Quantum dots ͑QDs͒ of high symmetry ͑e.g., C 3v ͒ have degenerate bright exciton states, unlike QDs of C 2v symmetry, making them intrinsically suitable for the generation of entangled photon pairs. Deviations from C 3v symmetry are detected in real QDs by polarization-resolved photoluminescence spectroscopy in side-view geometry of InGaAs/AlGaAs dots formed in tetrahedral pyramids. The theoretical analysis reveals both an additional symmetry plane and weak symmetry breaking, as well as the interplay with electron-hole and hole-hole exchange interactions manifested by the excitonic fine structure. DOI: 10.1103/PhysRevB.81.161307 PACS number͑s͒: 78.67.Hc, 71.70.Gm, 73.21.La, 78.55.Ϫm Semiconductor quantum dots ͑QDs͒ exhibit atomiclike energy spectra potentially useful in the area of quantuminformation processing. The indistinguishable radiation paths of the biexciton cascade decay have been proposed as the source of polarization-entangled photons.1 In the conventional QD fabrication process the nucleation of strained InAs QDs occurs spontaneously on the ͑001͒ plane of Zincblende crystals. The symmetry of these QDs is thus limited by the crystal to C 2v .2 The resulting anisotropy of the confined exciton breaks the degeneracy of its bright states, which prohibits entanglement and produces a fine structure splitting ͑FSS͒ characterized by the emission of two linearly polarized photons of unequal energies. Nevertheless, entangled photon pairs from such QDs have been detected by means of careful preselection of particular QDs, 3,4 by spectral postselection, 5 at the price of losing photons, or by the heavy use of external magnetic fields to restore the intermediate level degeneracy. 6 In the quest of more efficient QD sources of entangled photons, it was recently predicted that replacing the conventional GaAs barriers by InP significantly reduces the exciton FSS in such InAs self-assembled QDs. 7 Until now, however, studies of the FSS of neutral and charged exciton complexes have been limited to QDs of C 2v or lower symmetry. [2][3][4][5][6][7][8][9][10][11] In this Rapid Communication, we experimentally and theoretically investigate the FSS in QDs with high symmetry. Zincblende QDs of C 3v symmetry can ideally be achieved by choosing ͓111͔ as the crystallographic direction of crystal growth instead of the conventional ͓001͔ direction. For this growth geometry, including the lack of inversion symmetry in the crystal and the effects of strain and piezoelectric fields, the minimal symmetry is C 3v as long as the QD heterostructure has symmetrical shape. Here we utilize InGaAs/AlGaAs QDs that allow the simultaneous study of the FSS of dominating heavy-hole ͑hh͒ and light-hole ͑lh͒ excitons as well as a hybrid hh-lh trion by side-view polarization-resolved photoluminescence ͑PL͒ spectroscopy. We show how these trion states can probe a small symmetry breaking in otherwise ideal C 3v QDs due to exchange interactions.The polarization properties of the exciton fine structure depend on the symmetries of the initial and fi...

A systematic and simple theoretical approach is proposed to analyze true degeneracies and polarized decay patterns of exciton complexes in semiconductor quantum dots. The results provide reliable spectral signatures for efficient symmetry characterization, and predict original features for low C(2 nu) and high C(3 nu) symmetries. Excellent agreement with single quantum dot spectroscopy of real pyramidal InGaAs/AlGaAs quantum dots grown along [111] is demonstrated. The high sensitivity of biexciton quantum states to exact high symmetry can be turned into an efficient uninvasive postgrowth selection procedure for quantum entanglement applications

We demonstrate the dimensional tuning of InAs self-assembled quantum dots ͑QDs͒ by changing the growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequence during the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDs while keeping the wetting layer thickness constant. Using the same method but embedding the tuned InAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higher energies while keeping the wetting layer thickness constant. The main process responsible for the QDs size modification is consistent with a kinetically controlled materials redistribution of the InAs islands that minimizes the energy of the epitaxial layers at the start up of the GaAs capping deposition.

Vertically well-aligned ZnO nanorods on Si substrates were prepared by a two-step chemical bath deposition ͑CBD͒ method. The optical properties of the grown ZnO nanorods were investigated by time resolved photoluminescence spectroscopy. It was found that the effective decay time of the near bandgap recombination in the CBD grown ZnO nanorods strongly depends on the diameter of the ZnO nanorods. Typically, the decay curves obtained from these ZnO nanorods show a combination of two exponential decays. The experimental results show that the fast exponential decay is related to the surface recombination and the slow decay is related to the "bulk" decay. The measured decay time of the effective surface recombination decreases with decreasing diameter, while the bulk decay time remains unchanged. The results also show that an annealing treatment around 500°C significantly reduces the surface recombination rate. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps.

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