Optically Stimulated Electron Emission Research Articles

Electronic Work Function of Carbon Nanocomposite Films According to Vacuum and Atmospheric Photoemission

Using the methods of optically stimulated electron emission (OSEE) and ambient pressure photoemission spectroscopy (APS), the energy characteristics of carbyne-containing films on copper and silicon substrates have been studied. The average contact potential difference and the work function were determined, and the positions of the Fermi level for carbyne-containing films of various thicknesses were calculated. It was found that the electrons work function and the position of the Fermi level of the studied film samples do not depend on the film thickness and the type of substrate. The information content of the OSEE and APS methods for evaluating the quality of carbon coatings on substrates is shown.

Работа выхода электрона углеродных нанокомпозитных пленок по данным вакуумной и атмосферной фотоэмиссии

The energy characteristics of carbyne-containing films on copper and silicon substrates have been studied using the methods of optically stimulated electron emission (OSEE) and ambient pressure photoemission spectroscopy (APS). The average contact potential difference and the work function were determined, and the positions of the Fermi level for carbonaceous films of various thicknesses were calculated. It was found that the electron work function and the position of the Fermi level of the studied film samples are independent of the film thickness and the type of substrate. The information capabilities of the OSEE and APS methods for assessing the quality of carbon coatings on various substrates are shown.

Macroscopic Behavior and Microscopic Factors of Electron Emission from Chained Nanocarbon Coatings

The carbyne-containing films based on linear-chain carbon are promising materials for the manufacture of electronic equipment components. These carbyne-containing materials can be used as active elements of computational electronics and as ultra-miniature sensors of gaseous environment. The temperature studies of the electrical characteristics of carbyne-containing films by most of the scientific groups are limited to the low temperature range in which the quantum properties of nanostructures are most pronounced. We studied carbyne-containing films with a thickness of 20 and 400 nm on copper and silicon substrates using optically stimulated electron emission (OSEE) in the temperature range from room temperature (RT) to 400 °C. Theoretical modeling explains the dependence of work function on termination groups and substrate lattice. Experimental data revealed a relationship between the spectral characteristics of electron emission and temperature. The spectral contributions of both surface states and bulk interband transitions were clearly distinguishable.

Experimental Investigation of the Effect of Pre-Bond Contamination with Fingerprints and Ageing on the Fracture Toughness of Composite Bonded Joints

The effect of the pre-bond contamination with fingerprint (FP), both during the production and the application of adhesively bonded patch repairs on composite aircraft parts, and the cumulative effect of fingerprint and hygrothermal ageing on the fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints are experimentally investigated. To this end, mode I and mode II fracture toughness tests were conducted on FP contaminated coupons and mode II fracture toughness tests on FP contaminated and aged coupons. The artificial FP was supposed to mimic the accidentally applied fingerprints on the CFRP adherent and was applied on the adherent in the size of a human thumb. The two investigated FP formulations consisted of an artificial hand perspiration solution for the production scenario, and of Skydrol hydraulic-oil for the repair scenario. Three levels of contamination with FP were considered for each scenario. The hygrothermal ageing conditions applied until moisture saturation achieved were 70 °C/85% RH. Prior to bonding, the CFRP adherent surfaces were inspected using full-scale monitoring with optically stimulated electron emission (OSEE), an extended nondestructive testing method, in order to analyze the physicochemical variations at the contaminated surface of the adherents before bonding. The ANOVA statistical method was used to assist the evaluation of the results and the deduction or reliable conclusions. The results revealed that the pre-bond FP contamination of the adherents leads to the reduction of the fracture toughness of the CFRP bonded joints. After-bond hygrothermal ageing under the prescribed conditions had no significant influence on the moisture absorption and load carrying capacity of the joint. However, it influenced the bondline integrity as it led to a significant reduction of the fracture toughness of the joint. The effect of the combination of pre-bond contamination and after-bond hygrothermal ageing is more severe than the two separate effects.

Atomic and electronic structure of graphene oxide/Cu interface

The results of X-ray photoemission (XPS) and valence bands spectroscopy, optically stimulated electron emission (OSEE) measurements and density functional theory based modeling of graphene oxide (GO) placed on Cu via an electrophoretic deposition (EPD) are reported. The comparison of XPS spectra of EPD prepared GO/Cu composites with those of as prepared GO, strongly reduced GO, pure and oxidized copper demonstrate the partial (until C/O ratio about two) removal of oxygen-containing functional groups from GO simultaneously with the formation of copper oxide-like layers over the metallic substrate. OSEE measurements evidence the presence of copper oxide phase in the systems simultaneously with the absence of contributions from GO with corresponding energy gap. All measurements demonstrate the similarity of the results for different thickness of GO cover of the copper surface. Theoretical modeling demonstrates favorability of migration of oxygen-containing functional groups from GO to the copper substrate only for the case of C/O ratio below two and formation of Cu-O-C bonds between substrate and GO simultaneously with the vanishing of the energy gap in GO layer. Basing on results of experimental measurements and theoretical calculations we suggest the model of atomic structure for Cu/GO interface as Cu/CuO/GO with C/O ratio in gapless GO about two.

