Conventional High Vacuum Research Articles

Cleaning of Si (100) surface by As ionized cluster beam prior to epitaxial growth of GaAs

It has been found that the exposure of a Si (100) surface to an As ionized cluster beam (ICB) is effective in the preparation of the surface prior to epitaxial growth of GaAs under conventional high vacuum conditions of 2×10−5 Pa. This process is achieved at a temperature as low as 600 °C. A clear 1×2 or 2×2 reflection high energy electron diffraction pattern observed after the procedure indicates good ordering of the sample surface. The cleaning process is attributed to chemical and physical sputtering by As ICB in the first stage and to the subsequent As termination of Si dangling bonds. It has been found that the optimum preparation conditions are an accelerating voltage of the As ionized cluster beam of 1.3 kV and a substrate temperature of 600 °C. GaAs films deposited on As ICB treated Si (100) substrates show good crystal quality with single domain structure.

Photoluminescence spectra of Si1-xGex/Si quantum well structures grown by three different techniques

Photoluminescence (PL) spectra of Si1-xGex/Si multiple quantum wells have been measured at 4.2 K for the samples grown by three different techniques; conventional molecular beam epitaxy (MBE), gas-source MBE, and ultra high vacuum chemical vapor deposition (UHV-CVD). Only in the case of conventional MBE, strong emission bands appear about 80 meV below the band gap of Si1-xGex. These strong emission bands disappear after the annealing at 800° C. From the dependence of the PL intensity on the excitation power, strong emission is considered to be due to some recombination center. On the other hand, in the case of gas-source MBE and UHV-CVD, the strong emission bands are undetectable, although the band-edge PL lines of Si1-xGex are clearly observed. There is no significant change in the PL spectra after the annealing. The origin of the strong emission band is considered to be defects which are characteristic of conventional MBE.

Detector strategies for environmental scanning electron microscopy

Environmental SEM operate at specimen chamber pressures of ∼20 torr (2.7 kPa) allowing stabilization of liquid water at room temperature, working on rugged insulators, and generation of an environmental secondary electron (ESE) signal. All signals available in conventional high vacuum instruments are also utilized in the environmental SEM, including BSE, SE, absorbed current, CL, and X-ray. In addition, the ESEM allows utilization of the flux of charge carriers as information, providing exciting new signal modes not available to BSE imaging or to conventional high vacuum SEM.In the ESEM, at low vacuum, SE electrons are collected with a “gaseous detector”. This detector collects low energy electrons (and ions) with biased wires or plates similar to those used in early high vacuum SEM for SE detection. The detector electrode can be integrated into the first PLA or positioned at any other place resulting in a versatile system that provides a variety of surface information.

Principles of Low-Vacuum SEM

Only recently it became possible to expand scanning electron microscopy to low vacuum and atmospheric pressure through the introduction of several new technologies. In principle, only the specimen is provided with a controlled gaseous environment while the optical microscope column is kept at high vacuum. In the specimen chamber, the gas can generate new interactions with i) the probe electrons, ii) the specimen surface, and iii) the specimen-specific signal electrons. The results of these interactions yield new information about specimen surfaces not accessible to conventional high vacuum SEM. Several microscope types are available differing from each other by the maximum available gas pressure and the types of signals which can be used for investigation of specimen properties.Electrical non-conductors can be easily imaged despite charge accumulations at and beneath their surface. At high gas pressures between 10-2 and 2 torr, gas molecules are ionized in the electrical field between the specimen surface and the surrounding microscope parts through signal electrons and, to a certain extent, probe electrons. The gas provides a stable ion flux for a surface charge equalization if sufficient gas ions are provided.

Alloying Effects on Hydrogen Gettering Properties for Zr-Al System

This paper discusses alloying effects on absorption and desorption kinetics of deuterium for Zr-Al alloys, studied with mass analyzed thermal desorption spectroscopy using a conventional high vacuum system. It is found that the absorption rate of deuterium was proportional to the 1/2 power of deuterium gas pressure. On the other hand, the desorption process obeyed the second order kinetics with respect to the amount of absorbed deuterium. The temperature dependence of the rate constants revealed that the activation energies for both the absorption and desorption processes were lowered by the increase in the Al content in the alloys. Through potential diagrams for the absorption and desorption of deuterium, it was also found that the heat of deuterium (hydrogen) solution decreased with increasing Al composition. In addition, the x-ray diffraction spectroscopy showed the formation of a Zr{sub 4}Al{sub 3} phase in the Zr{sub 3}Al{sub 2} sample owing to repeated absorption and desorption cycles. The results suggest that the electronic factors, for example, work function, electron density, d-band character and so on, play an important role for the alloying effects rather than crystallographic structures.

