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We demonstrated nanopit formation by Ga-assisted local droplet etching technique in InGaAs metamorphic layers grown on vicinal GaAs(111)A substrates. We studied nanopit formation depending on the substrate temperature, Ga flux and Ga amount. The etched pits show a highly symmetrical pyramidal shape with an equilateral triangular base and the edges of the triangle are along <11 ̄0> directions. The observed behavior, in terms of nanopit density, depth and, aspect ratio is well described by a model taking into account the dynamics of the droplet etching process.

Dense arrays of self-assembled nanoholes are fabricated in GaAs(100) surfaces by As-free oxide cleaning thermal process. The formation of pit-like structures above 500 °C occurs by congruent evaporation of GaAs in the areas where the oxides have already desorbed. Thermally etched nanoholes exhibit inverted symmetric conical shape with an average depth/base diameter ratio of 0.21. Shallow and deep nanoholes are formed with this method, their depth ranging from 2 nm to 15 nm. Metallic Indium deposited onto the etched surfaces accumulates at the bottom of the nanoholes, thus making the etched pits a convenient template for the fabrication of high density quantum dot structures.

The development of quantum nanostructures is of primary importance for the advancement of optoelectronic technologies, and particularly the successful implementation of quantum dots (QDs) has the potential to result in a generational leap in the field. Despite the promises, the performance of QD-based optoelectronic devices is still lacking compared to the state-of-theart technology. One approach to overcome their limitations is to fully exploit the engineering possibilities allowed by QDs, by controlling not only their quantum states but also the carrier dynamics in the nanostructures. This approach allows control both wavefunction shaping and interlevel coupling, permitting to selectively enhance or suppress recombination channels. Our group is facing this challenge by designing a hybrid nanostructure driven by the concept of quantum confinement modulation. This approach exploits the ability of Droplet Epitaxy to grow homoepitaxial QDs on a quantum well, combining the advantages of 1D and 3D confinement while overcoming their individual drawbacks. In this talk I will discuss the impact of this approach on optoelectronic devices, focusing on the application in long-wave infrared detectors and intermediate band solar cells.

We investigate in detail the role of strain relaxation and capping overgrowth in the self-assembly of InAs quantum dots by droplet epitaxy. InAs quantum dots were realized on an In (Formula presented.) Al (Formula presented.) As metamorphic buffer layer grown on a GaAs(111)A misoriented substrate. The comparison between the quantum electronic calculations of the optical transitions and the emission properties of the quantum dots highlights the presence of a strong quenching of the emission from larger quantum dots. Detailed analysis of the surface morphology during the capping procedure show the presence of a critical size over which the quantum dots are plastically relaxed.

The efficient generation of entangled photons at telecom wavelength is essential for the success of many quantum communication protocols and the development of fiber-based quantum networks. Entangled light can be generated by solid-state quantum emitters with naturally low fine-structure splitting, such as highly symmetric InAs quantum dots (QDs) grown on (111)-oriented surfaces. The incorporation of these QDs into optical cavities is crucial to achieve sufficient signal intensities for applications but has so far shown major complications. In this work, we present droplet epitaxy of telecom-wavelength InAs QDs within an optical cavity on a vicinal (2° miscut) GaAs(111)A substrate. We show a remarkable enhancement of the photon extraction efficiency compared to previous reports together with a reduction of the density that facilitates the isolation of single spectral lines. Moreover, we characterize the exciton-fine-structure splitting and employ numerical simulations under the framework of the empirical-pseudopotential and configuration-interaction methods to study the impact of the miscut on the optical properties of the QDs. We demonstrate that the presence of the miscut steps influences the polarization of the neutral excitons and introduces a preferential orientation in the C3v symmetry of the surface.

The threshold voltage of a field-effect transistor (FET) determines its switching and limits the scaling of the supply voltage in the logic gates. Here we demonstrate a GaAs FET with a monolayer graphene gate in which the threshold voltage was externally controlled by an additional control gate. The graphene gate forms a Schottky junction with the transistor channel, modulating the channel conductivity. The control gate sets the work function of the graphene gate, controlling the Schottky barrier height and therefore the threshold voltage, and reduces the subthreshold swing down to ~60 mV dec−1. The change of the threshold voltage was large enough to turn the initially depletion mode FETs into the enhancement mode FETs. This allowed to realize logic gates with a positive switching threshold in which the threshold voltage of each transistor was independently set. The presented FETs can also be operated as dual-gate FETs, which was demonstrated by realizing frequency mixers.

We investigated the nucleation of Ga droplets on singular GaAs(111)A substrates in the view of their use as the seeds for the self-assembled droplet epitaxial quantum dots. A small critical cluster size of 1–2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around 350∘C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich–Schwöbel barrier at the terrace edges.

