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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. [ABSTRACT FROM AUTHOR]

We investigated the composition uniformity of InGaN epilayers in presence of metal droplets on the surface. We used Plasma Assisted MBE to grow an InGaN sample partially covered by metal droplets and performed structural and compositional analysis. The results showed a marked difference in indium incorporation between the region under the droplets and between them. Based on this observation we proposed a theoretical model able to explain the results by taking into account the vapour liquid solid growth that takes place under the droplet by direct impingement of nitrogen adatoms. [ABSTRACT FROM AUTHOR]

Given the recent increase in the demand for gallium nitride (GaN) in different markets like optoelectronics and power devices, the request for epitaxially grown GaN will further increase. To meet this high demand, higher throughput and more economical manufacturing technologies must be advanced. In this work, GaN thin films are deposited by reactive sputter deposition from a liquid gallium target at a substrate temperature of 900 °C. The layers are grown epitaxially on c-plane oriented sapphire in an industrial-scale sputter tool from Evatec AG. Due to the growth rate of >1 nm/s and the fast substrate heat-up time, the throughput in a production setup can be increased compared to other GaN growth techniques. The resistivity of the intrinsic insulating GaN can be lowered by intentional Si doping during the sputter deposition process by three orders of magnitude. Thereby, conductive n-type GaN can be grown with different dopant amounts. The carrier mobility of the sputter deposited film is 45 cm2 V−1 s−1 at a carrier concentration of 1.1 × 1020 cm−3 based on room temperature Hall measurements using a van der Pauw geometry. The lowest resistivity reaches 1300 μΩ cm, which is confirmed by sheet resistance measurements. Undoped films exhibit an x-ray diffraction rocking curve full width at half maximum of 0.2°, which increases up to 0.5° for highly Si-doped layers. The presented results show that GaN prepared by reactive sputter deposition from a liquid gallium source is a viable alternative to conventional deposition techniques for GaN. [ABSTRACT FROM AUTHOR]

With black phosphorus being a promising two-dimensional layered semiconductor for application to electronics and optoelectronics, an issue remains as to how heat diffusion is managed when black phosphorus is interfaced with metals, namely in a typical device heterojunction. We use Raman spectroscopy to investigate how the laser-induced heat affects the phonon modes at the interface by comparing the experimental data with a finite element simulation based on a localized heat diffusion. The best convergence is found taking into account an effective interface thermal conductance, thus indicating that heat dissipation at the Au-supported black phosphorus nanosheets is limited by interface effect. [ABSTRACT FROM AUTHOR]

The urgent quest to introduce the Xenes, a new family of graphene‐like materials, into everyday technological devices furthers demands for a specific understanding of their reactivity to different environments. Herein, the role of oxygen on blue phosphorene fragments alloyed with the Au(111) substrate, the so‐called BlueP–Au alloy, is investigated both on a microscopic scale and at the interface with Al2O3, commonly used to protect Xenes in ambient conditions. Although molecular oxygen does not affect the BlueP–Au alloy or intercalate below it, the role of oxygen is fundamental to relieve the charging effects at the very interface between Al2O3 and the BlueP–Au alloy when exposed to air. These findings highlight the importance of combining different tools, including microscopy and spectroscopy, to ascertain the stability of 2D materials in device‐ready configurations. [ABSTRACT FROM AUTHOR]

In the realm of two-dimensional material frameworks, single-element graphene-like lattices, known as Xenes, pose several issues concerning their environmental stability, with implications for their use in technology transfer to a device layout. In this Discussion, we scrutinize the chemical reactivity of epitaxial silicene, taken as a case in point, in oxygen-rich environments. The oxidation of silicene is detailed by means of a photoemission spectroscopy study upon carefully dosing molecular oxygen under vacuum and subsequent exposure to ambient conditions, showing different chemical reactivity. We therefore propose a sequential Al2O3 encapsulation of silicene as a solution to face degradation, proving its effectiveness by virtue of the interaction between silicene and a silver substrate. Based on this method, we generalize our encapsulation scheme to a large number of metal-supported Xenes by taking into account the case of epitaxial phosphorene-on-gold. [ABSTRACT FROM AUTHOR]

