Applications of Surface Science最新文献

This paper reviews a theory that justifies “lattice match” assumptions in epitaxy and the application of the theory to epitaxy at {111}fcc/{110}bcc metal interfaces. The basic assumptions of the model are (i) that the adatom-substrate interaction has the periodicity and symmetry of the substrate surface, (ii) that the overlayer is crystalline, (iii) that the adatom-adatom interaction is harmonic and (iv) that the ideal epitaxial configurations are the same as for rigid half crystals. The theory correlates epitaxy with lattice match and predicts the occurrence of the Nishiyama-Wassermann and Kurdjumov-Sachs orientations in thick overlayers with astonishing accuracy. In thin overlayers there is good qualitative agreement with empirical data. The theory stresses the need for quantifying the Fourier coefficients.

Amorphous silicon has been deposited at high rates using an RF-excited hollow cathode. Films of 0.6 μm have been formed in 6 min, thus overcoming an important barrier to economic fabrication of amorphous silicon devices. In addition, the films are resistant to abrasion, which may be a consequence of the extremely high ion bombardment which the growing films are subject to. Infrared spectra have confirmed that the bonding of hydrogen is predominantly in the preferred monohydride form, without any external substrate heating.

The effects of processes used in making thin film ITO/CdS/CdTe/Au heterojunction solar cells have been investigated. In particular, air annealing the ITO/CdS/CdTe layer is shown, from spectral evidence, to produce a heterojunction. XPS evidence shows that this heating forms CdO and TeO₂ on the CdTe surface and that these are removed by a KOH etch. A further bromine/methanol etch leaves a Te-rich surface which forms an injecting contact with the Au top electrode into which the Te mixes.

The chemisorption of oxygen at room temperature on polycrystalline zirconium has been studied using AES and ESCA as a function of sample pretreatment. It is shown that annealing at high temperature greatly reduces the ability of zirconium to chemisorb oxygen but that its activity can be restored by argon-ion bombardment. This suggests that chemisorption may be site specific and that the concentration of these sites can be manipulated by varying sample pretreatment.

Three different methods of oxidation - thermal, TCE, and anodic — were applied to n-type (111) silicon 10 ohm cm resistivity samples. MOS test samples were fabricated and their interface state properties were characterized by C-V and AC field effect techniques. From C-V measurements the interface state density at mid gap was found to be less in TCE (2 × 10¹⁰ cm⁻² eV⁻¹) and anodic (1 × 10¹⁰ cm⁻² eV⁻¹) samples than in dry (5 × 10¹⁰ cm⁻² eV⁻¹) oxidized samples. The mobile charges were also less in TCE (2 × 10¹⁰ cm⁻²) and anodic (5 × 10¹⁰ cm⁻²) samples. Using the AC field effect technique, the frequency (2–100 kHz) and temperature dependence of field effect mobility, μ_FE, were studied. By applying Garrett's theory of frequency dependence of μ_FE, the relaxation times of interface states were found to vary from 30 to 3 μs in dry, 8 to 4 μs in TCE, and 4.5 to 1.5 μs in anodic samples in the temperature range 230 to 370 K. Using Rupprecht's theory of temperature dependence of relaxation times, thedominant energy levels, E_c - E_T, were found to be shallower in TCE (0.04 eV) and anodic (0.06 eV) than in dry (0.1 eV) oxidized samples. The capture cross-section of these samples was found to be small, in the range 10⁻²⁰ to 10⁻²¹ cm². In TCE and anodic samples the shallow interface state levels indicate stronger interactions between silicon and oxygen atoms at the interface. The observed low densities of interface states and mobile charges in these samples also show improved passivation of silicon.

