Ga Flux Research Articles

Structural order and disorder within novel antiferromagnetic RCrxGa3-yGey and R4Cr1-xGa12-yGey intermetallic compounds (R = Tb, Dy)

Single crystals of intermetallic phases RCrxGa3−yGey (R = Tb, Dy; x = 0.19–0.23, y = 0–0.4) and R4Cr1−xGa12−yGey (R = Tb, Dy; x = 0–0.13, y = 3–4) were grown from Ga flux ensuring the absence of magnetic impurities. The X-ray diffraction experiments revealed that the compounds belong to the family of filled AuCu3-type gallides. The compounds are formed upon the insertion of Cr atoms into E6 octahedral voids (E = Ga/Ge), which can occur in either disordered (RCrxGa3-yGey) or ordered (R4Cr1-xGa12-yGey) fashion. The unique feature of the obtained compounds is the strong correlation between the level of Ge doping and the Cr concentration in the phases, which allows one to control both the structural and magnetic ordering. At low Ge concentration, the cubic RCrxGa3-yGey phases form, which then experience tetragonal distortion upon the increase in the Ge content, and the cubic R4Cr1-xGa12-yGey 2×2×2 superstructures are formed at even higher Ge concentrations. The X-ray diffraction analysis also showed a presence of structural defects in the phases, which are caused by different size of empty E6 and filled CrE6 octahedra. Magnetic measurements of the single crystals of the cubic phases reveal antiferromagnetic ordering of the rare earth sublattice with the Neel temperatures decreasing upon Ge doping from 22 K for RCrxGa3 to 10 K for R4Cr1−xGa12−yGey. The Ge doping causes gradual decrease in the strength of antiferromagnetic R-R interactions ultimately leading to noncollinear magnetic structure in the superstructure R4Cr1−xGa12−yGey phases.

Local droplet etching of a vicinal InGaAs(111)A metamorphic layer

We demonstrated nanopit formation by Ga-assisted local droplet etching technique in InGaAs metamorphic layers grown on vicinal GaAs(111)A substrates. We studied nanopit formation depending on the substrate temperature, Ga flux and Ga amount. The etched pits show a highly symmetrical pyramidal shape with an equilateral triangular base and the edges of the triangle are along <11¯0> directions. The observed behavior, in terms of nanopit density, depth and, aspect ratio is well described by a model taking into account the dynamics of the droplet etching process.

Epitaxial Growth of GaN Films on Chemical-Vapor-Deposited 2D MoS2 Layers by Plasma-Assisted Molecular Beam Epitaxy.

The van der Waals epitaxy of wafer-scale GaN on 2D MoS2 and the integration of GaN/MoS2 heterostructures were investigated in this report. GaN films have been successfully grown on 2D MoS2 layers using three different Ga fluxes via a plasma-assisted molecular beam epitaxy (PA-MBE) system. The substrate for the growth was a few-layer 2D MoS2 deposited on sapphire using chemical vapor deposition (CVD). Three different Ga fluxes were provided by the gallium source of the K-cell at temperatures of 825, 875, and 925 °C, respectively. After the growth, RHEED, HR-XRD, and TEM were conducted to study the crystal structure of GaN films. The surface morphology was obtained using FE-SEM and AFM. Chemical composition was confirmed by XPS and EDS. Raman and PL spectra were carried out to investigate the optical properties of GaN films. According to the characterizations of GaN films, the van der Waals epitaxial growth mechanism of GaN films changed from 3D to 2D with the increase in Ga flux, provided by higher temperatures of the K-cell. GaN films grown at 750 °C for 3 h with a K-cell temperature of 925 °C demonstrated the greatest crystal quality, chemical composition, and optical properties. The heterostructure of 3D GaN on 2D MoS2 was integrated successfully using the low-temperature PA-MBE technique, which could be applied to novel electronics and optoelectronics.