We investigated the nucleation of Ga droplets on singular GaAs(111)A substrates in the view of their use as the seeds for the self-assembled droplet epitaxial quantum dots. A small critical cluster size of 1–2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around 350∘C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich–Schwöbel barrier at the terrace edges.

We investigated the nucleation of Ga droplets on singular GaAs(111)A substrates in the view of their use as the seeds for the self-assembled droplet epitaxial quantum dots. A small critical cluster size of 1–2 atoms characterizes the droplet nucleation. Low values of the Hopkins-Skellam index (as low as 0.35) demonstrate a high degree of a spatial order of the droplet ensemble. Around 350∘C the droplet size distribution becomes bimodal. We attribute this observation to the interplay between the local environment and the limitation to the adatom surface diffusion introduced by the Ehrlich–Schwöbel barrier at the terrace edges.

Among different research areas connected to widespread use of quantum technology, Quantum Information and Communication is one of the most advanced. The term “Quantum Information” is used for any physical information that is encoded in quantum systems. Quantum Information Processing (QIP) deals with the manipulation of quantum information, to perform tasks, which are unachievable in a classical context, for instance, an absolute secure transmission of information, known as quantum cryptography. Nowadays, widely developed optical fiber networks provided an availability of commercial quantum cryptographic systems, where secure keys are encoded by quantum bits (qubits) – the basic unit of Quantum Information, which refers to the state space of a two-level quantum system. So, the long-term goal of the QIP is a creation of a large quantum network (the Quantum Internet), over which photons will transport quantum information to specific quantum nodes (Quantum Computers), where it would be processed and stored. Coming back to a simpler task of QIP, that is quantum cryptography, there are two main approaches for a quantum key distribution (QKD). The first based on the coding of the quantum state of a single particle and on the principle of the impossibility to distinguish reliably two non-orthogonal quantum states. Thus, security of this approach is based on the theorem prohibiting the cloning of an unknown quantum state. Due to the unitarity and linearity of quantum mechanics, it is impossible to create an exact copy of an unknown quantum state without affecting an initial state. There are many QKD protocols based on the approach, and the general one is BB84 (developed by Charles Bennet and Gilles Brassard in 1984), exploiting quantum states of single photons (for instance, polarization of photons). The second approach is based on the quantum entanglement. Two particles (including those separated in the space) can be in a correlation state, so the measurement of the selecte

We have successfully grown, through the detailed control of the growth kinetics, flat InAlAs metamorphic buffer layers on 2 degrees -off GaAs(111)A substrates using molecular beam epitaxy. Almost full plastic relaxation is obtained for a layer thickness > 40 nm. The control of an adatom diffusion length and a step ejection probability from the bunches permits a reduction of the InAlAs epilayer root-mean-square surface roughness to 0.55 nm.

We introduce a high–temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self–assemble with the desired emission wavelength and highly symmetric shape while maintaining a high optical quality. The role of the growth parameters during the droplet arsenization is discussed and modeled.

We introduce a high–temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self–assemble with the desired emission wavelength and highly symmetric shape while maintaining a high optical quality. The role of the growth parameters during the droplet arsenization is discussed and modeled.

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards (-1-12). At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards (-1-12). At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

We present self-assembly of InAs/InAlAs quantum dots by the droplet epitaxy technique on vicinal GaAs(111)A substrates. The small miscut angle, while maintaining the symmetries imposed on the quantum dot from the surface, allows a fast growth rate thanks to the presence of preferential nucleation sites at the step edges. A 100nm InAlAs metamorphic layer with In content >= 50% directly deposited on the GaAs substrate is already almost fully relaxed with a very flat surface. The quantum dots emit at the 1.3 mu m telecom O-band with fine structure splitting as low as 16 mu eV, thus making them suitable as photon sources in quantum communication networks using entangled photons., QC 20210517

Nucleation of Ge islands on a stepped Si(100) surface is studied. It is shown by diffraction of fast electrons that at a temperature of 600°C, constant flux of Si, and deposition rate of 0.652 Å/s, a series of the 1×2 superstructure reflections completely disappears, if the Si (100) substrate deviated by an angle of 0.35° to the (111) face is preliminarily heated to 1000°C. The disappearance of the 1×2 superstructure reflexes is due to the transition from the surface with monoatomic steps to that with diatomic ones. Investigations of the Ge islands’ growth were carried out on the Si(100) surface preliminarily annealed at temperatures of 800 and 1000°C. It is shown that the islands tend to nucleate at the step edges.

