Molecular Beam Intensity Research Articles

Molecular beam epitaxy of stoichiometric tin–telluride thin films

Tin–telluride (SnTe) thin films were grown on (001)-oriented GaAs substrates using molecular beam epitaxy. Samples were grown with various molecular beam intensity ratios at substrate temperatures of 225 °C and 240 °C. The crystallinity of the SnTe thin films was studied using X-ray diffraction θ–2θ measurements. Stoichiometric SnTe layers were achieved by tuning both molecular beam intensity ratio and substrate temperature. It was also found that the excess Te remained in films when the molecular beam intensity ratio was Te rich. The (111)-oriented domain was confirmed when the substrate temperature was 240 °C. Continuous film formation was confirmed from cross-sectional transmission electron microscopy (TEM) observation. The segregation of Te for certain samples were also confirmed from the TEM energy dispersive x-ray mapping.

Partial and Contingent Recoil-Frame Covariance-Map Imaging.

When applied to multimass velocity-map imaging data, covariance analysis reveals correlations between different fragment ions formed from the same parent molecule and can provide detailed insights into the fragmentation dynamics. Covariances between the time-of-flight signals for two different ions show that they are formed in the same event, while covariances between their velocity-map images, often referred to as "recoil-frame covariances", reveal details of the correlated motion of the two fragments. In many cases, covariance analysis is complicated by the fact that fluctuations in experimental parameters such as laser or molecular beam intensities can lead to apparent correlations between unrelated ions. In the context of time-of-flight covariance signals, this problem has been overcome by the introduction of partial covariance and contingent covariance approaches. Here, we apply these approaches to recoil-frame covariance-map images. We also demonstrate that in many cases the total signal within each experimental cycle can be used as a useful proxy for independent explicit measurements of the varying experimental parameter(s).

Characterization of CdTe thin films with different compositions obtained by CMBD for thin film solar cells

CdTe thin films with different compositions, i.e. Cd/Te ratio of ∼0.79, 0.88, 0.94, 1.01, 1.05, 1.08, were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) under hydrogen flow in atmospheric pressure. For this purpose, Cd and Te granules were used as precursors. Thin films were deposited on glass and metal substrates at 600°C. The growth rate was varied in the range of (10–30)Å/s. The composition ratio in the samples was controlled by varying the molecular beam intensity (MBI) ratio. The morphological, structural, electrical and optical properties of the CdTe thin films were investigated by SEM, XRD, Hall-effect and photoluminescence (PL) measurements, respectively.

Fragmentation of the adenine and guanine molecules induced by electron collisions.

Secondary electron emission is the most important stage in the mechanism of radiation damage to DNA biopolymers induced by primary ionizing radiation. These secondary electrons ejected by the primary electron impacts can produce further ionizations, initiating an avalanche effect, leading to genome damage through the energy transfer from the primary objects to sensitive biomolecular targets, such as nitrogenous bases, saccharides, and other DNA and peptide components. In this work, the formation of positive and negative ions of purine bases of nucleic acids (adenine and guanine molecules) under the impact of slow electrons (from 0.1 till 200 eV) is studied by the crossed electron and molecular beams technique. The method used makes it possible to measure the molecular beam intensity and determine the total cross-sections for the formation of positive and negative ions of the studied molecules, their energy dependences, and absolute values. It is found that the maximum cross section for formation of the adenine and guanine positive ions is reached at about 90 eV energy of the electron beam and their absolute values are equal to 2.8 × 10(-15) and 3.2 × 10(-15) cm(2), respectively. The total cross section for formation of the negative ions is 6.1 × 10(-18) and 7.6 × 10(-18) cm(2) at the energy of 1.1 eV for adenine and guanine, respectively. The absolute cross-section values for the molecular ions are measured and the cross-sections of dissociative ionization are determined. Quantum chemical calculations are performed for the studied molecules, ions and fragments for interpretation of the crossed beams experiments.

