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In(Ga)As quantum dot (QD) with uniform size and controlled sites have great potential in optical communications and quantum computing. In this review, we focus on the site-controlled preparation of In(Ga)As quantum dot arrays based on patterned substrates, including the improvements made by the researchers to enhance the quantum dot site-control capability and optical quality. Based on the current research on site-controlled In(Ga)As QDs, it has been possible to grow uniformly ordered In(Ga)As QD arrays, in which the size, morphology, and nucleus location of each quantum dot can be precisely controlled. In addition, the study of deoxidation treatment of patterned substrates has led to the performance enhancement of the prepared QD arrays. Finally, we propose that the future development of site-controlled In(Ga)As QD arrays lies in improving the optical quality and tuning their emission wavelength to the telecommunication band. [ABSTRACT FROM AUTHOR]

We report on the fabrication of a novel design of GaAs/(In,Ga)As/GaAs radial nanowire heterostructures on a Si 111 substrate, where, for the first time, the growth of inhomogeneous shells on a lattice mismatched core results in straight nanowires instead of bent. Nanowire bending caused by axial tensile strain induced by the (In,Ga)As shell on the GaAs core is reversed by axial compressive strain caused by the GaAs outer shell on the (In,Ga)As shell. Progressive nanowire bending and reverse bending in addition to the axial strain evolution during the two processes are accessed by in situ by x-ray diffraction. The diameter of the core, thicknesses of the shells, as well as the indium concentration and distribution within the (In,Ga)As quantum well are revealed by 2D energy dispersive x-ray spectroscopy using a transmission electron microscope. Shell(s) growth on one side of the core without substrate rotation results in planar-like radial heterostructures in the form of free standing straight nanowires. [ABSTRACT FROM AUTHOR]

We successfully fabricated two-dimensional metallic CoBi nanoislands on SrTiO3 (001) substrate by molecular beam epitaxy, and systematically investigated their electronic structures by scanning tunneling microscopy and spectroscopy in situ at 4.2 K. Coulomb blockade and Coulomb staircases with discrete and well-separated levels are observed for the individual nanoisland, which is attributed to single-electron tunneling via two tunnel junction barriers. They are in excellent agreement with the simulations based on orthodox theory. Furthermore, we demonstrated that the Coulomb blockade becomes weaker with increasing temperature and almost disappears at ∼22 K in our variable temperature experiment, and its full-width at half-maximum of d I /d V peaks with temperature is ∼6 mV. Our results provide a new platform for designing single-electron transistors that have potential applications in future microelectronics. [ABSTRACT FROM AUTHOR]

(In, Ga) alloy droplets are used to catalyse the growth of (In, Ga)As nanowires by molecular beam epitaxy on Si(111) substrates. The composition, morphology and optical properties of these nanowires can be tuned by the employed elemental fluxes. To incorporate more than 10% of In, a high In/(In+Ga) flux ratio above 0.7 is required. We report a maximum In content of almost 30% in bulk (In, Ga)As nanowires for an In/(In+Ga) flux ratio of 0.8. However, with increasing In/(In+Ga) flux ratio, the nanowire length and diameter are notably reduced. Using photoluminescence and cathodoluminescence spectroscopy on nanowires covered by a passivating (In, Al)As shell, two luminescence bands are observed. A significant segment of the nanowires shows homogeneous emission, with a wavelength corresponding to the In content in this segment, while the consumption of the catalyst droplet leads to a spectrally-shifted emission band at the top of the nanowires. The (In,Ga)As nanowires studied in this work provide a new approach for the integration of infrared emitters on Si platforms. [ABSTRACT FROM AUTHOR]

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5th and 6th generation telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO3 films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO3 layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO3, LiNbO3 and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO3/SrTiO3/Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm−1 as expected for the nearly stoichiometric LiNbO3. According to the theoretical calculations, 36°Y-X LiNbO3 films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr1− x Ti x O3 films. [ABSTRACT FROM AUTHOR]

Reliable and controllable growth of two-dimensional (2D) hexagonal boron nitride (h-BN) is essential for its wide range of applications. Substrate engineering is one of the critical factors that influence the growth of the epitaxial h-BN films. Here, we report the growth of monolayer h-BN on Ni (111) substrates incorporated with oxygen atoms via molecular beam epitaxy. It was found that the increase of incorporated oxygen concentration in the Ni substrate through a pretreatment process prior to the h-BN growth step would have an adverse effect on the morphology and growth rate of 2D h-BN. Under the same growth condition, h-BN monolayer coverage decreases exponentially as the amount of oxygen incorporated into Ni (111) increases. Density functional theory calculations and climbing image nudged elastic band (CI-NEB) method reveal that the substitutional oxygen atoms can increase the diffusion energy barrier of B and N atoms on Ni (111) thereby inhibiting the growth of h-BN films. As-grown large-area h-BN monolayer films and fabricated Al/h-BN/Ni (MIM) nanodevices were comprehensively characterized to evaluate the structural, optical and electrical properties of high-quality monolayers. Direct tunneling mechanism and high breakdown strength of ∼11.2 MV cm−1 are demonstrated for the h-BN monolayers grown on oxygen-incorporated Ni (111) substrates, indicating that these films have high quality. This study provides a unique example that heterogeneous catalysis principles can be applied to the epitaxy of 2D crystals in solid state field. Similar strategies can be used to grow other 2D crystalline materials, and are expected to facilitate the development of next generation devices based on 2D crystals. [ABSTRACT FROM AUTHOR]

