Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Electronic structure and Coulomb correlation energy in UO2 single crystal

High-resolution X-Ray Photoemission Spectroscopy (XPS) and Bremsstrahlung Isochromat Spectroscopy (BIS) measurements are reported for UO₂. Clear evidence is found for the localization of the 5f electrons. The Coulomb correlation energy and the p-d gap are determined to be 4.6 ± 0.8 and 5.0 ± 0.4 eV, respectively. A satellite observed 7 eV below the Uranium core levels is assigned to a final state excitation from the valence p band to the empty 5f³ state.     

Angular resolved ups of surface states on GaAs(1̄1̄1̄) prepared by molecular beam epitaxy

The As-rich (2 × 2), a newly found (√3 × √3) and the (√19 × √19) surfaces of GaAs(1̄1̄1̄) are studied by angular resolved UPS (ARUPS). The (2 × 2) surface is prepared by molecular beam epitaxy and the others by mild annealing. For the (2 × 2) surface emission from surface states is observed, which shows dispersion periodic within the (2 × 2) surface Brillouin zone. Using s-polarized light and the known symmetry selection rules the uppermost surface bands between 1 and 2 eV below the valence band maximum are assigned to the As dangling bond orbital. The bands near 4 and 7 eV assigned to the backbonds. From the strong decrease of emission intensity of the As-derived surface states between the (2 × 2) and the annealed surfaces it is concluded that the character of the As dangling bond orbital must have been changed from sp³-hybridic to s-like. This gives further evidence for our recently proposed model for the (√19 × √19) surface, which is particularly applicable for the (√3 × √3) surface.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.     

Luminescence and decay times of CsI:In+

Luminescence spectra of single crystals of CsI:In⁺ excited in the A(304 nm), B(288 nm), C(268 nm) and D(257 nm) absorption bands have been studied in the temperature range 4.2–300 K. Excitation in the A band at 4.2 K gives rise to the principal emission at 2.22 eV accompanied by a partly-overlapping weak band at 2.49 eV. An additional emission band at about 2.96 eV is observed on excitation in the B, C or D bands. Yet another emission band located at 2.67 eV is excited only in the D band. The relative intensities of the bands are very sensitive to excitation wavelength as well as to temperature. The origin of all these bands is assigned in terms of a model for the relaxed excited states (RES). All the luminescence spectra were resolved into an appropriate number of skew-Gaussian components. Moments analysis leads to a value of (1.35 ± 0.02) × 10¹³ rad s^-1 for the effective frequency (ω_eff) of lattice vibrations coupled to the RES. At the lowest temperature, the radiative decay times of each of the intracenter emission bands (2.22, 2.49 and 2.96 eV) show a slow decay ( ～ 10–100 μs) and a fast decay ( ～ 10–100 ns). The 2.96 eV band, which is assigned to an emission process which is the inverse of the D-band absorption, exhibits a single decay mode ( ～ 10 μs). The intrinsic radiative decay rates (k₁, k₂), the one-phonon transition rate (K) and the second-order spin-orbit splitting (D) for the RES responsible for the principal emission are: k₁ = (6.0±-0.3)×10³ s^-1, k₂ = (1.33±-0.06)×10⁵ s^-1, K = (2.4±-0.4)×10⁷ s^-1 and D = (13.8±-0.5) cm^-1.     

A photoelectron spectroscopic study of the second ionisation of the CF(X2Π) radical

The CF(X²Π) radical has been re-investigated with vacuum ultraviolet photoelectron spectroscopy. Two bands were observed and assigned to the ionisations CF⁺(X¹Σ⁺)←CF(X²Π) and CF⁺(a³Π)← CF(X²Π). The first band, which has been observed previously, has an adiabatic ionisation energy of (9.11±0.02) eV and a vertical ionisation energy of (9.55±0.02) eV. For the second band, three vibrational components were observed with the first, the most intense, at an ionisation energy of (13.94±0.02) eV. Analysis of the vibrational structure in the two observed bands allowed ω_e and r_e to be determined as 1810±30 cm⁻¹ and 1.154±0.005 Å, respectively for the first ionic state, CF⁺(X¹Σ⁺), and 1614±30 cm⁻¹ and 1.213±0.005 Å, respectively for the second ionic state, CF⁺(a³Π). Comparison of the ionisation energies and spectroscopic constants obtained has been made with values obtained from recent multi-reference configuration interaction calculations.     

