The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

Kinetics of the crystallization in amorphous NiTi thin films

The crystallization kinetics in Ni_50.54Ti_49.46 film was studied by differential scanning calorimetry through continuous heating and isothermal annealing. The activation energy for crystallization was determined to be 411 and 315 kJ/mol by Kissinger and Augis & Bennett method, respectively. In the isothermal annealing study, The Avrami exponents were in the range of 2.63–3.12 between 793 and 823 K, suggesting that the isothermal annealing was governed by diffusion-controlled three-dimensional growth for Ni_50.54Ti_49.46 thin films.     

Structural relaxation and crystallization of amorphous Ge films

The irreversible changes in conductivity occurring in evaporated amorphous Ge films just after deposition and during annealing are investigated in detail. Their analysis leads to a model for the structural relaxation of real amorphous networks. Electron diffraction and electron microscope experiments provide additional information on annealing effects and crystallization processes.     

Spatially localized current-induced crystallization of amorphous silicon films

Field-enhanced metal-induced solid phase crystallization (FE-MISPC) at room temperature is employed to create microscopic crystalline regions at predefined positions in hydrogen-rich amorphous silicon (a-Si:H) films. Electric field is applied locally using a sharp conductive tip in atomic force microscope (AFM) and nickel electrode below the a-Si:H film. The process is driven by a constant current of ?50 pA to ?500 pA while controlling the amount of transferred energy (1–300 nJ) as a function of time. Passing current leads to a formation of nanoscale pits in the a-Si:H films. Depending on the energy amount and rate the pits exhibit lower or orders of magnitude higher conductivity as detected by current-sensing AFM. High conductivity is attributed to a local crystallization of the films. This is confirmed by micro-Raman spectroscopy.     

Formation of amorphous Mn-Bi films and their crystallization process

Mn-Bi alloy films were prepared by simultaneous vacuum deposition at liquid N₂ temperature and their structures were investigated by transmission electron microscopy and electron diffraction. Films with compositions in the range of 34 to 63 at% Mn have the Bi lattice structure at room temperature. Films with composition in the range of 68 to 95 at% Mn show diffuse haloes at room temperature. When they are heated above room temperature, those in the range of 70 to 85 at% Mn show nucleation and grain growth crystallization, whereas the films of 68 and 95 at% Mn show only grain growth crystallization. The films in the range of 70 to 85 at% Mn are concluded to be amorphous at low and at room temperature. Some of the crystals obtained by nucleation and the grain growth crystallization grow as large as 10 μm in length. Their diffraction patterns are not ascribable to reported crystals in this alloy system.     

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

Flash lamp annealing (FLA) can form polycrystalline silicon (poly-Si) films with various microstructures depending on the thickness of precursor amorphous Si (a-Si) films due to the variation of crystallization mechanisms. Intermittent explosive crystallization (EC) takes place in precursor a-Si films thicker than approximately 2 μm, and the periodicity of microstructure formed resulting from the intermittent EC is independent of the thickness of a-Si films if their thickness is 2 μm or greater. In addition to the intermittent EC, continuous EC and homogeneous solid-phase crystallization (SPC) also occur in thinner films. These crystallization mechanisms are governed by the ignition of EC at Si film edges and the homogeneous heating of interior a-Si. The results obtained in this study could be applied to control the microstructures of flash-lamp-crystallized poly-Si films.     

Low-temperature metal-induced crystallization of Mn-containing amorphous Ge thin films

This work focuses on the crystallization of amorphous germanium (a-Ge) thin films induced by manganese species. A series of Mn-containing a-Ge films ([Mn] ~ 0?3.7 at.% range) was deposited at 150 °C by the cosputtering technique. After deposition, all films were submitted to isochronal thermal annealing treatments up to 600 °C and analyzed by Raman scattering, optical transmission spectroscopy and electrical resistivity measurements. The experimental results indicate that: (a) Mn impurity lowers the crystallization temperature of a-Ge in ~ 100 °C, as confirmed by the Raman analyses, (b) the optical properties of the films are affected by both the insertion of Mn and the temperature of thermal treatment, with the optical bandgap staying in the range of ~ 0.7?1 eV, and (c) the electrical resistivity of the samples is also influenced by the Mn concentration and by the temperature of annealing, varying between ~ 1.0×10¹ and 1.6×10⁴ Ω cm. These experimental observations were systematically studied and the possible reasons associated to them are presented and discussed.     

