Two-Dimensional In0.4Ga0.6As/GaAs Quantum Dot Superlattices Realized by Self-Organized Epitaxial Growth

We report on the realization of two-dimensional (2D) In0.4Ga0.6As/GaAs quantum dot superlattices (QDSLs) by self-organized epitaxial growth. The conditions for the formation of extended states or minibands are analyzed by treating QD arrays as disordered systems. Ordered quantum dot (QD) arrays are fabricated on GaAs (311)B substrates. High density and small size are achieved by decreasing the growth temperature. A large red shift of the photoluminescence (PL) peak energy and a dramatic narrowing of the linewidth are found when the dots become smaller and closer. The exciton coherence length in the high-density ordered QD array is confirmed to be much larger than the QD diameter by PL decay time measurements and by using beryllium impurities as scattering centers. As a comparison, the incoherent exciton motion dominated by nonresonant tunneling is discussed. The transition from coherent to incoherent, including the intermediate state, and the localization of excitons are demonstrated by various mechanisms.