Vertical order in stacked layers of self-assembled In(Ga)As quantum rings on GaAs (001)

Abstract

Stacked layers of self-assembled In͑Ga͒As quantum rings on GaAs grown by solid source molecular beam epitaxy are studied by ex situ atomic force microscopy ͑AFM͒, low temperature photoluminescence ͑PL͒ and cross-sectional transmission electron microscopy ͑XTEM͒. The influence of the strain field and InAs segregation on the surface morphology, optical properties and vertical ordering of three quantum ring layers is analyzed for GaAs spacers between layers from 1.5 to 14 nm. AFM and PL results show that samples with spacers Ͼ6 nm have surface morphology and optical properties similar to single layers samples. XTEM results on samples with 3 and 6 nm GaAs spacers show that the rings are preserved after capping with GaAs, and evidence the existence of vertically ordered quantum rings. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1866228͔ Molecular beam epitaxy ͑MBE͒ is a powerful technique for the fabrication of lattice mismatched semiconductor nanostructures. In particular, InAs on GaAs ͑001͒ selfassembled quantum dots ͑QDs͒ are one of the most studied systems. QDs are particularly interesting for their potential use in detectors, memories, quantum computing and photonic devices applications. [1][2][3][4] The development and design of QDs based devices requires a precise control of the morphology and composition of the QDs. As an example of the achieved capabilities to control QDs size and shape it has been proved that it is possible to self-assemble In͑Ga͒As quantum rings ͑QRs͒. 5 QRs are obtained by covering a layer of QDs with a thin cap ͑ϳ20% of the dot height͒ followed by a growth pause. In this way, the original islands reshape into ring-like structures. 6 Several authors have reported interesting differences in the optical and confining properties between QDs and QRs. Pettersson et al. 7 found an oscillator strength for the fundamental transition three times higher for QRs than for QDs. Warburton et al. 8 showed that the permanent dipole moment of exciton in QRs is three times higher and with opposite sign than those found for lens shaped QDs. Lorke et al. 9 proved that the ground state of QRs with zero angular momentum transits into a chiral state under the influence of an external magnetic field, concluding that ring shaped morphology translates into a ring-like electronic structure. Their nontrivial geometry, together with the possibility of controlling the energy levels, oscillator strength, polarizability and magnetic properties, make QRs good candidates for the development of new devices. The use of stacked layers increases quantum efficiency and avoids saturation gain effects in laser devices. 10 In the case of QRs, stacking the nanostructures is even more important, as the large in-plane mean dimensions of the rings ͑100 nmϫ 90 nm by AFM͒ require low density ring ensembles ͑1-9 ϫ 10 9 cm −2 ͒ to avoid overlap problems.In spite of all the work focused on optical characterization, little is known about the structural properties of embedded rings. This work mainly presents structural charact...

Interband Optical Properties of Concentric Type-I Nanorings in a Normal Magnetic Field

“…An interesting example is the one in the shape of a ring [1][2][3], which shows oscillations in the electron and hole ground state energy with varying magnetic field [4]. The oscillations are due to the Aharonov-Bohm effect, which is the result of double connected topology of the ring and wave nature of the charge carriers [5].…”

Characteristic molecular properties of one-electron double quantum rings under magnetic fields

“…Molecular states in vertically coupled QRs have been investigated [21,22,23,24,25,26,27], but the hybridized orbitals in such systems are aligned along the vertical direction. As a result, their response to vertical magnetic fields, which are responsible for AB effects, is very weak [23].…”

Oscillatory Persistent Currents in Self-Assembled Quantum Rings

“…The sample was grown by molecular beam epitaxy and contains 29 mutually decoupled periods [ Fig. 1(a)] [15]. Each period consists of a nanostructured InAs layer, between two 24 nm GaAs layers, and a 2 nm doped (7 10 16 cm ÿ3 Si) GaAs layer that provides electrons to the InAs nanostructures.…”