Ultra-high Vacuum Molecular Beam Research Articles

Development of High Temperature Electron Bombardment Evaporator for Ultra-High Vacuum

To improve the techniques of molecular beam epitaxy, the electron bombardment evaporator for high temperature evaporation in ultra-high vacuum is designed, and then its performances, such as power-temperature relationship, stability of beam flux as well as molecular beam distribution, are tested by using Ag source. Through adjusting the electric current of tungsten filament can achieve the remarkable heating power in high-voltage, and the crucible temperature rises with increasing heating power, and it exceeds 1600°C at around 60W. The evaporator can reach thermal equilibrium state in a quite short time and produce a highly stable beam flux of Ag at low deposition rate. A 9mm diameter homogeneous flux platform area is obtained at the position 60mm away from the nozzle, and this area can provide high quality beam flux for molecular beam epitaxy. These results show, the electron bombardment evaporator can meet the demands for ultra-high vacuum molecular beam epitaxy growth completely.

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.

Ge quantum dot arrays grown by ultrahigh vacuum molecular-beam epitaxy on the Si(001) surface: nucleation, morphology, and CMOS compatibility

Issues of morphology, nucleation, and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (≲600°C) and high (≳600°C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts--pyramids and wedges-- are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.

Microstructure and Magnetic Properties of Fe and Fe-alloy Thin Films Epitaxially Grown on MgO(100) Substrates

Fe, Fe50Co50, and Fe80Ni20 (at. %) single-crystal films with the (100)bcc plane parallel to the substrate surface were prepared on MgO(100) single-crystals heated at 300 °C by ultra high vacuum molecular beam epitaxy. The film growth mechanism, the film structure, and the magnetic properties were investigated. In-situ reflection high energy electron diffraction and X-ray diffraction analyses indicate that the strains in the films are very small though there are fairly large mismatches of −3.7∼−4.3% at the film/substrate interface. Cross-sectional high-resolution transmission electron microscopy shows that misfit dislocations are introduced in the film at the interface. Dislocations are also observed in the film up to around 10∼20 nm distance from the interface. The presence of such dislocation relieves the strain caused by the lattice mismatch. The in-plane magnetization properties of these films reflect the magnetocrystalline anisotropies of respective bulk Fe, Fe50Co50, and Fe80Ni20 crystals.

Preparation of Co(0001)hcp and (111)fcc Films on Single-Crystal Oxide Substrates

Co thin film were prepared on oxide single-crystal substrates of Al2O3(0001)D51, MgO(111)B1, and SrTiO3(111)E21 by ultra high vacuum molecular beam epitaxy. Effects of substrate material and substrate temperate on the film growth and the crystallographic properties were investigated. Co epitaxial thin films of hcp(0001) and/or fcc(111) orientations are obtained on all the substrate. With increasing the substrate temperature, the volume ratio of hcp to fcc increases for the Co films grown on Al2O3 and MgO substrates, whereas the ratio decrease for the Co film grown on SrTiO3 substrate. The in-plane lattice strain is larger than the out-of-plane strain due to accommodation of lattice mismatch between the film and the substrate

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.

An initial phase of Ge hut array formation at low temperature on Si(001)

We report a direct STM observation of Ge hut array nucleation on the Si(001) surface during ultrahigh vacuum molecular-beam epitaxy at 360 ∘C. Nuclei of pyramids and wedges have been observed on the wetting layer M×N patches starting from the coverage of about 5.1 Å (∼3.6 ML). Further development of hut arrays consists of simultaneous growth of the formerly appeared clusters and nucleation of new ones resulting in gradual rise of hut number density with increasing surface coverage. Huts nucleate reconstructing the patch surface from the usual c(4×2) or p(2×2) structure to one of two recently described formations composed by epitaxially oriented Ge dimer pairs and chains of four dimers.

