BaO Films Research Articles

GROWTH OF Ag ULTRAMICRO-PARTICLES IN BaO SEMICONDUCTOR FILM

The growth of Ag ultramicro-particles in BaO semiconductor film are investigated bv tran missive electron microscope. The fine growth of Ag ultramicro-particles in BaO film are discovered in the samples after proper heat treatment. We also contrasted the structure difference of single-layer-made and multilayer-made Ag + BaO films. The growth properties of Ag ultramicro-particles in Ag + BaO films with different Ag content are also studied. The Ag ultramicro-particles do not form maze structure in Ag + BaO film with high Ag content, but form isolated Ag particles. There are BaO layer of a few nm thick between every two Ag particles.

Interpretation of AES data of impregnated cathodes

Many surface studies of impregnated cathodes involve a measurement of the Ba 590, O 510 and W 170 Auger peak heights. This paper shows how these measurements plotted in terms of O Ba versus W Ba concentration ratios can be used to interpret experimental results on impregnated B-type cathodes. Data are presented from actual cathodes and simulated cathode surfaces using well defined BaO films on W. These results as well topographic data will be compared to various calculated models. It will be shown that an active B-type cathode consists of nearly a monolayer of BaO on W with a slight excess of O compared to Ba. This excess O, however, is associated with W rather than the BaO. Ageing increases the O Ba ratio and this takes place principally by the removal of Ba from the BaO surface, again with the excess O being associated with the W rather than the BaO. The work function increase during ageing corresponds only to the area-decrease of BaO. If the surface accumulates so much O that it exceeds the available empty Ba sites (i.e., all W is covered by Ba or O), then any additional O appears to sit on top of the remaining Ba and the surface will be poisoned. These conclusions are not only the result of work function observations but are also consistent with interatomic Auger analysis of BaO interactions as well as surface plasmon and ISS results. Toporgaphic data obtained from pore/impregnant regions are also in good agreement with calculated values of partial coverage of typical tungstate and impregnant residues.

Preparation and oxidation of a thin Ba film

A method of determining the thickness of vapor-deposited thin Ba films is described. The thickness is expressed in terms of numbers of Ba monolayers which were defined and characterized on W(100) and Ir(100) substrates using Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). The changes in the electronic properties of a 10-layer Ba film on Ir(100) during oxidation were studied using Low Energy Electron Reflectivity (LEER) measurements. When the oxidation of Ba was nearly complete (as shown by AES), the LEER patterns were similar to that found for a randomly oriented BaO layer with perhaps a partial layer of Ba on the surface. Additional O 2 exposure, however, resulted in the formation of a BaO crystal whose characteristic LEER patterns indicated a Fermi level position which suggested a high density of oxygen-vacancy donors and whose large electron affinity indicated non-stoichiometry at the surface. Further O 2 exposure resulted in a BaO-type surface dipole and oxygen vacancy donor densities more typical of BaO lattices. At this level of oxidation, a minimum work function of 1.4 eV was attained, which was identical to that of an active thin BaO film. Furthermore, AES indicated a Ba to O ratio identical to that of a BaO film. Any change from this stoichiometry through the addition of either oxygen or barium resulted in an increase in the work function.

Reaction products and subsequent thermal decomposition of Ba films exposed to CO, CO 2, H 2O and O 2

The reactions involving thin Ba films (monolayer as well as 10-layer films on Ir) and CO, CO 2, H 2O and O 2 have been studied using Auger Electron Spectroscopy (AES) and thermal desorption. Common to these reactions is the formation of BaO which is evident from the growth of the interatomic 68 eV (Ba 4d, 5p, O 2p) Auger line and decreases in 73 eV (Ba 4d, 5p, 6s) line. From CO and CO 2 exposures, BaC 2 has also been observed to form as indicated by the Auger peak shape in the carbon Auger lines. A quantitative analysis has been used to determine the chemical equation governing each reaction. The presence of BaC 2 in BaO has been observed to increase the work function to 2.0 eV (compared to 1.4 eV for an active BaO film). Although this poisoning is moderate compared to that resulting from BaCO 3 or Ba(OH) 2 formation, it requires higher temperature for reactivation. In addition to BaC 2, BaS has also been analyzed in terms of interatomic Auger transitions. The results showed that BaS like BaO exhibited such a transition indicating a charge transfer from Ba to S in the molecular bond. BaC 2 on the other hand showed no such transition.

