Amorphization and recrystallization of ion implanted Si nanocrystals probed through their luminescence properties

In this paper the amorphization of Si nanocrystals (nc) by ion beam irradiation and the subsequent recrystallization are investigated in detail. The luminescence properties of Si nanocrystals embedded within a SiO₂ matrix are used as a probe of the damaging effects generated by high-energy ion beam irradiation. Samples have been irradiated with 2 MeV Si⁺ ions at different doses, in the range between 1×10⁹ and 1×10¹⁶ cm⁻². By increasing the ion dose, the nc-related photoluminescence (PL) strongly decreases after a critical dose value. It is shown that the lifetime quenching alone cannot quantitatively explain the much stronger PL drop, but the total number of emitting centers has to diminish too. Moreover, we studied the recovery of the amorphized Si nc by performing thermal annealings. It is demonstrated that the recovery of the PL properties of completely amorphized Si nc is characterized by a single activation energy, whose value is 3.4 eV. Actually, this energy is associated to the transition between the amorphous and the crystalline phases of each Si grain. The recrystallization kinetics of Si nanostructures is demonstrated to be very different from that of a bulk system.     

基于Si-rich SiNx/N-rich SiNy多层膜结构的量子点构筑及发光特性

利用等离子体增强化学气相沉积法制备Si-rich SiN_x/N-rich SiN_y多层膜,分别使用热退火和激光辐照技术对多层膜进行退火,以构筑三维限制、尺寸可控、有序的硅纳米晶.实验结果表明,经退火后,纳米硅晶粒在Si-rich SiN_x子层内形成,其尺寸可由Si-rich SiN_x子层厚度调控.实验还发现,激光辐照技术相比于热退火能更有效地改善多层膜的微结构,提高多层膜的晶化率,以激光技术诱导晶化的Si-rich SiN_x/N-rich SiN_y多层膜作为有源层构建电致发光器件,在室温下观察到了增强的电致可见发光,并且发光效率较退火前提高了40%以上. 关键词： 氮化硅 多层膜 限制结晶 纳米晶硅     

Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type porous silicon (PS) are mainly attributed to some morphological changes related to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of the PS layers. It has been found that when PS is formed in pure aqueous HF solution, it can exhibit a non-conventional and reproducible “porosity – PL peak relationship”. By correlating the PL spectral behaviour and PS morphology throughout a quantum-confinement model, we explain both conventional and non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibit the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throughout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.     

纳米硫化镍增感的溴化银微晶中光电子衰减特性研究

利用微弱信号的微波吸收相敏检测技术, 获得了纳米硫化镍增感的立方体溴化银乳剂中, 在增感时间增加的条件下, 自由光电子和浅束缚光电子的时间衰减曲线.分析了采用纳米硫化镍进行增感的溴化银乳剂中光生电子随增感时间衰减的过程, 讨论了卤化银晶体中电子陷阱对光电子运动行为的影响, 分析了电子陷阱效应及陷阱深度同增感时间之间的关系. 通过未增感样品与增感样品的衰减曲线对比, 得到了在此实验条件下的最佳增感时间为80min. 关键词： 纳米硫化镍 衰减时间 电子陷阱 寿命     

Plasma synthesis of semiconductor nanocrystals for nanoelectronics and luminescence applications

Functional nanocrystals are widely considered as novel building blocks for nanostructured materials and devices. Numerous synthesis approaches have been proposed in the solid, liquid and gas phase. Among the gas phase approaches, low pressure nonthermal plasmas offer some unique and beneficial features. Particles acquire a unipolar charge which reduces or eliminates agglomeration; particles can be electrostatically confined in a reactor based on their charge; strongly exothermic reactions at the particle surface heat particles to temperatures that significantly exceed the gas temperature and facilitate the formation of high quality crystals. This paper discusses two examples for the use of low pressure nonthermal plasmas. The first example is that of a constricted capacitive plasma for the formation of highly monodisperse, cubic-shaped silicon nanocrystals with an average size of 35 nm. The growth process of the particles is discussed. The silicon nanocubes have successfully been used as building blocks for nanoparticle-based transistors. The second example focuses on the synthesis of photoluminescent silicon crystals in the 3–6 nm size range. The synthesis approach described has enabled the synthesis of macroscopic quantities of quantum dots, with mass yields of several mg/hour. Quantum yields for photoluminescence as high as 67% have been achieved.     

