NEXAFS and XPS study of GaN formation on ion-bombarded GaAs surfaces

Interaction of low-energy nitrogen ions (0.3-2 keV N₂⁺) with GaAs (100) surfaces has been studied by X-ray photoemission spectroscopy (XPS) around N 1s and Ga 3d core-levels and near-edge X-ray absorption fine structure (NEXAFS) around the N K-edge, using synchrotron radiation. At the lowest bombardment energy, nitrogen forms bonds with both Ga and As, while Ga-N bonds form preferentially at higher energies. Thermal annealing at temperatures above 350 °C promotes formation of GaN on the surface, but it is insufficient to remove disorder introduced by ion implantation. We have identified nitrogen interstitials and anti-sites in NEXAFS spectra, while interstitial molecular nitrogen provides a clear signature in both XPS and NEXAFS. The close similarity between NEXAFS spectra from thin GaN films and ion-bombarded GaAs samples supports our proposition about formation of thin GaN films on ion-bombarded GaAs.     

Growth and characterization of stoichiometric BCN films on highly oriented pyrolytic graphite by radiofrequency plasma enhanced chemical vapor deposition

Hexagonal boron carbonitride (h-BCN) hybrid films have been synthesized on highly oriented pyrolytic graphite by radiofrequency plasma enhanced chemical vapor deposition using tris-(dimethylamino)borane as a single-source molecular precursor. The films were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopic measurements. XPS measurement showed that the B atoms were bonded to C and N atoms to form the sp²-B-C-N atomic hybrid chemical environment. The atomic composition estimated from the XPS of the typical sample was found to be almost B₁C₁N₁. NEXAFS spectra of the B K-edge and the N K-edge had the peaks due to the π* and σ* resonances of sp² hybrid orbitals implying the existence of the sp² hybrid configurations of h-BCN around the B atoms. The G band at 1592 and D band at 1352 cm^− 1 in the Raman spectra also suggested the presence of the graphite-like sp²-B-C-N atomic hybrid bonds. The films consisted of micrometer scale crystalline structure of around 10 µm thick has been confirmed by the field emission scanning electron microscopy.     

Influence of substrate temperature on N-doped ZnO films deposited by RF magnetron sputtering

Nitrogen-doped ZnO films were deposited by RF magnetron sputtering in 75% of N₂ / (Ar + N₂) gas atmosphere. The influence of substrate temperature ranging from room temperature (RT) to 300 °C was analyzed by X-ray diffractometry (XRD), spectrophotometry, X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS) and Hall measurements setup. The XRD studies confirmed the hexagonal ZnO structure and showed that the crystallinity of these films increased with increasing substrate temperature (T_s). The optical studies indicate the average visible transmittance in the wavelength ranging 500-800 nm increases with increasing T_s. A minimum transmittance (9.84%) obtained for the films deposited at RT increased with increasing T_s to a maximum of 88.59% at 300 °C (500-800 nm). Furthermore, it was understood that the band gap widens with increasing T_s from 1.99 eV (RT) to 3.30 eV (250 °C). Compositional analyses (XPS and SIMS) confirmed the nitrogen (N) incorporation into the ZnO films and its decreasing concentration with increasing T_s. The negative sign of Hall coefficients confirmed the n-type conducting.     

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 LaO_1?x F _x 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.     

Orientation of B-C-N hybrid films deposited on Ni (111) and polycrystalline Ti substrates explored by X-ray absorption spectroscopy

