Ultra-high Vacuum Molecular Beam Research Articles

Synthesis and characterization of kinked GaAs nanowires by Sb surfactant

In this study, GaAs/GaSb and GaAs/GaSb/GaAs heterostructure nanowires (NWs) were grown on Si (111) substrates by self-catalyzed vapor-liquid-solid (VLS) mechanism using the ultrahigh vacuum (UHV) molecular beam epitaxy (MBE) system. The GaAs/GaSb/GaAs heterojunction NWs may kink away from [111] to another crystallographic direction, as observed by electron microscopy. The smallest NWs prefer growth along the ⟨220⟩ axis, while the larger GaAs NWs chose either ⟨111⟩ or ⟨002⟩ axis, and the kink angles of 55°, 71°, 109°, 125°, and 145°, respectively. Moreover, significant lateral growth was observed at the GaSb segment in the GaAs/GaSb heterojunction NWs, and lateral growth was clearly evidenced by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) techniques. This behavior was attributed to the surfactant effect of Sb, leading to a significant change in the contact angle that ultimately caused the expansion of the Ga droplets. This study is also widely applicable to other III-V nanomaterials and provides conditions for the diversification of optoelectronic devices, such as photodetector or field-effect transistor.

The adhesion of L-methionine amino acid through Dip Pen Nanolithography on silver thin films grown by Molecular Beam Epitaxy technique

The present work explore the fabrication of four silver thin films with controlled thicknesses of 30 nm, 60 nm, 90 nm and 120 nm, by using ultra-high vacuum (UHV) molecular beam epitaxy (MBE) technique. The peculiarities of their transformations depending on substrate quality, roughness and shape profile types were analyzed by Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and Atomic Force Microscopy (AFM) scanning approaches. Topographical characteristics and structure modifications were highlighted depending on the deposition thicknesses. Within the increase of silver layers, Ag particles started to form a more uniform, clustered layer on the glass substrate. The improved regularity of Ag particles inside clusters defines better the cluster profiles, emphasizing a preferred orientation and arrangement of Ag particles. For the thicker layers of 90 nm and 120 nm, a well-ordered particles arrangement is present in a much higher amount, unifying the clusters with preferred regular orientations. This phenomenon has conducted to the formation of cluster islands, thus increasing the surface roughness. The reactivity of Ag thin films toward L-methionine amino acid was explored by Dip Pen Nanolithography (DPN). A clear dependence between the formation of distinct surface properties and the ease of direct-write patterning was demonstrated. An optimum linkage preference of molecular L-methionine for the most flat surface of 30 nm growth has been emphasized, while for the other substrates the writing process is critically affected by the surface roughness.

Growth and optical properties of nanocrystalline Sb2Se3 thin-films for the application in solar-cells

Antimony selenide (Sb2Se3) is a promising material for thin-film solar-cells due to its attractive optoelectronic properties with the desirable band gap. In the present work, four different Sb2Se3 films were grown on Si/SiO2 (100) substrates at ambient temperature using ultra-high vacuum molecular beam evaporation technique and different evaporation temperatures. The compositional analysis with the help of wavelength-dispersive X-ray spectroscopy measurements demonstrate that there is a loss of Se in the as-deposited samples. However, the X-ray diffraction study indicates that the as-deposited films are nanocrystalline in nature, with average crystallite size slightly increasing with the evaporation temperature. The blue shift of Raman peaks with the increase of evaporation temperature indicates the possibility of the presence of small amount of compressive strain in the crystal lattice and it appears because of the anion vacancy (VSe) due to the loss of Se. The obtained band gap value ranges from 1.075 eV to 1.21 eV, which indicate that the as-deposited nanocrystalline Sb2Se3 films are suitable for application in thin-film photovoltaic solar-cells. The tuning of the band-gap with the evaporation temperature opens a new road to optimize the device efficiency for the low-cost thin-film photovoltaics.