Laser crystallization of compensated hydrogenated amorphous silicon thin films

Raman backscattering and hydrogen effusion measurements were performed on compensated, highly P- and B-doped laser crystallized polycrystalline silicon. From hydrogen effusion spectra the hydrogen chemical potential, μ_H, is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. Interestingly, hydrogen bonding is affected by doping of the amorphous starting material. Below the hydrogen transport states, four peaks are observed in the hydrogen density-of-states at 2.0, 2.2, 2.5 and 2.8 eV. The latest peak is not observed in B-doped samples. The hydrogen effusion results will be correlated with the results obtained from Raman backscattering measurements.     

Electrical properties of amorphous and crystalline InSb and InAs thin films

Amorphous films of indium antimonide (0.02–0.26 μm) and indium arsenide (0.03–0.3 μm) were formed on goldseal glass, freshly cleaved mica and NaCl substrates by using a “flash evaporation” technique. The post-deposition heat treatment was carried out on these films when the amorphous → crystalline transformation was observed. The transformation was characterized by a sudden and large fall in the resistance of the film at a particular temperature depending on the thickness. This transformation was confirmed by transmission electron micrographs and diffraction patterns obtained on the films before and after heat treatment. The transformation temperatures lie between 495–525 K for indium antimonide and 550–575 K for indium aresenide, for the thickness range involved in our investigations. The electrical conductivity measurements showed a temperature dependent activation in the high temperature region and hopping conduction in the low temperature region (Mott's theory). The activation energies, at different temperatures for various thicknesses were calculated and presented. While no Hall mobility could be observed in as-deposited films, very low mobilities were observed in annealed thick films (t > 2000 Å). Thermoelectric power for InSb films was found to vary from 0.075–0.17 mV/K for films of thickness ranging from 1000–2300 Å, whereas for InAs films, its value varied from 0.09–0.27 mV/K for the thickness range, 1250–2500 Å. These measurements indicated the conductivity to be n-type and supported the hopping conduction mechanism observed in low temperature conductivity measurements.     

RHEED investigations of unusual crystalline states in amorphous SiO2 films

Microcrystalline regions of silicon dioxide with structures unobserved in the macro-state are studied by Reflection High Energy Electron Diffraction (RHEED). Their formation can be understood by considering the existence of several possible relatively stable crystalline states of SiO₂, with free energies higher than those of the known silicon oxides. SiO₂ with structures and lattices identical to the structures and the lattices of the CaF₂-, of the rutile-type GeO₂, and of α-PbO₂ have been identified. Considerations for the state of silicon in some of these structures are also included.     

Effect of trace metals on reactive crystallization of gypsum

The effect of Fe²⁺, Fe³⁺, and Cr³⁺ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe²⁺, Fe³⁺, and Cr³⁺ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe³⁺ or Cr³⁺ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe³⁺ and Cr³⁺ ion concentrations. The change of morphology is related to the complex formation between Fe³⁺ or Cr³⁺ ions and citric acid at high ion concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impacts of growing inclusions on directional crystallization

The destructive influence of internally nucleated growing crystallites upon the growth of an oriented crystal front is examined. In particular, a simple two-dimensional model system is used to study the interaction of an oriented, planar growth front with a particle, initially of negligible size, which grows by circular growth. The locus of the intersection points of the fronts is obtained and used to derive an expression for the misoriented (ruined) area fraction produced by the presence of the growing particle. It is demonstrated that the reduced ruined area fraction is a function of two controlling parameters that describe the relative growth velocity of the interfering particle and its relative nucleation position. The sensitivity of the reduced ruined area fraction to these controlling parameters is ascertained and discussed.     

