GaN/AlN multiple quantum wells grown by molecular beam epitaxy: effect of growth kinetics on radiative recombination efficiency

Ultraviolet (UV) optoelectronic devices based on binary GaN quantum wells have been widely reported in the literature. The internal quantum efficiency (IQE) of such structures is relatively low due to the large dislocation densities generated during heteroepitaxial deposition on to non lattice-matched substrates. Enhancement of IQE is possible through the use of expensive lattice-matched substrates or by using complex dislocation density reducing mechanisms. In this paper we have investigated growth mechanisms of GaN/AlN multiple quantum wells (MQWs) using Plasma Assisted Molecular Beam Epitaxy. Specifically the modulation of the surface diffusivity of adatoms has been carried out through choice of appropriate growth parameters, such as the group-III to group-V flux ratio. Our results indicate that this leads to modification of not only the surface morphology, but also the abruptness of the well-barrier interface. Under conditions of growth where surface morphology was atomically flat, the interfaces are relatively diffuse. The IQE for such structures, as measured by the ratio of room temperature photoluminescence intensity to that measured at 4 K, is rather low typically similar to 10%. Use of near stoichiometric growth conditions however lead to a reduction of the surface diffusivity of adatoms, and the formation of spontaneous nanostructures in the form of nano-dots of about 20 nm in diameter and high levels of uniformity. The IQE for GaN/AlN MQWs grown under such conditions is increased to as high as 28% even for samples with large dislocation densities. Thus, growth under such conditions can mitigate the detrimental effects of non-radiative recombination centers associated with dislocations by spatial localization of electron-hole pairs. These results are important to many applications, including UV light emitting diodes.