Superradiant emission from self-assembled light emitters: From molecules to quantum dots

Abstract

Spin pumping and laser modulated inverse spin Hall effect in yttrium iron garnet/germanium heterojunctions

In this work, undoped semiconductors, germanium (Ge) and germanium tin (GeSn), were grown on ferrimagnetic insulator yttrium iron garnet (YIG) thin films using ultra-high vacuum molecular beam epitaxy. The crystallinity of the structure was determined from x-ray diffraction and high-resolution transmission electron microscopy combined with energy dispersive x-ray spectroscopy. Both spin pumping and inverse spin Hall effects (ISHEs) of YIG/Ge and YIG/GeSn heterojunctions have been investigated with the help of broadband ferromagnetic resonance (FMR). We observe that the spin mixing conductances of YIG/Ge (60 nm) and YIG/GeSn (60 nm) are 5.4 × 1018 m−2 and 7.2 × 1018 m−2, respectively, responsible for giant spin current injection. Furthermore, it is found that spin pumping injects giant spin current from ferrimagnetic YIG into the Ge semiconductor. The infrared laser modulated ISHE was examined using heavy metal platinum as a spin current collector. Also, it has been noted that the variation in the power of laser irradiation significantly changed the ISHE voltage of YIG/Ge/Pt spin junctions, saturated magnetization, FMR linewidth, and Gilbert damping parameter of YIG, which could be attributed to the laser-induced thermal effect. The outcomes from this study are promising for the development of Ge-based spintronic and magnonic devices.

Ultrahigh-vacuum organic molecular-beam deposition system for in situ growth and characterization.

A compact ultrahigh-vacuum molecular-beam deposition system has been developed for the in situ synthesis of organic thin films and multilayers. The system incorporates all the features (heater, thickness monitor, evaporators) necessary for controlled organic thin-film growth. It can be used independently, or it can be docked to the in situ growth system and transferred to other instruments of the PGM beamline, thus allowing extensive film preparation and characterization. A manipulator dedicated to specimen preparation and organic-film deposition with temperature control between 200 K and ∼800 K has been developed. The design and performance of the system are reported with emphasis on a novel solution of masks developed to achieve position-dependent film deposition. To demonstrate the enhanced capabilities of the PGM beamline in the growth and in the characterization of electronic-structure studies of organic molecular films and their heterostructures through synchrotron-based spectroscopies, this paper presents some preliminary results of a study of Fe-phthalocyanine growth on Si substrates and on in situ prepared La0.67Sr0.33MnO3 buffer layers on SrTiO3 single crystal.

Investigation and application of the optical field distribution in gradient organic heterojunction

Abstract Gradient organic heterojunction has emerging as a new type of semiconductors with advantages of anisotropy photocurrent output, flexibility and low material cost. Although the gradient heterojunctions are attractive to the scientific community, bottlenecks such as the difficulty in fabrication has significantly limited its further development. In this work, we have investigated in-depth mechanisms of optical field distributions inside of the gradient heterojunction. Based on the simulation model, an anisotropy photocurrent ratio was predicted when the heterojunction was designed in wedge-shaped geometry. According to the simulations, it was found that the photocurrent ratio was linear change along with the ITO displacement, where the thickness combination of donor and acceptor was tuned in gradient. In order to prove the simulation, a gradient heterojunction was fabricated based on ultra-high vacuum molecular beam epitaxy. The experimental photocurrent ratio was in accordance with the simulation. The experimental result exhibited a possibility of fabricating gradient heterojunction based on epitaxy technique. In summary, we have exhibited a possibility of gradient heterojunction structure which has potential applications in position sensing.

Exchange bias induced at a Co2FeAl0.5Si0.5/Cr interface

In order to engineer the strength of an exchange bias in a cubic Heusler alloy layer, crystalline strain has been induced at a ferromagnet/antiferromagnet interface by their lattice mismatch in addition to the conventional interfacial exchange coupling between them. Such interfaces have been formed in (Co2FeAl0.5Si0.5(CFAS)/Cr)3 structures grown by ultrahigh vacuum molecular beam epitaxy. The magnetic and structural properties have been characterised to investigate the exchange interactions at the CFAS/Cr interfaces. Due to the interfacial lattice mismatch of 1.4%, the maximum offset of 18 Oe in a magnetisation curve has been measured for the case of a CFAS (2 nm)/Cr (0.9 nm) interface at 193 K. The half-metallic property of CFAS has been observed to remain unchanged, which agrees with the theoretical prediction by Culbert et al (2008 J. Appl. Phys. 103 07D707). Such a strain-induced exchange bias may provide insight of the interfacial interactions and may offer a wide flexibility in spintronic device design.

