Showing papers in "Journal of Non-crystalline Solids in 1996"

Working hypothesis to explore novel wide band gap electrically conducting amorphous oxides and examples

Abstract: A working hypothesis for exploring optically transparent and electrically conducting amorphous oxides is proposed on the basis of simple considerations concerning chemical bonding. The hypothesis predicts that amorphous oxides composed of heavy metal cations with an electronic configuration of ( n − 1)d 10 ns 0 may be converted into transparent conducting amorphous oxides when doped by Li ion implantation or heating at temperatures below crystallization. Three new materials, amorphous Cd 2 GeO 4 , AgSbO 3 and Cd 2 PbO 4 , have been prepared as examples.

An infrared spectroscopic study of water-related species in silica glasses

Abstract: Infrared (IR) spectroscopic measurements were made on types I, III and IV silica glass specimens before and after they were subjected to heat treatments at 80–1150°C in low-pressure steam, ambient air and dry air. In addition to the standard silanol band at 3672 cm−1, IR features attributed to free, hydrogen-bonded and structurally bound molecular water and to silanol which is hydrogen-bonded to neighboring silanol groups, water molecules and chlorine impurities were found to exist even in glasses with maximum total water concentrations of less than approximately 500 ppm H2O by weight. The existence of these species and their behaviors during heat treatments suggest a specific mechanism for the reaction between silica glass and molecular water. In addition, it is shown that the glass-water reaction does not attain equilibrium instantaneously at temperatures below 750°C.

Analysis of OH absorption bands in synthetic silica

Abstract: The presence of bound hydroxyl (SiOH) in silica is well known to produce an optical fundamental absorption band at about 2.7 μm. For optical fiber applications the influence of the corresponding overtones and combination modes on the absorption spectrum are of significant importance. A literature review is presented which reveals uncertainties regarding the correct absorption band intensities as well as their spectral positions. We present precise data on spectral position and relative intensities of OH absorption bands in state of the art synthetic silica. Our investigations cover the influence of different manufacturing techniques, OH content, and a comparison of bulk and fiber data. With the knowledge of the conversion factors between the intensities of different OH absorption bands it is possible to predict the entire OH related transmission performance of an optical component by measurement of a single absorption band, e.g., the fundamental mode at 2.7 μm or the 1.38 μm band in the low loss range of optical fibers.

A structural model for phosphate glasses

Abstract: The structural component of the network-former oxide in phosphate glasses has a decreasing number of links between the PO4 tetrahedra. However, in addition to the common depolymerization process, further structural principles are in effect. In the present paper, it is proposed that all of the terminal oxygen atoms, including those doubly bonded within the PO4 branching groups, tend to coordinate a network-modifier cation. In a first range, that leads to an effect which can be called a re-polymerization based on modifier oxide additions. Due to the surfeit of terminal oxygen atoms in phosphate glasses with small metal oxide content, the modifier cations, Me, occupy places with high MeO coordination numbers surrounded by terminal oxygen atoms on MeOP bridges. The system tends to stabilize at the point at which all of the terminal oxygen atoms occupy MeOP positions. For even higher concentrations of metal oxide, the coordination number of modifying cations around terminal oxygens exceeds unity, and a modified random network ensues. The model is used for the explanation of the behaviour of packing densities and MeO coordination numbers. The phenomenon of drastic changes of glass properties at about 20 mol% metal oxide content has been related to a network reconstruction based on the change in the type of linkage between the metal cations and the PO4 middle groups.

Novel oxide amorphous semiconductors: transparent conducting amorphous oxides

Abstract: A working hypothesis to find wide gap oxide semiconductors was proposed on the basis of simple considerations. The hypothesis predicts that amorphous double oxides composed of heavy metal cations (HMCs) with an electronic configuration (n − 1)d10s0 are promising candidates for a novel class of amorphous semiconductors. Electrical and optical properties of three amorphous double oxides composed of the HMCs, a-AgSbO3, Cd2GeO4 and Cd2PbO4, were examined, following this hypothesis. It was found that when carrier electrons are generated via the formation of oxygen vacancies or doping of excess cations by ion implantation, these three wide band gap amorphous oxides show high electrical conductivities of 10−1 to 102 S cm−1 at ∼ 300 K and the conductivity remains almost constant down to 77 or 4 K for high carrier concentrations (> 1018 cm−3). This high conductivity originates primarily from a large Hall mobility of ∼ 10 cm2 V−1 s−1, which is higher by several orders of magnitude than that in amorphous transition metal oxides, Si:H and chalcogenides. A variety of chemical compositions for a novel oxide amorphous semiconductor are suggested.