Morphological and optical properties changes in nanocrystalline Si (nc-Si) deposited on porous aluminum nanostructures by plasma enhanced chemical vapor deposition for Solar energy applications

Photoluminescence (PL) spectroscopy was used to determine the electrical band gap of nanocrystalline silicon (nc-Si) deposited by plasma enhancement chemical vapor deposition (PECVD) on porous alumina structure by fitting the experimental spectra using a model based on the quantum confinement of electrons in Si nanocrystallites having spherical and cylindrical forms. This model permits to correlate the PL spectra to the microstructure of the porous aluminum silicon layer (PASL) structure. The microstructure of aluminum surface layer and nc-Si films was systematically studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD). It was found that the structure of the nanocrystalline silicon layer (NSL) is dependent of the porosity (void) of the porous alumina layer (PAL) substrate. This structure was performed in two steps, namely the PAL substrate was prepared using sulfuric acid solution attack on an Al foil and then the silicon was deposited by plasma enhanced chemical vapor deposition (PECVD) on it. The optical constants (n and k as a function of wavelength) of the deposited films were obtained using variable angle spectroscopic ellipsometry (SE) in the UV-vis-NIR regions. The SE spectrum of the porous aluminum silicon layer (PASL) was modeled as a mixture of void, crystalline silicon and aluminum using the Cauchy model approximation. The specific surface area (SSA) was estimated and was found to decrease linearly when porosity increases. Based on this full characterization, it is demonstrated that the optical characteristics of the films are directly correlated to their micro-structural properties.     

The influence of intrinsic and surface states on the emission properties of colloidal nanocrystals

Time Resolved Photoluminescence (TRPL) measurements on the picosecond time scale (temporal resolution of 17 ps) on colloidal CdSe and CdSe/ZnS Quantum Dots (QDs) were performed, to elucidate the role of intrinsic and surface states on the emission process. Transient PL spectra reveal three emission peaks with different lifetimes (60 ps, 460 ps and 9–10 ns, from the bluest to the reddest peak). The energy separations among the states, together with their characteristic decay times, allow us to attribute the two higher energy peaks to ±1^U and ±1^L bright states of the fine structure picture of spherical CdSe QDs, and the third one to surface states emission, respectively. We show that the contribution of surface emission to the PL results to be different for the two samples studied (67% in the CdSe QDs and 32% in CdSe/ZnS QDs), confirming the decisive role of the ZnS shell in the improvement of the surface passivation.     

Measurements of luminescence decay kinetics using an “Agat” streak-camera in a boxcar mode

Conclusion The specialized measuring setup described in the paper makes it possible to measure correctly and with reasonably high accuracy the luminescence decay kinetics of dyes within the time range from 40 psec to 5 nsec upon excitation in the visible range and at 1.064 μm. The system is highly flexible, having at the same time reasonably large data storage memory, and is completely automated. Such a system, undoubtedly, will be helpful for researchers working in the field of physics of laser media and dyes—passive modulators for femtosecond laser systems. The authors are grateful to M. V. Bondar for fruitful discussions and for help with the experiments. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 1, pp. 128–134, January–February, 1995.     

Investigation of defect luminescence from multicrystalline Si wafer solar cells using X‐ray fluorescence and luminescence imaging

Multicrystalline silicon wafer solar cells reveal performance‐ reducing defects by luminescence. X‐ray fluorescence spectra are used to investigate the elemental constituents from regions of solar cells yielding reverse‐bias or sub‐bandgap luminescence from defects. It is found that a higher concentration of metals is present in regions yielding reverse‐bias electroluminescence than in regions yielding sub‐bandgap electroluminescence. This suggests, dislocations do not create strong breakdown currents in the absence of impurity precipitates.

