Synchrotron investigations of the electron structure of silicon nanocrystals in a SiO2 matrix

Silicon ions are implanted into silicon oxide thin films obtained by the thermal oxidation of silicon wafers in wet oxygen. The implantation dose is accumulated either once or cyclically, and the samples are annealed in dry nitrogen every time after implantation. The second series of samples is prepared in a similar way, but the technology for obtaining the oxide films includes additional annealing at 1100°C in air for three hours. X-ray absorption near-edge structure (XANES) spectra are obtained using synchrotron radiation. In all the Si L _2,3 spectra, two absorption edges are observed, the first corresponding to elemental silicon, and the second corresponding to the SiO₂ matrix. The fine structure of the first edge indicates that nanocrystalline silicon (nc-Si) can form in the SiO₂ matrix, whose atomic and electronic structure depends on the technology of its formation. In both series, the cyclic accumulation of the total dose (Φ = 10¹⁷ cm^?2) and the annealing time (2 h) gives rise to the most pronounced fine structure in the region of the absorption edge of elemental silicon. The probability of forming silicon nanocrystals decreases for the denser silicon oxide in the second series of samples.     

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO<Subscript>2</Subscript> Matrix

Fused silica plates have been implanted with 40 keV Co⁺ or Ni⁺ ions to high doses in the range of (0.25–1.0) × 10¹⁷ ions/cm², and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO₂ matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO₂ matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.     

Iron nanoparticles in amorphous SiO<Subscript>2</Subscript>: X-ray emission and absorption spectra

The local structure of the chemical bond of iron ions implanted into SiO₂ glasses (implantation energy, 100 keV; fluence, 1 × 10¹⁶ cm^?2) is investigated using x-ray emission and absorption spectroscopy. The Fe L x-ray emission and absorption spectra are analyzed by comparing them with the corresponding spectra of reference samples. It is established that iron nanoparticles implanted into the SiO₂ vitreous matrix are in an oxidized state. The assumption is made that the most probable mechanism of transformation of iron nanoparticles into an oxidized state during implantation involves the breaking of Si-O-Si bonds with the formation of Si-Si and Fe-O bonds.     

Chemical and phase compositions of silicon oxide films with nanocrystals prepared by carbon ion implantation

The chemical and phase compositions of silicon oxide films with self-assembled nanoclusters prepared by ion implantation of carbon into SiO _x (x < 2) suboxide films with subsequent annealing in a nitrogen atmosphere have been investigated using X-ray photoelectron spectroscopy in combination with depth profiling by ion sputtering. It has been found that the relative concentration of oxygen in the maximum of the distribution of implanted carbon atoms is decreased, whereas the relative concentration of silicon remains almost identical over the depth in the layer containing the implanted carbon. The in-depth distributions of carbon and silicon in different chemical states have been determined. In the regions adjacent to the layer with a maximum carbon content, the annealing results in the formation of silicon oxide layers, which are close in composition to SiO₂ and contain silicon nanocrystals, whereas the implanted layer, in addition to the SiO₂ phase, contains silicon oxide species Si²⁺ and Si³⁺ with stoichiometric formulas SiO and Si₂O₃, respectively. The film contains carbon in the form of SiC and elemental carbon phases. The lower limit of the average size of silicon nanoclusters has been estimated as ∼2 nm. The photoluminescence spectra of the films have been interpreted using the obtained results.     

CEMS study of silicon implanted iron

Thin layers of iron-rich Fe-Si alloys were formed by silicon implantation into iron at room temperature with different energies (100, 200, and 300 keV) and ion doses (2 × 10¹⁷ to 1×10¹⁸ cm^–2). The produced layers were investigated by⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) to identify the phases formed by the ion implantation. Auger electron spectroscopy (AES) was used to measure the concentration depth profiles of the implanted silicon. Depending on the implantation parameters different disordered Fe-Si structures were detected. At low doses only magnetic phases were formed while at high doses a non-magnetic phase with a hitherto unknown structure appeared. Annealing of the samples resulted first in the formation of a D0₃-like short-range order and a slow decrease of the non-magnetic phase, and subsequently in the migration of Si out of the investigated depth range.     

Optical properties of xenon implanted CuInSe2 by photoacoustic spectroscopy

A theoretical relation is derived for the normalized photoacoustic amplitude signal of a gas-coupled cell for the case of double-layer solid samples with particular application given to ion implanted semiconductors. Numerical estimates for a solar cell of the type CdS/CuInSe₂ based on experimental measured data of these compounds are given to illustrate the photoacoustic effect originating from double-layer samples. In application to ion implanted semiconductors, we show that the absorption coefficient of the implanted layer can be very easily extracted by photoacoustic spectroscopy if the absorption coefficient of the untreated substrate is known. We also present the optical properties results obtained from the analysis of the effect of xenon implantation into CuInSe₂ single crystals with the energy of 40 keV and a dose of 5×10¹⁶ ions/cm².     

