Research

Our main Research fields

The Epitaxy and Physics of Nanostructures laboratory investigates the epitaxial growth mechanism and the physics of a variety of semiconductor structures. In particular, we undertake fundamental studies of the growth mechanism and the optical properties of site controlled quantum dots (QDs) obtained by growing in pre-patterned GaAs substrates. Our field of interest spans as far as optoelectronic and electronic devices, with special attention to light emitting sources at telecom wavelengths.

We are member of the Irish Photonic Integration Centre

What Is New?

We obtained entangled photons emission and Bell's inequalities violation with an electrically pumped LED based on our Pyramidal dots.

T. H. Chung, G. Juska et al., Selective carrier injection into patterned arrays of pyramidal quantum dots for entangled photon light-emitting diodes”, Nature Photonics 10, 782–787 (2016)

ABSTRACT
Scalability and foundry compatibility (as apply to conventional silicon-based integrated computer processors, for example) in developing quantum technologies are major challenges facing current research. Here we introduce a quantum photonic technology that has the potential to enable the large-scale fabrication of semiconductor-based, site-controlled, scalable arrays of electrically driven sources of polarization-entangled photons that may be able to encode quantum information.

The design of the sources is based on quantum dots grown in micrometre-sized pyramidal recesses along the crystallographic direction (111)B, which theoretically ensures high symmetry of the quantum dots—a requirement for bright entangled-photon emission. A selective electric injection scheme in these non-planar structures allows a high density of light-emitting diodes to be obtained, with some producing entangled photon pairs that also violate Bell’s inequality. Compatibility with semiconductor fabrication technology, good reproducibility and lithographic position control make these devices attractive candidates for integrated photonic circuits for quantum information processing.

G. Yuska et al., “Towards quantum-dot arrays of entangled photon emitters”, Nat. Photon. , 7 (2013).

ABSTRACT
To make photonic quantum information a reality, a number of extraordinary challenges need to be overcome. One challenge is to achieve large arrays of reproducible ‘entangled’ photon generators, while maintaining compatibility for integration with optical devices and detectors. Semiconductor quantum dots are potentially ideal for this as they allow photons to be generated on demand without relying on probabilistic processes. Nevertheless, most quantum-dot systems are limited by their intrinsic lack of symmetry, which allows only a small number (typically 1 out of 100, or worse) of good dots to be achieved per chip. The recent retraction of Mohan et al. seemed to question the very possibility of simultaneously achieving site control and high symmetry. Here, we show that with a new family of (111)-grown pyramidal sitecontrolled InGaAs1–dNd quantum dots it is possible to overcome previous hurdles and obtain areas with up to 15% of polarization- entangled photon emitters, with fidelities as high as 0.721+0.043.

A. Gocalinska et al., “Unusual nanostructures of “lattice matched” InP on AlInAs”, APPL. PHYS. LETT. 104, 141606

ABSTRACT
We show that the morphology of the initial monolayers of InP on Al0.48In0.52As grown by metalorganic vapor-phase epitaxy does not follow the expected layer-by-layer growth mode of lattice-matched systems, but instead develops a number of low-dimensional structures, e.g., quantum dots and wires. We discuss how the macroscopically strain-free heteroepitaxy might be strongly affected by local phase separation/alloying-induced strain and that the preferred aggregation of adatom species on the substrate surface and reduced wettability of InP on AlInAs surfaces might be the cause of the unusual (step) organization and morphology.

V. Dimastrodonato et al., “Self-limiting profile evolution of seeded two- and three- dimensional nanostructures during metalorganic vapor-phase epitaxy”, Phys. Rev. Lett. 96, 130501 (2012).

ABSTRACT
Extensive experimental data and an accompanying theoretical model are presented for the self-limiting profiles and Ga segregation on patterned GaAs(111)B substrates during metalorganic vapor-phase epitaxy of AlxGa1-xAs. Self-limiting widths and segregation of Ga produce quantum dots along the base of pyramidal recesses bounded by (111)A planes and quantum wires along the vertical axis of the template, respectively. Coupled reaction-diffusion equations for precursor and adatom kinetics reproduce the measured concentration and temperature dependence of the self-limiting width and segregation. Our model can be extended to other patterned systems, providing a new paradigm for predicting the morphology of surface nanostructures and inferring their quantum optical properties.