Vibrational spectra and structure of heavy metal oxide glasses

Abstract: The infrared reflectance spectra at near-normal and off-normal incidence and polarized Raman spectra have been investigated to characterize the structure of PbOBi2O3Ga2O3 glasses. Infrared reflectance bands at 400 cm−1, 470 cm−1 and 610 cm−1 have been assigned to PbO, BiO and GaO related vibrations, respectively. The polarized Raman spectra were deconvoluted into seven bands which have been classified into four sets of modes: (1) depolarized low-frequency Raman modes at 30–70 cm−1, (2) strongly polarized Pb and Bi heavy metal vibrations at 120 to 140 cm−1: (3) polarized GaOPb, GaOBi, BiOPb, GaOGa and BiOBi bridging-oxygen modes at 300–600 cm−1; (4) non-bridging-oxygen modes at 600–700 cm−1. The optic vibrations observed in the series of glasses indicate that most cations behave essentially as network-formers. The low-frequency Raman scattering was analyzed on the basis of Martin and Brenig's acoustic model, from which some intermediate range structural information is derived. The structural correlation and structural units which determine the high frequency modes in these glasses are discussed.

Silica in aqueous environments

Abstract: The aqueous chemistry of silica (SiO2) is regulated by a number of coupled processes such as dissolution/precipitation, complexation to cations and anions in the aqueous phase and complexation to cations and anions at the particle-water interface. Further, uptake by biota is significant in parts of the hydrosphere. The dissolution of silica is strongly pH-dependent in that silicic acid (Si(OH)4) is formed at pH values ≤ 9. In more alkaline solutions, monosilicates (SiO(OH)3−), SiO2(OH)22−) and polysilicates are formed. Composition and thermodynamic stability of these polysilicates are still not fully understood. The surface chemistry of silica has been extensively studied throughout the years. Based on recent surface complexation models, thermodynamic data describing acid/base properties, as well as cation (metal ion) and anion adsorption/desorption reactions have been presented. The acidic properties of a silica surface have been described by an acidity constant for surface SiOH groups with pKa(intr.) = 6.8. Further, surface complexation constants for several metal ions have been published showing a strong correlation between hydrolytic properties of the metal ion in aqueous solution and at the SiO2-water interface.

Strong and fragile liquids — a brief critique

Abstract: The classification scheme of strong and fragile liquid relaxation behavior introduced by Angell is examined, and the relations between the different fragility parameters are derived and discussed. The relation between fragility and non-linear relaxation behavior within and below the glass transition temperature range is explored. It is suggested that the minimum value of the fragility parameter, m, is proportional to the quotient of the glassy state activation enthalpy and Tg, and implications of this interpretation are discussed. It is argued that TgΔCp(Tg) is an inappropriate measure of the thermodynamic contribution to the Angell strength parameter, D, because it contains the kinetic quantity, Tg.

Spectroscopic properties and energy transfers in Tm3+ singly- and Tm3+Ho3+ doubly-doped glasses

Abstract: Spectroscopic measurements and analysis of energy transfer processes for Tm3+ singly- and Tm 3+ Ho 3+ doubly-doped glasses pumped with 790 nm diode laser have been performed. The emission cross-sections and the net gain have been determined from the measured absorption spectra for both the ions on the basis of principle of reciprocity. Tm3+ singly-doped fluorozircoaluminate glass, which has a higher fluorescence efficiency at 1.82 μm and a longer upper state lifetime, is a material suitable for Tm laser compared with oxide glasses. The quantum efficiency of Ho3+ fluorescence of 5I75I8 at 2.05 μm for the Tm 3+ Ho 3+ doubly-doped glass is dominated by three energy transfer processes: Tm3+Tm3+ cross-relaxation, net Tm3+Ho3+ energy transfer, and energy transfer upconversion. The first process allows pumping efficiency to approach 2, and the second one contributes to population of the upper 5I7 laser level, while the final process gives rise to a sublinear increase in upper state population with pump power. Optimum doping levels for laser performance are easily predicted based on spectroscopic measurements and are found to be 2–8Tm3+ (1020/cm3) for Tm3+ singly-doped and 2–8Tm3+/0.3–1Ho3+ (1020/cm3) for Tm3+/Ho3+ doubly-doped systems, respectively.

