T. Mano

Formation of InAs quantum dot arrays on GaAs (100) by self-organized anisotropic strain engineering of a (In,Ga)As superlattice template

We demonstrate the formation of well-defined InAs quantum dot (QD) arrays by self-organized engineering of anisotropic strain in a (In,Ga)As/GaAs superlattice (SL). Due to the accumulation and improvement of the uniformity of the strain-field modulation along [011], formation of InAs QD arrays along [0-11] with 140 nm lateral periodicity is clearly observed on the SL template when the number of SL periods is larger than ten. By enhancing the In adatom surface migration length at low growth rates, clear arrays of single InAs QDs are obtained. The QD arrays exhibit strong photoluminescence efficiency that is not reduced compared to that from InAs QD layers on GaAs. Hence, ordering by self-organized anisotropic strain engineering maintains the high structural quality of InAs QDs.

Direct imaging of self-organized anisotropic strain engineering for improved one-dimensional ordering of (In,Ga)As quantum dot arrays

Single (In,Ga)As quantum dot (QD) arrays are formed on GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template in molecular beam epitaxy. The crucial steps in QWR template evolution, i.e., elongated QD formation at elevated temperature, thin GaAs capping, annealing, and stacking, are directly imaged by atomic force microscopy (AFM). AFM reveals a very smooth connection of the QDs into QWRs upon annealing. In addition, AFM shows the presence of height and width fluctuations of the QWRs with a significant number of bends and branches. These are attributed to excess strain accumulation during formation of the QWR template. By reducing the amount of (In,Ga)As and increasing the GaAs separation layer thickness in each SL period, a dramatic improvement of the uniformity of the QWR template is achieved. On the improved QWR template, well-defined one-dimensional single (In,Ga)As QD arrays are formed which are straight over more than...

Complex quantum dot arrays formed by combination of self-organized anisotropic strain engineering and step engineering on shallow patterned substrates

One-dimensional (In,Ga)As quantum dot (QD) arrays are created on planar singular, vicinal, and shallow mesa-patterned GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs quantum wire (QWR) superlattice template in molecular beam epitaxy. On planar singular substrates, highly uniform single QD arrays along [0−11] are formed. On shallow [0−11] and [011] stripe-patterned substrates, the generated type-A and -B steps distinctly affect the surface migration processes which are crucial for QWR template development, i.e., strain-gradient-driven In adatom migration along [011] and surface-reconstruction-induced Ga∕In adatom migration along [0−11]. In the presence of both type-A and -B steps on vicinal substrates misoriented towards [101], the direction of adatom migration is altered to rotate the QD arrays. This establishes the relationship between self-organized anisotropic strain and step engineering, which is exploited on shallow zigzag-patterned substrates for the reali...

Effect of annealing on formation of self-assembled (In,Ga)As quantum wires on GaAs (100) by molecular beam epitaxy

The role of annealing for (In,Ga)As self-organized quantum wire (QWR) formation on GaAs (100) during growth of (In,Ga)As/GaAs superlattice (SL) structures is studied by x-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy. XRD and AFM evidence that annealing after the supply of each layer of elongated (In,Ga)As quantum dots (QDs) in the SL is the crucial process for QWR formation. We conclude that during annealing, the shape anisotropy of the QDs is enhanced due to anisotropic mass transport and the QDs become connected along the [0-11] direction. Strain reduction by In desorption, revealed by XRD and PL, which accompanies this process, then results in well defined, uniform QWR arrays by repetition in SL growth.

Self-organized anisotropic strain engineering: a new concept for quantum dot ordering

We have established a new concept for creating ordered arrays of quantum dots by self-organized epitaxy. The concept is based on self-organized anisotropic strain engineering of strained layer templates and is demonstrated for (In,Ga)As/GaAs superlattice structures on GaAs (100) and strain-induced (In,Ga)As growth instability on GaAs (311)B. The well-defined one- and two-dimensional networks of InAs quantum dots grown on top of these templates are of excellent structural and optical quality. This breakthrough, thus, allows for novel fundamental studies and device operation principles based on single and multiple carrier- and photon-, and coherent quantum interference effects.

Role of In desorption for formation of self-organized (In, Ga)As quantum wires on GaAs(100) during superlattice formation

Abstract We investigate the role of In desorption for (In,Ga)As self-organized quantum wire (QWR) formation on GaAs (1xa00xa00) based on elongated island formation, annealing, and stacking in (In,Ga)As/GaAs superlattice growth. We observe well-defined QWR formation only in the presence of thin GaAs capping of the elongated islands before annealing. Without thin GaAs capping, the In composition becomes too low for controlling the nucleation of (In,Ga)As islands in subsequent layers, necessary for QWR formation. Hence, thin GaAs capping is crucial to reproducibly balance In desorption for formation and vertical correlation of well-defined QWRs, serving as unique template for one-dimensional quantum dot ordering.

Dichroic reflection of InAs∕GaAs quantum dots

Polarization-resolved reflection measurements are performed on nearly circular InAs∕GaAs quantum dots (QDs) by means of time-resolved differential reflection spectroscopy. We observe linear polarization anisotropy of the differential absorption, revealing the dichroic character of the QD reflection. The observed magnitude of the dichroism is Θ[11¯0]∕Θ[110]=1.07. The polarization has a preferential direction orientated along the [11¯0] crystal axis, which is confirmed by polarization-resolved photoluminescence. We observe that the polarization anisotropy of the reflectivity is strongly dependent on the pump excitation density, decreasing from ρ=0.14 at low excitation to ρ=0.06 at high excitation. The pump power dependence is described by a binomial model taking into account the statistics of carrier capture into a limited number of QDs.

Bleaching Dynamics In InAs Self‐Assembled Quantum Dots

Carrier capture processes within the transient absorption spectrum of self‐assembled quantum dots are investigated by means of time‐resolved pump‐probe differential reflection spectroscopy at various temperatures. The bleaching dynamics depends strongly on the excitation density. At low excitation densities the carrier capture process is governed by single LO‐phonon emission whereas Auger effects dominate the overall capture process at high densities.

Picosecond time-resolved bleaching dynamics of self-assembled quantum dots

Picosecond bleaching dynamics of vertically stacked self-assembled quantum dots is investigated by means of time-resolved pump-probe differential reflection spectroscopy at room temperature. This allows us to study the dynamics of the pump generated carriers within the quantum dots. We observe that the absorption spectrum is shifted over 12 meV with respect to the photoluminescence spectrum. From the time-resolved measurements we detect that the carrier lifetime within the dots strongly depends on the energy within the absorption spectrum.

Ordered quantum dot arrays on self-organized strain engineered templates

In this paper, we have created highly uniform one-dimensional single (In,Ga)As QD arrays on planar GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template. During molecular beam epitaxy (MBE) of a strained (In,Ga)As/GaAs SL, elongated (In,Ga)As QDs develop into uniform QWR arrays with well-defined lateral periodicity. QWR formation relies on the anisotropic adatom surface migration and in desorption during annealing of the layers of elongated QDs after capping with a thin GaAs layer. The formation of highly uniform (In,Ga)As QD arrays with excellent PL properties is attributed to the smoothness of the strain field modulation on the dot-diameter and dot-to-dot distance length scales.