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Metamorphosis of a quantum wire into quantum dots

Abstract

Bound states of electron–hole pairs (excitons) in semiconductors possess desirable properties — such as an enhanced oscillator strength for radiative recombination — that hold promise for the next generation of optical devices. However, at typical device operating conditions (room temperature and moderate charge densities), excitons dissociate to form an electron–hole plasma. Dissociation may be prevented by confining excitons to lower dimensions, where their binding energy is expected to increase significantly1. But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire. As the temperature of the structure is lowered, we observe a striking transition from broad and fairly continuous photoluminescence to an intense set of emission peaks which are both energetically sharp and spatially localized. Such behaviour indicates that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated quantum dot, we observe an unusual decrease in the relaxation rate of excitons, such that they radiate (via recombination) from higher energy states before relaxing to their ground state. We argue that this is the manifestation of an exciton relaxation ‘bottleneck’, the existence of which could pose problems for the development of optical devices based on quantum dots.

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Figure 1: Schematic cross-section of the GaAs quantum wire, showing the centre-of-mass wavefunction of an exciton trapped at the T-intersection of the stem and arm quantum wells.
Figure 2: a, A schematic cross-section of the quantum wire (QWR) and related quantum wells.
Figure 3: Line scan of PL along the quantum wire at 20 K.
Figure 4: Line scan of PL along a 30-µm length of the quantum wire at several temperatures.
Figure 5: Magnification of the PL spectra from Fig. 4 at position 27.5 µm at several temperatures, showing the exciton relaxation bottleneck.

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Acknowledgements

We thank H. U. Baranger, T. D. Harris, P. B. Littlewood and R. Zimmermann for discussions.

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Correspondence to Joel Hasen.

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Hasen, J., Pfeiffer, L., Pinczuk, A. et al. Metamorphosis of a quantum wire into quantum dots. Nature 390, 54–57 (1997). https://doi.org/10.1038/36299

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