Thermoelectric Properties of Epitaxial β-FeSi2 Thin Films on Si(111) and Approach for Their Enhancement

We have investigated the intrinsic thermoelectric properties of epitaxial β-FeSi2 thin films and the impact of phosphorus (P) doping. Epitaxial β-FeSi2 thin films with single phase were grown on Si(111) substrates by two different techniques in an ultrahigh-vacuum molecular beam epitaxy (MBE) system: solid-phase epitaxy (SPE), where iron silicide films formed by codeposition of Fe and Si at room temperature were recrystallized by annealing at 530°C to form epitaxial β-FeSi2 thin films on Si(111) substrates, and MBE of β-FeSi2 thin films on epitaxial β-FeSi2 templates formed on Si(111) by reactive deposition epitaxy (RDE) at 530°C (RDE + MBE). Epitaxial SPE thin films based on codeposition had a flatter surface and more abrupt β-FeSi2/Si(111) interface than epitaxial RDE + MBE thin films. We investigated the intrinsic thermoelectric properties of the epitaxial β-FeSi2 thin films on Si(111), revealing lower thermal conductivity and higher electrical conductivity compared with bulk β-FeSi2. We also investigated the impact of doping on the Seebeck coefficient of bulk and thin-film β-FeSi2. A route to enhance the thermoelectric performance of β-FeSi2 is proposed, based on (1) fabrication of thin-film structures for high electrical conductivity and low thermal conductivity, and (2) proper choice of doping for high Seebeck coefficient.

Self-organized nano-structuring of CoO islands on Fe(001)

The realization of nanometer-scale structures through bottom-up strategies can be accomplished by exploiting a buried network of dislocations. We show that, by following appropriate growth steps in ultra-high vacuum molecular beam epitaxy, it is possible to grow nano-structured films of CoO coupled to Fe(001) substrates, with tunable sizes (both the lateral size and the maximum height scale linearly with coverage). The growth mode is discussed in terms of the evolution of surface morphology and chemical interactions as a function of the CoO thickness. Scanning tunneling microscopy measurements reveal that square mounds of CoO with lateral dimensions of less than 25nm and heights below 10 atomic layers are obtained by growing few-nanometers-thick CoO films on a pre-oxidized Fe(001) surface covered by an ultra-thin Co buffer layer. In the early stages of growth, a network of misfit dislocations develops, which works as a template for the CoO nano-structuring. From a chemical point of view, at variance with typical CoO/Fe interfaces, neither Fe segregation at the surface nor Fe oxidation at the buried interface are observed, as seen by Auger electron spectroscopy and X-ray Photoemission Spectroscopy, respectively.

Structure of Sm–Co/Fe–Co multilayer films with in-plane magnetic anisotropies prepared on MgO(110) single-crystal substrates

Sm17Co83/Fe65Co35 (at. %) multilayer films are prepared on MgO(110) substrates at 500°C by ultra-high vacuum molecular beam epitaxy. The film crystallographic properties during formation are investigated by in-situ reflection high-energy electron diffraction. The 1st Fe–Co layer epitaxially grows on the substrate with two variants whose orientations are rotated around the film normal by 180° each other. The 1st Sm–Co layer formed on the 1st Fe–Co layer involves a small volume of amorphous phase in addition to epitaxial (11¯00)) crystal of RT5 structure with the c-axis lying in the film plane. The crystallographic quality of 2nd Fe–Co layer is inferior to that of 1st Fe–Co layer. The 2nd Sm–Co layer is composed of polycrystal and amorphous phases. The crystallographic properties of Sm–Co and Fe–Co layers are delicately influenced by those of lower layers. The Sm–Co/Fe–Co multilayer films involving Sm–Co(11¯00)RT5 epitaxial crystal show in-plane uniaxial magnetic anisotropies reflecting the magnetocrystalline anisotropy of SmCo5 crystal. The coercivity increases with increasing the volume of Sm–Co(11¯00)RT5 crystal.

