Angular distributions of sputtered atoms for low-energy nitrogen irradiation of silicon

We studied the angular distributions of silicon and nitrogen atoms emitted from a Si target subjected to reactive sputtering by N ₂ ⁺ ions at primary energies of 0.5 and 2keV. The composition of the deposited material does not depend strongly on the substrate position. From a comparison with nonreactive sputtering, we show that the observed shift of the Si angular distribution is mainly due to the contribution of collision events occurring in the first monolayer. Contrary to the case of noble gas ions, the sharpness of the Si distribution depends on the N ₂ ⁺ energy. The behavior of the differential sputtering yield of silicon indicates that this effect is likely to be due to a loss of recoil atoms out of the preferential direction. A possible explanation of the observed phenomena consists in assuming an anisotropic emission of Si _x N _y radicals. This hypothesis is very attractive as it could satisfactorily explain the similarity we observed between the angular distributions of silicon and nitrogen.     

Molecular beam epitaxy of monotype CrSi2 on Si(111)

Low energy electron diffraction (LEED), angle-resolved ultraviolet (ARUPS), and X-ray (XPS) photoemission spectroscopy and work function measurements were used to investigate the growth of epitaxial CrSi₂ on a Si(111) surface. The CrSi₂layers ) (~ 100 Å) are formed by the MBE technique, in which Cr and Si are coevaporated in their stoichiometric ratio on the Si(111) substrate maintained at ~450°C. In comparison with the CrSi₂ epitaxy previously obtained by the SPE technique, where two kinds of CrSi₂ domains with equal formation probability are always observed, the epitaxial CrSi₂ layers obtained by the MBE technique essentially present one definite orientation characterized by CrSi₂(0001)∥Si(111) and CrSi₂[112̄0] ∥[112̄].     

Influence of heat treatment on the structure and thermoelectric properties of CrSi2

Textured CrSi₂ crystals obtained by heating finely dispersed constituents (Si, Cr) are studied. The use of a mixture of Cr and Si powders makes it possible to lower the CrSi₂ synthesis temperature by 100 K. Crystallization conditions and post-crystallization annealing are found to influence the thermoelectric properties and composition of samples.     

Substrate effect on electronic sputtering yield in polycrystalline fluoride (LiF, CaF2 and BaF2) thin films

Influence of substrate on electronic sputtering of fluoride (LiF, CaF₂ and BaF₂) thin films, 10 and 100 nm thin, under dense electronic excitation of 120 MeV Ag²⁵⁺ ions irradiation is investigated. The sputtering yield of the films deposited on insulating (glass) and semiconducting (Si) substrates are determined by elastic recoil detection analysis technique. Results revealed that sputtering yield is higher, up to 7.4 × 10⁶ atoms/ion for LiF film on glass substrate, than that is reported for bulk materials/crystals (∼10⁴ atoms/ion), while a lower value of the yield (2.3 × 10⁶ atoms/ion) is observed for film deposited on Si substrate. The increase in the yield for thin films as compared to bulk material is a combined effect of the insulator substrate used for deposition and reduced film dimension. The results are explained in the framework of thermal spike model along with substrate and size effects in thin films. It is also observed that the material with higher band gap showed higher sputtering yield.     

Migration of CrSi2 nanocrystals through nanopipes in the silicon cap

CrSi₂ nanocrystals (NC¹) were grown by reactive deposition epitaxy of Cr at 550 °C. After deposition the Cr is localized in about 20-30 nm dots on the Si surface. The NCs were covered by silicon cap grown by molecular beam epitaxy at 700 °C. The redistribution of NCs in the silicon cap was investigated by transmission electron microscopy and atomic force microscopy. The NCs are partly localized at the deposition depth, and partly migrate near the surface. A new migration mechanism of the CrSi₂ NCs is observed, they are transferred from the bulk toward the surface through nanopipes formed in the silicon cap. The redistribution of CrSi₂ NCs strongly depends on Cr deposition rate and on the cap growth temperature.     