Secondary emission properties of impregnated tungsten cathodes

The secondary emission properties of impregnated tungsten cathodes have been studied using high resolution Auger spectroscopy to correlate the local variations in secondary emission (δ) with composition of the surface. It is found that for a tungsten surface activated with approximately a monolayer of Ba and oxygen (such as is found over most of the surface of a normally activated dispenser cathode) δ max ∼ 2.2 with a first crossover energy ∼ 70 eV. Any improvement in δ on practical dispenser cathodes is due to either the high value of δ from impregnant in the exposed pores, or due to excess impregnant left on the surface by the surface preparation methods. The value of δ max from excess impregnants is ∼ 5.3 consistent with previous measurements on thick BaO films. From cathode to cathode the fraction of the surface area covered with excess impregnant varied considerably. Thus, the average δ from the cathodes was not very reproducible. Little or no difference in δ could be attributed to changes in impregnant composition. It was found that charging effects can limit the value of δ that can be obtained from the pore area exposed on the surface, unless the pore size is limited. Various types of surface treatments were tried to determine their effect on δ and the highest average δ over the cathode surface was obtained from surfaces that were gritblasted with Al 2O 3 as a final treatment after impregnation. The results obtained here are compared with previous work.

Poisoning and reactivation processes in oxide-type cathodes: Part II. Single crystal BaO films

The work described here is a continuation of the studies reported in part I of shelf-life poisoning and reactivation observed with mixed Ba/Sr oxide-type cathodes after exposure in a vacuum tube ambient. Emphasis in part II is placed on the identification and determination of the basic mechanisms involved in the individual processes that make up the overall effects observed in part I. In order to achieve this, single crystal BaO films deposited on an inert Ir substrate were studied with respect to poisoning and reactivation by the various individual gas components found in typical vacuum ambients. The electronic work function changes were separated into their components of surface dipole and bulk-type Fermi level changes using the analysis of shifts in the low energy electron reflection (LEER) patterns obtained with an incident electron beam. Corresponding compositional changes were correlated with the electronic changes using multiple Auger energy analysis and also by noting changes in the characteristic shapes of the LEER patterns. The results showed that poisoning during shelf-life takes place chiefly by O 2, CO 2 and H 2O. O 2 poisoning occurs mainly by neutralizing O-vacancy donors and is more of a bulk-type effect altering the Fermi level. CO 2 and H 2O, on the other hand, form partial monolayers of surface carbonates and hydroxides which alter the surface dipole, i.e. electron affinity. These electronic changes in band structure are verified by the compositional measurements indicating the presence of an adsorbed surface layer with CO 2 and H 2O but not with O 2. The surface layers have also been identified as the carbonates and hydroxides from characteristic LEER shapes as well as from comparison to dissociation temperatures and gas desorption results of part I. The poisoning rate for a 10 Langmuir exposure is about 1 eV change in Fermi level for O 2, about 1 eV change in electron affinity for CO 2 and about 1 3 eV change in electron affinity for H 2O. The actually observed vacuum ambient poisoning rates are in very good agreement with that expected from the above mentioned gas poisoning rates when combined in the ratios of the partial pressures of these gases found in typical vacuum ambients. Reactivation is largely a desorption of the gas adsorbed during shelf-life and this is usually completed for oxide-type cathodes when the reactivation temperature reaches ∼1200 K, a typical operating temperature. Reactivation can also be enhanced by choosing substrate materials which will aid in the generation of O-vacancy donors or in the formation of a Ba surface layer. Care must be taken though to avoid interaction effects such as the formation of an oxide residue at the interface which may impede the donor generation process.

Orientation dependence of work function: BaO on Ir

The dependence of work function (φ) on substrate crystal orientation is studied for monolayer BaO films on Ir. The substrate used was a recrystallized Ir sheet with a distribution of crystals exposed. Auger and φ measurements were made on individual crystals by use of a scanning Auger instrument and a thermionic emission microscope. Comparisons are made between the properties of films deposited with a cold substrate (static films), and films produced by allowing the substrate to intercept an incident flux of BaO while at a temperature of 1050°C. In the latter case, the properties are measured after a steady state condition is reached. It was found that BaO which was adsorbed on those planes that one expects to be atomically smooth (e.g., (100), (111), (110)) had higher φ than BaO adsorbed on planes for which an atomically rough surface is expected (e.g., (012)). Possible interpretations of these results are discussed in terms of existing models for the adsorption of alkali metals and alkaline earth oxides deposited (or adsorbed) on Si and other substrates. The dependence of φ on the concentration of a trace impurity of phosphorus in the Ir is measured, and explained in terms of a dipole model.

