Group III-Nitride-based wurtzite self-assembled quantum dots (QDs) have attracted much attention in the fundamental semiconductor physics and for potential device applications. An important characteristic of group III-Nitride compounds is the existence of a large polarization field, originating from both piezoelectric and pyroelectric polarizations. The charge polarization will create electric fields along the [0001] QD growth axis that will be screened in a complex manner when electrons and holes begin to fill the excited energy levels of the QD during sufficient levels of excitation. In this work, GaN/AlN self-assembled QDs were grown by the Stranski-Krastanov method on a Si(111) substrate using molecular beam epitaxy. During the subsequent cooling from growth temperatures, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing vertically stacked QDs leads to an additional tensile stress at the Si/III-Nitride interface, which is partially relaxed by the formation of microcracks that propagate parallel to the interface and mainly along the <11-20> directions. We demonstrate that these defects serve as excellent stressors which can modify the strain tensor of QDs in close proximity (i.e., within a few mm's) of the microcracks.^1,2 The excitonic luminescence of QDs with a uniaxial stress perturbation exhibits an in-plane linear polarization anisotropy. Localized cathodoluminescence (CL) spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. We have studied these emissions using time- and polarization-resolved CL for ensembles of QDs.^1,2 We have further exploited the thermal stress in the QD layers by etching stripes of varying widths in order to selectively control the polarization of optical emission. Inductively coupled Cl₂/Ar plasma reactive ion etching was used to etch stripes of varying thicknesses in the samples. SiO₂ and photo-resist mask patterns were formed on the samples, through which the GaN/AlN QD layers were etched. The results show that a suitable choice of stripe width and orientation can enable a selective and local control of polarized emission from the QDs. The effects of screening of the polarization field in the QD, state-filling, changes in the polarization anisotropy and lifetime with varying excitation were studied experimentally and modeled with a 6 x 6 k^.p calculation method.^3,4 Using the method of ensemble CL, we excite locally groups of vertically stacked QDs. The local strain tensors for such QDs, which are subject to an interfacial stress perturbation, have been determined by modeling the dependence of the QD excitonic transition energy on the interfacial stress. Varying levels of excitation are employed by adjusting the e-beam current to analyze the effects of carrier filling in the QDs and the resulting emission from excited states. Experimental results indicate that the polarization anisotropy vanishes at high temperatures with an increasing excitation of the QDs, while the anisotropy decreases more slowly with excitation at low temperatures.³ A theoretical modeling of the effect of carrier filling on the polarization anisotropy and lifetime was performed, as based on 3D self-consistent solutions of the Schrödinger and Poisson equations using the 6 x 6 k^.p and effective mass methods for the calculation of the e-h wavefunctions, including the effects of the interfacial stress, microcrack-induced stress perturbation, carrier-induced screening of the QD polarization field, temperature, and occupation of the QD excited sates through changes in the electron and hole quasi-Fermi levels. 

¹ G. Sarusi, O. Moshe, S. Khatsevich, D. H. Rich, and B. Damilano, Phys. Rev. B 75, 075306 (2007).

² O. Moshe, D. H. Rich, B. Damilano and J. Massies, Phys. Rev. B 77, 155322 (2008).

³ O. Moshe, D. H. Rich, B. Damilano and J. Massies, J. Vac. Sci. Technol. B 28 (4), C5E25 (2010).

⁴ O. Moshe, D. H. Rich, S. Birner, M. Povolotskyi, B. Damilano, and J. Massies, J. Appl. Phys. 108, 83510 (2010).