Here, two novel approaches for disassembling epitaxial silicene from the native substrate and transferring onto arbitrary target substrates are presented. From the processing perspective, the two methodologies open up a new route for handling silicene, and in general any epitaxial Xene, in view of establishing reliable process flows for the development of a Xene‐based nanotechnology. Integration of silicene in a back‐gated controlled device architecture is demonstrated and the built up of unique multi‐stack heterostructures between silicene, but potentially every Xene, and other technological relevant materials, like transparent conductive oxides and transition metal dichalcogenides is shown. [ABSTRACT FROM AUTHOR]

The large foreseeable use two-dimensional materials in nanotechnology consequently demands precise methods for their thickness measurements. Usually, having a quick and easy methodology is a key requisite for the inspection of the large number of flakes produced by exfoliation methods. An effective option in this respect relies on the measurement of the intensity of Raman spectra, which can be used even when the flakes are encapsulated by a transparent protective layer. However, when using this methodology, special attention should be paid to the crystalline anisotropy of the examined material. Specifically, for the case of black phosphorus flakes, the absolute experimental determination of the thickness is rather difficult because the material is characterized by a low symmetry and also because the Raman tensors are complex quantities. In this work, we exploited Raman spectroscopy to measure the thickness of black phosphorous flakes using silicon as reference material for intensity calibrations. We found out that we can determine the thickness of a flake above 5 nm with an accuracy of about 20%. We tested the reproducibility of the method on two different setups, finding similar results. The method can be applied also to other van der Waals materials with a Raman band characterized by the same Raman tensor. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Raman spectroscopy finds applications in many research fields to monitor stress on a micrometric scale. Unfortunately, the interpretation of Raman maps is always complicated by the tensorial nature of stress and by averaging effects due to the finite spatial resolution of the technique. Usually, the interpretation is either purely qualitative or based on thorough simplifications. In this paper we present an approach that compares the experimental data with a virtual experiment using a finite-element model. The method is applied to the study of the stress generated in a microelectronic device during the manufacturing process. The results are used both as a feedback and a validation for the modeling and also for the interpretation of the Raman data. [ABSTRACT FROM AUTHOR]

We present the development of a Raman spectroscopy technique for the measurement of the tensorial nature of stress in silicon on a micrometric scale. After the detailed description of the theoretical bases of the experiment, we measure the stress tensor of a silicon surface close to a scratch. Then, we apply this method to discern which models are suitable for the description of the stress tensor in shallow trench isolations for microelectronics. [ABSTRACT FROM AUTHOR]

This Special Issue collected contributions with respect to the modeling, synthesis, and characterization of two-dimensional materials, with special attention focused on their possible exploitation for energy-related applications. Two-dimensional (2D) materials are a class of materials that can be reduced to a thickness of a few layers, exhibiting peculiar and innovative properties relative to their three-dimensional solid counterparts. [Extracted from the article]

Transition metal dichalcogenide (TMDC) ultrathin layers have attracted considerable interest in the recent years. Their peculiar functional properties can be exploited in electronics, spintronics, optoelectronics, photonics, energy production, harvesting and storage. The availability of cost-effective, green, and efficient growth processes is of paramount importance and significant effort has been made in the research on various production methodologies. Here we report on a simple laser-based process which allows the direct writing of thin TMDC layers. In detail, by direct exposure to laser irradiation of a dip-coated MoS2 precursor, we obtained a three-dimensional arrangement of MoS2 nanoparticles in the form of platelets with a lateral dimension of about 50 nm and thickness down to bilayers. The characterization was assessed by AFM and Raman spectroscopy. The platelets are formed only in the central region of the laser spot, confirming that the material out of the spot is indeed an unprocessed precursor. By tuning the precursor deposition, we demonstrate the fabrication of MoS2 patterns with designed layer numbers. The proposed approach is highly versatile and can be applied also for the controlled growth of other TMDCs, as proved by the successful generation of WS2 layers. [ABSTRACT FROM AUTHOR]

