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Experimental method for obtaining exact isobars of the adsorption of rare gases on single crystal surfaces
Classified Classified abstracts 158-l
abstracts
158-241
64 on this page
Editor’s note The label immediately following the title of each item denotes country or origin of publication, that at the end of each abstract indicates country of origin of work (where known).
I. General 14. KINETIC
vacuum THEORY
science
and engineering
OF GASES
14 158. Method of measuring the intensity of a flow of metal vapor in vucuo. (USSR) A device for measuring the intensity of a flow of metal vapor in WZCUO, giving a much wider range of measurable flow and deposition rates than earlier devices, is described. The intensity of the flow is in fact determined by reference to the critical temperature below which the metal undergoes continuous deposition on a heated dielectric surface. The underlying principle of this method relies on the fact that the critical tern&&&e at which condensation begins depends on the properties of the condensing metal, the material and state of the substrate surface, and the intensity of the incident molecular beam. The substrate is first deliberately heated to 100°C above the expected critical temperature, then the substrate is cooled gradually until a continuous metal film appears between two electrodes on the dielectric surface; this determi&s the critical temperature. I L Roikh et al. Soviet Patent. ckzss C23 c. 13100. No. 383759. claimed 24th March, 1970, publd 17th Aug 1973 (A R&an). 14:15 159. Method of determining the magnitude of a gas flow at low pressures. (USSR) An improved method of determining the magnitude of a gas flow passing from a vessel under evacuation to a vessel of constant volume is described. In order to increase accuracy, and expand the range of flows covered, the gas is passed in finite small portions through a constant-volume chamber from the line connected to the source of flow and into a vacuum line. At the entrance and exit of the constant-volume chamber are slides (gates) which alternately open and shut, the pressure in the chamber being measured with a manometer. The flow of gas is simply related to the pressure reading and the number of portions of gas passing per second. Subject to an optimum choice of all the parameters this method gives an extremely high accuracy at low pressures. A M Grigor’ev et al, Soviet Patent, class G 01 cf. 3122, No. 3823931, claimed 23rd April 1971, publd 13th Sept. 1973 (in Russian). 15. FLUID
DYNAMICS
15 160. Device using controlled diffusion to prepare gas mixtures of uniform composition (GB) A simple diffusion cell was designed to prepare gas mixtures of uniform composition. because the performance of gas flow regulators and flow gauges commercially available at moderate prices makes it difficult to measure or control flow rates smaller than 0.1 ml s-l. Thus when gas mixtures are prepared by blending two or more streams there is a limit on the ratio of components imposed by the total flow of gas. The ratio of components could not exceed 10: 1 at a total gas flow of 1 ml s-’ but 10,000 : 1 would be possible at 1 1 s-l. However, such high flow rates are often far in excess of that required. In experiments involving the exposure of plant root systems to concentrations of ethylene in air ranging from 0.1 to 1,000 ppm very stable low flow rates of such wide ratio mixtures were obtained by allowing the ethylene component to diffuse across a permeable membrane into a stream of air. The high temperature coefficient of diffusion provided a convenient method of controlling the rate of addition of ethylene to air. R N Crossett and D J Campbell, Lab Practice, 23 (7), 1974, 369-370.
