Approaches to growth and study of properties of multilayer silicon-silicide heterostructures with buried semiconductor silicide nanocrystallites

Studies of nanosize (5-50 nm) island formation of Fe, Cr and Mg silicides on atomically clean silicon surfaces with (111) and (100) orientations, silicon growth atop nanosize silicide islands and multilayer repetition of developed growth procedure for all silicides have been carried out. Optimization of growth parameters has permitted to create multilayer monolithic heteronanostructures with buried nanocrystallites of iron and chromium disilicides. Only polycrystalline multilayer heteronanostructures with buried Mg₂Si nanocrystallites have been created after optimization of growth procedures. A new approach to study optical properties of multilayer heteronanostructures has been developed and tested.     

The solid state reaction of Fe with the Si(111) vicinal surface: splitting of bunched steps

The solid state reaction of deposited Fe (four monolayers, ML) with vicinal Si(111) substrate induced by subsequent thermal treatment has been studied using scanning tunnelling microscopy. At the lower range of annealing temperatures up to 400?°C the bunched steps of bare substrate are reproduced by the surface of the covering iron silicide layer. At 400?°C the onset of three-dimensional growth of iron silicide islands is observed. In comparison to the samples covered with smaller amounts of Fe it appears at a lower annealing temperature. Above 500?°C the bunched steps split into lower ones but more densely distributed due to proceeding reactions between Fe-rich iron silicide and Si substrate. As a consequence, at 700?°C the well-developed three-dimensional nanocrystallites of iron silicide are randomly distributed on the Si surface. This observation is in contrast to the formation of a regular array of iron silicide crystallites upon deposition of 2?ML of Fe.     

Silicon-based light emitting diodes on silicon and glass substrates using a low temperature multilayered nanocrystalline structure

Si nanocrystals have been prepared by hydrogenation and subsequent annealing of as-deposited amorphous Si layers on glass and Si substrates. The hydrogenation process has been performed at 350 °C under radio frequency hydrogen plasma. The nanocrystallites were processed by sequential reactive ion etching to allow light emission. Photoluminescence (PL) measurements demonstrate that the nanocrystallites emit light in the range of 500-570 nm. The evolution of nanocrystals has been studied using scanning electron microscopy, while atomic force microscopy and transmission electron microscopy have been utilized to examine the structure of the Si nanocrystals. Multilayer luminescent Si nanocrystals have been fabricated using alternating layers of Si nanocrystals and Si oxy-nitride. Bilayer structures have higher efficiency than a single layer structure, while multilayers with three layers of luminescent nanocrystals and above did not show a higher PL intensity. Transparent light emitting diodes have been realized based on multilayer luminescent Si nanocrystals that displayed bright emission which was visible to the naked eye in a bright room.     

The diffusion barrier effect of a vanadium layer in the formation of nickel silicides

The formation of nickel silicides and vanadium silicides and the diffusion barrier effect of a vanadium layer in the formation of nickel silicides were studied for annealed metal films deposited on silicon substrates by depth profiling using secondary ion mass spectrometry and X-ray diffraction. Nickel films more than 2500 Å thick react with silicon after annealing at 400 °C for 30 min in a vacuum. A vanadium layer 250 Å thick between nickel and silicon shows a barrier effect in the nickel silicide formation after annealing at 400 °C for 30 min. The barrier effect of a vanadium layer 250 Å thick becomes imperfect after annealing at 500 °C for 30 min.     

Thin films of rare earth metal silicides

The phase composition, conductance and surface morphology of thin films of silicides of rare earth metals (of the yttrium subgroup) were studied. The silicides were formed by annealing thin film structures of the rare earth metal (Ln) and silicon at 473–673 K for 1–120 min in vacuum. X-ray analysis revealed the formation of crystalline silicide phases of composition LnSi_2-x and of the AlB₂ structural type for all the metals concerned except scandium, gadolinium and lutetium.It was established that the formation of the crystalline silicide phase is determined by the relation between the crystallographic parameters of a rare earth metal hexagonal lattice and silicon; the critical value of the lattice mismatch a is ± 1.3%. The kinetics of formation of a silicide phase were determined by measuring the conductance of thin film structures. A model for the formation of rare earth metal silicides in thin film structures is proposed, which serves as a basis for establishing conditions for the formation of quasi-amorphous, polycrystalline or large-block rare earth metal silicide layers, with a perfect silicide-silicon interface, taking into account the crystallographic orientation and parameter relationship of the substrate and the silicide.     

