Time-of-flight Secondary Ion Mass Spectrometry Depth Profiles Research Articles

Ultra-Low-Power Hybrid Synaptic Memristor via Simultaneous-Controlled Mechanism of Cations and Anions for Hyper-Scale Synaptic Cores

Recently, memristive devices such as phase change random access memory (PCRAM), resistive random access memory (ReRAM), and ferroelectric random access memory (FeRAM) have been actively researched to implement hyper-scale synaptic cores of various ANNs, i.e., deep neural networks (DNNs), spiking neural networks (SNNs), convolutional neural networks (CNNs), and binarized neural networks (BNNs). In particular, ReRAM is currently being highlighted as an artificial synaptic device because of its multilevel capability (> 11 bits)1, high switching speed (< 100 ps)2, high endurance (> 1012 cycles)3, high scalability, and complementary metal-oxide-semiconductor (CMOS) compatibility4. ReRAM is generally divided into two types: valence change memory (VCM) cells having oxygen vacancy filaments and electrochemical metallization (ECM) cells having conductive metal filaments. VCM cells have great retention but exhibit high switching current (~ 100 μA). In contrast, ECM cells exhibit relatively lower switching current (~10 uA) than VCM cells but have poor retention due to diffusion phenomena at high external temperatures5. Due to their high switching current and poor reliability, VCM and ECM cells have limitations in their application as synaptic devices in next-generation neuromorphic systems for energy consumption similar to that of the human brain (~20 W). In this study, for the first time, we developed a Ru-based ultra-low-power (< 1μA) hybrid synaptic memristor having a simultaneous-controlled mechanism of Ru cations and oxygen anions in a resistive switching layer. To elucidate the simultaneous-controlled mechanism of Ru cations and oxygen anions, we fabricated the VCM, ECM, and Ru-based hybrid memristor and performed x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis depending on the resistance states (i.e., pristine, set, and reset). In addition, the mobility of mobile species in VCM, ECM, and Ru-based hybrid memristors was calculated indirectly via electrical properties (switching speed, operating voltage) and ToF-SIMS depth profiles, revealing the mechanism by which Ru-based hybrid memristor has low switching currents (< 1μA). Finally, the power consumption of active synaptic cores in the training and inference process of the designed DNN was evaluated for VCM, ECM, and Ru-based hybrid memristor. The simultaneous-controlled mechanism of Ru cations and oxygen anions of the Ru-based hybrid memristor and its application to DNN will be presented in detail. Acknowledgement This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. RS-2023-00260527) and Institute of Information & communications Technology Planning & Evaluation (IITP) under the artificial intelligence semiconductor support program to nurture the best talents (IITP-(2023)-RS-2023-00253914) grant funded by the Korea government(MSIT)". References Rao, M. et al. Thousands of conductance levels in memristors integrated on CMOS. Nature 615, 823–829 (2023).Yu, S., Wu, Y., Jeyasingh, R., Kuzum, D. & Wong, H. S. P. An electronic synapse device based on metal oxide resistive switching memory for neuromorphic computation. IEEE Trans. Electron Devices 58, 2729–2737 (2011).Lee, H. Y. et al. Evidence and solution of over-RESET problem for HfOX based resistive memory with sub-ns switching speed and high endurance. Tech. Dig. - Int. Electron Devices Meet. IEDM 7–10 (2010) doi:10.1109/IEDM.2010.5703395.Wong, H. S. P. et al. Metal-oxide RRAM. Proc. IEEE 100, 1951–1970 (2012).Yoon, J. H. et al. A Low-Current and Analog Memristor with Ru as Mobile Species. Adv. Mater. 32, 1–9 (2020). Figure 1

Quantitative and Qualitative Analyses of Mass Spectra of OEL Materials by Artificial Neural Network and Interface Evaluation: Results from a VAMAS Interlaboratory Study.

