Medium Energy Ion Scattering Spectroscopy Research Articles

Characterization of the Interfacial Structures of Core/Shell CdSe/ZnS QDs.

Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.

Traceable quantitative analysis of Ag x Cu1−x alloy films by ID ICP-MS, RBS and MEIS

The measurement traceability of Rutherford backscattering spectroscopy (RBS), medium energy ion scattering spectroscopy (MEIS) and isotope dilution inductively coupled plasma mass spectrometry (ID ICP-MS) was compared for the quantitative analysis of alloy thin films. A set of thin Ag x Cu1−x alloy films were selected as a model alloy system for the quantitative analysis of MEIS and ID ICP-MS. Two sets of five Ag x Cu1−x alloy films with different mole fractions were grown on Si (100) wafers by ion beam sputter deposition. The mole fractions of thick Ag x Cu1−x alloy films (100 nm) measured by RBS and ID ICP-MS showed a great agreement within 0.4% difference. The mole fractions of thin Ag x Cu1−x alloy films (10 nm) measured with MEIS and ID ICP-MS also showed a small difference of about 1.0%. As a result, ID ICP-MS, RBS and MEIS can be used to certify the mole fractions of thin alloy reference films. ID ICP-MS is an absolute method for the mole fraction analysis of thin Ag x Cu1−x alloy films. Although the contribution of sample homogeneity was included, the uncertainties of ID ICP-MS results were much smaller than those of RBS and MEIS.

Effect of the surface contamination layer on the thickness measurement of ultra-thin HfO2 films

Mutual calibration by a combination of a zero-offset method (ZOM) and a length-unit traceable method (LTM) was suggested as a method to determine the true thickness of ultra-thin oxide films. Although, medium energy ion scattering spectroscopy (MEIS) and x-ray photoelectron spectroscopy (XPS) were suggested as the ZOMs, the offset value of thickness is affected by the surface contamination layer.In this study, we compared the effect of the surface contamination layer on the thickness measurement of a series of HfO2 films by MEIS and XPS. The surface of the HfO2 films were contaminated by thin carbon layers with the thicknesses from 0.25 nm to 1.30 nm. In MEIS, the variation of the offset value by the surface carbon layer was negligibly small in the range of 0.020 nm. However, it was large in XPS in the range of 0.131 nm. In conclusion, MEIS can be a reliable ZOM to calibrate the offset values in the thickness measurement of ultra-thin oxide films with negligibly small effect of surface contamination.

Thickness measurement of nm HfO2 films

Main textA pilot study for the thickness measurement of HfO2 films was performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study was to ensure the equivalency in the measurement capability of national metrology institutes for the thickness measurement of HfO2 films. In this pilot study, the thicknesses of six HfO2 films with nominal thickness from 1 nm to 4 nm were measured by X-ray Photoelectron Spectroscopy (XPS), X-ray Reflectometry(XRR), X-ray Fluorescence Analysis (XRF), Transmission Electron Spectroscopy (TEM), Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectrometry (RBS). The reference thicknesses were determined by mutual calibration of a zero-offset method (Medium Energy Ion Scattering Spectroscopy (MEIS) of KRISS) and a method traceable to the length unit (the average thicknesses of three XRR data except the thinnest film). These reference thicknesses are traceable to the length unit because they are based on the traceability of XRR. For the thickness measurement by XPS, the effective attenuation length of Hf 4f electrons was determined. In the cases of XRR and TEM, the offset values were determined from a linear fitting between the reference thicknesses and the individual data by XRR and TEM. The amount of substance of HfO2, expressed as thickness of HfO2 films (in both linear and areal density units), was found to be a good subject for a CCQM key comparison.To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

Traceable thickness measurement of ultra-thin HfO2 films by medium-energy ion scattering spectroscopy

The thicknesses of a series of ultra-thin HfO2 films were precisely determined by mutual calibration by x-ray photoelectron spectroscopy (XPS) and x-ray reflectometry (XRR) in the recent Consultative Committee for Amount of Substance (CCQM) pilot study P-190. From these well-defined reference film thicknesses, the measurement capability of medium-energy ion scattering spectroscopy (MEIS) for the thickness of HfO2 films was investigated. The film thicknesses determined by MEIS showed a small difference, within 2%, from the reference thicknesses and an offset value of 0.017 nm. The MEIS thicknesses can also be determined by mutual calibration between the transmission electron microscopy (TEM) thicknesses and the MEIS intensity ratios in the region of the substrate and HfO2 film. From linear fitting with the reference thicknesses, the MEIS thicknesses determined by mutual calibration showed a slope value of 1.011 and an offset value of 0.015 nm. As a result, MEIS can be a traceable method to determine the absolute thickness of ultra-thin HfO2 films, and a zero-offset method for application of the mutual calibration method.

