Molecular beams
Molecular beam research - square-wave phase modulation in molecular beam resonance studies and search for shift in transverse electromagnetic mass of electron
MOLECULAR BEAM↗
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Planetary atmospheric simulation using molecular beams.
Molecular beams for simulating orbital flight through planetary atmospheres, considering various nozzle beam systems
French, J. B.↗
Synthesis of antiferromagnetic Weyl semimetal Mn 3 Ge on insulating substrates by electron beam assisted molecular beam epitaxy
The antiferromagnetic kagome semimetals Mn 3 X (X = Ge, Sn, Ga) are of great interest due to properties arising from their Berry curvature, such as large anomalous Nernst and anomalous Hall coefficients, and spin to charge conversion efficiencies at ambient temperatures. However, the synthesis of epitaxial thin films of Mn 3 Ge in the desired hexagonal phase has been challenging because they do not wet insulating substrates, necessitating the use of a metallic buffer layer. Furthermore, a ferrimagnetic tetragonal phase also forms readily under typical growth conditions, interfering with hexagonal phase properties. We have synthesized atomically smooth and continuous epitaxial thin films of hexagonal Mn 3 Ge directly on insulating LaAlO 3 (111) substrates using electron beam assisted molecular beam epitaxy, using a three-step process that mitigates the formation of the tetragonal phase. The anomalous Nernst coefficient is found to be more than six times larger in our films than in sputtered thin films of Mn 3 Ge and significantly larger than that of Fe. Our approach can be used to grow thin layers of kagome materials, without interference from a buffer layer in transport properties, and may be applicable to a broader range of materials with large surface energies that do not grow readily on insulating substrates.
36 MATERIALS SCIENCE↗
Electron beam source molecular beam epitaxial growth of analog graded Al(x)Ga(1-x)As ballistic transistors
A new method has been developed for the growth of graded band-gap Al(x)Ga(1-x)As alloys by molecular beam epitaxy which is based upon electron beam evaporation of the group III elements. The metal fluxes are measured and feedback controlled using a modulated ion gauge sensor. The system is computer controlled which allows precise programming of the Ga and Al evaporation rates. The large dynamic response of the metal sources enables growth of variable band-gap III-V alloys with arbitrary composition profiles. This new technique is demonstrated by synthesis of analog graded Al(x)Ga(1-x)As unipolar ballistic electron transistors.
Malik, Roger J.↗
In situ surface/interface x-ray diffractometer for oxide molecular beam epitaxy
In situ studies of oxide molecular beam epitaxy by synchrotron x-ray scattering has been made possible by upgrading an existing UHV/molecular beam epitaxy (MBE) six-circle diffractometer system. For oxide MBE growth, pure ozone delivery to the chamber has been made available, and several new deposition sources have been made available on a new 12 in. CF (ConFlat, a registered trademark of Varian, Inc.) flange. X-ray diffraction has been used as a major probe for film growth and structures for the system. In the original design, electron diffraction was intended for the secondary diagnostics available without the necessity of the x-ray and located at separate positions. Deposition of films was made possible at the two diagnostic positions. And, the aiming of the evaporation sources is fixed to the point between two locations. Ozone can be supplied through two separate nozzles for each location. Also two separate thickness monitors are installed. Additional features of the equipment are also presented together with the data taken during typical oxide film growth to illustrate the depth of information available via in situ x-ray techniques.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Delayed Shutters For Dual-Beam Molecular Epitaxy
System of shutters for dual-molecular-beam epitaxy apparatus delays start of one beam with respect to another. Used in pulsed-beam equipment for deposition of low-dislocation layers of InAs on GaAs substrates, system delays application of arsenic beam with respect to indium beam to assure proper stoichiometric proportions on newly forming InAs surface. Reflectance high-energy electron diffraction (RHEED) instrument used to monitor condition of evolving surface of deposit. RHEED signal used to time pulsing of molecular beams in way that minimizes density of defects and holds lattice constant of InAs to that of GaAs substrate.
Grunthaner, Frank J.↗
High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy
Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm2 is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm2 is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.
Qi, Meng (ORCID:0000000299430056)↗
Mapping growth windows in quaternary perovskite oxide systems by hybrid molecular beam epitaxy
Requisite to growing stoichiometric perovskite thin films of the solid-solution A ' 1-x A x B O 3 by hybrid molecular beam epitaxy is understanding how the growth conditions interpolate between the end members A'B O 3 and AB O 3 , which can be grown in a self-regulated fashion, but under different conditions. Using the example of La 1-x Sr x VO 3 , the two-dimensional growth parameter space that is spanned by the flux of the metal-organic precursor vanadium oxytriisopropoxide and composition, x , was mapped out. The evolution of the adsorption-controlled growth window was obtained using a combination of X-ray diffraction, atomic force microscopy, reflection high-energy electron-diffraction (RHEED), and Rutherford backscattering spectroscopy. It is found that the stoichiometric growth conditions can be mapped out quickly with a single calibration sample using RHEED. Once stoichiometric conditions have been identified, the out-of-plane lattice parameter can be utilized to precisely determine the composition x . This strategy enables the identification of growth conditions that allow the deposition of stoichiometric perovskite oxide films with random A-site cation mixing, which is relevant to a large number of perovskite materials with interesting properties, e.g., high-temperature superconductivity and colossal magnetoresistance, that emerge in solid solution A ' 1-x A x B O 3 .
