Shallow Core Levels Research Articles

Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO3.

Ternary lanthanide indium oxides LnInO3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO6 octahedra and highly distorted LnO12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4fn compounds PrInO3, NdInO3, and SmInO3, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4fn–1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr–Nd–Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO3 and SmInO3, but a lower gap of 3.6 eV for PrInO3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.

Mixed ground state in Fe-Ni Invar alloys

We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe1−xNix alloys (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing x-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe0.6Ni0.4 exhibit anomalous behavior. Moreover, the magneto-resistance of the Invar alloy becomes highly negative while the end members possess positive magneto-resistance. The core level spectra of the Invar alloy exhibit emergence of a distinct new feature below 20 K while all other Fe-Ni alloys exhibit no temperature dependence down to 10 K. Interestingly, the shallow core level spectra (3s, 3p) of Fe and Ni of the Invar alloy reveal stronger deviation at low temperatures compared to the deep core levels (2s, 2p) indicating crystal field effect. It appears that there is a large precipitation of antiferromagnetic γ′ phase below 20 K possessing low magnetic moment (0.5μB) on Fe within the α phase. The discovery of negative magneto-resistance, anomalous magnetization at low temperature and the emergence of unusual new features in the core levels at low temperature provide an evidence of mixed phase in the ground state of Invar alloys.

Comparison of optoelectronic properties of epitaxial and non-epitaxial GaN nanostructures

Structural, optical and surface properties of epitaxially grown 2D GaN nanowall network using Molecular Beam Epitaxy have been compared to those of non-epitaxially grown single-crystalline 1D GaN nanowires using Chemical Vapour Deposition. The kinetics of growth mechanisms and formed morphology are shown to significantly influence the respective band structures and consequently their luminescence properties. X-ray diffraction and Raman spectroscopy reveal that the epitaxial 2D nanowall network experiences a hydrostatic strain in addition to a compressive strain, whereas non-epitaxial 1D nanowires possess a morphology-dependent tensile/compressive strain and a negligible hydrostatic strain. Slightly blue-shifted photoluminescence emission from both these nanostructures is markedly enhanced compared to that from an epilayer. The epitaxial nanowall network exhibits the highest enhancement in the band edge emission among them. X-ray photoelectron spectroscopy spectra show shifts in the valence band features and in the hybridization of shallow core levels. Using the XRD, Raman, PL and XPS data, a variation in the band structure of these differently kinetically formed GaN nanostructures is also sketched.

Concepts for chemical state analysis at constant probing depth by lab‐based XPS/HAXPES combining soft and hard X‐ray sources

The greater information depth provided in hard X‐ray photoelectron spectroscopy (HAXPES) enables nondestructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si, and Ti, the combined access to the Al 1s, Si 1s, or Ti 1s photoelectron line and its associated Al KLL, Si KLL, or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X‐rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently, however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X‐ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk‐sensitive shallow core levels can be excited using either the hard or soft X‐ray source, whereas more surface‐sensitive deep core‐level photoelectron lines and associated Auger transitions can be measured using the hard X‐ray source. As demonstrated for thin Al2O3, SiO2, and TiO2 films, the local chemical state of the constituting ions in the oxide may even be probed at near‐constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X‐ray sources. We highlight the potential of lab‐based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.

The influence of shallow core levels on the shape of REELS spectra

It is well established that the shape of spectra acquired using reflection electron energy loss spectroscopy (REELS) is determined by the dielectric function. Extracting the dielectric function from REELS spectra is a real challenge as it is governed by strong multiple scattering. It is generally assumed that the contribution of shallow core levels (with binding energies over 100 eV) to the REELS data can be neglected when one interprets valence band REELS data that usually extends to, at most, 100 eV energy loss. Here we show that, especially when incoming energies over 1 keV are used, this is not the case and that the intensity of the REELS spectrum can only be calculated correctly if the shallow core levels are taken into account. The implications for this for the extraction of the dielectric function from REELS data is discussed.

