Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO₂ of 10⁻² Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O₂ exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.     

A study of barium ultra-thin films on the SrTiO3(1 0 0) surface by soft X-ray photoelectron spectroscopy

The growth and the electronic properties of Ba ultra-thin films (coverage ≤ 2 ML) on the SrTiO₃(1 0 0) surface was studied using synchrotron radiation facilities. The investigation was carried out mainly by soft X-ray photoelectron spectroscopy measurements at low core levels and the valence band region. The results show that the Ba overlayer develops in a layer-by-layer mode. The first two layers do not create any states in the band gap, thus leaving intact the insulating character of the substrate. This is due to the oxidation of Ba through transfer of O²⁻ anions from the substrate into the Ba overlayer. It is observed that the valency of the Ti atoms at the interface does not change. This is attributed to an outwards diffusion of O²⁻ anions from deeper layers promoted by substrate annealing. Within the coverage examined, no evidence for Ba metallization is found, contrary to what happens on metallic substrates. Comparing the results of this work with previous ones on similar metal–SrTiO₃(1 0 0) adsorption systems, it is concluded that a single factor such as charge transfer or thermodynamic stability is not always sufficient to determine the oxidation process of the adsorbate layer. Kinetics on surfaces as well as substrate doping and defects can also play a decisive role.     

XPS study of SnTe(1 0 0) oxidation by molecular oxygen

The interaction between the (1 0 0) surface of SnTe single crystal and molecular oxygen was studied by means of X-ray photoelectron spectroscopy (XPS). Analysis of the obtained spectra shows that the mechanism of surface oxidation does not change in the range of oxygen exposure 10⁸-10¹³ L. During the oxidation an additional component shifted 1.1 eV towards higher binding energies appears in the Sn 3d spectra. The Te 3d_5/2 spectra fitting reveals two additional components with binding energies close to Te⁰ and Te⁺⁴. The dependence of the additional components fraction in both Sn 3d and Te 3d_5/2 spectra on the oxygen exposure is semi-logarithmic. On the base of the experimental data two possible mechanisms are proposed.     

Valence band and surface core-level shift studies of the oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

Using synchrotron radiation, W 4f_7/2 and valence-band (VB) photoelectron spectroscopy was used to study the oxidation states of the p(3 × 1) reconstructed surface of W(1 0 0) produced by oxygen adsorption at 1500 K. The W 4f_7/2 study showed two features at 0.40 and 1.50 eV higher binding energies relative to the bulk feature which represent shifts to lower binding energies compared to the W 4f_7/2 features of the as-grown oxygen films on W(1 0 0). Co-existence of these features is a characteristic of the formation of the p(3 × 1) structure. The VB studies of this system and as-grown oxygen films at room temperature (RT) showed an oxygen-induced feature for the p(3 × 1) structure for which the peak maximum had shifted by 0.9 eV to higher binding energies compared to the as-grown films. The direction of shifts in the W 4f_7/2 and VB spectra for the p(3 × 1) structure confirmed the reduction of the charge transfer from W to oxygen compared to the as-grown films. A study of the VB changes with photon energy shows that the main component of the oxygen-induced feature in the p(3 × 1) VB at 6.6 ± 0.2 eV is related to a hybridized state between W 5d and O 2p electrons.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

Dissociative adsorption and adsorbate-induced reconstruction on Fe{2 1 1}

The products of CO, NO, O₂ and N₂ dissociation on Fe{2 1 1} have been studied by means of first-principles density functional theory. Preferred adsorption sites for adatoms C, N and O are identified, and trends in charge transfer and surface magnetism described. An experimentally observed (2 × 1) reconstruction induced by O is confirmed to be energetically stable, and a similar reconstruction induced by N is tentatively predicted. It is argued that these reconstructions may be important not only in the context of the catalytic reactivity of the Fe{2 1 1} surface, but also for the initial stages of surface nitridation and oxidation.     

BaO/W(1 0 0) thermionic emitters and the effects of Sc, Y, La, and the density functional used in computations

Density functional theory is used to predict workfunctions, φ. For relaxed clean W(1 0 0), the local density approximation (LDA) agrees with experiment better than the newer generalized gradient approximation, probably due to the surface electron self-energy. The large Ba metallic radius indicates it covers W(1 0 0) at about 0.5 monolayer (ML). However, Ba²⁺, O²⁻, and metallic W all have similar radii. Thus 1 ML of BaO (one BaO unit for each two W atoms) produces minimum strain, indicating commensurate interfaces. BaO (1 ML) and Ba (1/2 ML) have the sameφ to within 0.02 V, so at these coverages reduction or oxidation is not important. Due to greater chemical activity of ScO vs. highly ionic BaO, when mixing the latter with this suboxide of scandia, the overlayer always has BaO as the top layer and ScO as the second layer. The BaO/ScO bilayer has a rocksalt structure, suggesting high stability. In the series BaO/ScO/, BaO/YO/, and BaO/LaO/W(1 0 0), the latter has a remarkably low φ of 1.3 V (LDA), but 2 ML of rocksalt BaO also has φ at 1.3 V. We suggest BaO (1 ML) does not exist and that it is worthwhile to attempt the direct synthesis and study of BaO (2 ML) and BaO/LaO.     