The monolithic integration of photonic functionality into silicon microtechnology is widely advanced. Yet, there is no final solution for the realization of a light source compatible with the prevailing complementary metal-oxide-semiconductor technology. A lot of research effort focuses on germanium (Ge) on silicon (Si) heterostructures and tensile strain application to Ge is accepted as one feasible route to make Ge an efficient light emitter. Prior work has documented the special suitability of Ge membranes to reach the high tensile strain. We present a top-down approach for the creation of SiGe stressors on Ge micro-bridges and compare the obtained strain to the case of an attached bulk-like Ge layer. We could show that the Ge influenced by a SiGe stressor is under tensile strain; absolute strain values are of the order of 0.7% for both micro-bridge and bulk. The relative strain induced by the nanostructures in the micro-bridge is 1.3% due to the high sharing of elastic energy between nanostructures and bridges. [ABSTRACT FROM AUTHOR]

We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in lm-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission. [ABSTRACT FROM AUTHOR]

We show that a relatively simple top-down fabrication can be used to locally deform germanium in order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically predicted to transform germanium from an indirect to a direct gap semiconductor. These values of strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers. [ABSTRACT FROM AUTHOR]

Dense arrays of micrometric crystals, with areal filling up to 93%, are obtained by depositing GaAs in a mask-less molecular beam epitaxy process onto Si substrates. The substrates are patterned into tall, micron sized pillars. Faceted high aspect ratio GaAs crystals are achieved by tuning the Ga adatom for short surface diffusion lengths. The crystals exhibit bulk-like optical quality due to defect termination at the sidewalls. Simultaneously, the thermal strain induced by different thermal expansion parameters of GaAs and Si is fully relieved. This opens the route to thick film applications without crack formation and wafer bowing. [ABSTRACT FROM AUTHOR]

We studied the plastic deformation of an ultrathin silicon-on-insulator with epitaxial Si1-xGex by transmission electron microscopy, Raman spectroscopy, and finite-element method. We analyzed a top Si layer of 10 nm (testing also a 2 nm layer) with epitaxial Si0.64Ge0.36 stressors of 50 and 100 nm. SiGe plastically deforms the top Si layer, and this strain remains even when Si1-xGex is removed. For low dislocation densities, dislocations are gettered close to the Si/SiO2 interface, while the SiGe/Si interface is coherent. Beyond a threshold dislocation density, interactions between dislocations force additional dislocations to position at the Si1-xGex/Si interface. [ABSTRACT FROM AUTHOR]

The extension of SiGe technology towards new electronic and optoelectronic applications on the Si platform requires that Ge-rich nanostructures be obtained in a well-controlled manner. Ge deposition on Si substrates usually creates SiGe nanostructures with relatively low and inhomogeneous Ge content. We have realized SiGe nanostructures with a very high (up to 90%) Ge content. Using substrate patterning, a regular array of nanostructures is obtained. We report that electron microscopy reveals an abrupt change in Ge content of about 20% between the filled pit and the island, which has not been observed in other Ge island systems. Dislocations are mainly found within the filled pit and only rarely in the island. Selective chemical etching and electron energy-loss spectroscopy reveal that the island itself is homogeneous. These Ge-rich islands are possible candidates for electronic applications requiring locally induced stress, and optoelectronic applications which exploit the Ge-like band structure of Ge-rich SiGe. [ABSTRACT FROM AUTHOR]

We report on the dewetting process, in a high vacuum environment, of amorphous Ge thin films on SiO2/Si (001). A detailed insight of the dewetting is obtained by in situ reflection high-energy electron diffraction and ex situ scanning electron microscopy. These characterizations show that the amorphous Ge films dewet into Ge crystalline nano-islands with dynamics dominated by crystallization of the amorphous material into crystalline nano-seeds and material transport at Ge islands. Surface energy minimization determines the dewetting process of crystalline Ge and controls the final stages of the process. At very high temperatures, coarsening of the island size distribution is observed. [ABSTRACT FROM AUTHOR]

