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.     

Quantitative auger electron analysis of homogeneous binary alloys: Chromium in gold

Quantitative Auger electron analysis of Cr/Au alloys with up to 20% Cr has been accomplished. The surface composition of scribed areas were compared to bulk compositions and it was shown that corrections for variation of density, escape depth, and electron backscattering must be included; these corrections change the measured surface Cr concentrations by approximately 15%. Alloy sputter yield ratios have been calculated from surface concentrations after sputtering with Ar or Ne (0.5, 1.0, 1.5, and 2.0 keV). The sputter yield ratio of Cr to Au was 0.5 at 1% Cr (significant preferred sputtering) but was near unity at 20% Cr (no preferred sputtering). The sputter yield ratio was nearly independent of ion species and ion energy. The 2 keV argon ion sputter yields for pure Cr and Au were determined to be 2.0 and 7.9 atoms/ion, respectively. However, the 2 keV argon ion sputter yield for Au in the alloys drops rapidly from 7.9 atoms/ion for pure Au, to 5 atoms/ion at 10–20% Cr. The sputter yield for Cr in alloys (5 atoms/ion) is relatively independent of composition and is 2.5 times higher than the yield of pure Cr. No simple model is known by which pure elements sputter yields could be used to predict alloy sputtering behavior.     

Ion-induced kinetic electron emission from 6LiF,7LiF and MgF2 thin films

<正>Secondary electron yields for Ar~+ impact on ~6LiF,~7LiF and MgF_2 thin films grown on aluminum substrates are measured each as a function of target temperature and projectile energy.Remarkably different behaviours of the electron yields for LiF and MgF_2 films are observed in a temperature range from 25℃to 300℃.The electron yield of LiF is found to sharply increase with target temperature and to be saturated at about 175℃.But the target temperature has no effect on the electron yield of MgF_2.It is also found that for the ion energies greater than 4 keV,the electron yield of ~6LiF is consistently high as compared with that of ~7LiF that may be due to the enhanced contribution of recoiling ~6Li atoms to the secondary electron generation.A comparison between the electron yields of MgF_2 and LiF reveales that above a certain ion energy the electron yield of MgF_2 is considerably low as compared with that of LiF.We suggest that the short inelastic mean free path of electrons in MgF_2 can be one of the reasons for its low electron yield.     

(Si,Er)双注入热氧化硅的光致发光

采用强流金属蒸汽真空弧（MEVVA）离子源注入机，先将Si大束流注入热氧化SiO2/单晶硅，直接形成镶嵌在SiO2中的纳米晶Si，再小束流注入Er。Er离子在掺杂层中的浓度可达10^21cm^-3量级，大大地提高了作为孤立发光中心的Er^3 浓度。在77K和室温下，观察到了Er^3 的1.54цm特征发射。     

Role of mobile interstitial oxygen atoms in defect processes in oxides: interconversion between oxygen-associated defects in SiO(2) glass

The role of mobile interstitial oxygen atoms (O(0)) in defect processes in oxides is demonstrated by interconversion between the oxygen dangling bond and the peroxy radical (POR) in SiO2 glass. Superstoichiometric O(0) was created by F2 laser photolysis of the interstitial O2. On annealing above 300 degrees C, O(0) migrated and converted the oxygen dangling bond to POR. Exposure to 5.0 eV light converted POR back to a pair of the oxygen dangling bond and O(0) (quantum yield: approximately 0.1). These findings suggest that various defect processes typically occurring in SiO2 glass at approximately 300-500 degrees C are related to migration of O(0), which exists in the glass network in the peroxy linkage form.     

铒掺杂富硅热化SiO2／Si发光薄膜的显微结构

利用金属蒸气真空弧(MEVVA)离子源将稀土元素Er离子掺杂到富硅热氧化SiO2/Si薄膜中.卢瑟福背散射(RBS)和X-射线电子能谱仪(XPS)分析表明,Er浓度可达原子百分数(x)～10,即Er的原子体浓度为～1021·     

高效率的有机电致发光器件

有机电致发光器件 (OL EDs)的发光机理包括电子和空穴从电极的注入、激子的形成及复合发光 ,其中 ,空穴和电子的注入平衡是非常重要的。为了平衡载流子的注入以得到高效率和稳定性好的器件 ,人们不仅使用了电子注入更为有效的 L i F/ Al[1] 和 Cs F/ Al[2 ] 等复合电极 ,同时也使用了空穴缓冲层 ,如 S.A.Van Slyke等 [3]在ITO和 NPB之间使用 Cu Pc,使得器件的稳定性得到了明显的提高 ;A.Gyoutoku等[4 ] 用碳膜使器件的半寿命超过 3 5 0 0小时 ;最近 ,Y.Kurosaka等 [5]和 Z.B.Deng[6 ]分别在 ITO和空穴传输层之间插入一薄层 Al…     

