As4 Molecules Research Articles

Simulation of GaAs nanowire annealing

The GaAs nanowire (NW) morphology transformation during a high temperature annealing of densely packed wires was analyzed by Monte Carlo simulation. For long densely spaced NWs, the arsenic evaporation with its removal from the system occurs only in the upper NW part at such distance that As2 molecules can overcome due to the channeling between adjacent NWs before reaching vacuum. In the lower NW part, two oppositely directed processes proceed: destruction and restoration of the NW side wall due to the readsorbed material. Thus, the lower part of wire evaporation is close to a quasi-equilibrium process, and the upper part is close to free evaporation into vacuum. An excess of gallium primarily occurs at the top part of the crystal, since it is in the conditions close to a free arsenic evaporation. An excess gallium gathering leads to the Ga droplet formation. Droplets in the middle part of the NW side walls are formed later. A droplet-free region is formed on the sidewall of sufficiently long NWs with a length>345 nm and the distance of 28 nm between the wires after the annealing at T = 890 K. That means that the congruent evaporation temperature varies along the NW axis due to the variation of arsenic pressure, resulting in the congruent evaporation from the lower NW region and incongruent evaporation from the upper wire region. Simulation results predict the dendrite (thin nanowire branching off original wires) growth during a prolonged annealing under incongruent conditions.

GaP-=SUB=-x-=/SUB=-As-=SUB=-1-x-=/SUB=- solid solution growth by molecular beam epitaxy: phenomenological description of the x dependence from growth conditions on GaAs(001) substrate

The experimental dependences of the GaPxAs1-x solid solution phosphorus proportion on growth conditions by molecular beam epitaxy from As2 and P2 molecules on GaAs(001) substrate were described using the phenomenological model. The model was built on the well-established ideas about the III-V compounds MBE growth. The ratio of the arsenic and phosphorus atoms incorporation coefficients was considered as a function of the substrate temperature and molecular flux densitys. Empirical expressions were found that describe the behavior of the arsenic and phosphorus incorporation coefficients ratio depending on the indicated growth parameters. This makes it possible to estimate the V group molecule flux values to obtain the required x in a GaPxAs1-x solid solution at the given substrate temperature and the gallium atoms flow density. Keywords: molecular beam epitaxy, AIIIBV, solid solutions, GaPxAs1-x, phenomenological model.

Молекулярно-лучевая эпитаксия твердого раствора GaP-=SUB=-x-=/SUB=-As-=SUB=-1-x-=/SUB=-: феноменологическое описание зависимости x от условий роста на подложке GaAs(001)

The experimental dependences of the GaPx As1−x solid solution phosphorus proportion on growth conditions by molecular beam epitaxy from As2 and P2 molecules on GaAs(001) substrate were described using the phenomenological model. The model was built on the well-established ideas about the III−V compounds MBE growth. The ratio of the arsenic and phosphorus atoms incorporation coefficients was considered as a function of the substrate temperature and molecular flux densitys. Empirical expressions were found that describe the behavior of the arsenic and phosphorus incorporation coefficients ratio depending on the indicated growth parameters. This makes it possible to estimate the V group molecule flux values to obtain the required x in a GaPx As1−x solid solution at the given substrate temperature and the gallium atoms flow density.

One-Dimensional Pnictogen Allotropes inside Single-Wall Carbon Nanotubes.

The discovery of phosphorene, a single layer of black phosphorus, has accelerated the investigation of pnictogen nanomaterials, leading to the recent identification of arsenene and antimonene. These two-dimensional nanomaterials display physical properties superior to those of graphene for some applications. Recently, single-wall carbon nanotubes (SWCNTs) have been filled with P4 molecules from the melt and As4 molecules from the vapor phase. Confined within SWCNTs, polymerization reactions yielded new one-dimensional pnictogen allotropes. Here, we show using high-resolution electron microscopy that such nanostructures can also be observed upon filling SWCNTs from the vapor phase using red phosphorus as the source material. Using larger-diameter SWCNTs, the vapor phase favors the formation of double-stranded phosphorus zigzag ladders observed here for the first time. Overall, however, SWCNTs were generally found to fill more efficiently with liquid phosphorus; substantial decreases in the filling yields were observed for both phosphorus and arsenic filling of narrow SWCNTs using the vapor route. Attempts to extend the pnitogen series using molten antimony gave very low filling yields. However, the antimony zigzag ladder was observed on two occasions, suggesting that this structural motif dominates across the pnictogens. Computational predictions of the encapsulation energies of the various pnictogen nanostructures are consistent with the observed experimental trends, and band gap calculations predict that the single-stranded zigzag chains of all investigated pnictogens are fully metallic. Using SWCNTs with diameters of >1.5 nm revealed a plethora of complex new phosphorus nanostructures, which highlights an exciting new avenue for future work in this area.