Correlation of optically stimulated electron emission with failure mode of adhesively bonded epoxy composites

Carbon fiber reinforced polymers (CFRP) reduce weight in aerospace and automotive applications while maintaining or improving structural performance. Additional performance gains can be realized with composites if adhesive bonding is used in place of mechanical fasteners for structural assemblies. Surface preparation for adhesive bonding plays a critical role in the assembly process. Effective techniques for monitoring the pre-bonding surface conditions are crucial to obtain surfaces free from bond-degrading contaminants, e.g. mold release agents, which are widely used in CFRP manufacturing. In this work, optically stimulated electron emission (OSEE) was used prior to and after laser ablation to measure deposited levels of polydimethylsiloxane (PDMS) on CFRP (Torayca P2302-19). OSEE was also used to model the contamination layer thickness. After the specimens were adhesively bonded with a modified epoxy film adhesive (Loctite EA 9696 Aero), they were subjected to double cantilever beam (DCB) tests to investigate the hypothesis that surface contamination species and levels affect the fracture characteristics of adhesively bonded joints. This study relates OSEE photocurrent and the classification of the bond failure modes to their PDMS surface contamination levels prior to adhesive bonding. DCB test results show that (1) the region under the traces within the load and unloading boundaries consistently correlates with the bond strength and the probability of different failure modes, and (2) bond performance at fracture depends on the surface PDMS film thickness coating prior to laser surface treatment. The decrement in bond performance in this study correlates to the OSEE readings of contaminant levels on the adherent surfaces.

Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies

Two different types of boron-doped graphene/copper interfaces synthesized using two different flow rates of Ar through the bubbler containing the boron source were studied. X-ray photoelectron spectra (XPS) and optically stimulated electron emission (OSEE) measurements have demonstrated that boron-doped graphene coating provides a high corrosion resistivity of Cu-substrate with the light traces of the oxidation of carbon cover. The density functional theory calculations suggest that for the case of substitutional (graphitic) boron-defect only the oxidation near boron impurity is energetically favorable and creation of the vacancies that can induce the oxidation of copper substrate is energetically unfavorable. In the case of non-graphitic boron defects oxidation of the area, a nearby impurity is metastable that not only prevent oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction demonstrates high catalytic performance of these materials.

Investigations of biofilms formed on silica in contact with aqueous formulations containing laccase and maltodextrin

The formation of nano-scale biofilms on hydrophilic silica surfaces from aqueous polypeptide/polysaccharide mixtures containing laccase and maltodextrin was investigated in situ with quartz crystal microbalance with dissipation monitoring (QCM-D). Surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), optically stimulated electron emission (OSEE) and atomic force microscopy (AFM) were applied for characterizing the resulting layers obtained after periods varying from a few seconds to several hours of contact between the substrate and the biopolymers formulation. The biofilm formation in contact with the aqueous laccase/maltodextrin suspension was studied at pH level 4.75, under conditions close to the isoelectric point of the enzyme. The few nanometers rough biofilms obtained were composed of a laccase/maltodextrin mixture, and their thickness was observed to steadily increase during 4 h of contact with the aqueous mixture of biopolymers. Remarkably, the still adhesive films obtained after 1 h of contact with aqueous polypeptide/polysaccharide mixture resisted a 30 min rinsing with acetate buffer. The biofilms growing process was monitored using OSEE, due to the effected attenuation of the UV-induced electron emission from the SiO2/Si substrate, which was found to be more pronounced than the attenuation of the photoelectrons from the substrate which contribute to the XPS signals. Layers as thin as 1 nm were detected by the OSEE.