     

A density functional theoretic study of novel silicon-carbon fullerene-like nanostructures: Si40C20, Si60C20, Si36C24, and Si60C24

Fullerene-like silicon nanostructures with twenty and twenty-four carbon atoms on the surface of the Si₆₀ cage by substitution, as well as inside the cage at various orientations have been studied within the generalized gradient approximation to density functional theory. Full geometry optimizations have been performed without any symmetry constraints using the Gaussian 03 suite of programs and the LANL2DZ basis set. Thus, for the silicon atom, the Hay-Wadt pseudopotential with the associated basis set is used for the core electrons and the valence electrons, respectively. For the carbon atom, the Dunning/Huzinaga double zeta basis set is employed. Electronic and geometric properties of these nanostructures are presented and discussed in detail. Optimized silicon-carbon fullerene like nanostructures are found to have increased stability compared to the bare Si₆₀ cage and the stability depends on the number and the orientation of carbon atoms, as well as on the nature of silicon-carbon and carbon-carbon bonding.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Light emission from Si nanocrystals embedded in CaF2 epilayers on Si(1 1 1): Effect of rapid thermal annealing

Visible photoluminescence (PL) of nanocrystalline silicon (nc-Si) embedded in single crystal CaF₂ formed on Si(1 1 1) has been studied and the influence of ex situ rapid thermal annealing (RTA) on the surface morphology and PL spectra has been studied. It has been found that the PL intensity and uniformity was improved by RTA with appropriate temperature and short annealing time.     

Effects of dopant concentrations and firing temperatures on decay kinetics of manganese doped willemite nanopowders

Nanocrystallline willemite, Zn_2−xMn_xSiO₄ (0.5≤x≤5 mol%), doped with variable concentration of divalent manganese ions, phosphor powders were prepared using the simple wet-chemical sol-gel method combined with furnace firing at 800, 900, and 1000 °C. X-ray diffraction (XRD) and high resolution X-ray photoelectron (HR-XPS) scans confirm the presence of willemite phase of Zn₂SiO₄. Laser-induced phosphorescence decay measurements of Zn_2−xMn_xSiO₄ nanophosphors were investigated using high peak power pulsed UV nitrogen laser (λ=337.1 nm). The decay curves show non-single exponential behavior with long term decay rate. Various parameters describing the strength of optical transitions in atoms and molecules such as, Einstein's A and B coefficients, ‘f’, integrated cross-section, and transition dipole moment values have been calculated. The long term decay rate of optical transition parameters was found to be somewhat temperature and concentration dependent.     

Structural and optical properties study of nanocrystalline Si (nc-Si) thin films deposited on porous aluminum by plasma enhanced chemical vapor deposition

In this paper we report detail investigation and correlation between micro-structural and optical properties of nanocrystalline silicon (nc-Si) deposited by plasma enhancement chemical vapor deposition (PECVD) on porous aluminum structure. The influence of the microstructure of the nc-Si thin films on their optical properties was investigated through an extensive characterization. The effect of anodisation currents on the microstructure of aluminum surface layer and nc-Si films was systematically studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD). The optical constants (n and k as a function of wavelength) of the films were obtained using variable angle spectroscopic ellipsometry (SE) in the UV-vis-NIR regions. The silicon layer (SL) was modeled as a mixture of void, crystalline silicon and aluminum using the Bruggeman approximation. Based on this full characterization, it is demonstrated that the optical characteristics of the films are directly correlated to their micro-structural properties. A very bright photoluminescence (PL) was obtained and find to depend on anodisation current.     

Scaling dependence of memory windows and different carrier charging behaviors in Si nanocrystal nonvolatile memory devices

Based on the charge storage mode,it is important to investigate the scaling dependence of memory performance in silicon nanocrystal(Si-NC) nonvolatile memory(NVM) devices for its scaling down limit.In this work,we made eight kinds of test key cells with different gate widths and lengths by 0.13-μm node complementary metal oxide semiconductor(CMOS) technology.It is found that the memory windows of eight kinds of test key cells are almost the same of about1.64 V @ ±7 V/1 ms,which are independent of the gate area,but mainly determined by the average size(12 nm) and areal density(1.8×10~(11)/cm~2) of Si-NCs.The program/erase(P/E) speed characteristics are almost independent of gate widths and lengths.However,the erase speed is faster than the program speed of test key cells,which is due to the different charging behaviors between electrons and holes during the operation processes.Furthermore,the data retention characteristic is also independent of the gate area.Our findings are useful for further scaling down of Si-NC NVM devices to improve the performance and on-chip integration.     