A pulsed synthesis of β-FeSi2 layers on silicon implanted with Fe ions

Continuous layers and fine-grained films of β-FeSi₂ were synthesized using the implantation of Fe⁺ ions into Si (1 0 0) with subsequent pulsed nanosecond ion-beam treatment of the implanted layers. The X-ray diffraction studies showed that the pulsed ion-beam treatment brings about the formation of a mixture of two phases: FeSi and β-FeSi₂ with strained crystal lattices. Subsequent rapid thermal annealing led to the complete transformation of the FeSi phase into the β-FeSi₂ phase with the formation of a textured layer. The data obtained using Raman spectroscopy corroborate the formation of the β-FeSi₂ phase with a high degree of silicon crystallinity.The results of measuring the optical absorption point to the formation of β-FeSi₂ layers and precipitates with a direct-gap structure, an optical gap of E_g≈0.83 eV. The photoluminescence band peaked at λ≈1.56 μm and caused by direct band-to-band transitions in β-FeSi₂ was observed at temperatures lower than 210 K.     

Effect of High Temperature—Pressure on Buried Silicon Dioxide in Simox and Soi Structures

The effect of annealing at 1520–1570 K under high pressure (HP, up to 1.2GPa) on the structure of SiO₂ in oxygen implanted silicon (Si:O) and in silicon with buried SiO₂ layer (SOI) was investigated by TEM, X-Ray and FTIR methods. Depending on the implantation and treatment parameters, SiO₂ precipitates or continuous SiO₂ layers, sometimes with defects at the SiO₂/Si boundary, are created. A stress dependent shift of asymmetric stretching vibration mode associated with Si-O bonds towards lower frequencies is detected for SiO₂ in the HT—HP treated Si:O and SOI samples.     

Radiation defects introduced by MeV electrons in argon implanted MOS structures

The influence of MeV electron irradiation on the interface states of argon implanted thin oxide MOS samples has been studied by the thermally stimulated current (TSC) method. The oxide thickness of the structures is 18 nm. Two groups of n-type MOS structures non-implanted and implanted with 20 keV Ar⁺ ions and a dose of 5×10¹² cm⁻² are examined. Both groups are simultaneously irradiated by 23 MeV electrons with doses of 1.2×10¹⁶, 2.4×10¹⁶ or 6.0×10¹⁶ el/cm². The energy position and density of the interface states (generated by electron irradiation, ion implantation or both treatments of the samples) are determined. It is shown that MeV electron irradiation decreases the concentration of interface states (like an oxygen-vacancy and di-vacancy) slightly and creates additional interface states (like an impurity-vacancy) at the Si–SiO₂ interface of argon implanted MOS structures.     

Ground state magnetic properties of ultrathin fcc Fe and Co films on Cu(001) surfaces

The oxidation characteristics of silicon implanted with a low dose of nitrogen (1–3×10¹⁵cm^–2) have been studied for dry oxidation conditions at 1020°C. The wafers were subjected to a pre-oxidation annealing. Complete inhibition of the oxide growth occurs in the initial stage of oxidation, while the oxidation rate for prolonged oxidation is identical to that for pure silicon. The oxidation resistance increases with the implantation dose. The resistance is attributed to the formation of a nitrogen-rich surface film during annealing. This layer, which consists of only a few monolayers, is presumably composed of oxynitride. The electrical characteristics of MOS capacitors formed on implanted wafers show that the interface state density is not significantly increased by the low-dose N implantation.     

Ion dose and anneal temperature dependent studies of silicon implanted AlxGa1−xN

Electrical and optical activation studies of Al_xGa_1−xN (x = 0.11 and 0.21) implanted with silicon were made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ to 1 × 10¹⁵ cm⁻² at room temperature. The implanted samples were subsequently annealed from 1100 to 1300 °C for 20 min in a nitrogen environment. A maximum electrical activation efficiency of 91% was obtained for the Al_0.11Ga_0.89N implanted with the highest dose of 1 × 10¹⁵ cm⁻² even after annealing at 1150 °C. 100% activation efficiencies were successfully obtained for the Al_0.21Ga_0.79N samples after annealing at 1300 °C for both doses of 5 × 10¹⁴ and 1 × 10¹⁵ cm⁻². The mobility of the Si-implanted Al_xGa_1−xN increases with annealing temperature, and the highest mobilities are 109 and 98 cm²/V·s for Al_0.11Ga_0.89N and Al_0.21Ga_0.79N, respectively. The cathodoluminescence (CL) spectra for all the samples exhibited a sharp neutral-donor-bound exciton peak, and the CL intensity increases with annealing temperature, indicating successive improved implantation damage recovery as the annealing temperature is increased. These results provide the optimum annealing conditions for activation of implanted Si ions in Al_xGa_1−xN.     