Composition and optical properties of (In, Ga)As nanowires grown by group-III-assisted molecular beam epitaxy.

(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.

Dislocation density as a factor compensating the polarization doping effect in graded p-AlGaN contact layers

One of the approaches to improve p-doping properties in (Al)GaN-based materials is using a method called polarization doping. In this study, the graded p-AlGaN contact layers were deposited using plasma-assisted molecular beam epitaxy with different III/N ratios. To study the influence of Ga flux on the structural and electrical properties of the grown graded p-AlGaN structures, the samples were investigated using reflection high-energy electron diffraction, atomic force microscopy, secondary ion mass spectrometry, X-ray diffraction, and Hall effect measurements. The electronic structure of the samples was investigated by X-ray absorption near edge spectroscopy, while the polarity of the layers was analyzed by means of transmission electron microscopy. Our study reveals that growing with a higher Ga flux increases Mg incorporation and changes its distribution in the graded layer. However, it was shown that the main factor drastically affecting the magnitude and the type of conductivity in the graded p-AlGaN structures is the dislocation density. The highest hole concentration of ∼3.0×1018 cm−3 was observed for sample grown with a low Ga flux.

Impact of Ge Doping on Structural and Magnetic Ordering in RMnxGa3 and R4Mn1-xGa12-yGey (R = Tb, Dy; x ≤ 0.25, y ≈ 1.0-3.3).

Single crystals of RMnxGa3 and their new quaternary derivatives R4Mn1-xGa12-yGey (R = Tb, Dy, x ≤ 0.25, y ≈ 1.0-3.3) were grown from a Ga flux. The compounds are derivatives of cubic RGa3 phases, with Mn atoms filling the Ga6 voids. RMnxGa3 formally adopts a cubic ABO3 perovskite structure, in which the presence of Mn atoms results in a shift of the neighboring Ga atoms from their ideal position. A partial substitution of Ga by Ge leads to a higher Mn content, resulting in structural ordering of the latter and the formation of the superstructure phases R4Mn1-xGa12-yGey, which can be formally described in the Y4PdGa12 structure type. The presence of Mn vacancies, which was observed for R = Tb, and Ga/Ge mixing lead to a noticeable deviation from the idealized structure. The compounds contain two magnetic sublattices: the R sublattice, which orders antiferromagnetically near 20 K, and the Mn sublattice, which orders ferromagnetically at TC = 125-225 K with the Ge doping resulting in higher TC. The two sublattices are not independent, as the Mn sublattice induces partial ferromagnetic ordering of the rare earth atoms below TC, at least for the Ge-doped phases. Near TN, both magnetic susceptibility and heat capacity reveal complex behavior, indicating changes in magnetic structures below TN.

β -Ga2O3 trench Schottky diodes by low-damage Ga-atomic beam etching

β -Ga2O3 trench Schottky barrier diodes fabricated through a Gallium atomic beam etching technique with excellent field strength and power device figure of merit are demonstrated. Trench formation was accomplished by a low-damage Ga flux etch that enables near-ideal forward operating characteristics that are independent of fin orientation. The reverse breakdown field strength of greater than 5.10 MV/cm is demonstrated at a breakdown voltage of 1.45 kV. This result demonstrates the potential for Ga atomic beam etching and high-quality dielectric layers for improved performance in β-Ga2O3 vertical power devices.

Demonstration of gallium oxide nano-pillar field emitter arrays

We demonstrate field emission characteristics of β-Ga2O3 nano-pillar arrays fabricated using a damage-free etching technique. The technique utilizes Ga flux in an ultra-high vacuum environment (molecular beam epitaxy) to form high aspect ratio Ga2O3 nano-pillars with atomic-scale etching precision. Electrically conductive Ga2O3 nano-pillars with uniform widths of ∼200–300 nm were realized without the use of e-beam lithography. Furthermore, field emission characteristics on the nano-pillars displayed an emission current of 19 nA per tip at an electric field of 385 kV/cm. The field emission characteristics were modeled using the Murphy–Good model, and the measurements were validated with a field emission orthodoxy test.

