the case of narrow barriers (5 um), these fields favor the nonradiative escape of carriers from narrow wells into wider wells. When all wells have the same width, the field in such narrow barriers allow us to observe the recombination of long-lived "inter-well" excitons at energies close to those of the short-lived "intra-well" excitons. Our results also prove that the energies and the dynamics of excitonic recombinations depend on the parameters of the heterostructures in a complicated way, due to the interplay of piezoelectric and spontaneous polarizations. INTRODUCTION Nitride semiconductor quantum wells (QWs) are emerging as promising systems for the realization of blue-UV light emitters and detectors [1,2]. Their importance is such that it is crucial to obtain a better comprehension of mechanisms that rule optical processes in these heterostructures, especially from the dynamical viewpoint. In particular, it has been recently shown [3-8] that some original behaviors are due to the presence, along the growth axis of the QWs, of built-in electric fields of several hundred kV/cm. Considering the large lattice-mismatch between the constituents, these fields have been assigned [5-8] to the piezoelectric effect, due to very large piezoelectric coefficients. In fact, theoretical investigations [9,101 have established that two mechanisms, essentially similar in nature, should be considered to explain the large electric fields reported thus far. According to these studies, the values of the fields in the wells and in the barriers depend on the difference of piezoelectric polarizations between the two materials and on the difference of their so-called spontaneous (or equilibrium) polarizations. The latter polarizations are essentially provoked by the nonideal c/a ratio of lattice parameters in the hexagonal elementary cells. The magnitudes of these spontaneous polarizations have been found to be particularly large in group-Ill nitride semiconductors; in fact, they are the largest of all III-V compounds [9], with values comparable to those for ZnO or BeO. However, Bernardini and Fiorentini have also calculated that the spontaneous polarizations of GaN and InN are not G 3.69 Mat. Res. Soc. Proc. Vol. 537 © 1999 Materials Research Society
very different. Thus, for ideal (disorder-free) InGaN-GaN QWs, the main effect which induces built-in electric fields is the difference of piezoelectric polarizations. On the other hand, this is not the case for the GaN-AIGaN pair, because the spontaneous polarizations of the two compounds are really different. Both spontaneous and piezoelectric contribution
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