Photoluminescence induced by Si implantation into Si3N4 matrix

Up to the present, photoluminescence (PL) was obtained from near stoichiometric or amorphous Si nitride films (SiN_x) after annealing at high temperatures. As a consequence, the existence of PL bands has been reported in the 400–900 nm range. In the present contribution, we report the first PL results obtained by Si implantation into a stoichiometric 380 nm Si₃N₄ film. The Si excess is obtained by a 170 keV Si implantation at different temperatures with a fluence of Φ = 10¹⁷ Si/cm². Further, we have annealed the samples in a temperature range between 350 and 900 °C in order to form the Si precipitates. PL measurements were done using an Ar laser as an excitation source, and a broad PL band basically centered at 910 nm was obtained. We show that the best annealing condition is obtained at T_a = 475 °C for the samples implanted at 200 °C, with a PL yield 20% higher than the obtained at room temperature implantation. Finally, we have varied the implantation fluence and, consequently, the Si nanocrystals size. However, no variation was observed nor in the position neither in the intensity of the PL band. We concluded that the PL emission is due to radiative states at the matrix and the Si nanocrystals interface, as previously suggested in the literature.     

Strain enhancement in Si induced by direct bonding of a LaAlO3 film to a Si substrate

The depth profiles of lattice strain near the interface regions of LaAlO₃/Si and the SiO₂ interfacial layer/Si were investigated by the ion channeling technique using high-resolution Rutherford backscattering spectroscopy (HRBS). In the case of the LaAlO₃/Si stack, horizontal tensile strain in the Si near the interface was clearly observed. However, this strain was relaxed by formation of the interfacial layer through annealing in an oxygen ambient. These results suggest that the strain in Si induced by a dielectric strongly depends on the material in contact with Si.     

Strain-driven defect evolution in Sn+ implanted Si/SiGe multilayer structure

We report on secondary defect evolution in a multilayered Si/SiGe structure after 1 MeV Sn⁺-ion implantation to a fluence of 2 × 10¹⁴ cm^?2 followed by thermal annealing in a dry nitrogen atmosphere. Formation of a buried amorphous layer is registered after ion implantation. Thermal treatment leads to formation of dislocation loops in an EOR-defect band, and a mixture of tangle dislocations and “clamshell” defects at the depth of 200–500 nm. In addition, self-assembling of voids in a near-surface SiGe layer structure is observed. The voids are of nanometer size and are preferably located in thin SiGe layers. The results are discussed in terms of the separation of the vacancy and interstitial depth profiles attributed to the preferential forward momentum of recoiling Si atoms. The compressively strained SiGe layers play the role of vacancy accumulator, prevent in-surface diffusion of vacancies and, in this way, result in self-assembling of voids inside compressively strained SiGe layers.     

Damage creation in ion irradiated Si1−xGex/Si structures

Strained SiGe/Si structures have been proposed as substrates for fabrication of high speed metal oxide semiconductor transistors. However, influence of strain and/or presence of Ge atoms on damage creation during ion irradiation have not been explored to a significant extent. In this study, Rutherford backscattering spectrometry (RBS) was used to characterize Si_1−xGe_x/Si structures irradiated by 140 keV He⁺ ions at room temperature. When compared with pure Si, strained samples show enhanced damage accumulation as a function of He fluence. Channeling angular scans did not reveal any specific configuration of displacements. Possible mechanisms for enhanced damage in strained Si are discussed.     

Influence on the structural characteristic of defect solid solution GdxZr1−xO2−x/2 with 0.18 ? x ? 0.62 under longer term annealing studied by Gd Mössbauer spectroscopy

Three kinds of defect solid solution Gd_xZr_1−xO_2−x/2 with 0.18 ? x ? 0.62, including the three single crystal samples with x = 0.21, 0.26 and 0.30, were investigated by ¹⁵⁵Gd Mössbauer spectroscopy at 12 K. Difference in the structural characteristic under longer term annealing were confirmed by comparing the ¹⁵⁵Gd Mössbauer parameters of the polycrystalline samples sintered one time and twice at 1773 K for 16 h in air, respectively. The results indicated that the polycrystalline samples sintered twice have relatively equilibrated structure by comparing with the three single crystal samples. After being sintered twice, basically the local structure around the Gd³⁺ ions does not change, but the degree of the displacements of the six 48f oxygen ions from positions of cubic symmetry becomes slightly smaller, and distribution of the Gd³⁺ ions in the system becomes more homogeneous.     