Orientation of sp²-bonded boron carbonitride (BCN) hybrid films has been investigated. The films were synthesized on Ni (111) and polycrystalline Ti substrates by radio frequency plasma enhanced chemical vapor deposition using tris-dimethylamino borane as a single-source molecular precursor. The deposition was performed at the radiofrequency power 400-800 W at the working pressure 2.6 Pa. Formation of sp²-BCN hybrids in the samples was confirmed by X-ray diffraction (XRD). In the XRD profile, the peak at 26.3° revealed formation of crystalline phase in the samples in which the lattice planes are separated from each other by around 3.5 Å. The D band at ~ 1350 cm^− 1 and the G band at ~ 1570 cm^− 1 in Raman spectra also suggested presence of graphite-like sp²-B-C-N hybrid bonds. The films were composed of different B-N, B-C, and C-N bonds to form sp²-BCN atomic hybrids confirmed by X-ray photoelectron spectra. Orientation and local structures of the films were studied by near-edge X-ray absorption fine structure (NEXAFS) measurements. The dominant presence of π* and σ* resonance peaks of the sp² hybrid orbitals in B K-edge NEXAFS spectra revealed preferred formation of sp²-BCN atomic hybrids around B atoms like-BN₃ configuration in respect to the plane of Ni (111) substrate. Different orientations were suggested on the basis of polarization dependence of B K-edge and N K-edge of the NEXAFS spectra.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Oxidation study of polycrystalline InN film using in situ X-ray scattering and X-ray photoemission spectroscopy

Oxidation process of polycrystalline InN films were investigated using in situ X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS). The films were grown by dc sputter on sapphire (0001) substrates and were oxidized in air at elevated temperatures. The XRD data showed that the structure of the films changed to the bixbyite In₂O₃ (a = 10.11 Å) above 450 °C. Chemical configurations of the sample surfaces were investigated using high-resolution XPS. For the non-intentionally oxidized InN film, XPS analysis on the In 3d peak and the N 1s main peak at 396.4 eV suggests that indium and nitrogen are bound dominantly in the form of InN. An additional peak observed at 397.4 eV in the N 1s photoelectrons and the O 1s peaks indicate that the InN film surface is partly oxidized to have InO_xN_y configuration. After oxidation of the InN film at elevated temperature, the O 1s spectrum is dominated by In₂O₃ peak, which indicates that the structure is stable chemically with In₂O₃ configuration at least within the XPS probing depth of a few nm.     

XPS study of CZTSSe monograin powders

The surface and bulk composition of Cu₂ZnSn(Se_xS_1-x)₄ (CZTSSe) monograin powders were investigated by X-ray photoelectron spectroscopy (XPS). The concentration depth profiling of CZTSSe monograin powders was obtained by Ar⁺ ion etching.According to the XPS spectra of CZTSSe monograin powder, the binding energies of Zn 2p_3/2, Cu 2p_3/2, Sn 3d_5/2, S 2p_3/2 and Se 3d_5/2 core levels after surface cleaning are located at 1021.6 eV, 932.4 eV, 486.1 eV, 161.5 eV, 53.9 eV, respectively. From XPS depth profile analysis, Cu deficiency and the excess of chalcogenides on the powder crystals surface were observed.     

Structural and magnetic properties of Co-N thin films synthesized by direct current magnetron sputtering

Influence of nitrogen fractions [N_f = N₂/(N₂ + Ar)] and sputtering powers (P_s) on the structural and magnetic properties of Co-N thin films synthesized by direct current magnetron sputtering have been studied. With increasing N_f from 0 to 20%, a series of phases from β-Co, β-Co (N), Co₄N to Co₃N were obtained. However, when N_f was fixed at 10%, only Co₄N phase with different Co contents in the films was prepared, whose values of saturation magnetism (M_s) increased from 12.9 ± 8.2 Am²/kg to 103.9 ± 6.1 Am²/kg with the increase of P_s. Interstitial nitrogen caused the decrease of coercivity from 24.12 kAm^− 1 (for β-Co film) to 2.71 kAm^− 1. However, the addition of interstitial nitrogen was not observed to increase the M_s of β-Co.     

Crystal growth and physical properties of ternary bismuth molybdenum oxides Bi x MoO y (x=0.21, 0.25; y=3 + δ)

Large purple-brown crystals of two reduced bismuth molybdenum oxides have been grown from the MoO₃–Bi₂O₃ system by electrolytic reduction of the molten salts. The unit cell parameters and the compositions have been determined by X-ray diffraction and XPS microprobe analysis, respectively. The empirical formulae were refined to Bi_0.25MoO_3.1 and Bi_0.21MoO_3.2. Transport measurements (the resistivity and the thermoelectric power) showed that both crystals are semiconductors with p-type carriers, and the thermally activated energy gaps are respectively about 0.296 eV and 0.30 eV. The EPR spectra, the XPS core-level spectra and possible structural features were also considered.     