Relaxation of excited surface states of thin Ge-implanted silica films probed by OSEE spectroscopy

As an example of thin silica films, 30nm SiO2–Si heterostructures implanted with Ge+ ions (1016cm−2 fluence) and rapid thermally annealed (RTA) at 950°C are studied by means of optically stimulated electron emission (OSEE) in the spectral region of optical transparency for bulk silica. Quartz glass samples were used as references. Experimental data revealed a strong dependence between electron emission spectral features and RTA annealing time. The spectral contributions of both surface band tail states and interband transitions were clearly distinguished. The application of emission Urbach rule as well as Kane and Pässler equations allowed to analyze the OSEE spectra at different optical excitation energy ranges and to retrieve the important microstructural and energy parameters. The observed correlations between parameter values of Urbach- and Kane-related models suggest the implantation-induced conversion of both the vibrational subsystem and energy band of surface and interface electronic states.

Quality assurance concepts for adhesive bonding of composite aircraft structures – characterisation of adherent surfaces by extended NDT

In order to ensure the performance of adhesively joined load-critical composite structures, suitable technologies are needed to steadily monitor adherent surfaces prior to bonding and to detect adhesion properties of bonded components. A novel class of non-destructive testing (NDT) techniques, classified as extended non-destructive testing (ENDT), is required to ascertain selected physicochemical properties which are important for the performance of adhesive bonds in place of detecting material defects like conventional NDT methods do. The European FP7 project, ‘ENCOMB – Extended non-destructive testing of composite bonds’ aims in the identification, development, adaptation and validation of ENDT methods for characterisation of adherent surfaces and adhesive bond quality. Here, recent NDT techniques such as optically stimulated electron emission (OSEE) and aerosol wetting test (AWT) as well as laser-induced breakdown spectroscopy (LIBS) were advanced and applied in field, and without contacting carbon fibre-reinforced polymer (CFRP) surfaces for detecting different contamination layers such as release agent, moisture or hydraulic oil as well as thermal degradation of CFRP adherent surfaces before performing an adhesive bonding process. Sensitivity and accuracy of these techniques allow distinguishing surface states which are suitable for bonding of CFRP adherents from surface states which are unfavourable for bonding. ENDT using OSEE, AWT and LIBS facilitated the detection of layers of release agent as thin as one nanometre and thin layers resulting from hydraulic oil. OSEE investigations of adherent surfaces before adhesive bonding allowed the indication of all surface states of potential CFRP adherents, which according to previous studies, were related to application scenarios reducing the joint strength of resulting adhesive joints by 20–70%.

Optically Stimulated Electron Emission from Surface States of SiO<sub>2</sub>:Ge Films

Thin 30 nm SiO2films on silicon substrate implanted with Ge+ions and flash-annealed at 900°C are investigated by means of optically stimulated electron emission (OSEE). The materials under study may find a number of technological applications in optical devices and ultraviolet sensors. Samples attestation was performed by electron microscopy and x-ray photoelectron spectroscopy (XPS). XPS data revealed strong dependence between the germanium atoms state and annealing time. The application of Urbach rule and power Kane dependence allowed to fit OSEE spectra at different excitation energy ranges and to retrieve the important structure and energy parameters. Observed correlations between parameter values of Urbach-and Kane-related models suggest the implantation-induced changeover of both the vibronic subsystem and energy band structure.

Surface analytical approaches contributing to quality assurance during manufacture of functional interfaces

Assessing adhesion or strength of composites or adhesive joints in a non-destructive way is highly challenging. Therefore, instead of performing retrospective quality assurance, i.e. investigating manufactured joints, it is advantageous to safeguard performance and quality of each layer and each interface already during manufacture. This approach still is challenging, as it requires a systematic quantitative evaluation of threshold criteria, but appreciably it gets more and more feasible. We present approaches for an inline-capable and non-destructive quality assurance of steps in manufacturing processes used for tailoring the state of substrate surfaces. Benefits from applying techniques for inline surface analysis like Optically Stimulated Electron Emission (OSEE) and Aerosol Wetting Test (AWT) will be detailed. The performed procedures contribute to a novel class of non-destructive testing (NDT) techniques, classified as Extended NDT (ENDT). The principle of ENDT methods is based on the detection of selected physico-chemical properties which are important for the anticipated performance of the functional interfaces in the products to be manufactured. A prerequisite for obtaining reliable composite materials is to reproducibly prepare a suitable surface state of the substrates before the first step of a coating or bonding process. As demonstrative application scenarios, we highlight first an exemplary surface pretreatment process for steel substrates, and second the identification of a surface state for carbon fiber reinforced polymer (CFRP) adherents suitable for joining. Concerning steel substrates we investigated two types of steel both in the as-received state and a state after grinding. We demonstrate that the removal of the topmost material layer comprising the reaction layer and mechanically deformed metal grains strongly affects the properties of the resulting adherent surface. As a consequence, a material-specific time slot for a steel substrate exposure in air after grinding is suggested in which the surface properties probed by OSEE remain unchanged. Moreover, we work out that the sensitivity and accuracy of inline-capable NDT techniques allow distinguishing surface states suitable for bonding of CFRP adherents from surface states unfavourable for adhesive bonding, and we exemplarily verify this statement for bonding processes applying freshly ground CFRP or, respectively, CFRP covered with thin layers of release agents.