Si/Gex Si1−x heterojunction bipolar transistors formed by Ge ion implantation in Si. Narrowing of band gap and base width

Si/Ge_xSi_1−x heterojunction n-p-n bipolar transistors (HBT) with a double polycrystalline silicon (polysilicon) self-aligned structure were fabricated by using high dose Ge implantation for the formation of the Si/Ge_xSi_1−x heterostructure and As and BF₂ implantation for emitter and base doping. Improvements in electrical characteristics compared to reference Si transistors are demonstrated and related to a band gap narrowing in the base region and to a reduction of B diffusion.     

Endotaxial growth of InSb nanocrystals at the bonding interface of the In+ and Sb+ ion implanted SOI structure

Growth of InSb nanocrystals at the Si/SiO₂ bonding interface of silicon-on-insulator (SOI) structures has been studied as a function of the annealing temperature. SOI structures with the ion implanted regions above and below the bonding interface were produced as a result of the hydrogen transfer of the Sb⁺ ion implanted silicon layer from first silicon substrate to the In⁺ ion implanted SiO₂ layer thermally-grown on the second silicon substrate. Rutherford backscattering spectrometry and high-resolution transmission electron microscopy (XTEM) were used to study the properties of the prepared structures. Up-hill diffusion of In and Sb atoms from the implantation regions toward the bonding interface as well as subsequent interface-mediated growth of InSb nanocrystals were observed as the annealing temperature achieved 1100 °C. The strain minimizing orientations of the Si and InSb lattice heteropairs were obtained from XTEM analysis of the grown nanocrystals.     

Thermal solid phase epitaxial growth and ion-beam induced crystallisation of Ge ion implanted layers in silicon

Solid phase epitaxial growth (SPEG) of amorphous SiGe layers in Si has been investigated. The amorphous layers were formed by 40 keV ⁷⁴Ge⁺ ion implantation in Si(100) single crystals with doses giving 22 at.% Ge at the maximum of the ion implanted distribution of Ge. SPEG of the amorphous layers was achieved by either thermal SPEG or a combination of thermal SPEG and ion-beam induced crystallisation (IBIC). The crystal quality of the layers was investigated by Rutherford backscattering spectrometry and transmission electron microscopy. Fully crystallised SiGe alloy layers were obtained by annealing in a furnace at 550°C for 60 min or at 850°C for 20 min. However, the SiGe alloy layers contain extended defects formed at the relaxation of the built-in strain in the alloy layer. When the combination of thermal SPEG and IBIC was used for the SPEG very few of these defects were formed.     

Effect of oxygen on ion-beam induced synthesis of SiC in silicon

The properties of Si-structures with a buried silicon carbide (SiC) layer created by high-dose carbon implantation into Cz–Si or Fz–Si wafers followed by high-temperature annealing were studied by Raman and infrared spectroscopy. The effect of additional oxygen implantation on the peculiarities of SiC layer formation was also studied. It was shown that under the same implantation and post-implantation annealing conditions the buried SiC layer is more effectively formed in Cz–Si or in Si (Cz-or Fz-) subjected to additional oxygen implantation. So we can conclude that oxygen in silicon promotes the SiC layer formation due to SiO_x precipitate creation and accommodation of the crystal volume in the region where SiC phase is formed. Carbon segregation and amorphous carbon film formation on SiC grain boundaries were revealed.     

Atomic simulation of energetic fluorine interacting with Si(0 0 1)

In this study, Molecular dynamics simulations were performed to investigate F continuously bombarding silicon surfaces at normal incidence and room temperature. The simulated results show that with increasing incident energy and temperature, the etch yield of Si atoms increases, which is in good qualitative agreement with experiments. Accompanying reaching the steady-state F uptake and Si etching, a steady-state SiF_x (x = 1-4) reactive layer is formed whose thickness increases with increasing incident energy. In the reaction layer, SiF species are dominant and SiF₃ species decrease with increasing incident energy.     

HRBS/channeling studies of ultra-thin ITO films on Si

High-resolution Rutherford backscattering spectroscopy (HRBS)/channeling techniques have been utilized for a detailed characterization of ultra-thin indium tin oxide (ITO) films and to probe the nature of the interface between the ITO film and the Si(0 0 1) substrate. Channeling studies provide a direct measure of the lattice strain distribution in the crystalline Si substrate in the case of amorphous over layers. The measurements on DC magnetron sputtered ITO films have been carried out using the recently installed HRBS facility at the Centre for Ion Beam Applications (CIBA). The thickness of the ultra-thin (∼9.8 nm) ITO films was calculated from the HRBS spectra having an energy resolution of about 1.4 keV at the superimposed leading (In + Sn) edge of the ITO film. The films were near stoichiometric and the interface between ITO film and Si was found to include a thin SiO_x transition layer. The backscattering yields from (In + Sn) of ITO were equal in random and channeling directions, thereby revealing the non-crystalline nature of the film. Angular scans of HRBS spectra around the off-normal [1 1 1] axis clearly showed a shift in the channeling minimum indicative of compressive strain of the Si lattice at the SiO_x/Si interface. The observed strain was about 0.8% near the interface and decreased to values below our detection limits at a depth of ∼3 nm from the SiO_x/Si interface.     