In order to elucidate the mechanisms responsible for cathodoluminescence intensity variations at the scale of single InGaN/GaN nanowire heterostructures, a methodology is proposed based on a statistical analysis on ensembles of several hundreds of nanowires exhibiting a diameter of 180, 240 and 280 nm. For 180 nm diameter, we find that intensitiy variations are consistent with incorporation of point defects obeying Poisson's statistics. For wider diameters, intensity variations at the scale of single NWs are observed and assigned to local growth conditions fluctuations. Finally, for the less luminescent nanowires, a departure from Poisson's statistics is observed suggesting the possible clustering of non independent point defects. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The growth of two dimensional sp2-bonded boron nitride (2D-BN) was studied in a plasma-assisted molecular beam epitaxy set-up, using independent boron and nitrogen sources. We studied the growth conditions on polycrystalline Ni foils: B and N respective fluxes, growth temperature and time, which are influencing the surface morphology, stoichiometry and the 2D-BN domain size. Using a B/N precursor flux ratio ≫1 yields films with incorporated boron largely in excess and intermixed with 2D-BN. On the contrary, precursor flux ratios from moderately B-rich to moderately N-rich leads to stoichiometric 2D-BN. The optimum growth temperature is found to be 900 °C, a temperature for which the crystallographic quality is improved compared to lower temperatures thanks to the increased adatom surface mobility although a partial sublimation of BN occurs. Increasing the growth time under the optimized settings shows that the growth does not occur in a layer-by-layer mode, but rather by stacking BN domains on top of each other with a rather slow lateral extension of the domains. [ABSTRACT FROM AUTHOR]

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10 μ m−2) or ultralow (<1 μ m−2) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN x patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN x atoms prior to the GaN growth. By varying the amount of pre-deposited SiN x , the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN x patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III–V semiconductors nucleus grown on inert surfaces like 2D materials. [ABSTRACT FROM AUTHOR]

To exploit the promising properties of semiconductor nanowires and ensure the uniformity required to achieve device integration, their position on the growth substrate must be controlled. This work demonstrates the direct patterning of a SiO2/Si substrate using focused ion beam (FIB) patterning to control self-catalyzed GaAsSb nanowire growth in molecular beam epitaxy (MBE). Besides position control, FIB patterning parameters influence nanowire yield, composition and structure. Total ion dose per hole is found to be the most important parameter. Yield of single nanowires ranges from ≈34% to ≈83%, with larger holes dominated by multiple nanowires per hole. Areas exposed to low ion beam doses are selectively etched by routine pre-MBE HF cleaning, enabling patterning and nanowire nucleation with minimal damage to the Si substrate. The optical and electronic properties of nanowires are found to depend on the ion dose used during patterning, indicating the potential for FIB patterning to tune nanowire properties. These findings demonstrate the possibility for a FIB lithography protocol which could provide a rapid and direct patterning process for flexible controlled nanowire growth. [ABSTRACT FROM AUTHOR]

In-plane selective area growth (SAG) of III–V nanowires (NWs) has emerged as a scalable materials platform for quantum electronics and photonics applications. Most applications impose strict requirements on the material characteristics which makes optimization of the crystal quality vital. Alignment of in-plane SAG NWs with respect to the substrate symmetry is of importance due to the large substrate-NW interface as well as to obtain nanostructures with well-defined facets. Understanding the role of mis-orientation is thus important for designing devices and interpretation of electrical performance of devices. Here we study the effect of mis-orientation on morphology of selectively grown NWs oriented along the [1 1̅ 1̅] direction on GaAs(2 1 1)B. Atomic force microscopy is performed to extract facet roughness as a measure of structural quality. Further, we evaluate the dependence of material incorporation in NWs on the orientation and present the facet evolution in between two high symmetry in-plane orientations. By investigating the length dependence of NW morphology, we find that the morphology of ≈1 μ m long nominally aligned NWs remains unaffected by the unintentional misalignment associated with the processing and alignment of the sample under study. Finally, we show that using Sb as a surfactant during growth improves root-mean-square facet roughness for large misalignment but does not lower it for nominally aligned NWs. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy growth and optical properties of GaN quantum disks in AlN nanowires were investigated, with the purpose of controlling the emission wavelength of AlN nanowire-based light emitting diodes. Besides GaN quantum disks with a thickness ranging from 1 to 4 monolayers, a special attention was paid to incomplete GaN disks exhibiting lateral confinement. Their emission consists of sharp lines which extend down to 215 nm, in the vicinity of AlN band edge. The room temperature cathodoluminescence intensity of an ensemble of GaN quantum disks embedded in AlN nanowires is about 20% of the low temperature value, emphasizing the potential of ultrathin/incomplete GaN quantum disks for deep UV emission. [ABSTRACT FROM AUTHOR]