XRD and XPS characterisation of transition metal silicide thin films

Binary transition metal silicides based on the systems Ti–Si, Fe–Si, Ni–Si and Cr–Si were fabricated on Si wafers by means of ion-beam co-sputter deposition and subsequent annealing. The crystalline structures of the phases formed were identified from the characteristic patterns acquired by means of X-ray diffraction (XRD) measurements. The phase formation sequences were described by means of the Pretorius' effective heat of formation (EHF) model. For the Ti–Si, Fe–Si and Ni–Si systems, single phase thin films of TiSi₂, β-FeSi₂ and NiSi₂ were generated as the model predicts, while a mixture of CrSi + CrSi₂ phases was obtained for the Cr–Si system. The surface chemical condition of individual specimens was analysed by using X-ray photoelectron spectroscopy (XPS). The chemical shifts of transition metal 2p_3/2 peaks from their metallic to silicide states were depicted by means of the Auger parameters and the Wagner plots. The positive chemical shift of 2.0 eV for Ni 2p_3/2 peak of NiSi₂ is mainly governed by the initial-state effects. For the other silicide specimens, the initial-state and final-state effects may oppose one another with similar impact. Consequently, smaller binding energy shifts of both negative and positive character are noted; a positive binding energy shift of 0.3 eV for the Fe 2p_3/2 level was shown for β-FeSi₂ and negative binding energy shifts of 0.1 and 0.3 eV were determined for CrSi + CrSi₂ and TiSi₂, respectively.     

Surfactant effect of Sb on the growth of MnSi1.7 layers on Si(0 0 1)

Deposition of one monolayer of Sb prior to the deposition of Mn at 600 °C is observed to increase the MnSi_1.7 island density by about two orders of magnitude as well as to change the crystalline orientation of the silicide grains. The preferential epitaxial orientation of MnSi_1.7 grains grown by this process is determined to be MnSi_1.7(1 0 0)[0 1 0]||Si(0 0 1)[1 0 0]. This growth procedure results in the silicide growth into the Si matrix. For comparison, the same deposition process carried out without Sb leads to silicide formation on top of the substrate surface. The observed morphological changes of the MnSi_1.7 layers can be explained by a reduced surface diffusion of the Mn atoms on Si(0 0 1) in presence of the Sb monolayer. Additionally, lateral Si diffusion is considered to be nearly suppressed, which is responsible for the observed silicide growth into the substrate.     

Study of the interaction of argon and nitrogen ions with the silicon dioxide surface

Angular dependences of the sputtering yield, surface layer composition, and changes in the mass spectra of residual atmosphere upon irradiation of silicon dioxide with argon and nitrogen ions are measured. It is found that the sputtering yield of SiO₂ bombarded by N ₂ ⁺ ions is almost two times higher than for Si. The sputtering yields of SiO₂ and Si irradiated with Ar ions are identical, although the binding energy of atoms in SiO₂ is almost two times higher than in Si. An analysis of the surface composition and residual atmosphere near the sample during its irradiation suggests that a chemical reaction is involved in SiO₂ sputtering. Molecules of SiO and NO gases form in the surface layer, whose subsequent desorption increases the SiO₂ sputtering rate.     

Sputtering-induced nanometre hole formation in Ni3Al under intense electron beam irradiation

The irradiation damage of polycrystalline Ni₃Al thin foils of stoichiometric composition by a stationary nanoscale 200?keV field emission gun (FEG) electron probe in a FEI Tecnai F20 (S)TEM has been investigated. At current densities greater than 10⁷?A/m², nanometre holes are produced quickly with both ?001? and ?110? incident electron beam directions. EDX spectra from the irradiated volume have been collected simultaneously during the hole forming process. From the EDX results, preferential surface sputtering of aluminium from Ni₃Al has been demonstrated. To understand the underlying physical process of sputtering, modelling based on a combination of molecular dynamics and Monte Carlo simulation has been performed. It appears to reproduce faithfully the overall film sputtering and hole formation processes, but is not capable of predicting the detailed geometry of the hole. It predicts that the sputtering cross-section of Al atoms is much higher than that of Ni atoms, resulting in a very small concentration of Al at the surface. This, together with the increase of surface area during hole formation, explains the preferential Al loss observed from the specimen. Calculated sputtering rates agree well with experiment, and are of the order of magnitude of 10^?8?atoms/electron.     

Synthesis of chromium silicide with laser pulses

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm². Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi₂ layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.     