Blue-cathodoluminescent layers synthesis by high-dose N, C and B SiO2 implantation

Thermal silicon oxide layers have been implanted at 600 °C with N⁺+C⁺, N⁺+B⁺ and N⁺+C⁺+B⁺ ions. Two different implantation doses have been chosen in order to introduce peak concentrations at the projected range comparable to the SiO₂ density. Some pieces of the samples have been annealed in conventional furnace at 1200 °C for 3 h. After annealing, cathodoluminescence measurements show in all cases a main broad band centered at 460 nm (2.7 eV). High doses of C implantation give rise to an intensity attenuation. Phases formed in the oxides have been investigated by Fourier transform infrared spectroscopy before and after annealing. The spectra suggest that N incorporates as BN and probably as a ternary BCN phase in the triply implanted samples, while C seems to bond mainly with B. Boron is also bonded to O in B-O-Si configuration. Depth structure and quantitative composition of the films were deduced from fittings of the spectroscopic ellipsometry measurements.     

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.     

Optical and electrical properties of Ge-implanted SiO2 layers on n-Si and p-Si

Ge ions of 100 keV were implanted into a 120 nm-thick SiO₂ layer on n-Si at room temperature while those of 80 keV were into the same SiO₂ layer on p-Si. Samples were, subsequently, annealed at 500°C for 2 h to effectively induce radiative defects in the SiO₂. Maximum intensities of sharp violet photoluminescence (PL) from the SiO₂/n-Si and the SiO₂/p-Si samples were observed when the samples have been implanted with doses of 1×10¹⁶ and 5×10¹⁵ cm⁻², respectively. According to current–voltage (I–V) characteristics, the defect-related samples exhibit large leakage currents with electroluminescence (EL) at only reverse bias region regardless of the type of substrate. Nanocrystal-related samples obtained by an annealing at 1100°C for 4 h show the leakage at both the reverse and the forward region.     

Structure and micro-mechanical properties of helium-implanted layer on Ti by plasma-based ion implantation

The present paper concentrates on structure and micro-mechanical properties of the helium-implanted layer on titanium treated by plasma-based ion implantation with a pulsed voltage of −30 kV and doses of 3, 6, 9 and 12 × 10¹⁷ ions/cm², respectively. X-ray photoelectron spectroscopy and transmission electron microscopy are employed to characterize the structure of the implanted layer. The hardnesses at different depths of the layer were measured by nano-indentation. We found that helium ion implantation into titanium leads to the formation of bubbles with a diameter from a few to more than 10 nm and the bubble size increases with the increase of dose. The primary existing form of Ti is amorphous in the implanted layer. Helium implantation also enhances the ingress of O, C and N and stimulates the formations of TiO₂, Ti₂O₃, TiO, TiC and TiN in the near surface layer. And the amount of the ingressed oxygen is obviously higher than those of nitrogen and carbon due to its higher activity. At the near surface layer, the hardnesses of all implanted samples increases remarkably comparing with untreated one and the maximum hardness has an increase by a factor of up to 3.7. For the samples implanted with higher doses of 6, 9 and 12 × 10¹⁷ He/cm², the local displacement bursts are clearly found in the load-displacement curves. For the samples implanted with a lower dose of 3 × 10¹⁷ He/cm², there is no obvious displacement burst found. Furthermore, the burst width increases with the increase of the dose.     

Influence of Cr<Superscript>+</Superscript> ion implantation and pulsed ion-beam annealing on the formation and optical properties of Si/CrSi<Subscript>2</Subscript>/Si(111) heterostructures

The effect of pulsed ion-beam annealing on the surface morphology, structure, and composition of single-crystal Si(111) wafers implanted by chromium ions with a dose varying from 6 × 10¹⁵ to 6 × 10¹⁶ cm⁻² and on subsequent growth of silicon is investigated for the first time. It is found that pulsed ion-beam annealing causes chromium atom redistribution in the surface layer of the silicon and precipitation of the polycrystalline chromium disilicide (CrSi₂) phase. It is shown that the ultrahigh-vacuum cleaning of the silicon wafers at 850°C upon implantation and pulsed ion-beam annealing provides an atomically clean surface with a developed relief. The growth of silicon by molecular beam epitaxy generates oriented 3D silicon islands, which coalesce at a layer thickness of 100 nm and an implantation dose of 10¹⁶ cm⁻². At higher implantation doses, the silicon layer grows polycrystalline. As follows from Raman scattering data and optical reflectance spectroscopy data, semiconducting CrSi₂ precipitates arise inside the silicon substrate, which diffuse toward its surface during growth.     