Modelling the contribution of semi-core electrons to the dielectric function

Many aspects of the interaction of charged particles with matter can be expressed in terms of the dielectric function ε(q,ω). The dielectric function is relatively well known in the optical limit (q=0) but, for example, stopping power calculations require the knowledge of the dielectric function for q≠0. Several approaches have been used to extend the dielectric function to q≠0 for both the valence electrons, using variations of the Lindhard dielectric function and the core levels where an atomic description can be used. The intermediate case of shallow core levels is somewhat problematic. Here collective effects modify the atomic picture, and the hydrogenic approximation of the wave function is less accurate. In this paper we describe a new extension scheme of the contribution to the energy loss function of shallow core levels to q≠0 and show that tor Al and Si it describes the experimental stopping data somewhat better than previous approaches. It also describes reasonably well the proton and electron induced ionization cross section of these shallow core levels. Lastly we investigate the dielectric function in the limit of high momentum, where it can be interpreted as a Compton profile. An approximation scheme in terms of Mermin dielectric function with an amplitude that depends on q works in all cases quite well.

Evidence of Nitrogen Contribution to the Electronic Structure of the CH3 NH3 PbI3 Perovskite.

Despite fast development of hybrid perovskite solar cells, there are many fundamental questions related to the perovskite film which remain open. For example, there are contradicting theoretical reports on the role of the organic methylammonium cation (CH3 NH3+ ) in the methylammonium lead triiodide (CH3 NH3 PbI3 ) perovskite film. From one side it is reported that the organic cation does not contribute to electronic structure of the CH3 NH3 PbI3 film. From the other side, valence band maximum fluctuations, dependent on the CH3 NH3+ rotation, have been theoretically predicted. The resonant X-ray photoelectron spectroscopy results reported here show experimental evidence of nitrogen contribution to the CH3 NH3 PbI3 electronic structure. Moreover, the observed strong resonances of nitrogen with the I 5s and the Pb 5d-6s levels indicate that the CH3 NH3 PbI3 valence band is extended up to ≈18 eV below the Fermi energy, and therefore one should also consider these shallow core levels while modeling its electronic structure.

Electronic properties and bonding in ZrHx thin films investigated by valence-band x-ray photoelectron spectroscopy

The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x=0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at% are investigated by 4d valence band, shallow 4p core-level and 3d core-level X-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity towards the H 1s - Zr 4d hybridization region at about 6 eV below the Fermi level. For low hydrogen content (x=0.15, 0.30), the films consist of a superposition of hexagonal closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type structure) phases, while for x=1.16, the film form single phase ZrHx that largely resembles that of stoichiometric delta-ZrH2 phase. We show that the cubic delta-ZrHx phase is metastable as thin film up to x=1.16 while for higher H-contents, the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge-transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as function of hydrogen content is discussed in relation to the charge-transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.

Stopping power beyond the adiabatic approximation

Energetic ions traveling in solids deposit energy in a variety of ways, being nuclear and electronic stopping the two avenues in which dissipation is usually treated. This separation between electrons and ions relies on the adiabatic approximation in which ions interact via forces derived from the instantaneous electronic ground state. In a more detailed view, in which non-adiabatic effects are explicitly considered, electronic excitations alter the atomic bonding, which translates into changes in the interatomic forces. In this work, we use time dependent density functional theory and forces derived from the equations of Ehrenfest dynamics that depend instantaneously on the time-dependent electronic density. With them we analyze how the inter-ionic forces are affected by electronic excitations in a model of a Ni projectile interacting with a Ni target, a metallic system with strong electronic stopping and shallow core level states. We find that the electronic excitations induce substantial modifications to the inter-ionic forces, which translate into nuclear stopping power well above the adiabatic prediction. In particular, we observe that most of the alteration of the adiabatic potential in early times comes from the ionization of the core levels of the target ions, not readily screened by the valence electrons.

Quantifying small changes in uranium oxidation states using XPS of a shallow core level.

The U 4f line is commonly used to determine uranium oxidation states with X-ray photoelectron spectroscopy (XPS). In contrast, the XPS of the shallow core-levels of uranium are rarely recorded. Nonetheless, theory has shown that the U 5d (and 5p) multiplet structure is very sensitive to oxidation state. In this contribution we extracted the U(iv) and U(v) 5d XPS peak shapes from near stoichiometric and oxidized UO2 single crystal samples, respectively, where the oxidation state of U was constrained by fitting the 4f line. The empirically extracted 5d spectra were similar to the theoretically determined multiplet structures and were used, along with the relatively simple U(vi) component that was constrained by theory, to determine the oxidation states of UO2+x samples. The results showed a very strong correlation between oxidation states determined by the 5d and 4f line and suggested that the 5d might be more sensitive to minor amounts of oxidation than the 4f. Limitations of the methodology, as well as advantages of using the 5d relative to the 4f line are discussed.