12C(α,α)12C resonant elastic scattering at 5.7 MeV as a tool for carbon quantification in silicon-based heterostructures

The incorporation of carbon into substitutional sites in Si or Si_1−xGe_x attracts increasing interest due to the enhanced possibilities in strain and band gap engineering of group IV heterostructures. Precise and accurate measurement of carbon concentration is, however, quite difficult to achieve. We focused our attention on the study of the alpha resonant elastic scattering in the 5.7 MeV energy region. We measured the scattering cross-section in the range 5.4–6.0 MeV at a laboratory scattering angle of 170°. The results indicate that the cross-section value is enhanced with respect to the Rutherford one of an almost constant factor (×130) in an energy interval about 100 keV wide. This allows a more accurate measurement of carbon concentration than with the normally used 4.265 MeV resonance. The experimental procedure to deal with non-Rutherford scattering of Si has been also determined. The resonant scattering at 5.72 MeV has been used, in combination with Rutherford Backscattering Spectrometry (RBS) at 3.0 MeV, to determine the carbon content of three Si_1−x−yGe_xC_y samples. This has also been used, in channelling geometry, to determine the substitutional carbon fraction of the samples.     

Spatial distribution of defects in ion-implanted and annealed Si: The RP/2 effect

Gettering of metal impurities in ion-implanted Si occurs midway between the surface and the projected ion range, R_P, after annealing at temperatures in the range of 700–1000°C and vanishes at higher temperatures. This phenomenon, called the R_P/2 effect, seems to be a common feature of ion-implanted and annealed Si. The gettering ability of the damage at R_P/2 is commensurate with or may exceed that of the damage at R_P. The defects around R_P/2 acting as gettering sites have not yet been identified by other analysis techniques. They are formed after ion implantation in the process of defect evolution during annealing and, probably, consist of small complexes of intrinsic defects (vacancies or/and self-interstitials).     

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 × 10¹⁶/cm² and 110 keV H ions at a fluence of 1 × 10¹⁶/cm², singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 μm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.     

Xenon-ion-induced and thermal mixing of Co/Si bilayers and their interplay

Studies on ion-irradiated transition-metal/silicon bilayers demonstrate that interface mixing and silicide phase formation depend sensitively on the ion and film parameters, including the structure of the metal/Si interface. Thin Co layers e-gun evaporated to a thickness of 50 nm on Si(1 0 0) wafers were bombarded at room temperature with 400-keV Xe⁺ ions at fluences of up to 3 × 10¹⁶ cm⁻². We used either crystalline or pre-amorphized Si wafers the latter ones prepared by 1.0-keV Ar-ion implantation. The as-deposited or Xe-ion-irradiated samples were then isochronally annealed at temperatures up to 700 °C. Changes of the bilayer structures induced by ion irradiation and/or annealing were investigated with RBS, XRD and HRTEM. The mixing rate for the Co/c-Si couples, Δσ²/Φ = 3.0(4) nm⁴, is higher than the value expected for ballistic mixing and about half the value typical for spike mixing. Mixing of pre-amorphized Si is much weaker relative to crystalline Si wafers, contrary to previous results obtained for Fe/Si bilayers. Annealing of irradiated samples produces very similar interdiffusion and phase formation patterns above 400 °C as in the non-irradiated Co/Si bilayers: the phase evolution follows the sequence Co₂Si → CoSi → CoSi₂.     

Influence of annealing temperatures and time on the photoluminescence properties of Si nanocrystals embedded in SiO2

We report on the effects of annealing conditions on the photoluminescence from Si nanocrystal composites fabricated by implantation of Si ions into a SiO₂ matrix, followed by thermal treatment in a nitrogen atmosphere. The evolution of the photoluminescence under different annealing temperatures (900–1100 °C) and annealing time (0.5 up to 5 h) were systematically studied for the implanted samples. After annealing the spectra presented two photoluminescence bands: one centered at 610 nm and another around 800 nm. Combined with transmission electron microscopy, we conclude that the photoluminescence behavior of the two bands suggests different origins for their emissions. The 610 nm band has its origin related to matrix defects, while the 800 nm band can be explained by a model involving recombination via quantum confinement effects of excitons in the Si nanocrystals and the interfacial states recombination process confined in the interfacial region between nanocrystals and SiO₂ matrix.     

