Giant permanent dipole moments of excitons in semiconductor nanostructures

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...

mentioning

confidence: 99%

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

Granados¹,

García²,

Ben

et al. 2005

“…We have characterized similar QDs in other samples under the influence of vertical electric fields and find the average lateral extent of the electron ͑hole͒ wave function to be 5 ͑3͒ nm. 16 Using these values, we estimate ␤ =−4 eV/ ͑kV/ cm͒ 2 and for a Stark shift of 1 meV, F = 15 kV/ cm. Figure 3 displays the Stark shift as a function of laser power for four biases on a different QD than that in Fig.…”

mentioning

confidence: 99%

Manipulating exciton fine structure in quantum dots with a lateral electric field

Gerardot

Seidl

Dalgarno

et al. 2007

Self Cite

211

165

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2431758͔There is currently great interest in producing entangled photons on demand for applications in quantum information processing. 1 One proposal which spurred much research is using radiative biexciton-exciton cascade in semiconductor quantum dots ͑QDs͒ to produce pairs of polarization entangled photons. 2 In an idealized QD, the bright exciton states ͑M = ±1͒ are degenerate. In this case the two decay paths from the biexciton to the vacuum state via the intermediate single exciton are indistinguishable in energy; thus photons emitted in the radiative cascade are polarization entangled. However, in practice, the rotational symmetry of a self-assembled QD is broken and the electron-hole exchange interaction mixes the bright exciton states into a nondegenerate doublet ͓referred to as a fine-structure splitting ͑FSS͔͒. This leads to an energetically distinguishable recombination path for the biexciton-exciton cascade. Polarization correlations are observed in the linear basis but polarization entanglement is destroyed due to the FSS. 3,4 For photons to be polarization entangled using this scheme, the requirement that the FSS be less than the homogeneous linewidth must be met. The FSS is typically 10-100 eV, while the homogeneous linewidth of self-assembled InGaAs QDs is ϳ1 eV. 5 Techniques used to actively tune the FSS include an inplane electric 6 or magnetic 7 field and an in situ uniaxial stress. 8 Also, QDs which are smaller due to the growth process 9 or subsequent annealing 10 have a smaller FSS. Unfortunately, such QDs are higher in energy and the QD photons become difficult to distinguish from those produced in the wetting layer. Recently, polarization entangled photons have been reported from specific QDs with energetically overlapping bright exciton states 11 and initially nondegenerate states tuned via a magnetic field. 12 However, a robust approach that would allow one to actively tune the FSS from a large value to zero for each QD is still necessary to realize an event ready entangled photon pair source. To this end we further explore the effect of a lateral electric field on the FSS.There are three basic characteristics of an exciton in a lateral electric field attributed to the quantum confined Stark effect, as has been investigated for quantum wells 13 and QDs: 14 a redshift in recombination energy, decreased oscillator strength, and an increase in nonradiative carrier tunneling probability. Additionally, electric fi...

“…[17][18][19][20][21] The quantum rings are of much interest due to their electronic properties such as the large and negative excitonic permanent dipole moments, 9 the high oscillator strength of the band-to-band ground state transition, 10 and the possibility to tune their electronic states. 11 Initially, it was not clear whether the electronic structure inside the ring shaped nanostructures is indeed ring-like.…”

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confidence: 99%

Formation of InAs self-assembled quantum rings on InP

Raz

Ritter

Bahir

2003

122

Shape transformations of partially capped self-assembled InAs quantum dots grown on InP are studied. Atomic force microscopy (AFM) images show large anisotropic redistribution of the island material after coverage by a 1 nm thick InP layer. The anisotropic material redistribution occurs within a few minutes and leads to a change from lens-like to elongated ring-like islands. The shape transformation is not accompanied by dot material compositional change. The formation of InAs/InP quantum rings disagrees with a previous model of InAs/GaAs ring formation that assumes that the driving force for the dot to ring transformation is the difference in surface diffusion velocity of indium and gallium atoms.