Reflectance anisotropy spectroscopy of strain-engineered GaAsBi alloys

In this paper, we present results obtained by an optical technique, namely, reflectance anisotropy spectroscopy (RAS), applied to a series of GaAs1-xBix samples grown by molecular beam epitaxy (MBE) under different strain conditions with the increasing concentration of Bi, up to the higher value of about 7%. The epitaxial buffer layers for the growing GaAs1-xBix layer were prepared with either a compressive strain (as it is commonly done) or a tensile strain: The latter case has been proven to be a strategy that allows us to obtain a better crystalline quality [Tisbi et al., Phys. Rev. Appl. 14, 014028 (2020)]. A characteristic, well defined anisotropy signal below 2.5 eV is demonstrated to be connected to the presence of Bi and, in particular, to the strain produced in the sub-surface region by the voluminous Bi atoms. The amplitude of this signal directly relates to the Bi quantity, while its sign gives information about the local clustering/ordering of Bi atoms in the grown sample. We conclude that the detailed interpretation of RAS signatures and the knowledge of their origin offer the opportunity to utilize this technique to follow in real time the GaAsBi growth either in MBE or in metal organic vapor phase epitaxy processes

Anisotropic cation diffusion in the GaAs capping of InAs/GaAs(001) quantum dots

The effect of the As flux on the kinetics of Ga and In cations, and the role of the elastic strain were studied in the capping process of isolated InAs quantum dots with GaAs by molecular beam epitaxy. Using a fixed evaporation geometry and a suitable choice of growth parameters which enhance the anisotropic diffusion of In and Ga cations, we obtained, at variance with current results, the formation of asymmetric GaAs caps. The growth of a second InAs layer led to the formation of vertically aligned couples of dots (one buried, the other uncapped) placed on the right side of the GaAs caps on the surface, with no other dots in different locations. Published by AIP Publishing

Stress-determined nucleation sites above GaAs-capped arrays of InAs quantum dots

We studied the stress field at the surface of GaAs capping layers of variable thicknesses burying InAs quantum dot arrays using the Finite Element method to solve numerically the equations of the elastic field. The aim is to determine the stress-determined favorable sites for dot nucleation. We show that: (i) depending on the cap thickness, dot distances, and array orientation, sudden transitions in the stress-strain fields occur, leading from a vertical alignment of the dots to an antialigned correlation. We find that just few determined positions are favorable for dot nucleation and exclude some other sites previously indicated as favorable in the literature; (ii) the critical thicknesses at which the switch between the vertical alignment and the anti-aligned positions occurs depend on the distance between the dots in a square array and on the ratio between the two different distances if the arrays are rectangular; (iii) the transitions occur within a few nanometer range of the capping layer thickness, and the elastic field undergoes large changes in its properties before and after the transition. This behavior has been revealed by a very accurate fit of the tangential stress field using appropriate fit functions. The fit and parameter functions allow to easily reproduce the stress field in different contexts and are useful in growth simulation models. The results suggest that by properly engineering the capping layer thicknesses in the layers of a stack, it is possible to obtain different three-dimensional quantum dot lattices starting from an initial fixed dot array. Our results are in agreement with the available experimental data

Sensing sub-surface strain in GaAsBi(001) surfaces by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) is applied to study the reconstructed GaAsBi(001) surfaces at room temperature. Arsenic-capped GaAsBi samples with 7% Bi concentration are grown by molecular beam epitaxy (MBE) in nearly matched conditions on a proper buffer layer and annealed in ultra-high vacuum (UHV). Low energy electron diffraction (LEED) shows that, following the As decapping, a 2 x 3/1 x 3 phase (Bi-rich) is obtained after annealing the sample at 400 degrees C, while subsequent annealing at 450 degrees C yields a deterioration of the surface order. RAS spectra measured in situ allow to definitely confirm that the characteristic Bi-dependent anisotropy measured below 2.5 eV has not a true surface origin, although being connected to the surface: it is related to the strain of the directional bonds between Bi atoms existing at the surface and below the surface. This result has a twofold significance: it recommends that previous attributions to the surface of RAS anisotropy features in III-V semiconductors should be in some cases revisited; for the future, it shows that RAS is suitable to characterize 2D-layered materials, and to investigate the consequences of strain in the electronic properties of low-dimensional systems