Electronic structure of (In,Mn)As quantum dots buried in GaAs investigated by soft-x-ray ARPES

Electronic structure of a molecular beam epitaxy-grown system of (In,Mn)As quantum dots (QDs) buried in GaAs is explored with soft-x-ray angle-resolved photoelectron spectroscopy (ARPES) using photon energies around 1 keV. This technique, ideally suited for buried systems, extends the momentum-resolving capabilities of conventional ARPES with enhanced probing depth as well as elemental and chemical state specificity achieved with resonant photoexcitation. The experimental results resolve the dispersive energy bands of the GaAs substrate buried in ∼2 nm below the surface, and the impurity states (ISs) derived from the substitutional Mn atoms in the (In,Mn)As QDs and oxidized Mn atoms distributed near the surface. An energy shift of the Mn ISs in the QDs compared to (In,Mn)As DMS is attributed to the band offset and proximity effect at the interface with the surrounding GaAs. The absence of any ISs in the vicinity of the VBM relates the electron transport in (In,Mn)As QDs to the prototype (In,Mn)As diluted magnetic semiconductor. The SX-ARPES results are supported by measurements of the shallow core levels under variation of probing depth through photon energy. X-ray absorption measurements identify significant diffusion of interstitial Mn atoms out of the QDs towards the surface, and the role of magnetic circular dichroism is to block the ferromagnetic response of the (In,Mn)As QDs. Possible routes are drawn to tune the growth procedure aiming at practical applications of the (In,Mn)As based systems.

The origin of anisotropy and high density of states in the electronic structure of Cr2GeC by means of polarized soft x-ray spectroscopy and ab initio calculations

The anisotropy in the electronic structure of the inherently nanolaminated ternary phase Cr2GeC is investigated by bulk-sensitive and element selective soft x-ray absorption/emission spectroscopy. The angle-resolved absorption/emission measurements reveal differences between the in-plane and out-of-plane bonding at the (0001) interfaces of Cr2GeC. The Cr L2, 3, C K, and Ge M1, M2, 3 emission spectra are interpreted with first-principles density-functional theory (DFT) including core-to-valence dipole transition matrix elements. For the Ge 4s states, the x-ray emission measurements reveal two orders of magnitude higher intensity at the Fermi level than DFT within the General Gradient Approximation (GGA) predicts. We provide direct evidence of anisotropy in the electronic structure and the orbital occupation that should affect the thermal expansion coefficient and transport properties. As shown in this work, hybridization and redistribution of intensity from the shallow 3d core levels to the 4s valence band explain the large Ge density of states at the Fermi level.

Study of Band Structure Properties of Pnictide LaO1−xF x FeAs (x = 0, 0.2) Superconducting Compound

This paper reports on the band structure properties and changes in band structure of fluorine-doped LaO1−x Fx FeAs (x = 0, 0.2) compound, measured using X-ray photoemission spectroscopy (XPS). The band structure of the superconducting compound is compared with nonsuperconducting parent compound LaOFeAs. With fluorine doping, a shift of the shallow core level is observed in XPS spectra, which may be a response of the band structure due to fluorine doping in the system. The balance of the chemical potential shift with the screening effect of conduction electrons near the Fe and As ions is discussed using nearly unchanged Fe 2p and As 3d core-level spectra. The La 3d core-level spectra shift towards the high energy, ∼0.36 eV, may be due to the chemical potential shift caused by fluorine doping. In our valence band spectra, a small peak at around 0.2 eV is observed, which disappeared with the fluorine doping in the system, indicating a change of Fe 3d state from low spin to high spin states and also confirming the nature of Fe 3d electrons as itinerant, which is responsible for superconductivity in these compounds.