Efficient kinematical simulation of reflection high-energy electron diffraction streak patterns for crystal surfaces

An efficient program with a user-friendly graphical interface has been developed for simulating reflection high-energy electron diffraction patterns obtained on crystal surfaces. The calculations are based on a kinematical approach which considers single electron scattering events at the surface. This time-efficient approach is in most cases sensitive enough for distinguishing different structural models, even if they differ very subtly. The results are presented in realistic two-dimensional density plots which can be directly compared to experimental observations. This program provides a useful tool in studying different structural models for crystal surfaces. Program summaryProgram title: RHEEDsimCatalogue identifier: AEJC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJC_v1_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 2752No. of bytes in distributed program, including test data, etc.: 367 205Distribution format: tar.gzProgramming language: Matlab (version > 7.6.0)Computer: Personal ComputersOperating system: Windows with Matlab environmentRAM: Greater than 1 MBClassification: 7.4Nature of problem: Reflection high-energy electron diffraction (RHEED) has been widely used in studying crystal surface structures all over the world, especially in combination with ultra-high vacuum molecular beam epitaxy systems. In addition to determining the surface smoothness, RHEED is also a very useful tool in studying surface reconstructions, which are often encountered at the growth surfaces of semiconductors and alloys. While the positions of the fractional streaks can be used to determine the basic information of the surface supercells, the intensity modulations on the fractional streaks provide further insights on the details within these unit cells. Kinematic approach is an efficient method for simulating the RHEED patterns based on various surface structural models, which can help unraveling the surface atomic structure.Solution method: The kinematic approach utilized here assumes single scattering events. Furthermore, the Ewald sphere is approximated into a planar surface for computing the streak intensities (which are most relevant to real experiments). Structure factors are calculated based on a given input of atomic species and their coordinates, with user modifiable form factors. In addition to the intensity modulation within the surface plane, additional modulations extending into the z direction is also taken into consideration, resulting in realistic density maps of the RHEED streaks, which can be directly compared to experimental observations.Unusual features: The main program RHEEDsim.m calls several local subprograms for certain computational tasks. As a result, all programs should be extracted into a single folder, and that folder should be set as the main directory in Matlab.Running time: The computing time is computer and user parameter dependent, but typically ranges from few seconds to few minutes.

Preparation and characterization of Co epitaxial thin films on Al2O3(0001) single-crystal substrates

Co epitaxial thin films were prepared on Al2O3(0001) single-crystal substrates in a substrate temperature range between 50 and 500 °C by ultra high vacuum molecular beam epitaxy. Effects of substrate temperature on the structure and the magnetic properties of the films were investigated. The films grown at temperatures lower than 150 °C consist of fcc- Co(111) crystal. With increasing the substrate temperature, hcp-Co(0001) crystal coexists with the fcc crystal and the volume ratio of hcp to fcc crystal increases. The films prepared at temperatures higher than 250 °C consist primarily of hcp crystal. The film growth seems to follow island-growth mode. The films consisting primarily of hcp crystal show perpendicular magnetic anisotropy. The domain structure and the magnetization properties are influenced by the magnetocrystalline anisotropy and the shape anisotropy caused by the film surface roughness.

Structure Analysis of Co Epitaxial Thin Films Grown on Al2O3 Single-Crystal Substrates

Co thin films were prepared on Al2O3 single-crystal substrates with (0001) and (11-20) orientations by ultra-high vacuum molecular beam epitaxy. Effects of substrate temperature and substrate orientation on the film structure were investigated. Co epitaxial thin films with close-packed planes parallel to the substrate surfaces are obtained on Al2O3(0001) and Al2O3(11-20) substrates at 50-500 °C and 300-500 °C, respectively. The films grown on Al2O3(0001) substrates at temperatures lower than 100 °C consist of fcc-Co(111) crystal. With increasing the substrate temperature, hcp-Co(0001) crystal begins to coexist with the fcc crystal and the volume ratio of hcp to fcc crystal increases. The films grown at temperatures higher than 300 °C consist primarily of hcp crystal coexisting with a small volume of fcc crystal. On the contrary, the Co thin films grown on Al2O3(11-20) substrates at temperatures higher than 300 °C consist of only the hcp crystal. The in-plane lattice strains of these Co epitaxial films are larger than the out-of-plane lattice strains due to the accommodation of lattice mismatches between the films and the substrates. The film strain decreases as the substrate temperature increases.

Ga 극초박막의 계면특성과 초전도 물성제어에 대한 연구

Spin polarized tunneling studies were carried out with Al-Ga bilayer as a spin detector, by Meservey-Tedrow technique. The superconductor (SC)/Insulator (I)/Ferromagnet (FM) tunnel junctions were provided by ultra high vacuum molecular beam epitaxy (UHV-MBE) technique. The analysis of interfacial properties in the Al-Ga bilayer was also carried out by Auger electron spectroscopy. It was observed that the superconducting transition temperature and energy gap were raised in comparison with that of bulk Ga and pure ultrathin Al films. Current studies clearly show how one can modify the material properties at the interface just with a few monolayers.

