Evidence of Sn adatoms quantum tunneling at the alpha-Sn/Si(111) surface

We present a low-temperature scanning tunneling microscopy study of the alpha-Sn/Si(111) surface that demonstrates the fluctuating behavior of the Sn adatoms. The dynamical fluctuation model, successfully applied in describing the alpha-Sn/Ge(111) surface, is proposed for the related alpha-Sn/Si(111) surface too, although with a much lower transition temperature. In addition, a new phenomenon appears responsible for the unexpected evidence that the average oscillation frequency remains constant at temperatures lower than 15 K, in contradiction to the Arrhenius law. We explain this phenomenon as quantum tunneling of Sn adatoms.     

Explanation for the surface dependence of the tunneling gap in Nb3Sn

An explanation is proposed for the experimental results of Hoffstein and Cohen on tunneling into superconducting Nb₃Sn. In these experiments a strong dependence of the gap on the orientation of the surface was found. This dependence is explained here in terms of the linear chain model; a [100] surface cutting only one family of niobium atom linear chains, while a [111] surface cutting all three families of linear chains.     

Long-range surface reconstruction: Si(110)-(16 x 2)

A variety of reconstruction models is studied for the Si(110)-(16 x 2) surface using first-principles calculations. Assuming appropriate rebonding of edge atoms and surface chains buckled in antiphase, we show that steps along the [112] direction yielding a trench indeed lower the surface energy. We explain the long-range surface reconstruction and develop a geometry model based on steps, adatoms, tetramers, and interstitials. The model is able to explain the stripes of paired pentagons seen obviously in empty-state scanning tunneling microscopy images.     

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.     

Vicinal and on-axis surfaces of 6H-SiC(0001) thin films observed by scanning tunneling microscopy

Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (3.5° off (0001) towards [11 0]) and on-axis 6H---SiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were 0.25 nm corresponding to single bilayers containing one Si and one C layer. After annealing at T>1100°C for 3–5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the [ 110] or [1 10] orientations of the steps and energetic considerations. Additional deposition of very thin (2 nm) SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6H---SiC layers and formation of 3C---SiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.     

Chemically resolved structure of the surface

The structure and chemical states of the Sn/Ge(111) surface are characterized by x-ray standing waves combined with photoemission. For the room temperature square root 3xsquare root 3 phase two chemical components, approximately 0.4 eV apart, are observed for both Sn 3d and 4d core levels. Our model-independent, x-ray standing wave analysis shows unambiguously that the two components originate from Sn adatoms located at two different heights separated vertically by 0.23 A, in favor of a model composed of a fluctuating Sn layer. Contrary to the most accepted scenario, the stronger Sn 3d and 4d components, which appear at the lower binding-energy sides and account for 2/3 of the Sn adatoms, are identified to be associated with the higher Sn position, manifesting their filled valence state character.     

Adatom-adatom interaction mediated by an underlying surface phase transition

Low temperature scanning tunneling microscopy measurements on the adsorption of single Pb adatoms on Si(111)-(square root 3 x square root 3)-Pb surfaces reveal the vertical displacement patterns induced on the substrate by these Pb adatoms as well as a novel adatom-adatom interaction. The origin of both can be traced back to the (square root 3 x square root 3)<-->(3 x 3) phase transition taking place at lower temperatures. A Landau-like approach explains the displacement patterns as due to the corresponding order parameter and shows that the vicinity of a surface phase transition gives rise to a nonmonotonic adatom-adatom interaction.     

Perturbation of Ge(1 1 1) and Si(1 1 1)√3α-Sn surfaces by adsorption of dopants

We test the response of the √3 × √3α reconstructions formed by 1/3 monolayer of tin adatoms on silicon and germanium (1 1 1) surfaces upon doping with electrons or holes, using potassium or iodine as probes/perturbers of the initial electronic structures. From detailed synchrotron radiation photoelectron spectroscopy studies we show that doping with either electrons or holes plays a complimentary role on the Si and Ge surfaces and, especially, leads to complete conversion of the Sn 4d two-component spectra into single line shapes. We find that the low binding energy component of the Sn core level for both Si and Ge surfaces corresponds to Sn adatoms with higher electronic charge, than the Sn adatoms that contribute to the core level high binding energy signal. This could be analyzed as Sn adatoms with different valence state.     

STM observation of initial growth of Sn atoms on Ge(0 0 1) surface

We have studied initial growth of Sn atoms on Ge(0 0 1) surfaces at room temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto the Ge(0 0 1) substrate at room temperature, the Sn atoms form two kinds of one-dimensional structures composed of ad-dimers with different alignment, in the 〈3 1 0〉 and the 〈1 1 0〉 directions, and epitaxial structures. For Sn deposition onto the substrate at 80 K, the population of the dimer chains aligning in the 〈3 1 0〉 direction increases. The diffusion barrier of the Sn adatom on the substrate kinetically determines the population of the dimer chain. We propose that the diffusion barrier height depends on surface strain induced by the adatom. The two kinds of dimer chains appearing on the Ge(0 0 1) and Si(0 0 1) surfaces with adatoms of the group-IV elements are systematically interpreted in terms of the surface strain.     