The use of Raman spectroscopy to determine strain in microelectronic devices is intrinsically limited by optical diffraction. The critical issue is not the limited spatial resolution itself, but rather the averaging of inhomogeneously strained regions reducing the sensitivity significantly. To eliminate this effect, we took advantage of the near-field properties of an illuminated subwavelength periodic structure. As it is possible to restrict the investigated volume to the transistor channel only, the sensitivity increases significantly. The technique is advantaged by a very small pitch of the devices, and therefore can be also used in the future technological nodes. [ABSTRACT FROM AUTHOR]

This letter presents a method to measure stress by a Raman spectroscopic technique that combines high spatial resolution with tensorial analysis. The experiment is based on separating the contributions to the Raman spectrum from the marginal and paraxial rays of the collection cone of the objective. The stress tensor was measured over a 20 μm line scan with a resolution of 1 μm on a highly strained (001) silicon surface in proximity to a micrometric scratch by observing the different frequency shifts of three orthogonal crystal vibrations. [ABSTRACT FROM AUTHOR]

Silicene has recently been revisited as a 2D material with potential thermoelectric applications. Here, the thermal properties of supported silicene are determined by optothermal Raman spectroscopy. Single and multilayer silicene is grown either directly on silver (Ag) substrates or on an intermediate tin (Sn) monolayer, introduced to reduce the influence of the substrate on the physical properties of silicene. Experimental values of an effective cross‐plane thermal conductivity of 0.5 W/mK are obtained for silicene on Ag and 0.3 W/mK for silicene on stanene‐Ag. The values of the interfacial thermal conductance, on the other hand, are 0.3 and 0.5 GW/m2K, respectively. Heterostack engineering is confirmed as a versatile strategy for extracting relevant physical parameters in silicene and for modulating the thermal response in the 2D limit. [ABSTRACT FROM AUTHOR]

Germanium is known to become a direct band gap material when subject to a biaxial tensile strain of 2% (Vogl et al 1993 Phys. Scr.T49B 476) or uniaxial tensile strain of 4% (Aldaghri et al 2012 J. Appl. Phys.111 053106). This makes it appealing for the integration of optoelectronics into current CMOS technology. It is known that the induced strain is highly dependent on the geometry and composition of the whole system (stressors and substrate), leaving a large number of variables to the experimenters willing to realize this transition and just a trial-and-error procedure. The study in this paper aims at reducing this freedom. We adopt a finite element approach to systematically study the elastic strain induced by different configurations of lithographically-created SiGe nanostructures on a Ge substrate, by focusing on their composition and geometries. We numerically investigate the role played by the Ge substrate by comparing the strain induced on a bulk or on a suspended membrane. These results and their interpretation can provide the community starting guidelines to choose the appropriate subset of parameters to achieve the desired strain. A case of a very large optically active area of a Ge membrane is reported. [ABSTRACT FROM AUTHOR]

In this work we present an innovative approach to realise coherent, highly-mismatched 3-dimensional heterostructures on substrates patterned at the micrometre-scale. The approach is based on the out-of-equilibrium deposition of SiGe alloys graded at an exceptionally shallow grading rate (GR) of 1.5% µm −1 by low energy plasma enhanced chemical vapour deposition (LEPECVD). Fully coherent SiGe/Si crystals up to 6 µm in width were achieved as confirmed by defect etching and transmission electron microscopy (TEM) analyses. The experimental results are supported by calculations of the energy for dislocation formation which indicate that elastic relaxation is energetically favoured over plastic relaxation in the narrower heterostructures. X-ray diffraction measurements show that the SiGe crystals are strain-free irrespective of the stress relieving mechanism which changes from elastic to plastic by increasing their width. The impact of dislocations on the SiGe crystal quality is analysed by comparing the width of X-ray diffraction peaks as a function of the heterostructure size. [ABSTRACT FROM AUTHOR]

