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.     

Nanometer-thick SGOI structures produced by Ge+ ion implantation of SiO2 films and subsequent hydrogen transfer of Si layers

Nanometer-thick silicon-germanium-on-insulator (SGOI) structures have been produced by the implantation of Ge⁺ ions into thermally grown SiO₂ layer and subsequent hydrogen transfer of silicon film on the Ge⁺ ion implanted substrate. The intermediate nanometer-thick Ge layer has been formed as a result of the germanium atom segregation at the Si/SiO₂ bonding interface during annealing at temperatures 800–1100 ^оС. From a thermodynamic analysis of Si/Ge/SiO₂ system, it has been suggested that the growth of the epitaxial Ge layer is provided by the formation of a molten layer at the Si/SiO₂ interface due to the Ge accumulation. The effect of germanium on the hole mobility in modulation-doped heterostructures grown over the 3–20 nm thick SGOI layers was studied. An increase in the Hall hole mobility in SGOI-based structures by a factor of 3–5 was obtained in comparison with that in respective Ge-free SOI structures.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

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.     

Strain relaxation of epitaxial SiGe layers on Si(1 0 0) improved by hydrogen implantation

We propose a new method to fabricate strain relaxed high quality Si_1−xGe_x layers on Si by hydrogen implantation and thermal annealing. Hydrogen implantation is used to form a narrow defect band slightly below the SiGe/Si interface. During subsequent annealing hydrogen platelets and cavities form, giving rise to strongly enhanced strain relaxation in the SiGe epilayer. As compared to thermally induced strain relaxed Si–Ge epilayers, the hydrogen implanted and annealed samples show a greatly reduced threading dislocation density and a much higher degree of strain relaxation (90%). We assume that the hydrogen induced defect band promotes strain relaxation via preferred nucleation of dislocation loops in the defect band which extend to the interface to form misfit segments. The samples have been investigated by X-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Effect of implanted Si concentration on the Si nanocrystal size and emitted PL spectrum

Implantation of Si⁺ in excess into SiO₂ followed by annealing produces Si nanocrystals (Si-nc) embedded in the SiO₂ layer, which can emit a strong photoluminescence (PL) signal. Several samples have been characterized by means of ellipsometry and transmission electron microscopy (TEM). For local Si concentrations in excess of ∼2.4 × 10²² Si⁺/cm³, the Si-nc diameter ranges from ∼2 to ∼22 nm in the whole sample, the Si-nc in the middle region of the implanted layer being bigger than those near the surface or the bottom of the layer. The depth distribution of the Si-nc agrees relatively well with the SRIM simulation as well as with the depth distribution of the n and k components of the complex refractive index. For SiO₂ layers thermally grown on a Si wafer, the PL spectrum is modulated by optical interference of the pump laser and of the light emitted by the Si-nc in this layer. The good agreement between the results of the model calculations and experimental measurements indicates that for low and moderate Si concentration in excess (<8 × 10²¹ cm⁻³) the PL light emitters are localized in a layer situated at the same depth as the Si-nc depth distribution. However, for a Si concentration in excess of ∼2.3 × 10²² cm⁻³, the depth distribution of light emitters is narrow and situated mostly in the first half (relative to the surface) of the Si-nc depth distribution. This observation indicates that the recombination of the electron–hole pair at the interfaces could be responsible for the emitted PL spectrum.     

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.     

Nuclear resonant reflectivity with standing waves for the investigation of a thin Fe layer buried inside a superconducting Si/[Mo/Si]45/Fe/Nb multilayer

A special multilayer sample Si/[Mo/Si]₄₅/⁵⁷Fe/Nb has been prepared for the depth selective investigations of the hyperfine fields in thin iron layer at low temperatures above and below the superconducting transition in the top Nb layer (T_c ∼ 8 K) by means of the nuclear resonant reflectivity with standing waves. The periodic multilayer [Mo/Si]₄₅ below the iron layer in our sample was used as “a standing wave generator”. A weak magnetic hyperfine splitting in the ⁵⁷Fe layer was detected just at low temperature. A slight variation of the nuclear resonant reflectivity time spectra measured above and below T_c was observed. At first it was supposed that this change of the spectrum shape is caused by the spatial modulation of ferromagnetic domains in the ⁵⁷Fe layer caused by a proximity effect. A closer analysis, however, reveals that the spectrum variations are due to just the changes of the relative weights of the magnetic and paramagnetic phases in ⁵⁷Fe layer.     