V. Dimastrodonato et al., Impact of nitrogen incorporation on pseudomorphic site-controlled quantum dots grown by metalorganic vapor phase epitaxy, Applied Physics Letters 97, 072115 (2010)

ABSTRACT
We report on some surprising optical properties of diluted nitride InGaAs 1-eNe /GaAs (e<<1) pyramidal site-controlled quantum dots, grown by metalorganic vapor phase epitaxy on patterned GaAs (111)B substrates. Microphotoluminescence characterizations showed antibinding exciton/ biexciton behavior, a spread of exciton lifetimes in an otherwise very uniform sample, withunexpected long neutral exciton lifetimes (up to 7 ns) and a nearly zero fine structure splitting on a majority of dots.

E. Pelucchi et al., Decomposition, diffusion, and growth rate anisotropies in self-limited profiles during metalorganic vapor-phase epitaxy of seeded nanostructures, Physics Review B, 83 205409 (2011)

ABSTRACT
We present a model for the interplay between the fundamental phenomena responsible for the formation of nanostructures by metalorganic vapor phase epitaxy on patterned (001)/(111)B GaAs substrates. Experiments have demonstrated that V-groove quantum wires and pyramidal quantum dots form as a consequence of a self-limiting profile that develops, respectively, at the bottom of V-grooves and inverted pyramids. Our model is based on a system of reaction-diffusion equations, one for each crystallographic facet that defines the pattern, and include the group III precursors, their decomposition and diffusion kinetics (for which we discuss the experimental evidence), and the subsequent diffusion and incorporation kinetics of the group-III atoms released by the precursors. This approach can be applied to any facet configuration, including pyramidal quantum dots, but we focus on the particular case of V-groove templates and offer an explanation for the self-limited profile and the Ga segregation observed in the V-groove. The explicit inclusion of the precursor decomposition kinetics and the diffusion of the atomic species revises and generalizes the earlier work of Biasiol et al. [Biasiol et al., Phys. Rev. Lett. 81, 2962 (1998); Phys. Rev. B 65, 205306 (2002)] and is shown to be essential for obtaining a complete description of self-limiting growth. The solution of the system of equations yields spatially resolved adatom concentrations, from which average facet growth rates are calculated. This provides the basis for determining the conditions that yield self-limiting growth. The foregoing scenario, previously used to account for the growth modes of vicinal GaAs(001) and the step-edge profiles on the ridges of vicinal surfaces patterned with V-grooves during metalorganic vapor-phase epitaxy, can be used to describe the morphological evolution of any template composed of distinct facets.

G. Yuska et al., A study of nitrogen incorporation in pyramidal site-controlled quantum dots, Nanoscale Research Letters, 6:567 (2011)

ABSTRACT
We present the results of a study of nitrogen incorporation in metalorganic-vapour-phase epitaxy-grown sitecontrolled quantum dots (QDs). We report for the first time on a significant incorporation (approximately 0.3%), producing a noteworthy red shift (at least 50 meV) in some of our samples. Depending on the level of nitrogen
incorporation/exposure, strong modifications of the optical features are found (variable distribution of the emission homogeneity, fine-structure splitting, few-particle effects). We discuss our results, especially in relation to a specific reproducible sample which has noticeable features: the usual pattern of the excitonic transitions is altered and the fine-structure splitting is suppressed to vanishing values. Distinctively, nitrogen incorporation can be achieved
without detriment to the optical quality, as confirmed by narrow linewidths and photon correlation spectroscopy.

A. Gocalinska et al., Surface organization of homoepitaxial InP films grown by metalorganic vapor-phase epitaxy, PHYSICAL REVIEW B 86, 165307 (2012)

ABSTRACT
We present a systematic study of the morphology of homoepitaxial InP films grown by metalorganic vaporphase epitaxy which are imaged with ex situ atomic force microscopy. These films show a dramatic range of different surface morphologies as a function of the growth conditions and substrate (growth temperature, V/III ratio, and miscut angle <0.6◦ and orientation toward A or B sites), ranging from stable step flow to previously unreported strong step bunching, over 10 nmin height. These observations suggest awindowof growth parameters for optimal quality epitaxial layers.We also present a theoretical model for these growth modes that takes account of deposition, diffusion, and dissociation of molecular precursors, and the diffusion and step incorporation of atoms released by the precursors. The experimental conditions for step flow and step bunching are reproduced by this model, with the step bunching instability caused by the difference in molecular dissociation from above and below step edges, as was discussed previously for GaAs (001).