High resolution photoemission study of the formation and thermal stability of Mg silicide on silicon

The focus of this study is to use high resolution synchrotron based photoemission to investigate the initial growth mode of magnesium silicide which has been formed by both stepwise and continuous deposition of metallic Mg onto a thermally grown ultra-thin Si oxide surface. The findings suggest that stepwise deposition of Mg initially results in the growth of Mg silicide islands on the surface. Further magnesium deposition leads to the growth of metallic Mg on the surface of these silicide islands, along with the continued growth of silicide species on the uncovered oxide surface. However, it has been shown that continuous deposition of Mg results in considerably less silicide growth. The thermal stability of Mg silicide and a mechanism for high temperature silicide growth have also been studied using conventional X-ray photoelectron spectroscopy. The results suggest that the presence of oxidised Si acts as a barrier to Si diffusion during vacuum annealing, hence preventing the growth of further Mg silicide. It has also been shown that metallic Mg desorbs from the surface below 300 °C, while Mg silicide is not stable at temperatures above 500 °C in contrast to other metal silicides.     

Growth, structure and luminescence properties of multilayer Si/β-FeSi2NCs/Si/…/Si nanoheterostructures

Multilayer structures (up to 15 layers) with β-FeSi₂ nanocrystallites (NCs) buried in silicon crystalline lattice were grown by successive repetition of reactive deposition epitaxy (RDE) or solid phase epitaxy (SPE) of thin iron film on Si(100) or Si(111) substrates and silicon molecular beam epitaxy (MBE) (100-200 nm). Cross-section high resolution transmission electron microscopy (HR TEM) images and ex situ optical and Raman spectroscopy data prove that NCs formed in silicon matrix have the structure and optical properties of β-FeSi₂. The growth conditions provide no dislocations in silicon lattice were found in the course of TEM analysis. Two types of NCs depth distribution were observed: (i) layered that corresponds to iron RDE and (ii) uniform that occurs in the case of iron SPE. The uniform NCs distribution points out the fact that during a growth process NCs moves up to the surface. In spite of small nanocrystallites size (5-50 nm) and their distribution in silicon cap layers the significant photoluminescence (PL) signal at 0.8 eV was observed for all grown samples.     

Characterization of titanium silicide films formed by composite sputtering and rapid thermal annealing

Titanium silicide films were prepared by sputtering from a single composite TiSi_x source followed by rapid thermal annealing in N₂. The composition, resistivity, crystal structure and microstructure were investigated using Auger electron spectroscopy, Rutherford backscattering spectrometry, scanning and transmission electron microscopy, X-ray and electron diffraction and a four-point resistivity probe. As-deposited and fully annealed films were found to possess a TiSi_2.2 stoichiometry and to contain 6–7 at.% O and significant amounts of several metallic impurities (copper, iron and tungsten). Rapid thermal annealing at 850–1000°C for 10 s forms a polycrystalline equilibrium orthorhombic phase TiSi₂ structure with 0.25-0.50 μm grains. The annealed layer resistivity was reduced to 28 μΩ cm, i.e. 1.4Ω/▭ for a 0.20 μm film. No expulsion of silicon was observed after annealing, and it appears that excess silicon has trapped oxygen within the film in the form of silicon oxide precipitates. Titanium polycide layers were unable to be etched successfully in an SF₆-CCl₄ plasma because of the presence of the non-volatile copper and iron impurities within the silicide.     