The nanoscale intrinsic electrical properties of in-plane InAs nanowires grown by selective area epitaxy are investigated using a process-free method involving a multi-probe scanning tunneling microscope. The resistance of oxide-free InAs nanowires grown on an InP(111)B substrate and the resistance of InAs/GaSb core–shell nanowires grown on an InP(001) substrate are measured using a collinear four-point probe arrangement in ultrahigh vacuum. They are compared with the resistance of two-dimensional electron gas reference samples measured using the same method and with the Van der Pauw geometry for validation. A significant improvement of the conductance is achieved when the InAs nanowires are fully embedded in GaSb, exhibiting an intrinsic sheet conductance close to the one of the quantum well counterpart. [ABSTRACT FROM AUTHOR]

Nanoporous GaN layers were fabricated using selective area sublimation through a self-organized AlN nanomask in a molecular beam epitaxy reactor. The obtained pore morphology, density and size were measured using plan-view and cross-section scanning electron microscopy experiments. It was found that the porosity of the GaN layers could be adjusted from 0.04 to 0.9 by changing the AlN nanomask thickness and sublimation conditions. The room temperature photoluminescence properties as a function of the porosity were analysed. In particular, a strong improvement (>100) of the room temperature photoluminescence intensity was observed for porous GaN layers with a porosity in the 0.4–0.65 range. The characteristics of these porous layers were compared to those obtained with a Si x N y nanomask. Furthermore, the regrowth of p-type GaN on light emitting diode structures made porous by using either an AlN or a Si x N y nanomask were compared. [ABSTRACT FROM AUTHOR]

Nanoporous GaN layers were fabricated using selective area sublimation through a self-organized AlN nanomask in a molecular beam epitaxy reactor. The obtained pore morphology, density and size were measured using plan-view and cross-section scanning electron microscopy experiments. It was found that the porosity of the GaN layers could be adjusted from 0.04 to 0.9 by changing the AlN nanomask thickness and sublimation conditions. The room temperature photoluminescence properties as a function of the porosity were analysed. In particular, a strong improvement (>100) of the room temperature photoluminescence intensity was observed for porous GaN layers with a porosity in the 0.4–0.65 range. The characteristics of these porous layers were compared to those obtained with a Si x N y nanomask. Furthermore, the regrowth of p-type GaN on light emitting diode structures made porous by using either an AlN or a Si x N y nanomask were compared. [ABSTRACT FROM AUTHOR]

Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulk m - and c -plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices. [ABSTRACT FROM AUTHOR]

Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2 thin films requires high-temperature reduction of Nb2O5 films using H2 or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2 films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2 phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation on c -sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2 films without post-reduction steps which will be assumed to be a cost-effective process for NbO2 based devices. [ABSTRACT FROM AUTHOR]

Since the advent of atomically flat graphene, two-dimensional (2D) layered materials have gained extensive interest due to their unique properties. The 2D layered materials prepared on epitaxial graphene/silicon carbide (EG/SiC) surface by molecular beam epitaxy (MBE) have high quality, which can be directly applied without further transfer to other substrates. Scanning tunneling microscopy and spectroscopy (STM/STS) with high spatial resolution and high-energy resolution are often used to study the morphologies and electronic structures of 2D layered materials. In this review, recent progress in the preparation of various 2D layered materials that are either monoelemental or transition metal dichalcogenides on EG/SiC surface by MBE and their STM/STS investigations are introduced. [ABSTRACT FROM AUTHOR]

Nearly localized moirĂ© flat bands in momentum space, arising at particular twist angles, are the key to achieve correlated effects in transition-metal dichalcogenides. Here, we use angle-resolved photoemission spectroscopy (ARPES) to visualize the presence of a flat band near the Fermi level of van der Waals WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. This flat band is localized near the Fermi level and has a width of several hundred meVs. By combining ARPES measurements with density functional theory calculations, we confirm the coexistence of different domains, namely the reference 2H stacking without layer misorientation and regions with arbitrary twist angles. For the 2H-stacked heterobilayer, our ARPES results show strong interlayer hybridization effects, further confirmed by complementary micro- Raman spectroscopy measurements. The spin-splitting of the valence band at K is determined to be 470 meV. The valence band maximum (VBM) position of the heterobilayer is located at the Î" point. The energy difference between the VBM at Î" and the K point is of âˆ'60 meV, which is a stark difference compared to individual single monolayer WSe2 and monolayer WSe2, showing both a VBM at K. [ABSTRACT FROM AUTHOR]

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO x deposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range. [ABSTRACT FROM AUTHOR]

