Vacuum Molecular Beam Epitaxy System Research Articles

Ordered phase formation in Sm-Ni thin film deposited on Cr(100) single-crystal underlayer

Sm17Ni83 (at. %) alloy thin films are prepared on Cr(100) underlayers hetero-epitaxially grown on MgO(100) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The effect of substrate temperature on the ordered phase formation is investigated. The films deposited below 300 °C consist of amorphous phase, whereas formation of SmNi5 ordered phase is recognized for the films deposited above 400 °C. The SmNi5 films with ordered phase consist of two types of (110) variant whose c-axes are lying in the film plane and rotated around the film normal by 90° each other. With increasing the temperature from 400 to 500 °C, the long-range order degree increases from 0.65 to 0.80. The ordered film formed at 500 °C shows an in-plane magnetic anisotropy reflecting the magnetocrystalline anisotropy of SmNi5 crystal.

Effect of Substrate Temperature on the Ordered Phase Formation in Sm–Ni Thin Film Deposited on Cu(111) Underlayer

Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">83</sub> (at.%) alloy thin films are prepared on Cu underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The substrate temperature is varied from 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 500 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The film growth behavior and the detailed resulting film structure are respectively investigated by in situ reflection high-energy electron diffraction and by a combination of out-of-plane and in-plane X-ray diffractions. With increasing the temperature up to 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, formation of ordered SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase is recognized. The films deposited above 400 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C consist of only epitaxial SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (0001) crystal, whereas the films deposited below the temperature include amorphous phase and/or a hexagonal ordered phase other than SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . When the temperature is increased from 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 500 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, the long-range order degree of SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> film increases from 0.60 to 0.97. SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase formation is enhanced with increasing the temperature.

Metastable Sm(Fe,Cu)5ordered alloy films formed on Cu(111) underlayers

Sm-Fe thin films are prepared on Cu(111) underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates by using an ultra-high vacuum molecular beam epitaxy system. The Sm/Fe composition is varied from Fe-rich (10 at. % Sm - Fe) to Sm-rich (30 at. % Sm - Fe) region including SmFe5 stoichiometry. The influence of film composition on the film structure is studied by in-situ reflection highenergy electron diffraction and X-ray diffraction. Metastable Sm(Fe,Cu)5 ordered phase formation is recognized in the Sm-Fe films with the investigated compositions. Cu atoms diffuse from the underlayer into the Sm-Fe film and substitute the Fe site in SmFe5 structure forming an alloy compound of Sm(Fe,Cu)5 . The Sm-Fe films with Fe-rich compositions consist of Sm(Fe,Cu)5 and bcc-Fe phases, whereas the Sm-Fe films with Sm-rich compositions consist of Sm(Fe,Cu)5 and amorphous phases. Cu atom diffusion into Sm-Fe film is assisting the formation of ordered phase.

Influence of fcc Underlayer Facet on Microstructure of Co Thin Film

Co epitaxial thin films are prepared on (111)-oriented fcc-Au and fcc-Ag underlayers with island-like surfaces consisting of wide top (111) terraces surrounded by narrow side {111} and {100} facets by using an ultrahigh vacuum molecular beam epitaxy system. The influences of side facet of underlayer on crystallographic properties are investigated by pole figure X-ray diffraction and cross-sectional transmission electron microscopy. fcc-Co crystals grow epitaxially not only on top Au(111) terraces, but also on side Au{111} facets, involving stacking faults along the directions normal to the top Au(111) terraces and the side Au{111} facets, respectively. hcp-Co crystals are also formed on side Au{100} facets, where the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis of hcp-Co crystals is parallel to the side Au{100} facets. On the contrary, hcp-Co crystals grow epitaxially on top Ag(111) terraces and side Ag{111} facets, whereas fcc-Co crystals are formed on side Ag{100} terraces. As the surface roughness of underlayer increases, the Co film growth on the side facet is promoted. It is demonstrated that surface roughness reduction of underlayer is important in the preparation of well-oriented magnetic thin films.

Enhanced magnetic and electrical properties in amorphous Ge:Mn thin films by non-magnetic codoping

Amorphous Ge1−xMnx thin films have been prepared by co-depositing Ge and Mn on SiO2/Si using an ultrahigh vacuum molecular beam epitaxy system. Across a range of growth temperatures and Mn concentrations (2.8 at. %, 10.9 at. %, and 21.3 at. %), we achieved enhanced magnetic and electrical properties with non-magnetic codopants dispersed in the films. Self-assembled Mn-rich amorphous nanostructures were observed in the amorphous Ge matrix, either as isolated nanoclusters or as nanocolumns, depending on Mn concentration. The ferromagnetic saturation moments were found to increase with Mn concentration and reached a maximum of 0.7 μB/Mn in the as-grown samples. Two magnetic transition temperatures around 15 K and 200 K were observed in these amorphous MBE-grown samples. Coercivity is considered within the context of local magnetic anisotropy. The anomalous Hall effect confirmed a strong correlation between the magnetization and transport properties, indicating that global ferromagnetic coupling was carrier-mediated rather than through direct exchange. In addition, negative magnetoresistance was detected from 5 K to room temperature.