16. GASES AND
and
SOLIDS
16 161. Ellipsometry (Germany) Some aspects of ellipsometry have been reviewed, the review being of an introductory character. A number of applications have been considered: phsisorption in the sub-monolayer region of noble gas atoms on semiconductor substrates. The atomic polarizability of the gaseous atoms could be used to interpret the optical effects resulting from the adsorption. Chemisorption data on Ge, Si and GaP substrates indicated that the top atomic layers of these substrates are somewhat less well ordered than the bulk atoms of the crystals. Chemisorption has here a “healing” effect. Chemisorption thus essentially induces a substrate effect. This effect is consistent with earlier results obtained in studies of the surface conductance and the influence of chemisorption thereupon. Another class of system is that of Langmuir-Blodgett layers containing dye molecules. Ellipsometry here is capable of determining the optical anisotropy in these layers. This anisotropy leands to statements concerning the most probable position of the consituent molecules of these layers and also of the enclosed dye molecules. It is pointed out that in these systems transmission ellipsometry (instead of reflection ellipsometry) can be put to good use. Finally a number of recently published results, obtained by applying ellipsometry to various systems is briefly considered. (Netherlands). M J Spamaay, Vukuum-Tech, 23, (5), 1974, 141-151. 16 162. Field electrode: some improvements and possible applications in vacuum (France) This contribution deals with capacitance and conductance measurements of a capacitor, whose isolation is realised by a 0.01 mm evacuated space. One of the electrodes consists of the tested material, i.e. a semiconductor or an evaporated thin metallic film. It treats the mechanical problems caused by the necessity to maintain a constant small distance between the electrodes. Application for this measurement method are proposed for desorption-adsorption studies on semiconductors and for thin film sticking measurements. The measurements carried on Si with newly formed oxide showed the importance of the surface states on the oxide. (Switzerland) R. Juillerat, Vide, 29 (169), 1974, 289-292 (in French). 16 163. Experimental method for obtaining exact isobars of the adsorption of rare gases on single crystal surfaces (France) Isobars are plotted by measuring the flow of desorbed gas from a single crystal the temperature of which rises slowly in the adsorption vessel at a given pressure which is kept automatically constant to about 2x,. Fluxes as low as lo8 atoms per s can be evaluated. As an example the first three mixed virial coefficients obtained for xenon on nickel (110) are given as function of temperature. A Besnard and A Schram, Vide, 29 (169), 1974, 275-282, (in French). 18. GASEOUS ELECTRONICS 18 164. Monte Carlo approach to the spatial deposition of energy by electrons in molecular hydrogen (USA) The Monte Carlo (MC) and continuous slowdown approximation (CSDA) approaches to the spatial deposition of energy by electrons are compared using the same detailed atomic cross section (DACS). It is found that the CSDA method overestimates the amount of energy that is deposited near the end of the path for electrons above a few hundred electron volts. The MC results are in approximate agreement with experimental data in such a way as to be relatively 83
abstracts
158-241
64 on this page
Editor’s note The label immediately following the title of each item denotes country or origin of publication, that at the end of each abstract indicates country of origin of work (where known).
I. General 14. KINETIC
vacuum THEORY
science
and engineering
OF GASES
14 158. Method of measuring the intensity of a flow of metal vapor in vucuo. (USSR) A device for measuring the intensity of a flow of metal vapor in WZCUO, giving a much wider range of measurable flow and deposition rates than earlier devices, is described. The intensity of the flow is in fact determined by reference to the critical temperature below which the metal undergoes continuous deposition on a heated dielectric surface. The underlying principle of this method relies on the fact that the critical tern&&&e at which condensation begins depends on the properties of the condensing metal, the material and state of the substrate surface, and the intensity of the incident molecular beam. The substrate is first deliberately heated to 100°C above the expected critical temperature, then the substrate is cooled gradually until a continuous metal film appears between two electrodes on the dielectric surface; this determi&s the critical temperature. I L Roikh et al. Soviet Patent. ckzss C23 c. 13100. No. 383759. claimed 24th March, 1970, publd 17th Aug 1973 (A R&an). 14:15 159. Method of determining the magnitude of a gas flow at low pressures. (USSR) An improved method of determining the magnitude of a gas flow passing from a vessel under evacuation to a vessel of constant volume is described. In order to increase accuracy, and expand the range of flows covered, the gas is passed in finite small portions through a constant-volume chamber from the line connected to the source of flow and into a vacuum line. At the entrance and exit of the constant-volume chamber are slides (gates) which alternately open and shut, the pressure in the chamber being measured with a manometer. The flow of gas is simply related to the pressure reading and the number of portions of gas passing per second. Subject to an optimum choice of all the parameters this method gives an extremely high accuracy at low pressures. A M Grigor’ev et al, Soviet Patent, class G 01 cf. 3122, No. 3823931, claimed 23rd April 1971, publd 13th Sept. 1973 (in Russian). 15. FLUID
DYNAMICS
15 160. Device using controlled diffusion to prepare gas mixtures of uniform composition (GB) A simple diffusion cell was designed to prepare gas mixtures of uniform composition. because the performance of gas flow regulators and flow gauges commercially available at moderate prices makes it difficult to measure or control flow rates smaller than 0.1 ml s-l. Thus when gas mixtures are prepared by blending two or more streams there is a limit on the ratio of components imposed by the total flow of gas. The ratio of components could not exceed 10: 1 at a total gas flow of 1 ml s-’ but 10,000 : 1 would be possible at 1 1 s-l. However, such high flow rates are often far in excess of that required. In experiments involving the exposure of plant root systems to concentrations of ethylene in air ranging from 0.1 to 1,000 ppm very stable low flow rates of such wide ratio mixtures were obtained by allowing the ethylene component to diffuse across a permeable membrane into a stream of air. The high temperature coefficient of diffusion provided a convenient method of controlling the rate of addition of ethylene to air. R N Crossett and D J Campbell, Lab Practice, 23 (7), 1974, 369-370.
