verslWP2 - IAP 5/01 Quantum Size Effects in Nanostructured...

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Unformatted text preview: IAP 5/01 Quantum Size Effects in Nanostructured Materials ~ Scientific Report 2004 Section III.A. WP2 33 III. WP2: Electrical and optical properties of quantum dots KUL (P1) 1. Influence of temperature and magnetic field on carrier confinement in self-assembled InAs/GaAs quantum dots Given that any device based on self-assembled QDs should operate at room temperature we initiated a study of the PL of InAs QDs (grown University of Nottingham, UK) in pulsed mangetic fields. This study, which represents a significant technical challenge due to the decrease in PL intensity with temperature and the limited photon count times available ( ≤ 4 ms) was the first of its kind. With these experiments we were able to show that the lowering of the PL energy with increasing temperature, T, for T<100 K seen over and above that due to the change in the band gap [1] is due to thermal excitation to dots that are higher but not larger in the plane of the sample. For T >100 K we observed (see Fig. 1) a strong decrease in exciton size as measured by pulsed fields, whilst the zero-field PL energy varied according to the temperature dependence of the band gap implying no changes in excitonic properties. We attribute this apparent contradiction to confinement of carriers to the dots by the magnetic field. This mechanism is stronger for smaller dots, hence as T increases the size of the dots measured in field decreases. 2. Novel GaAs/Al x Ga 1-x As quantum dots fabricated by modified self-assembly The major obstacle preventing real comparison of sophisticated 8-band k.p or atomistic calculations with experimental data on self-assembled quantum dots is that the structural properties (size, shape, material composition, and hence strain profile) of the dots are not known. In order to overcome this problem we investigated novel GaAs/Al x Ga 1-x As quantum dots in collaboration with Max-Planck- Institut für Festkörperforschung (Dr. O. Schmidt), who grew the samples. The latttice mismatch between GaAs/Al x Ga 1-x As is negligible, hence self-assembled quantum dots cannot be formed by strain in the usual way. Instead Stranski-Krastanow grown InAs/GaAs dots are etched in situ in the MBE chamber, and the resulting holes are filled in with Al x Ga 1-x As/GaAs/Al x Ga 1-x As to form GaAs/Al x Ga 1-x As dots. Since the nanoholes can by precisely characterised by scanning probe microscopy, and the dots are known to be pure strain-free GaAs, there are, in principle, no adjustable 20 40 60 80 100 120 140 160 10 12 14 16 18 20 Temperature (K) Energy shift (meV) 5 6 7 8 B // z B ⊥ z B // z B ⊥ z 20 40 60 80 100 120 140 160 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 Bohr radius (nm) Temperature (K) (a) (b) 20 40 60 80 100 120 140 160 10 12 14 16 18 20 Temperature (K) Energy shift (meV) 5 6 7 8 B // z B ⊥ z B // z B ⊥ z B // z B ⊥ z 20 40 60 80 100 120 140 160 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 Bohr radius (nm) Temperature (K) (a) (b) 20 40 60 80 100 120 140 160 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.59....
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verslWP2 - IAP 5/01 Quantum Size Effects in Nanostructured...

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