Peculiarities of the electron structure of nanosized ion-implanted layers in silicon

The effect of implantation of Ba, P, B, and alkali element ions with a low energy (E ₀ < 5 keV) on the electron structure of a thin surface region of p- and n-type Si(111) single crystals is investigated by photoelectron and secondary electron-electron spectroscopy. It is shown that irrespective of the ion species, high-dose ion implantation leads to a sharp narrowing of the band gap in silicon, which can be explained by overlapping of the wavefunctions of electrons of impurity atoms and the formation of the impurity subband overlapping with the allowed band. After implantation thermal annealing, SiP, BaSi, and SiB3 films of nano-sized thickness are obtained for the first time. Optimal regimes of ion implantation and subsequent annealing for the formation of silicide films are determined, and their electron structure is investigated.     

Electron-excited luminescence of SiO2 layers on silicon

An analysis of cathodoluminescence and electroluminescence spectra of Si-SiO₂ structures suggests a conclusion concerning the processes involved in excitation of the luminescence centers generated in the UV spectral region and their localization. The electroluminescence observed in this region of the spectrum is generated in excitation of luminescence centers localized in the immediate vicinity of the Si-SiO₂ phase boundary. In the case of cathodoluminescence, the observed emission bands at ??4.3 and ??2.7 eV appear in excitation of the luminescence of silylene centers at the Si-SiO₂ phase boundary.     

Optical and compositional studies of buried oxide layers in silicon formed by high dose implantation

Samples of standard SIMOX material, dose 2.4×10¹⁸/cm² at 200 keV have been examined in the as-implanted and annealed condition. The samples were prepared by wet-chemical etching with the silicon overlayer being removed first. Samples were then etched to remove the buried oxide, revealing and successively exposing the interface. These samples were then examined by ellipsometry and SIMS with an emphasis on the optical properties and compositional nature of the interfaces. Ellipsometry results indicate that a final layer of the buried oxide with refractive index 2.5 to 3 and thickness 150 to 250 Å is very resistant to the oxide etch. This may be correlated with the compositional profile of the interfacial region obtained by SIMS. The interface between the buried oxide and bulk silicon includes two layers, the first of silicon with SiO₂ precipitates and the second of entirely SiO₂.     

Structural studies of thin silicon layers repeatedly implanted by carbon ions

The composition and structure of homogeneous SiC_1.4 and SiC_0.12 layers produced by multiple implantation of 40-, 20-, 10-, 5-, and 3-keV carbon ions into silicon were studied by electron microscopy, x-ray diffraction, Auger spectroscopy, and IR spectroscopy. The temperature dependences of the IR transmittance peak parameters obtained in the range 200–1400°C indicate that the increase in the number of carbon atoms that are bound to silicon atoms and are involved in absorption is caused by the formation and breaking of hexagonal, near-tetrahedral, and multiple Si-C bonds and by the decomposition of optically active strong carbon clusters. The high crystallization temperature of SiC (1200°C) in the SiC_1.4 layer is explained by the presence of stable multiple Si-C bonds and strong carbon clusters. Strong carbon clusters are shown to exist in the implanted SiC_0.12 layer, and their decomposition is found to affect the formation of tetrahedral bonds in the temperature range 1200–1400°C.     

Picosecond laser ablation of SiO2 layers on silicon substrates

In this work, we report on laser ablation of thermally grown SiO₂ layers from silicon wafer substrates, employing an 8–9 ps laser, at 1064 (IR), 532 (VIS) and 355 nm (UV) wavelengths. High-intensity short-pulse laser radiation allows direct absorption in materials with bandgaps higher than the photon energy. However, our experiments show that in the intensity range of our laser pulses (peak intensities of <2×10¹² W/cm²) the removal of the SiO₂ layer from silicon wafers does not occur by direct absorption in the SiO₂ layer. Instead, we find that the layer is removed by a “lift off” mechanism, actuated by the melting and vaporisation of the absorbing silicon substrate. Furthermore, we find that exceeding the Si melting threshold is not sufficient to remove the SiO₂ layer. A second threshold exists for breaking of the layer caused by sufficient vapour pressure. For SiO₂ layer ablation, we determine layer thickness dependent minimum fluences of 0.7–1.2 J/cm² for IR, 0.1–0.35 J/cm² for VIS and 0.2–0.4 J/cm² for UV wavelength. After correcting the fluences by the reflected laser power, we show that, in contrast to the melting threshold, the threshold for breaking the layer depends on the SiO₂ thickness.     

