MBE Method Research Articles

GaN micro- and nanostructures selectively grown on profiled sapphire substrates using PA-MBE without lithography

Purpose: Development of technology for the formation of ordered arrays of nanocolumns (NCs) of GaN microcrystals using plasma-activated molecular beam epitaxy from nitrogen (PA-MBE) on profiled sapphire substrates (SPS) of large diameter with a micro-cone profile. The proposed method eliminates the use of low-performance and expensive nanolithography methods. The article is aimed at a deeper understanding of the processes that determine the growth kinetics of III-N nanocolumns using PA MBE on patterned sapphire substrates with multiple orientations of various non-polar and polar planes. A new technological process for the fabrication of GaN NCs using PA-MPE is proposed, which ensures selectivity of their growth at the tops of PPS micro-cones and suppresses growth on the semipolar planes of these substrates. GaN NCs and microcrystals were grown using PA-MBE on commercially available PPS. A technology has been developed for the formation of discharged arrays of GaN nanocolumns without the use of lithographic procedures. Modes have been established that allow the formation of microcrystals and NCs with different diameters: from 30 nm to several microns. A diagram of the growth of GaN by the PA MBE method on PPS has been constructed, demonstrating the boundaries of the technological regimes for the formation of GaN NCs and microcrystals with different surface topography

Low magnetic damping constant in half-metallic Co2FeAl Heusler alloy thin films grown by molecular beam epitaxy

Half-metallic Co2FeAl Heusler alloy thin films with various thickness were prepared by MBE method and its magnetic and crystalline properties were investigated. At the thickness of 20 nm and above, good magnetic properties and high B2-order parameter was obtained. In addition, magnetic damping constant was evaluated from peak-to-peak linewidth measured by FMR and ultra-low magnetic damping constant below 0.001 was obtained at the film thickness of 30 nm. These results are thanks to the excellent half-metallicity in the Co2FeAl films prepared by MBE.

Genetically predicted childhood obesity and adult atrial fibrillation: A mendelian randomization study

Background and aimsIt is unclear whether the association of childhood obesity with adult atrial fibrillation observed in observational studies reflects causal effects. The aim of this study was to evaluate the association of childhood obesity with adult atrial fibrillation using genetic instruments. Methods and resultsWe used a two-sample Mendelian randomization (MR) design to evaluate the association between childhood obesity and adult atrial fibrillation. Two sets of genetic variants (15 single nucleotide polymorphisms [SNPs] for childhood body mass index [BMI] and 12 SNPs for dichotomous childhood obesity) were selected as instruments. Summary data on SNP-childhood obesity and SNP-atrial fibrillation associations were obtained from recently published genome-wide association studies. Effect estimates were evaluated using inverse-variance weighted (IVW) methods. Other MR analyses, including MR-Egger, simple and weighted median, weighted MBE and MR-PRESSO methods were performed in sensitivity analyses.The IVW models showed that both a genetically predicted one-standard deviation increase in childhood BMI (kg/m2) and higher log-odds of childhood obesity were associated with a substantial increase in the risk of atrial fibrillation (OR = 1.22, 95% CI: 1.11–1.34, P < 0.001; OR = 1.09, 95% CI: 1.04–1.14, P < 0.001). MR-Egger regression showed no evidence of genetic pleiotropy for childhood BMI (intercept = 0.000, 95% CI: −0.024 to 0.023), but for childhood obesity (intercept = −0.036, 95% CI: −0.057 to −0.015). Similar results were observed using leave-one-out and other MR methods in sensitivity analyses. ConclusionsThis MR analysis found a consistent association between genetically predicted childhood obesity and an increased risk of adult atrial fibrillation. Further research is warranted to validate our findings.

Accessing the applicability of the MBE approach for constructing potential energy surfaces of nitrogen clusters

The many-body expansion approach (MBE) is employed to build potential energy surfaces (PESs) for nitrogen clusters, which serve as a possible candidate of high energy density materials. The ground-state global PES of N 4 developed by Truhlar’s group is utilized to construct the two-body, three-body, and four-body interaction potentials. The energy of nitrogen clusters is thus obtained by summing the many-body interaction energies up to four-body term. This work found that such a many-body expansion could give reasonable energy predictions for loose chain nitrogen molecules while produce bad predictions for compact structures. The failure is caused by the neglect of high-order ( n ≥ 5) n -body interactions that are significant in the bonding area. The difficulties in applying the MBE method to construct the potential energy surface of nitrogen clusters are analyzed.

