We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [−110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the T 1/2 dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

We have developed a technique to control the stacking direction of InAs/GaAs quantum dots (QDs) grown on GaAs(001) by varying the direction of the In flux. Transmission-electron microscope images of the stacked QDs reveal that the stacking direction tilts along the [110] direction according to the projection of the In flux direction on the (−110) and does not tilt in the [−110] direction. This anisotropic tilting behavior of the stacked QDs is considered to be caused by an anisotropic migration of In atoms on the (001) growth front. The linear polarization feature of the edge-emitted photoluminescence (PL) demonstrates a strong anisotropy of the strain distribution attributable to the tilted direction of the stacked QDs. According to multidirectional observations of the polarized PL, anisotropic valence band mixing was caused by strain symmetry lowering owing to the tilted stacking direction.

We have controlled the electronic states of closely‐stacked InAs/GaAs quantum dots with a 4.0 nm spacer layer and investigated the optical gain characteristics. With an increase in the stacking‐layer number (SLN), the [001] transverse‐magnetic (TM) polarization component increases as well as the linear polarization anisotropy in the (001) plane becomes remarkable. These SLN‐dependent polarization characteristics result from the valence‐band mixing induced by the vertically‐coupled electronic states in stacked QDs. We have systematically studied polarized electroluminescence properties of a semiconductor‐optical amplifier devise containing 30‐stacked InAs/GaAs QDs. The net modal gain was analyzed by using the Hakki‐Paoli method. The injection current dependence of the gain spectra shows a state filling effect and a change in the contribution of the TM polarization component. The polarization insensitive gain feature within ±1 dB has been achieved in the low injection current condition. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)