G. Molnár

Thickness‐dependent formation of Gd‐silicide compounds

Gd‐silicide phases were investigated by x‐ray diffraction. The results showed that not only one phase exists in a Gd thin‐film and silicon substrate reactions. The first phase formed was hexagonal GdSi≊1.7, the second orthorhombic GdSi2. The ratio of the two phases depends on temperature of the heat treatment, and at a given temperature and time of annealing, a dependence of the thickness of the evaporated Gd layer was found. At ∼100‐nm Gd thickness the dominant phase was orthorhombic GdSi2, at ∼250 nm hexagonal GdSi≊1.7. In the 300–1000‐nm interval orthorhombic GdSi2 was the main component again. Rutherford backscattering analysis showed that the phases were found mixed within the layer. This thickness‐dependent formation could be described with a simple model proposed by Gosele and Tu [J. Appl. Phys. 53, 3252 (1982)].Gd‐silicide phases were investigated by x‐ray diffraction. The results showed that not only one phase exists in a Gd thin‐film and silicon substrate reactions. The first phase formed was hexagonal GdSi≊1.7, the second orthorhombic GdSi2. The ratio of the two phases depends on temperature of the heat treatment, and at a given temperature and time of annealing, a dependence of the thickness of the evaporated Gd layer was found. At ∼100‐nm Gd thickness the dominant phase was orthorhombic GdSi2, at ∼250 nm hexagonal GdSi≊1.7. In the 300–1000‐nm interval orthorhombic GdSi2 was the main component again. Rutherford backscattering analysis showed that the phases were found mixed within the layer. This thickness‐dependent formation could be described with a simple model proposed by Gosele and Tu [J. Appl. Phys. 53, 3252 (1982)].

Epitaxy of GdSi≊1.7 on 〈111〉Si by solid phase reaction

Epitaxial hexagonal GdSi≊1.7 was grown by in situ vacuum annealing of 50 and 250 nm Gd layers on 〈111〉 silicon. The epitaxy was investigated by x‐ray and electron diffraction measurements.

Effect of crystallization on the electrical and interface characteristics of GdSi2/p-Si Schottky junctions

Abstract The electrical and interface parameters of epitaxial orthorhombic, textured orthorhombic, polycrystalline orthorhombic, and polycrystalline hexagonal GdSi 2 /(100)p-Si Schottky structures prepared by solid phase epitaxy with different initial thickness of the evaporated Gd layer are compared. The Schottky barrier height, the ideality factor, the series resistance, the breakdown voltage, the relative interfacial layer thickness, the energy distribution spectra of interface states, and the equilibrium interface charge have been evaluated from the measured current-voltage and capacitance-voltage characteristics. The obtained results indicate that the electrical and interface parameters depend strongly on the epitaxial or polycrystalline form of the GdSi 2 . The crystal structure (orthorhombic or hexagonal) in polycrystalline samples has only a minor influence. The epitaxial and textured orthorhombic structures also show only small difference.

The effect of silicon substrate orientation on the formation of Gd-silicide phases

Abstract The formation and epitaxy of Gd-silicide compounds were investigated in the solid phase reaction of Gd thin film on (111) and (100) oriented Si substrate by X-ray diffraction, Rutherford backscattering and transmission electron microscopy. It was recognised that the effect of substrate orientation on the phase formation became dominant for Gd films thinner than 30 nm. To determine the role of the substrate orientation on the phase formation, 20 nm Gd films were annealed in-situ under the same conditions for (111) and (100) pairs. At low temperature (320°C for 5 min) the first phase (hexagonal GdSi2−x) was formed epitaxially on Si(111), while on Si(100) an amorphous alloy formed. At higher temperatures — 350, 430, 500, 550 and 650°C, and 5 min annealing — epitaxial hexagonal GdSi2−x was found on Si(111), that could not transform into the second phase (orthorhombic GdSi2), in contrast to the case, where films were thicker than 30 nm. Meanwhile on Si(100) epitaxial orthorhombic GdSi2 was found under the same annealing conditions. It was demonstrated, that under a certain thickness (30 nm) the formed phase was determined by the substrate orientation instead of the usual diffusion and reaction processes. Our results show the possibility of amorphous phase formation in GdSi reaction, which is reasonable because of the high mobility difference of the diffusing species.