Interface engineering of Ge using thulium oxide: Band line-up study

Thulium oxide layers on germanium were investigated.The valence band offset was found to be 2.95eV.The band gap of thulium oxide was found to be 5.3eV from the Tauc method.A sub-band gap absorption at 3.2eV points to a bulk thulium oxide defect.A permittivity of 14 was derived from electrical measurements. This paper investigates the band line-up and optical properties (dielectric function) of Tm2O3/Ge gate stacks deposited by atomic layer deposition. X-ray photoelectron spectroscopy has been performed to ascertain the shallow core levels (Ge3d and Tm4d) in ultra-thin and bulk Tm2O3/Ge stacks as well as valence band maxima in Ge and bulk Tm2O3. The valence band offset of Tm2O3/Ge has been found to be 2.95?0.08eV. Vacuum ultra violet variable angle spectroscopic ellipsometry studies reveal the indirect band gap nature of Tm2O3, with the value extracted from the Tauc method of 5.3?0.1eV. A distinct absorption feature is observed at ~3.2eV below the band gap of Tm2O3, and clearly distinguished from the Si and Ge critical points. A dielectric constant of 14 to 15 has been derived from the electrical measurements on 5nm Tm2O3/epi Ge/Si gate stacks. The band line-up study of Tm2O3/Ge implies an acceptable barrier for holes (2.95eV) and electrons (greater than 1.7eV) for Ge MOSFET engineering.

Multiplet resonance lifetimes in resonant inelastic x-ray scattering involving shallow core levels

Resonant inelastic x-ray scattering (RIXS) spectra of model copper- and nickel-based transition metal oxides are measured over a wide range of energies near the M edge ($h\ensuremath{\nu}=60$--80 eV) to better understand the properties of resonant scattering involving shallow core levels. Standard multiplet RIXS calculations are found to deviate significantly from the observed spectra. However, by incorporating the self-consistently calculated decay lifetime for each intermediate resonance state within a given resonance edge, we obtain dramatically improved agreement between data and theory. Our results suggest that these textured lifetime corrections can enable a quantitative correspondence between first-principles predictions and RIXS data on model multiplet systems. This accurate model is also used to analyze resonant elastic scattering, which displays the elastic Fano effect and provides a rough upper bound for the core hole shakeup response time.

2D XAFS–XEOL Mapping of Ga1–xZnxN1–xOx Nanostructured Solid Solutions

We have applied two-dimensional X-ray absorption fine structure–X-ray-excited optical luminescence (2D XAFS–XEOL) and X-ray absorption near-edge structure (XANES) spectroscopy to study the nature of the X-ray-excited optical luminescence process in GaN–ZnO solid-solution nanostructures, both indium-doped (GIZNO) and undoped (GZNO). XAFS–XEOL analysis of GZNO reveals that X-ray-excited optical luminescence (XEOL) is a strong function of the excitation energy, and hence penetration depth, as well as the turn-on of any shallow core levels. Indium K-edge XANES results reveal that indium in GIZNO forms a surface oxide but has limited solubility in the solid solution. This observation is consistent with the morphology change from nanoparticles in GZNO to nanosheets in GIZNO induced by the presence of a surface In layer, which ultimately becomes oxidized to form In2O3. XAFS–XEOL measurements suggest that the XEOL from GIZNO is primarily due to oxygen defects in the indium oxide surface layer and effective energy transfer from the GaN–ZnO solid solution.

Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface

Further understanding of the electronic structure at the ZnS/Cu(In,Ga)Se2 interface is necessary to enhance the electron injection across the interface in Cu(In,Ga)Se2 solar cells. The valence band structure and shallow core levels were investigated by ultraviolet photoelectron spectroscopy depth profile analysis with He II line excitation. ZnS film was grown by a chemical bath deposition on a Cu(In,Ga)Se2 absorber deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The discontinuity of 2.0 eV in the valence band edge at the ZnS/Cu(In0.7Ga0.3)Se2 interface was directly determined. This type of valence band offset yields a spike conduction band alignment of 0.25 eV. The positions of the VBM and the Zn 3d core-level emission of the buffer underwent the substantial shifts of 0.36 eV and 0.64 eV to a lower binding energy levels during the etching process. The shifts are ascribed to the contribution of the band bending in the ZnS buffer layer and its graded chemical composition. This study is the first to determine the small conduction band offset at the interface formed by the chemical bath deposited ZnS layer and the Cu(In0.7Ga0.3)Se2 absorber. Our results also provide information toward the design optimization of the optoelectronic properties of the ZnS/Cu(In0.7Ga0.3)Se2 interface. To enhance the electron injection from Cu(In0.7Ga0.3)Se2 absorber to ZnS layer further lowering of the energy barrier is required. For this purpose, the bandgap of ZnS should be reduced by controlling the crystal structure and composition or its Fermi level should be upward shifted by appropriate doping.

