ABSTRACT
There is also a weaker peak, on the red side of the red band. The origin of this multi-modal spectrum is unknown, but is most likely a multi-modal distribution in the vertical confinement.
We embed the quantum rings between an electron reservoir (n+-GaAs, the back contact) and the sample surface. The rings are just 25 nm from the back contact allowing fast tunneling of electrons from the back contact into the dots, but tunneling to the surface is blocked by 150nm of barrier material. Ohmic contacts are made to the back contact and a NiCr semi transparent gate electrode is deposited onto the surface. The high doping in the back contact pins the Fermi energy close to the GaAs conduction band edge, and by changing the gate voltage Vg we can alter the position of the ring levels relative to the Fermi energy, allowing us to alter the charge state of the rings. There exists a pronounced Coulomb blockade such that the charge remains constant over large regions of Vg [3]. We present here measurements of the PL from individual quantum rings at 4.2 K. The PL is excited by generating carriers in the wetting layer with a pump wavelength between 820 and 850 nm. A magnetic field is applied along the growth direction (the [001] direction). The results of a typical experiment are shown in Fig. 2, a plot of the PL energy against Vg representing the PL intensity with a grey scale. At large and negative Vg, the PL is quenched because of electric-field induced ionisation, but at a particular Vg emission from the neutral exciton (X°) appears. At more positive Vg, the PL abruptly red-shifts because the singly-charged exciton (X1-) is now favoured. At higher Vg still the transition from X1-to X2-is observed. The X2” shows a characteristic doublet because there are two possible final states split by the exchange energy [3]. The broadening at higher Vg is associated with the occupation of the wetting layer states with electrons.