Luminescence in GaN co-doped with carbon and silicon

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal-organic-chemical-vapor deposition. Previous electrical and optical data, together with Density Functional calculations, have suggested that carbon is incorporated at acceptor and donor substitutional sites in this material; the relative importance of each is determined by the Fermi level position and the growth conditions. Here the luminescence behavior of these materials is examined in more detail, including spectral, temperature, and time dependences under ultraviolet light and electron beam excitation conditions. Particular attention is given to the commonly observed “yellow band” at , a blue luminescence at seen only in samples where carbon is the majority dopant, and ultraviolet bands near . Our data suggest that the latter two bands are both donor-acceptor related with the final state being the negatively charged state of a carbon atom substituting for nitrogen. In samples where carbon is the majority dopant, extended luminescence excitation at low temperatures results in large changes in the brightness of the yellow and blue luminescence bands. These effects are similar to other recent observations of luminescence metastability in high resistivity GaN, and we suggest that carbon plays a crucial role in this phenomenon.     

The coalescence of silicon layers grown over SiO2 by liquid-phase epitaxy

We have investigated by liquid-phase epitaxy (LPE) the coalescence of defect-free silicon-on-insulator (SOI) layers. The SOI lamellae grow out laterally from neighbouring seeding windows and spread over the SiO₂. In our study, the seeding window edges are straight. The long window edges are parallel and extend in the (111) substrate plane in direction. Coalescence of SOI lamellae takes place without the formation of defects whenever it begins at one point and then proceeds in directions parallel to the longer edges of the windows in a zip-like mechanism. Defect-free coalescence seams reach lengths of up to 150 m.     

The coalescence of silicon layers grown over SiO2 by liquid-phase epitaxy

The coalescence of epitaxial silicon layers which are grown laterally over oxidized and patterned Si substrates is studied using various techniques of transmission electron microscopy (TEM). The epitaxial layers, the seam of coalescence and lattice defects formed by the coalescence are characterized. The epitaxial layer as a whole is found to be bent with respect to the substrate. Such misorientations, together with facetting of the growth fronts of the coalescing layers, may lead to the formation of solvent inclusions, dislocations and stacking faults at the seam of coalescence. However, under favourable conditions, the seam is found to be entirely defect-free.     

Photoluminescence of Er3+ ions in amorphous silicon layers under intensive laser excitation

Transients of the photoluminescence (1.54 μm) of Er³⁺ ions embedded in an amorphous silicon matrix excited with intensive laser pulses are simulated using a phenomenological model which takes into account both the defect-related excitation mechanism and stimulated optical transitions in the ions. The simulated transients are compared with the experimental ones observed in Er-doped amorphous silicon layers under pulsed laser excitation. The modeling and the experimental results demonstrate a possibility to realize a regime of superradiance in the system of Er³⁺ ions pumped via an electronic excitation of the amorphous matrix. Received: 7 August 2001 / Revised version: 1 November 2001 / Published online: 17 January 2002     

Phenomenology of picosecond heating and evaporation of silicon surfaces coated with SiO2 layers

Picosecond time-resolved reflectivity measurements on bare silicon surfaces and silicon surfaces with oxide layers reveal very fast heat diffusion and material evaporation on subnanosecond time scales. With a thick oxide layer resolidification of a molten silicon surface can take place in a few hundred picoseconds. At high laser fluences, vaporization processes take only a couple of 100 ps.     

Acoustic spectroscopy and electrical characterization of SiO2/Si structures with ultrathin SiO2 layers formed by nitric acid oxidation

Ultrathin silicon dioxide (SiO₂) layers formed on Si substrate with nitric acid have been investigated using both acoustic deep-level transient spectroscopy (A-DLTS) and electrical methods to characterize the interface states. The set of SiO₂/Si structures formed in different conditions (reaction time, concentrations of nitric acid (HNO₃), and SiO₂ thickness [3–9 nm]) was prepared. The leakage current density was decreased by post-oxidation annealing (POA) treatment at 250°C in pure nitrogen for 1 h and/or post-metallization annealing (PMA) treatment at 250°C in a hydrogen atmosphere for 1 h. All structures of the set, except electrical investigation, current-voltage (I - V), and capacitance — voltage (C - V) measurements, were investigated using A-DLTS to find both the interface states distribution and the role of POA and/or PMA treatment on the interface-state occurrence and distribution. The evident decreases of interface states and shift of their activation energies in the structures with PMA treatment in comparison with POA treatment were observed in most of the investigated structures. The results are analyzed and discussed.      

The properties of SiO2 layers produced by the thermal oxidation of silicon

The introduction contains a brief survey of the experimentally found properties of SiO₂ layers according to available sources of literature. The paper then deals with the procedure for determining the magnitude and number of imperfections in SiO₂ layers on the basis of measurements of the magnitude of the break-down voltage.     

