Thin Interfacial Region Research Articles

Lithium extraction/insertion in LiFePO 4: an X-ray diffraction and Mössbauer spectroscopy study

The extraction and insertion of lithium in solid-state synthesized LiFePO 4 has been followed by in situ X-ray diffraction and Mössbauer spectroscopy in ‘coffee-bag’ cells of type 〈Li-metal | liq. el. | LiFePO 4〉 during the first cycle. Two-phase Rietveld refinement of the X-ray diffractograms gives the triphylite (LiFePO 4) to heterosite (FePO 4) phase-ratios as charging and discharging of the cell proceeds. The Fe 3+/Fe 2+ ratios at each step, as measured by Mössbauer spectroscopy and X-ray diffraction, were in good general agreement with the amount of lithium calculated from the charge passed through the cell; there was, however, a slight tendency for the Mössbauer technique to record a higher concentration of the oxidized phase. The possible existence of a thin interface region at the phase boundary is discussed.

Schottky enhancement of contacts to n-GaAs via the exchange mechanism using NiAlxGa1−x as a metallization

Based on the thermodynamic/kinetic model of the exchange mechanism, the ternary intermetallic compound NiAlxGa1−x (where 0<x⩽1) was identified as a metallization that may be used to fabricate Schottky enhanced contacts to n-GaAs. Experimental phase equilibrium studies of the quaternary Al-Ga-Ni-As system, in conjunction with diffusion data available in the literature, indicated that the phase NiAlxGa1−x fulfills the thermodynamic and kinetic requirements necessary for participation in an exchange reaction with GaAs. Contacts to n-GaAs were fabricated by sputter deposition of NiAlxGa1−x metallizations, with compositions corresponding to x=0.00, 0.25, 0.50, 0.75 and 1.00. These contacts were subjected to rapid thermal processing, and analyzed using cross-sectional high resolution transmission electron microscopy and I-V characterization. Electron microscopy and concomitant electron dispersive spectroscopic analysis indicated that a very thin (2.5 nm) interfacial region of AlxGa1−xAs was formed in annealed contacts for which x>0.00, in accordance with the exchange mechanism model. Schottky barrier enhancement was also observed in all annealed contacts for which x>0.00. The degree of Schottky barrier enhancement was shown to be dependent upon the initial composition of the metallization, again in accordance with the prediction of the exchange mechanism model. Schottky barrier heights as high as 0.96 eV were obtained under the optimum annealing conditions of 400 °C for 1 min. However, these experimentally determined Schottky barrier heights were somewhat smaller than the values that were anticipated based upon the exchange mechanism model. Potential reasons for these discrepancies were discussed. Overall, it was demonstrated that the thermodynamic/kinetic model of the exchange mechanism is a powerful tool for identifying metallizations that may be used to enhance the Schottky barriers of contacts to n-GaAs.

Microstructure of Gd/W magnetic multilayered films

Gd/W multilayer films (MLF) have been grown by DC magnetron sputter deposition, and the variation in structure with layer thickness has been determined using high-resolution electron microscopy. The technique has shown some evidence for a thin interfacial region and strong [0002] texture along the film normal for Gd. The cubic GdH 2 is sometimes observed, with (111) planes (equivalent to Gd (0002) in the film plane. The relationship between structure and magnetic anisotropy is briefly discussed.

Current injection mechanism for porous-silicon transparent surface light-emitting diodes

We present a model for the injection of minority carriers into porous silicon films which results in visible dc electroluminescence. A thin interfacial dielectric region is postulated between the surface of the porous silicon layer and a transparent conductive oxide on the surface, which allows alignment of states between the two corresponding conduction bands of these materials under bias, and hence, overlap of electron wave functions and the passage of a tunneling current. Interface state densities are calculated and a parasitic nonradiative shunt current through such states is discussed.

Investigation and optimization of InGaAs/InP heterointerfaces grown by chemical beam epitaxy using spectroscopic ellipsometry and photoluminescence

Spectroscopic ellipsometry (SE) has been used to investigate transition layers for InGaAs/InP heterointerfaces. For the case of InGaAs on InP, we have found that the samples can be best modeled by a strained InxGa1-xAs film with the possible presence of a thin interface region (15A). We are unable to conclusively determine the existence of such a thin transition region. For InP on InGaAs, we find clear indications of As contamination in the bulk film, and that the addition of a thin interface region of In0.75Gao0.25As0.5P0.5 improves both the numerical fit and shape of the dielectric response curves, especially around E1 and E1+ Δ1 where the effects of a transition region are most pronounced. However, difficulties in modeling the dielectric response of the contaminated InP film make identification of an interface transition region only speculative at this point. Multiple single quantum well structures have also been grown and analyzed with 7K photoluminescence. The quality of the quantum wells shows strong dependence on the gas switching sequence used at the heterointerfaces. The best switching sequence produced a 0.5 nm well with a 7K FWHM of only 12.3 meV. Multiple quantum wells have also been grown to investigate the uniformity and repeatability of our system. Twenty period MQWs with a well width of 1.6 nm display a 14K FWHM of 7.9 meV.

Multiple low-temperature interface reactions: An alternative route into the amorphous state of metallic alloys.

