Thermal ionization of excitons in V-shaped quantum wires

Abstract

The exciton-to-free-carrier transition in GaAs and In x Ga 1Ϫx As V-shaped quantum wires is revealed by means of temperature-dependent magnetoluminescence experiments. The experimental results are in excellent agreement with the diamagnetic shift obtained from a solution of the full two-dimensional Schrödinger equation for electrons and holes including magnetic-field and excitonic effects. In the GaAs wires, the exciton-tofree-carrier transition is found to occur at temperature consistent with the exciton binding energies. In the In x Ga 1Ϫx As wires the diamagnetic shift of the luminescence is found to be free-carrier-like, independent of temperature, due to the weakening of the exciton binding energy induced by the internal piezoelectric field.

Free versus localized exciton in GaAs V-shaped quantum wires

“…1 the exciton binding energy is E b ϭ12.5 meV 4 ͒ and carrier escape from the wires into the barriers. 8 The activation energy for the sample of Fig. 1 is about 40 meV, which is consistent with the potential barrier height.…”

Nanoscale compositional fluctuations in multiple InGaAs/GaAs quantum wires

“…The wave functions are assumed to be factorized in a plane wave along the wire axis and an envelope function which is the solution of a 2D Schroedinger equation for this potential. 9 In the absence of In-enrichement at the apex, we find a very weak confinement that originates primarily from the bending of the quantum well, whereas the weak tapering of the profile does not influence appreciably the carrier wave functions ͓as opposed to the usual V-grooved wires with ͑111͒ sidewalls and stronger tapering͔. The corresponding calculated energy states do not account for the observed optical spectra.…”

Electro-optic processes in InGaAs/GaAs quantum wires grown by MBE on patterned substrates

“…Finally, we know from magnetoluminescence studies on InGaAs/GaAs quantum wires [5] that in a V-shaped wire of typical dimension of 10 nm (L z ) × 20 nm (L y ) the exciton binding energy is of the order of 5 meV taking also into account the effect of the internal piezoelectric field. In order to investigate the electro-optical properties of these excitons we performed luminescence measurements as a function of external electric fields.…”

“…This internal field is about one order of magnitude smaller than the external electric field and it is supposed not to influence the transport properties of the p-i-n quantum wire diodes. The net effect of the internal field is to partially ionize the confined excitons [5], resulting in a predominant contribution of free carriers in the electroluminescence lines, also at low temperature and at the lowest injection currents. We should mention that at low injection currents a slight blue shift of the order of 3 meV is observed, probably due to the screening of the internal piezoelectric field induced by the injected charges.…”