Low energy electron reflection (LEER) and electron affinity of MgO

LEER measurements have been made on epitaxial (100) MgO films. As was the case for previously reported measurements on BaO films, the reflection structure agrees with a Bragg reflection condition using a free electron dispersion law. Correlation of the reflection structure with LEED beam intensities verified that the LEER structure is primarily due to the (0, 0) reflection. Values of electron affinity χ obtained from LEER were consistent with values of χ obtained by measuring the threshold energy for secondary emission. These values were also consistent with estimates of χ in the literature.

Thermionic properties of BaO on iridium

The thermionic properties of BaO films on both single and polycrystalline Ir substrates have been measured using Auger, LEED, and thermionic emission microscopy to determine the chemical and structural effects that contribute to these thermionic properties. Measurements were made of the work function (φ) as a function of film thickness, and the temperature dependence of φ for different film thicknesses and substrate orientations. In addition, the effects of nucleation, carbon contamination, as well as evaporation and diffusion on the electron emission from the BaO-Ir system were examined, and quantitative data were obtained on the steady state coverage of BaO on an Ir surface at cathode operating temperatures. A comparison is made between the thermionic properties of the BaO-Ir system and other Ba/BaO-metal systems that have been used in cathodes.

Electronic and chemical behavior of oxygen in BaO films on Ir(100)

A new surface analysis technique involving low energy electron reflection (LEER) has been used to separate the electron affinity and position of the conduction band edge for BaO single crystal films epitaxially grown on Ir(100) substrates. AES and LEED were used in the study for characterizing chemical and structural effects. Active films have a work function of 1.3 eV with an electron affinity of 0.6 eV. Thermal activation was principally the result of the generation of O-vacancy donors by O diffusion into the substrate and not by evaporation of O from the coating. Donor generation could be retarded by the formation of an oxide diffusion barrier at the substrate interface or by pre-saturating the substrate surface region with oxygen. Electron bombardment desorption caused changes in surface stoichiometry which increased the worl function by increasing the electron affinity. Poisoning by oxygen took place by neutralization of O- vacancy donors until near intrinsic conditions occurred. Additional oxygen accumulates at the surface increasing the electron affinity. Since the Fermi level does not go below the mid-gap position, the formation of Ba-vacancies does not seem likely. Reactivation is possible but the initial donor density cannot be achieved due to O saturation of the substrate.

Determination of conduction band edge and electron affinity in surface potential measurements of BaO

A technique is described which utilizes low-energy electron reflections (LEER) to determine the electron affinity and to simultaneously locate the position of the conduction band edge with respect to the Fermi level at the surface. The method involves the use of very-low-energy incident electrons (<10 V) and subsequent analysis of relections using a kinematical approximation for the interference phenomena. The good agreement between the LEER analysis for thin epitaxial films of BaO and low-energy electron diffraction (LEED) lattice spacings indicates the validity of the kinematic approximation for this case. An electron affinity of ∼0.6 eV is obtained which is also in good agreement with previously assumed values. Examples of changes in LEER patterns due to changes in electron affinity are correlated with changes in surface stoichiometry from Auger spectroscopy results, while changes in conduction band edge are found to result from changing the temperature or altering the donor density. Because of the depth resolution which is in the range of five to six atomic layers, LEER can be a suitable complementary surface analysis technique to Auger and LEED studies.

Thermal stability of single crystal BaO films grown on (0001) rhenium face: G Ya Pikus and V F Shnyukov, Izv AN SSSR Ser Fiz, 37 (12), 1973, 2609–2612 ( in Russian)

Thermal stability of single crystal BaO films grown on (0001) rhenium face: G Ya Pikus and V F Shnyukov, Izv AN SSSR Ser Fiz, 37 (12), 1973, 2609–2612 ( in Russian)

Electron emission and related properties of amorphous thin films of mixed barium and silicon oxides

