This paper proposes a new approach to nanoscale patterning of GaAs substrates that allows formation of subcritical (below the Stranski-Krastanov critical thickness) low-density quantum dots. We demonstrate that the removal of the GaAs native oxide under molecular arsenic flux stimulates the formation of nanosized holes on the substrate surface. Geometric characteristics of these holes (diameter, depth and shape) can be controlled by the variation of the oxide thickness and the parameters of the oxide removal procedure. Importantly, the surface density of resulting holes shows a weak dependence on the listed parameters and remains around 1 x 109 cm-2. We show that the arsenic flux influences the processes of hole formation during the GaAs native oxide removal mainly by the modulation of the substrate thermal decomposition. We experimentally demonstrate the possibility of using GaAs substrates patterned in this way as templates for obtaining subcritical InAs quantum dot arrays with low density. We observe complete suppression of the wetting layer formation on such templates and a pronounced red shift of the emission wavelength of quantum dots grown on them (up to 1080 nm). Our results suggest that further optimization of the proposed approaches will make it possible to form low-density quantum dots emitting in the O- and C-bands.

The results of a study of the optical properties of self-assembled InAs/GaAs (001) quantum dots (QDs) overgrown under different arsenic pressures, obtained using photoluminescence (PL) and PL excitation spectroscopy, are presented. If the arsenic pressure during overgrowth is low (2.5.10-6 Pa), a series of pronounced QD-related peaks is observed in the 77-K PL spectrum over a 200-meV broad spectral interval with the brightest one located at 1.37 eV. With increasing arsenic pressure, the PL spectrum becomes smoother and is red-shifted to 1.16 eV at 1.10-5 Pa and to 1.26 eV at 3.10-5 Pa. We explain this behavior in terms of enhanced QD decomposition, the mechanism of which is strongly dependent on the arsenic deficiency or excess during the overgrowth process, determined by the arsenic pressure. Supported by the analysis of the wetting layer luminescence, we also conclude that the total indium content in the QD layer decreases with decreasing arsenic pressure during the overgrowth. This study reveals an essential role of the arsenic pressure in the overgrowth of InAs QDs.

We reveal a novel phenomenon observed after self-catalytic growth of GaAs nanowires (NWs) on Si(111) substrates treated with a Ga focused ion beam (FIB). Depending on the ion dose, NW arrays with various geometrical parameters can be obtained. A minor treatment of the substrate enables a slight increase in the surface density of NWs relative to an unmodified substrate area. As the ion dose is increased up to ∼0.1 pC μ m −2 , the growth of GaAs NWs and nanocrystals is suppressed. However, a further increase in the ion dose stimulates the crystal growth leading to the formation of extremely thin NWs (39 ± 5 nm) with a remarkably high surface density of up to 15 μ m −2 . Resting upon an analysis of the surface structure before and after stages of ion-beam treatment, ultra-high vacuum annealing and NW growth, we propose a mechanism underlying the phenomenon observed. We assume that the chemical interaction between embedded Ga ions and a native Si oxide layer leads either to the enhancement of the passivation properties of the oxide layer within FIB-modified areas (at low and middle ion doses), or to the etching of the passivating oxide layer by excess Ga atoms, resulting in the formation of pores (at high ion doses). Due to this behavior, local fabrication of GaAs NW arrays with a diverse range of characteristics can be implemented on the same substrate. This approach opens a new way for self-catalytic growth of GaAs NWs.

In this paper, for the first time, we report a strong effect of the arsenic pressure used for the high-rate GaAs capping of self-assembled InAs quantum dots on their optical properties. A 140 nm red shift of the photoluminescence peak position is observed when the overgrowth arsenic pressure increases threefold. We explain this behavior in terms of different intensities of quantum dot decomposition, which occurs during the overgrowth under different conditions. When the arsenic pressure is sufficiently high, a GaAs capping layer is formed by deposited species with a low impact on initial quantum dots. At a low arsenic pressure, arsenic deficiency leads to the intensive intermixing caused both by the enhanced Ga/In atom exchange and by the consumption of arsenic atoms belonging to quantum dots for the GaAs capping layer formation. As a result of the overgrowth, quantum dots are divided into families with a large (high pressure) and a small (low pressure) average size, yielding long-wave (1.23 µm) and short-wave (1.09 µm) photoluminescence peaks, respectively. Thus, a significant influence of the overgrowth arsenic pressure on the characteristics of InAs quantum dots is evidenced in this study.

