Effect of thermal annealing and strain engineering on the fine structure of quantum dot excitons

Abstract

The fine structure splitting of bright exciton states is measured for a range of thermally annealed InGaAs quantum dot (QD) samples with differing degrees of In/ Ga intermixing and also for a dot-in-a-well (DWELL) structure. Magnitudes of the fine structure splitting are determined in polarization-resolved differential transmission experiments from measurements of the period of quantum beats observed in QD exciton dynamics. The splitting is found to decrease in structures with weaker strain: both for In/ Ga intermixed QD's and also in dots surrounded by strain-reducing layers (DWELL's). Our findings pave the way to the achievement of entangled two photon sources based on emission from individual QD's, currently prevented since the fine structure splitting is larger than the radiative linewidth. In many QD systems the breakdown of a pure-spin picture for exciton states is observed and linearly polarized exciton eigenmodes form split by the electron-hole exchange interaction 1-10 (see Fig. 1). This prevents the realization of an entangled two photon source based on individual QD's. 11,12 A splitting smaller than the radiative broadening [of order of a few eV (Ref. 13)] is required for photon entanglement 11 in such sources of potential importance for implementation in quantum information systems. 14The fine structure splitting arises due to the long range electron-hole exchange interaction, and occurs in systems with lowered in-plane symmetry. 4,8,15 Due to the reduced atomistic symmetry of zinc blende semiconductors (C 2v symmetry), fine structure splitting is expected for self-assembled dots with cylindrical symmetry about the growth axis, although in this case splittings of only up to Ϸ9 eV are predicted, 8 considerably smaller than those found in most as-grown dot samples (see below). The splitting will then be enhanced by anisotropy in dot shape occurring during crystal growth. In a similar way it is known that the strain field is anisotropic for self-assembled dots, distinguishing the [110] and ͓−110͔ directions, even for dots of spherical shape. 16,17 The strain leads to piezoelectric fields in the samples, and when included in k · p theory treatments of electronic band structure, leads to markedly enhanced p-state splittings; 17 the s-like ground state is expected to show similarly enhanced splitting when strain and piezoelectric fields are enhanced. In accord with the above arguments, in unstrained dots in a glass matrix, the fine structure is zero. 18 Further support to the above reasoning is given by recent experiments where externally applied uniaxial strain was shown to produce a large splitting of bright excitons in quantum wells. 19Here we present systematic studies of the fine structure splitting in ensembles of InGaAs dots, providing experimental evidence for the factors determining the magnitude of the electron-hole exchange interaction. In particular, we show that E FS can be accurately tuned in InGaAs dots by postgrowth thermal annealing (enhancing the In/ Ga intermixing and reducing strain 20 ...

Toward weak confinement regime in epitaxial nanostructures: Interdependence of spatial character of quantum confinement and wave function extension in large and elongated quantum dots

“…The result is again counterintuitive when taking only the pronounced in-plane asymmetry of the In 0.3 Ga 0.7 As QDs into account. We would like to point out that the average FSS is significantly lower than values observed for standard InGaAs QDs in a GaAs matrix [49,[52][53][54]. It is even close to the values observed for strainfree GaAs/AlGaAs QDs [55] or In(Ga)As QDs grown on (111)-oriented GaAs [56][57][58][59], which makes the In 0.3 Ga 0.7 As appealing for the generation of polarization entangled photon pairs using the biexciton-exciton cascade [59,60].…”

Excitonic fine structure and binding energies of excitonic complexes in single InAs quantum dashes

“…We note however that there is yet another important factor affecting the FSS, which is intermixing and the related lattice randomness. From one point of view, intermixing increases the X emission energy as discussed earlier, on the other hand it also smears out the potential anisotropy 42,67 . In effect, already at x = 0.05 of barrier material intermixed into the QDash region (Fig.…”

Size-dependent binding energies and fine-structure splitting of excitonic complexes in single InAs/GaAs quantum dots

“…In the In(Ga)As/GaAs QD system they reach from 0 to 180 meV [24][25][26][27][28][29] and for the slightly different InAs/AlGaAs quantum dots from 500 to 1000 meV [30].…”