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7689. Comparative LEED and STM study of Au(430)
Classified abstracts 7685-7694
growth which gives rise to exponential variations. The resolution of these difficulties by a statistical analysis is discussed. Several sets of reported data for different adsorbate/substrate systems are examined. In many cases the analysis dos not agree with the reported mode. For a reliable determination of the growth mode and an accurate calibration of monolayers a statistical analysis is essential. L C G Rogers and G E Rhead, Thin Solid Films, 188, 1990, 109-122. 54. ELECTRON M~CROSCOPY 54 7685. An ultrahigh vacuum scanning tunneling microscope for surface science studies A uhv scanning tunneling microscope (STM) operating at 1 × 10- H mbar has been incorporated into a VG surface analysis system equipped with LEED, XPS, UPS, SEM/SAM, in addition to several sample and tip preparation facilities. Calibration of the STM can be done routinely by imaging the atomic lattice of graphite and its performance was tested by resolving the Si(111)7 × 7 surface structure. The main intended application is the investigation of well characterized, clean metal surfaces of single crystals, polycrystalline, quasicrystalline and amorphous metals. On a clean single crystal Au(111) surface, a series of monoatomic steps, the reconstruction of this surface as well as the atomic structure could be imaged. For comparison, we have also studied a polycrystalline Au film, vacuum deposited onto graphite. For the first time, atomic resolution could be obtained on a polycrystalline metal surface. R Wiesendanger et al, Vacuum, 41, 1990, 386-388. 54 7686. Simple STM theory A simple theory for the scanning tunneling microscope is presented. We calculate the tunneling current between the tip and the sample using Bardeen's formalism, as most of the actual theories do. It is usual in most of these theories to calculate the tunneling current under some approximations to make the analytical calculation easier: low temperature, small bias and to choose a particular description for the tip. We think that owing to the presently unknown tip structure and considering these approximations, it is useful to choose a simple model for describing the tip: a box with free electrons inside. In a similar way, the sample consists of extended plane waves. N Barniol et al, Vacuum, 41, 1990, 379-381. 54 7687. A low-cost, high-performance imaging system for scanning tunneling microscopy A low-cost, high-performance image acquisition and display system for scanning tunneling microscopy, based on a PC/AT compatible computer and a video frame grabber is described. The imaging system allows the use of fast scan rates and provides for viewing of the image as it is acquired. The imaging system could be used in any application involving single frame image acquisition at a rate at or below the standard video rate of 30 s '. A Brown and R W Cline, Rev Scient lnstrum, 61, 1990, 1484~1489. 54 7688. A scanning tunneling microscopy]spectroscopy system for crosssectional observations of epitaxial layers of semiconductors We have constructed a new scanning tunneling microscope (STM) using two mechanical stages. One (z stage) is for approaching the tip to the sample surface and the other (x stage) is for one-dimensional movement of the sample to observe a specific area of the sample surface. The stages move so precisely that the distance between the tip and the sample is constant during the sample movement. It enables us to find the specific area quickly. Another feature of the STM is a novel data accessing method which realizes high-speed scanning tunneling spectroscopy (STS) measurement. A great deal of data are accessed at high speed by a personal computer equipped with 32-megabyte random access memory (RAM). Using this system, STM and STS measurements of cleaved (1 I0) surfaces o f Gao.47In0 ~3As/InP multiquantum wells were performed in air. T Kato and I Tanaka, Rev Scient Instrum, 61, 1990, 1664-1667. 54 7689. Comparative LEED and STM study of An(430) Au(430) has been investigated by LEED and STM in order to study the influence of the step density on the reconstruction behaviour of Au(110). Both methods indicate the formation of steps as well as (1 x 3) and (I x 2)
reconstructed (110) terraces. The surface is not stable and after several hours in uhv rearranges to form small hillocks. M Borbonns et al, Vacuum, 41, 1990, 321 324. 54 7690. A transmission electron microscopic study of the topography of clean Si(111) surfaces Steps on Si(11 I) surfaces are shown to be preserved at the interfaces of epitaxial silicide layers grown at room temperature. Symmetry requires the formation of a dislocation at every step of this (type B) interface. This has enabled the examination, by transmission electron microscopy, of the topography of large areas of the Si surface after various treatments. Silicon molecular-beam epitaxy (MBE) is shown to occur via a step-flow mechanism at high temperatures, and through nucleation and growth of islands on the terraces at low growth temperatures. These observations point to the importance of including a nucleation mechanism into existing theories, especially at high supersaturation. A change of step character from (11~) to (TT2) at the initial stage of MBE is observed and is attributed to the stabilities of the two types of steps in relationship to the 7 x 7 structure. Preliminary results on vicinal Si(111) surfaces are also presented. R T Tung et al, J Vac Sci Technol, B8, 1990, 23~241. 