Extended excitons and compact heliumlike biexcitons in type-II quantum dots

Abstract

We have used magneto-photoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15nm and 1.5 meV respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the hetero-interface. The measurements are extended to over almost six orders of magnitude of laser excitation powers and to magnetic fields of up to 50 tesla. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots, and hence the strength of the electron-hole binding. The diamagnetic shift coefficient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogen-like) two-particle excitonic states to a regime where the emission from (helium-like) four-particle biexcitonic states also become significant.

Charge separation and temperature-induced carrier migration in Ga1−xInxNNuytten1, Hayne2, Bansal3 et al. 2011Phys. Rev. BSelf Cite

“…We observe a clear blueshift when increasing the excitation power density, a phenomenon that is commonly attributed to spatially indirect recombination. [15][16][17] However, since we are studying type-I two-dimensional structures, a spatial separation of the charge carriers is only possible by means of localized potential fluctuations that can separately trap electrons and holes. Indeed, the blueshift of the PL energy with increasing excitation power has also been attributed to carrier filling of the localized states.…”

Tuning the properties of exciton complexes in self-assembled GaSb/GaAs quantum rings

“…In both cases, a strong blueshift is seen, a feature that is characteristic of type-II systems and has been observed, for example, in GaSb/GaAs quantum wells, 17 GaSb/GaAs QD's, 1,15,16,18 and InP/GaAs QD's. 2 In the case of type-II quantum wells, it is believed that the shift is the result of band bending at the heterointerface due to charge separation. 17 The same explanation has also been invoked to explain the blueshift in the PL from type-II QD's.…”

“…It is important to note that because of the spatial separation of electrons and holes, such exciton complexes in these samples will be fundamentally different from those in type-I systems and are analogous to atoms and ions with electrons bound to a positively charged "nucleus." 2,14 The second possible reason for the difference in zero-field PL energy is a change in the hole confinement energies due to a difference in the size of the nanostructures between the two samples. However, confinement energies for holes are much less sensitive to size effects than for electrons, and the TEM data indicate no systematic change in QR base length or height between samples A and B.…”

“…The result is that type-II dots exhibit phenomena that are absent in type-I dots, such as Mott transitions 1 and the formation of excitonic helium. 2 In particular, the strongly staggered band alignment of GaSb/(Al)GaAs type-II nanostructures has attracted much interest for its potential to extend the absorption spectrum of GaAs solar cells beyond 1 μm, 3 and provide deep confining potentials capable of room-temperature charge storage for memory applications. 4,5 Intriguingly, Timm et al have recently demonstrated that, in stark contrast to GaSb quantum dots grown by metal-organic chemical vapor deposition (MOCVD), 6 capped GaSb nanostructures grown by molecular-beam epitaxy (MBE) tend to form quantum rings (QR's).…”

Hole migration and optically induced charge depletion in GaSb/GaAs wetting layers and quantum rings

“…14 Strong blueshifts of emission energy with increasing excitation power are regularly seen in type-II systems. Capacitive charging, [15][16][17] band bending, [18][19][20][21][22][23] and state filling 24 are commonly used to explain the blueshift of the type-II QD/QR emission energy. But capacitive charging is believed to be dominant in the sample investigated here.…”