Single Crystal Wires Research Articles

Morphology of porous n-type silicon obtained by photoelectrochemical etching II: Study of the tangled Si wires in the nanoporous layer

Visible luminescence observed from the nanoporous layer of the two (100)-orinted low doped and highly doped PEC-etched n-type Si is explained as being due to the existence of single crystal silicon quantum wires within their structure. The nanometer-size tangled Si structure is contained and attached to a regular geometric Si macroarray. TEM studies also reveal subtle variations in morphology between the two layers studied, which could explain the blueshift in the spectrum of the low-doped specimen — thinner and more rigid irregular wires — as compared to the highly doped specimen.

A liquid solution synthesis of single crystal germanium quantum wires

A liquid solution synthesis for preparing single crystal germanium quantum wires is presented. The synthesis is based on the reduction of GeCl 4 and phenyl—GeCl 3 by sodium metal in an alkane solvent at elevated temperature and pressure. The synthesis produces single crystal Ge quantum wires of 70–300 Å diameter and up to 10 μm in length. With an excess of sodium in the reaction, the diameter of the wires is reduced to below 50 Å. Implications for the chemical control of both size and shape are discussed.

Wire-like island growth of epitaxial NiAl on AlAs by molecular beam epitaxy

Laterally aligned ultrafine single-crystal metal wires were first attained in the epitaxial growth process of NiAl on AlAs at high temperature.

Large Barkhausen effect of 6.3% SiFe single crystal wire

A sensitive and stable large Barkhausen effect was observed for 4-8% SiFe single-crystal wires. New pulse generator elements were constructed using 6.3% SiFe single-crystal wire which has nearly zero magnetostriction ( lambda /sub 100/). Pulse voltage were induced at a pickup coil for an applied AC field of more than about 0.2 Oe with frequencies of 0.01 Hz to 10 kHz. The high reliability of pulse generation characteristics against disturbance stresses was estimated for the 6.3% SiFe single-crystal wire. A reliable and accurate magnetic-field sensor developed on the basis of the proposed technique is described. >

MICROPROCESSOR CONTROLLED REMOLDING OF FIELD EMITTERS

A fully automatic, microprocessor controlled remolding system is presented. I t has been tested with -oriented tip-shaped tungsten field electron emitters. Many of its features and capabilities wouldibe equally relevant for other types of field electron emitters or in the context of field ion emission. I t can produce with high reliability a single spot emission pattern, which is most frequently desired because i t normally represents highest brightness. Field emission tips that were in such bad conditions tha t they would normally have to be replaced could be restored to good emission by the system. Some results on the remolding process itself which were obtained through the use of the remolding system are presented. 1. I n t r o d u c t i o n Field emission electron guns with pointed emitters fabricated from single crystal tungsten wires electrolytically etched to a very fine tip can reach the highest brightness values (> 1 0 ~ ~ / c r n ~ s r ) known. However, a brightness as high as this is not normally achieved with a newly etched tip. Moreover, the brightness often deteriorates after a certain operation time. In these cases, the t ip can be restored to good emission conditions by a so-called remolding process, achieved by the simultaneous application of a high electric field and high temperature to the tip / I / . However, up to now remolding facilities are not used in commercial instruments due to the rather cumbersome and unreliable application of the process when performed manually. These difficulties have been described in detail in a previous paper / 2 / . In the same paper we presented a microprocessor aided remolding system. This system has now been developed into a fully automatic microprocessor controlled remolding system. It has been tested with -oriented tungsten t ip emitters. However, many of its features and capabilities would be equally relevant for other types of field electron emitters or in the context of field ion emission. 2. T h e m i c r o p r o c e s s o r r e m o l d i n g s y s t e m ( h a r d w a r e ) A block diagram of the system is shown in Fig. 1. All components are controlled by the microprocessor, which is a t present a 16 bit microprocessor Intel 8088. Any other suitable processor could be used as well. Two externally programmable high tension cascades provide the remolding and field emission voltages resp.. Both cascades are controlled by the microprocessor via 12 bit D/A-converters. The actual switching of the high voltages to the field emitter is performed by two high voltage Reed-relays. The total field emission current is measured by a sensing resistor in the extraction voltage lead. The resulting voltage drop is amplified and then Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986776 C7-454 JOURNAL DE PHYSIQUE

Direction of the cross-section ellipse and secondary slip in extended copper single crystal wires

A simple geometrical relation is found for predicting the orientation of the elliptic cross section of extended copper single crystal circular bars. Its sensitivity to the relative amount of secondary slip is used to obtain an estimate of the latter quantity. A particular orientation of the X-ray beam is found to reduce asterism of Laue photographs. On a trouve une relation geometrique qui permet do calculer d'une facon simple l'orientation de la coupe transversale elliptique de bars circulaires de cuivre monocristallin deformes par traction. La sensibilite de cette orientation en regard de la quantite relative de glissement secondaire a ete utilisee pour obtenir une estimation de cette quantite. II se trouve qu'une orientation speciale du faisceau des rayon X reduit l'asterisme des photographies Laue.