During the preparation of thin Au/In multilayers (0.39/1.2 nm, total thickness 48 nm) by ion-beam sputtering at 86 K, periodic interface reactions were observed by an Iin situP measurement of the electrical resistivity. The resulting phase was identified as amorphous ${\mathrm{Au}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ with compositions x which were controlled by the choice of the individual Au and In layer thicknesses. The formation of these amorphous phases was restricted to a thin interface region (6 nm) and, most important, could not be obtained by long-range diffusion of thick Au/In multilayers.

Electronic structure of silicide-silicon interfaces

Over the past few years the electronic structure of transition metal-silicon and silicide-silicon interfaces (and of bul;k silicide compounds) has been revealed for the first time, using surface spectroscopies (photoemission and Auger) and theoretical calculations. These investigations, which have included palladium, platinum, nickel, vanadium, chromium, molybdenum and tungsten, have elucidated the important role played by the chemical bond between the transition metal d and the Si 3p electrons. They have also shown the high chemical reactivity of the atomically clean transition metal-silicon interface, which leads to interfacial silicide formation at relatively low temperatures ( i.e. markedly below those needed for bulk silicide formation on chemically cleaned silicon surfaces). As a result, silicide-like chemical bonding dominates the interface electronic structure of such contacts under a wide variety of conditions. Detailed comparisons of interface spectra (observed at low metal coverages) with those of the bulk silicide reaction products give strong evidence for additional electronic states of relatively high density (about 0.1 states per interface atom) which lie in or near the silicon band gap region. These states are believed to be true interface states associated with localized bonding configurations unique to the atoms at the interface, and they could explain the silicon-rich silicide composition of a thin (about 3–5 Å) interfacial region which has been observed by Auger composition analysis and suggested by chemical shifts in core and valence electron densities of states. Finally, metal atom diffusion into the silicon substrate has been observed in ion channeling studies and suggested from surface spectroscopy results; these impurity atoms should produce defect states localized near the interface and lying within the silicon band gap, although these electronic states have not yet been directly observed. As a whole these results present a fairly detailed picture of the electronic structure and chemistry of the silicide-silicon interface. However, correlations with the interface electrical properties are needed to ascertain which electronic features and chemical mechanisms in fact determine the Schottky barrier height of the contact.

Electronic Structure of Silicide-Silicon Interfaces

Over the past few years the electronic structure of transition metal-silicon and silicide-silicon interfaces (and of bulk silicide compounds) has been revealed for the first time, using surface spectroscopies (photoemission and Auger) and theoretical calculations. These investigations, which have included palladium, platinum, nickel, vanadium, chromium, molybdenum and tungsten, have elucidated the important role played by the chemical bond between the transition metal d and the Si 3p electrons. They have also shown the high chemical reactivity of the atomically clean transition metal-silicon interface, which leads to interfacial silicide formation at relatively low temperatures (i.e. markedly below those needed for bulk silicide formation on chemically cleaned silicon surfaces). As a result, silicide-like chemical bonding dominates the interface electronic structure of such contacts under a wide variety of conditions. Detailed comparisons of interface spectra (observed at low metal coverages) with those of the bulk silicide reaction products give strong evidence for additional electronic states of relatively high density (about 0.1 states per interface atom) which lie in or near the silicon band gap region. These states are believed to be true interface states associated with localized bonding configurations unique to the atoms at the interface, and they could explain the silicon-rich silicide composition of a thin (about 3–5 Å) interfacial region which has been observed by Auger composition analysis and suggested by chemical shifts in core and valence electron densities of states. Finally, metal atom diffusion into the silicon substrate has been observed in ion channeling studies and suggested from surface spectroscopy results; these impurity atoms should produce defect states localized near the interface and lying within the silicon band gap, although these electronic states have not yet been directly observed. As a whole these results present a fairly detailed picture of the electronic structure and chemistry of the silicide-silicon interface.However, correlations with the interface electrical properties are needed to ascertain which electronic features and chemical mechanisms in fact determine the Schottky barrier height of the contact.

ChemInform Abstract: THE GROWTH AND CHARACTERIZATION OF VERY THIN SILICON DIOXIDE FILMS

AbstractOxidation von Si bei 900‐1000°C unter O2 ‐Drücken von 0.25‐2.0 Torr liefert Filme von nur 30‐140 Å Dicke, die ellipsometrisch, IR‐ und Auger‐spektroskopisch, durch Ätzgeschwindigkeitsexperimente sowie anhand ihrer Durchbruchspannungen charakterisiert werden.

The Growth and Characterization of Very Thin Silicon Dioxide Films

Very thin oxides (30–140Å) have been prepared by oxidizing silicon at 900°–1000°C at reduced pressure (0.25–2.0 Torr). The oxidation rate is parabolic. The oxides have been characterized by ellipsometry, infrared spectroscopy, Auger spectroscopy, etch rate variation, and breakdown voltages. The thin, low pressure oxides have properties similar to those of thicker oxides prepared at atmospheric pressure. The intrinsic breakdown fields are high (10–13 MV/cm) and distributed over a very narrow range. The ellipsometric, etch rate, and Auger data indicate a thin interfacial region <15Å thick, but such a region is not detected in the infrared spectra.