Phase transition between (2 × 1) and c(8 × 8) reconstructions observed on the Si(001) surface around 600°C

The Si(001) surface subjected to different treatments in ultrahigh vacuum molecular beam epitaxy chamber for SiO$_2$ film decomposition has been in situ investigated by reflected high energy electron diffraction (RHEED) and high resolution scanning tunnelling microscopy (STM). A transition between (2 x 1) and (4 x 4) RHEED patterns was observed. The (4 x 4) pattern arose at T <~600C during sample posttreatment cooling. The reconstruction was observed to be reversible. The c(8 x 8) structure was revealed by STM at room temperature on the same samples. The (4 x 4) patterns have been evidenced to be a manifestation of the c(8 x 8) surface structure in RHEED. The phase transition appearance has been found to depend on thermal treatment conditions and sample cooling rate.

Characterization of epitaxial MgO growth on Si(001) surface

MgO epitaxial growth on a Si(001) surface by ultrahigh-vacuum molecular beam epitaxy was investigated. Epitaxial orientation and crystalline quality were characterized based on the three-dimensional reciprocal map obtained by Weissenberg RHEED. The epitaxial orientation and crystallinity were strongly dependent on the initial condition of the substrate. When MgO was deposited on a clean Si(001) surface at room temperature a MgO(001) film grew on the Si(001) substrate with two in-plane orientations:MgO[110]//Si[100] and MgO[100]//Si[100]. This is the first observation of MgO epitaxy with the former orientation, which has a smaller mismatch than the latter orientation. When the substrate was exposed to O 2 or thermally oxidized, the latterorientation predominantly grew on the substrate. Deposition of Mg on the substrate also produced the latter orientation. These results imply that nucleation sites on the initial substrate play an important role in determining the epitaxial orientation.

Classification of Ge hut clusters in arrays formed by molecular beam epitaxy at low temperatures on the Si(001) surface

Ge hut clusters forming quantum dot arrays on the Si(001) surface in the process of low-temperature ultrahigh-vacuum molecular beam epitaxy are morphologically investigated and classified using in situ scanning tunnelling microscopy. It is found that two main Ge hut cluster types—pyramidal and wedge-shaped—have different atomic structures, and it is concluded that shape transitions between the two are impossible. Derivative cluster species — obelisks (or truncated wedges) and accreted wedges — are revealed and investigated for the first time and shown to start dominating at high Ge coverages. The uniformity of cluster arrays is shown to be controlled by the scatter in the lengths of wedge-like clusters. At low growth temperatures (360 °C), cluster nucleation during the growth of the array is observed for all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher temperatures (530 °C), no cluster nucleation is observed after the initial formation of the array.

Effects of fcc Noble Metal Underlayer and Substrate Temperature on the Formation of Ni(111) Epitaxial Thin Films

Ni(111)fcc epitaxial thin films were prepared on fee noble metal, X [X = Cu, Au, Ag], underlayers hetero-epitaxially grown on MgO(111) substrates by ultra-high vacuum molecular beam epitaxy. The microstructure and the lattice strain of Ni epitaxial films are influenced by the underlayer material and the substrate temperature. Ni films formed on Cu and Au underlayers are strained, which is interpreted to be caused by atomic mixing around the interface. The atomic mixing is enhanced with increasing the substrate temperature. On the contrary, Ni films with small strain are formed on Ag underlayers. The small film strain is possibly due to formation of sharp Ni/Ag interface resulting from immiscibility of the two elements and introduction of misfit dislocations. It is also shown that parts of Ag atoms segregate from the underlayer to the surface during Ni deposition and Ag(111)fcc crystal is coexisting with Ni(111)fcc crystal in the surface layer.

Structural characterization of metastable hcp–Ni thin films epitaxially grown on Au(100) single-crystal underlayers

Ni ( 11 2 ¯ 0 ) epitaxial thin films with hcp structure were prepared on Au(100) single-crystal underlayers at 100 °C by ultra high vacuum molecular beam epitaxy. The detailed film structure is studied by in situ reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy. The hcp–Ni film consists of two types of variants whose c-axes are rotated around the film normal by 90° each other. An atomically sharp boundary is recognized between the film and the underlayer, where misfit dislocations are introduced. Presence of such dislocations seems to relieve the strain caused by the lattice mismatch between the film and the underlayer.