Evolution of Electrical Properties with Thermal Annealing for Seeded Heteroepitaxial InN Thin Films

ABSTRACTTwo related studies of the effects of elevated temperature annealing on reactive rf magnetron sputtered InN films have been conducted. In the first study, thin films of InN were deposited and annealed in 5 mTorr of nitrogen gas using a conventional high-vacuum (HV) magnetron sputtering system. Films were grown at 100°C and following annealing at an elevated temperature (Ta) for 4 hours were removed from the system for physical characterization. The physical properties of the annealed films improved with thermal treatment and were strikingly coincident with the properties of a second set of films grown at Ta. In the second study, films were deposited at 400°C in 5 mTorr of nitrogen gas using an ultrahigh-vacuum (UHV) sputtering system. Samples were annealed (at 550°C) and electrically characterized under UHV conditions using a custom designed annealing furnace with an integral Van der Pauw probe. Contrary to expectations, the carrier concentration of these films showed a steady decrease with extended annealing and the carrier mobility nearly doubled after 15 hours of treatment.

Pulsed Laser Deposition and Atomic Scale Characterization of Perovskite Oxide Films

ABSTRACTPerovskite oxide films inclusive of high Tc cuprates were fabricated by pulsedlaser deposition (PLD) under both conventional Po2(0.01∼1Torr) and high vacuum(≤10−6 Torr) conditions and their surface morphology, crystal quality, and conductivity were evaluated. Under optimized conventional PLD conditions, we obtained high quality YBa2Cu3O7−δ (YBCO) films that showed clear energy gaps and atomically flat surface image in the small area (4x4nm) STM/STS measurements. AFM and SEM ana!yscs on wider film surfaces (∼1x1μm), however, revealed granular structure of about 250nm grains with their root mean square roughness of 5nm.Under high vacuum PLD, i.e. laser MBE conditions, not only epitaxial but also two dimensional growth has been verified for such films as SrTiO3−x, SrVO3−y, and (AE)CuO2−z(AE=alkaline earth metal) by the RHEED observation of fine streak patterns and intensity oscillations. AFM demonstrated atomically flat surfaces of these films. Oxgen or nitric oxide pressure at the laser MBE growth gave crucial effects on the electrical conductivity of SrTiO3−x. and predominant crystal phase of SrCuO2−z.Insulating BaTiO3 films with atomically flat surfaces could be fabricated by both conventional and high vacuum PLD conditions. RBS channeling measurements revealed very high epitaxial quality (χmin−2%) of our SrTiO3 and BaTiO3 films. The factors controlling the surface morphology and electrical properties of perovskite oxide epitaxial films,are discussed.

Borazine adsorption and decomposition at Pt(111) and Ru(001) surfaces

The adsorption and decomposition of borazine (B 3N 3H 6) was examined over the substrate temperature range 140–1200 K on Pt(111) and Ru(0011) surfaces. The surface reactivity was examined by conventional ultra high vacuum (UHV) methods that included Auger electron spectroscopy (AES), thermal desorption mass spectroscopy (TDMS), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED), and low energy ion surface scattering (LEISS). For room temperature adsorption, borazine adsorbed irreversibly with effective BN coverages of 0.36 monolayers (ML) on Pt(111) and 0.15 ML on Ru(001) (determined from attenuation of substrate AES signals). The drastic difference in room temperature borazine saturation coverage between the two substrates is attributed to a significant degree of dissociation on Pt(111) compared to the Ru surface. For room temperature adsorption, the TDMS data indicated only hydrogen (H 2) desorption from both surfaces over a very broad temperature range (e.g., 350–900 K). Borazine exposures of > 5.0 × 10 −5 Torr. s at either surface at 1000 K produced ordered close-packed hexagonal BN overlayers with saturation BN surface coverages of 1.22 ML on Pt(111) and 1.16 ML on Ru(001). LEED from the BN M ( M=Pt(111) or Ru(001)) surfaces prepared in this manner gave hexagonal spot splittings of approximately 10th and 12th order, respectively. This is consistent with the formation of a coincidence lattice h-BN overlayer on both substrates. Producing thicker BN overlayerrs by thermal exposures in the high vacuum regime was not possible. The evidence for h-BN overlayers is derived from EELS and LEED data from the annealed BN M surfaces. Multilayer adsorption/desorption of borazine was observed to occur at ∼ 150 K for initial substrate adsorption temperatures of 140 K.