2D boron nitride (2D-BN) was synthesized by gas-source molecular beam epitaxy on polycrystalline and monocrystalline Ni substrates using gaseous borazine and active nitrogen generated by a remote plasma source. The excess of nitrogen atoms allows to overcome the thickness self-limitation active on Ni when using borazine alone. The nucleation density and the shape of the 2D-BN domains are clearly related to the Ni substrate preparation and to the growth parameters. Based on spatially-resolved photoemission spectroscopy and on the detection of the π plasmon peak, we discuss the origin of the N1s and B1s components and their relationship with an electronic coupling at the interface. After optimization of the growth parameters, a full 2D-BN coverage is obtained, although the material thickness is not evenly distributed. The 2D-BN presents a granular structure on (111) oriented Ni grains, showing a rather poor cristallographic quality. On the contrary, high quality 2D-BN is found on (101) and (001) Ni grains, where triangular islands are observed whose lateral size is limited to ∼20 μ m. [ABSTRACT FROM AUTHOR]

Selective area epitaxy (SAE) provides the path for scalable fabrication of semiconductor nanostructures in a device-compatible configuration. In the current paradigm, SAE is understood as localized epitaxy, and is modelled by combining planar and self-assembled nanowire growth mechanisms. Here we use GaAs SAE as a model system to provide a different perspective. First, we provide evidence of the significant impact of the annealing stage in the calculation of the growth rates. Then, by elucidating the effect of geometrical constraints on the growth of the semiconductor crystal, we demonstrate the role of adatom desorption and resorption beyond the direct-impingement and diffusion-limited regime. Our theoretical model explains the effect of these constraints on the growth, and in particular why the SAE growth rate is highly sensitive to the pattern geometry. Finally, the disagreement of the model at the largest pitch points to non-negligible multiple adatom recycling between patterned features. Overall, our findings point out the importance of considering adatom diffusion, adsorption and desorption dynamics in designing the SAE pattern to create pre-determined nanoscale structures across a wafer. These results are fundamental for the SAE process to become viable in the semiconductor industry. [ABSTRACT FROM AUTHOR]

Remote epitaxy is an emerging materials synthesis technique which employs a 2D interface layer, often graphene, to enable the epitaxial deposition of low defect single crystal films while restricting bonding between the growth layer and the underlying substrate. This allows for the subsequent release of the epitaxial film for integration with other systems and reuse of growth substrates. This approach is applicable to material systems with an ionic component to their bonding, making it notably appealing for IIIâ€"V alloys, which are a technologically important family of materials. Chemical vapour deposition growth of graphene and wet transfer to a IIIâ€"V substrate with a polymer handle is a potentially scalable and low cost approach to producing the required growth surface for remote epitaxy of these materials, however, the presence of water promotes the formation of a IIIâ€"V oxide layer, which degrades the quality of subsequently grown epitaxial films. This work demonstrates the use of an argon ion beam for the controlled introduction of defects in a monolayer graphene interface layer to enable the growth of a single crystal GaAs film by molecular beam epitaxy, despite the presence of a native oxide at the substrate/graphene interface. A hybrid mechanism of defect seeded lateral overgrowth with remote epitaxy contributing the coalescence of the film is indicated. The exfoliation of the GaAs films reveals the presence of defect seeded nucleation sites, highlighting the need to balance the benefits of defect seeding on crystal quality against the requirement for subsequent exfoliation of the film, for future large area development of this approach. [ABSTRACT FROM AUTHOR]

In this manuscript, we have shown the growth and extensive structural and optical characteristic of the uniformly Mg-doped Al0.23Ga0.77N (UV-A region, λ ∼ 323 nm) nanowire. The Kelvin probe force microscopy was employed to determine the profile of holes in p-type AlGaN nanowires by measuring the work function changes induced by Mg incorporation. The influence of surface band bending on doping concentration has thoroughly been discussed. Our experiment confirms the homogeneous incorporation of Mg throughout the nanowire without any top surface Mg segregation. In this work, we have also demonstrated a comprehensive analysis of acceptor states induced thermal quenching behaviour in the optical transition of Mg-doped AlGaN nanowire. We propose a phenomenological model, based on the rate equation which confirms that achieving higher ‘hole’ (p-doping) concentration in AlGaN nanowire (>1018 cmâˆ'3) is more conducive than the planar counterpart if the growth of NWs is carried out at optimized process conditions. This rate equation-based model has also demonstrated the influence of sidewall surface passivation in those AlGaN nanowires. [ABSTRACT FROM AUTHOR]

