Low Electron Energy Loss Spectroscopy Research Articles

Segregation and morphology on the surface of ferritic stainless steel (0 0 1)

The temperature dependence of the segregation and morphology of ferritic steel (001) surfaces has been examined by a combination of Auger electron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and low-energy electron loss spectroscopy. Upon annealing ferritic steel at 500°C, the topmost layer was observed to be mainly composed of Fe–Cr alloy. Oxygen segregation was also detected locally in the STM images. LEED showed a (1×1) pattern and a weak (√5×√5)R27° reconstruction corresponding to Fe and Cr4O5, respectively. Upon annealing at 600°C, carbon and chromium co-segregated to the surface, forming two different regions composed of CrC and Cr-based steel, while the Cr4O5 domains disappeared. Upon annealing at 700°C, nitrogen segregated to the surface, and the topmost layer was observed to be mainly composed of CrN domains with local CrC domains.

Limits of spatial and compositional resolution of electron energy loss spectroscopy of soft materials

Abstract We investigate the spatial resolution limit of low electron energy loss spectroscopy (EELS) for imaging of electron beam sensitive materials, particularly for the case of composite materials that undergo phase separation. In order to make optimum use of the information contained in noisy spectra we modify the multiple least squares (MLS) fitting algorithm, which is widely used for fitting experimental spectra with a linear combination of reference spectra. Our approach, which uses the iteratively reweighted least squares (IRLS) routine, allows one to accommodate the non-constant variance in the noise. Assuming that the noise has a Poisson distribution we examine the performance of IRLS fitting. We introduce a parameter that reflects the difference between the spectra of the material components and computationally examine the relation between this parameter and the accuracy of the fitting algorithm. Use of this parameter allows us to derive an equation that relates the spatial resolution of imaging and the achievable level of composition uncertainty, for an ideal detector, based on the experimental parameters. These parameters include not only irradiation exposure, the difference between the spectra and the thickness of the sample but also the composition of the sample. The results presented will guide the proper choice of experimental conditions to obtain the best quality data from radiation sensitive materials.

On the nucleation of the cubic phase in boron nitride films

Boron nitride films were produced on silicon substrates by r.f. magnetron sputtering with pure argon as working gas. The cubic BN ( c-BN) phase was achieved through an r.f. substrate bias which set up a controlled ion bombardment on the growing surface. The dependence of film structure on the deposition time was studied at fixed growth parameters by means of Fourier transform infrared spectroscopy, Auger electron spectroscopy, low energy electron loss spectroscopy, atomic force microscopy, and ellipsometry. The results show for the first time that the nucleation into c-BN takes place through a phase transformation process of the upper part of an h-BN base layer formed in the initial growth stage. Special features correlating with this transformation are presented, particularly a shrinking of the film's volume and a transient extraordinarily high roughness of the growing surface. The results indicate that c-BN nucleation is not directly governed by the instantaneous ion bombardment of the growing film.

Study of the electronic structure of carbon materials near the bandgap using electron energy loss spectroscopy

The detailed understanding of the electronic properties of semiconducting materials requires the determination of their joint density of states (JDOS) and dielectric constant with high spatial resolution. Low electron energy loss spectroscopy (EELS) provides a continuous spectrum which represents all electronic excitations with energies ranging between zero and about 100 eV. Therefore EELS is potentially more powerful than conventional optical spectroscopy which has an intrinsic upper information limit of about 6 eV due to absorption of light from the optical components of the system or the ambient. However, when analysing EELS data, the extraction of the single scattered data needed for Kramers–Kronig calculations is subject to the deconvolution of the zero loss peak from the raw data. This procedure is particularly critical when attempting to study the near-bandgap region of materials with a bandgap below 1.5 eV. In this paper calculation of the electronic properties of some widely studied carbon materials; namely amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C), C60 fullerite crystal as well as synthetic and natural diamond, are presented. Extrapolation of a fit to the absorption data at the onset of an interband transition was used to get an accurate value for the bandgap energy and to determine whether the bandgap is direct or indirect in character. Particular problems relating to the extraction of the single scattered data for these materials are also discussed. The ta-C and C60 fullerite materials are found to be direct bandgap-like semiconductors, having a bandgap of 2.63 and 1.59 eV, respectively. On the other hand, the electronic structure of a-C was unobtainable because it had such a small bandgap that most of the information is contained in the first 1.2 eV of the spectrum, which is a region removed during the zero loss peak deconvolution. The processing of the data obtained from the ta-C material revealed another benefit of using EELS: the electrons of the beam can transfer momentum and energy to the electrons of the material, thus enabling the probing of indirect interband transitions. Use of this extra feature of EELS coupled with the extremely high spatial resolution obtainable in a scanning electron microscope has indicated that tetrahedral carbon thin films might not be amorphous after all, implying the need for a fresh look at their use as electronic materials.