The oxidation of Gd0.95Si0.05 layers

Abstract Gd films are important for different applications, but their handling is very difficult because they are extremely reactive, mainly towards oxygen. This reactivity can be decreased by addition of small amount of Si (∼ 5 at%) diffused into Gd layer. Oxidation of the films and the chemical state of the components (Gd, O, Si) characterized by chemical shifts and by Gd/O ratio, were measured by X-ray photoelectron spectroscopy (XPS) and by Rutherford Backscattering (RBS) method. The diminished oxidation in presence of Si can be explained by the enrichment of Si at the grain boundaries and thereby forming a barrier against the oxygen diffusion into the Gd layer such that the oxidation decreased.

Current-voltage anomalies on polycrystalline GdSi2/ p-Si Schottky junctions due to grain boundaries

Abstract The current-voltage characteristics of epitaxial orthorhombic, textured orthorhombic, polycrystalline orthorhombic, and polycrystalline hexagonal GdSi 2 /(100)p-Si Schottky structures prepared by solid phase epitaxy are compared. It is concluded that the anomalous I–V characteristics measured in the polycrystalline GdSi 2 /p-Si junctions, and the much higher values are scatters of the ideality factor and of the series resistance obtained for the polycrystalline structures may be explained by the oxidation of the Gd remaining near the grain boundaries in the GdSi 2 layer.

Kinetic pattern formation of Gd‐silicide films in lateral growth geometry

The solid phase reaction of gadolinium thin film and silicon substrate was investigated in lateral growth geometry with the help of periodic titanium protective stripes by optical microscopy. In the lateral reaction zone the shape of the interface between gadolinium and Gd silicide was very complicated and showed pattern formation. This silicide growth can be described as a kinetic process modified by the structure of the Gd film in contrast to the previously proposed simple nucleation.

Nanoscale morphology and photoemission of arsenic implanted germanium films

Germanium films of 140nm thickness deposited onto Si substrate were implanted with 70keV arsenic ions with a dose of 2.5×1014cm−2. The morphology of the implanted films was determined by Rutherford backscattering and cross-sectional transmission electron microscopy. Concentration of oxygen and carbon impurities and their distribution in the implanted layer were detected by secondary-ion-mass spectroscopy and nuclear reaction analysis using the O16(He4,He4)O16 reaction. The depth dependence of the valence band density of states was investigated by measuring the energy distribution curve of photoelectrons using Ar ion etching for profiling. The morphology of As implanted film was dominated by nanosized (10–100nm) Ge islands separated by empty bubbles at a depth of 20–50nm under the surface. At depth ranges of 0–20 and 70 to a measured depth of 140nm, however, morphology of the as-evaporated Ge film was not modified. At a depth of 20–50nm, photoelectron spectra were similar to those obtained for Ge amorphize...

Solid phase reaction and epitaxy of Gd-silicide films on Si substrates

The solid phase reaction of e-beam evaporated Gd thin film and Si substrate was investigated by X-ray diffraction and scanning electron microscopy. A description has been developed for the kinetics of the reaction, which showed sequential, selective growth of two equilibrium phases. By strict control of the growth parameters it was possible to reach hexagonal GdSi1.7 epitaxy on 〈111〉 and orthorhombic GdSi2 epitaxy on 〈100〉 Si substrates. In the lateral growth mode with a Ti step, Gd-Si showed a rapid and nonuniform reaction.

Si 3 N 4 based non-volatile memory structures with embedded Si nanocrystals

Memory structures with an embedded sheet of separated Si nanocrystals were prepared by low pressure chemical vapour deposition using a Si₃N₄ control layer and SiO₂ or Si₃N₄ tunnel layers. It was obtained that a properly located layer of Si nanocrystals improves the charging behaviour of the MNOS structures. Memory window width of about 6.6 V and retention time of 41 years has been achieved for charging pulses of plusmn15 V, 10 ms.