Quantitative cleaning characterization of a lithium-fluoride iondiode

An ion source cleaning testbed was created to test plasma-cleaning techniques, and to provide quantitative data on plasma-cleaning protocols prior to implementation on the SABRE accelerator. The testbed was designed to resolve issues regarding the quantity of contaminants absorbed by the anode source (LiF), and the best cleaning methodology. A test chamber was devised containing a duplicate of the SABRE diode. Radio-frequency (RF) power was fed to the anode, which was isolated from ground and thus served as the plasma discharge electrode. RF plasma discharges in 1-3 mtorr of Ar with 10% O₂ were found to provide the best cleaning of the LiF surface. X-ray photoelectron spectroscopy (XPS) showed that the LiF could accrue dozens of monolayers of carbon just by sitting in a 2×10^-5 vacuum for 24 h. Tests of various discharge cleaning protocols indicated that 15 min of an Ar/O₂ discharge was sufficient to reduce this initial 13-45 monolayers of carbon impurities to 2-4 monolayers. Rapid recontamination of the LiF was also observed. Up to ten monolayers of carbon returned in 2 min after termination of the plasma discharge and subsequent pumping back to the 10^-5 torr range. Heating of the LiF also was found to provide anode cleaning. Application of heating combined with plasma cleaning provided the highest cleaning rates     

Surface defects created by low energy (20 < E < 240 eV) ion bombardment of Ge(001)

Surface defects created on Ge(001) exposed to low energy Xe ions are characterized by in situ scanning tunneling microscopy (STM). The temperature of the sample during ion bombardment is 165 C and ion energies range from 20 to 240 eV. The ion collisions create defects (vacancies and adatoms) which nucleate and form vacancy and adatom islands. For fixed total vacancy creation, the vacancy island number density increases with increasing ion energy: the vacancy island number density is 1.6 × 10⁻²⁰ cm⁻² for 40 eV ion bombardment and increases to 4.4 × 10⁻²⁰ cm⁻² for 240 eV ion bombardment. The increased nucleation rate for vacancies is attributed to clustering of defects. The sputtering yield of Ge(001) is also measured by STM. The sputtering yield for 20 eV ions is approximately 10⁻³ per ion but the net yield for surface defects (sum of adatoms and vacancies) is an order of magnitude higher, 10⁻², due to adatom-vacancy pair creation.     

Bound state resonances in atom-solid scattering

Theoretical studies and numerical calculations of bound state resonances (selective adsorption) in the elastic scattering of low-energy light atoms from a perfect crystalline surface are made. The surface-atom potential used is the sum of Yukawa-6 pair potentials with the pair potential parameters fixed to yield the correct bound state energies for the HeLiF system. The scattering equation is numerically integrated using basis sets of open and closed channels of varying number. It is found that numerically accurate solutions are obtained only if the basis set includes the bound state in resonance, all diffracted channels coupled strongly to it by the periodic surface potential, and the specular and other low-order diffracted channels. The calculations yield specular minimum and (11) and (21) diffraction maximum under (01) selective adsorption conditions in agreement with experimental observations for the HeLiF and HeNaF systems. These results appear independent of the potential model. The atomic band structure due to the surface periodic potential is found to yield splitting of the specular minima and/or broadening of the minima. It is suggested that studying such splittings will yield information on the periodic surface potential.     

Investigation and simulation of the structural and electronic properties of LiF:Mg,LiF:Cu,LiF:P nano-layer dosimeters

In this study, electronic structure of lithium fluoride thin films in pure state and doped with magnesium (Mg), copper (Cu) and phosphorus (P) impurities was studied using WIEN2K Code. The structural and electronic properties of two LiF thin films with 1.61 and 4.05?nm thicknesses were studied and compared. Results show that the distance of atoms in the surface and central layers of pure LiF are 1.975 and 2.03?nm, respectively. Electronic density of the valence band around the surface atoms is greater than that around middle atoms of the supercell. The band gap of bulk LiF is 9?eV. But, in the case of thin films, it is reduced to 2?eV. Electronic and hole-traps were not observed in composition of LiF thin films doped with Mg and P with 1.61 and 4.05?nm thickness and in fact, metallic properties were observed. When Cu atoms were doped in composition of an LiF thin film, the thin film was converted to semiconductor.     