Hydroxylated α-Al2O3 (0 0 0 1) surfaces and metal/α-Al2O3 (0 0 0 1) interfaces

X-ray photoelectron spectroscopy was applied to study the hydroxylation of α-Al₂O₃ (0 0 0 1) surfaces and the stability of surface OH groups. The evolution of interfacial chemistry of the α-Al₂O₃ (0 0 0 1) surfaces and metal/α-Al₂O₃ (0 0 0 1) interfaces are well illustrated via modifications of the surface O1s spectra. Clean hydroxylated surfaces are obtained through water- and oxygen plasma treatment at room temperature. The surface OH groups of the hydroxylated surface are very sensitive to electron beam illumination, Ar⁺ sputtering, UHV heating, and adsorption of reactive metals. The transformation of a hydroxylated surface to an Al-terminated surface occurs by high temperature annealing or Al deposition.     

Pb induced charge accumulation on InAs(1 1 1)B

The Pb/InAs(1 1 1)B interface has been studied by synchrotron radiation photoelectron spectroscopy (SR-PES) of valence band and In4d, As3d and Pb5d core levels. Room temperature deposition of ∼1 ML of Pb on InAs(1 1 1)B leads to an ordered overlayer that induces a metallic channel at the surface, as seen through a weak emission in the vicinity of the Fermi level. Its narrow localization in reciprocal space supports the formation of a two-dimensional free electron gas (2DEG) in the surface region. It is proposed that the adsorbed metal layer swaps the initial polarisation of the surface and thus pulls electrons back to the surface. This charge re-arrangement increases the charge density in the accumulation layer and reduces the screening length and thus the depth of the potential well at the surface.     

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

Surface chemistry of HfI4 on Si(1 0 0)-(2 × 1) studied by core level photoelectron spectroscopy

The chemistry of HfI₄ adsorbed on the Si(1 0 0)-(2 × 1) surface has been studied by core level photoelectron spectroscopy in ultra-high vacuum. Two stable surface intermediates are identified: HfI₃ and HfI₂, both of which remain upon heating to 690 K. The dissociation of HfI₄ is accompanied by the formation of SiI. In addition, HfI₄ is observed up to 300 K. Complete desorption of iodine occurs in the temperature regime 690-780 K. Deposition of HfI₄ at 870 K results in a layer consisting of metallic Hf, whereas deposition at 1120 K results in the formation of Hf silicide. The results indicate that the metallic Hf formed at 870 K is in the form of particles. Oxidation of this film by O₂ at low pressure does not result in complete Hf oxidation. This suggests that complete oxidation of Hf is a critical step when using HfI₄ as precursor in atomic layer deposition.     

Pt interactions with annealed and chemically-etched Nb-doped SrTiO3(0 0 1) surfaces: Metal/oxide surface chemical effects on band bending behavior

XPS and LEED have been used to characterize the interaction of sputter-deposited Pt (maximum coverage <5 ML) with Nb-doped SrTiO₃(0 0 1) surfaces prepared either by annealing in O₂ and then UHV, or by chemical-etching in aqua regia. The annealed surface exhibits an ordered (1 × 1) LEED pattern, with additional diffraction spots and streaks indicating the presence of oxygen vacancies. Increasing Pt coverage results in the decrease of the observed Pt(4f_7/2) binding energy and the uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to smaller binding energies, as expected for Pt cluster growth and surface-to-Pt charge donation on an n-type semiconductor. The etched surface is disordered, and exhibits a hydroxylated surface with a contaminant C film of ∼23 ? average thickness. Pt deposition on the etched surface results in an immediate decrease in the intensity of the OH feature in the O(1s) spectrum, and a uniform shift of the Sr(3d), Ti(2p) and O(1s) levels to larger binding energies with increasing Pt coverage. The observed Pt(4f_7/2) binding energy on the etched surface (∼72 eV) is independent of Pt coverage, and indicates substantial electronic charge donation from the Pt to surface hydroxyl species. The observation of band bending towards higher binding energies upon Pt deposition (behavior normally associated with p-type semiconductors) demonstrates that sub-monolayer quantities of adsorbates can alter metal/oxide interfacial charge transfer and reverse the direction of band bending, with important consequences for Schottky barrier heights and device applications.     

Behavior of the (0 0 0 1) surface of sapphire upon high-temperature annealing

Evolution of the (0 0 0 1) α-Al₂O₃ surface morphology upon annealing was studied using atomic force microscopy. The annealing protocol included temperatures of 1200 and 1500 °C and different time. Vicinal Al₂O₃ (0 0 0 1) surfaces annealed at 1200 °C exhibit initial localized step coalescence that evolves into terrace-and-step with island morphology that persists for several hours. Annealing at 1500 °C results in initial step coalescence on a global scale, and yields a terrace-and-step morphology with an indication of step bunching after longer annealing times.     