Synthesis and Growth Mechanism of Stable Prenucleated (≈0.8 nm Diameter) PbS Quantum Dots by Medium Energy Ion Scattering Spectroscopy.

In the current work, stable prenucleated PbS quantum dots (QDs) with a sub-nanometer (0.8 nm) size have been successfully synthesized via a systematically designed experiment. A detailed analysis of critical nucleation, growth, and stability for such ultrasmall prenucleated clusters is done. The experimental strategy is based on controlled concentration, temperature and injection of respective precursors, thus enabling us to control nucleation rate and separation of stable sub-nanometer PbS QDs with size 0.8 nm. Significantly, by providing additional thermal energy to sub-nanometer PbS QDs, we achieved the fully nucleated cubic crystalline structure of PbS with size of around 1.5 nm. The size and composition of the prenucleated QDs are investigated by sophisticated tools like X-ray photoelectron spectroscopy (XPS) and medium energy ion scattering (MEIS) spectroscopy which confirms the synthesis of PbS with Pb2+ rich surface while the UV-Vis spectroscopy and X-ray diffraction (XRD) data suggests an alternative crystallization path. Non-classical nucleation theory is employed to substantiate the growth mechanism of prenucleated PbS QDs.

Label-free NanoBio Imaging for New Biology and Medical Science

We have been developing a new label-free nanobio imaging platform using non-linear optics such as Coherent Anti-Stokes Raman Spectroscopy (CARS) and ion beam techniques based on sputtering and scattering such as Secondary Ion Mass Spectrometry (SIMS) and Medium Energy Ion Scattering Spectroscopy (MEIS), which have been widely used for atomic and molecular level analysis of semiconductors and nanomaterials. To apply techniques developed for semiconductors and nanomaterials for biomedical applications, the convergence of nano-analysis and biology were tried. Our activities on label-free nanobio imaging during the last decade are summarized in this review about non-linear optical 3D imaging, ellipsometric interface imaging, SIMS imaging, and TOF-MEIS nano analysis for cardiovascular tissues, collagen thin films, peptides on microarray, nanoparticles, and cell adhesion studies and finally the present snapshot of nanobio imaging and the future prospect are described.

Quantitative Compositional Profiling of Conjugated Quantum Dots with Single Atomic Layer Depth Resolution via Time-of-Flight Medium-Energy Ion Scattering Spectroscopy

We report the quantitative compositional profiling of 3-5 nm CdSe/ZnS quantum dots (QDs) conjugated with a perfluorooctanethiol (PFOT) layer using the newly developed time-of-flight (TOF) medium-energy ion scattering (MEIS) spectroscopy with single atomic layer resolution. The collection efficiency of TOF-MEIS is 3 orders of magnitude higher than that of conventional MEIS, enabling the analysis of nanostructured materials with minimized ion beam damage and without ion neutralization problems. The spectra were analyzed using PowerMEIS ion scattering simulation software to allow a wide acceptance angle. Thus, the composition and core-shell structure of the CdSe cores and ZnS shells were determined with a 3% composition uncertainty and a 0.2-nm depth resolution. The number of conjugated PFOT molecules per QD was also quantified. The size and composition of the QDs were consistent with those obtained from high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. We suggest TOF-MEIS as a nanoanalysis technique to successfully elucidate the core-shell and conjugated layer structures of QDs, which is critical for the practical application of QDs in various nano- and biotechnologies.