Selective Area Epitaxy of GaN Nanostructures: MBE Growth and Morphological Analysis

This work presents the selective area epitaxy of GaN nanostructures grown on Ga-polar GaN/sapphire substrates by plasma-assisted molecular beam epitaxy. We demonstrate three types of nanostructures, including nanowires, nanofins, and nanorings on GaN-on-sapphire templates as well as investigate the ways of controlling their morphology, and orientation of sidewall plus top facets. A range of growth conditions including low to high Ga flux were employed during selective area epitaxy to develop these nanostructures with homogenous geometry and near vertical and smooth sidewalls. Using appropriate growth conditions and mask patterning orientations, nonpolar nanofin grids containing both/either m-/a-planes with as low as 260 nm fin width and up to six interconnects are demonstrated with smooth sidewalls. Based on these experimental results, we developed a growth model that takes different sidewall facets and orientations into account. The model generalizes the experimental results well and explains the growth conditions for the nanostructures. This study serves to advance the understanding of selective area epitaxy for defining complex III-nitride nanostructures that constitute an active area of research and development in the fields of nanotechnology and nanoscience.

Epitaxial growth of atomically thin Ga2Se2 films on c-plane sapphire substrates

Broadening the variety of two-dimensional (2D) materials and improving the synthesis of ultrathin films are crucial to the development of semiconductor industry. As a state-of-the-art 2D material, Ga2Se2 has attractive optoelectronic properties when it reaches the atomically thin regime. However, its van der Waals epitaxial growth, especially for atomically thin films, has seldom been studied. In this paper, we used molecular beam epitaxy to synthesize Ga2Se2 single-crystal films with a surface roughness down to 1.82 nm on c-plane sapphire substrates by optimizing the substrate temperature, Se:Ga flux ratio, and growth rate. Then, we used a three-step mode to grow Ga2Se2 films with a thickness as low as three tetralayers and a surface roughness as low as 0.61 nm, far exceeding the performance of direct growth. Finally, we found that surface morphology strongly depends on the Se:Ga flux ratio, and higher growth rates widened the suitable flux ratio window for growing Ga2Se2. Overall, this work advances the understanding of the vdW epitaxy growth mechanism for post-transition metal monochalcogenides on sapphire substrates.

Investigation of growth dynamics of β-Ga2O3 LPCVD by independently controlling Ga precursor and substrate temperature

We present results on low-pressure chemical vapor deposition (LPCVD) of β-Ga2O3 on a c-sapphire substrate with independent control of Ga precursor (T P) and substrate (T SUB) temperatures, allowing independent tuning of the Ga flux and thermal energy of the adatoms on the substrate surface. Experiments with constant T P = 900 °C with varying T SUB (600 °C–1050 °C) and varying T P (800 °C–1000 °C) with constant T SUB = 900 °C are reported. Island/nanorod formation on top of β-Ga2O3 thin film was observed at T SUB = 600–750 °C, suggesting the Stranski–Krastanov mode of growth, while thin film growth was observed for T SUB = 825–1050 °C. The growth rate decreased at higher T SUB, whereas it increased sharply for T P = 800–850 °C followed by a quasi-saturation for T P = 800–1000 °C. The growth rate evolution in both experiments reveals the significant role of gallium suboxide formation and desorption at the precursor/film during β-Ga2O3 LPCVD. This study provides useful insights into the growth dynamics involved in LPCVD of β-Ga2O3.

Tremendous Acceleration of Plant Growth by Applying a New Sunlight Converter Sr4 Al14- x Gax O25 :Mn4+ Breaking Parity Forbidden Transition.