The M_2,3M_4,5V Auger transition obtained from vacuum-cleaved Ge(111) surfaces has been debroadened to remove inelastic scattering, instrumental effects and core level lifetime broadening. The resultant spectrum was further separated into its component - M₂M_4,5V, M₃M_4,5V - transitions to obtain the transition density of states function (TDOS). For the 15.3 eV wide valence band, four definite peaks were found at -4.3, -8.0, -11.9 and -13.7 eV and the main TDOS peak at - 4.3 eV is at variance with the accepted bulk value of approximately - 2.7 eV (analogous to previous silicon results). Detailed AES measurements for excited-oxygen exposed cleaved germanium surfaces are also reported. For oxygen exposures in the range 0.1 to 10³ L a continuous shift was observed in the negative excursion of the M_2,3M_4,5V [N'(E)] transition and the derived TDOS curves showed that a number of oxygen-induced peaks appeared at well-defined energies, -7.7, -8.4, -11.3, -13.5, -14.9, -17.8 and -19.7 eV, below the vacuum level. Three of these peaks, -11.3, -14.9 and -19.7 eV, are consistent with GeO₂ formation. For the range of exposures studied, the valence-band spectroscopic data reported have been interpreted as showing the simultaneous existence of both atomic and molecular chemisorption states and their coexistence with GeO₂ formation during the early oxidation stage.

The excess-carrier charge transfer velocity, υ_cp, is an important parameter describing the efficiency of charge transfer across a solid-liquid interface. By using recent theory to analyze current-voltage curves of photoelectrochemical cells, we have been able to derive values of υ_cp for CdSe films in aqueous polysulfide electrolytes and measure the effects of varying the sulfur concentration. The parameter, υ_cp, is found to vary at short circuit conditions, from 1.2 × 10⁶ to 1.36 × 10⁶ cm s⁻¹ on increasing the sulfur concentration from 0 to 3.0M. In addition it has been found that υ_vp has only a weak voltage dependence described by an equation linear in voltage.

Crystalline films composed of mixtures (1) Fe, Fe₃O₄ and FeO, (2) FeO and Fe₃O₄, and (3) Fe₃O₄ were obtained by evaporating FeO powder (1) in a vacuum of 10⁻⁵ Torr; (2) in an oxygen atmosphere of 10⁻⁴−10⁻³ Torr; and (3) in an oxygen atmosphere of 10⁻¹ Torr, respectively. Films of γ-Fe₂O₃ were obtained by evaporating Fe₃O₄ powder in an oxygen atmosphere of 10⁻⁴ Torr, or by evaporating α-Fe₂O₃ powder in a vacuum of 10⁻⁵ Torr. The γ-Fe₂O₃ films gave only amorphous haloes in electron diffraction patterns. High resolution electron microscope images of the films showed crossed lattice fringes (whose spacings were assigned to the γ-Fe₂O₃ crystal) in areas 2 nm in extent. The formation of various iron oxide films is discussed in connection with experimental results from the oxidation of iron.

The Lifshitz theory is applied to calculate the long-range dispersion force between two semi-infinite half planes of dissimilar metals. It is shown that the asymptotic form of the force F, for large separation d between the half planes is F = C₁₂/d³, where an explicit expression is given for C₁₂ in terms of the two plasma frequencies ω_p2 and ω_p1 of the interacting metals. Attention is then given to the contributions to the interfacial energy of the composite system. In an ideal situation, in which the two metals had (a) the same crystal structure, (b) identical lattice parameters and (c) no charge transfer, the interfacial energy is the sum σ₁ + σ₂ of the surface energies of the two pure metals involved. It is argued that the charge transfer contribution (c) takes the form (ΔW)²/ρ²l_c, where ΔW is the difference in work functions of the two pure metals and l_c is a characteristic length. Contributions arising from departures for ideal lattice matching, embodied in point (b) above can be estimated from the isothermal compressibility, κ_T, of the pure metals. Further, invoking for a pure metal the known approximate relation that $\text{σκ}{}_{\mathrm{<mtext>T</mtext>}}\text{∼ 1}\text{A}\text{̊}$. this lattice mismatch contribution is expressed again in terms of surface energies. It will usually alter the contribution σ₁ + σ₂ of ine interfacial energy by a multiplying factor less than, but quite near to unity.

Low energy ion scattering can provide atom location and structural information from a number of different measurements. Low angle scattering can yield information on atom location and surface crystallography from the measurement of the azimuthal angular distribution of scattered particles. Large angle backscattering provides a direct measurement of shadow cone geometry and combined with the angular dependence of the backscattered yield, this large angle scattering event can provide accurate information on the surface structure. Studies of the angular distribution of recoil ions from the surface can also provide information on the site from which the recoil came. These three techniques have been applied to the study of the Ni(110) surface and to oxygen adsorption on that surface. A comparison of the information available from each technique is presented.