Strain-engineered arrays of InAs quantum dots on GaAs(001): Epitaxial growth and modeling

Working under critical conditions for dot nucleation in a Molecular Beam Epitaxy chamber, we were able to drive the formation of InAs dot chains to precise locations in multilayered samples grown on a rippled GaAs(001) surface. We discussed the role of the elastic field and the surface curvature in determining the dot arrangement at each stacked layer, proving a new mechanism of self-organization of the dots. In particular, we succeeded in controlling the interplay between elastic and curvature effects and we showed how a selection process is achievable in the chain formation. The role of the stress field was also studied by means of Finite Element Method simulations, and we gained a valuable understanding of the interlayer dot correlations for dot arrays with variable cap thicknesses. We proved the existence of an anisotropy in the cap formation of isolated dots, which appeared to be directly related to our peculiar growth geometry and experimental set-up

Tuning the growth for a selective nucleation of chains of Quantum Dots behaving as single photon emitters

Single and two-layer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical conditions, leading to the selective growth of self-assembled InAs Quantum Dot chains over mounded GaAs surfaces. Changing the thickness of the spacer layer and the InAs deposition made it possible to tune the nucleation of 2-fold or single chains in the second layer. Finite Element Method simulations evidenced the major role of the strain field in favoring the formation of single stacked chains. On the other hand, tuning properly the As4/In flux ratio contributed to improving the QD ordering along the chains. Microphotoluminescence experiments demonstrated single photon emission properties of the observed QDs. Our growth approach did not degrade the optical quality of the InAs QDs, allowing a significant spatial correlation between the QDs aligned along the chain

Textured Sb2Te3 films and GeTe/Sb2Te3 superlattices grown on amorphous substrates by molecular beam epitaxy

The realization of textured films of 2-dimensionally (2D) bonded materials on amorphous substrates is important for the integration of this material class with silicon based technology. Here, we demonstrate the successful growth by molecular beam epitaxy of textured Sb2Te3 films and GeTe/Sb2Te3 superlattices on two types of amorphous substrates: carbon and SiO2. X-ray diffraction measurements reveal that the out-of-plane alignment of grains in the layers has a mosaic spread with a full width half maximum of 2.8 degrees . We show that a good texture on SiO2 is only obtained for an appropriate surface preparation, which can be performed by ex situ exposure to Ar+ ions or by in situ exposure to an electron beam. X-ray photoelectron spectroscopy reveals that this surface preparation procedure results in reduced oxygen content. Finally, it is observed that film delamination can occur when a capping layer is deposited on top of a superlattice with a good texture. This is attributed to the stress in the capping layer and can be prevented by using optimized deposition conditions of the capping layer. The obtained results are also relevant to the growth of other 2D materials on amorphous substrates. (C) 2017 Author(s)

Tuning the growth for a selective nucleation of chains of Quantum Dots behaving as single photon emitters

Single and two-layer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical conditions, leading to the selective growth of self-assembled InAs Quantum Dot chains over mounded GaAs surfaces. Changing the thickness of the spacer layer and the InAs deposition made it possible to tune the nucleation of 2-fold or single chains in the second layer. Finite Element Method simulations evidenced the major role of the strain field in favoring the formation of single stacked chains. On the other hand, tuning properly the As-4/In flux ratio contributed to improving the QD ordering along the chains. Microphotoluminescence experiments demonstrated single photon emission properties of the observed QDs. Our growth approach did not degrade the optical quality of the InAs QDs, allowing a significant spatial correlation between the QDs aligned along the chain. (C) 2016 Elsevier B.V. All rights reserved

Interplay between Structural and Thermoelectric Properties in Epitaxial Sb_2+xTe₃ Alloys

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb2 and Sb4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned