Self-assembled Semiconductor Quantum Dots Research Articles

Heterogeneous integration of single InAs/InP quantum dots with the SOI chip using direct bonding.

Quantum information processing with photons in small-footprint and highly integrated silicon-based photonic chips requires incorporating non-classical light sources. In this respect, self-assembled III-V semiconductor quantum dots (QDs) are an attractive solution, however, they must be combined with the silicon platform. Here, by utilizing the large-area direct bonding technique, we demonstrate the hybridization of InP and SOI chips, which allows for coupling single photons to the SOI chip interior, offering cost-effective scalability in setting up a multi-source environment for quantum photonic chips. We fabricate devices consisting of self-assembled InAs QDs embedded in the tapered InP waveguide (WG) positioned over the SOI-defined Si WG. Focusing on devices generating light in the telecom C-band compatible with the low-loss optical fiber networks, we demonstrate the light coupling between InP and SOI platforms by observing photons outcoupled at the InP-made circular Bragg grating outcoupler fabricated at the end of an 80 µm-long Si WG, and at the cleaved edge of the Si WG. Finally, for a device with suppressed multi-photon generation events exhibiting 80% single photon generation purity, we measure the photon number outcoupled at the cleaved facet of the Si WG. We estimate the directional on-chip photon coupling between the source and the Si WG to 5.1%.

Wafer-Scale Emission Energy Modulation of Indium Flushed Quantum Dots

Semiconductor self-assembled quantum dots (QDs) have garnered immense attention for their potential in various quantum technologies and photonics applications. Here, we explore a novel approach for fine-tuning the emission wavelength of QDs by building upon the indium flush growth method: Submonolayer variations in the capping thickness reveal a non-monotonic progression, where the emission energy can decrease even though the capping thickness decreases. indium flush, a well-known technique for inducing blue shifts in quantum dot emissions, involves the partial capping of QDs with GaAs followed by a temperature ramp-up. However, our findings reveal that the capping layer roughness, stemming from fractional monolayers during overgrowth, plays a pivotal role in modulating the emission energy of these QDs. We propose increased indium interdiffusion between the QDs and the surrounding GaAs capping layer for a rough surface surrounding the QD as the driving mechanism. This interdiffusion alters the indium content within the QDs, resulting in an additional emission energy shift, counterintuitive to the capping layer’s thickness increase. We utilize photoluminescence spectroscopy to generate wafer maps depicting the emission spectrum of the QDs. Using thickness gradients, we produce systematic variations in the capping layer thickness on 3″ wafers, resulting in modulations of the emission energy of up to 26 meV.

Culling a Self-Assembled Quantum Dot as a Single-Photon Source Using X-ray Microscopy.

Epitaxially grown self-assembled semiconductor quantum dots (QDs) with atom-like optical properties have emerged as the best choice for single-photon sources required for the development of quantum technology and quantum networks. Nondestructive selection of a single QD having desired structural, compositional, and optical characteristics is essential to obtain noise-free, fully indistinguishable single or entangled photons from single-photon emitters. Here, we show that the structural orientations and local compositional inhomogeneities within a single QD and the surrounding wet layer can be probed in a screening fashion by scanning X-ray diffraction microscopy and X-ray fluorescence with a few tens of nanometers-sized synchrotron radiation beam. The presented measurement protocol can be used to cull the best single QD from the enormous number of self-assembled dots grown simultaneously. The obtained results show that the elemental composition and resultant strain profiles of a QD are sensitive to in-plane crystallographic directions. We also observe that lattice expansion after a certain composition-limit introduces shear strain within a QD, enabling the possibility of controlled chiral-QD formation. Nanoscale chirality and compositional anisotropy, contradictory to common assumptions, need to be incorporated into existing theoretical models to predict the optical properties of single-photon sources and to further tune the epitaxial growth process of self-assembled quantum structures.