Majority of Mn4+ activated oxide phosphors have the wavelength of excitation and emission suitable for acceleration of plant growth as light converter from sunlight to deep red. Here, it is observed that 60% increase of red emission of Sr4 Al14 O25 :0.01Mn4+ is found by substituting 0.1Ga3+ . It is clarified that the increase is originated from a unique mechanism of breaking parity forbidden transition under the substitution of cation in d-d transition by using the tool of special aberration corrected transmission electron microscope(AC-STEM), pre-edge peak (1s→3d) Mn K-edge X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), Rietveld analysis of X-ray diffraction (XRD) patterns, and reflection spectra. Further, a combination of substituted Ga, Mg, and special double flux H3 BO3 /AlF3 is found to tremendously increase the emission intensity (355% up). Actual growth of chlorella and rose is examined by a combination of the cheap Sr4 Al14 O25 :0.01Mn4+ ,0.007Mg2+ ,0.1Ga3+ and a unique reflection typed phosphor-film system as sunlight converting system. Optical density of chlorella and height of rose grass is increased by 36±14% and 174±80% compared with nonphosphor-film, respectively.

Multiscale Kinetic Monte Carlo Simulation of Self-Organized Growth of GaN/AlN Quantum Dots.

A three-dimensional kinetic Monte Carlo methodology is developed to study the strained epitaxial growth of wurtzite GaN/AlN quantum dots. It describes the kinetics of effective GaN adatoms on an hexagonal lattice. The elastic strain energy is evaluated by a purposely devised procedure: first, we take advantage of the fact that the deformation in a lattice-mismatched heterostructure is equivalent to that obtained by assuming that one of the regions of the system is subjected to a properly chosen uniform stress (Eshelby inclusion concept), and then the strain is obtained by applying the Green’s function method. The standard Monte Carlo method has been modified to implement a multiscale algorithm that allows the isolated adatoms to perform long diffusion jumps. With these state-of-the art modifications, it is possible to perform efficiently simulations over large areas and long elapsed times. We have taylored the model to the conditions of molecular beam epitaxy under N-rich conditions. The corresponding simulations reproduce the different stages of the Stranski–Krastanov transition, showing quantitative agreement with the experimental findings concerning the critical deposition, and island size and density. The influence of growth parameters, such as the relative fluxes of Ga and N and the substrate temperature, is also studied and found to be consistent with the experimental observations. In addition, the growth of stacked layers of quantum dots is also simulated and the conditions for their vertical alignment and homogenization are illustrated. In summary, the developed methodology allows one to reproduce the main features of the self-organized quantum dot growth and to understand the microscopic mechanisms at play.

Crystal structure and superconductivity of NaAlSi1–x Ge x single crystals

NaAlSi and NaAlGe are nodal-line semimetals with an anti-PbFCl type structure. The former is a superconductor with a transition temperature Tc of 7 K, while the latter does not exhibit superconductivity. We prepared single crystals of solid solution NaAlSi1–x Ge x by the Na–Ga flux method and investigated their crystal structures and superconductivity by X-ray structural analysis and magnetization measurements, respectively. The crystal structure parameters vary continuously with x, while the Tc initially decreases gradually from 6.9 K for x = 0 to 5.6 K for x = 0.33 and then decreases rapidly to below 1.8 K for x = 0.45. This sudden variation in Tc at x ~ 0.4 may not be explained by the decrease in phonon frequency due to the Ge substitution, but suggests an unusual change in the electronic structure, in spite that the calculated band structures of the end members are almost identical with nearly equal density of states.