Selective Growth of Semiconducting Silicide Phase Based on the Growth Parameters

Os silicide thin films were grown on (001) Si substrate using MBE method. The structural characterizations of the grown films were investigated using x-ray diffraction (XRD) measurements, Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). In addition, the surface morphology was depicted by AFM micrographs, and the nature of the grown epilayers was recognized using Transmission electronic microscope (TEM). All the samples had rough surface with various size of hillocks and intermediate valley like regions with a remarkable decrease in surface roughness after annealing. The measurements revealed a significant difference in the formed phases and crystalline quality with the growth parameters. The influence of Si buffer layer with thickness of 20 nm which deposited at relatively low temperature of 650 °C, and high temperature annealing (at 1050 °C) of the grown Os silicide epilayer was investigated. It was confirmed that isolating a metastable phase OsSi2 was established by selecting the required growth parameters.

Energy-Screened Many-Body Expansion: A Practical Yet Accurate Fragmentation Method for Quantum Chemistry.

We introduce an implementation of the truncated many-body expansion, MBE(n), in which the n-body corrections are screened using the effective fragment potential force field, and only those that exceed a specified energy threshold are computed at a quantum-mechanical level of theory. This energy-screened MBE(n) approach is tested at the n = 3 level for a sequence of water clusters, (H2O)N=6-34. A threshold of 0.25 kJ/mol eliminates more than 80% of the subsystem electronic structure calculations and is even more efficacious in that respect than is distance-based screening. Even so, the energy-screened MBE(3) method is faithful to a full-system quantum chemistry calculation to within 1-2 kJ/mol/monomer, even in good quality basis sets such as aug-cc-pVTZ. These errors can be reduced by means of a two-layer approach that involves a Hartree-Fock calculation for the entire cluster. Such a correction proves to be necessary in order to obtain accurate relative energies for conformational isomers of (H2O)20, but the cost of a full-system Hartree-Fock calculation remains smaller than the cost of three-body subsystem calculations at correlated levels of theory. At the level of second-order Møller-Plesset perturbation theory (MP2), a screened MBE(3) calculation plus a full-system Hartree-Fock calculation is less expensive than a full-system MP2 calculation starting at N = 12 water molecules. This is true even if all MBE(3) subsystem calculations are performed on a single 40-core compute node, i.e., without significant parallelization. Energy-screened MBE(n) thus provides a fragment-based method that is accurate, stable in large basis sets, and low in cost, even when the latter is measured in aggregate computer time.

An advanced approach to control the electro-optical properties of LT-GaAs-based terahertz photoconductive antenna

This work reports on an advanced approach to the design of THz photoconductive. antenna (PCA). The LT-GaAs thin films used for the PCA fabrication were synthesized by MBE method on GaAs (100) substrate by adjusting the As pressure, As/Ga fluxes ratio, growth/annealing temperatures and annealing time. These parameters crucially affect electro-optical properties of the PCA samples as evidenced by the THz radiation power and time-domain spectroscopy measurements. The annealing temperature of 670 °C was found to be optimal for constructing a PCA possessing high amplitude of the THz radiation over the spectral range up to 1 THz at the resonance of 0.1 THz. The comparison of this PCA with the reference ZnTe crystal reveals a 2-fold increase in THz power. Furthermore, this antenna attains a 1.5-, 3-, and 2-fold increase in THz power, photocurrent efficiency, and actuating dc BV, as compared with the commercial ZOMEGA antenna. These results pave the way towards the creation of highly efficient LT-GaAs-based PCAs.

Can we always control the thickness layer in the MBE method with atomic precision? Analysis of the problem on the MQWs GaN/AlN example

The GaN/AlN multiple-quantum-wells (MQWs) structures were studied using high resolution scanning transmission electron microscopy simulations (HR STEM) and the experimental data from HR STEM measurements. GaN/AlN MQWs were synthesized by plasma-assisted molecular beam epitaxy (PAMBE). The electron microscopy methods were used to examine both interfaces. It was shown that AlN /GaN interfaces are sharp while the GaN/AlN are diffuse over two atomic layers. The latter diffusional disorder is not related to the basic limitation of the PAMBE method, but to the chemical growth properties of GaN. The three cases were investigated: sharp interface, diffuse single monolayer (ML) and diffusive two MLs.