Quantum Light Source Based on Semiconductor Quantum Dots: A Review

Quantum light sources that generate single photons and entangled photons have important applications in the fields of secure quantum communication and linear optical quantum computing. Self-assembled semiconductor quantum dots, also known as “artificial atoms”, have discrete energy-level structures due to electronic confinement in all three spatial dimensions. It has the advantages of high stability, high brightness, deterministic, and tunable emission wavelength, and is easy to integrate into an optical microcavity with a high-quality factor, which can realize a high-performance quantum light source. In this paper, we first introduce the generation principles, properties, and applications of single-photon sources in the field of quantum information and then present implementations and development of quantum light sources in self-assembled semiconductor quantum dot materials. Finally, we conclude with an outlook on the future development of semiconductor quantum dot quantum light sources.

Widely Tunable Single-Photon Frequency Conversion in a Nanomechanically Reconfigurable Si-Rich Nitride Waveguide

An on-demand single-photon source based on self-assembled semiconductor quantum dots (QDs) is a key building block for quantum communication and quantum information processing. To scale up, a challenge lies in frequency converting the emitted photons to the telecom wavelengths and eliminating the wavelength difference between multiple QDs. In this paper, we propose a novel nanomechanical silicon-rich nitride waveguide device, which can be reconfigured to realize efficient frequency conversion from the emission wavelength band of InAs/GaAs quantum dots at 900 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 950 nm to the telecom C-band around 1550 nm, by exploiting the Bragg scattering process in nonlinear four-wave mixing. Numerical results show that an optimal conversion efficiency over 99% and a signal-to-noise ratio up to 27 dB can be achieved when the waveguide loss is negligible, and the efficiency only reduces to 78% when a realistic loss of 1 dB/cm is considered. We conduct a detailed study on the unwanted noise processes, and provide design guidelines to improve the conversion efficiency and suppress the noises, e.g., by optimizing the power and wavelength of the pump laser.

Bright Quantum Dot Single-Photon Emitters at Telecom Bands Heterogeneously Integrated on Si.

Whereas the Si photonic platform is highly attractive for scalable optical quantum information processing, it lacks practical solutions for efficient photon generation. Self-assembled semiconductor quantum dots (QDs) efficiently emit photons in the telecom bands (1460–1625 nm) and allow for heterogeneous integration with Si. In this work, we report on a novel, robust, and industry-compatible approach for achieving single-photon emission from InAs/InP QDs heterogeneously integrated with a Si substrate. As a proof of concept, we demonstrate a simple vertical emitting device, employing a metallic mirror beneath the QD emitter, and experimentally obtained photon extraction efficiencies of ∼10%. Nevertheless, the figures of merit of our structures are comparable with values previously only achieved for QDs emitting at shorter wavelength or by applying technically demanding fabrication processes. Our architecture and the simple fabrication procedure allows for the demonstration of high-purity single-photon generation with a second-order correlation function at zero time delay, g(2)(τ = 0) < 0.02, without any corrections at continuous wave excitation at the liquid helium temperature and preserved up to 50 K. For pulsed excitation, we achieve the as-measured g(2)(0) down to 0.205 ± 0.020 (0.114 ± 0.020 with background coincidences subtracted).

Electroabsorption in gated GaAs nanophotonic waveguides

We report on the analysis of electroabsorption in thin GaAs/Al0.3Ga0.7As nanophotonic waveguides with an embedded p–i–n junction. By measuring the transmission through waveguides of different lengths, we derive the propagation loss as a function of electric field, wavelength, and temperature. The results are in good agreement with the Franz–Keldysh model of electroabsorption extending over 200 meV below the GaAs bandgap, i.e., in the wavelength range of 910–970 nm. We find a pronounced residual absorption in forward bias, which we attribute to Fermi-level pinning at the waveguide surface, producing over 20 dB/mm loss at room temperature. These results are essential for understanding the origin of loss in nanophotonic devices operating in the emission range of self-assembled InAs semiconductor quantum dots toward the realization of scalable quantum photonic integrated circuits.