54 7691. Measurement of the local density of states on a metal surface: scanning tunneling spectroscopic imaging of Au(l 11) We describe a method for measurement of the local density of states on metals with the scanning tunneling microscope and apply it to the study of the sp-gap surface state on Au(l 11). Local surface state peak intensity changes are found to correlate with surface features. Spectroscopic images o f the surfaces are produced. The surface state intensity is substantially reduced at step edges as compared to the value for large terraces. A change in the intensity by a factor of 2 over the 23 × ~/3 reconstruction unit cell is also observed. These effects are attributed to a spatial variation of the surface state intensity with the local potential. Upward shifts of the surface state energy have also been observed on narrow terraces, however more detailed measurements are necessary to understand this effect. M P Everson et al, J Vac Sci Technol, A8, 1990, 3662 3665. 54 7692. The orientation and morphology of niobium deposits on N i { l l l } substrates Transmission electron microscopy and electron diffraction were applied to study vacuum-deposited Nb/Ni{ 111 } bilayer films. The thicknesses of the niobium deposits and the substrate temperatures were in the ranges 1.5-10 nm and 25-300°C, respectively. Depending on substrate temperature, the theoretically predicted Nishiyama Wassermann (NW) orientation, orientations with an azimuthal rotation of 30 ~' with respect to NW, and a parallel orientation (Nb(111) (T10) II Ni(l 1 I)(T10)) were observed. In addition to NI~Ni compound ((phase) was identified. The deposits appeared continuous for all thicknesses and substrate temperatures. H L Gaigher and N G Van Der Berg, Thin Solid Films, 187, 1990, 111 119. 54 7693. Scanning tunnelling microscopy investigations of organic thin film topography We present scanning tunnelling microscopy images of the surface topography of thin films o f erbium bis(phthalocyanine) and tetrathiafulvalen~tetracyanoquinodimethane (TTF TCNQ). The images are obtained under low vacuum conditions for specimens which have been stored in the open air for several days. Comparison is made with scanning electron microscopy and transmission electron microscopy micrographs. Reproducible features can be interpreted as microcrystallites and in the case of TTF TCNQ even as molecular growth steps. A M Truyanovskij et al, Thin Solid Films, 188, 1990, 329 333. 54 7694. Development and application of a computer control system for an analytical electron microscope The present paper is concerned with the use of microprocessor techniques in the field of electron microscopy, in particular with the computer control of an analytical electron microscope (AEM) and the digital recording of the image. The design of a comprehensive computer control system is 695
growth which gives rise to exponential variations. The resolution of these difficulties by a statistical analysis is discussed. Several sets of reported data for different adsorbate/substrate systems are examined. In many cases the analysis dos not agree with the reported mode. For a reliable determination of the growth mode and an accurate calibration of monolayers a statistical analysis is essential. L C G Rogers and G E Rhead, Thin Solid Films, 188, 1990, 109-122. 54. ELECTRON M~CROSCOPY 54 7685. An ultrahigh vacuum scanning tunneling microscope for surface science studies A uhv scanning tunneling microscope (STM) operating at 1 × 10- H mbar has been incorporated into a VG surface analysis system equipped with LEED, XPS, UPS, SEM/SAM, in addition to several sample and tip preparation facilities. Calibration of the STM can be done routinely by imaging the atomic lattice of graphite and its performance was tested by resolving the Si(111)7 × 7 surface structure. The main intended application is the investigation of well characterized, clean metal surfaces of single crystals, polycrystalline, quasicrystalline and amorphous metals. On a clean single crystal Au(111) surface, a series of monoatomic steps, the reconstruction of this surface as well as the atomic structure could be imaged. For comparison, we have also studied a polycrystalline Au film, vacuum deposited onto graphite. For the first time, atomic resolution could be obtained on a polycrystalline metal surface. R Wiesendanger et al, Vacuum, 41, 1990, 386-388. 54 7686. Simple STM theory A simple theory for the scanning tunneling microscope is presented. We calculate the tunneling current between the tip and the sample using Bardeen's formalism, as most of the actual theories do. It is usual in most of these theories to calculate the tunneling current under some approximations to make the analytical calculation easier: low temperature, small bias and to choose a particular description for the tip. We think that owing to the presently unknown tip structure and considering these approximations, it is useful to choose a simple model for describing the tip: a box with free electrons inside. In a similar way, the sample consists of extended plane waves. N Barniol et al, Vacuum, 41, 1990, 379-381. 54 7687. A low-cost, high-performance imaging system for scanning tunneling microscopy A low-cost, high-performance image acquisition and display system for scanning tunneling microscopy, based on a PC/AT compatible computer and a video frame grabber is described. The imaging system allows the use of fast scan rates and provides for viewing of the image as it is acquired. The imaging system could be used in any application involving single frame image acquisition at a rate at or below the standard video rate of 30 s '. A Brown and R W Cline, Rev Scient lnstrum, 61, 1990, 1484~1489. 