Self-Healing Phenomenon and Dynamic Hardness of C60-Based Nanocomposite Coatings

The phenomenon of surface self-healing in C60-based polymer coatings deposited by ion-beam assisted physical vapor deposition was investigated. Nanoindentation of the coatings led to the formation of a protrusion rather than an indent. This protrusion was accompanied by an abnormal shape of the force-distance curve, where the unloading curve lies above the loading curve due to an additional force applied in pulling the indenter out of the media. The coatings exhibited a nanocomposite structure that was strongly affected by the ratio of C60 ion and C60 molecular beam intensities during deposition. The coatings also demonstrated the dynamic hardness effect, where the effective value of the hardness depends significantly on the indentation speed.

Effect of CdCl2 treatment on physical properties of CdTe films with different compositions fabricated by chemical molecular beam deposition

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by revolutionary novel and low cost chemical molecular beam deposition (CMBD) method in the atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO2: Al2O3) substrates at 600°C. The growth rate was varied in the range of 20–30 A/s. The composition of the samples was changed by controlling the molecular beam intensity (MBI) ratio Cd/Te. Effect of CdCl2 treatment on morphology, photoluminescence and electrical properties of CdTe films was investigated by AFM, Raman, photoluminescence (PL) and Hall methods.

Inelastic collisions of the uracil molecules with electrons

Ionization and excitation of the uracil molecules by electron impact is investigated. Production of positive ions of uracil molecules (nucleic acid base) was studied using a crossed electron and molecular beam technique. The method developed by the authors enabled the molecular beam intensity to be measured and the electron dependences and the absolute values of the total cross sections of production of both positive ions to be determined. It is shown that the total positive uracil ion production cross section reaches its maximal value of (1.0 ± 0.1) × 10(-15) cm(2) at the 95 eV electron energy. Dissociative ionization cross sections were also determined. The luminescence spectra of isolated uracil molecules in the wavelength range of 200-500 nm under the influence of slow electrons are obtained. In the spectrum, more than 20 spectral bands and lines at 100 eV electron energy are observed. It is shown that the uracil radiation spectrum is formed by the processes of molecules dissociative excitation, dissociative excitation with ionization, excitation of electronic levels of the initial molecule and molecular ion.

Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide

Rotational state-selection of the asymmetric-top molecule propylene oxide was carried out using an electrostatic hexapole field of 85-cm length. Molecular beam intensities were monitored by a quadrupole mass spectrometer. It was found that beam intensities of molecular beams for pure propylene oxide and those seeded in He and in Ar increased with increasing hexapole voltages. The hexapole voltage dependence of the beam intensity, which is called the focusing curve, was interpreted by computer simulation of the trajectories of molecules in the hexapolar field due to the Stark effect, as a function of rotational temperatures of molecular beams. The calculated best fit focusing curves, when compared with the experimental results, demonstrated that the rotational temperatures, associated with the distribution of states of a given rotational angular momentum J, are similar to the translational temperatures. It was found that the M = 0 states (where M is the projection of J along the direction of the electrostatic field) and negative values of the pseudoquantum number tau of propylene oxide can be selected using our experimental setup. These results suggest that the hexapole electric field is a tool even for the selection of rotational and orientation states of asymmetric-top molecules.

Formation of positive and negative ions of thymine molecules under the action of slow electrons

The formation of positive and negative molecules of thymine—a base of nucleic acids—under the action of slow electrons is investigated by the method of crossed electron and molecular beams. The method developed makes it possible to measure the molecular beam intensity and determine the energy dependences and absolute values of total cross sections for the formation of positive and negative ions of thymine molecules. It is found that the maximal cross section for the formation of positive ions is reached at an energy of 95 eV and its absolute value is, accordingly, 1.4 × 10−15 cm2. The total cross section for the formation of negative ions is 8.2 × 10–18 cm2 at an energy of 1.1 eV. The mass spectra of thymine molecules are measured and the cross sections of dissociative ionization are determined.