High-performance deterministic in situ electron-beam lithography enabled by cathodoluminescence spectroscopy

The application of solid-state quantum emitters in real-world quantum information technologies requires precise nanofabrication platforms with high process yield. Self-assembled semiconductor quantum dots with excellent emission properties have proven to be among the best candidates to meet the needs of a number of novel quantum photonic devices. However, their spatial and spectral positions vary statistically on a scale that is far too large for their system integration via fixed lithography and inflexible processing schemes. We solve this severe problem by introducing a flexible and deterministic manufacturing scheme based on precise and convenient cathodoluminescence spectroscopy followed by high-resolution electron-beam lithography. The basics and application examples of this advanced in situ electron-beam lithography are described in this article. Although we focus here on quantum dots as photon emitters, this nanotechnology concept is very well suited for the fabrication of a variety of quantum nanophotonic devices based on quantum emitters that exhibit suitably strong cathodoluminescence signals.

Strain-driven quantum dot self-assembly by molecular beam epitaxy

Research into self-assembled semiconductor quantum dots (QDs) has helped advance numerous optoelectronic applications, ranging from solid-state lighting to photodetectors. By carefully controlling molecular beam epitaxy (MBE) growth parameters, we can readily tune QD light absorption and emission properties to access a broad portion of the electromagnetic spectrum. Although this field is now sufficiently mature that QDs are found in consumer electronics, research efforts continue to expand into new areas. By manipulating MBE growth conditions and exploring new combinations of materials, substrate orientations, and the sign of strain, a wealth of opportunities exist for synthesizing novel QD nanostructures with hitherto unavailable properties. As such, QDs are uniquely well positioned to make critical contributions to the development of future quantum technologies. In this tutorial, we summarize the history of self-assembled QDs, outline some examples of quantum optics applications based on QDs, discuss the science that explains the spontaneous formation of QDs, and provide recipes for successful QD growth by MBE for some of the most commonly used semiconductor materials systems. We hope that compiling this information in one place will be useful both for those new to QD self-assembly and for experienced researchers, ideally supporting the community’s efforts to continue pushing the boundaries of knowledge in this important field.

Correlation between exciton polarized lifetime and fine structure splitting in InAs/GaAs quantum dots

Semiconductor self-assembled single quantum dots (QDs) provide a promising solid-state light source for single photons and entangled photons. The structural asymmetry in QDs results in fine structure splitting (FSS) of exciton and biexciton emission lines. Here, we propose a method to study QD symmetry by measuring the difference in two different polarized lifetimes of QD excitonic emission lines under applied stress. The method can be reasonably correlated with the direct FSS measurement. Actually, due to the limitation of the resolution of the spectrometer, the zero value of FSS is difficult to measure. Instead, the lifetime measurement is an effective method to judge the symmetry condition of single QDs.

Tuning the Emission Colors of Self-Assembled Quantum Dot Monolayers via One-Step Heat Treatment for Display Applications

Homogeneously self-assembled colloidal semiconductor quantum dot monolayers (QD-SAMs) over large areas are promising materials for thin film optoelectronic device applications, especially for displ...

Droplet epitaxy of semiconductor nanostructures for quantum photonic devices.

The long dreamed 'quantum internet' would consist of a network of quantum nodes (solid-state or atomic systems) linked by flying qubits, naturally based on photons, travelling over long distances at the speed of light, with negligible decoherence. A key component is a light source, able to provide single or entangled photon pairs. Among the different platforms, semiconductor quantum dots (QDs) are very attractive, as they can be integrated with other photonic and electronic components in miniaturized chips. In the early 1990s two approaches were developed to synthetize self-assembled epitaxial semiconductor QDs, or 'artificial atoms'-namely, the Stranski-Krastanov (SK) and the droplet epitaxy (DE) methods. Because of its robustness and simplicity, the SK method became the workhorse to achieve several breakthroughs in both fundamental and technological areas. The need for specific emission wavelengths or structural and optical properties has nevertheless motivated further research on the DE method and its more recent development, local droplet etching (LDE), as complementary routes to obtain high-quality semiconductor nanostructures. The recent reports on the generation of highly entangled photon pairs, combined with good photon indistinguishability, suggest that DE and LDE QDs may complement (and sometimes even outperform) conventional SK InGaAs QDs as quantum emitters. We present here a critical survey of the state of the art of DE and LDE, highlighting the advantages and weaknesses, the achievements and challenges that are still open, in view of applications in quantum communication and technology.