Indium as a surfactant: Effects on growth morphology and background impurity in GaN films grown by ammonia-assisted molecular beam epitaxy

We report on the improvement of the surface morphology of c-plane GaN films grown at high growth rates (∼1 µm/h) using ammonia molecular beam epitaxy through a series of growth optimizations as well as the introduction of indium as a surfactant. The indium surfactant was expected to help with the adatom mobility and, thus, provide smoother growth surfaces. Through a combination of varying V/III ratios, In flux, and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved. At higher Ga fluxes for fast growth rates (∼1 µm/h and beam equivalent pressures of ∼5 × 10−7 Torr), higher ammonia flows were necessary to preserve the surface morphology. In addition, indium was an effective surfactant—reducing the roughness and improving the overall surface morphology. However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant also resulted in a reduction of background Si impurity concentrations in the film. These effects allow for the growth of thick drift layers with low background dopant concentrations for vertical GaN power devices.

Selective Area Epitaxy of GaN Nanowires on Si Substrates Using Microsphere Lithography: Experiment and Theory.

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiOx/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N2 flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N2 flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.

Detailed surface studies on the reduction of Al incorporation into AlGaN grown by molecular beam epitaxy in the Ga-droplet regime

A better understanding of the molecular beam epitaxy (MBE) grown AlGaN layers is crucial for the development of UV emitters based on III-nitrides. In this work, the growth kinetics of AlGaN structures deposited by plasma-assisted MBE under Ga-rich conditions on AlGaN/AlN/sapphire templates is studied in detail. The surface quality of the layers and the Al concentration was investigated in situ by reflection high-energy electron diffraction, atomic force microscopy and X-ray photoelectron spectroscopy. The Al content in the grown AlGaN layers was also determined by high-resolution X-ray diffraction, while optical properties were examined by photoluminescence and cathodoluminescence. Obtained results reveal that an increase of the Ga flux influences the Al incorporation probability only when stepping into the Ga-droplet regime. This leads to a decrease of the Al concentration in AlGaN layers, but to a different extent in the bulk and at the surface.

Hyperbolic-tan graded composition InxGa1-xAs layers for THz radiation emitters

The growth and characterization of InxGa1-xAs layers with hyperbolic tangent In concentration profiles is presented. Along the growth the molecular beam fluxes of Ga and In, and the substrate temperature were varied. Since the concentration of In is varied during growth process the strain is expected to increase monotonically, and eventually be relaxed via dislocation formation. Positively, reordering of the ternary alloy and the improvement of its crystal quality were propitiated by annealing the samples. The gradual In concentration propitiated along the thickness of the film conduced to variations of the near-surface band bending. The THz emission from GaAs, InAs, and graded samples was investigated by femtosecond laser pumping excitation. The band-bending from hyperbolic-tan graded composition InxGa1-xAs layers causes depth dependence of effective mass and built-in electric fields modifying accordingly the THz emission.

Effect of mask-film properties on the initial GaAs nanowire growth stages

The initial self-catalyze GaAs nanowire (NW) growth stages were studied using Monte Carlo simulation. We analyzed the effect of the mask-film etching rate with liquid gallium for different thicknesses on the initial nanowire formation stages. At a high etching rate NWs do not form on thick mask-films. A high etching rate of a thin film-mask can lead to lateral Ga drop motion over the crystalline substrate surface, which delays the nanowire formation onset. It is shown that, for the NW formation, it is necessary to maintain the correct ratio between the film thickness, etching rate, Ga and As2 flux intensities.

Structural Properties of Superlattices {LTG-GaAs/GaAs : Si} on GaAs (100) and (111)A Substrates

The proposed new structure for photoconductive antennas is a multilayer epitaxial film grown on a GaAs (111) A substrate and consisting of alternating layers of undoped GaAs grown at low temperature (LTG-GaAs) and GaAs layers formed in the standard high-temperature regime and doped with silicon (GaAs : Si). The ratio of As4 and Ga fluxes (γ) was chosen such that the GaAs : Si layers hadp-type conductivity. LTG-GaAs layers were grown at an increased γ value. A similar structure was grown on a (100) substrate as a reference sample. Phase composition was investigated using high-resolution X-ray diffractometry, surface relief — by mean of atomic force microscopy and distribution of Si impurity over thickness — using secondary ion mass spectrometry.