Optically-induced darkening and crystallization in amorphous Ag–Sb–S films

The amorphous films of Sb₃₃S₆₇ and Ag_x(Sb_0.33S_0.67)_100−x composition, where x was between 0 and 25 at.% Ag, were prepared by different techniques, i.e. by vacuum evaporation, optically-induced silver dissolution into the binary Sb–S chalcogenides deposited by vacuum evaporation or by spin-coating techniques. Ternary Ag–Sb–S amorphous films were also prepared directly by pulsed laser deposition. We have studied effect of silver content in the host materials and Ar⁺ ion laser exposure on optical properties and structure of the films. The optically-induced darkening and crystallization was proved in studied films and their potential application in optical memories could be expected.     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Order and disorder in edge-supported pure amorphous Si and pure amorphous Si on Si(001)

We report results from an investigation into hidden anisotropy in pure fully-dense amorphous silicon. For amorphous silicon in intimate contact with a crystalline Si(001) substrate, one can reasonably expect that the interface with the substrate may impose anisotropy in the form of distorted ordering within the film. Indeed, we found four-fold periodic intensity variations, with bimodal intensity centered along the substrate c-Si < 110> directions, in the X-ray scattering from a-Si on Si(001). These well-defined intensity variations disappeared entirely in X-ray scattering from edge-supported a-Si films, where there was no detectable anisotropy.     

Magnetic properties of amorphous metals

Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe_xNi_1?x)₇₅P₁₆B₅Al₃ and Fe_xB_1?x. The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mössbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.     

Effect of hydrogen on the intrinsic stress in ion beam sputtered amorphous silicon films

The results of measurements of the intrinsic stress in hydrogenated amorphous silicon (a-Si:H) films deposited by ion beam sputtering are presented. The data show that the total intrinsic stress is composed of growth stress and hydrogen induced stress. At high hydrogen concentration (> 24%), an abrupt decrease in stress is observed which correlates with a microstructural change as shown by SEM and spectrosoopic ellipsometry measurements. The change in microstructure is accompanied by a modification in the IR transmission spectrum at 2000 cm⁻¹ indicating a change in hydrogen bonding configuration or in local environment near SiH bonds. A model is proposed which reconciles the observation of both tensile and compressive stress in unhydrogenated silicon reported in the literature.     

Cross-sectional TEM study on Ni-mediated crystallization of amorphous silicon

Structural characteristics of polycrystalline silicon (poly-Si) made by Ni-mediated crystallization of amorphous silicon (a-Si) were investigated by cross-sectional transmission electron transmission (XTEM) according to various a-Si thickness. The Ni area density of ∼10¹⁴ cm⁻² was deposited onto a-Si and it was annealed at 500 °C in the presence of an electric field of 10 V/cm. It is found that NiSi₂ precipitates form at the top and bottom interfaces of a-Si during annealing. After reaching its critical size the crystallization proceeds from the top and bottom interfaces. The growth of needle-like Si crystallites has been seen, showing a migration of NiSi₂ precipitates through the a-Si network. 1700 nm thick a-Si can be crystallized within 30 min which is longer than that (10 min) of 50 nm thick a-Si. However, the quality of 50 nm thick poly-Si is better than that of 300 nm or 1700 nm thick poly-Si.     

Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens

Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH₄) and H₂Se/H₂S gas mixtures in an RF plasma glow discharge on 7059 corning glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (σ_ph/σ_d) of a-Si, Se:H thin films decreases as the gas ratio H₂Se/SiH₄ increased from 10^?4 to 10^?1, while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H₂S/SiH₄ increased from 6.8 × 10^?7 to 1.0×10^?4.