Measurement Of Electron Affinity Research Articles

Accurate band alignment of sputtered Sc2O3 on GaN for high electron mobility transistor applications

Sc2O3 is a promising gate dielectric for surface passivation in GaN-based devices. However, the interface quality and band alignment of sputtered Sc2O3 on GaN has not been fully explored. In this work, x-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry were performed to extract the discontinuities in the valence and conduction bands of the Sc2O3/GaN system. Sc2O3 films were deposited on GaN using radio frequency sputtering. The valence band offset of Sc2O3/GaN was determined to be 0.76 ± 0.1 eV using Kraut’s method. The Sc2O3 band gap of 6.03 ± 0.25 eV was measured using O 1s energy loss spectroscopy. The electron affinity measurements of GaN and Sc2O3 using XPS secondary electron cut-off spectra provided an additional degree of accuracy to the derived band line-up for the Sc2O3/GaN interface. The band alignment results were compared with literature values of band offsets determined experimentally and theoretically for differently grown Sc2O3 films on GaN.

Measurement, calculation, and applications of ionization energies and electron affinities of hydrogen and atomic ions

Measurement, calculation, and applications of ionization energies and electron affinities of hydrogen and atomic ions

Electron affinity measurements of lanthanide atoms: Pr, Nd, and Tb

Electron affinities (EAs) of many lanthanide elements still remain unknown due to their complicated electronic structures and relatively low EA values. In the present work, we utilized the slow-velocity map-imaging method combined with an ion trap to resolve conundrums. The EA values of praseodymium, neodymium, and terbium are determined to be $881.0(37)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.109 23(46) eV, $786.3(26)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.097 48(31) eV, and $1059.1(64)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.131 31(79) eV, respectively. We also observed several excited states of Pr, Nd, and Tb anions.

Measurement of electron affinity of atomic lutetium via the cryo-SEVI Method

Electron affinities (EAs) of most lanthanide elements still remain unknown due to their relatively low EA values. In the present work, the cryogenically controlled ion trap is used for accumulating atomic lutetium anion Lu−, which makes the measurement of electron affinity of lutetium become practicable. The high-resolution photoelectron spectra of Lu− are obtained via the slow-electron velocity-map imaging method. The electron affinity of Lu is determined to be 1926.2(50) cm−1 or 0.23882(62) eV. In addition, two excited states of Lu− are observed.

24-channel dual microcontroller-based voltage controller for ion optics remote control

The design of a 24-channel voltage control instrument for Wenzel Elektronik N1130 NIM modules is described. This instrument is remote controlled from a LabVIEW GUI on a host Windows computer and is intended for ion optics control in electron affinity measurements on negative ions at the CERN-ISOLDE facility. Each channel has a resolution of 12 bits and has a normally distributed noise with a standard deviation of <1 mV. The instrument is designed as a standard 2-unit NIM module where the electronic hardware consists of a printed circuit board with two asynchronously operating microcontrollers.

Experimental and DFT Studies of the Electron-Withdrawing Ability of Perfluoroalkyl (RF ) Groups: Electron Affinities of PAH(RF )n Increase Significantly with Increasing RF Chain Length.

Two series of aromatic compounds with perfluoroalkyl (RF ) groups of increasing length, 1,3,5,7-naphthalene(RF )4 and 1,3,5,7,9-corannulene(RF )5 , have been prepared and their electronic properties studied by low-temperature photoelectron spectroscopy (PES) (for gas-phase electron affinity measurements). These and many related compounds were also studied by DFT calculations. The data demonstrate unambiguously that the electron-withdrawing ability of RF substituents increases significantly and uniformly from CF3 to C2 F5 to n-C3 F7 to n-C4 F9 .

Precision measurement of electron affinity of Zr and fine structures of its negative ions.

The high-resolution photoelectron spectra of Zr- were obtained via the slow-electron velocity-map imaging method. The electron affinity of Zr was measured to be 3494.67(72) cm-1 or 0.433 283(89) eV. The accuracy has been improved by a factor of 160 compared with the previous result. The fine structures of Zr- were also well resolved: 251.0(37) (4F5/2), 579.6(8) (4F7/2), and 971.7(12) cm-1 (4F9/2) above the ground state 4F3/2.

Ambiguity in the magnitude and direction of the derived interface dipole in lanthanum aluminate heterostructures: Implications and proposed solution

The electric field strength of interface dipoles cannot be measured directly but can be derived from measurable electronic properties such as the valence band offset (VBO) and electron affinity using photoemission techniques. In this study, we found that the measurements of these two values are affected by differential charging and surface contaminants, respectively. This can affect both the polarity and the strength of the derived interface dipole and therefore might have implications regarding the understanding of oxide-semiconductor band alignment. Our overall band lineup and derived interface dipole in lanthanum aluminate (LAO) heterostructures agree excellently with a popular charge-neutrality level model. This would not be possible without the accurate measurement of VBO and electron affinity in LAO heterostructures.

