Site-controlled lateral arrangements of InAs quantum dots grown on GaAs(001) patterned substrates by atomic force microscopy local oxidation nanolithography

Modeling of Masked Droplet Deposition for Site-Controlled Ga Droplets

Site-controlled Ga droplets on AlGaAs substrates are fabricated using area-selective deposition of Ga through apertures in a mask during molecular beam epitaxy (MBE). The Ga droplets can be crystallized into GaAs quantum dots using a crystallization step under As flux. In order to model the complex process, including the masked deposition of the droplets and a reduction of their number during a thermal annealing step, a multiscale kinetic Monte Carlo (mkMC) simulation of self-assembled Ga droplet formation on AlGaAs is expanded for area-selective deposition. The simulation has only two free model parameters: the activation energy for surface diffusion and the activation energy for thermal escape of adatoms from a droplet. Simulated droplet numbers within the opening of the aperture agree quantitatively with the experimental results down to the perfect site-control, with one droplet per aperture. However, the model parameters are different compared to those of the self-assembled droplet growth. We attribute this to the presence of the mask in close proximity to the surface, which modifies the local process temperature and the As background. This approach also explains the dependence of the model parameters on the size of the aperture.

Highly uniform and symmetric epitaxial InAs quantum dots embedded inside Indium droplet etched nanoholes

III-V semiconductor quantum dots (QDs) obtained by local droplet etching technology provide a material platform for generation of non-classic light. However, using this technique to fabricate single emitters for a broad spectral range remains a significant challenge. Herein, we successfully extend the QD emission wavelength to 850-880 nm via highly uniform and symmetric InAs QDs located inside indium-droplet-etching nanoholes. The evolution of InGaAs nanostructures by high temperature indium droplet epitaxy on GaAs substrate is revealed. By carefully designing the appropriate growth conditions, symmetric QDs with the a small fine structure splitting of only ∼4.4 ± 0.8 μeV are demonstrated. Averaging over the emission energies of 32 QDs, an ensemble broadening of 12 meV is observed. Individual QDs are shown to emit nonclassically with clear evidence of photon antibunching. These highly uniform and symmetric nanostructures represent a very promising novel strategy for quantum information applications.

Role of re-growth interface preparation process for spectral line-width reduction of single InAs site-controlled quantum dots

We present growth and optical characterization measurements of single InAs site-controlled quantum dots (SCQDs) grown by molecular beam epitaxy on GaAs (001) patterned substrates by atomic force microscopy oxidation lithography. InAs SCQDs directly grown on the patterned surface were used as a seed layer and strain template for the nucleation of optically active single InAs SCQDs. The preservation of the initial geometry of the engraved pattern motifs after the re-growth interface preparation process, the lack of buffer layer growth prior to InAs seed layer deposition and the development of suitable growth conditions provide us an improvement of the SCQDs' active layer optical properties while retaining a high ratio of single occupation (89%). In this work a fivefold reduction of the average optical line-width from 870 μeV to 156 μeV for InAs SCQDs located 15 nm from the re-growth interface is obtained by increasing the temperature of the initial thermal treatment step of the re-growth interface from 490 °C to 530 °C.

Advanced scanning probe lithography

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

Fundamental role of arsenic flux in nanohole formation by Ga droplet etching on GaAs(001)

Nanoholes with a depth in the range of tens of nanometers can be formed on GaAs(001) surfaces at a temperature of 500°C by local etching after Ga droplet formation. In this work, we demonstrate that the local etching or nanodrilling process starts when the Ga droplets are exposed to arsenic. The essential role of arsenic in nanohole formation is demonstrated sequentially, from the initial Ga droplets to the final stage consisting of nanoholes surrounded by ringlike structures at the surface and Ga droplets consumed. The kinetics of local etching depends on the arsenic flux intensity, while the ringlike structures are basically the same as those formed underneath the droplets in the absence of arsenic. These structures show motifs with well-defined crystalline facets that correspond to those expected from surface energy minimization. These experimental results are qualitatively analyzed for a better understanding of the nanohole formation underlying processes.

High quality factor GaAs-based photonic crystal microcavities by epitaxial re-growth

We investigate L7 photonic crystal microcavities (PCMs) fabricated by epitaxial re-growth of GaAs pre-patterned substrates, containing InAs quantum dots. The resulting PCMs show hexagonal shaped nano-holes due to the development of preferential crystallographic facets during the re-growth step. Through a careful control of the fabrication processes, we demonstrate that the photonic modes are preserved throughout the process. The quality factor (Q) of the photonic modes in the re-grown PCMs strongly depends on the relative orientation between photonic lattice and crystallographic directions. The optical modes of the re-grown PCMs preserve the linear polarization and, for the most favorable orientation, a 36% of the Q measured in PCMs fabricated by the conventional procedure is observed, exhibiting values up to ~6000. The results aim to the future integration of site-controlled QDs with high-Q PCMs for quantum photonics and quantum integrated circuits.