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

The GeSiSn, SiSn layer growth mechanisms on Si(100) were investigated and the kinetic diagrams of the morphological GeSiSn, SiSn film states in the temperature range of 150 °C-450 °C at the tin content from 0% to 35% were built. The phase diagram of the superstructural change on the surface of Sn grown on Si(100) in the annealing temperature range of 0 °C-850 °C was established. The specular beam oscillations were first obtained during the SiSn film growth from 150 °C to 300 °C at the Sn content up to 35%. The transmission electron microscopy and x-ray diffractometry data confirm the crystal perfection and the pseudomorphic GeSiSn, SiSn film state, and also the presence of smooth heterointerfaces between GeSiSn or SiSn and Si. The photoluminescence for the multilayer periodic GeSiSn/Si structures in the range of 0.6-0.8 eV was detected. The blue shift with the excitation power increase is observed suggesting the presence of a type II heterostructure. The creation of tensile strained Ge films, which are pseudomorphic to the underlying GeSn layer, is confirmed by the results of the formation and analysis of the reciprocal space map in the x-ray diffractometry. The tensile strain in the Ge films reached the value in the range of 0.86%-1.5%. The GeSn buffer layer growth in the Sn content range from 8% to 12% was studied. The band structure of heterosystems based on pseudomorphic GeSiSn, SiSn and Ge layers was calculated and the valence and conduction band subband position dependences on the Sn content were built. Based on the calculation, the Sn content range in the GeSiSn, SiSn, and GeSn layers, which corresponds to the direct bandgap GeSiSn, SiSn, and Ge material, was obtained.

We show that the density of indium islands on GaAs(111)A substrates have a non-monotonic, reentrant behavior as a function of the indium deposition temperature. The expected increase in the density with decreasing temperature, indeed, is observed only down to 160 ∘C, where the indium islands undertake the expected liquid-to-solid phase transition. Further decreasing the temperature causes a sizable reduction of the island density. An additional reentrant increasing behavior is observed below 80 ∘C. We attribute the above complex behavior to the liquid-solid phase transition and to the complex island-island interaction which takes place between crystalline islands in the presence of strain. Indium solid islands grown at temperatures below 160 ∘C have a face-centered cubic crystal structure.

We show that the density of indium islands on GaAs(111)A substrates have a non-monotonic, reentrant behavior as a function of the indium deposition temperature. The expected increase in the density with decreasing temperature, indeed, is observed only down to 160 ∘C, where the indium islands undertake the expected liquid-to-solid phase transition. Further decreasing the temperature causes a sizable reduction of the island density. An additional reentrant increasing behavior is observed below 80 ∘C. We attribute the above complex behavior to the liquid-solid phase transition and to the complex island-island interaction which takes place between crystalline islands in the presence of strain. Indium solid islands grown at temperatures below 160 ∘C have a face-centered cubic crystal structure.

This work deals with elastically strained GeSiSn films and GeSiSn islands. The kinetic diagram of GeSiSn growth for different lattice mismatches between GeSiSn and Si has been drawn. The multilayered periodic structures with pseudomorphic GeSiSn layers and GeSiSn island arrays have been obtained. The density of the islands in the GeSiSn layer is 1.8 · 1012 cm-2 for an average island size of 4 nm. Analysis of the rocking curves has shown that the structures contain smooth heterointerfaces, and no abrupt changes of composition and thickness between periods have been found. Photoluminescence has been demonstrated and calculation of band diagram with the model-solid theory has been carried out. Luminescence presented for sample with pseudomorphic Ge0.315Si0.65Sn0.035 layers in the narrow range 0.71–0.82 eV is observed with the maximum intensity near 0.78 eV corresponding to 1.59 µm wavelength. Based on the band diagram calculation for Si/Ge0.315Si0.65Sn0.035/Si heterocomposition we have concluded that 0.78 eV photon energy luminescence corresponds to interband transitions between the X-valley in Si and the heavy hole subband in the Ge0.315Si0.65Sn0.035 layer.

In the paper we investigate the Ge island nucleation on a Si(100) surface which was annealed to obtain the diatomic steps. It was observed that the islands tend to nucleate at the step edges.

Growth of SiSn compounds with a Sn content from 10 to 35% is studied. The morphology and surface structure of the SiSn layers are examined and the kinetic diagram of the morphological state of SiSn films is established in the temperature range of 150–450°C. During the growth of SiSn films from 150 to 300°C, oscillations of specular beam were observed. For the first time, periodic multilayer SiSn/Si structures with pseudomorphic monocrystalline SiSn layers with the Sn content from 10 to 25% are grown. The c(8×4) and (5×1) superstructures are identified during the growth of Si on the SiSn layer and the conditions are determined for the formation of the desired Si surface structure by controlling the growth temperature. From the diffraction reflection curves, the lattice parameter, the SiSn composition, and the period in the multilayer periodic structure are defined, which with high precision correspond to the specified values.