Decomposition of nitrous oxide on palladium crystal planes

The catalysed decomposition of N2O in the range 830–1000 K and 0.05–1.0 Torr (1 Torr ≈ 133 Pa) has been examined on Pd single-crystal surfaces and polycrystalline wires and compared with earlier work. The relative reaction velocities at 1000 K and 0.2 Torr are as follows: 0.01 cm diameter wire, 21; 0.025 cm diameter wire, 10.9; (610) plane, 2.2; (100) plane (thin disc), 1.8; (110) plane, 1.8; (100) plane (normal thickness disc), 1.7; and (111) plane, 1.0. Kinetic equations describing the first-order inhibition in oxygen pressure derived earlier by Eley and Knights for O2 chemisorbed as molecules have been rederived in terms of an adsorption–desorption reaction between chemisorbed oxygen atoms and ‘transitional’ or ‘subsurface’ oxygen atoms. The apparent activation energies for Pd (and for the earlier PdAu wires) have been discussed in terms of this mechanism. From the above, ‘stepsites’ on (610) furnish sites ca. 2.7 times more active than terrace sites on (100), while grain boundaries may well be responsible for the more active sites on the Pd wires. By implication, since pure Au is known to show negligible chemisorption of O2 at 1000 K, it seems possible that grain boundaries furnish the only active sites for N2O decomposition on that metal.

Size-induced deviations from Matthiessen's rule in gallium single crystals

Size-effect data are presented for gallium single-crystal wires oriented for current flow along the $b$ axis. The enhancement of the temperature-dependent resistivity is deduced with the use of straightforward assumptions concerning the common high $\frac{d}{{l}_{b}}$ behavior of all theoretical enhancement functions. Comparison is made with the theoretical curves of Sambles et al., wherein an angularly dependent specularity parameter is employed.

Resistivity anomaly in thin Bi wires: Possibility of a one-dimensional quantum size effect

Thin Bi single-crystal wires down to ~0.2 Μm in diameter were prepared by pressing liquid Bi into fine Pyrex capillaries. Electron microphotographs were taken to determine the diameter of the wires. Anisotropic magnetoresistance together with Shubnikov-de Haas oscillations were observed at helium temperatures. An anomalous drop in resistivity ϱ(T) was found at T < 50 K. Possible explanations are considered, including one based on the quantum size effect.

Measurements of quenched-in resistivity in thin single crystal tungsten wires

Abstract We have measured the electrical resistivities of a series of thin polycrystal and single-crystal tungsten wires annealed and quenched in both ultra-high vacuum and superfluid helium. For a given sample the quenched-in resistivities were very nearly the same for both ultra-high vacuum and superfluid quenches. We infer from this result that the quenched-in resistivities are not due either to quenohed-in helium atoms or to atmospheric contamination as recently suggested by Evans. On the other hand, the magnitude of the quenched-in resistivity varied somewhat from sample to sample. Measurements with an optical pyrometer suggest that this variation is not due primarily to differences in temperature of the samples just prior to quenching. Analysing both our data and other data recently published we conclude that impurities must play an important, but not yet adequately understood, role in determining the magnitude of the resistivity retained in quenched tungsten.

A Study of Remolding Process of Tungsten Tip with FEM and 3MV Electron Microscope

The remolding proxess of <110>, <310>, <111> and <100> oriented tungsten single crystal tips have been observed using FEM and 3 MV electron microscope. In the pressure of 10-10 Torr, the following planes {110}, {211} and {100} are enlarged at initial stage ofremolding process in all of the single crystal wires, then the ridges will form on the {111} and {310} planes and a spot is at last formed at and near the (310) plane. Field emission current also varied in three steps according to the migration process of which have activation energy of 1.9, 1.26 and 0.64 eV respectively. The activation energy for the blunting process of the spot thus formed is 2.3 eV, which means to be a defective crystal. By the formation of faceted surfaces upon heating the tip in the pressure of less than 10-9 Torr and the change in relative surface energy of planes around the {100} planes due to chemisorption of gases, a spot is formed on the {100} planese. Observations of remold tip with 3 MV electron microscope reveal dislocation along the wife axis, the ridge direction, the slip plane and the direction normal to surface. A spot radius measured by FEM is larger than the value obtained by 3 MV electron microscope.