In this study, molecular beam epitaxial growth of strain-driven three-dimensional self-assembled Ge/GeSi islands on silicon-on-insulator (SOI) substrates, along with their optical and photodetection characteristics, have been demonstrated. The as-grown islands exhibit a bimodal size distribution, consisting of both Ge and GeSi alloy islands, and show significant photoluminescence (PL) emission at room temperature, specifically near optical communication wavelengths. Additionally, these samples were used to fabricate a Ge/GeSi islands/Si nanowire based phototransistor using a typical e-beam lithography process. The fabricated device exhibited broadband photoresponse characteristics, spanning a wide wavelength range (300-1600 nm) coupled with superior photodetection characteristics and relatively low dark current (∼ tens of pA). The remarkable photoresponsivity of the fabricated device, with a peak value of ∼11.4 A W -1 ( λ ∼ 900 nm) in the near-infrared region and ∼1.36 A W -1 ( λ ∼ 1500 nm) in the short-wave infrared (SWIR) region, is a direct result of the photoconductive gain exceeding unity. The room-temperature optical emission and outstanding photodetection performance, covering a wide spectral range from the visible to the SWIR region, showcased by the single layer of Ge/GeSi islands on SOI substrate, highlight their potential towards advanced applications in broadband infrared Si-photonics and imaging. These capabilities make them highly promising for cutting-edge applications compatible with complementary metal-oxide-semiconductor technology., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Semiconductor nanowires (NWs) are believed to play a crucial role for future applications in electronics, spintronics and quantum technologies. A potential candidate is HgTe but its sensitivity to nanofabrication processes restrain its development. A way to circumvent this obstacle is the selective area growth technique. Here, in-plane HgTe nanostructures are grown thanks to selective area molecular beam epitaxy on a semi-insulating CdTe substrate covered with a patterned SiO 2 mask. The shape of these nanostructures is defined by the in-plane orientation of the mask aperture along the <110>, <11¯0>, or <100> direction, the deposited thickness, and the growth temperature (GT). Several micron long in-plane NWs can be achieved as well as more complex nanostructures such as networks, diamond structures or rings. A good selectivity is achieved with very little parasitic growth on the mask even for a GT as low as 140 °C and growth rate up to 0.5 monolayer per second. For <110> oriented NWs, the center of the nanostructure exhibits a trapezoidal shape with {111}B facets and two grains on the sides, while <11¯0> oriented NWs show {111}A facets with adatoms accumulation on the sides of the top surface. Transmission electron microscopy observations reveal a continuous epitaxial relation between the CdTe substrate and the HgTe NW. Measurements of the resistance with four-point scanning tunneling microscopy indicates a good electrical homogeneity along the main NW axis and a thermally activated transport. This growth method paves the way toward the fabrication of complex HgTe-based nanostructures for electronic transport measurements., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

The methodology of statistical analysis of cathodoluminescence (CL) intensity mappings on ensembles of several hundreds of InGaN/GaN nanowires (NWs) used to quantify non-radiative recombination centres (NRCs) was validated on InGaN/GaN NWs exhibiting spatially homogeneous cathodoluminescence at the scale of single NWs. Cathodoluminescence intensity variations obeying Poisson's statistics were assigned to the presence of randomly incorporated point defects acting as NRCs. Additionally, another type of NRCs, namely extended defects leading to spatially inhomogeneous cathodoluminescence intensity at the scale of single InGaN/GaN NWs are revealed by high resolution scanning transmission electron microscopy, geometrical phase analysis and two-beam diffraction conditions techniques. Such defects are responsible for deviations from Poisson's statistics, allowing one to achieve a rapid evaluation of the crystallographic and optical properties of several hundreds of NWs in a single cathodoluminescence intensity mapping experiment., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

We have studied electronic transport in undoped GaAs/SrTiO3 coreâ€"shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/ ÎĽ m. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO3 shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3 shells as gate insulators. [ABSTRACT FROM AUTHOR]

This work evaluates the passivation efficacy of thermal atomic layer deposited (ALD) Al2O3 dielectric layer on self-catalyzed GaAs1- x Sb x nanowires (NWs) grown using molecular beam epitaxy. A detailed assessment of surface chemical composition and optical properties of Al2O3 passivated NWs with and without prior sulfur treatment were studied and compared to as-grown samples using x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature photoluminescence (PL) spectroscopy. The XPS measurements reveal that prior sulfur treatment followed by Al2O3 ALD deposition abates IIIâ€"V native oxides from the NW surface. However, the degradation in 4K-PL intensity by an order of magnitude observed for NWs with Al2O3 shell layer compared to the as-grown NWs, irrespective of prior sulfur treatment, suggests the formation of defect states at the NW/dielectric interface contributing to non-radiative recombination centers. This is corroborated by the Raman spectral broadening of LO and TO Raman modes, increased background scattering, and redshift observed for Al2O3 deposited NWs relative to the as-grown. Thus, our work seems to indicate the unsuitability of ALD deposited Al2O3 as a passivation layer for GaAsSb NWs. [ABSTRACT FROM AUTHOR]

The early stage of growth of semiconductor nanowires is studied in the case where the sidewall adatoms have a short diffusion length due to a strong desorption. Experimental results are described for the growth of ZnSe nanowires by molecular beam epitaxy. They are discussed and interpreted using the Burtonâ€"Cabreraâ€"Frank description of the propagation of steps along the sidewalls, and compared to other IIâ€"VI and IIIâ€"V nanowires. The role of the growth parameters and the resulting shape of the nanowires (cylinder, cone, or both combined) are highlighted. [ABSTRACT FROM AUTHOR]

In-plane InGaAs/Ga(As)Sb heterojunction tunnel diodes are fabricated by selective area molecular beam epitaxy with two different architectures: either radial InGaAs core/Ga(As)Sb shell nanowires or axial InGaAs/GaSb heterojunctions. In the former case, we unveil the impact of strain relaxation and alloy composition fluctuations at the nanoscale on the tunneling properties of the diodes, whereas in the latter case we demonstrate that template assisted molecular beam epitaxy can be used to achieve a very precise control of tunnel diodes dimensions at the nanoscale with a scalable process. In both cases, negative differential resistances with large peak current densities are achieved. [ABSTRACT FROM AUTHOR]

