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.     

Enhancement of luminescence from encapsulated Si nanocrystals in SiO2 with rapid thermal anneals

Potentialities of rapid thermal annealing to enhance the photoluminescence emission of Si nanocrystals in SiO₂ have been investigated. Ion implantation was used to synthesize specimens of SiO₂ containing excess Si with different concentrations. Si precipitation to form nanocrystals in implanted samples takes place with a conventional furnace anneal. The photoluminescence intensity and the peak energy of emission from Si nanocrystals depend on implanted ion fluence. Moreover, the luminescence intensity is strongly enhanced with a rapid thermal anneal prior to a conventional furnace anneal. The luminescence intensity, however, decreases with a rapid thermal anneal following a conventional furnace anneal. It is found that the order of heat treatment is an important factor in intensities of the luminescence. Moreover, the luminescence peak energy is found to be dependent, but a little, on thermal history of specimens. Based on our experimental results, we discuss about the mechanism of an enhancement of the photoluminescence, together with the mechanism of photoemission from encapsulated Si nanocrystals produced in a SiO₂ matrix by ion implantation and annealing.     

N、O、Ga、Mg和Si离子注入n型GaN样品的光致发光谱中蓝光发射带的研究

采用光致发光手段研究了几种不同注入离子N、O、Mg、Si和Ga对n型GaN蓝光发射带的影响.其中离子的注入剂量分别为1013、1014、1015和1016cm-2,注入温度为室温.注入后的样品在900℃流动氮气环境下进行10 min的热退火.通过对实验测得的光致发光谱的分析,给出了不同注入离子对n型GaN蓝光发射带的影响随注入剂量的变化关系以及该影响的相对强弱,进而确定蓝光发射起源于注入离子引入的间位缺陷.     

Structure and morphology of ion irradiated Au nanocrystals in SiO2

We have investigated the effect of ion irradiation on the structure and morphology of Au nanocrystals (NCs) fabricated by ion beam synthesis in a thin SiO₂ layer on a Si substrate. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements show a significant drop in the average Au–Au coordination, as well as a loss of medium and long range order with increasing irradiation dose. Small angle X-ray scattering (SAXS) measurements reveal a concomitant reduction in average NC size. These observations are a consequence of structural disorder and collisional mixing induced by the irradiation. The observed reduction in average Au–Au coordination by EXAFS differs significantly from that estimated from the average NC sizes evaluated using SAXS. This behavior can be explained by the dissolution of Au NCs into the SiO₂ matrix. A significant bond-length contraction indicates that part of this material forms small Au clusters (dimers, trimers, etc.) during irradiation that cannot be detected by SAXS. Combining the results from SAXS and EXAFS measurements, we estimate the volume fraction of such clusters.     

Influence of UV irradiation and RTA process on optical properties of Si implanted SiO2

Si ion implantation was widely used to synthesize specimens of SiO₂ containing supersaturated Si and subsequent high temperature annealing induces the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to enhance the photoluminescence and to achieve low temperature formation of Si nanocrystals have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluence of 7.5 × 10¹⁶ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp. After the process, the samples were rapidly thermal annealed before furnace annealing (FA). Photoluminescence spectra were measured at various stages at the process. We found that the luminescence intensity is strongly enhanced with excimer-UV irradiation and RTA. Moreover, effective visible photoluminescence which is not observed with a simple FA treatment, is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA. Based on our experimental results, we discuss the effects of excimer-UV lamp irradiation and RTA process on Si nanocrystals related photoluminescence.     

C注入GaN光致发光和微区拉曼散射的研究

使用光致发光谱和微区拉曼散射谱的测量,研究了C离子注入原生无黄光发射的GaN。C离子的注入剂量范围为1013-1017cm-2。发光谱的研究表明,C注入的GaN经950℃高温退火后出现了黄光发射,而近带边发射峰的峰位则由于C注入产生的某种缺陷而发生了蓝移。拉曼谱的测量表明,GaN薄膜的应力不随C注入而改变。当注入剂量增加至1015cm-2时,出现了与无序激活拉曼散射相关的300cm-1峰,但随着注入剂量进一步增加,300cm-1峰减弱并未消失,这被归因于注入束流强度随注入剂量增大。     

Radiation effect on the photoluminescence properties of Si/SiO2 thin films

Silicon ions were implanted into SiO₂ thin films with various doses and energies. For the films implanted with various ion doses the photoluminescence (PL) intensity of 470 nm firstly increased with the increase of Si ion dose, which is similar to the variation trend of displacement per atom (DPA) number during ion radiation. Further increasing Si ion dose the PL intensity of 470 nm decreased gradually since the neutral oxygen vacancy centers were destroyed. For the samples implanted with different energy the variation trend of PL intensity for 470 nm peak is similar to the result of DPA under different radiation energy according to SRIM2006 simulation. With the increase of radiation energy a new PL peak at 550 nm appeared because of the variation of defect type. Combining with the simulation results and PL spectra the radiation effect on Si/SiO₂ thin films were proposed.     

