J.M. Bermond

The meandering of steps and the terrace width distribution on clean Si(111): An in-situ experiment using reflection electron microscopy

In situ reflection electron microscopy experiments have been done on steps in thermodynamic equilibrium on clean Si(111) surfaces (900° C). Isolated steps and step trains (mean step separation from 20 to 140 nm) have been studied. From the thermal fluctuations of isolated steps we deduce a value of (roughly) 1 × 10 −10 Jm−10 for the line tension of a step at 900°C. The terrace width distribution has a standard deviation which varies linearly with the mean separation between steps up to mean step distances of 70 nm at least. Over the whole range of step mean separation the distributions are best fitted by a Gaussian law. This is attributed to step interactions decaying as Ax−2 (x normal distance to the step edge). The value of A is determined (A = 4.6 × 10−30 J mat(900°C). The nature of these interactions is briefly discussed.

The equilibrium shape of silicon

Abstract In order to obtain the equilibrium shape of silicon, small monocrystalline silicon columns (diameter ∼ 1–5 μ m, height ∼ 10 μ m) have been formed on a silicon (111) substrate by photolithography. Indashsitu observation, in a UHV transmission electron microscope, of the shape changes of these columns upon heating them shows that the equilibrium shape of clean silicon can be installed over the apices, once the columns have become bulbous by evaporation and surface diffusion. Equilibrium profiles, along the 〈110〉 and 〈112〉 zones, have been visualised indashsitu at 1323 K, recorded and analysed. Well-characterised {111} and {113} facets exist on the equilibrium shape at this temperature. They are separated by rounded regions that display a tangential merging into the facets. Hence, all orientations belong to the equilibrium shape at 1323 K. The γ-plot has been constructed for the 〈110〉 zone. It shows cusps at 〈111〉 and 〈113〉 and rather broad minima at 〈110〉 and 〈100〉. However, within the resolution of our micrographs, it is not possible to decide whether flat facets exist at 〈110〉 and 〈100〉 or whether the crystal surface is merely slightly rounded. The anisotropy of the surface specific free energy is found surprisingly weak (∼ 4% maximum). Within the experimental accuracy (∼ 1%), the hierarchy is γ 111 ≥ γ 110 > γ 113 > γ 100 , the relative anisotropies with respect to {111} being 0.99, 0.98 and 0.97 for {110}, {113} and {100} respectively. An order of magnitude for the step-free energy β has been obtained ( β 111 ≈ 3 × 10 −11 J m −1 , β 113 ≈ 1 × 10 −11 J m −1 ). Our results are compared to those obtained by other authors by using voids in silicon.

A measurement of the surface energy anisotropy of nickel by transmission electron microscopy of field emitter crystals

Abstract A combination of field emission (FEM) and transmission electron microscopy (TEM) is used to measure the anisotropy of the surface tension (γ) of a metal (nickel). The stationary form of a clean nickel tip is produced and controlled in a field electron microscope, under ultrahigh vacuum. The shape of the tip is visualized in a TEM and then analysed. It is shown that the cap around the apex of a bulbous tip closely approximates the equilibrium shape of a nickel crystal. By using the inverse Wulff construction, the anisotropy of γ can be measured as a function of crystallographic orientation. Values of γ normalized to (111) have been obtained for the 〈200〉 and the 〈220〉 zones. The maximum anisotropy is found at [026] along the 〈200〉 zone ( γ 026 γ 111 = 1.05 ). The results agree with the available experimental or theoretical data reasonably well.

The roughening transition of the Si{113} and Si{110} surfaces – an in situ, real time observation

The roughening transition of the silicon {110} and {113} compact faces has been visualized, in situ and in real time, by transmission electron microscopy (TEM) and reflection electron microscopy (REM) under UHV. By using a silicon evaporation source in the close vicinity of the sample, it has been possible to control the silicon super (or under) saturation over the sample surface and, in particular, to obtain thermodynamic equilibrium at high temperatures (up to, roughly, 1400°C). REM pictures of the {110} and {113} surfaces at their roughening temperatures (thermodynamic equilibrium) are displayed. These roughening temperatures have been determined (1370°C and 1340°C for {110} and {113}, respectively). The kinetic broadening of the roughening of {110} and {113} has been studied by TEM, by observing the growth shapes of small silicon crystals, and by REM, by observing the topography of {110} and {113} surfaces, as a function of the growth temperature. Under our experimental conditions the {110} facet was kinetically roughened less than the {113}. In contrast, REM and TEM showed that the {111} surface does not become rough up to 1400°C.

A field emission measurement of the isosteric heat of adsorption of metallic adatoms on single crystal faces of a metal (Pb/W)

Abstract A method to measure the isosteric heat of adsorption of metallic adatoms on single crystal faces of a metal is described. An adsorption equilibrium is established between a flux of metal atoms impinging on a crystal face and the thermal desorption of the adsorbed atoms. In isosteric measurements, the constancy of the degree of coverage on one face is controlled by measuring the field electron current of this face. The method is tested on the adsorption system Pb/W. Definite coverages are produced by the deposition of Pb from a calibrated Pb source. The variation of the isosteric heat with the coverage is measured. Low coverage values (θ → 0) are: 85 solkcal/mole on (112), 85 kcal/mole on (113), 115 kcal/mole on (111). The second Pb layer is formed with an adsorption energy close to the vaporisation enthalpy of bulk Pb. For comparison, the activation energy of desorption is measured also. Experimental and theoretical data show a reasonable agreement.

A measurement of the local electric field on field emitter crystals using T-F emission

Abstract A method is introduced to measure in situ the absolute value of the local field strength on the surface of a field emitter tip by using T − F emission. The method is based on the unified theory of electron emission (Christov) which is now experimentally well confirmed. The method is tested on several single crystal faces of tungsten tips. The absolute value of the local field can be determined within an error of about 5%. Relative field strengths at different points on one single crystal face can be measured with an error of 2%. Once the absolute value of the field strength is thus measured, the absolute value of the work function can be obtained additionally, but so far only with a fairly great error.

Surface step dynamics: Experimental observations

Publisher Summary The thermodynamics of silicon surfaces has attracted a renewed interest during the past few years because of the availability of modem observational techniques. The direct in situ imaging of steps opened a first line of approach. The in situ visualization of the motion of steps and of step trains over lengths of a few micrometers and possibly under conditions of thermo dynamical equilibrium is their common feature. One of the important applications of this emerging capability will be in understanding the kinetics of step motion on surfaces, which is important in the growth and equilibration of surface structure. The simplest conditions under which step motion can be observed are equilibrium conditions in which the step position is fluctuating under thermal excitations. Themacroscopic analysis of the equilibrium shape of Si is a very fruitful way to get information on the step free energy and their interactions. Under thermo dynamical equilibrium conditions, the wandering of an isolated step and of step trains observed by reflection electron microscopy (REM) can be analyzed at 1170K on Si vicinals.