Structure and optical properties of AlxZn1−xO alloys by sol-gel technique

Al_xZn_1−xO (x = 0-0.5) thin films were prepared on quartz glass substrates by sol-gel technique. X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) were employed for microstructure characterization of these thin films. In films with up to 20 at.% Al incorporation, compound nano-crystal phase was observed while wurtzite structure disappeared. Zn3d electron binding energy and Zn LMM‘s chemical shift were both increased by more than 0.4 eV. Transmittance spectra revealed that these films possessed high transmittance in the visible region, and the end of UV absorption edge shifted to less than 300 nm when Al content exceeds 20 at.% due to quantum confinement effect.     

Temperature-dependent high-resolution X-ray photoelectron spectroscopic study on Ge1Sb2Te4

Oxygen-free and amorphous Ge₁Sb₂Te₄ thin film was obtained in an ultra-high vacuum and then annealed in situ to the stable-phase temperature. High-resolution X-ray photoelectron spectroscopy using synchrotron radiation was performed on the film at the different annealing temperatures of 100, 130, 150, 180, and 250 °C. The Te 4d, Sb 4d, and Ge 3d shallow core levels as well as the valence-band spectra were acquired. In the shallow core-level spectra, we observed distinguishable changes in the Sb 4d and Ge 3d levels as the film phase changed. As the temperature increased, a higher binding-energy (BE) component appeared at the Sb 4d level, the intensity of the component increased, and the spin-orbit split feature was enhanced at the Ge 3d level. In the valence-band spectra, a slight increase was observed at 0-1, ~ 3, ~ 9, and ~ 12 eV BE, and a decrease, at ~ 1.5 and ~ 4.5 eV BE. The energy resolution employed in this study was about 150 meV.     

Investigation on the Pyrazine Molecular Junction Studied by Conductance Measurement and Near Edge X-ray Absorption Fine Structure

A pyrazine molecular junction was investigated using mechanically controllable break junction (MCBJ) technique at 10 K. The conductance measurements revealed the single pyrazine molecular junctions showed two distinct conductance values of 0.27 ± 0.04 and 1.0 ± 0.2 G ₀ (2e ²/h). The conductance value of the single pyrazine molecular junction was comparable with that of the metal atomic junction. The interface between pyrazine molecule and Pt surface was investigated by near edge X-ray absorption fine structure (NEXAFS). The broadening of the π* peak in N K-edge NEXAFS spectra suggested that the pyrazine molecule connected to Pt surface via a nitrogen atom. Based on the measurements of the conductance and NEXAFS, we could propose the structural models of two distinct conductance states for the single pyrazine molecular junction.     

Spectroscopic study of Zn1−xCoxO thin films showing intrinsic ferromagnetism

Dilute magnetic semiconductors are widely studied due to their potential applications in spin-resolved electronics. We report the direct evidences of intrinsic ferromagnetism in the primarily ferromagnetic ZnO:Co thin films using near-edge X-ray absorption fine structure (NEXAFS) and soft X-ray magnetic circular dichroism (XMCD). The single phase Zn_1−xCo_xO thin films with nominal compositions (0.00 ≤ x ≤ 0.15) were synthesized by a spray pyrolysis technique, which exhibit room temperature ferromagnetism as revealed by alternating gradient force magnetometer (AGFM) measurements. The spectroscopic measurements indicate that most of Co dopants have substituted for Zn sites in ZnO matrix and they are present in divalent Co²⁺ (d⁷) state with tetrahedral symmetry according to the atomic multiplet calculations. The O 1s NEXAFS spectra suggest strong hybridization between O 2p and Co 3d electrons within ZnO matrix. The Co 2p XMCD measurements rule out the magnetism due to the presence of Co clusters, and show that Co–O–Co bonding provides localized magnetic moments leading to ferromagnetism.     