In this paper, we analyze superstructural transitions during epitaxial growth of two-dimensional layers and the formation of quantum dots by the Stranskiâ€"Krastanov mechanism in elastically stressed systems by the reflection high-energy electron diffraction method. Detailed dependences of the periodicity parameter N of the 2 × N reconstruction on the effective thickness of the deposited material in a wide range of growth temperatures during epitaxy of germanium on a silicon surface with a crystallographic orientation (001) are obtained. Superstructural transitions and the change in the value of the parameter N at low temperatures of epitaxy in this system have been investigated for the first time. It is shown that the length of dimer rows in such a reconstruction during the growth of pure germanium on silicon can reach a value of no less than N  = 11. A relationship is found between the value of the parameter N, determined by elastic strains in the system, and the critical thickness of the transition from two-dimensional to three-dimensional growth. Based on this relationship, a physical mechanism is proposed that explains the nature of the temperature dependence of the critical thickness of the Stranskiâ€"Krastanov transition, which has been the subject of constant scientific disputes until now. [ABSTRACT FROM AUTHOR]

We report on the growth, structural, and optical properties of GaAs/GaAsPBi coreâ€"shell nanowires (NWs) synthesized by molecular beam epitaxy (MBE). The structure presents advantageous optical properties, in particular, for near- and mid-infrared optical applications. Scanning electron microscopy shows that although the stems of GaAs/GaAsP and GaAs/GaAsBi coreâ€"shell NWs preserve the hexagonal prism shape, the GaAs/GaAsPBi coreâ€"shell NWs develop a quasi-three-fold orientational symmetry affected by the hexagonal prismatic core. Detailed structural analyses of a GaAs/GaAsPBi coreâ€"shell stem show that it crystallized with zincblende structure with a nominal shell composition of GaAs0.617P0.362Bi0.021. Photoluminescence of GaAs/GaAsPBi coreâ€"shell NWs shows the luminescent peak at 1.02 eV with high internal quantum efficiency at room temperature (IQERT ∼ 6%) superior to those of MBE-grown GaAs core NWs and GaAsPBi multiple quantum wells earlier reported. Energy-dispersive x-ray spectroscopy performed on the GaAs/GaAsPBi coreâ€"shell NWs yields an estimated bandgap different from the optically measured value. We attribute this discrepancy to the NW compositional fluctuations that also may explain the high IQERT. [ABSTRACT FROM AUTHOR]

Among ultrathin monoelemental two-dimensional (2D) materials, bismuthene, the single layer of heavier group-VÎ' element bismuth (Bi), has been predicted to have large non trivial gap. Here, we demonstrate the growth of Bi films by molecular beam epitaxy on 2D-HfTe2 template. At the initial stage of Bi deposition (1â€"2 bilayers, BL), both the pseudocubic Bi(110) and the hexagonal Bi(111) phases are formed. When reaching 3 BL Bi, a transformation to pure hexagonal Bi(111) occurs. The electronic band structure of 3 BL Bi(111) films was measured by angle-resolved photoemission spectroscopy showing very good matching with the density functional theory band structure calculations of 3 BL free standing Bi(111). The grown Bi(111) thin film was capped with a protective Al2O3 layer and its stability under ambient conditions, necessary for practical applications and device fabrication, was confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Wurtzite GaP nanowires are interesting for the direct bandgap engineering and can be used as templates for further growth of hexagonal Si shells. Most wurtzite GaP nanowires have previously been obtained with Au catalysts. Here, we show that long (∼500 nm) wurtzite sections are formed in the top parts of self-catalyzed GaP nanowires grown by molecular beam epitaxy on Si(111) substrates in the droplet consumption stage, which is achieved by abruptly increasing the atomic V/III flux ratio from 2 to 3. We investigate the temperature dependence of the length of wurtzite sections and show that the longest sections are obtained at 610 °C. A supporting model explains the observed trends using a phase diagram of GaP nanowires, where the wurtzite phase is formed within a certain range of the droplet contact angles. The optimal growth temperature for growing wurtzite nanowires corresponds to the largest diffusion length of Ga adatoms, which helps to maintain the required contact angle for the longest time. [ABSTRACT FROM AUTHOR]

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor–liquid–solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron–hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure. [ABSTRACT FROM AUTHOR]