Quantitative analysis of binary mixtures of tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) and tris(8-hydroxyquinolinato)aluminum (Alq3) by using an artificial neural network (ANN) system to mass spectra was attempted based on the results of a VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study (TW2 A31) to evaluate matrix-effect correction and to investigate interface determination. Monolayers of binary mixtures having different Ir(ppy)3 ratios (0, 0.25, 0.50, 0.75, and 1.00), and the multilayers containing these mixtures and pure samples were measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS) with different primary ion beams, OrbiSIMS (SIMS with both Orbitrap and ToF mass spectrometers), laser desorption ionization (LDI), desorption/ionization induced by neutral clusters (DINeC), and X-ray photoelectron spectroscopy (XPS). The mass spectra were analyzed using a simple ANN with one hidden layer. The Ir(ppy)3 ratios of the unknown samples and the interfaces of the multilayers were predicted using the simple ANN system, even though the mass spectra of binary mixtures exhibited matrix effects. The Ir(ppy)3 ratios at the interfaces indicated by the simple ANN were consistent with the XPS results and the ToF-SIMS depth profiles. The simple ANN system not only provided quantitative information on unknown samples, but also indicated important mass peaks related to each molecule in the samples without a priori information. The important mass peaks indicated by the simple ANN depended on the ionization process. The simple ANN results of the spectra sets obtained by a softer ionization method, such as LDI and DINeC, suggested large ions such as trimers. From the first step of the investigation to build an ANN model for evaluating mixture samples influenced by matrix effects, it was indicated that the simple ANN method is useful for obtaining candidate mass peaks for identification and for assuming mixture conditions that are helpful for further analysis.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Interdiffusion behaviors observation in TiN/ZrOxNy bilayer by XAS and ToF-SIMS

Interface behavior plays an important role in multilayer structure. The deep understanding of interface diffusion and reaction is necessary to improve the performance of device with multilayer structure. In the present work, we perform a detailed systematic study on interdiffusion and interaction between the TiN and ZrOxNy layers in the TiN/ZrOxNy bilayer system. A series of TiN/ZrOxNy bilayers with reverse deposition orders or with varied composition have been prepared by radio-frequency reactive magnetron sputtering. Synchrotron radiation x-ray absorption spectroscopy (XAS) and time of flight secondary ion mass spectrometry (ToF-SIMS) are both used to understand well interface diffusion behavior. The XAS results indicate that interaction and interdiffusion extent between TiN and ZrO2 (ZrOxNy) layers depend on deposition order of both layer. Herein the O atoms enter easily into the TiN layer by substituting the N atoms when the ZrO2 or ZrOxNy layer is deposited onto the TiN layer. No significant interaction between TiN and ZrN layers takes place in the TiN/ZrN sample. Both XAS results and ToF-SIMS depth profiles reveal that the interaction between TiN and ZrOxNy layers depends on the deposition order of the layers.

Evaluation of sputtering induced surface roughness development of Ni/Cu multilayers thin films by Time-of-Flight Secondary Ion Mass Spectrometry depth profiling with different energies O2+ ion bombardment

As a follow up on a previous paper on the quantitative evaluation of sputtering induced surface roughness and its influence on Auger electron spectroscopy depth profiling of polycrystalline Ni/Cu multilayers thin films, the ion sputtering induced surface roughness of these multilayered Ni/Cu polycrystalline thin films under low energy (0.5, 1.0 and 2.0 keV) O2+ ion bombardment and 1.0 keV Cs+ ion sputtering were investigated with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Atomic Force Microscopy (AFM). The lowest energy (0.5 keV) O2+ ion sputtering gave the best depth resolution for the different energy ion sputtering. Furthermore the 1.0 keV Cs+ ion sputtering led to a better resolution than the O2+ ion sputtering. The ToF-SIMS depth resolution was also better than the AES depth profiling with a 2.0 keV Ar+ ion sputtering. The Mixing-Roughness-Information depth (MRI) model used for fitting of the ToF-SIMS depth profiles accounts for the broadening upon experimental depth profiling owing to the effects of atomic mixing, surface roughness and information depth. The MRI fitted roughness parameters obtained from the ToF-SIMS depth profiling are in good agreement with the root-mean-square roughness obtained from the AFM topography scans of the crater bottom. Ion sputtering induced roughness and depth resolution during ToF-SIMS depth profiling of Ni/Cu multilayers were quantitatively evaluated according to the MRI fitting parameters.

