Molecular-dynamics simulation of Si1−xGex epitaxial growth on Si(100)

Molecular dynamics study of the Si_1−xGe_x epitaxial growth on Si(1 0 0) substrate utilizing the Stillinger–Weber two- and three-body interaction potentials was carried out. The Stranski–Krastanov growth mechanism of the Si_1−xGe_x strain layers on Si(1 0 0) was studied and compared with experiment results. The influence of different x on the epitaxial growth layers morphology was investigated. The structure properties of the Si_1−xGe_x layers were evaluated.     

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.     

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.     

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.     

The di-vacancy in particle-irradiated,strain-relaxed SiGe

The di-vacancy is known to introduce three energy levels in the energy-band gap of Si. Using deep level transient spectroscopy (DLTS) on particle-irradiated p⁺n and n⁺p diodes, we have followed these levels in epitaxially grown, strain-relaxed Si_1−xGe_x as a function of x for 0⩽x⩽0.5. Both the single- and double-acceptor levels located in the upper half of the band gap in Si move gradually deeper into the gap with increasing x. While the double-acceptor level remains in the upper half of the band gap of the alloys, the single-acceptor level crosses the mid-gap for x≈0.25. The donor level becomes gradually more shallow, but remains pinned to the conduction band. The anneal temperature of the di-vacancy is found to be independent of composition in the investigated composition range.     

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.     

Matrix effects in SIMS analysis of high-dose boron implanted silicon wafers

Secondary ion emission from ¹¹B⁺ implanted silicon wafers with dose of 1 × 10¹²−5 × 10¹⁶ cm⁻² has been investigated. Experiments were performed using O₂⁺ primary ions with an impact energy of 8.0 keV and an incident angle of 39° from the surface normal. The emission of ¹¹B⁺ is enhanced and ²⁸Si⁺ is suppressed at the peak region of boron profile for the high-dose sample, such as at doses ⩾ 5 × 10¹⁵ cm⁻² (peak concentration ∼5 × 10²⁰ cm⁻³). The secondary ion energy distribution of ¹¹B⁺ is broadened and ²⁸Si⁺ is sharpened with increasing the boron concentration. The mechanisms of these phenomena are also considered.     

Electrically active defects in BF2+ implanted and germanium preamorphized silicon

Ultra-shallow p⁺-n junctions have been formed using 15 keV/10¹⁵ cm⁻² BF₂⁺ implantation into both Ge⁺-preamorphized and crystalline 〈1 0 0〉 silicon substrates. Rapid thermal annealing (RTA) for 15 s at 950°C was used for dopant electrical activation and implantation damage gettering. The electrically active defects present in these samples were characterized using Deep Level Transient Spectroscopy (DLTS) and isothermal transient capacitance (ΔC(t, T)). Two electron traps were detected in the upper half of the band gap at, respectively, E_c - 0.20 eV and E_c - 0.45 eV. They are shown to be related to Ge⁺ implantation-induced damage. On the other hand, BF₂⁺ implantation along with RTA give rise to a depth distributed energy continuum which lies within the forbidden gap between E_c - 0.13 eV and E_c - 0.36 eV. From isothermal transient capacitance (ΔC(t, T)), reliable damage concentration profiles were derived. They revealed that preamorphization induces not only defects in the regrown silicon layer but also a relatively high concentration of electrically active defects as deep as 3.5 μm into the bulk.     

Damage accumulation and annealing in 6H–SiC irradiated with Si+

Damage accumulation and annealing in 6H-silicon carbide (α-SiC) single crystals have been studied in situ using 2.0 MeV He⁺ RBS in a 〈0 0 0 1〉-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30° off normal) at a temperature of −110°C, and the damage recovery was investigated by subsequent isochronal annealing (20 min) over the temperature range from −110°C to 900°C. At ion fluences below 7.5 × 10¹³ Si⁺/cm² (0.04 dpa in the damage peak), only point defects appear to be created. Furthermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences, amorphization occurs; however, partial damage recovery at RT is still observed, even at a fluence of 6.6 × 10¹⁴ Si⁺/cm² (0.35 dpa in the damage peak) where a buried amorphous layer is produced. At an ion fluence of 6.0 × 10¹⁵ Si⁺/cm² (−90°C), an amorphous layer is created from the surface to a depth of 0.6 μm. Because of recovery processes at the buried crystalline–amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from −90°C to 600°C.     

Improvement on thermoelectric properties of multilayered Si1−xGex/Si by ion beam bombardment

We made n-type nano-scale thin film thermoelectric (TE) devices that consist of multiple periodic layers of Si_1−xGe_x/Si. The period is about 10 nm. The structure was modified by 5 MeV Si ion bombardment that formed a nano-scale cluster structure. In addition to the effect of confinement of the phonon transmission, formation of nanoclusters by the ionization energy of incident MeV Si ions further increases the scattering of phonons, increasing the chance of inelastic interaction of phonons, resulting in more annihilation of phonons. This limits phonon mean free path. Phonons are absorbed and dissipated along the layers rather than in the direction perpendicular to the layer interfaces, therefore cross plane thermal conductivity is reduced. The increase of the density of electronic states due to the formation of nanocluster minibands increases the cross plane Seebeck coefficient and increases the cross plane electric conductivity of the film. Eventually, the thermoelectric figure of merit of the TE film increases.     