Structural, photoluminescent and electrical properties of CdTe films with different compositions fabricated by CMBD

CdTe films with different compositions (Cd-rich, Te-rich and stoichiometric) were fabricated by a novel and low cost chemical molecular beam deposition method (CMBD) in atmospheric pressure hydrogen flow. Cd and Te granules were used as precursors. The films were deposited on ceramic (SiO 2:Al 2O 3) substrates at 580 °C and 600 °C. The growth rate was varied in the range of 9–30 Å/s. The composition (Cd/Te) of the samples was changed by controlling the molecular beam intensity (MBI) ratio. Three samples fabricated at MBI ratios Cd/Te = 0.5, 1.0 and 1.16 were investigated by XRD, AFM, EDX, SEM, photoluminescence (PL) and Hall methods.

Dielectric Constants on Mixture Thin Films of Barium Carbide and Barium Nitrate Deposited on Silicon Wafers by Barium Carbonate Electron Beam Evaporation Using Nitrogen Molecular and Ion Beams

Thin barium carbide (BaC6) films were deposited on Si substrates by the electron beam evaporation of barium carbonate (BaCO3). Mixture films of BaC6 and barium nitrate (BaN2O4) were deposited on Si substrates using N2 molecules and ion beams during the evaporation of BaCO3 by electron beams. The fraction of BaN2O4 in the mixture films increased with increasing intensity of N2 molecular and nitrogen ion beams. The prepared films had a large number of C–O bonds, because the BaCO3 used was a source material. Highly O-doped BaC6 films had very low dielectric constants of (1–2)ε0. However, the dielectric constants of the mixture films varied from 1.3 to 5.8ε0 with increasing proportion of BaN2O4.

Absolute cross sections of positive- and negative-ion production in electron collision with cytosine molecules

Production of positive and negative ions of cytosine molecules (nucleic acid base) has been studied using a crossed electron and molecular beam technique. The method developed by the authors enabled the molecular beam intensity to be measured and the electron dependences and the absolute values of the total cross sections of production of both positive and negative cytosine ions to be determined. It has been shown that the total positive cytosine ion production cross section reaches its maximal value of 7.8 × 10−16 cm2 at the 78 eV electron energy. Dissociative ionization cross sections have also been determined. The maximum total negative cytosine ion production cross section was measured to be 4.2 × 10−18 cm2 at 1.5 eV.

A double-stage pulsed discharge fluorine atom beam source

Molecular-beam intensity and speed ratio are two major limiting factors in many molecular-beam experiments. This article reports a high-intensity, high-speed-ratio, pulsed supersonic fluorine atom beam source using a double-stage discharge beam source. Its performance is indicated by the high-resolution time-of-flight spectrum in the crossed beam experiment of F(P2)+para-H2.

Absolute cross sections for the electron-impact formation of cytosine anions

The process of negative ion formation by electron impact on cytosine (a nitrous base of nucleic acids) has been studied in crossed molecular and electron beams for electron energies in the interval from 0.4 to 5.0 eV. Using a specially developed method, the molecular beam intensity was determined and the energy dependence of the absolute cross section Q for the formation of cytosine anions was studied. The maximum ionization cross section σ = 4.2 × 10−18 cm2 was observed for an electron energy of 1.5 eV.

A novel chemical molecular beam deposition method for fabrication of II–VI low dimensional structures

CdSe, CdTe and ZnxCdTe1−x films were fabricated by novel and low cost chemical molecular beam deposition method in the atmospheric pressure H2 and Ar flows. Zn, Cd, Se and Te were used as precursors. The kinetics of the growth process of the films depended strongly on the molecular beam intensity (MBI) ratio of the metal IA and the chalcogen IB and on the nature of the gas medium. Films did not condense for MBI ratios IA/IB<1.0 and IA/IB>1.0. Films were fabricated for 10−2<IA/IB<10.0.The growth rate increased from 0.2 to 0.3Å/s with equalization of MBI beginning when the ratios IA/IB<5–7. The value of the growth rate was saturated and equal to 1–2Å/s at the congruent evaporation IA/IB=1.0—1.5. The value of the growth rate did not significantly change when IA/IB<0.2–0.5. The results of this study of the growth process of CdTe films in H2 and Ar flows showed that the growth rate was 2–3 times higher in the first case than in the second. The composition of ZnxCd1−xTe films depended linearly on the MBI ratio of the constituent elements.