Interaction of point defects with and on dislocations in electron-irradiated platinum

Abstract Polycrystalline and 〈100〉-oriented single-crystal wires of high-purity platinum have been irradiated with 2.5-MeV electrons at ∼20 K and above liquid-nitrogen temperature. Following each irradiation, measurements of the logarithmic decrement were taken as a function of temperature. In general, the irradiations resulted in pronounced pinning and depinning effects as well as anelastic effects due to the irradiation-produced interstitials alone. The former are interpreted as, on the one hand, pinning of dislocations by interstitials which arrive at the dislocations and are immobilized, and on the other hand, the annihilation of pre-existing vacancy pinners by interstitials that become mobile on the dislocations. Aa increase in the decrement at a temperature of ∼75 K was observed in both samples, and this increase is related to the onset of interstitial pipe diffusion which brings about the mutual annihilation of the interstitial and vacancy pinners. In addition to some structure in the background, a...

The influence of crystalline anisotropy on neck growth during the sintering of zinc

Polycrystalline zinc wires of two grain sizes and zinc single crystal wires of four orientations all of them 0.010 in. in diameter were grown. Pairs of wires from each of the six groups were sintered together at 365°C for 8 hr in a pressurized dry hydrogen atmosphere and the widths of the necks formed between pairs were measured. In the single crystal pairs, the neck width depended upon the relative orientation of the basal planes in the wire with the largest width up to 2.7 times as large as the smallest. In the polycrystalline samples those with fine grain size, of the order of neck width, had small necks, while those with coarse grain size, of the order of the wire diameter, had large necks. The observed variations in the width of the neck with relative orientation and with grain size must be caused by the anisotropy of those properties of zinc which determine the rate of material transport during sintering. For sintering by diffusional flow these are specific surface free energy and diffusivity, for sintering by evaporation and condensation they are specific surface free energy and vapor pressure. Since for these mechanisms approximate equations for the rate of neck growth exist, the maximum ratios of neck width due to the anisotropies of specific surface free energies, diffusivities and vapor pressures can be estimated. They are considerably smaller than the observed ratio of up to 2.7. The remaining material transport mechanism for the early stages of sintering is plastic flow which, in zinc, is highly anisotropic, since even at temperatures near the melting point the critical resolved shear stress for slip on the basal slip system is five to ten times lower than for slip on any other system. The measured neck width of the single crystal wire pairs can be directly related to the ease of basal slip. Those pairs which are oriented so that no basal slip is possible have small necks, while those which are favorably oriented for basal slip have large necks. The small neck width in the fine-grained wires must be due to constraint which inhibits basal slip. No such constraint exists in the necks of the coarse grained wires. Material transport by plastic flow must play an important role in the sintering of the zinc wires.

Pressure effect on the Fermi surfaces of Ag and Cu

Abstract The effect of pressure on the detailed shape of the Fermi surfaces of Ag and Cu has been investigated by a measurement of the transverse magnetoresistance of high purity single crystals at 4·2°K and 18 kilogauss under hydrostatic pressures up to 2·5 kilobar. No change in size of the ‘necks’ relative to the rest of the Fermi surface was observed in either metal. The precision was such that a change of about 0·2% per kilobar and 0·3% per kilobar could have been detected in Cu and Ag respectively. A theoretical estimate based on Ziman's model for the noble metals gives a change just less than the limits of detection in our experiment. A technique for obtaining single crystal wires with resistance ratios (R 300°K/R 4°K) in the range 1000–1500 is outlined.

The Surface Impedance of Superconducting Aluminium at 3 Gc/s

In an attempt to find anisotropy of the penetration depth in super-conducting aluminium, such as would confirm a non-local relation between field and current, high-frequency measurements have been made on single-crystal wires of different orientations. No strong evidence for anisotropy has appeared, and there is some evidence from the measurements of surface resistance that the crystals used may not have had the best orientations to reveal any effect. The behaviour of the surface resistance and reactance has been carefully studied very near the transition temperature, and the latter has been found to show a maximum just below Tc. There is qualitative but not quantitative agreement with the predictions of theory.