Control of complex quantum structures in droplet epitaxy

We report the controllable growth of GaAs quantum complexes in droplet molecular-beam epitaxy, and the optical properties of self-assembled AlxGaAs quantum rings embedded in a superlattice. We found that Ga droplets on a GaAs substrate can retain their geometry up to a maximum temperature of 490 °C during post-growth annealing, with an optimal temperature of 320 °C for creating uniform and symmetric droplets. Through controlling only the crystallisation temperature under As4 in the range of 450 °C to 580 °C, we can reliably control diffusion, adsorption and etching rates to produce various GaAs quantum complexes such as quantum dots, dot pairs and nanoholes. AlxGaAs quantum rings are also realised within these temperatures via the adjustment of As beam equivalent pressure. We found that crystallisation using As2 molecules in the place of As4 creates smaller diameter quantum rings at higher density. The photoluminescence of As2 grown AlxGaAs quantum rings embedded in a superlattice shows a dominant emission from the quantum rings at elevated temperatures. This observation reveals the properties of the quantum ring carrier confinement and their potential application as efficient photon emitters.

The Growth of InAsxSb1 –x Solid Solutions on Misoriented GaAs(001) Substrates by Molecular-Beam Epitaxy

The effect of a substrate misorientation degree from a singular face on the composition and morphology of layers of InAsxSb1 –x solid solutions obtained by molecular-beam epitaxy on a GaAs surface has been investigated. The substrates of GaAs wafers with the orientation (100) misoriented in the direction [110] by 0°, 1°, 2°, and 5° are used. The heterostructures are grown at temperatures of 310°C and 380°C (respectively, the lower and upper boundaries of the temperature range in which structurally perfect InAsxSb1 –x films form). The effect of the molecular form of arsenic (As2 or As4) on the composition of the layers is studied. The composition and structural properties are investigated using high-resolution X-ray diffractometry (HRXRD) and atomic-force microscopy (AFM). It is established that, in the series of misorientation angles 0° → 5°, the arsenic fraction x increases consecutively when using fluxes of both As2 and As4 molecules. With the As2 molecular flux, the fraction x increases only a little (1.05 times) with increasing degree of misorientation, while, when using the As4 flux, the increase in x is 1.75 times. An increase in the growth temperature leads to growth in the arsenic fraction in the solid solution. The surface morphology improves with an increasing degree of misorientation at a low growth temperature and degrades at a high temperature.

The Chemistry of Yellow Arsenic.

The generation and handling of the light-sensitive and metastable yellow arsenic (As4) is extremely challenging. In view of recent breakthroughs in synthesizing As4 storage materials and transfer reagents, the more intensive use of yellow arsenic as a source for further reactions can be expected. Given these aspects, the current stage of knowledge of the direct use of As4 is comprehensively summarized in the present review, which lists the activation of As4 by main group elements as well as transition metal compounds (including the f-block elements). Moreover, it also partly compares the reaction outcomes in relation to the corresponding reactions of P4. The possibility of using alternative sources for generating arsenic moieties and compounds is also discussed. The release of As4 molecules from precursor compounds and the use of transfer reagents for polyarsenic entities open up new synthetic pathways to avoid the direct generation of yellow arsenic solutions and to ensure its smooth usage for subsequent reactions.