In environmental Scanning Electron Microscopy, the secondary electron signal reveals surface information not accessible by conventional backscattered electron signals

Environmental scanning electron microscopes (ESEM) operate at high as well as at low vacuum (<2.5 kPa: ~20 Torr) but utilize all advantages of conventional high vacuum SEM (large specimen size, high depth of focus and specimen tilt capability, TV-rate scanning for imaging dynamic events). They have the advantage of imaging wet specimens as well as insulators without the need of any specimen preparation. Previously, environmental scanning microscopy was restricted to the BSE signal collected with BSE detectors. SE signals cannot be collected with the Everhart-Thornley detector because it cannot operate at low vacuum. Using positively biased electron collectors, it is now possible to collect an SE signal. However, the origin and quality of this signal need to be further characterized.An ElectroScan ESEM was used equipped with SE and BSE detectors and operated at 7-30 kV with partial water pressures of 0.1-2.5 kPa (∼1-20 Torr).

History of the modern cryopump

Fast clean pumpdown to 10−7 Torr range from high crossover pressure was a luxury a decade ago. Today the manufacturing/equipment engineer has at hand reliable, economical, and maintainable cryopumps alternative to conventional high vacuum pumps. This paper traces the nonliquid cryogen, mechanical refrigeration supported pump package from its inception to acceptance within IBM.

Characterization of Ag Wire Brush/Cu Slip Ring Electrical Contacts Rotating in Wet CO<inf>2</inf>

Current-carrying silver wire brushes were run on the same track against a rotating copper slip ring in a conventional stainless steel ultra high vacuum system. The experiments were performed after sputter cleaning of the slip ring in a H 2 0-CO 2 ambient held at one atmosphere of pressure. The electrical contact resistance and the frictional coefficient were measured in situ. Subsequently, scanning electron microscopy, electron probe microanalysis, and elemental mapping studies were carried out ex situ on the slip ring, brush end, and wear particle surfaces. Both low (50 mA) and high (40 A) currents were used for comparison. Interestingly, it was found that not only was silver transferred to the copper slip ring, but copper was also transferred to the wire brush ends. Moreover, the wear particles were mainly of two types, one being rich in silver and the other rich in copper, the relative concentrations depending on the current flowing through the contacts. These results demonstrate clearly that mechanical mixing of brush and slip ring material and the incorporation of both materials in wear particles play important roles in rotating current-carrying electrical contact phenomena.

Quantitative recovery of volatiles from fats and oils by combined high vacuum degassing and thin film molecular distillation

AbstractAn apparatus is described which in one unit combines conventional high vacuum degassing on cold finger with falling film molecular distillation. The upper part of the cold finger is for the collection of the most volatile substances, while the lower part is for the condensing of substances which are molecular distilled from a wiped falling film. All the connections in the glass apparatus are made from Teflon or Viton‐O‐rings, and the turning of the rotor in the molecular distillation part is carried out by means of a magnetic coupling. In this way, the risk for contamination is reduced greatly. As all volatile substances after the treatment of the oil are on the same cold finger, the collection can be made by a high vacuum distillation into a small glass chilled with liquid nitrogen. In the bp range of 40–450 C, the apparatus gives recoveries of volatiles in or below the ppm range between 71–114% with coefficients of variance between 5–23%. The recoveries are found with internal standards added before treatment of the oil. Below 70 C, the recoveries drop to some extent. The volatiles are substances with different kinds of functional groups, e.g. hydrocarbons, aldehydes, ketones, acids, esters, lactones, and sulfur containing substances.

Tools for the Industry

A Selection of Equipment of Use in the Production and Maintenance of Aircraft, Missiles, Space Vehicles and Components. A range of five new Scot‐Vac high vacuum resistance furnaces for heat treatment and brazing of high purity steels and aerospace alloys incorporate an adjustable gas quenching system which can reduce cooling time to about half that of conventional high vacuum furnaces. This rapid quenching system makes it possible to use the furnaces for hardening tool steels and can be employed to effect considerable reductions in cycling times for brazing operations. The new range has been developed and manufactured by Vacuum Engineering (Scotland) Ltd., Motherwell, Lanarkshire, Scotland, for hardening tool steels and tempering, bright annealing and brazing stainless steels, nickel‐based alloys and titanium. They can also be used for surface degassing these materials and for sintering metal powders such as nickel, tantulum, tungsten and uranium.