Growth of anatase films on vicinal and flat LaAlO 3 (110) substrates by oxygen plasma assisted molecular beam epitaxy

The heteroepitaxial growth and surface structures of TiO 2 films on vicinal and flat LaAlO 3 (110) were characterized. The films were grown by oxygen plasma assisted molecular beam epitaxy at low growth rates and temperatures between 825 and 875 K. Bulk characterization by low energy electron loss spectroscopy indicated that only the anatase structure formed. Both X-ray diffraction and scanning transmission electron microscopy indicated that the films grew with anatase (102) planes parallel to the interface. The surfaces of the films were characterized by reflection high energy electron diffraction (RHEED) during growth, and photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM) following growth. The photoelectron spectra were consistent with the growth of stoichiometric TiO 2. Both RHEED and LEED, however, showed patterns expected of anatase (101) surfaces, not anatase (102). Further, STM images revealed the oblique unit cell expected of anatase (101) along with many parallel steps. The (101) surface is the lowest energy anatase surface and it is suggested that the film surface facets towards (101) to reduce the surface energy. Anatase (101) is also a much better geometric match to LaAlO 3 (110) than anatase (102); however, unlike the [102] orientation, the [101] orientation places Ti cations directly above substrate cations increasing the interfacial energy. Thus the film structure and surface morphology is determined by the interplay between geometric and electronic matching at the interface and the energy of the free surface.

Calculation of the electronic structure of carbon films using electron energy loss spectroscopy

The detailed understanding of the electronic properties of carbon-based materials requires the determination of their electronic structure and more precisely the calculation of their joint density of states (JDOS) and dielectric constant. Low electron energy loss spectroscopy (EELS) provides a continuous spectrum which represents all the excitations of the electrons within the material with energies ranging between zero and about 100 eV. Therefore, EELS is potentially more powerful than conventional optical spectroscopy which has an intrinsic upper information limit of about 6 eV due to absorption of light from the optical components of the system or the ambient. However, when analysing EELS data, the extraction of the single scattered data needed for Kramers–Kronig calculations is subject to the deconvolution of the zero loss peak from the raw data. This procedure is particularly critical when attempting to study the near-bandgap region of materials with a bandgap below 1.5 eV. In this paper, we have calculated the electronic properties of three widely studied carbon materials; namely amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C) and C 60 fullerite crystal. The JDOS curve starts from zero for energy values below the bandgap and then starts to rise with a rate depending on whether the material has a direct or an indirect bandgap. Extrapolating a fit to the data immediately above the bandgap in the stronger energy loss region was used to get an accurate value for the bandgap energy and to determine whether the bandgap is direct or indirect in character. Particular problems relating to the extraction of the single scattered data for these materials are also addressed. The ta-C and C 60 fullerite materials are found to be direct bandgap-like semiconductors having a bandgaps of 2.63 and 1.59 eV, respectively. On the other hand, the electronic structure of a-C was unobtainable because it had such a small bandgap that most of the information is contained in the first 1.2 eV of the spectrum, which is a region removed during the zero loss deconvolution.