On the Secondary Droplets of Self-Running Gallium Droplets on GaAs Surface

Self-running droplets by thermal evaporation GaAs (001) surface are studied and analyzed using a scanning electron microscope. The sample is prepared under high-temperature annealing in an ultrahigh vacuum molecular beam epitaxy system. Particularly, secondary droplets which formed along primary droplet running trails are investigated. The secondary droplets are found to initially move along the [1 ̅10] instead of [110] direction, but these droplets tend to turn into [110] direction as they grow bigger. The scanning electron microscope also captures nanoscale footprints of secondary droplets different from the main droplets.

Synthesis and characterization of compositionally graded Si1−xGex layers on Si substrate

Thin film of silicon germanium (Si1−xGex) with tailored composition was grown on Si(100) substrate at 650°C in an ultrahigh vacuum molecular beam epitaxy system. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and atomic force microscopy. The nominal x-value ranged from 0 to 0.14. RBS/channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. The film shows island growth, which is driven by interface misfit dislocation.

Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.

Si(100) Surface Cleaning Using Sr and SrO

AbstractA method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as received Si(100) wafer in a ultrahigh vacuum molecular beam epitaxy system. The substrate is then heated to ∼800°C for about 5 minutes, the SiO2 is removed to leave behind a Sr terminated Si(100) surface. This Sr terminated Si(100) surface is well suited for the growth of crystalline high k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle was mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalyst reaction SiO2 + Sr (or SrO) → SiO (g) + O + Sr (or SrO). The reaction happened through several intermediate steps. The reaction SiO2 + Si → 2SiO (g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and preventing further etching and the formation of pits in the substrate.

Structure and Magnetism of MBE-Grown Co/Cu Multilayers

Structural and magnetic properties of Co/Cu multilayers deposited in the ultra-high vacuum molecular beam epitaxy system on glass substrates with different modulations periods were investigated. A structural characterization was performed by means of RHEED and Auger spectroscopy (in situ), small angle X-ray reflectivity and scanning tunneling microscopy. The samples obtained have a textured, polycrystalline layered structure for deposition at room temperature. Magnetization and in-plane magnetoresistance measurements were performed as a function of Cu and Co layer thicknesses. An influence of different buffers and of interface quality on magnetic properties was investigated.

High vacuum molecular beam epitaxy for the growth of IV–VI compounds

The molecular beam epitaxy growth of IV–VI compounds using background pressures of about 10 -7 Torr (during growth) instead of ultra-high vacuum background conditions is investigated. A simple high vacuum molecular beam epitaxy system is described. Calculations of the growth rate for the binary and pseudobinary compounds are compared with experimental data. Results on the growth of PbTe, PbSe and dilute magnetic Pb 1− x Mn x Se and Pb 1− x Mn x Se epitaxial films as well as on PbTe/Pb 1− x Mn x Te and PbSe/Pb 1− x Mn x Se multi-quantum-well structures are presented. Structural properties have been checked by X-ray diffraction. The electronic properties are investigated by conductivity and Hall effect measurements. These data are compared with that of samples grown under ultra-high vacuum conditions.

Epitaxial growth of superconducting niobium thin films by ultrahigh vacuum evaporation

A series of thin films of niobium have been grown on various orientations of single crystal sapphire by e-beam evaporation in an ultrahigh vacuum molecular beam epitaxy system. The films were grown at temperatures between 600 and 950 °C and at growth rates from 0.1 to 10 Å/s. The films were single-crystal as determined by both in situ reflection high energy electron diffraction patterns and by Read photographs. The best films had rocking curve widths of the order of 1/10 of a degree. The superconducting transition temperature was measured as a function of film thickness and crystal orientation and ranged up to 9.35 K for a thick sample grown on [11̄02] sapphire.

Modified Schottky Barrier Heights by Interfacial Doped Layers: MBE Al on GaAs

Modifications to the Schottky barrier heights of Al: GaAs diodes were achieved through the use of heavily doped semiconductor interfacial layers. These contacts were grown entirely in an ultra-high vacuum molecular beam epitaxy system, allowing the preparation of nearly ideal metal: semiconductor structures. We used p+-GaAs layers (NA=1018-1020 cm-3, tp=50–360 Å) at Al: n-GaAs interfaces (ND=5×1016 cm-3) to reproducibly increase the barrier height from 0.79 eV to 1.24 eV, with minimal degradation of the diode ideality factor (n=1.03–1.21). Our modeling of these diodes utilized the results of the unified defect model for surface and interface states on III-V compound semiconductors. The results of current-voltage and capacitance-voltage measurements on these diodes are compared with theory.