Size dependence of acoustic and optical vibrational modes of CdSe nanocrystals in glasses

Abstract: The size dependence of resonant Raman scattering from acoustic and optical vibrational modes in CdSe nanocrystals (size less than 10 nm) embedded in a glassy matrix has been investigated. In the low-frequency Raman scattering range ( −1 ) a fine structure due to the confinement of acoustic modes is observed. The Raman line corresponding to the interaction with the optical modes (new 210 cm −1 ) shifts and widens with a decreasing of the particle size. These experimental results can be described by a single model based on the size dependence of the eigenvibration modes of spherical particles in a matrix.

Structural role of lead in lead silicate glasses derived from XPS spectra

Abstract: The X-ray photoelectron spectroscopy (XPS) spectra of lead silicate glasses with different PbO content were measured and analyzed and the structural role of lead in the glass is discussed. It was found that the binding energy of Pb4f 7/2 decreases with the increase of PbO content before the concentration of PbO reaches 40 mol% and is maintained at a low constant when the PbO content is higher than 40 mol%. It was inferred that the polarizability of Pb is concentration-dependent and grows with the increase of PbO content in the glass. This suggests an increase in the covalent degree of PbO bond, which accounts for the formation of the PbO 4 pyramid polymeric chain network. It is proposed that in the low PbO content region (PbO 2 is the main glass former and the three-dimensional network built by SiO 4 tetrahedra is still maintained, with PbO increase in the glass. PbO 4 pyramids tend to assemble together to form another structure network. In the high PbO content region (PbO > 40 mol%), the three-dimensional network built by SiO 4 tetrahedra is seriously destroyed; the PbO 4 pyramid polymeric chains connect together through SiO 4 tetrahedra to form the glass structure network skeleton.

High-quality polycrystalline silicon thin film prepared by a solid phase crystallization method

Abstract: We succeeded in fabricating high-quality polycrystalline silicon (poly-Si) thin films with no boundary from the bottom surface to the top, and achieved an extremely high electron mobility of 808 cm2/V s by a solid phase crystallization (SPC) method. This film was obtained by using a new nucleation layer with 1000 A wide single-crystalline grains embedded in a matrix of amorphous tissue. A poly-Si thin-film solar cell fabricated using this film as an active layer demonstrated a total area conversion efficiency of 9.2% (active area efficiency: 9.7%), which is the world's highest value for crystalline silicon solar cells fabricated below 600°C on metal substrates.

An XPS Study of Oxygen Bonding in Zinc Phosphate and Zinc Borophosphate Glasses

Abstract: X-ray photoelectron spectroscopy has been used to determine oxygen bonding in a series of x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.67) glasses. Curve fitting of the O 1s spectra leads to a quantitative measure of the bridging-to-non-bridging oxygen ratio which is shown to depend on composition according to a simple structural depolymerization model. A third peak, present in the O 1s spectra collected from y B 2 O 3 · (1 − y )Zn(PO 3 ) 2 (0.00 ≤ y ≤ 0.40) glasses, is due to oxygens which link borate and phosphate tetrahedra. The relative concentrations of POP, POZn and POB bonds are shown to be in good agreement with a structural model which assumes that borophosphate units (BPO 4 ) form when B 2 O 3 is added to Zn metaphosphate glass.

An XPS study of iron sodium silicate glass surfaces

Abstract: A series of (SiO2)0.7−x(Na2O)0.3(Fe2O3)x glasses (0.0 ≤ x ≤ 0.18) were prepared and investigated by means of X-ray photoelectron spectroscopy (XPS). The quantitative ratio [Fe2+]/[Fetotal], for each glass has been determined from an analysis of the Fe 3p spectra. For low Fe2O3 content both iron valencies are present, however, it was found that Fe3+ is the dominant species for high Fe2O3. From an analysis of the O 1s spectra, it was possible to discriminate between bridging and non-bridging oxygen atoms. It was found that the ratio of the non-bridging oxygen content to the total oxygen content increases with increasing iron concentration. It has also been shown that the non-bridging oxygen contribution to the O 1s spectra can be simulated by summing the contributions from the SiONa, SiOFe(II) and SiOFe(III) components present in the glass.