Joining C/C composite to copper using active Cu–3.5Si braze

A simple technique was developed to join C/C composite to Cu using active Cu–3.5Si braze for nuclear thermal applications. The brazing alloy exhibited good wettability on C/C substrate due to the reaction layer formed at the interface. A strong interfacial bond of the brazing alloy on C/C with the formation of TiC + SiC + Ti₅Si₃ reaction layer was obtained. The produced CC/Cu/CuCrZr joint exhibited shear strength as high as 79 MPa and excellent thermal resistance during the thermal shock tests.     

Light particle irradiation effects in Si nanocrystals

Si nanocrystals, formed by Si ion implantation into SiO₂ layers and subsequent annealing at 1150°C, were irradiated at room temperature either with He⁺ions at energies of 30 or 130 keV, or with 400 keV electrons. Transmission electron microscopy (TEM) and photoluminescence (PL) studies were performed. TEM experiments revealed that the Si nanocrystals were ultimately amorphized (for example at ion doses ∼10¹⁶ He cm⁻²) and could not be recrystallized by annealing up to 775°C. This contrasts with previous results on bulk Si, in which electron- and very light ion-irradiation never led to amorphization. Visible photoluminescence, usually ascribed to quantum-size effects in the Si nanocrystals, was found to decrease and vanish after He⁺ ion doses as low as 3 × 10¹²–3 × 10¹³ He cm⁻² (which produce about 1 displacement per nanocrystal). This PL decrease is due to defect-induced non-radiative recombination centers, possibly situated at the Si nanocrystal/SiO₂ interface, and the pre-irradiation PL is restored by a 600°C anneal.     

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.     

Defect- and strain-enhanced cavity formation and Au precipitation at nano-crystalline ZrO2/SiO2/Si interfaces

Defect- and strain-enhanced cavity formation and Au precipitation at the interfaces of a nano-crystalline ZrO₂/SiO₂/Si multilayer structure resulting from 2 MeV Au⁺ irradiation at temperatures of 160 and 400 K have been studied. Under irradiation, loss of oxygen is observed, and the nano-crystalline grains in the ZrO₂ layer increase in size. In addition, small cavities are observed at the ZrO₂/SiO₂ interface with the morphology of the cavities being dependent on the damage state of the underlying Si lattice. Elongated cavities are formed when crystallinity is still retained in the heavily-damaged Si substrate; however, the morphology of the cavities becomes spherical when the substrate is amorphized. With further irradiation, the cavities appear to become stabilized and begin to act as gettering sites for the Au. As the cavities become fully saturated with Au, the ZrO₂/SiO₂ interface then acts as a gettering site for the Au. Analysis of the results suggests that oxygen diffusion along the grain boundaries contributes to the growth of cavities and that oxygen within the cavities may affect the gettering of Au. Mechanisms of defect- and strain-enhanced cavity formation and Au precipitation at the interfaces will be discussed with focus on oxygen diffusion and vacancy accumulation, the role of the lattice strain on the morphology of the cavities, and the effect of the binding free energy of the cavities on the Au precipitation.     

Positron annihilation on the surfaces of SiO2 films thermally grown on single crystal of Cz-Si

Two-detector coincidence system and mono-energetic slow positron beam has been applied to measure the Doppler broadening spectra for single crystals of SiO₂, SiO₂ films with different thickness thermally grown on single crystal of Cz-Si, and single crystal of Si without oxide film. Oxygen is recognized as a peak at about 11.85 × 10⁻³m₀c on the ratio curves. The S parameters decrease with the increase of positron implantation energy for the single crystal of SiO₂ and Si without oxide film. However, for the thermally grown SiO₂-Si sample, the S parameters in near surface of the sample increase with positron implantation energy. It is due to the formation of silicon oxide at the surface, which lead to lower S value. S and W parameters vary with positron implantation depth indicate that the SiO₂-Si system consist of a surface layer, a SiO₂ layer, a SiO₂-Si interface layer and a semi-infinite Si substrate.     

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.     