Different charging behaviors between electrons and holes in Si nanocrystals embedded in SiN_x matrix by the influence of near-interface oxide traps

Si-rich silicon nitride films are prepared by plasma-enhanced chemical vapor deposition method,followed by thermal annealing to form the Si nanocrystals(Si-NCs)embedded in Si Nx floating gate MOS structures.The capacitance–voltage(C–V),current–voltage(I–V),and admittance–voltage(G–V)measurements are used to investigate the charging characteristics.It is found that the maximum flat band voltage shift(△VFB)due to full charged holes(～6.2 V)is much larger than that due to full charged electrons(～1 V).The charging displacement current peaks of electrons and holes can be also observed by the I–V measurements,respectively.From the G–V measurements we find that the hole injection is influenced by the oxide hole traps which are located near the Si O2/Si-substrate interface.Combining the results of C–V and G–V measurements,we find that the hole charging of the Si-NCs occurs via a two-step tunneling mechanism.The evolution of G–V peak originated from oxide traps exhibits the process of hole injection into these defects and transferring to the Si-NCs.     

Fabrication and characterization of ZnO micro and nanostructures prepared by thermal evaporation

A simple method of thermal evaporation to fabricate micro and nanostructures of zinc oxide was presented. ZnO micro and nanostructures, prepared under different quantity of O₂, were characterized by techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and analytical transmission electron Microscope. The SEM images indicated that the products prepared under the condition of sufficient O₂ were needle-like microrods and the samples synthesized under the condition of deficient O₂ were nanorods and nanowires with very high aspect ratio. The results of XRD and Raman shifts revealed that the ZnO micro and nanostructures synthesized under different quantity of O₂ were both single crystalline with the hexagonal wurtzite structure. The HRTEM images indicated that the ZnO nanowire prepared under the condition of deficient O₂ was single crystalline and grown along the direction of [0 0 1]. Photoluminescence measurement was carried out and it showed that the spectra of ZnO micro and nanostructures prepared under different quantity of O₂ exhibited similar emission features. In addition, the growth mechanism of ZnO micro and nanostructures was preliminarily discussed.     

Relative Distribution of Au48 ～ Au52 in Au Plasma by Ionization Dynamics

The present work proposes a theoretical method called ionization dynamics to derive the ionic charge state distribution. Using relativistic quantum mechanics to calculate the energy level lifetime and average ionic lifetime of each ion, the first-order ionization rate constant can be obtained. Based on these data, from the solution of differential equations for consecutive-irreversible ionization reactions, one will be able to derive the ionic charge state distribution.The calculated average positive charge 49.24 of Au48 ～ Au52 and their relative distribution are in good agreement with the results of Lawrence Livermore National Laboratory.     

On the slow decay ofO(2) correlations in the absence of topological excitations: Remark on the Patrascioiu-Seiler model

For spin models withO(2)-invariant ferromagnetic interactions, the Patrascioiu-Seiler constraint is |arg(S(x))–arg(S(y))|₀ for all |x–y|=1. It is shown that in two-dimensional systems of two-component spins the imposition of such contraints with ₀ small enough indeed results in the suppression of exponential clustering. More explicitly, it is shown that in such systems on every scale the spin-spin correlation function obeys S(x)·S(y)1/(2|x–y|²) at any temperature, includingT=. The derivation is along the lines proposed by A. Patrascioiu and E. Seiler, with the yet unproven conjectures invoked there replaced by another geometric argument.Dedicated to Oliver Penrose on the occasion of his 65th birthday.     

Enhancement of the visible luminescence from the ZnO nanocrystals by Li and Al co-doping

Nanocrystals of ZnO, doped with aluminum and lithium, were synthesized by chemical co-precipitation from an aqueous solution mixture of Zn(NO)₃, AlCl₃ and LiCl. With an aim to produce strong white luminescence, the intensity of the visible luminescence was tailored as a function of dopant concentration. We observed a significant enhancement of the yellowish-white photoluminescence from ZnO nanocrystals due to co-doping with aluminum and lithium. The luminescence intensity increases and the peak position shifts to lower wavelength with increase in the dopant concentration. Although the exact mechanism of the enhancement of the luminescence could not be established, still a possible shallow donor-to-deep acceptor recombination was proposed.