The reactive adsorption of CCl4 on GaAs(100) surfaces at 300 K

Auger electron spectroscopy has been used to monitor the adsorption of CCl₄ on an As-rich GaAs(100) surface at 300 K. Intensities of the Ga (55 eV), As (34 eV), C (270 eV) and Cl (181 eV) transitions have been used to estimate surface number densities at saturation and relative C : Cl stoichiometry of the surface species. Number densities of (4.3 ± 0.2) × 10¹⁴ and (2.0 ± 0.2) × 10¹⁴ cm ⁻² are obtained for carbon and chlorine respectively, suggesting that coverage saturates near one theoretical monolayer and that the C : Cl stoichiometry is approximately 2:1. These data are discussed in terms of a reactive adsorption mechanism.     

Electronic properties of the polar GaAs(111)As surface

Photoemission yield spectroscopy (PYS) has been used to investigate the electronic properties of the polar GaAs(111)As surface cleaned by ion bombardment with subsequent annealing at 770 K under UHV conditions of about 10⁻⁷ Pa. The elemental composition was recorded simultaneously by means of Auger electron spectroscopy (AES). After the above described cleaning procedure As has been completely removed from the first surface monolayer which corresponds to the (1×1) atomic surface structure whereas the values of the work function ø and absolute band bending were 4.00±0.02 and −0.16±0.06 eV respectively. Moreover two filled electronic surface state bands were observed which have been described as the Ga-derived surface state band and As-derived “back-bond” surface state band respectively.     

Structural and Chemical Modification in a Polymer by Metal Ion Implantation

Physical and chemical properties changes in a polymer have been studied for polycarbonate (PC) implanted with 100 keV Ni⁺ ions with varying fluence from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The changes in the surface morphology and composition have been observed with atomic force microscopy (AFM) and X-ray diffraction (XRD). The ions implanted induce changes in topography of PC and indicate that the roughness increases dramatically with ion fluence. Implanted metal ions shows direct evidence of compound formation on the surface. The chemical changes in the surface region have been carried out by Raman Spectroscopy and UV-VIS spectroscopy. UV-VIS absorption analysis indicates a drastic decline in optical band gap from 5.46 eV to 1.76 eV at an implanted dose of 1 × 10¹⁶ ions/cm². It could be shown that the partial destruction of chemical bonding under ion implantation leads to the creation of new amorphous and graphite-like structures, which is confirmed by Raman spectroscopy.     

Emission cross sections of the second positive and first negative systems of N2 and N+2 excited by electron impact

Cross sections have been measured for emission of the 2nd positive system of N₂ and the 1st negative system of N⁺₂ by electron excitation. The electron energy ranged from the threshold to 400 eV. To obtain absolute values, comparison with the emission cross section of the 5¹S He level has been used.The results for the (0, 2) band of the 2nd positive system and for the (0, 1) band of the 1st negative system are (2.87 ± 0.40) × 10^-18 and (5.35 ± 0.5) × 10^-18cm², respectively, at the peak. The cross sections are given for 23 bands of the 1st negative systems.Use of the (0, 1) band of the 1st negative system is proposed for reference in obtaining absolute values.     

Chlorine adsorption on copper: III. Angle-resolved electron energy loss spectroscopy

Electronic excitations on Cu(001) and Cu(001)c(2 × 2)-Cl have been investigated by angle-resolved electron energy loss spectroscopy at an angular resolution of Δθ = ±1° and an energy resolution of ΔE = 60 meV. Primary energies in the 50–100 eV range were chosen and the specular reflection was studied for angles 35° ? θ ? 71° with respect to the surface normal. The results are summarized as follows: The specular Cu(001) spectra are compared to optical data and good agreement is found for the energetic position of direct transitions. The electronic losses observed for the ordered overlayer system may be interpreted by one-electron excitations from occupied surface bands (known from angle-resolved photoemission results) into an empty band with a minimum energy at 0.4 eV above the Fermi level.     