Determination of Contributions to the Residual Resistivity of Gallium at 4.2°K

Resistivity measurements have been carried out on single-crystal wires of high-purity gallium in the temperature range of 1.27\ifmmode^\circ\else\textdegree\fi{} to 43\ifmmode^\circ\else\textdegree\fi{}K, and also at 4.2\ifmmode^\circ\else\textdegree\fi{}K on gallium with added impurities, and with varying wire diameter. The last measurements indicated the electron mean free path to be 1.2 mm at 4.2\ifmmode^\circ\else\textdegree\fi{}K at a resistivity of about 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$ esu, implying that the effective number of free electrons per atom in gallium is 0.2. Diffuse surface scattering was found to be an important scattering mechanism in wires of high-purity gallium at 4.2\ifmmode^\circ\else\textdegree\fi{}K, and its contribution to the ratio of resistivity at 4.2 to 295\ifmmode^\circ\else\textdegree\fi{}K in a 1-mm diam crystal is 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$. The temperature measurements indicated that no resistivity minimum occurs, and also that along the $b$ axis, the lattice scattering contributes 0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ to the resistivity ratio. The effect of grain boundaries on the resistivity ratio is negligible. The presence of as little as one atomic part per million of impurities such as Cu, Ag, Zn, Tl, Ge, Sn, Pb, and As can be detected by residual resistivity measurements.

An abnormal after-effect in metals

A metal wire that has been subjected to plastic flow in torsion generally untwists at a rate such that equal strains occur in equal intervals of log time. It was found that this is true with single crystal and polycrystalline wires of iron and of zinc, both when coated with oxide films and when freshly cleaned of oxide. But the normal after-effect following plastic twisting of these wires is replaced by an abnormal after-effect (twisting instead of untwisting), which occurs as a transient, when an etchant is suddenly applied to an oxide-coated wire that has been twisted. The transient reversal of the after-effect was predicted on the basis that adherent oxide films on metals act as barriers to the movement of dislocations. When a wire is twisted, dislocations pile up beneath the oxide-metal interface; these escape through the surface when acid is applied. An after-effect in the normal direction but with a reduced rate follows the transient effect and is also accounted for by dislocations escaping that had previously been piled up beneath the surface.

The Influence of Electrolytes on the Mechanical Properties of Certain Metal Single Crystals

The contact of electrolytes has a marked effect on the mechanical behaviour of single crystals of cadmium and zinc which have been exposed to the air This is due to the disruption, or formation, of a thin surface film, probably of oxide or hydroxide. If ordinary single crystal cadmium wires are loaded so as to flow very slowly, the contact of solutions containing free cadmium ions increases the rate of flow a similar effect exists in the case of zinc treated with solutions of zinc salts If the surface of the wire is cleaned by preliminary thermal evaporation there is no such effect, establishing that it is due to a surface film normally present. Solutions of cadmium or zinc nitrate stop the flow and raise the critical shear stress appreciably, which is attributed to the formation of a surface film, probably of hydroxide With the cadmium nitrate there is a temporary increase of flow before the cessation, which is attributed to the cadmium ion, for there is no such effect with the zinc nitrate With polycrystalline wires there is no electrolyte effect The work described supports the view that a thin oxide or hydroxide film increases the mechanical resistance of single crystal wires, and that with cadmium and zinc wires such a film is normally present.

XXXVIII. Twin formation, in cadmium

Summary Experiments on the formation of twins in single crystal wires of cadmium under tension demonstrate the essentially discontinuous nature of the process of twin growth, and produce evidence in favour of a critical resolved shear stress law for twinning. Twinning can contribute to creep at room temperature but not at 90° a. A dislocation mechanism for the growth of a twin is discussed in some detail.

The Thermal Etching of Single Crystals of Cadmium

The heating of single crystal wires of cadmium in circumstances that permit free evaporation produces a development of crystalline planes. When the basal plane makes a small angle with the axis of the wire there is a characteristic formation of hexagonal pits, the bottoms of which are mirror-like basal planes: when the basal planes make larger angles, characteristic elliptical traces appear, reminiscent of those formed by mechanical extension. The thermal etching reveals, by the formation of steps, thin laminae existing in the unstrained wire: the thickness of these, of the order of 1 μ, is the same as the spacing of the glide planes revealed by mechanical extension. The etched figures, in general, indicate that the substructure revealed by straining is already present in the unstrained crystal. The secondary pyramidal faces revealed agree with the calculations of Stranski, Kaischew and Krastanov. The mirror-like basal planes are sufficiently large to permit the application of an optical method to reveal a scatter in their inclination to the axis. This method shows that there is a variation, round about 20 minutes, of orientation among the basal planes about the direction of growth of the crystals, which were grown by the method of Andrade and Roscoe and showed good single crystal properties by the usual criteria.