The electron distribution in superconducting alloys: II. Room temperature analysis of Cr3Si and comparison with V3Si

The charge distribution in Cr₃Si a non-superconducting alloy of the A-15 family was obtained from accurate X-ray diffraction data. The deformation density in the bond between two adjacent Cr atoms in Cr₃Si is 0.17 eA^?3, about three times lower than the density observed in the isomorphous V₃Si structure. Integration of the charge around the Cr and Si atoms leads to a best estimate of 1.3–1.9 electrons for the charge transfer from the Si to three Cr atoms as compared to 1.8 – 2.4 electrons transfered from the Si to the three V atoms in V₃Si.     

On the chemical sputtering of oxygen-exposed molybdenum

The sputtering of oxygen-exposed molybdenum was studied by means of mass analysis of emitted neutral and charged particles. The irradiation was performed with 8 keV Ar⁺ ions at temperatures of 25° and 485°C. It was found that the enhanced sputtering yield at elevated temperature during oxygen exposure is due to beam-induced desorption of MoO₂ and cascade sputtering of MoO. At this temperature considerable oxygen incorporation also takes place owing to recoil mixing and diffusion.EURATOM Association     

Mechanisms of ion-beam-enhanced diffusion in amorphous silicon

We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×10²⁰ atoms/cm³. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B.     

AES analysis of nitridation of Si(100) by 2–10 keV N+2 ion beams

Ion beam nitridation of Si(100) as a function of N⁺₂ ion energy in the range of 2–10 keV has been investigated by in-situ Auger electron spectroscopy (AES) analysis and Ar⁺ depth profiling. The AES measurements show that the nitride films formed by 4–10 keV N⁺₂ ion bombardment are relatively uniform and have a composition of near stoichiometric silicon nitride (Si₃N₄), but that formed by 2 keV N⁺₂ ion bombardment is N-rich on the film surface. Formation of the surface N-rich film by 2 keV N⁺₂ ion bombardment can be attributed to radiation-enhanced diffusion of interstitial N atoms and a lower self-sputtering yield. AES depth profile measurements indicate that the thicknesses of nitride films appear to increase with ion energy in the range from 2 to 10 keV and the rate of increase of film thickness is most rapid in the 4–10 keV range. The nitridation reaction process which differs from that of low-energy (< 1 keV) N⁺₂ ion bombardment is explained in terms of ion implantation, physical sputtering, chemical reaction and radiation-enhanced diffusion of interstitial N atoms.     

Simulations of C60 bombardment of Si, SiC, diamond and graphite

Molecular dynamics simulations of the 20-keV C₆₀ bombardment at normal incidence of Si, SiC, diamond and graphite targets were performed. The unique feature of these targets is that strong covalent bonds can be formed between carbon atoms from the C₆₀ projectile and atoms in the solid material. The mesoscale energy deposition footprint (MEDF) model is used to gain physical insight into how the sputtering yields depend on the substrate characteristics. A large proportion of the carbon atoms from the C₆₀ projectile are implanted into the lattice structure of the target. The sputtering yield from SiC is ∼twice that from either diamond or Si and this can be explained by both the region of the energized cylindrical tract created by the impact and the number density. On graphite, the yield of sputtered atoms is negligible because the open lattice allows the cluster to deposit its energy deep within the solid. The simulations suggest that build up of carbon with a graphite-like structure would reduce any sputtering from a solid with C₆₀⁺ bombardment.     

Surface oxidation investigation of Ni36Fe32Cr14P12B6 glass using angle resolved XPS

Native oxide and in-situ prepared, dry oxides of Ni₃₆Fe₃₂Cr₁₄P₁₂B₆ metallic glass have been investigated using angle resolved X-ray photoelectron spectroscopy (XPS or ESCA). The core-level binding energies of the various constituents of clean and oxidized samples have been determined accurately. A qualitative as well as quantitative estimation of elements in the outermost surface layers with and without oxidation is given by comparing XPS results obtained at normal and grazing emission angles. Stepwise oxidation leads to growing thickness of the surface oxide layer and to identification of different oxide species. The maximum thickness of the in-situ prepared oxide was determined as 3.5 nm compared to 4.5 nm for the native oxide. The sequence of oxidation is found to be Cr, Fe, B, P and Ni, but only some of the P and Ni atoms in the surface region are oxidized. The oxidation reaction induces diffusion of the constituents in the surface region as monitored by the change of relative intensities of the various peaks. For instance, P and especially Ni are strongly depleted in the oxide layer whereas Fe, Cr, and especially B are enriched. Differences between native and dry oxide have been observed and are discussed. The main difference is the abundance of carbon and oxygen containing species other than oxides in the native layer. Ar⁺ sputtering of the dry oxide layer leads to stochiometric changes in the surface region which are due to preferential sputtering.     