Deposition of polycrystalline Si thin films on glass substrates by direct negative Si ion beam deposition

Polycrystalline Si (Poly-Si) thin films were deposited on a glass substrate by direct negative Si (Si⁻) ion beam deposition. The glass substrate temperature was kept constant at 500 °C for all depositions. Prior to deposition, the ion energy spread and ion-to-atom arrival ratio were evaluated as a function of the ion beam energy.The Si⁻ ion energy spread was less than 10% regardless of the ion energy, while the ion-to-atom arrival ratio increased proportionally from 1.3 to 1.6 according to the ion beam energy.Atomic force microscopy images showed that a relatively rough surface was obtained at 50 eV of Si⁻ ion energy and it is also concluded that the Si⁻ ion beam irradiation at 50 eV is effective to deposit Si thin film with small grains as shown in Fig. 3.     

N、0、Ga、Mg和Si离子注入n型GaN样品的光致发光谱中蓝光发射带的研究

采用光致发光手段研究了几种不同注入离子N、O、Mg、Si和Ga对n型GaN蓝光发射带的影响。其中离子的注入剂量分别为10^13、10^14、10^15和10^16cm^-2，注入温度为室温。注入后的样品在900℃流动氮气环境下进行10min的热退火。通过对实验测得的光致发光谱的分析，给出了不同注入离子对n型GaN蓝光发射带的影响随注入剂量的变化关系以及该影响的相对强弱，进而确定蓝光发射起源于注入离子引入的间位缺陷。     

A study of the blue photoluminescence emission from thermally-grown, Si-implanted SiO2 films after short-time annealing

Thermal SiO₂ films have been implanted with Si⁺ ions using double-energy implants (200 + 100 keV) at a substrate temperature of about −20°C to total doses in the range 1.6 × 10¹⁶−1.6 × 10¹⁷ cm⁻² followed by short-time thermal processing, in order to form a Si nanostructure capable of yielding blue photoluminescence (PL). The intensity and the peak position of the PL band have been investigated as a function of ion dose, manner of heat treatment, anneal time and anneal temperature. For the formation of blue PL emitting centres, optimum processing conditions in terms of excess Si concentration and overall thermal budget are mandatory. The nature of the observed blue emission is discussed.     

Ion beam induced formation of nanocrystalline silicon in pulsed laser deposited SiOX thin films

Synthesis and structural studies of nanocrystalline silicon grown in pulsed laser deposited SiO_X films is reported. The effect of high energy heavy ion beam irradiation on these films is studied using 100 MeV Ag ions. The structural studies were carried out using micro Raman spectroscopy, GAXRD, FTIR, TEM, HRTEM, SAED and EDX. The occurrence of phase separation in non-stoichiometric silicon oxide by means of ion beam irradiation leading to the formation of silicon nanocrystals in the films is confirmed by the results. HRTEM results reveal the structure of silicon phase formed after ion beam treatment and the particle size can be controlled up to 2-3 nm. A detailed analysis by micro Raman and HRTEM studies suggest the presence of crystallite size distribution. The results of GAXRD and SAED confirm the formation of cubic phase of silicon with two different lattice parameters. The studies conclude that the size of the nanocrystals can be controlled by varying deposition and ion irradiation parameters.     

Pb离子辐照注碳a-SiO2薄膜的光致发光性能

用湿氧化法在单晶硅表面生长了非晶态SiO2薄膜,进行120 keV C离子注入和950 MeV Pb离子辐照,用荧光光谱分析样品发光特性的改变.结果发现,C离子注入和高能Pb离子辐照均能显著影响样品的发光特性,且荧光光谱的改变强烈依赖于注入和辐照剂量,预示不同注入和辐照剂量将导致不同的发光结构形成.对注入和辐照造成薄膜...     

Channeling characterization of defects in silicon: an atomistic approach

We review here the possibilities opened by a recent development of the Monte Carlo binary collision approximation (MC-BCA) simulation of Rutherford backscattering spectrometry-channeling (RBS-C) spectra for the study of radiation damage in monocrystalline materials. The ion implantation of silicon has been chosen as a case study. Atomic-scale modeling of defect structures was used to determine the location of interstitial atoms in the host lattice. Among possible candidate defects, we have considered the elementary hexagonal, tetrahedral, 1 1 0-split interstitials, the Bond-defect and one type of tetra-interstitial cluster. For each defect model a large Si supercell was populated with a proper defect depth distribution and then it was structurally relaxed by the application of the classical EDIP potential. This model system was then given as an input to the MC-BCA simulation code and the spectra corresponding to nine different axial and planar alignments were calculated. For low defect concentration (a few atomic percent), the scattering yields are strongly dependent on the orientation and a distinct signature characteristic of the limited number of allowed interstitial positions in Si could be found. The comparison of simulations and experiments in the case of 180 keV self ion implantation allowed the identification of the dominant interstitial defect whose structural properties are represented by the split-1 1 0 interstitial. By increasing the concentration of defects (and their mutual interaction) the technique looses sensitivity and, at the same time, the contribution of lattice relaxation becomes important. Under these conditions, although the RBS-C response becomes similar to the one obtained from a random distribution of displaced atoms, the major structural features of a heavily damaged sample could be still observed.     