Influence of hole shape/size on the growth of site-selective quantum dots

: The number of quantum dots which nucleate at a certain place has to be controllable for device integration. It was shown that the number of quantum dots per nucleation site depends on the size of the hole in the substrate, but other dimensions of the nucleation site are vague. We report on the influence of hole shape on site-selectively grown InAs quantum dots (QDs) by molecular beam epitaxy. Dry etching of the GaAs wafers was used because of its high anisotropic etching characteristic. Therefore, it was possible to verify the influence of several hole shape parameters on the subsequent QD growth independently. We show that the nucleation of these QDs depends on several properties of the hole, namely its surface area, aspect ratio of the surface area, and depth. Especially, the aspect ratio shows a big influence on the number of nucleating QDs per site. With knowledge of these dependencies, it is possible to influence the number of QDs per site and also its distribution.

Leveraging crystal anisotropy for deterministic growth of InAs quantum dots with narrow optical linewidths

Crystal growth anisotropy in molecular beam epitaxy usually prevents deterministic nucleation of individual quantum dots when a thick GaAs buffer is grown over a nanopatterned substrate. Here, we demonstrate how this anisotropy can actually be used to mold nucleation sites for single dots on a much thicker buffer than has been achieved by conventional techniques. This approach greatly suppresses the problem of defect-induced line broadening for single quantum dots in a charge-tunable device, giving state-of-the-art optical linewidths for a system widely studied as a spin qubit for quantum information.

Site-controlled self-assembled InAs quantum dots grown on GaAs substrates

Atomically-flat surfaces are obtained after thin GaAsSb buffer layer growth on GaAs substrates with regular-distributed nano-holes formed after oxide desorption of the local atomic-force-microscopy anode oxidation. Different from the samples with GaAsSb buffer layers, increasing surface root-mean-square roughness is observed for the GaAs-buffered samples with increasing GaAs buffer layer thickness. The phenomenon is attributed to the enhanced adatom migration resulting from the incorporation of Sb atoms. By using the substrates with nano-holes after buffer layer growth, site-controlled self-assembled InAs quantum dots (QDs) are observed with the deposition of a below-critical-thickness InAs coverage of 1.3 monolayer (ML).

Single site-controlled In(Ga)As/GaAs quantum dots: growth, properties and device integration

Results obtained by an advanced growth of site-controlled quantum dots (SCQDs) on pre-patterned nanoholes and their integration into both photonic resonators and nanoelectronic memories are summarized. A specific technique has been pursued to improve the optical quality of single SCQDs. Quantum dot (QD) layers have been vertically stacked but spectrally detuned for single SCQD studies. Thereby, the average emission linewidth of single QDs could be reduced from 2.3 meV for SCQDs in a first QD layer close to the etched nanoholes down to 600 microeV in the third InAs QD layer. Accurate SCQD nucleation on large QD distances is maintained by vertical strain induced QD coupling throughout the QD stacks. Record narrow linewidths of individual SCQDs down to approximately 110 microeV have been obtained. Experiments performed on coupled photonic SCQD-resonator devices show an enhancement of spontaneous emission. SCQDs have also been integrated deterministically in high electron mobility heterostructures and flash memory operation at room temperature has been observed.

Dense arrays of ordered pyramidal quantum dots with narrow linewidth photoluminescence spectra

Arrays of site-controlled, pyramidal InGaAs/GaAs quantum dots (QDs) grown by organo-metallic chemical vapour deposition with densities comparable to those of self-assembled QDs (5 x 10(9) cm(-2)) are demonstrated. The QDs exhibit high quality photoluminescence spectra with inhomogeneous broadening of only 6.5 meV. The QD dipole moment was estimated through the analysis of time-resolved photoluminescence measurements. Such ordered QD arrays should be useful for applications in active nanophotonic systems such as QD lasers, modulators and switches requiring high overlap of the optical modes with the QD active region.

Single photon emission from site-controlled InAs quantum dots grown on GaAs(001) patterned substrates

We present a fabrication method to produce site-controlled and regularly spaced InAs/GaAs quantum dots for applications in quantum optical information devices. The high selectivity of our epitaxial regrowth procedure can be used to allocate the quantum dots only in positions predefined by ex-situ local oxidation atomic force nanolithography. The quantum dots obtained following this fabrication process present a high optical quality which we have evaluated by microphotoluminescence and photon correlation experiments.