Bulk Oxide Formation Research Articles

Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.

Fundamental studies of III–V surfaces and the (III–V)-oxide interface

Work using photoemission with synchrotron radiation (8 eV < hv < 300 eV) to investigate chemisorption and the formation of thin oxide layers on GaAs is reviewed and preliminary correlations with thick oxides are made. An advantage of synchrotron radiation is that it allows examination of both core and valence electrons of the last few molecular layers of the solid. By studying both the core levels and the valence band as a function of oxygen exposure, insight can be obtained into the bonding of oxygen and into oxide formation on an atomic scale. It is found that oxygen can bind to the Group V elements on the (110) surface without the necessity of breaking III–V bonds. Thus, it is suggested that optimum bonding of oxides or other insulators to III–V surfaces will be through the Group V elements. The stability of the surface against the breaking of III–V bonds and the formation of bulk oxides is found to correlate very well with the heat of formation of the III–V compound. For example InP is more stable than GaAs. This appears to correlate with the ease of forming metal-oxide-semiconductor (MOS) or metal-insulator-semiconductor (MIS) type devices. Even when it is possible to bond oxygen to a surface without the necessity of breaking III–V bonds, Fermi level pinning is produced by submonolayer quantities of oxygen. This is associated with defects induced by the strain produced by oxygen chemisorption. The pinning position is near the conduction band minimum and near mid-gap for (110) InP and (110) GaAs surfaces respectively. A striking correlation is found between these pinning positions and those found for thick oxides on practical device structures. This suggests that the pinning mechanism may be the same for both types of interfaces. It is suggested that lattice defects play a key role in the pinning. A novel approach to forming the oxide or insulation layer for MOS or MIS structures is suggested.

Surface oxidation of nickel metal as studied by X-Ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to study the oxidation of polycrystalline nickel metal. The results indicate that the oxidation process takes place in three stages; associative adsorption of molecular oxygen, followed by the combination of oxygen atoms with surface nickel atoms and, ultimately, the formation of bulk oxide. At room temperature only the first two stages can be detected. For exposures below 1 L the O 1s photoelectron spectrum is considered to be characteristic of an associatively adsorbed oxygen species, but for exposures above this value evidence for the formation of a monolayer of “NiO” is suggested by the development of an O 1s peak at 529.9 eV. Incorporation of oxygen into the nickel lattice is observed at temperatures >500°K. The activation energy for this place-exchange process was estimated at 1.80±0.06 eV.

X-ray ? Electron investigation of the oxidation of iron in oxygen and water vapor

1. By means of x-ray photoelectron spectroscopy it is shown that oxidation of iron in dry oxygen (at pressures between 10−8 and 10−6 torr) leads to formation of Fe3O4 on the surface. 2. Two stages are found in the growth of surface oxide in oxygen: In the first, the thickness depends linearly on the time; in the second, logarithmically. 3. Exposure of iron to water vapor (PH2O = 10−8−10−5 torr) does not cause the formation of bulk oxide. Calculation of the degree of surface coverage by the dissociation products of water molecules and construction of kinetic graphs reveal the presence of strong interaction between the adsorbate molecules.

The thorium (100) crystal face: A study of its cleaning, surface structure, and interaction with oxygen and carbon monoxide

The cleaned Th(100) crystal face was studied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The various thorium Auger transitions have been identified and assigned. Chemisorption of oxygen and CO was monitored by LEED, AES, and work function change (WFC) measurements. The work function of thorium was found to decrease with the chemisorption of oxygen and CO. Continued oxygen adsorption leads to its dissolution in the bulk and subsequent oxide formation at the surface. The oxide may be ordered by heating up to 1000 K while heating to higher temperatures leads to dissolution of the oxide by oxygen diffusion into the bulk. The data suggest dissociative chemisorption of CO on the studied (100) surface.

Multilayer Oxide Films on Anodized Platinum

A voltammetric study is described of conditions governing the production and stability of anodically formed bulk oxide on Pt in dilute . The range of anodizing conditions covered in 1 or was: current‐density, up to 10 A/cm2; temperature, 11°–93°C; duration, up to 18 hr. The oxidizability of the Pt anode (with respect to formation of bulk oxide) was subject to considerable hysteresis. Provided a potential of about 2.15V, RHE was exceeded, electrode oxidizability seemed to be controlled largely by the amount of strain energy present in the Pt surface. The electrolytic formation and reduction of thick bulk oxide layers effected large increases in electrode roughness factor (up to 100X or more).