Characterization of sputter deposited cBN-films by low energy electron loss spectroscopy LEELS

The characteristic energy losses of inelastically backscattered electrons of 0.2−2 keV measured by low energy electron loss spectroscopy (LEELS) are shown to enable a destruction free determination of the sp 2- and the sp 3-coordinated fractions in BN-films. The specific loss structures from plasmon excitation in hBN and cBN, and from the π→ π ∗ -transition in hBN are evaluated accordingly. In contrast to the commonly employed Fourier transform infrared spectroscopy (FTIR) the LEELS-method as surface sensitive technique is not influenced by contributions from the substrate or the commonly present hBN-baselayer for cBN-nucleation. Thus, the near surface region of properly deposited films could be shown to consist of pure cBN. When controlling the mean penetration depth of the primary electrons by changing their energy, phase sensitive depth profiling becomes possible. This is applied to determine the destruction depth of the cubic phase of BN under bombardment with Ar +-ions of 0.5−3 keV.

Passivation of GaAs surface by GaS

The thermal evaporation of a GaS single crystal has been used to passivate the GaAs surface. The passivation effect of GaAs surface by GaS has been studied by low-energy electron-loss spectroscopy (LEELS) and photoluminescence (PL). The LEELS spectra shows that the surface related peaks of GaAs disappeared immediately after starting the deposition, suggesting the effective passivation of dangling bonds of GaAs surface by S atoms included in the evaporated species. PL measurements suggest that the GaAs surface is effectively passivated by GaS films.

Structure and electronic properties of ionic nano-layers MBE-grown on III–V semiconductors

Thin epitaxial NaCl and KBr layers of various thickness (from 3 to 100 monolayers) have been deposited on (100) surfaces of GaAs and InSb semiconductors by means of molecular beam epitaxy (MBE). Electronic and structural properties of the freshly prepared films were subsequently investigated in the attached UHV analytical chamber by means of low energy electron diffraction, low electron energy loss spectroscopy, Auger electron spectroscopy and an electron holography.Alkali halide growth mode was found to be a two-dimensional layer-by-layer type (Franck van der Merve growth mode). It was possible to demonstrate that the first monolayer of alkali halide on the AIIIBV semiconductor is arranged by the strong bond formed between the halogen ion and the AIII metallic element. In case of NaCl/GaAs(100) system a local atomic configuration was found for an early stage of epitaxy (three to five monolayers) by means of the electron holography. A detailed analysis of the reconstructed diffraction patterns revealed that an initial formation of the Cl-Ga bond occurred in the system, and after deposition of five monolayers NaCl, the substrate was uniformly covered by a layer at least three monolayers thick.

Interface formation between GaS and CVD diamond films

Thin GaS films were deposited on B-doped diamond (as-grown) and on oxygen-annealed B-doped diamond (O-ann.) films grown by plasma-assisted chemical vapor deposition on p +-type Si(001) substrate and the interface formation has been characterized by Auger electron spectroscopy (AES), low-energy electron-loss spectroscopy (LEELS) and X-ray photoelectron spectroscopy (XPS). The thermal evaporation of a GaS single crystal is used for the deposition. LEELS spectra taken at low E P (≤200 eV) indicate that the surface-related peaks are dominant for both as-grown and O-ann. films. Whereas, Surface- and bulk-plasmon peaks of diamond are dominant in the LEELS spectra taken at high E P for both as-grown and O-ann. films. After the deposition of GaS at 500–550°C, some new loss peaks consisting of superposition of the peaks due to GaS appear in the LEELS spectra for both as-grown and O-ann. films. XPS spectra indicate a downward band bending due to oxygen treatment. The downward band bending decreases as a result of GaS deposition and increases as a result of post-annealing at higher temperature. CS and CGa bonds are observed for GaS-deposited diamond films. Secondary electron emission spectra indicate that the S-terminated diamond films seem to possess a positive electron affinity surface.

Non-metal-to-metal transition in alkaline earth monolayers

The non-metal-to-metal transition (NMT) in alkaline earth (AE) adsorbed films on increasing coverage is simulated by the film linearized augmented plane wave (LAPW) calculations of the electronic structure of hexagonal AE monolayers for various lattice periods. For low density of films, the absolute gap at the Fermi energy reveals a non-metallic state. As the density increases, the band hybridization makes the gap close, thus performing the metallization of the monolayers. The NMT is found to be of abrupt character, while the critical lattice period depends on the particular AE element. A qualitative relation between the calculated evolution of electronic structure for AE monolayers and experimental ultraviolet photoelectron spectroscopy (UPS) and low-energy electron loss spectroscopy (LEELS) data on the NMT in AE films adsorbed on transition metal surfaces is discussed in the framework of estimated dielectric function.