Understanding the Interface Reactions of Rutile TiO2 Grown by Atomic Layer Deposition on Oxidized Ruthenium

The atomic layer deposition of titanium oxide TiO2 on ruthenium and oxidized ruthenium with titanium methoxide as metal precursor and H2O and O3 as oxidant was investigated by Rutherford backscattering (RBS) and time of flight secondary ion mass spectrometry (TOFSIMS). An ultra-thin layer of TiO2 deposited a priori with H2O plays the role of protection of Ru(Ox) substrates against etching by O3. Information about thin films (∼3 nm) interfacial reactions, thickness and structure was brought by Medium Energy Ion Scattering Spectroscopy (MEIS) and X-ray absorption spectroscopy (XAS) measurements. The growth enhancements observed in the first stages of the deposition depends on the pre-treatment (pre-oxidation, H2O based interlayer thickness) of the Ru substrate. Thick films (∼14 nm) were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The as deposited TiO2 films are crystalline with rutile structure, as resulted from structural analyzes. However, the presence of small amounts of anatase was detected by soft X-ray absorption spectroscopy (XAS) and is strongly influenced by the surface pre-treatment of the Ru substrate. The electrical properties (equivalent oxide thickness and leakage current density) correlate with a different rutile/anatase ratio present in the films.

MEIS and ToF‐SIMS analyses of organic ultrathin films for absolute quantification of functional groups

AbstractComplementary use of medium‐energy ion‐scattering spectroscopy (MEIS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) analyses was proposed as a new methodology for absolute quantification of functional groups in organic ultrathin films. First, MEIS analysis was used to determine the absolute areal densities of Ru dye molecules in organic ultrathin films, which were made by spin coating Ru 535‐bis TBA (C58H86O8N8S2Ru) solutions with varying concentrations. The thicknesses of the films were less than 2 nm each and the densities of these ultrathin layers were linearly correlated with the solution concentrations. The determined absolute areal densities of the Ru organic molecules were then used to calculate the areal densities of the stoichiometric functional groups. Secondly, the ToF‐SIMS intensities of the characteristic peaks such as Ru+, C16H35N+, CO2−, CSN−, CSNRu−, etc, associated with the functional groups of a Ru 535‐bis TBA molecule were correlated by the densities of the Ru organic molecules. Our results show that the correlation curves obtained through the complementary use of MEIS and ToF‐SIMS techniques could be useful to quantify functional groups on organic ultrathin films. Copyright © 2010 John Wiley & Sons, Ltd.

ALD growth of Al2O3 on GaAs: Oxide reduction, interface structure and CV performance

AbstractWe have studied the effect of the trimethylaluminum (TMA) precursor on the reduction of surface “native” oxides from GaAs substrates using medium energy ion scattering spectroscopy (MEIS), X‐ray photoelectron spectroscopy (XPS) and electrical measurements. Our data show that after one single TMA pulse a substantial part of the native oxide is reduced and an oxygen‐rich aluminum oxide layer is formed. Al2O3 films grown with the normal atomic layer deposition cycles of TMA and water show that the growth rate of the Al oxide during this initial reduction of the native oxides is faster than the rate once this reduction is completed. Furthermore, the results of C‐V measurements of Al2O3/GaAs grown under the same conditions along with post deposition annealing indicate a good quality interface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of native oxides on GaAs during atomic layer growth of Al2O3

The reduction of surface “native” oxides from GaAs substrates following reactions with trimethylaluminum (TMA) precursor is studied using medium energy ion scattering spectroscopy (MEIS) and x-ray photoelectron spectroscopy (XPS). MEIS measurements after one single TMA pulse show that ∼65% of the native oxide is reduced, confirmed by XPS measurement, and a 5 Å thick oxygen-rich aluminum oxide layer is formed. This reduction occurs upon TMA exposure to as-received GaAs wafers.

Chemical and optical profiling of ultra thin high- k dielectrics on silicon

The thickness of HfO 2 and hafnium silicate Hf x Si 1 − x O y thin films with a range of compositions are investigated using three complementary analytical techniques. We compare results obtained from Medium Energy Ion Scattering spectroscopy, spectroellipsometry and high-resolution Transmission Electron Microscopy. Our results demonstrate that the thickness of the silicate layers decreases with the Hf content.

Interface strain profiling in ultrathin SiO2 gate oxides with medium energy ion scattering spectroscopy

Medium energy ion scattering spectroscopy (MEIS) could identify ∼1 nm interface layer with compressive strain, which depends sensitively on the interface treatment conditions such as oxynitridation, ozone oxidation, tilt of Si(0 0 1) substrates. The interface strain relaxation always shows improvements in gate oxide reliability. Atomic scale investigations of strain profiles with MEIS are reviewed for SiO 2 /Si(0 0 1) interfaces.