Refractory metal silicide formation by ion implantation

A systematic study of interface mixing of transition metal-Si structures by ion implantation was carried out. High resolution Auger analyses at the mixed region of the implanted samples showed an energy shift of the Auger Si LVV transition and a change in the spectrum relative to that of elemental silicon. The elemental depth distribution obtained using Auger electron spectroscopy combined with ion sputtering illustrated a composition plateau for the ion-implanted structures. The X-ray diffraction data show the presence of silicides. These results illustrated metal silicide formation and a reduction in the thermal reaction barrier for forming refractory metal silicides by ion implantation.     

Rod milling and thermal annealing of graphite: Passing the equilibrium barrier

The change in graphitic carbon structure induced by mechanical milling has been monitored by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction. It is well known that progressive rod milling of graphite results in an increase in structural disorder. Here, it has been found that a milling time of around 80 h is crucial in producing maximum nanocrystallite formation and this affects the nature of the products formed before or after annealing. At about 80 h equilibrium forms and no further production of nanocrystallites is possible although if additional energy is added amorphous carbon begins to form. Annealing produces different nanographitic carbons depending on the milling conditions because the material may be milled to an equilibrium concentration of nanocrystallites or less, or with additional energy transformed further past equilibrium to new product. Linear morphological structures and trace amounts of carbon nanotubes were found on milling for 80 h and annealing, but concentric layers of carbons were observed in samples milled as long as 240 h.     

Growth of Fe on Si (100) at room temperature and formation of iron silicide

Low-energy electron diffraction (LEED), Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS) investigations of both the growth of an iron film on silicon (100) at room temperature and the subsequent formation of iron silicide are the subjects of this paper. An in-situ cleaned silicon (100) wafer without carbon or oxygen contamination exhibiting the known 2 × 1 reconstruction in the LEED pattern served as the substrate. Iron was deposited on this reconstructed surface at 300 K. The comparison of theoretical calculations based on three growth mechanisms with XPS data obtained with take-off angles of 0° and 50° clearly demonstrates a layer-by-layer growth of the iron film on silicon (100). At 300 K no formation of iron silicide was observed, although an interaction between iron and silicon could be detected at the interface. The formation of iron silicide was observed at annealing temperatures of 630–730 K. Quantitative XPS analysis yields the presence of FeSi₂, when the thickness is large enough. Neither the iron film on silicon nor the silicide shows any LEED pattern.     

Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

Abstract

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α₂ + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.     

Ion-induced transformations of a W–Si interface

Tungsten silicide formation in multilayer tungsten/silicon structure was investigated. The W–Si multilayers were deposited on thermally oxidized silicon wafers using the dual-target magnetron sputtering. Deposition of the whole stack of sublayers was carried out without breaking vacuum in order to eliminate contamination or oxidation of the interfaces between sublayers. Samples were annealed in the RTA furnace at temperatures ranging from 700 °C up to 1050 °C. Some of the structures were irradiated with argon ion beam before annealing. Reactions between sublayers were studied using SEM imaging of cross-sectional cleavages and by X-ray diffraction analysis. Influence of the irradiation with argon ion beam on structural transformations was investigated using RBS analysis. It has been found that tungsten silicide formation depends on the deposition sequence. The reaction was more effective on interfaces between silicon layer deposited on tungsten then on interface between tungsten deposited on silicon. Ion beam mixing experiment showed that ion–target interaction promotes formation of the WSi₂ phase.     

A simple method of depositing oxygen-free titanium silicide films using vacuum evaporation

A novel technique for depositing oxygen-free low resistivity titanium silicide films with a controllable amount of excess silicon has been developed and is reported in this paper. This method employs electron beam evaporation of titanium and silicon in an ultrahigh vacuum environment to deposit alternate layers of metal and silicon in an appropriate ratio. The composite structure thus formed is finally covered with a relatively thick layer of evaporated silicon to eliminate the possibility of contamination of titanium during annealing. Heating in an inert ambient above 600°C results in a low resistivity titanium silicide layer. Auger analysis reveals the absence of oxygen contamination. Detailed analysis of the annealed layer from its reflection electron diffraction pattern indicates possible phase changes with varying annealing temperature.     

