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 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 to obtain a better crystalline quality [E.

The search for semiconducting materials with improved optical properties relies on the possibility to manipulate the semiconductors band structure by using quantum confinement, strain effects, and by the addition of diluted amounts of impurity elements such as Bi. In this study, we explore the possibility to engineer the structural and physical properties of the Ga(As, Bi) alloy by employing different stress conditions in its epitaxial growth. Films with variable concentration of Bi are grown by molecular beam epitaxy on bare GaAs(001) crystals and on partially relaxed (In, Ga)As double buffer layers acting as stressors aiming to control the Bi incorporation into the alloy and improving the optical properties in terms of efficiency. A combination of several structural and electronic characterization techniques and dedicated density-functional-theory calculations allows us a systematic comparison between the samples grown under compressive and tensile strain. We demonstrate the possibility to grow Ga(As, Bi) under different strain conditions without affecting its crystal quality. The different strain conditions strongly impact the Bi incorporation in the GaAs matrix and the luminescence properties of the sample. We find (i) a striking improvement of the photoluminescence with a strongly increased radiative efficiency when Ga(As, Bi) is grown under tensile strain and (ii) an interesting higher redshift with respect to Ga(As, Bi) grown compressively on GaAs. These two effects allow us to reach the important photoluminescence emission at 1.3 µm with a Bi concentration as low as 4.9% compared to 7.5% needed for samples grown directly on GaAs. This is a significant achievement for the application of the Ga(As, Bi) material in optoelectronic devices.

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°. 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.

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 Sb 2 Te 3 /GeTe heterostructures leveraging on the peculiar behavior of Sb 2 Te 3 , in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb 2+x Te 3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb 2 Te 3 and Sb 4 Te 3 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 Sb 2 and Sb 4 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 Sb 2+x Te 3 /GeTe SLs by controlling at the nanoscale the 2D character of the Sb 2+x Te 3 alloys is envisioned.

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 × 3/1 × 3 phase (Bi‐rich) is obtained after annealing the sample at 400 °C, while subsequent annealing at 450 °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.

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 anti-aligned 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

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

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