Band Gap Mapping Research Articles

Correlative Electron Energy-Loss Spectroscopy Bandgap Mapping and DFT Modeling in AlGaN Diodes

Correlative Electron Energy-Loss Spectroscopy Bandgap Mapping and DFT Modeling in AlGaN Diodes

From early to present and future achievements of EELS in the TEM

This paper reviews the implementation of Electron Energy Loss Spectroscopy (EELS) in a Transmission Electron Microscope (TEM), as an essential tool for advanced analytical studies, exhibiting a unique level of performance in terms of spatial resolution down to the interatomic distances for imaging and sensitivity down to the single atom for elemental identification. In terms of spectral resolution, it offers access with a resolution as good as a few meV, to a very broad spectral domain extending from tens of meV (in the IR) up to a few keV (in the X-ray). This new generation of instrument (EELS+(S)TEM) is now routinely used to investigate the structural, spectral, electronic and chemical properties of a wide range of materials and to broaden spectacularly the field of novel information which it provides. A first part of the paper describes the major progress in advanced instrumentation brought by the novel pieces of equipment (spectrometers, monochromators, aberration correctors and detectors) together with the newly elaborated tools for the acquisition and processing of huge data collections. The second part is devoted to the description of the information contained in a global EELS spectrum: (i) from the core-loss domain implying excitations from inner-shell atomic electrons and its application in elemental, chemical and electronic mapping; (ii) from the low-energy domain exhibiting individual or collective excitations of the valence and conduction electron gas, with its most recent developments in band gap mapping and nanoplasmonics; (iii) in the ultra-low energy domain, which is now in its infancy, the surface collective electron excitations, molecular bonds and the vibrations of phonons at surfaces and in the bulk of nanostructures. The third part is devoted to the exploration of unconventional domains of applications, which in many cases associate the EELS acquisition with the generation and the capture of other signals in various environments, in situ operation (temperature, pressure...), absorption or generation of photons (cathodoluminescence, X-ray emission), acquisition and handling of multidimension data (space, energy, momentum, time). In conclusion, EELS fifty years after its first recognition as a useful actor in the development and promotion of the analytical microscopy, has nowadays become an essential tool for the acquisition of many physical parameters with ultimate resolution, thus opening new routes in nanophysics to be explored.

Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale

Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1−xCdxO/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.

Automated approaches for band gap mapping in STEM-EELS

Band gap variations in thin film structures, across grain boundaries, and in embedded nanoparticles are of increasing interest in the materials science community. As many common experimental techniques for measuring band gaps do not have the spatial resolution needed to observe these variations directly, probe-corrected Scanning Transmission Electron Microscope (STEM) with monochromated Electron Energy-Loss Spectroscopy (EELS) is a promising method for studying band gaps of such features. However, extraction of band gaps from EELS data sets usually requires heavy user involvement, and makes the analysis of large data sets challenging. Here we develop and present methods for automated extraction of band gap maps from large STEM-EELS data sets with high spatial resolution while preserving high accuracy and precision.

Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

Compositionally Dependent Nonlinear Optical Bandgap Behavior of Mixed Anodic Oxides in Niobium-Titanium System.

Optical bandgap mapping of Nb-Ti mixed oxides anodically grown on a thin film parent metallic combinatorial library was performed via variable angle spectroscopic ellipsometry (VASE). A wide Nb-Ti compositional spread ranging from Nb-90 at.% Ti to Nb-15 at.% Ti deposited by cosputtering was used for this purpose. The Nb-Ti library was stepwise anodized at potentials up to 10 V SHE, and the anodic oxides optical properties were mapped along the Nb-Ti library with 2 at.% resolution. The surface dissimilarities along the Nb-Ti compositional gradient were minimized by tuning the deposition parameters, thus allowing a description of the mixed Nb-Ti oxides based on a single Tauc-Lorentz oscillator for data fitting. Mapping of the Nb-Ti oxides optical bandgap along the entire compositional spread showed a clear deviation from the linear model based on mixing individual Nb and Ti electronegativities proportional to their atomic fractions. This is attributed to the strong amorphization and an in-depth compositional gradient of the mixed oxides. A systematic optical bandgap decrease toward values as low as 2.0 eV was identified at approximately 50 at.% Nb. Mixing of Nb2O5 and TiO2 with both amorphous and crystalline phases is concluded, whereas the possibility of complex NbaTibOy oxide formation during anodization is unlikely.

Detection and Characterization of OH Vibrational Modes using High Energy Resolution EELS

The recent detection of vibrational excitations in monochromated electron energy-loss spectroscopy recorded from scanning transmission electron microscopes has opened up new opportunities for nanoscale materials characterization [1]. The enhanced energy resolution has the greatest impact on the low-loss EELS and it is now possible to probe vibrational and electronic excitations at the nanometer level. For example, localized bandgap mapping and detection of interband states is now possible providing a new tool to correlate optical properties with atomic structure [2,3]. Vibrational spectroscopy allows hydrogen containing species to be identified and correlated with materials structure. Detection of water and OH species on nanoparticle surfaces is important for developing a fundamental understanding of solar water splitting catalysts. The delocalized nature of the low-loss spectrum also makes it possible to use the aloof beam spectral acquisition mode (i.e. with the electron probe positioned outside the sample) dramatically reducing electron beam damage. To investigate the feasibility of OH detection, a series of hydroxide and hydrates have been investigated.