CO2 SPRAY CLEANING AND OSEE NON-CONTACT INSPECTION FOR WIRE BOND PAD PREPARATION

Surface pad contamination is a major cause of poor performance for wire bonding operations. Examples of the wide range of contaminants that can degrade wire bond pull strength include, for example:Halogens and hydrocarbons: plasma etching, epoxy outgassing (dry processing), photoresist strippers, cleaning solvents.Contaminants from plating operations: thallium, brighteners, lead, iron, chromium, copper, nickel, hydrogen.Sulfur compounds: packing containers, ambient air, cardboard and paper, rubber bands.Miscellaneous organic contaminants: epoxy outgassing, photoresist, general ambient air (poor storage).Miscellaneous inorganic compounds: sodium, chromium, phosphorous, bismuth, cadmium, moisture, glass, vapor, nitride, carbon, silver, copper, tin.Human sources of contamination: skin particles, hair, sweat, spittle, mucus, cosmetics, hand lotions, facial make-up and fibers from clothing. As can be seen, there are many types of surface contaminations that may challenge a wire bonding operation, all of which must be removed to insure reliable and strong bonds. In this regard, conventional precision cleaning processes for high reliability surface pad preparation typically involve multiple steps, chemistries, and equipment to accomplish complete decontamination. Moreover, conventional cleaning methods are sometimes non-selective for the surface contaminants and substrates. For example, conventional vacuum plasma using Ar/O2 is typically used to clean bond pads. Vacuum plasmas are usually performed off-line, taking up to 30 minutes to complete, and are non-selective for the organic contamination. The entire organic substrate (i.e., PCB) is etched away during the plasma cleaning process to remove the bond pad contamination. During treatment, secondary organic surface contaminations (plasma treatment by-products from reacted substrate) are produced which can re-contaminate bonding surfaces. Advanced carbon dioxide (CO2) spray cleaning technology provides various methods for consistently preparing bond pads for critical wire bonding operations. A patented hybrid CO2 particle-plasma spray is presented in this paper that has demonstrated efficacy for selectively treating bond pad surfaces to remove a wide range of challenging surface contaminations. Moreover, a novel non-contact surface inspection technology called Optically Stimulated Electron Emission (OSEE) - developed to address surface cleaning and inspection issues that led to the 1986 Challenger Spacecraft explosion - is used to measure the effectiveness of the new CO2 surface cleaning processes. Statistically significant studies have been performed to determine the effectiveness of the selective CO2 particle-plasma surface treatment process for preparing bond pads for gold ribbon bonding operations. One such study compared and contrasted the performance of this new single-step CO2 surface treatment method with that of a conventional multi-step solvent-plasma method. The two treatment methods were used to prepare the surface of a metalized ceramic wafer that simulated bond pad surfaces and treatment areas representative of an actual high-reliability electronic board. The test results of this evaluation demonstrated that the CO2 particle-plasma surface treatment process is statistically similar to or sometimes better than a solvent-plasma hybrid cleaning process. CO2 spray cleaning was determined to be better for some types of contaminants as well – and in particular more relevant mixed-contaminant challenge tests. The CO2 cleaning process demonstrates a lower defect-per-million (DPM) level and an improved CpK. Finally, in this study OSEE surface quality analysis was performed before and after surface cleaning. OSEE analysis provided a reliable non-contact means of determining the proper level of surface pad preparation.

Photoelectron emission from implanted SiO2: Se+ films

Silica films implanted with selenium ions (330 keV, 5 × 1016 cm−2) were studied. The method of optically stimulated electron emission (OSEE) was used to analyze features specific to disordering of the film structure. Spectrum dependences of the OSEE are described in terms of the formalism of Urbach’s emission rule. It was found that the discrete-continuum disturbances arising upon implantation considerably distort the spectrum of the electronic and vibrational states of the SiO2 matrix. Results of the experiment and the numerical simulation provide evidence for the activation tunneling of electrons from optically excited surface states of SiO2 to a vacuum.