分子線エピタキシー法によるＧａＡｓ（１１１）Ｂ上へのＣｄＳ薄膜の最適作製条件

Optimum growth conditions of CdS thin films depositted on GaAs (111) B substrate by MBE were investigated. The films were characterized by full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve, photoluminescence (PL), and surface morphology by scanning electron microscopy. The FWHM became minimum when the film was grown under the following conditions : The substrate temperature of 280°C, the ratio of molecular beam intensity of S to that of Cd about 1/1.5 and the interruption period of Cd supply for about 0.1 second. Although buffer layer depositted at 150°C for 15 seconds did not seem to affect the FWHM of the film, PL spectra indicated that the film with the buffer layer had better quality than that without the buffer layer. The roughness of the film surface was found to decrease by insertion of the buffer layer.

Valved molecular beam skimmer

Under routine source conditions, the optimum nozzle-skimmer distance to achieve maximum molecular beam intensities is within the range of a few millimeters. In cases where double skimming is additionally required, the distance between the skimmers should be kept small in order to sample a sufficiently large solid angle of the beam and hence maintain a good enough intensity. These two facts make it normally difficult to isolate the first from the second expansion chamber using a commercial vacuum gate valve due to the lack of remaining space. This note presents the design of a vacuum-tight valve which allows the aperture of a skimmer to be closed by plugging a needle directly against its internal conical wall. The valve can be driven manually or pneumatically from outside the vacuum chamber. The helium conductance of the valve was measured to be better than 1×10−8 mbar×l×s−1 for a helium partial pressure difference of 1 bar.

Focusing glass capillary array molecular beam inlet for a high sensitivity mass spectrometer system

A molecular beam is formed using a focusing glass capillary array to serve as the gas inlet of a mass spectrometer beam system. The glass disk has a diameter of 13 mm. The number of pores in the capillary array is about a half-million. The pore diameter is 10 μm and the pore length is 0.5 mm. The focal length of the glass capillary array is 9.5 cm. Results show that within the 0–30 mTorr input range, the molecular beam intensity is proportional to the inlet pressure. High sample transmission is obtained for low pressure gas input. Beyond this range, the beam intensity levels off rapidly. As opposed to the orifice beam formation, a relatively narrow operational window exists which limited the ability of the capillary array to obtain high sample transmission efficiency for high pressure gas input applications.

A laser micro-chopper approach for molecular beam probe

A laser micro-chopper has been designed to respond fast to a molecular beam. A method of spectral structure measurement is established to analyse the responding signals. In experiments, the minimum detectable signals correspond to an air molecular-beam intensity of 5 × 108/s

Optimum growth conditions of molecular beam epitaxial growth of ZnSe at a low temperature

ZnSe epilayers were grown on GaAs(100) substrates at 250°C by molecular beam epitaxy (MBE) to establish optimum growth conditions. Molecular beam intensities and beam intensity ratios of the VI element to the II element ( J VI / J II ) were taken as growth parameters. The beam intensity dependence of the growth rates at a J VI / J II around unity was investigated. At the low growth rates region, the rates increase nearly linearly with the beam intensity, which is the normal MBE growth mode. In contrast, at the high growth rates region, the rates increase super-linearly with the beam intensity. This anomalous increase can be explained rather well by taking account of the contribution of Zn atoms on the Zn-exposed surface to the ZnSe growth. The crystallinity can be improved by decreasing the growth rate at a J VI / J II around unity. However, the optimum J VI / J II is revealed to be around 1.6 at a conventional growth rate of around 0.5 μm/h, judging from the photoluminescence spectra, surface morphologies and reflection high-energy electron diffraction (RHEED) patterns.