Molecular Beam Flux Research Articles

MBE growth of SnTe films on GaAs substrates with ZnTe buffer layers

SnTe (100) single-domain thin films have been previously grown on GaAs substrates by molecular beam epitaxy but obtaining excellent crystallinity can be difficult. In this paper, ZnTe buffer layers are introduced to improve the crystallinity of SnTe films. The effects of the molecular beam flux ratio (JTe/JSn) and growth temperature (Tg) on the crystallinity are investigated to achieve high-quality layers. The surface morphology and electrical properties of the SnTe thin films are also evaluated in addition to the conventional X-ray diffraction (XRD)measurements. The crystallographic properties of the grown samples are shown to be significantly improved by introducing a ZnTe buffer layer. XRD (θ–2θ) full width at half maximum of the lowest value obtained so far is about 0.14°. It is also found that lowering JTe/JSn to 0.6 results in a reduced growth rate, and an increased growth temperature (Tg = 220 °C) promotes the migration of atoms at the growth front. Both effects contribute to improvement in the crystallographic quality, including the layer surface smoothness. The cross-sectional transmission electron microscopy observation of the sample indicates the abrupt interface formation with different lattice structures. It is also found that low growth temperatures (below 200 °C) are not favorable for the SnTe layer growth, which is confirmed by the surface morphology.

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.

Processes of formation of epitaxial arrays of self-catalytic GaP nanowires on Si (111)

This study is devoted to the investigation of the effect of growth conditions (growth temperature, values of molecular beam fluxes) on the formation of self-catalytic GaP NW on Si(111), namely surface density, orientation and NW morphology. Nanowire arrays were grown on Si (111) by the plasma-assisted molecular beam epitaxy. It was determined that an increase of the temperature and a decrease of the Ga flux, while maintaining the V/III ratio, reduces the inclined NWs and parasitic islands nucleation probability.

Characterization of eigenstates interface-modulated in GaAs (631) multi-quantum well heterostructures

By taking advantage of the GaAs (631) corrugation self-assembled on top of multi-quantum well heterostructure interfaces, the modulation of the confined state wave functions (eigenstates) has been achieved, attaining quasi-one-dimensional or fractional dimension eigenstates. Two different theoretical approaches were used to compute the energy shift of subband optical transitions as a function of the interface corrugation geometrical configuration. For large nominal quantum well widths and small corrugation amplitude, the perturbation theory was employed, while a modified Lanczos algorithm assisted us to calculate the shifts when the corrugation amplitude was comparable to the nominal quantum well width. Experimentally, the heterostructures were grown by molecular beam epitaxy on (001) and (631) oriented substrates, where the quasi-one-dimensional ordering was reached by changing the As to Ga molecular beam fluxes ratio. It was found that the corrugated interfaces (i) break the wave function's in-plane symmetry, allowing transitions that, in principle, must be forbidden and (ii) induce blue shifts or red shifts in the order of 10 meV to the energy spectrum of the quantum wires depending on the lateral and vertical periodicities, exhibiting the presence of a lateral confinement system. The main result is the effective modulation of eigenstates through the interface corrugation control. Additionally, it was found that the interface modulation effect is greater for harmonic (n > 1) heavy (and light) hole subbands than for the ground states.

A comparison of indium arsenide antimonide and mercury cadmium telluride as long wavelength infrared detector materials

The basic material parameters that govern the performance of bulk alloys for long wavelength infrared detection have been calculated for indium arsenide antimonide and mercury cadmium telluride. The numbers show that similar performance could be expected from the two materials provided they can be synthesized at similar degrees of perfection. In order to approach parity, the quality of InAsSb must be optimized, which requires careful variations of molecular beam epitaxy (MBE) growth parameters, namely, substrate temperature and flux magnitudes and ratios. Molecular dynamics-based simulations of MBE growth offer a means to optimize these parameters by providing constants for first-principles or empirical growth models or by directly relating the qualities of the simulation-grown crystals to the MBE parameters.

THz behavior originates from different arrangements of coalescent GaN nanorods grown on Si (111) and Si (100) substrates

We investigate the coalescent GaN nanorods grown on Si (100) and Si (111) substrates. Our results clearly show that GaN nanorods grown on both substrates have the same structural, optical and morphological properties. However, we observed a clear difference in terahertz (THz) radiation between the two sets of GaN nanorods. With high gallium molecular beam flux around 6 × 10−7 Torr, coalescent GaN nanorods grown on Si (111) substrates exhibit observable THz radiation, while the ones grown on Si (100) substrates do not. The inactive THz behavior of the GaN nanorods grown on Si (100) substrate is due to the presence of randomly-rotated GaN nanorods during coalescence. The dissimilarity in THz radiation behavior between the two GaN nanorods, i.e. interfering incident optical pulse thus exhibiting inactive THz radiation from GaN nanorods grown on (100) substrate indicate that the nanorods are attractive for further THz applications not limited to III-N materials system but also other materials systems.