A scanning tunneling microscopy study of water on Si(111)-(7 × 7): adsorption and oxide desorption

Scanning tunneling microscopy and temperature-programmed desorption have been used to investigate the chemistry of water on Si(111)-(7 × 7) substrates which were misoriented 2° toward the [ 10] direction. Upon room temperature exposure to water, the adatoms of the (7 × 7) unit cell are still evident even after high exposures, implying that major modifications of the substrate do not occur. At high coverages, the distribution of reacted adatoms shifts from one controlled by dissociative adsorption across the adatom-rest atom pair to a statistical distribution based on the availability of dangling bonds. Desorption of the oxide layer which remains after water adsorption and the desorption of hydrogen have also been characterized. The oxide desorption occurs along well-defined wavefronts which originate at step edges and advance in directions consistent with the underlying substrate symmetry, primarily the [ 2] direction (i.e. the wave vector points in the [ 2] direction). In regions of the surface where the oxide has desorbed, the (7 × 7) unit cell can be seen clearly. Vacancies resulting from the loss of surface silicon atoms (via the etching) coalesce into islands in the clean regions of the terraces, but unlike desorption of oxide layers from Si(100), the desorption does not occur from the boundaries of these vacancy islands.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Observing spin polarization of individual magnetic adatoms

We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.     

Surface soft phonon and the sqrt[3] x sqrt[3] <--> 3x3 phase transition in Sn/Ge(111) and Sn/Si(111)

Density functional theory calculations show that the reversible Sn/Ge(111) sqrt[3]xsqrt[3]<-->3x3 phase transition can be described in terms of a surface soft phonon. The isovalent Sn/Si(111) case does not display this transition since the sqrt[3]xsqrt[3] phase is the stable structure at low temperature, although it presents a partial softening of the 3x3 surface phonon. The rather flat energy surfaces for the atomic motion associated with this phonon mode in both cases explain the experimental similarities found at room temperature between these systems. The driving force underlying the sqrt[3]xsqrt[3]<-->3x3 phase transition is shown to be associated with the electronic energy gain due to the Sn dangling bond rehybridization.     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

The influence of ions and radiation on tunneling of electrons from the surface of liquid helium

In work previously reported by Saville et al. [1] and Saville [2], we found that electrons tunneling from their shallow potential well above the surface of liquid helium behave as an almost ideal 1D hydrogen atom. Computations [3], without approximation or adjustable parameters, using the time dependent Schrödinger equation are in excellent agreement with measured tunneling rates. The same computation method was used to compute the influence of AC fields on the tunneling rates [3] as preparation for planned experiments. In this paper I discuss phenomena which appear to set upper and lower limits on the observable tunneling rates. Lower limits were determined by photo assisted escape due to thermal radiation and by cosmic ray generation of ions in the bulk liquid helium. Upper limits were set by the creation of excitations in the superfluid film on the top of the sample chamber when electrons impacted with energy greater than 21.6 eV. Through its influence on the tunneling rate, we were also able to measure very small changes in the electron potential at the surface due to a sub-monolayer coverage of³He.     

Step instability of the In_0.2Ga_0.8As （001） surface during annealing

Anisotropic evolution of the step edges on the compressive-strained In0.2Ga0.8 As/GaAs(001) surface has been investigated by scanning tunneling microscopy (STM). The experiments suggest that step edges are indeed sinuous and protrude somewhere a little way along the [110] direction, which is different from the classical waviness predicted by the theoretical model. We consider that the monatomic step edges undergo a morphological instability induced by the anisotropic diffusion of adatoms on the terrace during annealing, and we improve a kinetic model of step edge based on the classical Burton–Cabrera–Frank (BCF) model in order to determine the normal velocity of step enlargement. The results show that the normal velocity is proportional to the arc length of the peninsula, which is consistent with the first result of our kinetic model. Additionally, a significant phenomenon is an excess elongation along the [110] direction at the top of the peninsula with a higher aspect ratio, which is attributed to the restriction of diffusion lengths.     

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.     

Step instability of the In0.2Ga0.8As （001） surface during annealing

Anisotropic evolution of the step edges on the compressive-strained In0.2Ga0.8 As/GaAs(001) surface has been investigated by scanning tunneling microscopy (STM). The experiments suggest that step edges are indeed sinuous and protrude somewhere a little way along the [110] direction, which is different from the classical waviness predicted by the theoretical model. We consider that the monatomic step edges undergo a morphological instability induced by the anisotropic diffusion of adatoms on the terrace during annealing, and we improve a kinetic model of step edge based on the classical Burton–Cabrera–Frank (BCF) model in order to determine the normal velocity of step enlargement. The results show that the normal velocity is proportional to the arc length of the peninsula, which is consistent with the first result of our kinetic model. Additionally, a significant phenomenon is an excess elongation along the [110] direction at the top of the peninsula with a higher aspect ratio, which is attributed to the restriction of diffusion lengths.