Electrical properties of the Hg0.7Cd0.3Te films grown by MBE method on Si(0 1 3) substrates

The electrical properties of undoped and doped with indium Hg1−xCdxTe (x ≈ 0.3) films grown by the MBE method on Si (0 1 3) were investigated. In as-grown films, staking faults and trending dislocations with densities of ∼106 cm−2 and ∼107 cm−2, respectively, are observed. Annealing films under conditions causing the formation of mercury vacancies leads to drastic decrease of the staking faults (<104 cm−2). Filling vacancies leads to a decrease in the contribution of Shockley-Read recombination and to an increase in the lifetime. The dominant generation-recombination level in the as-grown Hg1−xCdxTe/Si(0 1 3) is the level associated with the vacancies. The magnetic field dependences of the Hall effect in the magnetic field range of 0.05–1.0 T at 77 K were explained by the fact that, in the films, there are two types of electrons with high and low mobilities.

Developing a new solar radiation estimation model based on Buckingham theorem

Abstract While the value of solar radiation can be expressed physically in the days without clouds, this expression becomes difficult in cloudy and complicated weather conditions. In addition, solar radiation measurements are often not taken in developing countries. In such cases, solar radiation estimation models are used. Solar radiation prediction models estimate solar radiation using other measured meteorological parameters those are available in the stations. In this study, a solar radiation estimation model was obtained using Buckingham theorem. This theory has been shown to be useful in predicting solar radiation. In this study, Buckingham theorem is used to express the solar radiation by derivation of dimensionless pi parameters. This derived model is compared with temperature based models in the literature. MPE, RMSE, MBE and NSE error analysis methods are used in this comparison. Allen, Hargreaves, Chen and Bristow-Campbell models in the literature are used for comparison. North Dakota’s meteorological data were used to compare the models. Error analysis were applied through the comparisons between the models in the literature and the model that is derived in the study. These comparisons were made using data obtained from North Dakota’s agricultural climate network. In these applications, the model obtained within the scope of the study gives better results. Especially, in terms of short-term performance, it has been found that the obtained model gives satisfactory results. It has been seen that this model gives better accuracy in comparison with other models. It is possible in RMSE analysis results. Buckingham theorem was found useful in estimating solar radiation. In terms of long term performances and percentage errors, the model has given good results.

Fabrication and characterization of a Pt/MgxZn1−xO/ZnO Schottky barrier photodiode utilizing a field plate structure

We fabricated a Pt/MgxZn1−xO/ZnO Schottky barrier photodiode (SBPD) utilizing a field plate structure and investigated the characteristics of its electric property toward realization of a high-gain avalanche photodiode for feeble ultraviolet ray detection. A MgxZn1−xO film on the Zn-face of a c-ZnO substrate was grown by the plasma-assisted MBE method. The fabricated Pt/MgxZn1−xO/ZnO SBPD showed a low reverse current and a high breakdown voltage of −380 V, and several SBPDs showed high breakdown voltage in excess of the measurement limitation of −500 V. The high breakdown voltage was achieved by a low density of defects in both the MgxZn1−xO film and ZnO substrate and the field plate structure, and therefore a depletion layer reached the bulk of the ZnO substrate through the MgxZn1−xO film.

Degradation of subcells and tunnel junctions during growth of GaInP/Ga(In)As/GaNAsSb/Ge 4‐junction solar cells

AbstractA GaInP/Ga(In)As/GaNAsSb/Ge 4J solar cell grown using the combined MOVPE + MBE method is presented. This structure is used as a test bench to assess the effects caused by the integration of subcells and tunnel junctions into the full 4J structure. A significant degradation of the Ge bottom subcell emitter is observed during the growth of the GaNAsSb subcell, with a drop in the carrier collection efficiency at the high energy photon range that causes a ~15% lower Jsc and a Voc drop of ~50 mV at 1‐sun. The Voc of the GaNAsSb subcell is shown to drop by as much as ~140 mV at 1‐sun. No degradation in performance is observed in the tunnel junctions, and no further degradation is neither observed for the Ge subcell during the growth of the GaInP/Ga(In)As subcells. The hindered efficiency potential in this lattice‐matched 4J architecture due to the degradation of the Ge and GaNAsSb subcells is discussed.

A nonnegativity preserved efficient chemical solver applied to the air pollution forecast

Air pollution forecast is becoming more and more important nowadays. The numerically sticky chemical ordinary differential equations (ODEs) is a critical component of air pollution models. Various solvers have been designed for the chemical ODEs in the past. However, they are either slow or imprecise. In our previous work, we have designed a nonnegativity preserved efficient chemical solver MBE, which is an acronym for Modified-Backward-Euler. In this paper, we review MBE method and prove its convergence and stability mathematically, which guarantee that MBE results converge to the exact solutions as the step-size becomes smaller and MBE results with relatively small step-size can be used as the standard. Then we apply MBE to the Nested Air Quality Prediction Modeling System (NAQPMS). Comparison between MBE and the most popular solver LSODE is also made. Considering the speed and precision, MBE is a better choice for the air pollution forecast.