Dealing with image shifting in 3D ToF-SIMS depth profiles.

The high sputter efficiency and low damage of gas cluster ion beams have enabled depth profiling to greater depths within organic samples using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Due to the typically fixed geometry of the ion sources used in ToF-SIMS, as one digs into a surface, the position sampled by ion beams shifts laterally. This causes a lateral shift in the resulting images that can become quite significant when profiling down more than one micron. Here, three methods to compensate for this image shifting are presented in order to more accurately stack the images to present a 3D representation. These methods include (1) using software to correct the image shifts post-acquisition, (2) correcting the sample height during acquisition, and (3) adjusting the beam position during acquisition. The advantages and disadvantages of these methods are discussed. It was found that all three methods were successful in compensating for image shifting in ToF-SIMS depth profiles resulting in a more accurate display of the 3D data. Features from spherical objects that were ellipsoidal prior to shifting were seen to be spherical after correction. Software shifting is convenient as it can be applied after data acquisition. However, when using software shifting, one must take into account the scan size and the size of the features of interest as image shifts can be significant and can result in cropping of features of interest. For depth profiles deeper than a few microns, hardware methods should be used as they preserve features of interest within the field of view regardless of the profile depth. Software shifting can also be used to correct for small shifts not accounted for by hardware methods. A combination of hardware and software shift correction can enable correction for a wide range of samples and profiling depths. The scripts required for the software shifting demonstrated herein are provided along with tutorials in the supplementary material.

Metal–Phosphate Bilayers for Anatase Surface Modification

Compared to many other metal oxides, anatase TiO2 shows relatively lower reactivity toward carboxylic acid anchor groups. The latter is crucial for applications, for example, in dye-sensitized solar cells (DSSCs), where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. To improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes' depth, and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces toward carboxylic acid-terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat-treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer-modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all modified electrodes with phosphate-Co treatment, leading to the highest JSC values and an efficiency improvement of 48%.

A perspective on two chemometrics tools: PCA and MCR, and introduction of a new one: Pattern recognition entropy (PRE), as applied to XPS and ToF-SIMS depth profiles of organic and inorganic materials

X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are much used analytical techniques that provide information about the outermost atomic and molecular layers of materials. In this work, we discuss the application of multivariate spectral techniques, including principal component analysis (PCA) and multivariate curve resolution (MCR), to the analysis of XPS and ToF-SIMS depth profiles. Multivariate analyses often provide insight into data sets that is not easily obtained in a univariate fashion. Pattern recognition entropy (PRE), which has its roots in Shannon’s information theory, is also introduced. This approach is not the same as the mutual information/entropy approaches sometimes used in data processing. A discussion of the theory of each technique is presented. PCA, MCR, and PRE are applied to four different data sets obtained from: a ToF-SIMS depth profile through ca. 100 nm of plasma polymerized C3F6 on Si, a ToF-SIMS depth profile through ca. 100 nm of plasma polymerized PNIPAM (poly (N-isopropylacrylamide)) on Si, an XPS depth profile through a film of SiO2 on Si, and an XPS depth profile through a film of Ta2O5 on Ta. PCA, MCR, and PRE reveal the presence of interfaces in the films, and often indicate that the first few scans in the depth profiles are different from those that follow. PRE and backward difference PRE provide this information in a straightforward fashion. Rises in the PRE signals at interfaces suggest greater complexity to the corresponding spectra. Results from PCA, especially for the higher principal components, were sometimes difficult to understand. MCR analyses were generally more interpretable.