Molecular beam deposition of high-density InAs quantum dots on SiOx films

InAs quantum dots (QDs) were grown on SiOx films by molecular beam deposition (MBD), and then an SiOx capping layer was deposited by rf magnetron sputtering. High arsenic pressure and As2 molecular beam flux enhanced InAs nucleation and the dot growth. Ultrahigh-density InAs QDs with 8.5 × 1011 cm−2 was obtained for As2 molecular beam of 3.0 × 10−5 Torr. The amount of InAs growth grown by As2 was about three times higher than that by As4. Photoluminescence (PL) properties depended on InAs QD structures. The vacuum transport process between InAs MBD and SiOx sputtering was effective to improve the quality of InAs QDs on a SiOx film. Furthermore, strong PL intensity was obtained from InAs QDs in a SiOx matrix, fabricated by the vacuum transport process.

Molecular beam brightening by shock-wave suppression.

Supersonic beams are a prevalent source of cold molecules used in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling, and more. The triumph of this method emanates from the high densities produced in relation to other methods; however, beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. We show experimentally that this shock interaction can be reduced or even eliminated by cryocooling the interacting surface. An increase of nearly an order of magnitude in beam density was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order-of-magnitude brighter, cold molecular beams.

Abrupt GaP/GaAs Interfaces in Self-Catalyzed Nanowires.

We achieve the self-catalyzed growth of pure GaP nanowires and GaAs1-xPx/GaAs1-yPy nanowire heterostructures by solid-source molecular beam epitaxy. Consecutive segments of nearly pure GaAs and GaP are fabricated by commuting the group V fluxes. We test different flux switching procedures and measure the corresponding interfacial composition profiles with atomic resolution using high-angle annular dark field scanning transmission electron microscopy. Interface abruptness is drastically improved by switching off all the molecular beam fluxes for a short time at the group V commutation. Finally, we demonstrate that the morphology of the growth front can be either flat or truncated, depending on the growth conditions. The method presented here allows for the facile synthesis of high quality GaP/GaAs axial heterostructures directly on Si (111) wafers.

Adsorption dynamics of water on Pt{110}−(1×2)

The dynamics of H(2)O adsorption on Pt{110}-(1 x 2) is studied using supersonic molecular beam and temperature programed desorption techniques. The sticking probabilities are measured using the King and Wells method at a surface temperature of 165 K. The absolute initial sticking probability s(0) of H(2)O is 0.54+/-0.03 for an incident kinetic energy of 27 kJmol. However, an unusual molecular beam flux dependence on s(0) is also found. At low water coverage (theta<1), the sticking probability is independent of coverage due either to diffusion in an extrinsic precursor state formed above bilayer islands or to incorporation into the islands. We define theta=1 as the water coverage when the dissociative sticking probability of D(2) on a surface predosed with water has dropped to zero. The slow falling H(2)O sticking probability at theta>1 results from compression of the bilayer and the formation of multilayers. Temperature programed desorption of water shows fractional order kinetics consistent with hydrogen-bonded islands on the surface. A remarkable dependence of the initial sticking probability on the translational (1-27 kJ/mol) and internal energies of water is observed: s(0) is found to be essentially a step function of translational energy, increasing fivefold at a threshold energy of 5 kJ/mol. The threshold migrates to higher energies with increasing nozzle temperature (300-700 K). We conclude that both rotational state and rotational alignment of the water molecules in the seeded supersonic expansion are implicated in dictating the adsorption process.

Dislocations as sources of ions from NaCl and KCl (1 0 0) surfaces

The thermal emission of atomic M + and molecular M 2Cl + ions (M: alkali) from (1 0 0) surfaces of NaCl and KCl crystals under the exposure to alkali chloride molecular beams is studied. Transient emission currents after release and interruption of the impinging fluxes are recorded for different crystal temperatures and molecular beam fluxes. The fact that the host ion currents can be effectively suppressed even by relatively low impinging guest fluxes leads us to the conclusion that not the terrace, ledge or kink sites but dislocations ending at the evaporating surfaces represent the sources and sinks for the ions. Similarities between the ion emission and diffusion at dislocations and grain boundaries as well as the mathematical analysis of the courses of the transients support this assumption. Different models for the formation of the molecular ions on the terraces are discussed.

The deposition angle-dependent density of amorphous solid water films

The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz–Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are presumed to be fully dense (0.94 g/cm3). At glancing incidence (86°) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle-dependent film density is in semiquantitative agreement with the results of ballistic deposition simulations of ASW film growth.