Spray pyrolysis deposition of indium sulphide thin films

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using the pneumatic spray set-up and compressed air as a carrier gas. Aqueous solutions containing InCl₃ and SC(NH₂)₂ at a molar ratio of In/S = 1/3 and 1/6 were deposited onto preheated glass sheets at substrate temperatures T_s = 205-410 °C. The obtained films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM,) optical transmission spectra, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). According to XRD, thin films deposited at T_s = 205-365 °C were composed of the (0 0 12) orientated tetragonal β-In₂S₃ phase independent of the In/S ratio in the spray solution. Depositions performed at T_s = 410 °C led to the formation of the In₂O₃ phase, preferably when the 1/3 solution was sprayed. Post-deposition annealing in air indicated that oxidation of the sulphide phase has a minor role in the formation of In₂O₃ at temperatures up to 450 °C. In₂S₃ films grown at T_s below 365 °C exhibited transparency over 70% in the visible spectral region and E_g of 2.90-2.96 eV for direct and 2.15-2.30 eV for indirect transitions, respectively. Film thickness and chlorine content decreased with increasing deposition temperatures. The XPS study revealed that the In/S ratio in the spray solution had a significant influence on the content of oxygen (Me-O, BE = 530.0 eV) in the In₂S₃ films deposited in the temperature range of 205-365 °C. Both XPS and EDS studies confirmed that oxygen content in the films deposited using the solution with the In/S ratio of 1/6 was substantially lower than in the films deposited with the In/S ratio of 1/3.     

Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures

Nitrogen-doped titanium dioxide (TiO_2 − xN_x) thin films desirable for visible light photocatalysts were prepared by reactive sputtering using air/Ar mixtures. Using air as the reactive gas allows the process to conduct at high base pressures (low vacuum), which reduces substantially the processing time. The obtained films transformed from mixed phases to anatase phase as the air/Ar flow ratio increased. Substitutional doping of nitrogen verified by X-ray photoelectron spectroscopy accounts for the red-shift of absorption edge in the absorption spectra. Anatase TiO_2 − xN_x films could incorporate up to about 7.5 at.% substitutional nitrogen and a maximum of 23 at.% nitrogen was determined in the films with mixed phases. The optical band gaps of the TiO_2 − xN_x films calculated from Tauc plots varied from 3.05 to 3.11 eV and those of the mixed phase ranged from 2.77 to 3.00 eV, which are all lower than that for pure anatase TiO₂ and fall into the visible light regime.     

Self-assembling peptides: A combined XPS and NEXAFS investigation on the structure of two dipeptides Ala–Glu,Ala–Lys

The two dipeptides AE (Lalanine–Lglutamic acid) and AK (Lalanine–Llysine), that constitute the “building blocks” of the 16-unit self-complementary amphiphilic oligopeptide EAK16, have been investigated by XPS (X-ray photoelectron spectroscopy) and NEXAFS (near-edge X-ray absorption fine structure) spectroscopy. Thin films of both dipeptides on TiO₂, a distinguished biocompatible surface, were prepared by incubation from aqueous solutions. Thick films of dipeptides on inert Au substrates were also studied for comparison. The chemical structure and composition were investigated by XPS spectroscopy; furthermore, molecular orientation of dipeptides on TiO₂ was checked by angular dependent NEXAFS measurements at both C–K and N–K edges. In order to yield some insight on adsorption geometry and molecular orientation MD (molecular dynamic) simulations were also carried out.The performed molecular and electronic characterization of AE and AK provides an excellent model for the interpretation of more complex peptide spectra.     