Dynamics and control of shock shells in the coulomb explosion of very large deuterium clusters

The explosion dynamics of very large (approximately 10(6)-10(7) atoms) deuterium clusters irradiated by ultraintense laser pulses (I approximately 10(18) W/cm(2)) is analyzed self-consistently with one-to-one three-dimensional and two-dimensional fully relativistic particle-in-cell simulations. Small-scale shock shells in the expanding ion cloud are observed. A technique to induce the formation of large shock shells inside a single cluster, increasing the probability of intracluster nuclear reactions, is proposed and demonstrated.     

The scattering of He atoms from an ordered layer of Xe atoms on Cu(001) with energy analysis

The scattering of He atoms from an ordered layer of Xe atoms deposited on Cu(001) is described. Energy analysis was carried out using a LiF crystal analyser. Energy gain and loss events were observed, the values of which were independent of the parallel momentum transfer. Results are shown for varying degrees of coverage where at large coverages a change in the energy spectrum of the scattered atoms was observed. Adsorption of Xe on a Cu surface damaged by ion bombardment yielded no inelastic information — only an elastic component was detected.     

高能Xe离子辐照引起的注碳a:SiO2中新结构的形成

先用120keV的碳离子注入非晶二氧化硅a:SiO2薄膜，再用能量为1754MeV的Xe离子辐照。注碳量为5．0×10^16—8．6×10^17ion／cm^2，Xe离子辐照剂量为1．0×10^11和5．0×10^11ion／cm^2。辐照后的样品中形成的新结构用显微傅立叶变换红外光谱仪进行测试分析。结果表明，Xe离子辐照引起了注碳a:SiO2中Si—C，C—C，Si—O—C键以及CO和CO2分子的形成与演化。在注碳量较高时，Xe离子辐照在样品中产生了大量的Si—C键。与注入未辐照和辐照的低注碳量样品比较，增强的Si—C键的形成，预示着辐照可引起注碳a:SiO2样品中的SiC结构相变。Amorphous silicon-dioxide （a:SiO2） films were firstly implanted at room temperature （RT） with 120 keV C-ions to doses ranging from 5.0 × 10^16 to 8.6 × 10^17 ion/cm^2, and then the C-doped a:SiO2 films were irradiated at RT with 1 754 MeV Xe ions to 1.0 × 10^11 and 5.0 × 10^11 ion/cm^2, respectively. The information of new tex- ture formation in the C-doped SiO2 films after high-energy Xe ion irradiation was investigated using micro-FTIR measurements. The obtained results showed that Si--C, C--C, Si--O--C bonds as well as CO and CO2 molecules were formed in the C-doped a-SiO2 films after Xe ion irradiation. Furthermore, Xe-ion irradiation induced a plenteous formation of Si--C bonds in the high dose C-ion implanted a:SiO2 films. Compared with the C-implanted sampies without Xe-ion irradiation and the low dose C-implanted samples with Xe-ion irraddiation, the enhanced and plenty of Si--C bond formation implied that the phase of SiC structures may be produced by Xe-ion irradiation in the high dose C-ion implanted a:SiO2 films.     

Quantitative study of oxygen enhancement of sputtered ion yields. I. Argon ion bombardment of a silicon surface with O2 flood

A novel quantification approach is applied to determine in situ the amount of surface oxygen within the sputtered particle escape depth during steady-state sputter depth profiling of silicon under simultaneous oxygenation with an oxygen flood gas or with an oxygen primary ion beam. Quantification is achieved by comparing the secondary ion intensities of ¹⁶O that is adsorbed or implanted at the Si surface with the measured peak intensities of a calibrated ¹⁸O ion implant used as a reference standard. Sputtered ion yields can thereby be related to surface oxygen levels. In the present work the dependences of the partial silicon sputter yield Y and of the positive and negative secondary ion useful yields UY(X^±) (X = B, O, Al, Si, P) on the oxygen/silicon ratio, O/Si, in the sputtered flux are studied for ⁴⁰Ar⁺ bombardment of Si with simultaneous O₂ flooding. The silicon sputter yield is found to decrease with increasing flood pressure and O/Si ratio by up to a factor of 3. Both positive and negative secondary ion yields are enhanced by the presence of oxygen at the silicon surface. The useful ion yield of Si⁺ scales non-linearly with the atom fraction of surface oxygen; this behavior is shown to invalidate models that suggest that Si⁺ ion yield enhancement is dominated either by isolated oxygen atoms or by formation of SiO₂ precipitates. In contrast a microscopic statistical model that assumes that local Si⁺ ion formation depends only on the number of oxygen atoms coordinated to the Si atom to be ejected fits the ion yield data quantitatively.     