Structure,phase composition,and nanohardness of vacuum-annealed multilayer fullerite/aluminum films

The influence of the number of layers and thermal annealing on the structure, elemental and phase compositions, and nanohardness of multilayer fullerite/aluminum films has been studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray microanalysis, and nanoindentation. The results demonstrate that sequential growth of five aluminum layers and four fullerite layers, each 50 nm in thickness, on oxidized single-crystal silicon substrates leads to the formation of textured films, which retain 111 texture after vacuum annealing at 620 K (τ = 5 h). In the case of the growth of bilayer films of greater thickness, C₆₀(200 nm)/Al(300 nm), the fullerite and aluminum have a polycrystalline structure with no growth texture. Thermal annealing of the bilayer films leads to the formation of a new phase, Al_xC₆₀. The materials studied here possess enhanced nanohardness compared to pure aluminum and fullerite films.     

Cu-Ni and Ni-Cu bilayers on silicon: reduction of formation temperature and phase separation

The solid state reaction between Ni-Cu and Cu-Ni bilayers on Si(100) has been studied using X-ray diffraction, transmission electron microscopy, Auger electron spectroscopy and sheet resistance measurements. The bilayers were produced by evaporation and annealed at temperatures between 200 and 500 °C. In the Ni-Cu sequence, a strong intermixing of silicides and a 33% reduction in the formation temperature of the NiSi₂ phase compared with Ni-Si(100) was observed. When nickel was in direct contact with the silicon substrate limited intermixing and reaction of Cu was observed in the Cu-NiSi and NiSi-Si interfaces at 500 °C, due to the large grain size of the nickel silicide.     

Evolution of microstructure during annealing of low-dose SIMOX wafers implanted at 65 keV

The microstructural changes that occur during annealing of ultra-thin oxygen-implanted silicon-on-insulator have been studied using transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and Auger electron spectroscopy (AES). Silicon substrates were implanted at 65 keV with a dose of 4.5×10¹⁷ O⁺ cm^–2, followed by annealing at various temperatures. TEM results show that the defects observed in the as-implanted material (stacking faults and {1 1 3} defects) were reduced after annealing at 900 °C for 2 h and were eliminated after annealing at 1100 °C for 2 h. A continuous buried oxide (BOX) layer was formed after annealing at 1300 °C for 6 h. Numerous silicon islands were present in the BOX layer. The silicon islands can be traced to a precursor structure that developed at the implantation step. RBS results indicate that the crystallinity of the top Si layer is significantly restored after annealing at 1100 °C for 2 h and is completely restored after annealing at 1300 °C for 6 h. It was also found through AES analysis that the redistribution of oxygen during annealing is initiated at 1100 °C.     

Formation and characterization of tungsten silicide layers

Tungsten silicide films formed via furnace annealing were studied. The tungsten layers were deposited either by evaporation or by r.f. sputtering onto Si(100) substrates as well as onto silicon layers deposited in situ. Tungsten deposited at room temperature yields poor silicides owing to the lack of permeability at the interface with silicon. This as well as the formation of voids in the substrate are discussed. Deposition onto substrates heated to 500 °C, however, always allows the formation of a silicide during subsequent annealing.     

Morphology of porous silicon under long anodic etching in electrolyte with internal current source

The results of electron microscopy investigation of morphology of porous silicon (PS) received under long anodic etching by using internal current source in electrolytes such as HF:H₂O₂:H₂C₅OH and HF:H₂O₂ are presented. Mosaic structure of nanoporous silicon is observed as the islands separated by silicon ledges. It was shown that these islands are presented as assemble of oxidized nanocrystallites and silicon ledges. The results of elemental analysis of islands of oxidized nanocrystallites and silicon ledges are presented.     

Auto-correlation function analysis of phase formation in iron ion-implanted amorphous silicon layers

High-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis have been applied to investigate the evolution of structural order in iron ion-implanted amorphous silicon layers. β-FeSi₂ nanocrystallites as small as 5 nm in size were detected in 600 °C annealed for 60 min a-Si layers. The embedded nanocrystalline β-FeSi₂ was found to grow in the interlayer with annealing temperature.