Recombination Dynamics in Nitride Quantum Boxes and Quantum Wells for Colors Ranging from the UV to the Red.

ABSTRACT Time-resolved photoluminescence experiments at varying temperature are performed on a series of In x Ga 1-x N/GaN quantum well and quantum box samples of similar compositions (0.15 < x < 0.20). The results are analyzed by using envelope-function calculations of transition energies and oscillator strengths, accounting for internal electric fields. The respective influences of localization and electric fields on radiative and nonradiative lifetimes and on the Stokes shift are deduced. The results indicate that the spatial extension of localization centers is much smaller than the size of the quantum boxes (~10 × 3 nm, typically). The room-temperature radiative efficiency of both quantum well and quantum box samples is enhanced by replacing the topmost GaN barrier by an AlGaN one. INTRODUCTION Despite large densities of nonradiative defects, InGaN/GaN quantum wells (QWs) are known as efficient light-emitting systems [1-3] covering a large part of the visible spectrum. This has been assigned in recent works [4] to carrier localization on deep potential valleys corresponding to the strong potential fluctuations induced by random distribution of indium atoms. The possibility of In-rich nanoclusters induced by de-mixing effects has also been invoked for explaining such localization [4-7]. The complex carrier transfers between these "zero-dimensional" [7,8] objects and towards nonradiative recombination centers induce complex energy relaxation [9-11] between energy levels below some "mobility edge" [12]. Whatever their exact origin, these localized potential minima increase the nonradiative lifetime, limited by the escape of carriers towards nonradiative recombination centers, most probably related to dislocations in group-III nitrides grown on sapphire or SiC substrates. Such a localization induces a large red-shift of optical transitions and a strong increase of the radiative lifetime, by expansion of the exciton wave-function in k-space away from the photon wave-vector [13]. Now, it has been also understood recently that internal electric fields of several hundred kV/cm are present along the growth direction of nitride-based QWs, due to the difference of piezoelectric and spontaneous polarizations in the well and barrier materials [14-23]. These electric fields, too, induce a large red-shift of emission lines, due to the quantum-confined Stark effect, and they separate the electron and hole wave-functions so that the radiative lifetime is drastically increased [18,19,22].