Рост твердых растворов InAs-=SUB=-x-=/SUB=-Sb-=SUB=-1-x-=/SUB=- на отклоненных подложках GaAs(001) методом молекулярно-лучевой эпитаксии

The influence of substrate misorientation degree on the composition and morphology of layers during molecular-beam epitaxy of InAsxSb1−x solid solutions on the GaAs surface was studied. The GaAs wafers with orientation (001), which were miscuted in the [110] direction by 0, 1, 2 and 5◦, were used as substrates. The growth of heterostructures was performed for temperatures of 310◦C and 380◦C (lower and upper boundaries for the temperature range of structurally perfect InAsx Sb1−x films formation, respectively). The influence of the arsenic molecular form (As2 or As4) on the composition of layers was studied. Studies of composition and structural properties were carried out using high-resolution X-ray diffractometry (HRXRD) and atomic force microscopy (AFM). It was established that in the series of misorientation from 0 to 5◦ the arsenic fraction x increases consecutively with the use of both flux of As2 and flux of As4 molecules. When the flux of As2 molecules is used, the fraction x increases insignificantly (in 1.05 times) with a rise of misorientation degree, but when using As4 molecules, x increases in 1.75 times. The increase of the growth temperature leads to the rise of the arsenic fraction in the solid solution. The morphology of the surface improves during increasing of misorientation degree at a low growth temperature and degrades at high temperatures.

Correlation between photoluminescence and morphology for single layer self-assembled InGaAs/GaAs quantum dots

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

One-Dimensional Arsenic Allotropes: Polymerization of Yellow Arsenic Inside Single-Wall Carbon Nanotubes.

The pnictogen nanomaterials, including phosphorene and arsenene, display remarkable electronic and chemical properties. Yet, the structural diversity of these main group elements is still poorly explored. Here we fill single-wall carbon nanotubes with elemental arsenic from the vapor phase. Using electron microscopy, we find chains of highly reactive As4 molecules as well as two new one-dimensional allotropes of arsenic: a single-stranded zig-zag chain and a double-stranded zig-zag ladder. These linear structures are important intermediates between the gas-phase clusters of arsenic and the extended sheets of arsenene. Raman spectroscopy indicates weak electronic interaction between the arsenic and the nanotubes which implies that the formation of the new allotropes is driven primarily by the geometry of the confinement. The relative stabilities of the new arsenic structures are estimated computationally. Band-gap calculations predict that the insulating As4 chains become semiconducting, once converted to the zig-zag ladder, and form a fully metallic allotrope of arsenic as the zig-zag chain.

One‐Dimensional Arsenic Allotropes: Polymerization of Yellow Arsenic Inside Single‐Wall Carbon Nanotubes

AbstractThe pnictogen nanomaterials, including phosphorene and arsenene, display remarkable electronic and chemical properties. Yet, the structural diversity of these main group elements is still poorly explored. Here we fill single‐wall carbon nanotubes with elemental arsenic from the vapor phase. Using electron microscopy, we find chains of highly reactive As4 molecules as well as two new one‐dimensional allotropes of arsenic: a single‐stranded zig‐zag chain and a double‐stranded zig‐zag ladder. These linear structures are important intermediates between the gas‐phase clusters of arsenic and the extended sheets of arsenene. Raman spectroscopy indicates weak electronic interaction between the arsenic and the nanotubes which implies that the formation of the new allotropes is driven primarily by the geometry of the confinement. The relative stabilities of the new arsenic structures are estimated computationally. Band‐gap calculations predict that the insulating As4 chains become semiconducting, once converted to the zig‐zag ladder, and form a fully metallic allotrope of arsenic as the zig‐zag chain.

Low-Temperature epitaxial growth of InGaAs films on InP(100) and InP(411)A substrates

The structural and electrical characteristics of In0.53Ga0.47As epitaxial films, grown in the low-temperature mode on InP substrates with (100) and (411)A crystallographic orientations at flow ratios of As4 molecules and In and Ga atoms of γ = 29 and 90, have been comprehensively studied. The use of InP(411)A substrates is shown to increase the probability of forming two-dimensional defects (twins, stacking faults, dislocations, and grain boundaries), thus reducing the mobility of free electrons, and AsGa point defects, which act as donors and increase the free-electron concentration. An increase in γ from 29 to 90 leads to transformation of single-crystal InGaAs films grown on (100) and (411)A substrates into polycrystalline ones.