Quantitative infrared spectroscopic measurement of hydroxyl concentrations in silica glass

Abstract: Infrared (IR) spectroscopic analysis of high hydroxyl content silica glasses was compared with both weight loss measurements and nuclear reaction analysis (Helmich and Rauch, 1993) to obtain the molar absorptivity of the 3673 cm−1 hydroxyl band in silica glass Agreement between the resulting molar absorptivity values (764 ± 28 and 77 ± 6 1glass/molOHcmglass for weight loss and nuclear reaction the experiments, respectively) and those from the literature gives a best estimate of the molar absorptivity to be 775 ± 15 1glass/molOHcmglass Proportionality between the amplitudes of the fundamental OH stretching band at 3673 cm−1 and the combination SiOH bending-OH stretching band at 4522 cm−1 allowed the molar absorptivity of the combination band to be determined (153 ± 003 1glass/molOHcmglass) and comparison of the amplitudes and areas for both the 3673 and 4522 cm−1 bands gives the integrated molar absorptivities to be 10,100 ± 200 and 232 ± 5 1glass/molOHcmglass2, respectively Nonlinear absorbance loss versus weight loss per surface area curves observed for some batches of type III silica glass suggest that hydroxyl can be simultaneously removed via multiple mechanisms

Properties of sol—gel-derived Al2O3SiO2 glasses using Eu3+ ion fluorescence spectra☆

Abstract: Eu 3+ ion fluorescence spectra were compared in glasses of the Al 2 O 3 SiO 2 system prepared by a sol—gel process. The gels synthesized by the hydrolysis of Si(OC 2 H 5 ) 4 , Al(OC 4 H 9 sec ) 3 , and EuCl 3 ·6H 2 O were heated in air from 200 to 1000°C to form x Al 2 O 3 ·(100 - x )SiO 2 ( x = 0, 5, 10 mol%) glasses containing 1, 2 and 5 wt% of Eu 2 O 3 . Eu 3+ -doped Al 2 O 3 SiO 2 glasses had an intense fluorescence, two times larger than that of glass without Al 2 O 3 . The phonon sideband (PSB) peaks associated with the 7 F 0 → 5 D 2 transition of Eu 3+ ion were observed at 910 and 600 cm −1 , which were assigned to the SiO − and AlO − bonds, respectively. As the content ratio of Al 3+ to Eu 3+ ion decreases, the PSB intensities decrease and the band at 910 cm −1 disappears in samples of 1 and 2 wt% of Eu 2 O 3 . It is concluded that Eu 3+ ion is preferentially coordinated with AlO − bonds, where Al 3+ ions constitute the AlO 6 octahedra.

A 29Si NMR study of the sol-gel polymerisation rates of substituted ethoxysilanes

Abstract: 29 Si nuclear magnetic resonance spectroscopy has been used to generate reaction profiles for the acid-catalysed hydrolysis and condensation of various substituted ethoxysilanes which may be used in sol-gel processes. Experiments were performed at acid concentrations that enabled hydrolysis rate determinations to be made both with and without the influence of the condensation process. The relative rates of both hydrolysis and condensation reactions were generally predictable when the electronic and steric effects of the substituents were considered.

Spectroscopic properties of Eu3+ and Tb3+ ions for local structure investigations of fluoride phosphate and phosphate glasses

Abstract: The relationships between glass structure and spectroscopic properties of the doped ions Eu3+ and Tb3+ are of interest. Eu3+ and Tb3+ ions are used as structure-sensitive ions. The Ω t - parameters of Eu3+ were determined from absorption and emission spectra. The lifetime of the 5D0 state of Eu3+ was calculated from Ω t and compared with the experimentally determined lifetime. The electron-phonon coupling strength and phonon energy were obtained from phonon sideband spectroscopy. The f → d absorption transition of Tb3+ was used for determination of the nephelauxetic parameter. With increasing phosphate content the covalency between rare earth ions and surrounding ligands increases due to substitution of fluoride ions by oxygen ions having higher electron polarizability. This effect leads to increasing nephelauxetic shift, Ω 2 - parameter and electron-phonon coupling strength. In fluoride phosphate glasses, the diphosphate groups dominate the spectroscopic properties of the rare earth ions.