Preparation and structural characterization of FeCo epitaxial thin films on insulating single-crystal substrates

FeCo epitaxial films were prepared on MgO(111), SrTiO3(111), and Al2O3(0001) single-crystal substrates by ultrahigh vacuum molecular beam epitaxy. The effects of insulating substrate material on the film growth process and the structures were investigated. FeCo(110)bcc films grow on MgO substrates with two type domains, Nishiyama–Wassermann (NW) and Kurdjumov–Sachs (KS) relationships. On the contrary, FeCo films grown on SrTiO3 and Al2O3 substrates include FeCo(111)bcc crystal in addition to the FeCo(110)bcc crystals with NW and KS relationships. The FeCo(111)bcc crystal consists of two type domains whose orientations are rotated around the film normal by 180° each other. The out-of-plane and the in-plane lattice spacings of FeCo(110)bcc and FeCo(111)bcc crystals formed on the insulating substrates are in agreement with those of the bulk Fe50Co50 (at. %) crystal with small errors ranging between +0.2% and +0.4%, showing that the strains in the epitaxial films are very small.

Microstructure of NiFe Epitaxial Thin Films Grown on MgO Single-Crystal Substrates

NiFe epitaxial thin films were prepared on MgO(100) and MgO(110) single-crystal substrates heated at 300° C by ultra-high vacuum molecular beam epitaxy. In the early stage of film growth on MgO(100) substrate, formation of NiFe (112?0) epitaxial thin film with hcp structure is observed by in-situ RHEED. With increasing the film thickness, fcc-NiFe(100) phase appears and the RHEED intensity from fcc-NiFe(100) crystal increases. X-ray diffraction and high-resolution transmission electron microscopy indicate that the resulting film structure is fcc-NiFe(100), suggesting that the metastable hcp-NiFe (112?0) phase is observable only during film growth process. On the contrary, NiFe(110) fcc single-crystal film grows epitaxially on MgO(110) substrate. Atomically sharp boundaries are recognized between NiFe thin films and MgO substrates, where misfit dislocations are periodically introduced in the films at the NiFe/MgO interfaces reducing the lattice mismatches. Out-of-plane and in-plane lattice spacings of these films agree with the values of bulk NiFe crystal with very small errors of less than 0.5%, suggesting that the strains in the NiFe films are very small.

Structure and Magnetic Properties of CoNi Thin Films Epitaxially Grown on MgO(100) and SrTiO$_{3}$(100) Substrates

CoNi epitaxial thin films were prepared on MgO(100) and SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (100) single-crystal substrates by ultra-high vacuum molecular beam epitaxy. Effects of substrate temperature on the structural and magnetic properties of epitaxial films were investigated. On MgO(100) substrate, CoNi epitaxial films consisting of hcp(112¿0) and fcc(100) crystals are obtained whereas on SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (100) substrate, CoNi(100) single-crystal films with fcc structure are formed. CoNi film growth on both substrates seems to follow the Volmer-Weber mode. CoNi (100) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fcc</sub> single-crystal film with smaller strain is obtained on SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate rather than on MgO(100) substrate. The film strain decreases with increasing the substrate temperature. The magnetic hysteresis loops of CoNi epitaxial thin films reflect the magnetocrystalline anisotropies of hcp-CoNi and fcc-CoNi crystals.

Epitaxial growth of bcc–Fe xCo 100- x thin films on MgO(1 1 0) single-crystal substrates

Fe x Co 100– x ( x=100, 65, 50 at%) epitaxial thin films were prepared on MgO(1 1 0) single-crystal substrates heated at 300 °C by ultra-high vacuum molecular beam epitaxy. The film structure and the growth mechanism are discussed. FeCo(2 1 1) films with bcc structure grow epitaxially on MgO(1 1 0) substrates with two types of variants whose orientations are rotated around the film normal by 180° each other for all compositions. Fe x Co 100– x film growth follows the Volmer Weber mode. X-ray diffraction analysis indicates the out-of-plane and the in-plane lattice spacings are in agreement with the values of respective bulk Fe x Co 100– x crystals with very small errors less than ±0.4%, suggesting the strains in the films are very small. High-resolution cross-sectional transmission electron microscopy shows that periodical misfit dislocations are preferentially introduced in the film at the Fe 50Co 50/MgO interface along the MgO[1 1¯ 0] direction. The presence of such periodical dislocations decreases the large lattice mismatch of about −17% existing at the FeCo/MgO interface along the MgO[1 1¯ 0] direction.