InAs/InP quantum dots stacking: Impact of spacer layer on optical properties

The optical properties of a vertical multilayer stack of semiconductor self-assembled quantum dots (QDs) depend on the coupling status. We present an improved modeling method for analysis of QD stacks, consisting of several separate steps in the elastic strain analysis, to reproduce the realistic unidirectional strain accumulation along the QD growth direction from lower and upper dots. Based on this modeling method, by varying the spacer thickness, we systematically study a large variety of QD stack structures, for better understanding the influence of strain and electronic coupling mechanisms. A “quasi continuum band” or highly degenerate excited state (ES) is found in the closely multi-stacked QDs, probably accounting for the occurrence of distinct blueshifting of the photoluminescence peak wavelength for the decreasing spacer thickness. The enhanced ES emission from this type of stacking QD modeled by using rate equations may pave a way for designing potential high-performance laser devices operating on ES.

Reinvestigation of quantum dot symmetry: the symmetric group of the 8-band k⋅p theory Hamiltonian

Self-assembled semiconductor quantum dots, usually formed in pyramid or lens shapes, have an intrinsic geometric symmetry. However, the geometric symmetry of a quantum dot is not identical to the symmetry of the associated Hamiltonian. It is a well-accepted conclusion that the symmetric group of the Hamiltonians for both pyramidal and lens-shaped quantum dots is C2v; consequently, the eigenstate of the Hamiltonian is not degenerate because C2v has only one-dimensional irreducible representations. In this paper, we show the above conclusion is wrong. Using the 8-band k · p theory model and considering the action of group elements on both spatial and electron spin parts of the wavefunction, we find the symmetric group of the Hamiltonian is the C2v double group not C2v. C2v is the symmetric group of the spatial part of the conduction band Hamiltonian only when the inter-band coupling is totally ignored. Employing the C2v double group symmetry, we prove that although the C2v double group has both one-dimensional and two-dimensional irreducible representations, the eigenstates of the 8 × 8 Hamiltonian are always two-fold degenerate and that these degenerate states only correspond to the two-dimensional irreducible representation of the C2v double group. The double group symmetry originates from the coupling between spatial potential and electron half spin. This coupling causes a full 2π rotation in the wavefunction space or the Hilbert space not equal to the unity operation. Finally, the connection between the two-fold degeneracy due to the C2v double group symmetry and Kramers’ degeneracy due to the time inversion symmetry is explored.

Fundamental Intrinsic Lifetimes in Semiconductor Self-Assembled Quantum Dots

The self-assembled quantum dots (QDs) provide an ideal platform for realization of quantum information technology because it provides on demand single photons, entangled photon pairs from biexciton cascade pro- cess, single spin qubits, and so on. The fine structure splitting (FSS) of exciton is a fundamental property of QDs for thees applications. From the symmetry point of view, since the two bright exciton states belong to two different representations for QDs with C2v symmetry, they should not only have different energies, but also have different lifetimes, which is termed exciton lifetime asymmetry. In contrast to extensively studied FSS, the investigation of the exciton lifetime asymmetry is still missed in literature. In this work, we carried out the first investigation of the exciton lifetime asymmetry in self-assembled QDs and presented a theory to deduce lifetime asymmetry indirectly from measurable qualities of QDs. We further revealed that intrinsic lifetimes and their asymmetry are fundamental quantities of QDs, which determine the bound of the extrinsic lifetime asymmetries, polarization angles, FSSs, and their evolution under uniaxial external forces. Our findings provide an important basis to deeply understanding properties of QDs.