Vertically grown nanowires (NWs) are a research interest in optoelectronics and photovoltaic applications due to their high surface to volume ratio and good light trapping capabilities. This study presents the effects of process and design parameters on self-catalyzed GaAsSbN NWs grown by plasma-assisted molecular beam epitaxy on patterned silicon substrates using electron beam lithography. Vertical alignment of the patterned NWs examined via scanning electron microscopy show the sensitivity of patterned NW growth to the parameters of NW diameter, pitch, dose time, etching techniques and growth plan. Diameters range from 90 nm to 250 nm. Pitch lengths of 200 nm, 400 nm, 600 nm, 800 nm, 1000 nm, and 1200 nm were examined. Dry etching of the oxide layer of the silicon substrate and PMMA coating is performed using reactive ion etching (RIE) for 20 s and 120 s respectively. Comparisons of different HF etch durations performed pre and post PMMA removal are presented. Additionally, the report of an observed surfactant effect in dilute nitride GaAsSbN NWs in comparison to non-nitride GaAsSb is presented. Optimizations to patterning, RIE, and HF etching are presented to obtain higher vertical yield of patterned GaAsSbN NWs, achieving ∼80% of the expected NW µ m 2 . Room temperature and 4 K photoluminescence results show the effect of nitride incorporation for further bandgap tuning, and patterned pitch on the optical characteristics of the NWs which gives insights to the compositional homogeneity for NWs grown at each pitch length., (© 2024 IOP Publishing Ltd.)

InGaN nanostructures are among the most promising candidates for visible solid-state lighting and renewable energy sources. To date, there is still a lack of information about the influence of the growth conditions on the physical properties of these nanostructures. Here, we extend the study of InGaN nanowires growth directly on Si substrates by plasma-assisted molecular beam epitaxy. The results of the study showed that under appropriate growth conditions a change in the growth temperature of just 10 °C leads to a significant change in the structural and optical properties of the nanowires. InGaN nanowires with the areas containing 4%–10% of In with increasing tendency towards the top are formed at the growth temperature of 665 °C, while at the growth temperatures range of 655 °C–660 °C the spontaneously core–shell NWs are typically presented. In the latter case, the In contents in the core and the shell are about an order of magnitude different (e.g. 35% and 4% for 655 °C, respectively). The photoluminescence study of the NWs demonstrates a shift in the spectra from blue to orange in accordance with an increase of In content. Based on these results, a novel approach to the monolithic growth of InxGa1−xN NWs with multi-colour light emission on Si substrates by setting a temperature gradient over the substrate surface is proposed. [ABSTRACT FROM AUTHOR]

The nanoflakes of SnS2/SnO2 heterostructure and SnS2 were synthesized by a one-step SnO2-templated chemical vapor deposition method. The metal oxide-assisted growth mechanism of SnS2/SnO2 heterostructure and SnS2 nanoflakes were realized through investigating serial microstructures of products with varied growth time. Furthermore, the photocatalytic activity for MB dyes degradation of varied growth time products was used to explore the effect of product microstructure under the visible light irradiation. The SnO2/SnS2 heterostructure and the oxide vacancies of nanoflakes demonstrated an improved visible light photocatalytic performance for MB degradation, which was around twice of the pure SnS2 nanoflakes and better than P25. The results of different scavengers on the degradation efficiency for MB indicate the·O2−, and ·OH are the main active species in the photodegradation reaction. The one-step growth mechanism of SnS2/SnO2 could prove a facile process to grow metal oxide-metal sulfide heterostructure. [ABSTRACT FROM AUTHOR]

GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor−liquid−solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume. [ABSTRACT FROM AUTHOR]

Point defects may significantly influence the electrical and optoelectronic properties of two-dimensional (2D) tungsten diselenide (WSe2), while precise information about point defects distribution (e.g. species and concentration) in monolayer (ML-) WSe2 are hard to obtain. In this letter, we tried to partly fill this knowledge gap via performing quantitative and statistical analysis of intrinsic point defects in WSe2 monolayers prepared by three so-called main-stream approaches i.e. mechanical exfoliation (ME), chemical vapor deposition (CVD), and molecular beam epitaxy (MBE), which are promising for providing high-quality samples. Via a conjunction of statistic atomic-resolution annular dark-field scanning transmission electron microscopy imaging, software-based automated defect identification and counting, together with image simulations, defect species and concentrations were quantitatively determined. Seven types of intrinsic point defects were identified in ML-WSe2 and the most dominant one is selenium mono-vacancy (VSe) (corresponding to one Se atom missing), irrespective of the synthetic route and growth conditions. Exact contents and diversity of point defects depend on the specific preparation method: CVD grown ML-WSe2 is the most defective (for example, the density of VSe reaches 1.48% in atomic ratio), followed by ME (∼0.85 at% for VSe) and MBE grown samples (∼0.49 at% for VSe). Our results, though still with limited sampling, provide preliminary quantitative information of point defects in ML-WSe2, which can serve as a reference to achieve the precisely controlled large-scale sample growth and establish the structure-property relationship of 2D transition-metal dichalcogenides materials. [ABSTRACT FROM AUTHOR]

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy. [ABSTRACT FROM AUTHOR]