MBE-grown InSb photodetector arrays

The MBE method has been applied to grow InSb layers on InSb substrates. These layers have served as a basis for fabricating mid-wave IR photodetector arrays. The characteristics of photodetector arrays on epitaxial InSb layers have been compared with those of series-produced single-crystal InSb arrays.

Ion-beam synthesis and thermal stability of highly tin-concentrated germanium – tin alloys

A 9at%Sn Ge-Sn alloy of good crystalline quality has been achieved by ion implantation followed by pulsed laser melting and resolidification. The concentration and crystallinity of the alloys are fully characterised by Rutherford backscattering spectrometry, X-ray diffraction, transmission electron microscopy and Raman spectroscopy. At high Sn concentrations, oxygen intermixing from a capping oxide layer, which is used to prevent ion-beam induced porosity, can interfere with the solidification process and compromise overall crystal quality. This indicates that the near surface layer containing oxygen after ion implantation must be removed before pulsed laser melting in order to obtain good crystal quality. The alloy's crystallinity is thermally stable under annealing up to 400℃ for 30min. This thermal budget is comparable to that of Ge-Sn produced by conventional MBE or CVD methods and suitable for subsequent device fabrication and post-processing.

Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La3Ga5SiO14 (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion from langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~1011 cm–2 and low surface roughness (<2 nm) is obtained.

MBE growth of ultra-thin GeSn film with high Sn content and its infrared/terahertz properties

Ultra-thin GeSn films with high Sn contents were epitaxial grown on single silicon substrates with different substrate temperatures by MBE method. It is found that the GeSn film with substrate temperature lower than 450 °C still represents amorphous or partially amorphous, which might lead to a volatile optical properties or I–V curve in infrared and terahertz bands. XRD measurement shows stronger GeSn single crystal peak (400) and narrower full width at half maximum (FWHM) when increase the substrate temperature during the growth. Mid and far infrared (FT-IR) test suggests that the transmission of the GeSn films get improved for about 46% when the substrate temperature increases from 200 to 450 °C. Terahertz transmission of the GeSn samples also improved with a maximum rising rate of 25%/50 °C as the substrate temperature increases. This study of GeSn substrate temperature during the growth demonstrates it is one of the key factors to control the structure stability of the GeSn films and their infrared and terahertz properties.

InP-based Sb-free Lasers and Photodetectors in 2-3 μm Band

Zhang et al. efforts on the explore of InP-based Sb-free 2-3 μm band lasers and photodetectors are introduced, including the 2-2.5 μm band type I InGaAs MQW lasers under pseudomorphic triangle well scheme, 2.5-3.0 μm band type I InAs MQW lasers under metamorphic strain compensated well scheme, as well as InGaAs photodetectors of high indium contents with cut-off wavelength large than 1.7 μm. All device structures are grown using gas source MBE method, and CW operation above room temperature have been reached for the lasers with wavelength less than 2.5 μm. Pulse operation of 2.9 μm lasers at TE temperature also have been reached The dark current of 2.6 μm InGaAs photodetectors have been decreased notably with the inserting of supperlattice electron barriers, those types of epitaxial materials have been used to the development of FPA modules for space remote sensing applications.

Growth of high-indium-content InGaN:Mg thin films by MBE method with dual RF nitrogen plasma cells

Mg-doped high-In-content InxGa1−xN (x ∼ 0.35) thin films were grown on a c-face sapphire substrate using an MBE apparatus having dual RF nitrogen plasma cells. The films showed p-type conduction under certain cell temperatures of Mg. The p-type conduction was confirmed by electrolyte-based capacitance–voltage measurement on an InxGa1−xN:Mg/In2O3/sapphire structure, in addition to thermo-electromotive force measurements.

Epitaxial silicon carbide on a 6″ silicon wafer

The results of the growth of silicon-carbide films on silicon wafers with a large diameter of 150 mm (6″) by using a new method of solid-phase epitaxy are presented. A SiC film growing on Si wafers was studied by means of spectral ellipsometry, SEM, X-ray diffraction, and Raman scattering. As follows from the studies, SiC layers are epitaxial over the entire surface of a 150-mm wafer. The wafers have no mechanical stresses, are smooth, and do not have bends. The half-width of the X-ray rocking curve (FWHMω−θ) of the wafers varies in the range from 0.7° to 0.8° across the thickness layer of 80–100 nm. The wafers are suitable as templates for the growth of SiC, AlN, GaN, ZnO, and other wide-gap semiconductors on its surface using standard CVD, HVPE, and MBE methods.