Reassessment of 182Hf AMS measurements at VERA

The radioisotope 182Hf ( t 1/2 = 8.9 Ma) is of great interest for astrophysical applications as a chronometer for the early solar system or as possible live supernova remnant on earth. However, AMS measurements of 182Hf are seriously influenced by the presence of the stable isobar 182W, which cannot be separated at typical AMS energies. Previous studies revealed a possible suppression of 182W against 182Hf by extracting the negatively charged pentafluoride HfF 5 - from the ion source, leading to a detection limit for 182Hf/ 180Hf in the order of 10 −11. However, this suppression behavior is in contrast to theoretical calculations of the electron affinity and recent measurements using SIMS instruments, where the achieved suppression cannot be reproduced. The aim of our study is to determine the effects of ion source background as well as further investigate the suppression of tungsten against hafnium by extracting negatively charged fluoride ions from different sample materials. The previously reported suppression factor of about 6000 could be increased to 36000 by careful tuning of the ion source using HfF 4 as sample material. The trend of the theoretical electron affinities could be reproduced using atomic tungsten and hafnium instead of HfF 4 as sample material. This supports the assumption that the major contribution of the tungsten background is not sputtered from the target matrix but comes from somewhere else in the ion source. Measurements from the second ion source show a higher background of tungsten and a lower suppression factor, i.e. careful design of the ion source is crucial. Moving the sputter beam over the target surface extending over the wheel holding the targets revealed the highest tungsten background was detected outside the sputter target position. Further investigations are necessary to locate the origin of the tungsten background in the ion source. Possible sources are the material used for the ion source construction or contaminations in the cesium used for sputtering.

Effect of a magnetic field in photodetachment microscopy

The effect of an external static magnetic field of arbitrary orientation with respect to the electric field, on the electron interference ring patterns observed by the photodetachment microscope is studied both experimentally and theoretically. The design of the interaction chamber has been modified to superimpose a controlled uniform magnetic field on the whole volume accessible to the interfering electron. Contrary to a previous study in weaker fields, where the overall dimension of the interferogram was not modified, the effect of the magnetic field here encompasses a regime of magnetic refocusing. A quantitative analysis is carried out using a closed-orbit perturbative calculation of the interference phase at the centre of the ring pattern. The essential result of this work is still the invariance of the extreme interference phase whatever the direction and magnitude of the applied magnetic field, up to values 100 times larger than in the previous experimental study. This property can be applied to revise former electron affinity measurements. Partly due to the previously unsuspected robustness of the electron interferograms vs. magnetic fields, partly thanks to the 2006 CODATA revision of the energy conversion factors, one can update the values of the electron affinities of 16O, 28Si and 32S to 1.4611134(9), 1.3895210(7) and 2.0771040(6) eV respectively.

Images of a photoelectron interfering with itself: the photodetachment microscope

An electron emitted from a pointlike source, when submitted to uniform acceleration by an electric field, can follow several trajectories to the point where it is detected. The corresponding interference pattern has been observed in a photodetachment experiment. Fitting the pattern with the squared Green function of the uniform acceleration problem makes it possible to perform electron spectrometry with a 1 µeV accuracy. This has been used for electron affinity measurements, both on atomic anions and with a molecular one.

Electron Affinities of 1,1-Diaryl-2,3,4,5-tetraphenylsiloles: Direct Measurements and Comparison with Experimental and Theoretical Estimates

We present a comprehensive experimental and theoretical characterization of the electronic structure of four 1,1-diaryl-2,3,4,5-tetraphenylsiloles (aryl = phenyl, 2-(9,9-dimethylfluorenyl), 2-thienyl, pentafluorophenyl). Solid-state electron affinities and ionization potentials of these siloles were measured using inverse-photoelectron spectroscopy (IPES) and photoelectron spectroscopy (PES), respectively; the density of electronic states obtained from calculations performed at the density functional theory (DFT) level corresponds very well to the PES and IPES data. The direct IPES measurements of electron affinity were then used to assess alternative estimates based on electrochemical and/or optical data. We also used DFT to calculate the reorganization energies for the electron-transfer reactions between these siloles and their radical anions. Additionally, optical data and ionization potential and electron affinity data were utilized to estimate the binding energies of excitons in these siloles.

The electron affinities of O, Si, and S revisited with the photodetachment microscope

Photodetachment microscopy has been performed on a beam of 32S- ions. Analysing the electron images obtained, we find that the electron affinity measurements performed with the photodetachment microscope contain a small bias, due to the difference between the actual and assumed values of the applied electric field. Having a measure of this bias, we can reanalyse older data recorded on the negative ions O- and Si- along similar lines. As a consequence, the values of the electron affinities of Oxygen, Silicon and Sulfur can be given with an improved accuracy. The recommended values (with expanded uncertainties) are now 11784.676(7)cm-1 for 16O, 11207.246(8)cm-1 for 28Si, and 16752.974(5)cm-1 for 32S, i.e. 1.461 113 5(12), 1.389 521 3(13) and 2.077 104 0(9) eV, respectively.