Change in work function during phase transition of Sc-O/W(1 0 0) system at high temperatures

The change in work function during the phase transition of a Sc-O/W(1 0 0) system at high temperatures of 1500-1700 K was investigated in order to elucidate the mechanism of the decrease in the work function of a (1 × 1)-Sc-O/W(1 0 0) surface. For the measurement of the work function using a secondary electron method at high temperatures, a sample holder was uniquely designed in order to suppress contributions of a large number of thermionic electrons emitted from a low-work-function surface. The obtained work function revealed that the change in work function strongly correlates with the coverage of Sc-O complexes, the stoichiometry of which is Sc₁O₁. The higher the coverage of ScO is, the lower the work function is, suggesting that the work function of the (1 × 1)-Sc-O/W(1 0 0) surface is reduced by the formation of ScO electric dipoles. Furthermore, the decrease in work function was modeled in combination with a previously reported kinetic model describing surface phenomena, i.e., the oxidation, oxygen desorption, diffusion and surface segregation of Sc-O complexes, during the phase transition of the Sc-O/W(1 0 0) surface at high temperatures. It was confirmed that the proposed model can explain the experimentally obtained change in work function very well. The value of the dipole moment of a ScO electric dipole was determined to be 2.7 × 10⁻³⁰ C m.     

Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO2(1 1 0) substrates

Growth modes and electronic properties were analyzed for Au nano-particles grown on stoichiometric and reduced TiO₂(1 1 0) substrates by medium energy ion scattering (MEIS) and photoelectron spectroscopy(PES) using synchrotron-radiation-light. Initially, two-dimensional islands (2D) with a height of one and two atomic layers grow and higher coverage increases the islands height to form three-dimensional (3D) islands for the stoichiometric TiO₂(1 1 0) substrate. In contrast, 3D islands start to grow from initial stage with a small Au coverage (?0.1 ML, 1 ML = 1.39 × 10¹⁵  atoms/cm²: Au(1 1 1)) probably due to O-vacancies acting as a nucleation site. Above 0.7 ML, all the islands become 3D ones taking a shape of a partial sphere and the Au clusters change to metal for both substrates. We observed the Au 4f and Ti 3p core level shifts together with the valence band spectra. The Ti 3p peak for the O-deficient surface shifts to higher binding energy by 0.25 ± 0.05 eV compared to that for the stoichiometric surface, indicating downward band bending by an electron charge transfer from an O-vacancy induced surface state band to n-type TiO₂ substrate. Higher binding energy shifts of Au 4f peaks observed for both substrates reveal an electron charge transfer from Au to TiO₂ substrates. The work functions of Au nano-particles supported on the stoichiometric and reduced TiO₂ substrates were also determined as a function of Au coverage and explained clearly by the above surface and interface dipoles.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Growth and oxidation of Cr films on the W(1 0 0) surface

The growth and oxidation of Cr films on the W(1 0 0) surface have been studied with low energy electron microscopy (LEEM) and diffraction (LEED). Cr grows in a Stranski-Krastanov (SK) mode above about 550 K and in a kinetically limited layer-by-layer mode at lower temperature. Stress relief in the highly strained pseudomorphic (ps) Cr film appears to be achieved by the formation of (4 × 4) periodic inclusions during the growth of the third layer between 575 and 630 K and by growth morphological instabilities of the third layer at higher temperature. Kinetic or stress-induced roughening is observed at lower temperature. In the SK regime, three-dimensional (3D) Cr islands nucleate after the growth of three Cr layers. 3D island nucleation triggers dewetting of one layer from the surrounding Cr film. Thus, two ps Cr layers are thermodynamically stable. However, one and two layer ps Cr films are unstable during oxidation. 3D clusters, that produce complex diffraction features and are believed to be Cr₂O₃, are formed during oxidation of one Cr layer at elevated temperature, T ? 790 K. The single layer Cr film remains intact during oxidation at T ? 630 K. 3D bulk Cr clusters are formed predominantly during oxidation of two ps Cr layers.     

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

Surface restructuring process on a Ag/Ge(0 0 1) surface studied by photoelectron spectroscopy

The atomic structure and charge distribution of Ag adsorbed Ge(0 0 1) surfaces have been investigated by means of Ge 3d core- and Ag 4d core-levels photoelectron spectroscopy. A mono-atomic layer of Ag was deposited on the clean Ge(0 0 1) c(4×2) surface at 80 K. The Ge 3d spectrum measured at 80 K was deconvoluted into two surface components, which is consistent with the previously proposed Ag ad-dimer model. After annealing the surface at room temperature, the rearrangement of the charge distribution was revealed to include electron transfer from Ge to Ag in conjunction with the surface restructuring process by the annealing.