ToF-SIMS Depth Profiling of PS-b-PMMA Block Copolymers Using Arn+, C60++, and Cs+ Sputtering Ions.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a high performance tool for molecular depth profiling of polymer films, in particular when they are structured in microphases. However, a major issue is the degradation of polymer materials under ion irradiation in reactions such as cross-linking, chain breaking, or reorganization processes of polymers which have been demonstrated for materials such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). This work aims at comparing ToF-SIMS molecular depth profiling of structured polymers (polystyrene (PS)-b-polymethyl methacrylate (PMMA) block copolymers (BCP)) using either ultralow energy cesium or the more recently introduced C60++ (under NO dosing and with sample cooling) and argon cluster ion beams (using Ar1500+ ions at 5 keV). The latter improved the quality of the depth profiles, especially the argon cluster ion beam, as it is characterized by a greater homogeneity for the sputter yields of PS and PMMA. No significant artifacts were observed, and this was confirmed by the comparison of depth profiles obtained from films with variable thickness, annealing time, and morphology (cylindrical blocks vs spherical blocks). Comparison to a theoretical model (hexagonal centered pattern) ensured that the ToF-SIMS depth profiles described the real morphology and may thus be a relevant characterization tool to verify the morphology of the films as a function of the deposition parameters.

Decomposition of ultrathin LiF cathode underlayer in organic-based devices evidenced by ToF-SIMS depth profiling

In this work we investigate the chemical composition of an archetypal thin-film organic device with the Ag/LiF cathode using the time-of-flight secondary ion mass spectrometry (ToF-SIMS) with depth profiling. The LiF cathode underlayer is partly decomposed because a significant amount of lithium is released into the bulk of the multilayer device. The released lithium diffuses all the way to the substrate, accumulating, as revealed by ToF-SIMS depth profiles, at the interfaces rather than uniformly doping the underlying layers. Particularly, the bottom anode becomes chemically modified.

Interphase Chemistry and Li-Ion Transport in Negative Electrode Materials Studied by XPS and ToF-SIMS

Understanding the electrode processes occurring at the electrode/electrolyte interface and in the bulk electrode material is necessary for developing the electrochemical performances of lithium-ion (LIB), sodium-ion, sulfur or metal-air batteries. The main electrode processes are insertion/extraction reactions that induce changes in the host electrode materials; they are accompanied by decomposition of electrolyte that leads to formation of a solid electrolyte interphase (SEI) layer [1]. Apart from the electrode processes, the electrochemical performance of batteries can also be significantly dependent on ionic transport, thus a good understanding of diffusion processes is also necessary.Two principal surface-sensitive techniques are particularly suitable for analyzing surface reactions at electrodes: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). As XPS provides information on chemical composition on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of thicker surface layers and bulk materials. Depth profiling can be either performed by XPS or ToF–SIMS using Ar+ or Cs+ ions, respectively, for sputtering. ToF-SIMS is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS can also serve as a new and more direct method than the commonly used electrochemical methods (i.e. electrochemical impedance spectroscopy, cyclic voltammetry or galvanostatic methods) for precise measurement of ion diffusion in electrode materials.Our approach is to use model thin-film electrodes (i.e. from few tens to few hundreds of nm directly grown on a current collector) with a good surface finishing and having a roughness suitable for analysis by surface sensitive techniques. The application of model thin-film electrode having enlarged surface-to-volume ratio provides clearer and more comprehensible insight into electrode/electrolyte interface reactions without complications from current percolators or binding agents that are used in bulk composite electrode materials. This allows for studies of intrinsic electrochemical and interfacial processes occurring on the electrode surfaces.In the present work, XPS and ToF-SIMS have been used to investigate the chemical and volume modifications of thin-film negative-electrode materials. As an example, two types of high capacity negative-electrode materials will be presented:a conversion-type electrode consisting of iron oxide (with a theoretical capacity of 1007 mAh/g), andan alloying-type electrode consisting of silicon (with a theoretical capacity of 3579 mAh/g). Both negative-electrode materials are interesting and important candidates for application in LiBs due to their abundance and environmental friendliness.For the first time, ToF-SIMS depth profiling have been applied to measure an apparent diffusion coefficient of lithium ions (DLi+) into these electrodes. The DLi+was calculated from the finite integration of Fick’s second law for one-dimensional diffusion.Regardless of the type of electrode material, XPS analysis shows strong intensity attenuation and binding energy positive shift of the principal core level peak corresponding to the main electrode component (Si2p in the case of Si-electrode and Fe2p in the case of Fe2O3-one), confirming the formation of the SEI layers on the surfaces after cycling in PC-LiClO4. The chemical composition of the SEI formed on the two different electrodes is essentially identical, with a principal Li2CO3 constituent and a minor quantity of alkyl carbonates (ROCO2Li) as evidenced by the C1s, O1s and Li1s core level peaks [2,3]. The SEI layer chemistry principally depends on the electrolyte composition [2].Both, XPS and ToF-SIMS, evidence a dynamic increase/decrease of the SEI layer thickness upon lithiation/delithiation, superimposed to a continuous increase upon cycling. The data reveal the formation of duplex-like SEI layer structure, with the organic carbonate species in the outer region and inorganic salt decomposition products in the inner part of the SEI layer. The ToF-SIMS depth profiles also show irreversible Li trapping in the bulk electrodes and volume variations with the degree of lithiation and the number of cycles [2,4]. These modifications can be detrimental to electrode cycling and capacity retention.A low DLi+ in the order of 10-15 cm2/s calculated from ToF-SIMS depth profiles for these two types of electrodes, indicates that the rate of lithiation is slower than for intercalation-type materials and can have an important influence on their electrochemical performance.