RF-스퍼터링 기법으로 제작한 Fe<sub>3</sub>O<sub>4</sub> 박막에 Ta 기저층이 미치는 효과

<TEX>$SiO_2$</TEX> 산화막이 제거되지 않은 Si(100) 기판 위에 실온에서 5 nm Ta과 5 nm MgO 기저층을 증착하고, 그 위에 RF 스퍼터링 기법으로 실온에서 약 35 nm 두께의 <TEX>$Fe_3O_4$</TEX> 박막을 적층하였다. 진공 후열처리에 따라 향상된 <TEX>$Fe_3O_4$</TEX> 박막의 결정성과 그에 따른 자기적 특성의 변화 양상을 관찰하였다. <TEX>$500^{\circ}C$</TEX>에서 1시간 동안 후열처리한 시료에 대해, 실온에서 강자성 특성을 보았을 뿐만 아니라, <TEX>$Fe_3O_4$</TEX> 박막의 고유한 특성으로 알려진 Verwey 상전이 현상 또한 관찰되었다. 후열처리에 의해 MgO 박막 위에 적층된 <TEX>$Fe_3O_4$</TEX>에 미치는 Ta 기저층의 영향에 대해 Ta이 삽입되지 않은 경우와 비교하여 논의 할 것이다. <TEX>$Si(100){\backslash}200nm$</TEX> <TEX>$SiO_2{\backslash}5nm$</TEX> <TEX>$Ta{\backslash}5nm$</TEX> <TEX>$MgO{\backslash}35nm$</TEX> <TEX>$Fe_3O_4$</TEX> multi-layers were prepared by using RF-sputtering and ultra-high vacuum molecular beam epitaxy (UHV-MBE) techniques. After post-annealing the multi-layers at <TEX>$500^{\circ}C$</TEX> for 1 hour under the high vacuum of <TEX>${\sim}1{\times}10^{-6}Torr$</TEX>, we observed ferromagnetic properties at room temperature as well as the Verwey transition which is the typical features of magnetite crystals formed. We have carried out a comparative study of the effect of Ta buffered layer on the crystallinity and magnetic properties of <TEX>$Fe_3O_4$</TEX> thin films prepared under different growth and annealing conditions.

Fabrication and characterization of spin injector using a high-quality B2-ordered-Co2FeSi0.5Al0.5/MgO/Si(100) tunnel contact

We successfully fabricate a (100)-orientated B2-type-Co2FeSi0.5Al0.5 (CFSA)/MgO/Si(100) tunnel contact that is promising for an efficient spin injector for Si channels. The MgO barrier is formed by radical oxidation of an Mg thin film deposited on a Si(100) surface at room temperature and successive radical oxygen annealing at 400 °C. The CFSA electrode is grown on the MgO barrier at 400 °C by ultrahigh-vacuum molecular beam deposition, and it exhibits a (100)-orientated columnar polycrystalline structure with a high degree (63%) of B2-order. The MgO barrier near the interface of the CFSA/MgO junction is crystallized with the (100) orientation, i.e., the spin filter effect due to the MgO barrier could be expected for this junction. A three-terminal Si-channel spin-accumulation device with a CFSA/MgO/Si(100) spin injector is fabricated, and the Hanle effect of accumulated spin polarized electrons injected from this contact to the Si channel is observed.

Direct band-gap measurement on epitaxial Co2FeAl0.5Si0.5 Heusler-alloy films

In this study, a newly developed band-gap measurement technique has been used to characterise epitaxial Co2FeAl0.5Si0.5 (CFAS) films. The CFAS films were deposited on MgO(001) substrate by ultra high vacuum molecular beam epitaxy. The band-gap for the as deposited films was found to be ∼110 meV when measured at room temperature. This simple technique provides a macroscopic analysis of the half-metallic properties of a thin film. This allows for simple optimisation of growth and annealing conditions.

Interfaces anisotropy in single crystal V/Fe/V trilayer

The value and sign of V/Fe interface anisotropy are investigated. Epitaxial V/Fe/V/Au layers with different iron thicknesses were grown on single-crystalline (001) MgO substrate by ultra-high vacuum molecular beam epitaxy. Magnetometry was used to measure magnetization and out-of-plane anisotropy field. From these values, we quantify the number of dead layers due to V/Fe or Fe/V interfaces, and compare it with the literature. We deduce that dead layers occur mostly at the bottom V/Fe interface. An average value for V/Fe and Fe/V interface anisotropy around 0±0.1erg/cm2 (mJ/m2) was thus deduced.

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 fcc-Fe film epitaxially grown on Cu(100) single-crystal underlayer

Fe film of 40 nm thickness is prepared on fcc-Cu(100) single-crystal underlayer at room temperature by ultra-high vacuum molecular beam epitaxy. The film growth and the detailed structure are investigated by reflection high-energy electron diffraction, cross-sectional high-resolution transmission electron microscopy (HR-TEM), and x-ray diffraction (XRD). An Fe single-crystal with metastable fcc structure nucleates on the underlayer. The HR-TEM shows that fcc lattice is formed from the Fe/Cu interface up to the film surface. A large number of misfit dislocations are introduced around the Fe/Cu interface due to an accommodation of lattice mismatch. Dislocations exist up to the film near surface. The lattice constant is estimated by XRD to be a = 0.3607 nm. The film shows a ferromagnetic property, which reflects the property of fcc-Fe crystal with high-spin ferromagnetic state.

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.