The property of Si/SiGe/Si heterostructure during thermal budget characterized by HRXRD

Si/SiGe/Si heterostructures grown by ultra-high-vacuum chemical vapor deposition (UHVCVD) were characterized by Rutherford backscattering/Channeling (RBS/C) together with high resolution X ray diffraction (HRXRD). High quality SiGe base layer was obtained. The Si/SiGe/Si heterostructures were subject to conventional furnace annealing and rapid thermal annealing with temperature between 750℃ and 910℃. Both strain and its relaxation degree in SiGe layer are calculated by HRXRD combined with elastic theory, which are never reported in other literatures. The rapid thermal annealing at elevated temperature between 880℃ and 910℃ for very short time had almost no influence on the strain in Si0.84Ge0.16 epilayer. However, high temperature (900℃@) furnace annealing for 1h prompted the strain in Si0.84Ge0.16 layer to relax.     

Structural analysis of DC magnetron sputtered and spin coated thin films using RBS, TEM and X-ray reflectivity methods

Metallic thin films such as Au, Cr, Ag, etc., on silicon substrate have many technologically important applications as contact layers in microelectronic industry, as reflecting mirrors in synchrotron radiation research, etc. The native oxide layer on crystalline silicon surface inhibits wetting of few nm thick Au or Ag on native oxide/silicon systems. To obtain continuous thin metallic films (a few nm thick), a Cr layer was first deposited as a adhesion layer on the Si substrate. In this paper, Rutherford backscattering analysis (RBS) of Si/Cr/SiO₂/Si, Si/Au/SiO₂/Si, Si/Au/Cr/SiO₂/Si and Polystyrene (PS) polymer coated on some of these bi- or tri-layer structures has been reported. The X-ray reflectometry and transmission electron microscopy studies were carried out to complement the RBS measurements. The thickness, surface and interface roughness, and crystalline quality have been determined.     

Characterization of the interaction layer grown by interdiffusion between U-7wt%Mo and Al A356 alloy at 550 and 340 °C

U(Mo) alloys are under study to get a low-enriched U fuel for research and test reactors. Qualification experiments of dispersion fuel elements have shown that the interaction layer between the U(Mo) particles and the Al matrix behaves unsatisfactorily. The addition of Si to Al seems to be a good solution. The goal of this work is to identify the phases constituting the interaction layer for out-of-pile interdiffusion couples U(Mo)/Al(Si). Samples γU-7wt%Mo/Al A356 alloy (7.1 wt%Si) made by Friction Stir Welding were annealed at 550 and 340 °C. Results from metallography, microanalysis and X-ray diffraction, indicate that the interaction layer at 550 °C is formed by the phases U(Al,Si)₃, U₃Si₅ and Al₂₀Mo₂ U, while at 340 °C it is formed by U(Al,Si)₃ and U₃Si₅. X-ray diffraction with synchrotron radiation showed that the Si-rich phase, previously reported in the interaction layer at 550 °C near U(Mo) alloy, is U₃Si₅.     

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.     

Ion beam studies on reactive DC sputtered manganese doped indium tin oxide thin films

Indium based transparent conducting oxides doped with magnetic elements have been studied intensively in recent years with a view to develop novel ferromagnetic semiconductors for spin-based electronics. In the present work, we have grown manganese doped indium tin oxide (Mn:ITO) thin films, onto Si and Si/SiO₂ substrates by DC reactive sputtering of a composite target containing indium-tin alloy and manganese, in a gas mixture of oxygen and argon. Glancing angle X-ray diffraction (GXRD) studies reveal the polycrystalline nature of the films. Magnetic measurements carried out using vibrating sample magnetometer (VSM) suggest that the films are ferromagnetic at room temperature, with a saturation magnetization of ∼22.8 emu/cm³. The atomic percentages of In, Sn, Mn and O, as estimated using Rutherford backscattering spectrometry (RBS) are 37.0, 4.0, 1.6 and 57.4, respectively. RBS measurements reveal that the interface of the films with Si substrate has a ∼30 nm thick intermediate layer. This layer consists of oxygen, silicon, indium, tin and manganese, in the ratio 1:0.56:0.21:0.07:0.03, indicative of diffusion of elements across the interface. The films on Si/SiO₂, on the other hand, have a sharp interface.