Structure and optical properties of Zr1−xSixN thin films on sapphire

A series of zirconium silicon nitride (Zr_1−xSi_xN) thin films were grown on r-plane sapphire substrates using reactive RF magnetron co-sputtering of Zr and Si targets in a N₂/Ar plasma. X-ray diffraction pole figure analysis, X-ray reflectivity, X-ray photoelectron spectroscopy (XPS), optical microscopy, and optical absorption spectroscopy were used to characterize the film stoichiometries and structures after growth at 200 °C and post-deposition annealing up to 1000 °C in ultra-high vacuum. The atomically clean r-plane sapphire substrates induce high quality (100) heteroepitaxy of ZrN films rather than the (111) orientation observed on steel and silicon substrates, but the addition of Si yields amorphous films at the 200 °C growth temperature. After the annealing treatment, films with Si content x < 0.15 have compressive stress and crystallize into a polycrystalline structure with (100) fiber texture. For x > 0.15, the films are amorphous and remain so even after ultra-high vacuum annealing at 1000 °C. XPS spectra indicate that the bonding changes from covalent to more ionic in character as Si―N bonds form instead of Zr―N bonds. X-ray reflectivity, atomic force microscopy (AFM) and optical microscopy data reveal that after post-deposition annealing the 100 nm thick films have an average roughness < 2 nm, except for Si content near x = 0.15 corresponding to where the film becomes amorphous rather than being polycrystalline. At this stoichiometry, evidence was found for regions of film delamination and hillock formation, which is presumably driven by strain at the interface between the film and sapphire substrate. UV-visible absorption spectra also were found to depend on the film stoichiometry. For the amorphous Si-rich films (x > 0.15), the optical band gap increases with Si content, whereas for Zr-rich films (x < 0.15), there is no band gap and the films are highly conductive.     

Electronic structure of carbon nitride thin films studied by X-ray spectroscopy techniques

Magnetron-sputtered carbon nitride thin films with different structures and compositions were analyzed by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), as well as X-ray emission spectroscopy (XES). In all techniques, the carbon spectra are broad and featureless with little variation depending on growth conditions. The nitrogen spectra, on the other hand, show more distinct features, providing a powerful tool for structural characterization. By comparing the experimental spectra with calculations on different model systems, we are able to identify three major bonding structures of the nitrogen—N1: nitrile (CN) bonds; N2: Pyridine-like N, i.e., N bonded to two C atoms; and N3: graphite-like N, i.e., N bonded to three C atoms as if substituted in a graphitic network, however, possibly positioned in a pentagon and/or with sp³ carbon neighbors. The presence of N2 and N3 are best detected by XPS, while N1 is better detected by NEXAFS. The calculated XES spectra also give good indications how valence band spectra should be interpreted. Films grown at the higher temperatures (≥350 °C) show a pronounced angular dependence of the incoming photon beam in NEXAFS measurements, which suggests a textured microstructure with standing graphitic basal planes, while amorphous films grown at low temperatures show isotropic properties.     

On the zinc nitride properties and the unintentional incorporation of oxygen

Zinc nitride films were prepared by radio-frequency magnetron sputtering in N₂/Ar ambient using different substrates (glass and thermally-oxidized-Si) and buffer layers (low-temperature Zn₃N₂ and ZnO). Resonant Rutherford backscattering (RBS) allowed determining Zn_xN_y stoichiometry and thickness. Despite the sputtering system was operated in high vacuum conditions, unintentional oxygen incorporation during growth was detected. Calculations of the relative oxygen concentration showed that the oxygen content was very dependent on the growth rate. Ex-situ oxidation was also analyzed by resonant RBS and compared with the results of as-grown layers. Scanning electron microscopy and X-ray diffraction revealed that surface morphology and crystal orientation were strongly dependent on the substrate temperature (T_s). In addition, optical transmission measurements show a reduction of the optical energy band gap from 1.46 to 1.25 eV as T_s increased. The electrical properties were examined as a function of growth rate, total working gas flux and T_s aiming to maximize electron mobility. From those studies, it was found that Hall mobility increased significantly as the growth rate decreased. A maximum mobility of 100 cm²/Vs and a minimum carrier concentration of 3.2 × 10¹⁸ cm⁻³ were achieved at a T_s of 423 K and a growth rate of 4.44 nm/min.