Effect of the depolarization field on coherent optical properties in semiconductor quantum dots

We study the photon echo spectrum of self-assembled semiconductor quantum dots using femtosecond light pulses. The spectrum shape changes from a single-peaked to a double-peaked structure as the time delay between the two excitation pulses is increased. The spectrum change is reproduced by numerical calculations, which include the depolarization field induced by the biexciton-exciton transition as well as the conventional local-field effect for the exciton--ground-state transition in a quantum dot. Our findings suggest that various optical transitions in tightly localized systems generate a depolarization field, which renormalizes the resonant frequency with a change in the polarization itself, leading to unique optical properties.

Correlating Photoluminescence and Structural Properties of Uncapped and GaAs-Capped Epitaxial InGaAs Quantum Dots

The understanding of the correlation between structural and photoluminescence (PL) properties of self-assembled semiconductor quantum dots (QDs), particularly InGaAs QDs grown on (001) GaAs substrates, is crucial for both fundamental research and optoelectronic device applications. So far structural and PL properties have been probed from two different epitaxial layers, namely top-capped and buried layers respectively. Here, we report for the first time both structural and PL measurements from an uncapped layer of InGaAs QDs to correlate directly composition, strain and shape of QDs with the optical properties. Synchrotron X-ray scattering measurements show migration of In atom from the apex of QDs giving systematic reduction of height and enlargement of QDs base in the capping process. The optical transitions show systematic reduction in the energy of ground state and the first excited state transition lines with increase in capping but the energy of the second excited state line remain unchanged. We also found that the excitons are confined at the base region of these elliptically shaped QDs showing an interesting volume-dependent confinement energy scaling of 0.3 instead of 0.67 expected for spherical dots. The presented method will help us tuning the growth of QDs to achieve desired optical properties.

Growth of Semiconductor Self-assembled Quantum Dots by Molecular Beam Epitaxy and the Dynamics of Photo-excited Carriers

Growth of Semiconductor Self-assembled Quantum Dots by Molecular Beam Epitaxy and the Dynamics of Photo-excited Carriers

Coupling a single solid-state quantum emitter to an array of resonant plasmonic antennas

Plasmon resonant arrays or meta-surfaces shape both the incoming optical field and the local density of states for emission processes. They provide large regions of enhanced emission from emitters and greater design flexibility than single nanoantennas. This makes them of great interest for engineering optical absorption and emission. Here we study the coupling of a single quantum emitter, a self-assembled semiconductor quantum dot, to a plasmonic meta-surface. We investigate the influence of the spectral properties of the nanoantennas and the position of the emitter in the unit cell of the structure. We observe a resonant enhancement due to emitter-array coupling in the far-field regime and find a clear difference from the interaction of an emitter with a single antenna.

Limited accuracy of conduction band effective mass equations for semiconductor quantum dots

Effective mass equations are the simplest models of carrier states in a semiconductor structures that reduce the complexity of a solid-state system to Schrödinger- or Pauli-like equations resempling those well known from quantum mechanics textbooks. Here we present a systematic derivation of a conduction-band effective mass equation for a self-assembled semiconductor quantum dot in a magnetic field from the 8-band k · p theory. The derivation allows us to classify various forms of the effective mass equations in terms of a hierarchy of approximations. We assess the accuracy of the approximations in calculating selected spectral and spin-related characteristics. We indicate the importance of preserving the off-diagonal terms of the valence band Hamiltonian and argue that an effective mass theory cannot reach satisfactory accuracy without self-consistently including non-parabolicity corrections and renormalization of k · p parameters. Quantitative comparison with the 8-band k · p results supports the phenomenological Roth-Lax-Zwerdling formula for the g-factor in a nanostructure.