6H-SiC（0001）-6[KF（]3[KF）]×6[KF（]3[KF）]R30°重构表面的同步辐射角分辨光电子能谱研究

利用同步辐射角分辨光电子能谱（SRARPES）对6H-SiC（0001）-6[KF（]3[KF）]×6[KF（]3[KF）] R30°重构表面的电子结构和表面态进行了研究.通过鉴别价带谱中来自于体态的信息,可以推断出重构表面的费米能级位于体态价带顶之上（2.1±0.1）eV处.实验测出的体能带结构与理论计算的结果较为符合.在重构表面上发现三个表面态,分别位于结合能-0.48 eV（S₀）,-1.62 eV（S₁）和-4. 关键词： 角分辨光电子能谱 碳化硅（SiC） 电子结构 表面态     

γ辐照室温短波HgCdTe光伏器件的导纳谱研究

利用变频导纳谱研究了γ辐照前后Hg_1－xCd_xTe(x=0.6)n⁺-on-p结中的深能级缺陷．辐照前其缺陷能级位置在价带上0.15 eV,俘获截面σ_p＝2.9×10^-18cm²,缺陷密度N_t＝6.5×10¹⁵cm^-3,初步认为是Hg空位或与其相关的复合缺陷;经过10⁴Gy的γ辐照后其能级变得更深,在价带上0.19 eV,同时其俘获截面增加了近一个数量级,而缺陷密度基本上没有变化．γ辐照引入的这种能级变化最终使器件的性能(探测率)下降了1/2以上． 关键词：     

Dissociative Excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by Controlled Electron Impact

The optical emission spectra in the wavelength region 200–800 nm produced by electron impact on the silicon-organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single-collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH(A²Δ → X²Π) emission, the so-called CH ?4300 Å”? band, was found to have the largest emission cross section with values (at 100 eV) of 5.5 × 10^?19 cm² and 6.1 × 10^?19 cm² for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B²∑^? → X²Π transition (the CH ?3900 Å”? system) and the C²∑ → X²Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near-threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single-step dissociative excitation processes are unlikely to play a dominant role in low-temperature processing plasmas containing TMS and HMDSO.     

Photoemission yield spectroscopy of electronic surface states on germanium (111) surfaces

The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures.     

Measurement of lifetimes for high spin states in 152Sm, 154Gd and 156Gd by the Doppler broadened lineshape method

Lifetimes of the ground band levels up to spin 10⁺ in ¹⁵²Sm, ¹⁵⁴Gd and ¹⁵⁶Gd have been measured by the Doppler broadened lineshape (DBLS) analysis of peaks observed following Coulomb excitation of enriched metallic targsts by 132–143 MeV ³⁵Cl beams. The γ-ray lineshapes were measured in time coincidence with backscattered ions and were analyzed with a computer program incorporating tabulated electronic stopping powers. The nuclear stopping power of Lindhard et al and multiple scattering treatment by Blaugrund were assumed. Renormalization of the electronic stopping powers given by Northclifie and Schilling was found necessary to reproduce the accurately known lifetime of the 6⁺ state in ¹⁵²Sm. Stopping powers for Sm in Sm inferred from the tabulated ⁴He stopping power of Ziegler and Chu support this renormalization. The stretching parameter a derived from the lifetimes of the ground band are (2.0 ± 0.6) × 10^?3, (1.85 ± 0.40) × 10^?3 and (0.0 ± 0.45) × 10^?3, in ¹⁵²Sm, ¹⁵⁴Gd and ¹⁵⁶Gd, respectively.     

Surface photovoltage,band-bending and surface states on a-Si : H

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (～min), saturation at photon fluxes of about 10¹¹/cm² · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 10¹¹/ cm², a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

A shock tube determination of the CN ground state dissociation energy and the CN violet electronic transition moment

The CN ground state (X²∑⁺) dissociation energy and the electronic transition moment of the CN violet B²∑⁺ ? X²∑⁺ bands have been simultaneously determined from spectral emission measurements behind incident shock waves. The unshocked test gases were composed of various CO₂-CO-N₂-Ar mixtures, and the temperatures behind the incident shocks ranged from 3500 to 8000°K. The dissociation energy was determined to be 7.89 eV with a statistical precision of ±0.02 eV; a conservative estimate of the absolute error was ±0.13 eV. The value obtained for the Δυ = 0 sequence electronic transition moment was 0.90 ± 0.14, corresponding to an electronic absorption f-number of 0.035 ± 0.005 at a wavelength of 3860 Å. The electronic transition moment variation with internuclear separation was also measured.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.