36 MATERIALS SCIENCE↗
Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy
Heterostructures with CdTe and CdTe{sub 1-x}Se{sub x} (x ∼ 0.01) absorbers between two wider-band-gap Cd{sub 1-x}Mg{sub x}Te barriers (x ∼ 0.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6 μm, suggesting that recombination is limited by diffusion dynamics. For heterostructures with a low concentration of extended defects, the bulk lifetime was determined to be 2.2 μs with an interface recombination velocity of 160 cm/s and an estimated radiative lifetime of 91 μs.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Molecular-beam time-of-flight spectroscopy applied to molecular-beam performance measurements
Time of flight spectroscopic measurements in arc heated supersonic molecular beam using algebraic relations, sensing modulated beam by Orbitron type detector
Alcalay, J. A.↗
Self-regulated growth of LaVO{sub 3} thin films by hybrid molecular beam epitaxy
LaVO{sub 3} thin films were grown on SrTiO{sub 3} (001) by hybrid molecular beam epitaxy. A volatile metalorganic precursor, vanadium oxytriisopropoxide (VTIP), and elemental La were co-supplied in the presence of a molecular oxygen flux. By keeping the La flux fixed and varying the VTIP flux, stoichiometric LaVO{sub 3} films were obtained for a range of cation flux ratios, indicating the presence of a self-regulated growth window. Films grown under stoichiometric conditions were found to have the largest lattice parameter, which decreased monotonically with increasing amounts of excess La or V. Energy dispersive X-ray spectroscopy and Rutherford backscattering measurements were carried out to confirm film compositions. Stoichiometric growth of complex vanadate thin films independent of cation flux ratios expands upon the previously reported self-regulated growth of perovskite titanates using hybrid molecular beam epitaxy, thus demonstrating the general applicability of this growth approach to other complex oxide materials, where a precise control over film stoichiometry is demanded by the application.
36 MATERIALS SCIENCE↗
High-mobility BaSnO{sub 3} grown by oxide molecular beam epitaxy
High-mobility perovskite BaSnO{sub 3} films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO{sub 3} films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnO{sub x}. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO{sub 3}. We demonstrate room temperature electron mobilities of 150 cm{sup 2} V{sup −1} s{sup −1} in films grown on PrScO{sub 3}. The results open up a wide range of opportunities for future electronic devices.
36 MATERIALS SCIENCE↗
Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy
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99 GENERAL AND MISCELLANEOUS↗
Electron transport in unipolar InGaN/GaN multiple quantum well structures grown by NH{sub 3} molecular beam epitaxy
Unipolar-light emitting diode like structures were grown by NH{sub 3} molecular beam epitaxy on c plane (0001) GaN on sapphire templates. Studies were performed to experimentally examine the effect of random alloy fluctuations on electron transport through quantum well active regions. These unipolar structures served as a test vehicle to test our 2D model of the effect of compositional fluctuations on polarization-induced barriers. Variables that were systematically studied included varying quantum well number from 0 to 5, well thickness of 1.5 nm, 3 nm, and 4.5 nm, and well compositions of In{sub 0.14}Ga{sub 0.86}N and In{sub 0.19}Ga{sub 0.81}N. Diode-like current voltage behavior was clearly observed due to the polarization-induced conduction band barrier in the quantum well region. Increasing quantum well width and number were shown to have a significant impact on increasing the turn-on voltage of each device. Temperature dependent IV measurements clearly revealed the dominant effect of thermionic behavior for temperatures from room temperature and above. Atom probe tomography was used to directly analyze parameters of the alloy fluctuations in the quantum wells including amplitude and length scale of compositional variation. A drift diffusion Schrödinger Poisson method accounting for two dimensional indium fluctuations (both in the growth direction and within the wells) was used to correctly model the turn-on voltages of the devices as compared to traditional 1D simulation models.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy
A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I∝P{sup k} dependence, where P is the incident power density, yields characteristic coefficients k = 1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature dependence, is observed. These trends are characteristic of spatially-indirect recombination associated with compositional variations. The energy depth of the confining potential, as derived from the thermal quenching of the photoluminescence, is 0.14 eV for the 325 °C sample, which is consistent with the red-shift of the PL emission peak relative to the expected bandgap energy. This suggests that compositional variation is the primary cause of the anomalously low PL emission peak energy. The higher energy PL emission of the 325 °C sample, relative to the 450 °C sample, is consistent with a reduction of the compositional fluctuations. The lower growth temperature is therefore considered more favorable for further growth optimization.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Perspective: Extremely fine tuning of doping enabled by combinatorial molecular-beam epitaxy
Chemical doping provides an effective method to control the electric properties of complex oxides. However, the state-of-art accuracy in controlling doping is limited to about 1%. This hampers elucidation of the precise doping dependences of physical properties and phenomena of interest, such as quantum phase transitions. Using the combinatorial molecular beam epitaxy, we improve the accuracy in tuning the doping level by two orders of magnitude. We illustrate this novel method by two examples: a systematic investigation of the doping dependence of interface superconductivity, and a study of the competing ground states in the vicinity of the insulator-to-superconductor transition.
36 MATERIALS SCIENCE↗
Structure and optical band gaps of (Ba,Sr)SnO{sub 3} films grown by molecular beam epitaxy
Epitaxial growth of (Ba{sub x}Sr{sub 1−x})SnO{sub 3} films with 0 ≤ x ≤ 1 using molecular beam epitaxy is reported. It is shown that SrSnO{sub 3} films can be grown coherently strained on closely lattice and symmetry matched PrScO{sub 3} substrates. The evolution of the optical band gap as a function of composition is determined by spectroscopic ellipsometry. The direct band gap monotonously decreases with x from to 4.46 eV (x = 0) to 3.36 eV (x = 1). A large Burnstein-Moss shift is observed with La-doping of BaSnO{sub 3} films. The shift corresponds approximately to the increase in Fermi level and is consistent with the low conduction band mass.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