Challenges and Opportunities in Materials Science with Next Generation Monochromated EELS

The recent development of monochromated scanning transmission electron microscopes (STEM) offering energy resolutions of better than 20 meV and electron probes of 1 A in size provides a completely new tool to materials characterization. Unique opportunities opened by access to ultra-high energy resolution low loss EELS include determination of optical properties in the IR, bandgap mapping, detection of defect interband states and localized vibrational spectroscopy. While techniques such as Raman and IR spectroscopy have been routinely used to characterize phonon modes in solids for many years, the vibrational excitations probed with focused fast electron beams is largely unexplored. At ASU we are currently applying ultra-high energy resolution low-loss EELS to a variety of materials that are important in fields such as energy, environmental science and information technology. Here we show representative initial results on carbons, oxides and metal hydrides. All data were acquired on a newly installed Nion UltraSTEM equipped with a probe corrector and monochromator [1].

Photoemission and density functional theory study of Ir(111); energy band gapmapping

We have performed combined angle-resolved photoemission spectroscopy (ARPES)experiments and density functional theory (DFT) calculations of the electronicstructure of the Ir(111) surface, with the focus on the existence of energy bandgaps. The investigation was motivated by the experimental results suggestingIr(111) as an ideal support for the growth of weakly bonded graphene. Therefore,our prime interest was electronic structure around the symmetry point. In accordance with DFT calculations, ARPES has shown a wide energyband gap with the shape of a parallelogram centred around the point. Within the gap three surface states were identified; one just below the Fermi leveland two spin–orbit split surface states at the bottom of the gap.

VEELS band gap measurements using monochromated electrons

With the development of monochromators for transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) has become a powerful technique to study the band structure of materials with high spatial resolution. However, artefacts such as Cerenkov radiation and surface effects pose a limit for interpretation of the low-loss spectra; also the inelastic delocalisation restricts the spatial resolution on band gap mapping. For direct semiconductors, spectra acquired at thin regions can efficiently minimize the Cerenkov effects. Examples of h-GaN spectra acquired at different thickness showed that a correct band gap onset value can be obtained for sample thicknesses up to about 60 nm. For indirect semiconductors, the correct band gap onset can be obtained in the dark-field mode when the required momentum transfer for indirect transition is satisfied. At low energy-loss range the spatial resolution of this technique, which is mainly limited by the inelastic delocalisation, can be improved by dark-field VEELS at high collection angles.

Scanning probe microscopy of oxide surfaces: atomic structure and properties

The intersection of two fields, oxide surface science and scanning probe microscopy (SPM), has yielded considerable insight on atomic processes at surfaces. Oxide surfaces, especially those containing transition metals, offer a rich variety of structures and localized physical phenomena that are exploited in a wide range of applications. Nonlinear optics, superconductivity, ferroelectricity and chemical catalytic activity are but a few. Furthermore, the challenges and solutions associated with the chemistry of these surfaces and particularly the solutions to these problems have led to important understanding of tip–surface interactions that can inform SPM studies of all materials. Here, the development of understanding of the model systems TiO2 and SrTiO3 are considered in detail, to demonstrate the role of nonstoichiometry in surface structure evolution and the approach to interpreting structure at the atomic level. Then a combination of scanning tunneling microscopy, noncontact atomic force microscopy and theory are applied to a variety of oxide systems including Al2O3, NiO, ferroelectric BaTiO3, tungstates and molybdates. Recently developed sophisticated probes of local properties include spin-polarized tunneling, Fourier mapping of charge density waves, band gap mapping of superconductors and ultra fast imaging of atomic diffusion. The impact of these studies on our understanding of the behavior of oxides and of tip–surface interactions is summarized.

Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

In situ band gap mapping of the V2O5(001) crystal surface revealed a reversible metal-to-insulator transition at 350-400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl (V=O) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyl-oxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as V2O5(001) --> V6O13(001) --> V2O3(0001).

Bandgap mapping for III-V quantum well by electron spectroscopy imaging

The concept of 'bandgap mapping' was proposed originally to map the inhomogeneity of band energy in III-V semiconductors with a spatial resolution of a few nanometres in a scanning transmission electron microscope with the focus beam mode. In this paper, several techniques were developed to demonstrate the possibility to map the distribution of bandgap energies for GaN/AlN quantum-well structures using electron spectroscopy imaging (ESI). The phase correlation function was used to register different energy-loss images among ESI series with an accuracy of 1 pixel. The energy dispersion of ESI series was improved by a fast Fourier transform interpolation method. An iterative multivariable least square algorithm was derived to refine the fitting of the single scattering distribution to an analytic form of the density of states function a(E - E(g))(0.5). The inhomogeneity of the band energy of the quantum well can be revealed from the band-energy map. A threshold filter method is applied to estimate the average value and SD of the bandgap energy from barrier and well regions in the energy map. The average bandgap energy of AlN and GaN is determined to be 5.62 +/- 0.35 and 3.87 +/- 0.36 eV, respectively. The effect of delocalization on the accuracy of band-energy determination is discussed. The 2sigma(E(g)) accuracy of this analysis is comparable to half of the energy resolution of the ESI experiment.

Assessment of electron energy-loss spectroscopy below 5 eV in semiconductor materials in a VG STEM

The possibilities of obtaining information about interband scattering processes from electron energy-loss spectra, taken in a VG601UX scanning transmission microscope, are investigated. With the help of precise simulations of the zero-loss peak it is feasible to process, extract and analyse data in the electron volt regime. The accuracy of the results is restricted predominantly by instrumental limitations. It is possible to extract band gaps of > 2 eV (e.g. for GaN) correctly. In the case of band gaps between 1 and 2eV (i.e. in GaAs), the uncertainty is larger, since the tail of the zero-loss peak interferes with the interband losses, and further refinements in the data analysis as well as the measurement techniques are discussed. ‘Band-gap mapping’ in the VG601UX can be carried out with a spatial resolution of a few nanometres.