Resonant photoemission shake‐up satellites from semiconductors with shallow 3d and 4d core levels

AbstractPhotoemission spectra from shallow 3d and 4d levels of ZnTe, ZnSe, CdTe, CdS, and tin‐doped In2O3 (ITO) using synchrotron radiation from the BESSY II storage ring are presented. All compounds show pronounced resonant shake‐up satellites at the excitation threshold of the 3p and 4p‐levels. The resonance intensity profiles are compared to 3p and 4p photoemission. Electron correlation energies U of 10.15, 10.75, 6.15, and 7.3 eV are derived for ZnTe, ZnSe, CdTe, and ITO, respectively.

High-resolution soft x-ray spectroscopic study on amorphous gallium indium zinc oxide thin films

Amorphous gallium indium zinc oxide (a-GIZO) thin films of different compositions (Ga2O3:In2O3:ZnO=1:1:1,2:2:1,3:2:1,4:2:1) on Si substrates were investigated by high-resolution x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (XAS) using synchrotron radiation. The O 1s, Ga 3d, In 4d, Zn 3d core, and shallow-core levels as well as the valence band maxima and O K-edge XAS were investigated. Each O 1s spectrum could be deconvoluted by a main component (O1 in the text) representing the Ga–In–Zn–O quaternary system along with two other higher-binding energy (BE) components (O2 and O3 in the text). The O2+O3 intensity increased as the Ga2O3 content increased. For the as-prepared samples, the spectral peak separations between the Ga 3d (∼20 eV) and Zn 3d (∼11 eV) orbitals and between the In 4d (∼18 eV) and Zn 3d orbitals became larger, respectively, as the Ga2O3 content increased. For the surface-cleaned samples, this trend was the same but with smaller increases in their separations. The sputter-cleaning effectively reduced the Zn 3d intensity by ∼30% relative to those of Ga 3d and In 4d. The valence band maximum shifted toward higher BE, up to ∼0.5 eV for the as-prepared samples and ∼0.25 eV for the cleaned samples, and the conduction band minimum (measured at the O K-edge) was measured at photon energies ranging upwards to ∼0.2 eV as the Ga2O3 content increased, demonstrating that the band gap can be tailored by increasing the Ga2O3 content. The effects of increasing Ga2O3 contents on the local chemical states and the corresponding electrical conduction are discussed in this paper.

Low-energy electron-stimulated desorption of cations and neutrals from Si(111)-(7×7):C2D2

The interactions of low-energy (5-50 eV) electrons with acetylene-d(2) (C(2)D(2)) adsorbed on the Si(111)-(7x7) surface have been examined by monitoring the stimulated desorption products. These include primary cation desorbates, D(+) and C(2)D(2)(+) (C(2)HD(+)), the fragment ion C(2)D(+), smaller amounts of C(2)(+), CDH(+) (CH(3)(+)), and neutral D((2)S). The approximately 23-25 eV threshold energies for D(+) and hydrocarbon fragment ion detection indicate involvement of two-hole or two-hole one electron final states that Coulomb explode. These multihole states can be created via Auger decay of single holes in shallow core levels localized on C or Si surface atoms. The approximately 12 eV appearance threshold for the C(2)D(2)(+) molecular ion can be correlated with direct excitation of an adsorbate-induced surface state, which may initially possess character of the A(3) surface state of Si. The 18 eV threshold for C(2)D(+) correlates with decomposition of C(2)D(2)(+) with excess vibronic energy. C(2)D(+) desorption via direct excitation of the dissociative (2)Sigma(u)(+)-type state of the C(2)D(2)(+) ion is also possible. The approximately 8 eV threshold energy for production and desorption of neutral D((2)S) may correlate with excitation of the perturbed/mixed F (1)Sigma(u)(+)<--X (1)Sigma(g)(+) and E (1)Sigma(u)(+)<--X(1)Sigma(g)(+) dissociative transitions of adsorbed acetylene molecules. Time-of-flight distributions of D((2)S) indicate both nonthermal (557 and 116 meV; 4300 and 900 K) and thermal (17 meV; 130 K) components. The two fast components can be related to the geometry of di-sigma bonded acetylene on the Si(111)-(7x7) surface.