Study of thin film deposition processes employing variable kinetic energy, highly collimated neutral molecular beams

Supersonic molecular beams have been investigated as alternative sources for thin film deposition employing a custom designed ultrahigh vacuum reactor. Molecular beam flux produced in this reactor has been measured as a function of gas flow rate, gas composition, and nozzle temperature. An efficient method to measure kinetics of thin film deposition has been developed that allows a large amount of kinetic data (i.e., deposition rate and incubation time) to be gathered per deposition experiment on a single substrate. Film thickness uniformity has been measured under two limiting conditions, which permitted the estimation of both flux and temperature spatial variations across the substrate. The kinetics of epitaxial silicon thin film deposition using Si2H6 has been examined as a function of incident beam kinetic energy (0.5–2.2 eV) and substrate temperature (550–750 °C). Calculated Si incorporation probabilities agree favorably with reaction probabilities previously measured in our laboratory employing a different apparatus and an alternative technique. The kinetics of Si1−xGex thin film growth using mixtures of Si2H6 and GeH4 were also investigated as a function of substrate temperature. In this case the Ge thin film composition was measured as a function of Ge composition of the beam. The incubation period associated with polycrystalline Si deposition on SiO2 has been investigated as a function of substrate temperature and incident beam kinetic energy. The incubation period decreases with both increasing substrate temperature and incident beam kinetic energy. SiC thin film deposition on Si(100) using SiH3CH3 (Ei=2.0 eV) has been investigated and the growth rate depends rather weakly on substrate temperature. Thin film morphology has been characterized using atomic force microscopy, while film crystallinity for polycrystalline and epitaxial films has been examined using x-ray diffraction and low energy electron diffraction, respectively. Epitaxial Si films exhibit a strong (2×1)+(1×2) pattern and a root-mean-square (rms) roughness of &amp;lt;1 nm, while polycrystalline films show 〈111〉, 〈220〉, and 〈311〉 reflections and a rms roughness of 8–25 nm, which increases with film thickness and deposition temperature.

Facet formation and characterization of III–V structures grown on patterned surfaces

In selective area growth there is a lateral transition from growth to non-growth areas. At this point the growth is determined by the lowest growing crystal planes. This review summarizes the mechanisms during the facet formation in the InP/GaInAsP material system with respect to the growth conditions in metalorganic molecular beam epitaxy. The effect of interfacet diffusion and the anisotropic surface diffusion process as well as the molecular beam flux density at the facets is discussed. Planar selective area epitaxy (SAE), where the facets can evolve freely, is selected as the starting point. Low lateral growth rates at side wall (01 1 ) planes of the structure are achieved under perpendicular molecular beam geometry. The results are transferred to embedded SAE for the lateral coupling of heterostructures having constant material compositions up to the lateral contact. Applications for SAE-grown waveguides and laser-waveguide integration are presented.

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. III. The system — epitaxial growth of LiBaF3

Abstract The development of overlayers during the interaction of molecular beams of BaF 2 with (0 0 1) surfaces of LiF is studied for crystal temperatures T = 573–673 K and for impinging BaF 2 fluxes j on = 1 × 10 12 −1.5 × 10 13 cm −2 s −1 . As long as the LiF supply from the substrate is not hindered formation and epitaxial growth of LiBaF 3 dominates strongly, and growth of BaF 2 in the orientation BaF 2 (0 0 1)[1 0 0]|LiF(0 0 1)[1 1 0] with a misfit m = + 8.8% occurs only as a transient and altogether negligible phenomenon. Grwoth of LiBaF 3 proceeds in the orientation with parallel axes, i.e. with LiBaF 3 (0 0 1)[1 0 0]|LiF(0 0 1)[1 0 0], with a misfit m = − 0.82%. As growth mode, island growth with nucleation, growth and coalescence of three-dimensional islands is found. The formation of closed layers can be accelerated by decreasing the crystal temperature or increasing the impinging molecular beam flux. With a BaF 2 flux only, layer thicknesses, because of the necessary supply of LiF from the substrate, are limited, while with congruent LiF and BaF 2 fluxes, layers of virtually unlimited thicknesses can be grown. By evaporation of LiF onto already grown LiBaF 3 layers, epitaxial growth of LiF in an orientation corresponding to that of the underlying LiF crystal is found. Closed LiF layers are achieved for T = 573 K and j on (LiF) = 1 × 10 12 cm −2 s −1 at a thickness of ≈ 30 nm. This would, in principle, enable the fabrication of superlattices LiF/LiBaF 3 /LiF etc. with layer thicknesses suitable for optical applications.