In this paper we present the exceptional thermal strain release provided by micrometric Si pillar arrays to Ge epitaxial patches suspended on them, for different pillar aspect ratios and patch sizes. By combining 3D and 2D Finite Element Method simulations, low-energy plasma-enhanced chemical vapor deposition on patterned Si substrates, μ-Raman, μ-photoluminescence and XRD measurements, we provide a quantitative and consistent picture of this effect with the patch sizes. Strain relaxation up to 85% of the value for the corresponding planar films can be obtained for a squared patch 100 μm in size. Finally, the enhanced thermal strain relaxation is analytically explained in terms of the Si pillar lateral tilting, critically dependent on the pillar aspect ratio, very similarly to the well-known case of a deflected beam. Our results are transferable to any material deposited, or wafer bonded at high temperature, on any patterned substrate: wafer bowing can be controlled by micrometric patterned features well within the present capabilities of deep reactive ion etching. [ABSTRACT FROM AUTHOR]

In the drive towards increased lithium based battery capacity, germanium is an attractive material due to its very high lithium storage capacity, second only to silicon. The persistent down-side is the considerable embrittlement accompanying its remarkable volume expansion of close to 300 %. A proven method to accommodate for this lattice expansion is the reduction of the size towards the nanoscale at which the fracturing is prevented by "breathing". In this work we employed a novel magnetron sputtering gas aggregation nanoparticle generator to create unprecedented layers of well-defined germanium nanoparticles with sizes below 20 nm. The electrochemical lithium intercalation was monitored by a suite of techniques under which Raman spectroscopy, which provided clear evidence of the presence of lithium inside the germanium nanoparticles. Moreover, the degree of lattice order was measured and correlated to the initial phases of the lithium-germanium alloy. This was corroborated by electron diffraction and optical absorption spectroscopy, of which the latter provided a strong dielectric change upon lithium intercalation. This study of low lithium concentrations inside layers of well-defined and very small germanium nanoparticles, forms a new avenue towards significantly increasing the lithium battery capacity., (© 2024 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Laser ablation in conjunction with Raman spectroscopy can be used to attain a controllable reduction of the thickness of exfoliated black phosphorus flakes and simultaneous measurement of the local temperature. However, this approach can be affected by several parameters, such as the thickness-dependent heat dissipation. Optical, thermal, and mechanical effects in the flakes and the substrate can influence the laser ablation and may become a source of artifacts on the measurement of the local temperature. In this work, we carry out a systematic investigation of the laser thinning of black phosphorus flakes on SiO 2 /Si substrates. The counterintuitive results from Raman thermometry are analyzed and elucidated with the help of numerical solutions of the problem, laying the groundwork for a controlled thinning process of this material., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

We investigate light-matter interactions in periodic silicon microcrystals fabricated combining top-down and bottom-up strategies. The morphology of the microcrystals, their periodic arrangement, and their high refractive index allow the exploration of photonic effects in microstructured architectures. We observe a notable decrease in reflectivity above the silicon bandgap from the ultraviolet to the near-infrared. Finite-difference time-domain simulations show that this phenomenon is accompanied by a ∼2-fold absorption enhancement with respect to a flat sample. Finally, we demonstrate that ordered silicon microstructures enable a fine tuning of the light absorption by changing experimentally accessible knobs as pattern and growth parameters. This work will facilitate the implementation of optoelectronic devices based on high-density microcrystals arrays with optimized light-matter interactions.

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route toward quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared with previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations.

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

We present a combined experimental and theoretical analysis of the dielectric and vibrational properties of crystalline lutetium oxide in its ground-state bixbyite structure. The vibrational dielectric function of Lu2O3 thin films grown by atomic-layer deposition was studied by infrared transmission and reflection-absorption spectroscopies, selectively accessing transverse and longitudinal optical frequencies. The static dielectric constant is extracted analyzing the infrared response. We also present first-principles density-functional linear-response calculations, which are in close agreement with experiment, and provide insight into the microscopic nature of vibrational spectra and dielectric properties.