Atomistic modeling of the effects of dose and implant temperature on dopant diffusion and amorphization in Si

We discuss atomistic simulations of ion implantation and annealing of Si over a wide range of ion dose and substrate temperatures. The DADOS Monte Carlo model has been extended to include the formation of amorphous regions, and this allows simulations of dopant diffusion at high doses. As the dose of ions increases, the amorphous regions formed by cascades eventually overlap, and a continuous amorphous layer is formed. In that case, most of the excess interstitials generated by the implantation are swept to the surface as the amorphous layer regrows, and do not diffuse in the crystalline region. This process reduces the amount of transient enhanced diffusion (TED) during annealing. This model also reproduces the dynamic annealing during high temperature implants. In this case, the local amorphous regions regrow as the implant proceeds, without the formation of a continuous amorphous layer. For sufficiently high temperatures, each cascade is annealed out independently; interstitials and vacancies can escape from the cascade and thus increase dopant diffusion.     

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.     

退火温度对掺铒硅1.54μm光致发光的影响

利用金属蒸气真空弧（MEVVA）离子源交稀土金属Er离子注入单晶硅中，经快速退火制备出掺铒硅发光薄膜。RBS分析表明掺铒硅的浓度接近10at%,即可达10^21cm^-3量级。XRD分析薄膜物相结构发现，退火温度升高将导致Er偏析。通过RHEED和AFM显微分析可得，退火温度影响辐照损伤的恢复程度，Si固相外延再结晶和显微形貌。这些结构变化将影响掺铒硅发光薄膜1.54μm光致发光。     

离子注入对菲胁迫下的苜蓿萌发的影响

为研究离子注入对多环芳烃(Polycyclic aromatic hydrocarbons,PAHs)修复植物的诱变效应,利用离子注入苜蓿草种,探讨了离子注入后苜蓿在不同含菲质量分数环境中的萌发率和芽根长度分布.结果发现,苜蓿萌发率随注入剂量的增大,呈“鞍形”曲线特征;以植物主根长度为诱变筛选条件,发现5×1015 c...     

The formation and evolution of vacancy-type defects in Ar-implanted silicon studied by slow-positron annihilation spectroscopy

The Doppler broadening spectrum of a silicon wafer was measured using a variable-energy positron beam to investigate the effects of vacancy-type defects induced by 180 keV Ar ion implantation. The S-parameter in the damaged layer decreases with annealing temperature up to 673 K, and then increases with annealing temperature from 673 to 1373 K. At low annealing temperatures ranging from room temperature to 673 K, argon-decorated vacancies are formed by argon atoms combining with open-volume defects at inactive positron sites. With further increase of annealing temperature, argon-decorated vacancies dissociate and subsequently migrate and coalesce, leading to an increase of S-parameter. Furthermore, the buried vacancy-layer becomes narrow with increasing annealing temperature. At 1373 K, the buried vacancy-layer moved towards the sample surface.     

Structural stability of Cu nanocrystals in SiO2 exposed to high-energy ion irradiation

Cu nanocrystals (NCs) were synthesized in SiO₂ by ion implantation and thermal annealing. Annealing at two different temperatures of 950 °C and 650 °C yielded two different nanocrystal size distributions with an average diameter of 8.1 and 2.5 nm, respectively. Subsequently the NCs were exposed to 5.0 MeV Sn³⁺ ion irradiation simultaneously with a thin Cu film as a bulk reference. The short-range atomic structure and average NC diameter was measured by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS), respectively. Consistent with the high regeneration rate of bulk elemental metals, no irradiation induced defects were observed for the reference, whereas the small NCs (2.5 nm) were dissolved as Cu monomers in the matrix. The latter was attributed to irradiation-induced mixing of Cu, Si and O based on dynamic binary collision simulations. For the large NCs (8.1 nm) only minor structural changes were observed upon irradiation, consistent with a more bulk-like pre-irradiation structure.     

Mean excitation energy for the stopping power of light elements

We have evaluated the mean excitation energy or I value for Coulomb excitations by swift charged particles passing through carbon, aluminum and silicon. A self-consistent Kramers–Kronig analysis was used to treat X-ray optical spectra now available from synchrotron light sources allowing us to carry out Bethe’s original program of evaluating I from the observed dielectric response. We find that the K and L shell are the dominant contributors to I in these light elements and that the contribution of valence electrons is relatively small, primarily because of their low binding energy. The optical data indicate that Si and Al have nearly equal I values, in contrast to Bloch’s Thomas–Fermi result, I ∝ Z. The optically based I values for C and Al are in excellent agreement with experiment. However, the dielectric-response I value for Si is 164 ± 2 eV, at variance with the commonly quoted value of 173 ± 3 eV derived from stopping-power measurements.