Sonochemical preparation of amorphous nickel

Abstract: A method for the preparation of amorphous Ni powder with particle size of about 10 nm is reported. The sonolysis was performed on neat Ni(CO)4 and on its solution in decane. The nickel nanoparticles were subjected to scanning and transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and SQUID magnetization measurements. These measurements confirm the amorphous state of the particles.

The hydrogenated amorphous silicon/nanodisperse metal (Simal) system - films of unique electronic properties

Abstract: The a-Si:H/metal systems ( Simals ) were prepared as a-Si:H/V/a-Si:H or a-Si:H/Mo/a-Si:H trilayers and multilayers of a-Si:H/Mo or a-Si:H/Ti by PCVD, sputtering and thermal evaporation, respectively. Structural investigations performed on these films by scanning force microscopy, cross-sectional TEM and X-ray techniques indicate that Simals contain metal nanoclusters and an intermixed silicon-metal phase. The temperature dependence of the conductivity is described by σ = σ o exp [−( T o T ) 1 2 ] with To ranging 100 K to 4 × 104 K. An electric field, typical of 100 V/cm applied to the samples in a coplanar electrode configuration results in an increase of the conductivity at room temperature from 10−6 to 1 (Ω cm)−1, which is reversible by annealing. This switching is accompanied by large fluctuation of the current. The electrical behavior of the Simals is compared to in-situ measurements on Mo-films on a-Si:H.

Structures of CeO2Al2O3SiO2 glasses

Abstract: Structures of glasses containing (20–30) mol% CeO2(16–23) mol% Al2O3(47–64) mol% SiO2 prepared at 1550°C for 3 h in an atmosphere of N2 have been investigated via the analyses of FT-IR and 27Al MAS-NMR spectroscopy. Results show that cerium ions act primarily as the glass modifiers. The number of NBO/T on average in the structures of CAS glasses increases with increasing ceria content. The aluminum ions in the structures of CAS glasses exist in both Al(6) and Al(4) coordination, and the fraction of Al(6) is more than Al(4).

Silver nanocrystals in silica by sol-gel processing

Abstract: Silver nanocrystal doped silica films were prepared by the sol-gel process. The sol was prepared from 1:0.12:12:0.2:6:7 molar ratios of Si(OC2H5)4:AgNO3:H2O:HNO3:C3nH7OH:C4iH9OH. The glassy, highly transparent film with high dopant concentration (silver/silicon atomic ratio = 0.12) was successfully prepared by the dip-coating method. After drying in air at 60°C for 30 min, samples were heat-treated, in air, at 300, 350, 400, 450, 500 and 550°C using 30–100 min soaking periods for each step in a cumulative heating procedure. Measurements on the films were made by ultraviolet-visible and infrared spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry and transmission electron microscopy. Mechanisms of silver colloid formation in the densified silica matrix with respect to the thermal treatment are discussed. To understand the formation of silver nanocrystals from the silver silicate network, the corresponding doped bulk gel samples were analyzed by X-ray diffraction and differential scanning calorimetry.

Characterisation of sol-gel-derived silica films

Abstract: Sol-gel-derived silica films were fabricated by dip-coating onto silicon and glass substrates. Film properties such as thickness and temporal stability were monitored as a function of dip speed, water:precursor ratio, sol aging time and time after dipping. Film behaviour was interpreted in terms of the dependence of hydrolysis and condensation rates on the interplay between sol pH and water:precursor ratio. Film thickness was found to increase by approximately a factor of two as water:precursor ratio increased from two to six. Film thickness also increased with sol pre-polymerisation time. The time to achieve film stability was decreased at higher water:precursor ratios. Surface quality was correlated with processing conditions.

Redox equilibria in glass

Abstract: Data for redox equilibria in silicate glass melts, for the ion couples: Fe2+/Fe3+, Cr3+/Cr6+, Ce3+/Ce4+, Sn2+/Sn4+ and As3+/As5+, can be related to the glass composition and indicate that increasing basicity favours the upper oxidation state. The relationships are simplified by making the correlation with the optical basicity of the glass rather than with its composition. The optical basicity model is concerned with the electron density environment provided by oxygen atoms for the hosted metal ions and, in principle, allows comparison between any oxidic environment. Comparison between glass melts and aqueous solution establishes a relationship between the redox behaviour in the melt and standard electrode potentials in aqueous solution. The applications of this relationship are discussed.