Growth temperature dependence of GaS thin films on GaAs(001) surface

The growth of GaS films on GaAs(001) surfaces by using thermal evaporation of layered-compound GaS has been investigated by Auger electron spectroscopy, low-energy electron-loss spectroscopy (LEELS), x-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) as a function of deposition temperature. The LEELS spectrum of the films grown at lower temperatures (⩽400 °C) resembles that of a GaS single crystal, whereas that of the films grown at 450 °C reveals that GaAs surface was terminated by Ga2S3 layer. XPS spectra suggest that after annealing at 500 °C, S atoms moved from As atoms to Ga atoms to form stable Ga–S bonds at the interface and As–S bonds are observed to be less stable. The band discontinuity at the GaS/GaAs(001) interface estimated by XPS showed the straddling-type I band alignment. Surface morphology of the films studied by AFM reveals the layer-by-layer initial growth of GaS.

Interface formation between layered-compound GaS and GaAs(111)A surface

The thermal evaporation of a GaS single crystal has been used for the first time to passivate the GaAs surface. The interface formation in the initial stage of the growth of GaS on a GaAs(111)A surface has been studied by low-energy electron-loss spectroscopy (LEELS), X-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) as a function of deposition temperature. Photoluminescence (PL) measurements of GaAs(111)A with and without GaS thin films have also been carried out at low temperature (5.5 K) as a function of aging time in the atmosphere. The LEELS spectra shows that the surface-related peaks of GaAs disappeared immediately after starting the deposition, suggesting the effective passivation of dangling bonds of the GaAs surface by S atoms included in the evaporated species. The LEELS spectrum of the films grown at 350 and 400°C resembles that of a GaS single crystal, whereas that of the films grown at 450°C and annealed at 500°C reveals that the GaAs surface was terminated by only an S layer. LEELS and XPS spectra suggest that the crystal quality of GaS films grown at 350 and 400°C is improved by post-annealing at 450°C. The band discontinuity between GaS and GaAs is estimated by XPS, and the results demonstrate a staggered-type band alignment. Surface morphology of the films studied by AFM reveals the layer-by-layer initial growth of GaS. PL measurements suggest that the GaAs(111)A surface is effectively passivated by a GaS films.

Initial stage of the growth of GaS thin films on GaAs

The initial stage of the growth of GaS on GaAs(111)A has been studied by Auger electron spectroscopy (AES), low-energy electron-loss spectroscopy (LEELS) and X-ray photoemission spectroscopy (XPS) as a function of deposition and annealing temperatures. The thermal evaporation of GaS single crystal is used for the deposition. The LEELS spectrum of the films with thickness of about 20Ågrown at 400°C resembles that of GaS single crystal, while there are some missing loss peaks in the films grown at 350°C due to the poor crystal quality. LEELS and XPS spectra suggest that the crystal quality of films grown at 350 and 400°C is improved by the post annealing at 450°C.

Substrate orientation dependence of the growth of GaSe thin films on GaAs

The initial stages of the heteroepitaxial growth of GaSe thin films on GaAs(001), (111)A and (111)B surfaces have been studied by means of LEELS (low-energy electron-loss spectroscopy) and AFM (atomic force microscopy). GaSe films were grown on the substrates at 400°C using a single evaporation source of GaSe. The evolution of a LEELS spectrum with increase in film thickness reveals a difference in deposition depending on the surfaces. While only GaSe grows on GaAs(111)A and (111)B surfaces, the GaAs(001) surface first undergoes a deposition of Ga 2Se 3; subsequently GaSe grows on the thin Ga 2Se 3. In the AFM images of films with thickness of about 20 Å on GaAs(111)A and (111)B surfaces, steps with a height of a primitive layer thickness of GaSe are observed, thereby suggesting a primitive layer-by primitive layer growth of GaSe.