Chemical beam epitaxy of GaAs1-xNx using MMHy and DMHy precursors, modeled by ab initio study of GaAs(100) surfaces stability over As2, H2 and N2

Using ab initio calculations, a simple model for GaAs1-xNx vapor-phase epitaxy on (100) surface of GaAs was created. By studying As2 and H2 molecules adsorptions and As/N atom substitutions on (100) GaAs surfaces, we obtain a relative stability diagram of all stable surfaces under varying As2, H2, and N2 conditions. We previously proved that this model could describe the vapor-phase epitaxy of GaAs1-x Nx with simple, fully decomposed, precursors. In this paper, we show that in more complex reaction conditions using monomethylhydrazine (MMHy), and dimethylhydrazine (DMHy), it is still possible to use our model to obtain an accurate description of the temperature and pressure stability domains for each surfaces, linked to chemical beam epitaxy (CBE) growth conditions. Moreover, the different N-incorporation regimes observed experimentally at different temperature can be explain and predict by our model. The use of MMHy and DMHy precursors can also be rationalized. Our model should then help to better understand the conditions needed to obtain an high quality GaAs1-xNx using vapor-phase epitaxy.

InAsSb on GaAs (001): influence of the arsenic molecules form on composition and crystalline properties of MBE layers

The influence of As molecular form on the composition and crystalline properties of InAsxSb1-x solid solutions with MBE has been experimentally investigated. A series of samples has been grown at different growth temperatures. The grown samples were studied with the HRXRD and TEM methods. The incorporation coefficient of As4 and As2 molecules were determined at different growth temperatures. It has been found that the incorporation coefficient of As4 much more dependent on growth temperature compared to As2. It has been found that at a low growth temperature a step-like increase of Sb fraction in an InAsxSb1-x film leads to a decrease of threading dislocations density in a layer with a smaller x.

Ab initio study of GaAs(100) surface stability over As2, H2 and N2 as a model for vapor-phase epitaxy of [formula omitted

GaAs(100) c(4×4) surfaces were examined by ab initio calculations, under As2, H2 and N2 gas mixed conditions as a model for GaAs1−xNx vapor-phase epitaxy (VPE) on GaAs(100). Using a simple model consisting of As2 and H2 molecules adsorptions and As/N atom substitutions, it was shown to be possible to examine the crystal growth behavior considering the relative stability of the resulting surfaces against the chemical potential of As2, H2 and N2 gases. Such simple model allows us to draw a picture of the temperature and pressure stability domains for each surfaces that can be linked to specific growth conditions, directly. We found that, using this simple model, it is possible to explain the different N-incorporation regimes observed experimentally at different temperatures, and to predict the transition temperature between these regimes. Additionally, a rational explanation of N-incorporation ratio for each of these regimes is provided. Our model should then lead to a better comprehension and control of the experimental conditions needed to realize a high quality VPE of GaAs1−xNx.

First-principles studies on molecular beam epitaxy growth ofGaAs1−xBix

We investigate the molecular beam epitaxy (MBE) growth of GaAs1-xBix film using density functional theory with spin-orbit coupling to understand the growth of this film, especially the mechanisms of Bi incorporation. We study the stable adsorption structures and kinetics of the incident molecules (As₂ molecule, Ga atom, Bi atom, and Bi₂ molecule) on the (2 x 1)-Gasub||Bi surface and a proposed q(1 x 1)-Gasub||AsAs surface has a quasi-(1 x 1) As layer above the Ga-terminated GaAs substrate and a randomly oriented As dimer layer on top. We obtain the desorption and diffusion barriers of the adsorbed molecules and also the reaction barriers of three key processes related to Bi evolution, namely, Bi incorporation, As/Bi exchange, and Bi clustering. The results help explain the experimentally observed dependence of Bi incorporation on the As/Ga ratio and growth temperature. Furthermore, we find that As₂ exchange with Bi of the (2 x 1)-Gasub||Bi surface is a key step controlling the kinetics of the Bi incorporation. Finally, we explore two possible methods to enhance the Bi incorporation, namely, replacing the MBE growth mode from codeposition of all fluxes with a sequential deposition of fluxes and applying asymmetric in-plane strain to the substrate.