SiO/SiO_2超晶格结构界面发光的研究

利用热蒸发技术在硅衬底上制备了层厚不同的SiO/SiO2超晶格样品.对其光致发光谱进行研究发现,随着SiO/SiO2超晶格中SiO层厚度的增加,发光峰在400～600 nm之间移动.研究表明,样品的发光中心来自于SiO/SiO2界面处的缺陷发光(界面态发光).即在样品沉积的过程中,在SiO/SiO2的界面处由于晶格的不连续性,会形成大量的Si-O悬挂键,这些悬挂键本身相互结合可以形成一定数量的缺陷,同时由于O原子容易脱离Si原子的束缚而产生扩散,因此,这些悬挂键可以与扩散的O原子结合,随着SiO层厚度的增加,在SiO/SiO2的界面处先后出现WOB(O3<≡Si-O-O·),NOV(O3≡Si-Si≡O3),E'中心(O≡Si·),NBOHC(O3≡Si-O·)等缺陷,这些缺陷在SiO层厚度增大的过程中对发光先后起到主导作用,从而使得发光峰产生红移.     

Fractional ionization in plasmas produced by pulsed laser ablation

A pulsed infrared laser (Q-switched Nd:Yag) is employed to irradiate different metal targets having atomic number from Z =13 up to Z =82. The high laser fluence deposited on the metals, of the order of 100 v J/cm, produces high ablation yield and a plasma generation at the target surface. The emitted species are neutral and ionized atoms. Both components have been investigated in terms of yield emission, time-of-flight measurements and angular distribution. Results indicate that the main emission occurs mainly along the normal to the target surface, that the etching, at high fluence, is of the order of 10 v atoms/ pulse, that the atoms velocities are of the order of 10 v m/s, that the maximum ion energies are of the order of keV. During the laser irradiation, expanding and non-equilibrium plasma is produced in front of the target. The plasma has a fraction ionization depending by the metal species and generally within 10% and 80%. The plasma'temperature, at high fluence, can be theoretically calculated and reaches about 10 v K. The fractional ionization of the plasma, experimentally measured, has been investigated as a function of the laser fluence and of the energy binding of the target molecules. The ion emission yield is presented and discussed in view of the possibility to realize a laser ion source for ion accelerators.     

Bound excitons in time-dependent density-functional theory: optical and energy-loss spectra

A robust and efficient frequency dependent and nonlocal exchange correlation f(xc)(r,r(');omega) is derived by imposing time-dependent density-functional theory (TDDFT) to reproduce the many-body diagrammatic expansion of the Bethe-Salpeter polarization function. As an illustration, we compute the optical spectra of LiF, SiO2, and diamond and the finite momentum transfer energy-loss spectrum of LiF. The TDDFT results reproduce extremely well the excitonic effects embodied in the Bethe-Salpeter approach, both for strongly bound and resonant excitons. We provide a working expression for f(xc) that is fast to evaluate and easy to implement.     

TiO2/SiO2的表面化学修饰及其DRS和Raman光谱分析

以氯化醇钛盐表面反应法制备系列TiO2/SiO2,根据XRD,Raman和DRS表征分析,载体表面具有分子级分散的锐钛矿型TiO2微晶粒子和非晶TiOx物种.与本体TiO2相比,TiO2/SiO2的吸收带边显著蓝移,能隙增大为3.96 eV.当金属M(M:Pd,Cu和Ni)负载于TiO2/SiO2表面,可使其光吸收域扩展到可见光区,并引起吸收带边红移.相对Pd的负载,Cu,Ni的负载对TiO2/SiO2的LMCT带影响更大,其中Cu-TiO2/SiO2的能隙减小为3.68 eV.当金属氧化物MoO3负载于TiO2/SiO2上时,可以调变TiO2/SiO2的吸收带边并增强对可见光的吸收;随MoO3载量的增加,表面物种的相互作用增强,形成Mo-O-Ti复合结构,增强了LMCT带的吸收强度,并使能隙减小为3.81 eV.     

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave is analyzed. It is shown that using repulsive light forces that push atoms towards dark regions and thus minimizes heating, cold atoms can be adiabatically compressed by more than an order of magnitude to yield background-free sub-10-nm (rms) spots. The optimal parameters for the atomic lens and the maximal compression ratio are predicted using an analytic model and found to be in agreement with the exact results of our Monte Carlo simulations. A combined adiabatic-coherent scheme is proposed and shown to yield 8.8 nm spot size even for a thermal atomic beam. Received: 1 October 1999 / Revised version: 13 December 1999 / Published online: 5 April 2000