54 7688. A scanning tunneling microscopy]spectroscopy system for crosssectional observations of epitaxial layers of semiconductors We have constructed a new scanning tunneling microscope (STM) using two mechanical stages. One (z stage) is for approaching the tip to the sample surface and the other (x stage) is for one-dimensional movement of the sample to observe a specific area of the sample surface. The stages move so precisely that the distance between the tip and the sample is constant during the sample movement. It enables us to find the specific area quickly. Another feature of the STM is a novel data accessing method which realizes high-speed scanning tunneling spectroscopy (STS) measurement. A great deal of data are accessed at high speed by a personal computer equipped with 32-megabyte random access memory (RAM). Using this system, STM and STS measurements of cleaved (1 I0) surfaces o f Gao.47In0 ~3As/InP multiquantum wells were performed in air. T Kato and I Tanaka, Rev Scient Instrum, 61, 1990, 1664-1667. 54 7689. Comparative LEED and STM study of An(430) Au(430) has been investigated by LEED and STM in order to study the influence of the step density on the reconstruction behaviour of Au(110). Both methods indicate the formation of steps as well as (1 x 3) and (I x 2)
reconstructed (110) terraces. The surface is not stable and after several hours in uhv rearranges to form small hillocks. M Borbonns et al, Vacuum, 41, 1990, 321 324. 54 7690. A transmission electron microscopic study of the topography of clean Si(111) surfaces Steps on Si(11 I) surfaces are shown to be preserved at the interfaces of epitaxial silicide layers grown at room temperature. Symmetry requires the formation of a dislocation at every step of this (type B) interface. This has enabled the examination, by transmission electron microscopy, of the topography of large areas of the Si surface after various treatments. Silicon molecular-beam epitaxy (MBE) is shown to occur via a step-flow mechanism at high temperatures, and through nucleation and growth of islands on the terraces at low growth temperatures. These observations point to the importance of including a nucleation mechanism into existing theories, especially at high supersaturation. A change of step character from (11~) to (TT2) at the initial stage of MBE is observed and is attributed to the stabilities of the two types of steps in relationship to the 7 x 7 structure. Preliminary results on vicinal Si(111) surfaces are also presented. R T Tung et al, J Vac Sci Technol, B8, 1990, 23~241. 54 7691. Measurement of the local density of states on a metal surface: scanning tunneling spectroscopic imaging of Au(l 11) We describe a method for measurement of the local density of states on metals with the scanning tunneling microscope and apply it to the study of the sp-gap surface state on Au(l 11). Local surface state peak intensity changes are found to correlate with surface features. Spectroscopic images o f the surfaces are produced. The surface state intensity is substantially reduced at step edges as compared to the value for large terraces. A change in the intensity by a factor of 2 over the 23 × ~/3 reconstruction unit cell is also observed. These effects are attributed to a spatial variation of the surface state intensity with the local potential. Upward shifts of the surface state energy have also been observed on narrow terraces, however more detailed measurements are necessary to understand this effect. M P Everson et al, J Vac Sci Technol, A8, 1990, 3662 3665. 54 7692. The orientation and morphology of niobium deposits on N i { l l l } substrates Transmission electron microscopy and electron diffraction were applied to study vacuum-deposited Nb/Ni{ 111 } bilayer films. The thicknesses of the niobium deposits and the substrate temperatures were in the ranges 1.5-10 nm and 25-300°C, respectively. Depending on substrate temperature, the theoretically predicted Nishiyama Wassermann (NW) orientation, orientations with an azimuthal rotation of 30 ~' with respect to NW, and a parallel orientation (Nb(111) (T10) II Ni(l 1 I)(T10)) were observed. In addition to NI~Ni compound ((phase) was identified. The deposits appeared continuous for all thicknesses and substrate temperatures. H L Gaigher and N G Van Der Berg, Thin Solid Films, 187, 1990, 111 119. 54 7693. Scanning tunnelling microscopy investigations of organic thin film topography We present scanning tunnelling microscopy images of the surface topography of thin films o f erbium bis(phthalocyanine) and tetrathiafulvalen~tetracyanoquinodimethane (TTF TCNQ). The images are obtained under low vacuum conditions for specimens which have been stored in the open air for several days. Comparison is made with scanning electron microscopy and transmission electron microscopy micrographs. Reproducible features can be interpreted as microcrystallites and in the case of TTF TCNQ even as molecular growth steps. A M Truyanovskij et al, Thin Solid Films, 188, 1990, 329 333. 54 7694. Development and application of a computer control system for an analytical electron microscope The present paper is concerned with the use of microprocessor techniques in the field of electron microscopy, in particular with the computer control of an analytical electron microscope (AEM) and the digital recording of the image. The design of a comprehensive computer control system is 695