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. II. The system [formula omitted

Epitaxial growth of CaF 2 on the (0 0 1) surface of LiF is studied for crystal temperatures between 573 and 773 K and for molecular beam fluxes of 1 × 10 12–1 × 10 13 CaF 2 molecules per cm 2 and s. For T = 573 K layer growth with the orientation CaF 2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 1 0] is found. The layer growth mode is interpreted by a restricted mobility of CaF 2 molecules at this low temperature. LEED studies suggest a rearrangement of the (0 0 1) plane of CaF 2 leading to the formation of nm-scaled fourfold pyramids with (1 1 1) facets or, alternatively, an ab initio formation of such pyramids accompanied by a fast coalescence to a closed layer. For temperatures T ⩾ 673 K growth of islands occurs. With increasing temperature the orientation changes to CaF 2(0 0 1)[1 0 0]∥LiF(0 0 1)[1 0 0]. This latter orientation may be attributed to an increasing mixing of Li + and Ca 2+ ions, enforcing in this way a continuation of the cation arrangement of the underlying LiF crystal into the growing CaF 2 islands. As for MgF 2 LiF(0 0 1) , for the present system growth of LiF crystallites — presumably on already grown CaF 2 layers — is found. The epitaxial growth of these crystallites with the (1 1 1) surface parallel to the original (0 0 1) surface of the LiF crystal is interpreted by a space filling arrangement of F − anions above Ca 2+ ions.

Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. I. The system [formula omitted

Epitaxial growth of MgF 2 on the (0 0 1) surface of LiF crystals is studied for crystal temperatures T = 700–762 K and for molecular beam fluxes of 7.6 × 10 13–6.0 × 10 14 MgF 2 molecules/(cm 2s). MgF 2 grows mainly in the form of needle-like crystallites whose axes correspond to the c-axis of this tetragonal rutile structure type. The orientations MgF 2(1 0 0)[0 0 1]∥LiF(0 0 1)[1̄ 1 0] and MgF 2(1 1 0)[0 0 1]∥LiF(0 0 1)[1̄ 1 0] are found to prevail. Besides growth of MgF 2 also non-oriented growth of lithium fluoride crystallites is observed. The growth of LiF crystallites may be explained by an increase of the activity of the lithium fluoride crystal by incorporation of Mg 2+ ions into the crystal lattice and by the existence of adsorption sites with a higher binding energy at MgF 2 clusters on or in the outermost surface layer of the LiF crystal.

Mechanisms and applications of selective area growth by metalorganic molecular beam epitaxy (CBE)

In selective area growth there is a lateral transition from growth to non-growth areas. At this point the growth is determined by the lowest growing crystal planes. This review summarizes the mechanisms during the facet formation with respect to the growth conditions in metalorganic molecular beam epitaxy (MOMBE). The effect of interfacet diffusion and molecular beam flux density at the facets is discussed. Planar selective area epitaxy (SAE) where the facets can evolve freely is selected as starting point. Low lateral growth rates at side wall-(0 1 1)-planes of the structure are achieved under perpendicular molecular beam geometry. The results are transferred to embedded SAE for the lateral coupling of heterostructures having constant material compositions up to the lateral contact. Applications for SAE-grown lasers and laser-waveguide integration are presented.

Real-time simultaneous optical-based flux monitoring of Al, Ga, and In using atomic absorption for molecular beam epitaxy

We have developed a multichannel atomic absorption measurement system for real-time simultaneous monitoring of Al, Ga, and In molecular beam fluxes. In our configuration, distinct atomic emission lines from three hollow cathode lamps are combined into one beam, thus requiring only one pair of through view ports for the optical probe beam. Based on the dual beam optical configuration, the reference arm compensates for intensity drift of the light sources. In this work, we demonstrate the use of reflection high energy electron diffraction oscillations for calibrating the absorption signal.

A specialized torsion balance designed to measure the absolute flux density of hyperthermal molecular beams containing reactive species

A torsion balance is described that has been used to measure the absolute flux density of seeded hyperthermal molecular beams containing reactive and/or condensable species with uncertainties of approximately ±5%. The balance is ultrahigh-vacuum compatible and can be used in corrosive environments. A specially designed beam stop mounted on the torsion balance lever arm, traps the incoming beam and allows only completely thermalized molecules to exit. The thermalized molecules exit the beam stop in equal numbers per unit time in opposite directions, ensuring that the force exerted on the beam stop by the exiting molecules is zero. The balance was suspended from a 25-μm-diam gold-coated tungsten wire that had a torsion constant of 6.04×10−8 N m/rad and a period of slightly larger than 400 s. Absolute molecular beam flux density measurements were made using both the torsion balance and the effusive method for a variety of pure and seeded molecular beams. The beams were composed of gases that could easily be measured by the effusive method. Measured fluxes ranged from 2.41×1015 mol/s cm2 for a beam of CO seeded in H2 to 1.25×1017 mol/s cm2 for a beam of pure H2. A comparison of the two methods establishes the ability of the torsion balance to accurately measure the flux of molecular beams.