Magnetization in Ni and Ni-Fe thin films

Nickel films have been deposited in conventional high vacuum, ultra-high vacuum, and in controlled contaminant, ultra-high vacuum environments. High field saturation magnetization M s measurements on the films made in high vacuum and an oxygen-contaminated ultra-high vacuum system show that they posses low values of M s for film thicknesses of 1000 A. The normal moments observed for ultra-high vacuum deposited films lead to the conclusion that the low M s values are associated with the oxygen and are caused by a true loss of effective magnetization rather than coupled superparamagnetism or a ripple blocking field. Supplementary torque and ferromagnetic resonance studies on the Ni and two different Ni-Fe compositions also show that the grain boundary perpendicular anisotropy models are not particularly relevant for films made under such nominal technical vacuum conditions.

The Initial Stages of Growth of a Metal Film on a Single-Crystal Metal Substrate

The initial stages of growth of copper, nickel, and cobalt, deposited in ultrahigh vacuum onto a clean (111) silver single-crystal film have been studied by reflection high-energy electron diffraction in situ. Oriented nuclei and single-crystal films (with double positioning) were obtained for all substrate temperatures and thicknesses. No signs of a pseudomorphic growth were detected even in the thinnest deposits. The detection limit of the deposits was 0.05 Å. The sticking coefficient of the first metal nuclei on the substrate was determined as a function of substrate temperature. The present results differ considerably from those obtained previously in conventional high vacuum, demonstrating the influence of contaminants in the latter.

The use of amplitrons as microwave amplifiers in high power radar systems with frequency agility†

ABSTRACT There are two important problems that arise in the use of amplitrons, as broadband microwave amplifiers in high power radar systems with frequency agility. The first one is to modify the conventional high vacuum tube modulator to obtain proper operation with good efficiency in case of very high power levels. The second one is to modify the modulator circuit by increasing its output impodance and thus limiting the poak current variations in the transmitted frequency band. In this paper analyses are made of both these problems, some practical results are given, and the implication in actual transmitter design is examined.

Reaction between Molybdenum Disulphide and Water

MOLYBDENUM disulphide, MoS2, undergoes oxidation in bulk at temperatures above 450° C, which may be demonstrated1 by gravimetry. However, the substance must ordinarily be covered with a surface layer of oxide, since heating samples in a good vacuum leads to the evaporation of molybdenum trioxide. It is possible that such layers are involved in the frictional transients observed2 when molybdenum disulphide is used as a lubricant in wet atmospheric environments. It is germane to inquire as to the origin of the oxide layers, and we wish to comment on the possible extent of the hydrolytic decomposition of molybdenum disulphide to yield hydrogen sulphide and molybdenum dioxide. Haltner found2 that when an MoS2 film, supported on certain metals, and particularly copper, was exposed to rubbing and shearing forces in a wet atmosphere, hydrogen sulphide could be detected. If no metal is present, however, the reaction appears to be different, for Cannon3 found that the gravimetry of adsorption of water on fine particle molybdenum disulphide showed a strong and extensive irreversible interaction at 0° C. The latter also found that for very finely dispersed samples, if the specific surface area, rather than the mass of the solid, were taken as the extensive variable, the observed gravimetry conformed most closely with the reaction : rather than a hydrolysis. While no search for hydrogen evolution was made in the earlier experiments1–3, an examination of the thermal behaviour3 of the putative oxysulphide indicated that a temperature of 350° C could be attained before any appreciable loss in weight was observed, and that this loss appeared to be associated with the formation of sulphur dioxide rather than hydrogen sulphide. It thus seemed likely that the reaction of molybdenum disulphide with water, studied in the absence of metals under conventional high vacuum conditions, was not a hydrolytic process of sulphide to oxide plus hydrogen sulphide, but a net oxidation. We have now extended the work to higher temperatures, with the following interesting results. The water adsorption work was repeated at 180° C, 20µ water vapour pressure, in quartz microbalance equipment like that described before1,3. The surface-area limited reaction with a permanent weight increase proportional to the specific surface area was seen again. 20 g samples of molybdenum disulphide (Σ = 15 m2/g) were then exposed to 20 mm water vapour at progressively higher temperatures between 200° and 500° C. The experiments were conducted in a quartz envelope to permit pre-degassing the solid. Pressures were monitored through the seven-day holding period of each of the experiments and gas samples were withdrawn for mass spectrometric study after one day each at various temperatures. The results are summarized in Table 1.