Photosensitive defects in silica layers implanted with germanium ions

Abstract Ge-implanted silica layers have been investigated by high-power pulsed synchrotron-photoluminescence (PL), photoluminescence excitation spectroscopy (PLE), and optically stimulated electron emission (OSEE) with respect to association of excitation and absorption bands to respective emission bands and lifetimes of excited defect states. In this way singlet–singlet (4.35 eV) and triplet–singlet (3.18 eV) radiative transitions from excited states of oxygen-deficient centers (ODC) in Ge-doped silica glass are characterized by their absorption and emission bands as well as their lifetimes. The main channel for non-radiative relaxation of photoexcitation is electron emission by the OSEE effect. The OSEE shows non-radiative transitions of surface E s ′ and bulk E′-centers found with concentrations of (2.7–3.4) × 1012 cm−2 and (2–4) × 1016 cm−3, respectively.

Characteristics of the electron-emission defects introduced in Si–SiO 2 structures by MeV electron irradiation

Defects induced by high-energy electrons in Si–SiO 2 structure have been studied by the optically stimulated electron emission (OSEE) method. Si–SiO 2 structures with oxide thickness of 100 nm are irradiated with 23 MeV electrons for different durations. It is shown that most of the defects created by electron irradiation at the interface and in the oxide bulk are vacancies like E′-centers. Most of the photoemission activity changes are observed during low doses electron irradiation. Some uncharged defects like diamagnetic oxygen-deficient centers are also observed, together with E′-centers.

Electron emission from excited states of E′ centers in SiO 2

The applicability of the non-stationary optically stimulated electron emission (OSEE) to the spectroscopy of E′ centers in different SiO 2 modifications was demonstrated. Spectral dependences of the OSEE from E′ centers in samples of silica glass, crystalline α-quartz, SiO 2 films and nanoceramics exposed to fast electrons and ions were obtained. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two non-radiative relaxation channels (intracenter and ionization) is discussed. This model was used to substantiate the mechanism of the photothermal decay of E′ centers and to determine the quantum yields of OSEE for different types of E′ centers.

Photoelectron spectroscopy of E′ centers in crystalline and glassy silicon dioxide

Radiation-induced E′ centers in SiO2 were studied to test the possibility of applying optically stimulated electron emission (OSEE) to the spectroscopy of excited states of point defects in dielectrics. The spectral responses of the OSEE of crystalline α quartz and silica glass irradiated by 10-MeV electrons were measured and studied. It was established that volume E′ centers in the crystalline and glassy SiO2 modifications are dominant emission-active defects. Surface E’s (1) centers were also detected in glassy SiO2. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two nonradiative (intracenter and ionization) relaxation channels is proposed. This model was used to explain the mechanism of photothermal decay of the E′ centers and to determine the ionization activation barriers and quantum yields of these centers. The emission, spectral, and kinetic parameters of the volume and surface E′ centers in glassy SiO2 were obtained, showing the excited states of these defects to have identical atomic configurations.

Taking Account of the Nonstationarity in Analyzing the Optically Stimulated Electron Emission of Irradiated Dielectrics

The distortions of the optically stimulated electron emission (OSEE) spectra resulting from the exponential decay in the recording time have been analyzed. It has been shown that taking account of the emission current nonstationary in analyzing the emission spectra permits obtaining isochronal temperature dependences of intensity for selective OSEE bands. The procedure for calculating the quantum yield of the OSEE and the activation barriers of thermal ionization of the excited states of defects is described. Analysis of the photoemission kinetics permits estimating the concentration of active OSEE centers with unknown parameters (optical transition oscillator strength, quantum yield). For diamagnetic oxygen-deficient centers in glassy SiO2, the photoemission kinetics has been calculated and the thermal ionization barrier of the photoexcited singlet state and the quantum yield of the OSEE have been determined.

Method for the analysis of nonselective spectra of optically stimulated electron emission from irradiated dielectrics

AbstractA method has been proposed and substantiated for processing of nonselective optically stimulated electron emission (OSEE) spectra of irradiated crystalline and amorphous dielectrics. This method allows the separation of the emission contribution of discrete centers taking into account effects of radiation‐induced electrization of materials and the unsteady state of the electron emission. Separation of OSEE selective bands facilitates the establishment of the origin of discrete emission‐active centers and makes it possible to estimate the concentration and parameters of radiation defects localized in surface layers. The quantum yields of OSEE and ionization barriers of optically excited defects are calculated using kinetic and corrected isochronal temperature dependences of the emission current. Some capabilities of the method have been demonstrated by an example of glassy SiO2 and crystalline Be2SiO4. Oxygen‐deficient centers (ODCs), bulk and surface E′‐centers, and also non‐bridging oxygen hole centers (NBOHCs) have been recorded. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)