Effect of Humidity on Scintillation Performance in Na and Tl Activated CsI Crystals

Time dependent photoluminescence and radioluminescence for sodium (Na) and thallium (Tl) activated cesium iodide (CsI) single crystals exposed to 50% and 75% relative humidity (RH) has been investigated. These results indicate that Tl activated crystals are more robust than the Na activated crystals against humidity induced scintillation degradation. The development of “etching pits” and “inactive” domains are the characteristics of deteriorated Na activated CsI crystals. These “inactive” domains, bearing a resemblance to a polycrystalline appearance beneath the crystal surface, can be readily detected by a 250 nm light emitting diode. These features are commonly observed at the corners and deep scratched areas where moisture condensation is more likely to occur. Mechanisms contributing to the scintillation degradation in Na activated CsI crystals were investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS depth profiles indicate that Na has been preferentially diffused out of CsI crystal, leaving the Na concentration in these “inactive” domains below its scintillation threshold.

Surface characterization of Zr/Ti/Nb tri-layered films deposited by magnetron sputtering on Si(111) and stainless steel substrates

Among metallic materials, commercially pure titanium and titanium alloys are very often used as biomaterials for implants. Among these alloys, titanium-aluminum-vanadium alloy Ti-6 A-4 V is one of the most commonly used due to its excellent biocompatibility and ability to allow bone-implant integration. A new class of Ti alloys employs Zr for solid-solution hardening and Nb as β-phase stabilizer. Metals such as Ti, Nb, and Zr—known as valve metals—usually have their surfaces covered by a thin oxide film that forms spontaneously in air. This oxide film constitutes a barrier between the metal and the medium. The Ti-Nb-Zr alloys have mechanical and corrosion resistance characteristics which make them suitable for use as implants. Tri-layered films of Ti-Nb-Zr were deposited on both Si(111) and stainless steel (SS) substrates using dc magnetron sputtering equipment, under an argon atmosphere according to the following methodology: a 100 nm thick layer of Nb was deposited on the substrate, followed by a 200 nm thick layer of Ti, and finally a 50 nm thick layer of Zr, on top of the multilayer stack. The morphology and chemical composition of the films were analyzed by atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). AFM images showed that the Zr/Ti/Nb tri-layer films presented nanostructured grains and low roughness. The ToF-SIMS depth profiles confirmed the formation of a three-layered film on Si(111) with well-defined and sharp interfaces between the layers, while the deposition on the stainless steel substrate caused slight intermixing at the different alloy/Nb, Nb/Ti and Ti/Zr interfaces, reflecting the greater roughness of the raw substrate. The XPS results for the Zr/Ti/Nb layers deposited on Si(111) and SS confirmed that the outermost layer consisted of Zr only, with a predominance of ZrO2, as the metal layer is passivated in air. An oxidation treatment of 1000 °C in air for 1 h for the multilayer stacks caused some dewetting with Si(111) films, and alloying in the layers. The latter effect was more pronounced for the stainless steel substrate.