Temperature dependent preferential sputtering in CoSi2 and NbSi2

Sputtering of CoSi₂ and NbSi₂ has been carried out by Xe ion bombardment at room temperature, as well as at elevated temperatures putting these systems in their radiation-enhanced diffusion regimes. The range of the Xe ions (at 200–260 keV) was appreciably less than the thickness of the silicides. The samples were analyzed by 2 MeV He⁺ backscattering spectrometry, x-ray diffraction and optical microscopy. The ratio of the sputtering yield of Si to that of the metal (i.e., Co or Nb) always exceeds the stoichiometric ratio 21, leading to Si depleted surface layers. The amount of the sputtered species increases almost linearly with dose until intermixing of the silicide with the underlying Si becomes appreciable. This happens at lower doses in the radiation-enhanced diffusion regime than at room temperature. Irradiation of CoSi₂ samples at high temperature leads to a broadening of the implanted Xe profile compared to the room temperature profile. No such phenomenon has been found in NbSi₂. The effect of Xe broadening on the sputtering yields is discussed.     

Optical properties of silicon-silicide nanoheterostructures grown by consecutive plasma-epitaxy synthesis

Consecutive plasma-epitaxial synthesis on silicon wafers is used for the first time to fabricate monolithic nanoheterostructures with embedded nanocrystals (NC) of chromium disilicide (Si–NC CrSi₂–Si). It is found that, initially, nanoislands form on the surface and within a coating layer of silicon, followed by the formation of small (10–15 nm) nanocrystals of semiconducting chromium disilicide (CrSi₂) at a high occupation density ((2–3)⋅10¹¹ cm^–2). During formation of silicon-silicide-silicon heterostructures, CrSi₂ nanocrystallites “float up” into the near surface area of the covering silicon layer.     

Generation mechanism and thermal stability of high carrier concentrations by KrF-excimer-laser doping of Si into GaAs

The generation mechanism and thermal stability of high carrier concentrations in GaAs formed by KrF-excimer-laser doping with Si using SiH₄ gas are investigated. The channeling Particle-Induced X-ray Emission (PIXE) analysis reveals that a high substitutional fraction of over 90% and preferential replacement of Si atoms on Ga sites result in the generation of carrier concentrations as high as 5×10¹⁹ cm^–3. In addition, the thermal stability of the doped regions is studied. The high carrier concentrations in a nonthermal equilibrium state return to a thermal equilibrium state by postannealing.Presented at LASERION'93, Munich, June 21–23, 1993     

Laser-assisted synthesis of semiconductor chromium disilicide films

Different photo-assisted techniques were employed for chromium disilicide (CrSi₂) semiconductor film fabrication. Flash evaporation of CrSi₂ powder on the Si substrate heated to ∼740 K leads to the formation (according to XRD study) of amorphous films. Post-annealing at 920 K leads to the formation of polycrystalline CrSi₂ phase. Crystallization is improved by further annealing with 1500 Q-Switched Nd:YAG laser pulses. Optical properties of the as deposited and annealed CrSi₂ films have been investigated in the 240-1100 nm spectral range by using spectroscopic ellipsometry. The formation of CrSi₂ semiconductor phase was additionally confirmed by the temperature dependence of electrical resistance of the films treated by Q-switched Nd:YAG laser. The band gap for intrinsic conductivity results E_g ≅ 0.2 eV. Backward laser-induced film transfer (LIFT) was also used for CrSi₂ film deposition from bulk material on Si substrates. Pulsed CO₂ laser was employed for this purpose, because of transparency of silicon at the 10.6 μm wavelength. Measurements of the electrical resistance of the deposited films as a function of temperature showed their semiconductor behavior (E_g = 6 × 10⁻⁴ eV). Chromium disilicide films were also deposited by congruent pulsed laser ablation deposition on Si substrates either at room temperature or heated to about 740 K. In this last case the deposit exhibits semiconducting properties with E_g ≅ 0.18 eV.