Effect of arsenic cracking on In incorporation into MBE‐grown InGaAs layer

AbstractThe effect of arsenic cracking on In incorporation in MBE‐grown InGaAs selective growth was systematically studied with changing the cracking cell temperature. Incorporation of In was greatly enhanced by the use of As2 molecular beam during the selective growth at 600 °C. On the other hand, almost no In was incorporated into the grown layer by the use of a conventional As4 molecular beam. The relation between InAs mole fraction and the cracking cell temperature was found to monotonously increase with the cracking cell temperature. It is probably because the ratio of As2 molecules increases with the cracking cell temperature. Not only at 600 °C but also at 580 °C, a selective growth was also successfully performed using As2 molecular beam by the formation of the “denuded zone”, with the InAs mole fraction of 10%. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modified energetics and growth kinetics on H-terminated GaAs (110)

Atomic hydrogen modification of the surface energy of GaAs (110) epilayers, grown at high temperatures from molecular beams of Ga and As4, has been investigated by friction force microscopy (FFM). The reduction of the friction force observed with longer exposures to the H beam has been correlated with the lowering of the surface energy originated by the progressive de-relaxation of the GaAs (110) surface occurring upon H chemisorption. Our results indicate that the H-terminated GaAs (110) epilayers are more stable than the As-stabilized ones, with the minimum surface energy value of 31 meV∕Å(2) measured for the fully hydrogenated surface. A significant reduction of the Ga diffusion length on the H-terminated surface irrespective of H coverage has been calculated from the FFM data, consistent with the layer-by-layer growth mode and the greater As incorporation coefficient determined from real-time reflection high-energy electron diffraction studies. Arsenic incorporation through direct dissociative chemisorption of single As4 molecules mediated by H on the GaAs (110) surface has been proposed as the most likely explanation for the changes in surface kinetics observed.

Multireference configuration interaction study on spectroscopic properties of low-lying electronic states of As2 molecule

The potential energy curves (PECs) of four electronic states (X1Σg+, e3Δu, a3Σu−, and d3Πg) of an As2 molecule are investigated employing the complete active space self-consistent field (CASSCF) method followed by the valence internally contracted multireference configuration interaction (MRCI) approach in conjunction with the correlation-consistent aug-cc-pV5Z basis set. The effect on PECs by the relativistic correction is taken into account. The way to consider the relativistic correction is to employ the second-order Douglas-Kroll Hamiltonian approximation. The correction is made at the level of a cc-pV5Z basis set. The PECs of the electronic states involved are extrapolated to the complete basis set limit. With the PECs, the spectroscopic parameters (Te, Re, ωe, ωexe, ωeye, αe, βe, γe, and Be) of these electronic states are determined and compared in detail with those reported in the literature. Excellent agreement is found between the present results and the experimental data. The first 40 vibrational states are studied for each electronic state when the rotational quantum number J equals zero. In addition, the vibrational levels, inertial rotation and centrifugal distortion constants of d3Πg electronic state are reported which are in excellent agreement with the available measurements. Comparison with the experimental data shows that the present results are both reliable and accurate.

Ab initio-based approach to novel behavior of InAs wetting layer surface grown on GaAs(001)

The novel behavior of the InAs(001) wetting layer(WL) surfaces grown on GaAs(001)−c(4×4)α are systematically investigated by our ab initio-based approach and Monte Carlo (MC) simulations under the growth conditions. The ab initio-based approach incorporates the chemical potentials of In atom and As4 molecule in the gas phase as functions of temperature T and beam equivalent pressure p. The calculated adsorption–desorption boundary curve of In elucidates that In adsorption does not occur on the (2×4)α2, (1×3), and (2×3) WLs but is allowed on the fully relaxed (FR) surface at equilibrium. These results suggest that In adsorption proceeds InAs growth on the initial GaAs(001) and the final InAs(001) FR surfaces, while the novel growth process should be considered on the InAs(001) WL surfaces. The MC simulation results suggest that In adatoms do not reside in the lattice sites on the (1×3)/(2×3) to quickly desorb from the surfaces under the growth conditions, whereas In adatoms can adsorb and migrate during a certain period on the (2×4)α2. On the basis of these results, the novel behavior of InAs(001) WL surfaces is found in In adsorption with As desorption from the (2×3) WL surfaces during InAs growth on GaAs(001).