Unique Aggregate Structure of Fluoroperylene Diimide Thin Film

A perfluorinated perylene diimide, N,N‘-diperfluorophenyl-3,4,9,10-perylenetetracarboxylic diimide (DFPP), and its nonfluorinated analogue, N,N‘-diphenyl-3,4,9,10-perylenetetracarboxylic diimide (DPP), showed similar UV−vis absorption and fluorescence spectra when they existed as monomeric dyes in solutions, whereas in solid films they exhibited great differences. Absorption band splitting was observed for DPP film; however, for DFPP film, the fine structure of absorptions similar to that in solution still remained in addition to the absorption peaks becoming broader and shifting to longer wavelength regions. Furthermore, it was found that DFPP molecules aggregated to form a crystalline film when the temperature of the substrate was elevated to 200 °C. Through the simulation of the molecular conformation, the theoretical examinations of molecular interactions, the measurement of electron affinity, the observation of the film morphology by AFM and SEM, and the crystalline structure characterization by XRD,...

Electronic Properties of GaN (0001) – Dielectric Interfaces

The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO 2, Si 3 N 4, and HfO 2 have been investigated. Layers of SiO 2, Si3 N 4, or HfO 2 were carefully deposited to limit the reaction between the film and clean GaN surfaces. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0±0.2 eV with a conduction band offset (CBO) of 3.6±0.2 eV was determined for the GaN / SiO 2 interface. The large band offsets suggest SiO 2 is an excellent candidate for passivation of GaN . For the GaN / Si 3 N 4, interface, type II band alignment was observed with a VBO of -0.5±0.2 eV and a CBO of 2.4±0.2 eV . While Si3 N 4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.3±0.2 eV with a CBO of 2.1±0.2 eV was determined for the annealed GaN / HfO 2 interface. An instability was observed in the HfO 2 film, with energy bands shifting ~0.4 eV during a 650°C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9±0.1 eV for GaN , SiO 2, Si 3 N 4, and HfO 2 surfaces, respectively. The deduced band alignments were compared to the predictions of the electron affinity model and deviations were attributed to a change of the interface dipole. Interface dipoles contributed 1.6, 1.1 and 2.0±0.2 eV to the band alignment of the GaN / SiO 2, GaN / Si 3 N 4, and GaN / HfO 2 interfaces, respectively. It was noted that the existence of Ga-O bonding at the heterojunction could significantly affect the interface dipole, and consequently the band alignment in relation to the GaN .

Predicted infrared and Raman spectra for neutralTi8C12isomers

Using a density-functional based algorithm, the full IR and Raman spectra are calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically unstable. The calculated properties of the relaxed structure having C3v symmetry are found to be in excellent agreement with experimental gas phase infrared results, ionization potential and electron affinity measurements. Consequently, the results presented may be used as a reference for further experimental characterization using vibrational spectroscopy.

AlN films on GaN: Sources of error in the photoemission measurement of electron affinity

This communication corrects an error in the value previously reported by one of the authors for the electron affinity (EA) of AlN. A brief discussion is given of the potential errors in photoemission measurements of EA which affect this and other studies. Finally, a recommendation is given for 1.9 eV as the “true” EA of wurtzite AlN.

Isotope shift in the oxygen electron affinity

The present paper reports the {ital ab initio} evaluation of the electron affinity of oxygen, and the value of its isotope shift. The theoretical results are in good agreement with the recent electron affinity measurements for {sup 16}O and{sup 18}O by photodetachment microscopy, revealing an {open_quotes}anomalous{close_quotes} isotope shift. The theoretical fine structure of the negative ion 2p{sup 5}thinsp{sup 2}P{sub 1/2{minus}3/2}{sup o} is also in very good agreement with observation. {copyright} {ital 1999} {ital The American Physical Society}

Electron Affinity of Some Endohedral Lanthanide Fullerenes

We report the first electron affinity measurements for some endohedral gadolinium metallofullerenes. Knudsen cell mass spectrometry has been used for the study of the gas-phase composition over the KF−Gd@Cn system. Both neutral and negatively charged species C60, C70+2n (n = 0, 1, ..., 10), Gd@C74, Gd@C76, Gd@C78, Gd@C80, and Gd@C82 were detected at 931−960 K, which permitted determination of equilibrium constants of electron exchange reactions. The electron affinities of gadolinium metallofullerenes were found to be greater than those of the parent fullerenes. In contrast to the empty fullerenes, endohedral metallofullerenes revealed no strong dependence of their electron-accepting ability on the size of the fullerene cage.

Measurement of electron affinity in boron-doped diamond from capacitance spectroscopy

Boron-doped diamond film sample has been grown on (100) silicon substrate using the microwave enhanced chemical vapor deposition method. It is found that the sample has very good material qualities and an excellent (100) surface morphology. Au/diamond Schottky was fabricated on the (100) surface to study electron affinity of the diamond sample. By measuring frequency dependence capacitance–voltage spectroscopy of the Schottky sample in high vacuum and at room temperature, a very small electron affinity of about 0.025 eV and a work function of about 5.165 eV have been obtained for the (100) surface of the diamond sample supposing the diamond band gap energy is 5.5 eV.