A review of critical experimental facts in electrical relaxation and ionic diffusion in ionically conducting glasses and melts

Abstract: Rapid progress has been made in the last decade in the area of the fundamental mechanisms of electrical relaxation and ionic transport in ionically conducting glasses. Many new experimental findings and computer simulations have become available during this period. These more recent developments should not be viewed as overshadowing the importance of earlier works. Instead, many earlier works, having laid the foundation and paved the way for the recent developments, are of immense importance. The phenomenologies that have been firmly established and the critical experimental facts which have been found will be the subjects of this review. One purpose of this work is to promote interest and debate among workers in this field. Hopefully, this will lead to some consensus among workers in the field on a list of critical experimental facts. For theories proposed in the future, these critical experimental facts can be employed collectively as an acid test of whether the theory is viable.

Theory of room temperature quantized resistance effects in metal-a-Si:H-metal thin film structures

Abstract: A theoretical model of room temperature quantized resistance steps is described in terms of movement of a free electron Fermi surface into a quantized k -space (associated with electron confinement in real space) under the influence of applied electric field. The model is in good accordance with experimental observations such as the non-periodicity in quanta and voltage space and gives very realistic values for the parameters of electron lifetime, Fermi wavevector and geometry. Room temperature quantization is observed when the value of free electron lifetime (∼ 10 −14 s) is matched with a small size for electron confinement (∼ 7 nm or less).

Absorption and mid-infrared emission spectroscopy of Dy3+ in Ge-As(or Ga)-S glasses

Abstract: Ge-As-S and Ge-Ga-S glasses doped with Dy 3+ were prepared and the emission characteristics of four infrared fluorescent emissions at 133, 175, 290 and 438 μm were evaluated High oscillator strengths and large intensity parameters of Dy 3+ originate from the highly covalent nature of the bonds in the glasses Measured lifetimes in the Ge-As-S glass are 728 and 4550 μs for the 6 H 11/2 → 6 H 15/2 (175 μm) and 6 H 13/2 → 6 H 15/2 (290 μm) transitions, respectively Quantum efficiencies of 328 and 872% of the above transitions compare well with the values reported previously Multiphonon relaxation rates in Ge-As-S and Ge-Ga-S glasses are4 orders of magnitude less than those in silicate glasses, which provide good opportunities for future laser applications

On the origin of p-type conductivity in amorphous chalcogenides

Abstract: An explanation for p-type conductivity in amorphous chalcogenides is given based on the role played by lone-pair electrons and charged defects. Valency alternation results in thermal excitation of free holes while free electrons are not created. Neutral dangling bonds are also a source of free holes. Existence of three-fold coordinated sites also explains the anomaly of the Hall effect in amorphous chalcogenides.

Apatite-mullite glass-ceramics

Abstract: Two series of glasses based on (2 − x )SiO 2 · x P 2 O 5 · Al 2 O 3 · CaO · y CaF 2 were studied. The values of x and y were varied from 0 to 1. The nucleation and crystallisation processes that occurred in these glasses were measured. Glasses in which x = 0.5 and y was greater than 0.5 crystallised to fluorapatite and mullite. Crystallisation occurred by a bulk nucleation mechanism involving prior amorphous phase separation. The glass with x = 0.5 and y = 0.5 was converted to a ceramic to give a material consisting of elongated fluorapatite crystals that had high fracture toughness.

Surface oxidation of an AlPdMn quasicrystal, characterized by X-ray photoelectron spectroscopy

Abstract: X-ray photoelectron spectroscopy (XPS) is used to determine the extent of oxidation of each of the three metals which comprise a quasicrystalline alloy. A single-grain sample, oriented with the fivefold axis perpendicular to the surface plane, and with nominal bulk composition Al 70 Pd 21 Mn 9 is used. The oxide which results from exposure to ambient gas at room temperature is compared with that which results from exposure to pure oxygen in ultrahigh vacuum at temperatures up to 870 K. XPS probes the near-surface region (ca. top 100 A), and shows that only the Al can be oxidized. The depth of the oxide layer depends systematically upon the conditions of treatment, but is always very thin — in the range of about 5–30 A. Taken together, the data suggest that the surface forms a thin, passivating, surface layer of aluminum oxide.