Diffusion of As ions and self-diffusion in silicon during implantation

Experimental concentration profiles of As ions in a silicon substrate at temperatures of 20, 600, and 1050°C and ion current of 40 μA/cm², as well as at 1050°C and 10 μA/cm², are presented. On the basis of our and previously published experimental data, the process of radiation-stimulated ionic diffusion and self-diffusion in silicon is simulated. A number of interesting dependences, which are discussed in the conclusion of this study, are obtained.     

Getter formation in silicon by implantation of antimony ions

A porous silicon layer as a getter of uncontrolled impurities has been prepared by implantation of Sb⁺ into silicon and subsequent thermal treatments. The lifetime of nonequilibrium charge carriers in n- and p-type silicon wafers with a getter layer is 3–4 times longer than that without a getter.     

Properties of Al2O3: nc-Si nanostructures formed by implantation of silicon ions into sapphire and amorphous films of aluminum oxide

Physics of the Solid State - Photoluminescence, infrared Fourier spectroscopy, Raman scattering, transmission electron microscopy, and electron diffraction were used to study the luminescent,...     

Formation of cubic silicon carbide layers on silicon under the action of continuous and pulsed carbon ion beams

The structure and infrared absorption of cubic silicon carbide (β-SiC) layers produced by the continuous high-dose implantation of carbon ions (C⁺) into silicon (E=40 keV and D=5×10¹⁷ cm⁻²), followed by the processing of the implanted layers with a high-power nanosecond pulsed ion beam (C⁺, τ=50 ns, E=300 keV, and W=1.0–1.5 J/cm²), are investigated. Transmission electron microscopy and electron diffraction data indicate the formation of a coarse-grained polycrystalline β-SiC layer with grain sizes of up to 100 nm. A characteristic feature of such a layer is the dendritic surface morphology, which is explained by crystallization from the melt supercooled well below the melting point of β-SiC.     

Formation and microstructural investigation of Ag-Cu alloy nanoclusters embedded in SiO2 formed by sequential ion implantation

We report on the formation of alloy nanoclusters in silica by Ag/Cu ion sequential implantation. The formation of alloy nanocluster has been evidenced by optical absorption spectra, selected area electron diffraction and energy dispersive X-ray spectra. The microstructure characters of nanoclusters have been studied by high resolution transmission electron microscopy. The lattice distortion of some nanoclusters has been observed. A model has been given to explain the distortion. Some defects (partial dislocation, stacking faults) have been found in nanoclusters.     

Photoluminescence and structural defects in silicon layers implanted by iron ions

The photoluminescence spectra of silicon samples implanted by ⁵⁶Fe⁺ ions [energy, 170 keV; dose, 1×10¹⁶, (2–4)×10¹⁷ cm^?2] and annealed at temperatures of 800, 900, and 1000°C are measured. The structure of the samples at each stage of treatment is investigated using transmission electron microscopy (TEM). It is found that the phase formation and morphology of crystalline iron disilicide precipitates depend on the dose of iron ions and the annealing temperature. A comparison of the dependences of the intensity and spectral distribution of the photoluminescence on the measurement temperature, annealing temperature, and morphology of the FeSi₂ phase revealed the dislocation nature of photoluminescence.     

ESR of residual defects in silicon from recoil implantation of oxygen in Si/SiO2 systems

In ionic crystals or other insulators with partial ionic binding, the ion-target interaction differs from that of neutral atoms due to different electronic distributions and overall electrical charges Consequently, the nuclear stopping power and defect production by recoiling atoms will deviate from standard values, obtained from e g Moliere-potentials In the present paper, realistic potentials between projectile ion and target ion are determined by the free electron gas model of overlapping Hartree-Fock-Slater or Lenz-Jensen ions (and neutral atoms for comparison) With the new potentials, the transferred energies T and the range of interaction is determined for either damage production (T>E_d) and for nuclear stopping (T>hω> for bound ions) In addition the excitation of optical phonons is taken into account which are excited by the transient electrical field of the charged projectile     

Mechanical characteristics of composites of polylactide and nanosized calcium phosphates formed in supercritical carbon dioxide

Composite materials for biomedical applications based on polylactide, a bioresorbable polymer, and nanosized calcium phosphate (hydroxyapatite) were produced using supercritical (SC) carbon dioxide. For composites with different polymer-to-mineral ratios, the bending stress-strain diagrams were investigated. The elasticity moduli of composites obtained in SC-CO₂ from powdered bioresorbable polymer and bioactive nanoceramics ranged elastic modulus of natural bone. Experiments on adhesion of human embryonic fibroblasts demonstrated the suitability of generated samples for use in bone tissue scaffold engineering and regenerative medicine.