Quantitative depth profiling of nitrogen in ultrathin oxynitride film with low energy SIMS

It was shown that the thickness of ultrathin gate oxides measured with low energy O 2 + SIMS shows linear relationships with medium energy ion scattering spectroscopy (MEIS) and HRTEM results in the range of 9–2.5 nm. For quantitative N depth profiling, N profiles in ∼3 nm Si oxynitrides were measured by low energy O 2 + and Cs + SIMS and calibrated with MEIS analysis results of the thickness and the N areal density. N SIMS profiles observed for low energy O 2 + and Cs + SIMS showed some difference in the shape and distribution.

Measurement of the physical and electrical thickness of ultrathin gate oxides

To evaluate the reliability in measurements of the thickness of ultrathin gate oxides in the range of 2–9 nm, various techniques based on different methodologies were used for comparison. The physical thickness was determined with medium energy ion scattering spectroscopy (MEIS), high-resolution transmission electron microscopy (HRTEM), and spectroscopic ellipsometry (SE). The physical thickness was compared with the electrical thickness measured with current–voltage (I–V) and capacitance–voltage (C–V) measurements with quantum effect corrections. The physical thickness of amorphous SiO2 layers in the range of 2–9 nm determined with MEIS and HRTEM is in a good agreement with the corresponding electrical thickness from C–V and I–V measurements within 0.3 nm. For SE, which is the main technique used for in-line monitoring, we observed that it can be used for 2–9 nm ultrathin gate oxides but is more sensitive to the details of the oxide characteristics.

Ultrathin nitrided-nanolaminate (Al2O3/ZrO2/Al2O3) for metal–oxide–semiconductor gate dielectric applications

An ultrathin nanolaminate (Al2O3/ZrO2/Al2O3) film prepared by atomic layer chemical vapor deposition was investigated for use in metal–oxide–semiconductor field effect transistor (MOSFET) gate dielectric applications. Based on transmission electron microscopy and medium-energy ion scattering spectroscopy (MEIS) analysis, an abrupt interface between stoichiometric top-layer Al2O3 and ZrO2 was found. Interfacial layers such as Zr–Al–O and Al–Si–O were also observed. An electrical equivalent oxide thickness as thin as 10.2 Å with a quantum mechanical correction was obtained. Additional plasma nitridation of nanolaminte in N2 led to a significant reduction in the interfacial oxidation of nanolaminate which was confirmed by x-ray photoelectron spectroscopy, MEIS, and capacitance–voltage (C–V) analysis. The nanolaminate film represents a promising alternative for gate dielectric applications of future sub-100 nm MOSFET.

Strain-induced anisotropic Ge diffusion in SiGe/Si superlattices

Anisotropic diffusion of Ge induced by nonuniform strain in SiGe/Si interfaces in the range of 700–850 °C is directly observed with medium-energy ion-scattering spectroscopy through its composition and strain profiles of atomic-layer depth resolution. For SiGe/Si interfaces with identical composition profiles but with different strain distributions, the anisotropic diffusion of Ge can be clearly correlated with the anisotropic relaxation of the nonuniform strain in the near-surface layer of several nm depth. The results suggest that atomic-scale strain control is critical to maintain abrupt SiGe/Si interfaces under thermal budget.

Establishing an accurate depth-scale calibration in the top few nanometers of an ultrashallow implant profile

A method to accurately determine the sputter yield of the matrix from the earliest stages of a sputter profile is described. Using the technique of medium-energy ion-scattering spectroscopy, this method provides data that enable a depth scale to be established from subnanometer depths onward. It may be adapted to samples containing a thin amorphous surface layer (e.g., a preamorphized shallow implant) or to crystalline surfaces containing a heavy-element marker layer. In this Brief Report we have used this method to interpret the near-surface profile using erosion-rate data obtained from a 1-keV boron implant into a germanium preamorphized silicon (001) surface.

Estimation of the electronic straggling using delta-doped multilayers

It is proposed that the electronic straggling of projectile ion beams for medium energy ion scattering spectroscopy (MEIS) can be estimated with delta-doped multilayers. From the depth dependence of the full width half maximum (FWHM) of the Ta peak of a Ta delta-doped Si multilayer, the electronic straggling of 100 keV H + in Si could be estimated. The electronic straggling can be estimated without information on the exact thickness and the interface abruptness of the Ta delta-doped multilayers. The reduced electronic straggling, Ω e /Ω B was estimated to be 0.59. With the experimentally determined electronic straggling, the thickness of the Ta delta layer could be estimated and compared with the nominal thickness based on the growth rate determined by transmission electron microscopy (TEM).