A novel (SiC)1−x(AlN)x compound synthesized using ion beams

The formation of a novel (SiC)_1−x(AlN)_x compound (x=0.2) at low temperatures within the miscibility gap of the SiC/AlN phase diagram by hot, high-dose co-implantation of N⁺ and Al⁺ ions into 6H–SiC substrates is investigated. The compound layers have been studied by Rutherford backscattering spectrometry/ion channelling (RBS/C), Auger electron spectroscopy (AES), polarized infrared reflection spectroscopy (PIRR) and cross sectional electron microscopy (XTEM) and the temperature dependence of their fabrication has been examined. An optimum temperature window has been established within which the structure of the synthesized material retains good crystallinity during implantation.     

Surface analytical studies of ion-implanted uni-directionally aligned silicon nitride for tribological applications

Uni-directionally aligned silicon nitride, which exhibits both high strength and high toughness, was implanted with B⁺, N⁺, Si⁺ and Ti⁺ ions at a fluence of 2 × 10¹⁷ ions/cm² and an energy of 200 keV. The effect of ion implantation on the surface structure of the uni-directionally aligned silicon nitride has been studied, in terms of surface analyses such as atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray absorption near edge structure (XANES). It was clarified that the ion-implanted layer was amorphized and the implantation profile showed good agreement with that estimated from a TRIM simulation. It was found that BN and TiN were formed in B⁺- and Ti⁺-implanted Si₃N₄, respectively. There was a slight difference in ion implantation depth among different structures of Si₃N₄, considered to be due to differences in ion channeling.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.     

Ionization probability of secondary ions sputtered from Si(1 1 1) and Ge(1 1 1) surfaces

We have measured the energy distributions of the secondary ions sputtered from the Si(1 1 1) and Ge(1 1 1) surfaces and investigated the ionization probabilities of sputtered Si⁺ and Ge⁺ ions for clarifying their ionization mechanisms. The observed ionization probabilities depend on the velocity of Si⁺ and Ge⁺ ions. This velocity dependence can be successfully analyzed by a theoretical expression, which was proposed originally for the metal surfaces. This implies that the ionization mechanism of Si⁺ and Ge⁺ ions is the same as ions sputtered from the metal surface, i.e., the resonant electron transfer in the high velocity regime and the thermal excitation process in the low velocity regime. The difference in the ionization probability between Si⁺ and Ge⁺ ions is well explained by the difference in the band gap energy.     

Proton beam induced luminescence of silicon dioxide implanted with silicon

Light emission from a silicon dioxide layer enriched with silicon has been studied. Samples used had structures made on thermally oxidized silicon substrate wafers. Excess silicon atoms were introduced into a 250-nm-thick silicon dioxide layer via implantation of 60 keV Si⁺ ions up to a fluence of 2 × 10¹⁷ cm⁻². A 15-nm-thick Au layer was used as a top semitransparent electrode. Continuous blue light emission was observed under DC polarization of the structure at 8-12 MV/cm. The blue light emission from the structures was also observed in an ionoluminescence experiment, in which the light emission was caused by irradiation with a H₂⁺ ion beam of energy between 22 and 100 keV. In the case of H₂⁺, on entering the material the ions dissociated into two protons, each carrying on average half of the incident ion energy. The spectra of the emitted light and the dependence of ionoluminescence on proton energy were analyzed and the results were correlated with the concentration profile of implanted silicon atoms.     

Heteroepitaxial Er0.49Gd0.51Si1.7 layers formed by channeled ion beam synthesis

Epitaxial ternary silicide Er_0.49Gd_0.51Si_1.7 layers with a good crystalline quality (χ_min of Er and Gd is 3.7%) have been formed by 60 keV Er and Gd ion implantation into Si(1 1 1) substrates to a total dose of 1.0 × 10¹⁷/cm² at 450°C using channeled ion beam synthesis (CIBS). The composition, the structure, the strain and the thermal stability of these layers have been studied using energy dispersive spectroscopy (EDS), Rutherford backscattering (RBS)/channeling and X-ray diffraction (XRD). It is shown that the perpendicular and parallel elastic strains of the Er_0.49Gd_0.51Si_1.7 epilayer are e^⊥=−0.46% ± 0.02% and e⁶=+0.73% ± 0.19%. The layer is stable up to 900°C. Annealing at 950°C results in a phase transformation.     

Reactive sputtering of simple condensed gases by kev ions III: Kinetic energy distributions

Condensed gas layers of H₂O, NH₃ and CO at 15–20 K have been bombarded by 6 keV H⁺₂ and 3 keV He⁺ and Ar⁺ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H₂O frozen on top of the CO-residue H₂CO was detected.     

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

Condensed gas layers of H₂O, NH₃ and CO at 15–20 K have been bombarded by 6 keV H⁺₂ and 3 keV He⁺ and Ar⁺ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H₂O frozen on top of the CO-residue H₂CO was detected.     

Temperature effect study of silicon-on-insulator structures prepared by high dose implantation of nitrogen

The temperature effect on the microstructure of the N⁺-ion implantation-induced Si₃N₄ buried layer was investigated. The underlying silicon nitride layers were formed in a Si (1 1 1) wafer after implantation of 50 keV nitrogen ions (fluence: 1 × 10¹⁷, 2 × 10¹⁷ and 5 × 10¹⁷ ions/cm²). It was observed that a continuous amorphous layer of about 200 nm thickness was formed in all implanted samples due to the irradiation damage. After 30 min annealing at 900 °C, poly-crystalline Si₃N₄ products were found by TEM examination in the specimen implanted with 5 × 10¹⁷ ions/cm² dose. In the case of annealing at 1200 °C a continuous single-crystalline α-Si₃N₄ buried layer was formed indicating that the amorphous layer in the implanted samples could be transformed into three successive layers, which are amorphous SiO₂, single-crystal α-Si₃N₄ and retained defects from surface to inner substrate, respectively.     

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.