Abstract Co-evaporated thin films of SiO and BaO were prepared and used as the insulator material in MIM and MIMIM structures, where the metal M was Al, Ag or Cu. Electroforming processes, resulting in an increase in the circulating current after the application of a direct forming voltage, were slow when Al electrodes were used but quite rapid with Ag or Cu electrodes. For devices formed with Cu or Ag electrodes the peak circulating current observed was at a voltage of the order of 2.6-3.2 V. At this applied voltage the electron emission current rose sharply and reached values comparable with those measured from MIM structures using SiO as the insulator. The probable existence of high field regions in the insulator was further tested by investigating triode structures and the forming processes were found to be quite complex. Although the polyfilamentary model can explain most of the diode properties, a refinement to this theory is needed to explain the forming effects observed with triode structures and this will involve a more careful consideration of the role of oxygen, as well as the usually considered metal and vacancy diffusion.

The study of structure and work function of barium oxide films and the BaO double system on the (0001) face of rhenium

Abstract The structure of barium oxide films on the (0001) face of rhenium has been investigated by the techniques of low energy electron diffraction (LEED) and contact potential difference method in a wide range of coverages. The changes in surface structure of the BaO films have been observed after annealing at various temperatures. It was shown that the structure and orientation of the films depended essentially on the interaction of barium oxide with the substrate. The result of this interaction was equivalent to the oxidation of the (0001) face of rhenium. This fact was confirmed by experiments with the adsorption of BaO films on an oxidized substrate. The structures observed after heating to high temperatures (900°–1000°C) seem to be formed by the products of chemical interaction of barium oxide with rhenium (rhenates).

The study of structure and work function of barium oxide films and the BaO double system on the (0001) face of rhenium

Abstract The double systems BaORe (0001) and OBaRe (0001) have been investigated by LEED and contact potential difference techniques. It is shown that the diffraction patterns in both cases were identical to those obtained for BaO films on the (0001) Re face described in the preceding paper. The analysis of the changes of the work function during the deposition of barium on an oxygen covered substrate suggests the existence of two phase adsorption of oxygen on that crystal face. The results obtained are compared with data for double LiOW (110) and NaOW (112) systems.

Electron Spin Resonance Absorption in Evaporated BaO Films

Electron Spin Resonance Absorption in Evaporated BaO Films

Electron Emission from Metal-BaO Systems

A model is proposed to explain the electron-emission properties of metals coated with thin evaporated films of BaO. The model considers the BaO-metal interface to be rectifying, and the interior of the BaO layer to have a large density of donor sites. Experimental data from both this laboratory and the literature, pertaining to both thermionic and photoelectric emission properties, are shown to be consistent with predictions based on the proposed model. A striking enhancement is reported in the photoemission from BaO-coated cathodes that are either bombarded by electrons or heated. The theory developed explains the observed dependence of work function on BaO-layer thickness, the observed independence of emission work function on the work function of the substrate metals, and the effects of nonuniform BaO deposits on electron-emission properties.

684. Absorption bands in evaporated BaO films: T. Tomotika, J. Phys. Soc. Japan, 17 (5), May 1962, 872–873.

684. Absorption bands in evaporated BaO films: T. Tomotika, J. Phys. Soc. Japan, 17 (5), May 1962, 872–873.

Absorption Bands in Evaporated BaO Films

Absorption Bands in Evaporated BaO Films

Evolution of Gases and Ions from Different Anodes Under Electron Bombardment

With the aid of a bakeable mass spectrometer a study has been made of the material evolved from Ag, Cu, Ni, Mo, Ta, Ti, and W anodes exposed to oxide cathodes. Chlorine was found to be evolved from all the anodes during the first period of bombardment. No evidence of the dissociation of the thin films of BaO or SrO on the anodes was obtained, suggesting it does not play a significant role in oxide cathode poisoning. It is believed that the Cl evolution may be responsible for the early slump in emission commonly observed at low anode voltages from oxide cathodes. Evolution of O+ ions was observed from oxidized Cu, Ni, Mo, Ta, and Ti with efficiencies up to approximately 10−5 ions/electron for electron energies of 90 ev and current densities of 10 ma/cm2. This O+ evolution reduced considerably the dc emission capabilities of the oxide cathodes. It was found that heating the anodes from 700° to 1000°C for long periods of time does not always eliminate all evolution products caused by electron bombardment. Hydrogen was observed to be evolved from Ti and O+ ions were observed from formerly oxidized Cu, Ni, Mo, and Ta immediately after outgassing at temperatures less than 1000°C.