Ion implantation studies on VOx films prepared by pulsed dc reactive sputtering

Vanadium oxide (VO_x) thin films find extensive use in room-temperature bolometers for IR imaging. It is desirable to control and modify the electronic properties of this temperature-sensitive material with treatments such as ion implantation and thermal annealing. In this work, we report on the modification of structural and electrical properties of VO_x thin films of varying compositions, deposited by pulsed dc reactive sputtering using a vanadium target under different oxygen flow rates. The as-deposited resistivities of the films ranged from 0.1 Ω cm to 100 Ω cm and the temperature coefficient of resistance (TCR) values varied from ?1.1% to ?2.7%. VO_x films used in microbolometers need to have a high TCR (>2%) and low resistivity values (1–10 Ω cm) in order to maximize sensitivity in conjunction with the read-out integrated circuit (ROIC). However, one usually finds a high TCR associated with high resistivity. Hence ion implantation followed by annealing was performed with the goal of improving the trade-off between TCR and resistivity. Two species – hydrogen (active) and helium (inert) – were chosen for implantation. Hydrogen is strongly electroactive and is well known for passivating defect states in a wide variety of electronic materials. As inert species, helium was chosen mainly to study the effects of bombardment on the film. The implanted films were annealed in an inert atmosphere to allow defect control and redistribution of atoms, and then characterized by current–voltage measurements over a wide temperature range. An order of magnitude change in resistance, and significant variations in TCR were observed. Further characterization has been done by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) to correlate these resistivity changes with the structure of the films.     

Effects of Li ion implantation energies on the structure, photoluminescence, and ferromagnetism properties of (Li, Co) co-implanted ZnO films

Room temperature ferromagnetism was observed in (Li, Co) co-implanted ZnO films. The implantation energy for Co ions was 400 keV, while for Li ions were 50, 100 and 200 keV, respectively. The ion implantation induced defects and disorder has been observed by the XRD, PL and TEM experiments. For the co-implanted ZnO films with Li ion implantation energies of 100 and 200 keV, the band energy emission disappears and the defect related emission with wavelength of 500-700 nm dominates, which can be attributed to defects introduced by implantation. Co-implanted ZnO Films with Li ion implantation energies of 200 keV show a saturation magnetization value (M_S) of over 9 × 10⁻⁵ emu and a positive coercive field of 60 Oe. The carrier concentration is not much improved after annealing and in the order of 10¹⁶ cm⁻³, which suggests that FM does not depend upon the presence of a significant carrier concentration. The origin of ferromagnetism behavior can be explained on the basis of electrons and defects that form bound magnetic polarons, which overlap to create a spin-split impurity band.     

Diffusion and strain relaxation in Si/Si1−xGex/Si structures studied with Rutherford backscattering spectrometry

The thermal stability of strained Si/Si_1−xGe_x/Si structures grown by molecular beam epitaxy was investigated by resistive heating and in situ Rutherford backscattering spectrometry. Ge profiles obtained from a 50 nm Si_1−xGe_x layer on a Si(100) substrate capped with 50 nm Si were evaluated for different Ge concentrations after sequential heating periods at a particular temperature between 850 and 1010° C. The diffusion coefficients, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable to the value for Ge in bulk Si. A more pronounced decrease of the signal at the center of the Ge profile indicated a faster diffusion within the SiGe layer which was confirmed by analysis of the FWHM of the Ge profile. Ion channeling measurements were used to characterize tetragonal strain in the buried SiGe layers. Angular scans through the 111 direction were interpreted with Monte Carlo channeling calculations and used to study strain relaxation in dislocation-free and partially relaxed layers.     

Epitaxial 3C-SiC nanocrystal formation at the SiO2/Si interface after carbon implantation and subsequent annealing in CO atmosphere

3C-SiC nanocrystallites were epitaxially formed on a single crystalline Si surface covered by a 150 nm thick SiO₂ capping layer after low dose carbon implantation and subsequent high temperature annealing in CO atmosphere. Carbon implantation is used to introduce nucleation sites by forming silicon–carbon clusters at the SiO₂/Si interface facilitating the growth of 3C-SiC nanocrystallites.