Synthesis and characterization of DyScO films deposited on Si and Si-rich SiN by atomic layer deposition for blocking layer replacement in TANOS stack

In the view of improving standard TANOS stacks, a possible route is the replacement of Al2O3 blocking oxide with materials with higher dielectric constant κ, as this would increase the electric field across the tunnel oxide. A possible solution is to integrate rare earth scandates. Among the scandates, DyScO3 appears as an attractive material due to the reported high-κ value. Films with 10–30 nm nominal thickness were grown by atomic layer deposition (ALD) on Si substrates for process optimization and structural characterization. Sc(thd)3 and Dy(thd)3 were used as Sc and Dy precursors and O3 as oxidizing agent. First, Sc2O3 and Dy2O3 growth process was optimized by changing growth temperature (Tg) and ALD cycle. The best conditions for the deposition of the ternary DyScO films were found to be Tg=350 °C and Dy:Sc=1:1 pulsing ratio. Optimized films were also grown on Si-rich SiN for integration as blocking oxide. After deposition, films were subjected to rapid thermal annealing up to 1030 °C to check their thermal stability. Film thickness was checked by x-ray reflectivity, together with roughness and electron density. Film crystallinity was investigated by grazing incidence x-ray diffraction. Film uniformity and thermal stability were explored by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiles. Dielectric constant κ was extracted from capacitance-voltage (C-V) electrical measurements. As deposited amorphous DyScO films on silicon remain amorphous after annealing up to 900 °C. Film electron density reduces while thickness increases with annealing, surface roughness remaining at ∼1.5 nm. The measured Sc:Dy atomic ratio by total x-ray fluorescence analyses is 1.33 in the as deposited film. ToF-SIMS depth profiles showed that film uniformity and composition is not preserved at 900 °C, in films deposited on Si, as major Si diffusion affects the ternary oxide. Only at 600 °C the diffusion phenomena and film composition are preserved. The extracted DyScO dielectric constant is κ∼20 in films annealed at 600 °C on Si. When deposited on Si-rich SiN, DyScO uniformity remains well preserved up to 900 °C, with an improvement of the thermal stability with respect to deposition on Si. Si diffusion is evident at 1030 °C only. The κ-value extracted from C-V resulted to be κ∼20.

Annealing ultra thin Ta2O5 films deposited on bare and nitrogen passivated Si(100)

Ta2O5 films, deposited on bare and nitrogen passivated Si(100) surfaces, were annealed in the presence of three different oxidizing agents, NO, N2O, and O2, and then examined using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). These films were thin enough to allow spectroscopic examination of the interface between the silicon and the oxide. During deposition of Ta2O5 on bare Si(100), the substrate is oxidized and the amount of oxide increases during subsequent annealing. No nitrogen accumulates when annealing in NO or N2O. Angle-resolved XPS and TOFSIMS depth profiles reveal, after annealing, oxidized silicon at the vacuum–solid interface and throughout the oxidized tantalum layer. For Ta2O5 on nitrided Si(100), annealing at 820°C depletes nitrogen in the silicon nitride and increases silicon oxide, regardless of the oxidant used. Temperature programmed reaction spectra of Ta2O5 deposited on previously oxidized Si(100) show SiO desorption above ∽950°C. The annealing results are accounted for in terms of chemical reactions at the buried interface that form SiO and, for the nitrided samples, NO species that migrate toward the vacuum interface. Oxidizing species from the gas phase also migrate to the buried interface to produce additional silicon oxide.