16. GASES AND
and
SOLIDS
16 161. Ellipsometry (Germany) Some aspects of ellipsometry have been reviewed, the review being of an introductory character. A number of applications have been considered: phsisorption in the sub-monolayer region of noble gas atoms on semiconductor substrates. The atomic polarizability of the gaseous atoms could be used to interpret the optical effects resulting from the adsorption. Chemisorption data on Ge, Si and GaP substrates indicated that the top atomic layers of these substrates are somewhat less well ordered than the bulk atoms of the crystals. Chemisorption has here a “healing” effect. Chemisorption thus essentially induces a substrate effect. This effect is consistent with earlier results obtained in studies of the surface conductance and the influence of chemisorption thereupon. Another class of system is that of Langmuir-Blodgett layers containing dye molecules. Ellipsometry here is capable of determining the optical anisotropy in these layers. This anisotropy leands to statements concerning the most probable position of the consituent molecules of these layers and also of the enclosed dye molecules. It is pointed out that in these systems transmission ellipsometry (instead of reflection ellipsometry) can be put to good use. Finally a number of recently published results, obtained by applying ellipsometry to various systems is briefly considered. (Netherlands). M J Spamaay, Vukuum-Tech, 23, (5), 1974, 141-151. 16 162. Field electrode: some improvements and possible applications in vacuum (France) This contribution deals with capacitance and conductance measurements of a capacitor, whose isolation is realised by a 0.01 mm evacuated space. One of the electrodes consists of the tested material, i.e. a semiconductor or an evaporated thin metallic film. It treats the mechanical problems caused by the necessity to maintain a constant small distance between the electrodes. Application for this measurement method are proposed for desorption-adsorption studies on semiconductors and for thin film sticking measurements. The measurements carried on Si with newly formed oxide showed the importance of the surface states on the oxide. (Switzerland) R. Juillerat, Vide, 29 (169), 1974, 289-292 (in French). 16 163. Experimental method for obtaining exact isobars of the adsorption of rare gases on single crystal surfaces (France) Isobars are plotted by measuring the flow of desorbed gas from a single crystal the temperature of which rises slowly in the adsorption vessel at a given pressure which is kept automatically constant to about 2x,. Fluxes as low as lo8 atoms per s can be evaluated. As an example the first three mixed virial coefficients obtained for xenon on nickel (110) are given as function of temperature. A Besnard and A Schram, Vide, 29 (169), 1974, 275-282, (in French). 18. GASEOUS ELECTRONICS 18 164. Monte Carlo approach to the spatial deposition of energy by electrons in molecular hydrogen (USA) The Monte Carlo (MC) and continuous slowdown approximation (CSDA) approaches to the spatial deposition of energy by electrons are compared using the same detailed atomic cross section (DACS). It is found that the CSDA method overestimates the amount of energy that is deposited near the end of the path for electrons above a few hundred electron volts. The MC results are in approximate agreement with experimental data in such a way as to be relatively 83