GaSe films grown on a GaAs(001) surface at high temperature using a thermal evaporation of GaSe

The initial stage of heteroepitaxial growth of GaSe films on GaAs(001) surface using a single evaporation source of GaSe has been studied as a function of growth temperature by means of in-situ LEELS (low-energy electron-loss spectroscopy), XPS (X-ray photoemission spectroscopy) and AES (Auger electron spectroscopy). The thick films were studied ex-situ by XRD (X-ray diffraction). In the LEELS spectra, the film grown at 450°C had a different structure from the film grown at 400°C. In the XRD pattern, weak but clear signals of Ga 2Se 3 were observed. The AES intensity ratio of Ga and Se also showed that the films were composed of Ga-rich Ga 2Se 3. The film grown at 430°C evolved the structure from high temperature phase (mainly Ga-rich Ga 2Se 3) to low temperature phase (GaSe) with increase in thickness. At 450°C, not the Ga 2Se 3 but the crystalline GaSe film grew on a GaAs(111)A surface.

Growth of GaSe layered compound on a GaAs (001) surface

The initial stage of the growth of GaSe on GaAs (001) has been studied by Auger electron spectroscopy, X-ray photoemission spectroscopy (XPS), low-energy electron-loss spectroscopy (LEELS) and X-ray diffraction (XRD) as a function of deposition temperature: room temperature, 250 and 400 °C. The thermal evaporation of GaSe single crystal was used with the rate 0.1 Å s −1 for the deposition. The LEELS spectrum of the film grown at 250 °C with thickness of only 3 Å resembles that of GaSe single crystal, while there are some missing loss peaks in the films grown at room temperature and 400 °C. The grown films were the crystalline GaSe (001) parallel to the substrate surface of GaAs (001). The XRD pattern of films with thickness of 1000 Å grown at 250 °C coincided with that of GaSe single crystal. Since the broadening of Se 3d XPS peak was observed for the film grown at 400 °C, it is considered that the passivation should take place at the GaAs surface for 400 °C deposition.

EELS investigation of small gold clusters on mica substrates

Gold clusters with diameters from 2 to 10 nm are prepared by evaporation on mica substrates. They are investigated with low energy electron loss spectroscopy in the reflected beam and characterised in a transmission electron microscope. The energy loss spectra show a broadening of the plasma peak with decreasing particle size. The plasma frequency shifts to higher energies. The size dependence of the half width and of the plasma frequency is compared to known models. The results support the quantum box model of Genzel et al.

Plasma nitridation of thin SiO 2 films: AES, ELS and IR study

Auger electron spectroscopy, low-energy electron loss spectroscopy and infrared spectroscopy are used to investigate the nitridation of thin (10–22 nm) thermal SiO 2 in RF soft NH 3 plasma. It is found that plasma action at a substrate temperature of 573 K can completely nitridate the thermal oxide to an oxynitride layer. The layers obtained are macroscopic mixtures of two phases SiO 2 and Si 3N 4, rather than amorphous polymers of Si, O and N.

(011̄0) α-quartz surface: a LEED, XANES and ELS study

We studied the structure of the (011̄0) α-quartz surface after different treatments (900°C vacuum or air heating and ion bombardment). The long-range order was investigated by low-energy electron diffraction (LEED), the mean-range order by X-ray absorption spectroscopy at the oxygen K edge (XANES) and the electronic structure of the surface band gap by low-energy electron loss spectroscopy (ELS). The (1 × 1) structure is obtained only after a slight chemical etching in HF. A 900°C air-annealed sample exhibits a (3 × 1) or (1 × 3) LEED pattern associated with reconstructed surface regions, which structure is either related to the bulk α → β transition phase occurring at 573°C, or to a new SiO 2 phase present only at the surface (tridymite). Finally, a 900°C heating in vacuum provides a glassy surface, also containing oxygen vacancies. These three structures lead to different surface electronic properties: the reconstructed surface presents a larger surface band gap than the (1 × 1) (9.5 instead of 8.6 eV). The disordered surface shows an 8.8 eV wide surface band gap, containing two electronic levels associated with surface oxygen vacancies.