离子注入形成SOI结构的界面及埋层的俄歇能谱研究

本文中研究了O⁺(200keV,1.8×10¹⁸/cm²)和N⁺(190keV,1.8×10¹⁸/cm²)注入Si形成SOI(Silicon on Insulator)结构的界面及埋层的化学组成。俄歇能谱的测量和研究结果表明:注O⁺的SOI结构在经1300℃,5h退火后,其表层Si和氧化硅埋层的界面存在一个不饱和氧化硅状态,氧化硅埋层是由SiO₂相和这不饱和氧化硅态组成,而且氧化硅埋层和体硅界面不同于表层Si和氧化硅埋层界面;注N⁺的SOI结构在经1200℃,2h退火后,其氮化硅埋层中存在一个富N的疏松夹层,表层Si和氮化硅埋层界面与氮化硅埋层和体硅界面性质亦不同。这些结果与红外吸收和透射电子显微镜及离子背散射谱的分析结果相一致。还对两种SOI结构界面与埋层的不同特征的原因进行了分析讨论。 关键词：     

Photon emission from clean and oxygenated Si and SiO2 surfaces bombarded by 5 keV krypton ions

The effect of oxygen on the light emission from a Si (1 0 0) semiconductor bombarded by energetic Kr⁺ ions has been studied in the 200–300 nm wavelength range. The influence of oxygen was verified by studying the optical spectra of SiO₂ bombarded under similar experimental conditions. It has been found that the measured intensities of the emitted photons are always higher in the presence of oxygen, even higher than those obtained for SiO₂. The electron-transfer model can explain our experimental observations. We do believe that in the presence of oxygen, an intermediate structure of silicon sub-oxide SiO_X<2 is formed on silicon surface, which is responsible for the increase of photon emission. In addition, the radiative dissociation process and breaking of chemical bond seems contribute to the enhancement of emitted photons intensity.     

Luminescence and structure of nanosized inclusions formed in SiO2 layers under double implantation of silicon and carbon ions

Luminescent and structural characteristics of SiO₂ layers exposed to double implantation by Si⁺ and C⁺ ions in order to synthesize nanosized silicon carbide inclusions have been investigated by the photoluminescence, electron spin resonance, transmission electron microscopy, and electron spectroscopy methods. It is shown that the irradiation of SiO₂ layers containing preliminary synthesized silicon nanocrystals by carbon ions is accompanied by quenching the nanocrystal-related photoluminescence at 700–750 nm and by the enhancement of light emission from oxygen-deficient centers in oxide in the range of 350–700 nm. Subsequent annealing at 1000 or 1100°C results in the healing of defects and, correspondingly, in the weakening of the related photoluminescence peaks and also recovers in part the photoluminescence of silicon nanocrystals if the carbon dose is less than the silicon dose and results in the intensive white luminescence if the carbon and silicon doses are equal. This luminescence is characterized by three bands at ～400, ～500, and ～625 nm, which are related to the SiC, C, and Si phase inclusions, respectively. The presence of these phases has been confirmed by electron spectroscopy, the carbon precipitates have the sp ³ bond hybridization. The nanosized amorphous inclusions in the Si⁺ + C⁺ implanted and annealed SiO₂ layer have been revealed by high-resolution transmission electron microscopy.     

Defects formation in the dual B+ and N+ ions implanted silicon

Silicon wafers were implanted with 40 keV B⁺ ions (to doses of 1.2×10¹⁴ or 1.2×10¹⁵ cm^–2) and 50 or 100 keV N⁺ ions (to doses from 1.2×10¹⁴ to 1.2×10¹⁵ cm^–2). After implantations, the samples were furnace annealed at temperatures from 100 to 450 °C. The depth profiles of the radiation damages before and after annealing were obtained from random and channeled RBS spectra using standard procedures. Two damaged regions with different annealing behaviour were found for the silicon implanted with boron ions. Present investigations show that surface disordered layer conserves at the annealing temperatures up to 450 °C. The influence of preliminary boron implantation on the concentration of radiation defects created in subsequent nitrogen implantation was studied. It was shown that the annealing behaviour of the dual implanted silicon layers depends on the nitrogen implantation dose.The authors would like to thank the members of the INP accelerator staff for the help during the experiments. The work of two authors (V.H. and J.K.) was partially supported by the Internal Grant Agency of Academy of Science of Czech Republic under grant No. 14805.