Gradual relaxation of elastic deformations in a silicon layer at the growth of a covering layer on strained layers was established. The dependence of the thickness of a silicon film, where full elastic strain relaxation occurs, on the germanium layer thickness was determined. The dependence of the critical thickness of 2D–3D transition of temperature and composition of the GeSiSn film on Si(100) was studied. Regularities of the formation of multilayer structures on quantum wells comprising pseudomorphous GeSiSn layers without relaxed buffer layers but creating the structures directly on Si. A possibility of synthesizing multilayer structures by molecular beam epitaxy was shown, and the crystal lattice constants using the high-resolution transmission electron microscopy were determined. Based on multilayer GeSiSn/Si structures the p-i-n-diodes, which demonstrated the photoresponse increasing by several orders of magnitude compared to the Sn-free structures at an increase in the Sn content, were created.

Tensile-strained germanium films in Ge/GeSn/Si/GeSnSi multilayer heterostructures grown by molecularbeam epitaxy on Si(001) substrates are investigated by Raman spectroscopy. Biaxial tensile strains in the films reach 1.5%, which exceeds values previously obtained for this system. Splitting of frequencies of long-wavelength optical phonons is experimentally observed; i.e., the shift of the frequency of the singlet induced by biaxial tensile strains is larger than the shift of the frequency of the doublet in agreement with calculations. The strain-induced shift of Raman scattering peaks from two-phonon scattering in germanium is also detected.

Temperature and composition dependencies of the critical thickness of transition from two-dimensional to three-dimensional growth for GeSiSn films on Si(100) with a lattice mismatch of 1 - 5% were experimentally determined. To understand the Sn influence on growth of SiGeSn/Si multi-quantum wells, the phase diagram of surface superstructures during the growth of pure Sn on Si(100) was created. A possibility of synthesizing multilayer structures by molecular beam epitaxy was shown, and the crystal lattice constants were determined using high-resolution transmission electron microscopy. We obtained GeSiSn/Si MQW structures which demonstrated photoluminescence for the Sn content in GeSiSn layers of up to 6%.

In the paper we investigate the Ge island nucleation on a Si(100) surface which was annealed to obtain the diatomic steps. It was observed that the islands tend to nucleate at the step edges.

The control over the spectral broadening of an ensemble of emitters, mainly attributable to the size and shape dispersion and the homogenous broadening mechanisms, is crucial to several applications of quantum dots. We present a convenient self-assembly approach to deliver strain-free GaAs quantum dots with size dispersion below 10%, due to the control of the growth parameters during the preliminary formation of the Ga droplets. This results in an ensemble photoluminescence linewidth of 19 meV at 14 K. The narrow emission band and the absence of a wetting layer promoting dot-dot coupling allow us to deconvolve the contribution of phonon broadening in the ensemble photoluminescence and study it in a wide temperature range.

The temperature and composition dependences of the critical thickness of the 2D–3D transition for a GeSiSn film on Si(100) have been studied. The regularities of the formation of multilayer structures with pseudomorphic GeSiSn layers directly on a Si substrate, without relaxed buffer layers, were investigated for the first time. The possibility of forming multilayer structures based on pseudomorphic GeSiSn layers has been shown and the lattice parameters have been determined using transmission electron microscopy. The grown structures demonstrate photoluminescence for Sn contents from 3.5 to 5% in GeSiSn layers.

Temperature dependence of the critical thickness of the transition from two-dimensional to threedimensional growth of the Ge1–5xSi4xSnx films grown on Si (100) by molecular-beam epitaxy in the temperature range 150–450°C has been experimentally determined. This dependence is nonmonotonic and is similar to that of the critical thickness for the transition from two-dimensional to three-dimensional growth in the case of the deposition of pure Ge on Si (100) and is caused by a change in the mechanism of two-dimensional growth. Data on the average size and the density of islands, and the ratio between the height of the islands and their lateral size are obtained by the methods of atomic force microscopy and scanning tunneling microscopy. As the growth temperature is increased from 200 to 400°C, the average size of the nanoislands increases from 4.7 to 23.6 nm.

Results of investigations into the synthesis of heterostructures based on Ge-Si-Sn materials by the method of low-temperature molecular beam epitaxy are presented. The formation of epitaxial films during structure growth has been controlled by the reflection high-energy electron diffraction method. Films with Ge/GeSn, Ge/GeSiSn, and GeSn/GeSiSn heterojunctions are grown with Sn content changing from 2 to 10% at temperatures in the interval 150-350°C. The stressed state, the composition, and the lattice parameter are studied by the x-ray diffraction method using Omega-scan curves and reciprocal space maps. A tensile strain in the Ge film during Ge/Ge09Sn01/Si structure growth has reached 0.86%.