In this paper, we study local etching of the GaAs(001) surface by Ga droplets at various technological conditions. Effects of the deposition temperature and thickness, interruption time, annealing temperature and arsenic background pressure are discussed. A minimum deposition thickness of 1.5 monolayer of Ga is found to be sufficient to etch the GaAs surface. We demonstrate that an increase in the annealing temperature leads to a decrease in the hole depth and an increase in their diameter. For the first time, we obtain symmetrical nanoholes of pyramidal shape on the GaAs(001) surface with a low surface density (similar to 1 center dot 10(8) cm(-2) and below) allowing subsequent formation of single quantum dots for high-efficiency quantum photonic devices.

In order to consider quantum dots as single objects and to use them in modern electronic and photonic devices, they must be well-isolated from each other and have an appropriate size and structural quality. This is a big challenge that is difficult to achieve with traditional technological methods, such as the Stranski-Krastanov growth mechanism. In this paper, we present a novel droplet epitaxial technique for the fabrication of small-sized (similar to 25 nm) InAs/GaAs nanostructures with a low surface density (<1.10(8) cm(-2)). To achieve this result, we develop a growth method based on two-stage crystallization in the arsenic flux. At the first stage, the droplet size is reduced by spreading the droplet material over the surface due to the diffusion decay of droplets in an ultra-low arsenic flux. At the second stage, crystallization is carried out in a large arsenic flux while heating the substrate in order to fix the size and shape of nanodroplets and prevent them from further decaying. We demonstrate that the size dispersion of nanostructures is small and the process is well-reproducible. Thus, the presented approach makes it possible to obtain low-density quantum dots with an ultra-small size required for advanced optical applications.

We studied the influence of the arsenic pressure during low-temperature GaAs overgrowth of InAs quantum dots on their optical properties. In the photoluminescence spectrum of quantum dots overgrown at a high arsenic pressure, we observed a single broad line corresponding to unimodal size distribution of quantum dots. Meanwhile, two distinct peaks (~1080 and ~1150 nm) at larger wavelengths are found in the spectra of samples with quantum dots overgrown at a low arsenic pressure. We attributed this phenomenon to the high-pressure suppression of atom diffusion between InAs islands at the overgrowth stage, which makes it possible to preserve the initial unimodal size distribution of quantum dots. The same overgrowth of quantum dots at the low arsenic pressure induces intensive mass transfer, which leads to the formation of arrays of quantum dots with larger sizes. Integrated photoluminescence intensity at 300 K is found to be lower for quantum dots overgrown at the higher arsenic pressure. However, a difference in the photoluminescence intensity for the high- and low-pressure overgrowths is not so significant for a temperature of 77 K. This indicates that excess arsenic incorporates into the capping layer at high arsenic pressures and creates numerous nonradiative recombination centers, diminishing the photoluminescence intensity.

This paper presents the results of experimental studies of the effect of Si(111) surface modification by Ga-focused ion beam (FIB) at 30 kV accelerating voltage on the features of the epitaxial GaAs nanowire (NW) growth processes. We experimentally established the regularities of the Ga ions’ dose effect during surface modification on the structural characteristics of GaAs NW arrays. Depending on the Ga ion dose value, there is one of three modes on the surface for subsequent GaAs NW growth. At low doses, the NW growth is almost completely suppressed. The growth mode of high-density (up to 6.56 µm−2) GaAs NW arrays with a maximum fraction (up to 70%) of nanowires normally oriented to the substrate is realized in the medium ion doses range. A continuous polycrystalline base with a dense array of misoriented short (up to 0.9 µm) and thin (up to 27 nm) GaAs NWs is formed at high doses. We assume that the key role is played by the interaction of the implanted Ga ions with the surface at various process stages and its influence on the surface structure in the modification region and on GaAs NW growth conditions.