In this review paper, we focus on the contribution of hydride vapor phase epitaxy (HVPE) to the growth of III-As nanowires (NWs). HVPE is the third epitaxial technique involving gaseous precursors together with molecular beam epitaxy (MBE) and metal-organic VPE (MOVPE) to grow III–V semiconductor compounds. Although a pioneer in the growth of III–V epilayers, HVPE arrived on the scene of NW growth the very last. Yet, HVPE brought different and interesting insights to the topic since HVPE is a very reactive growth system, exhibiting fast growth property, while growth is governed by the temperature-dependent kinetics of surface mechanisms. After a brief review of the specific attributes of HVPE growth, we first feature the innovative polytypism-free crystalline quality of cubic GaAs NWs grown by Au-assisted vapor–liquid–solid (VLS) epitaxy, on exceptional length and for radii down to 6 nm. We then move to the integration of III–V NWs with silicon. Special emphasis is placed on the nucleation issue experienced by both Au-assisted VLS MOVPE and HVPE, and a model demonstrates that the presence of Si atoms in the liquid droplets suppresses nucleation of NWs unless a high Ga concentation is reached in the catalyst droplet. The second known issue is the amphoteric behavior of Si when it is used as doping element for GaAs. On the basis of compared MBE and HVPE experimental data, a model puts forward the role of the As concentration in the liquid Au–Ga–As–Si droplets to yield p-type (low As content) or n-type (high As content) GaAs:Si NWs. We finally describe how self-catalysed VLS growth and condensation growth are implemented by HVPE for the growth of GaAs and InAs NWs on Si. [ABSTRACT FROM AUTHOR]

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications. [ABSTRACT FROM AUTHOR]

We report a comprehensive study of the growth dynamics in highly periodic, composition tunable InAsSb nanowire (NW) arrays using catalyst-free selective area molecular beam epitaxy. Employing periodically patterned SiO2-masks on Si (111) with various mask opening sizes (20–150 nm) and pitches (0.25–2 μm), high NW yield of >90% (irrespective of the InAsSb alloy composition) is realized by the creation of an As-terminated 1 × 1-Si(111) surface prior to NW nucleation. While the NW aspect ratio decreases continually with increasing Sb content (xSb from 0% to 30%), we find a remarkable dependence of the aspect ratio on the mask opening size yielding up to ∼8-fold increase for openings decreasing from 150 to 20 nm. The effects of the interwire separation (pitch) on the NW aspect ratio are strongest for pure InAs NWs and gradually vanish for increasing Sb content, suggesting that growth of InAsSb NW arrays is governed by an In surface diffusion limited regime even for the smallest investigated pitches. Compositional analysis using high-resolution x-ray diffraction reveals a substantial impact of the pitch on the alloy composition in homogeneous InAsSb NW arrays, leading to much larger xSb as the pitch increases due to decreasing competition for Sb adatoms. Scanning transmission electron microscopy and associated energy-dispersive x-ray spectroscopy performed on the cross-sections of individual NWs reveal an interesting growth-axis dependent core–shell like structure with a discontinuous few-nm thick Sb-deficient coaxial boundary layer and six Sb-deficient corner bands. Further analysis evidences the presence of a nanoscale facet at the truncation of the (111)B growth front and {1-10} sidewall surfaces that is found responsible for the formation of the characteristic core–shell structure. [ABSTRACT FROM AUTHOR]

We analyse the electrical and optical properties of single GaN nanowire p–n junctions grown by plasma‐assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p–n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p–n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 1018 cm−3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology. [ABSTRACT FROM AUTHOR]

The mechanisms of plasma-assisted molecular beam epitaxial growth of GaN on muscovite mica were investigated. Using a battery of techniques, including scanning and transmission electron microscopy, atomic force microscopy, cathodoluminescence, Raman spectroscopy and x-ray diffraction, it was possible to establish that, in spite of the lattice symmetry mismatch, GaN grows in epitaxial relationship with mica, with the [11–20] GaN direction parallel to [010] direction of mica. GaN layers could be easily detached from the substrate via the delamination of the upper layers of the mica itself, discarding the hypothesis of a van der Waals growth mode. Mixture of wurtzite (hexagonal) and zinc blende (ZB) (cubic) crystallographic phases was found in the GaN layers with ratios highly dependent on the growth conditions. Interestingly, almost pure ZB GaN epitaxial layers could be obtained at high growth temperature, suggesting the existence of a specific GaN nucleation mechanism on mica and opening a new way to the growth of the thermodynamically less stable ZB GaN phase. [ABSTRACT FROM AUTHOR]

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq−1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles. [ABSTRACT FROM AUTHOR]

Topological crystalline insulators (TCIs) are interesting for their topological surface states, which hold great promise for scattering-free transport channels and fault-tolerant quantum computing. A promising TCI is SnTe. However, Sn-vacancies form in SnTe, causing a high hole density, hindering topological transport from the surface being measured. This issue could be relieved by using nanowires with a high surface-to-volume ratio. Furthermore, SnTe can be alloyed with Pb reducing the Sn-vacancies while maintaining its topological phase. Here we present the catalyst-free growth of monocrystalline PbSnTe in molecular beam epitaxy. By the addition of a pre-deposition stage before the growth, we have control over the nucleation phase and thereby increase the nanowire yield. This facilitates tuning the nanowire aspect ratio by a factor of four by varying the growth parameters. These results allow us to grow specific morphologies for future transport experiments to probe the topological surface states in a Pb 1- x Sn x Te-based platform., (Creative Commons Attribution license.)