Stability of MgO(1 1 1) films grown on 6H-SiC(0 0 0 1) by molecular beam epitaxy for two-step integration of functional oxides

Crystalline magnesium oxide (MgO) (1 1 1), 20 Å thick, was grown by molecular beam epitaxy (MBE) on hydrogen cleaned hexagonal silicon carbide (6H-SiC). The films were further heated to 740 °C and 650 °C under different oxygen environments in order to simulate processing conditions for subsequent functional oxide growth. The purpose of this study was to determine the effectiveness and stability of crystalline MgO films and the MgO/6H-SiC interface for subsequent heteroepitaxial deposition of multi-component, functional oxides by MBE or pulsed laser deposition processes. The stability of the MgO films and the MgO/6H-SiC interface was found to be dependent on substrate temperature and the presence of atomic oxygen. The MgO films and the MgO/6H-SiC interface are stable at temperatures up to 740 °C at 1.0 × 10⁻⁹ Torr for extended periods of time. While at temperatures below 400 °C exposure to the presence of active oxygen for extended periods of time has negligible impact, exposure to the presence of active oxygen for more than 5 min at 650 °C will degrade the MgO/6H-SiC interface. Concurrent etching and interface breakdown mechanisms are hypothesized to explain the observed effects. Further, barium titanate was deposited by MBE on bare 6H-SiC(0 0 0 1) and MgO(1 1 1)/6H-SiC(0 0 0 1) in order to evaluate the effectiveness of the MgO as a heteroepitaxial template layer for perovskite ferroelectrics.     

Study of the early stages of Cr/6H-SiC(0 0 0 1) interface formation

The early stages of the Cr/6H-SiC(0 0 0 1) interface formation at room temperature were investigated using XPS, LEED and work function (WF) measurements. Upon stepwise Cr evaporation in UHV up to a thickness of 5-10 monolayers (ML) at RT, the binding energy of the XPS Cr 2p_3/2 core level peak shifted from 576.1 eV, at submonolayer coverage, to 574.7 eV (corresponding to metallic Cr) for the final Cr deposit, while the binding energies of the substrate XPS core level peaks remained stable. The WF exhibited a steep decrease of about 0.5 eV from the initial SiC substrate value, upon submonolayer coverage, but then increased gradually to saturation at a value of about 4.8 eV (polycrystalline Cr film with some chemisorbed oxygen). The growth of the ultrathin film was via 3D-cluster formation. The height of the Schottky barrier for the Cr/6H-SiC(0 0 0 1) contact was found by XPS to be 0.5 ± 0.1 eV. The results, generally, indicate the absence of any extended interfacial silicide-like interaction at RT.     

First-principles study of CoO films on MnO (1 1 1): Stability and interfacial structure

Based on the generalized gradient approximation, full potential linearized augmented plane-wave (FP-LAPW) calculations have been performed to study the stability and the interfacial structure of CoO/MnO (1 1 1). The surface energy, the strain energy and the binding energy are calculated and discussed. The calculations revealed that the CoO/MnO (1 1 1) is a stable interface structure. Also examined were the electronic properties and the atomic spin magnetic moments of the interface. It was found that the interface exhibited half-metallic property and the atomic magnetic moments were obviously weakened at the interface for metal atoms compared with the corresponding magnetic moments in bulk material.     

Electronic and geometric structure of methyl oxirane adsorbed on Si(1 0 0)2 × 1

Electronic and geometric properties of the adsorbate-substrate complex formed upon adsorption of methyl oxirane on Si(1 0 0)2 × 1 at room temperature is reported, obtained with synchrotron radiation-induced valence and core-level photoemission. A ring-opening reaction is demonstrated to occur, followed by a five-membered ring formation involving two of the Si surface atoms bound to a surface dimer. Core-level photoemission spectra support the ring-opening reaction and the SiO and SiC bond formation, while from the valence spectra a more extended molecular fragmentation can be ruled out. We discuss the most likely geometry of the five-membered ring.     

Characterization of Cr/6H-SiC(0 0 0 1) nano-contacts by current-sensing AFM

The electrical properties and interface chemistry of Cr/6H-SiC(0 0 0 1) contacts have been studied by current-sensing atomic force microscopy (CS-AFM) and X-ray photoelectron spectroscopy (XPS). Cr layers were vapor deposited under ultrahigh vacuum onto both ex situ etched in H₂ and in situ Ar⁺ ion-bombarded samples. The Cr/SiC contacts are electrically non-uniform. Both the measured I-V characteristics and the modeling calculations enabled to estimate changes of the Schottky barrier height caused by Ar⁺ bombardment. Formation of ohmic nano-contacts on Ar⁺-bombarded surfaces was observed.     

Theoretical investigations for zinc blende-wurtzite polytypism in GaAs layers at Au/GaAs(1 1 1) interfaces

The zinc blende (ZB)-wurtzite (W) polytypism of GaAs layers at the Au/GaAs(1 1 1)B interfaces is investigated based on total-energy electronic-structure calculations within density functional theory. The calculations for the abrupt interfaces including a GaAs top layer with ZB and W stacking sequences reveal that the ZB sequence is energetically favorable, but the energy of W sequence with an interstitial Au atom at the top GaAs layer of the interface is lower than that of ZB sequence. This is because electrons accumulate around the interstitial region due to the hybridization between Au-6s and As-4p orbitals, resulting in the reduction of Ga-As bond charges. As a result, the relative stability at the top GaAs layer is determined by the electrostatic energy due to ionic charges. The results imply that the stabilization of W sequence at the Au catalyst-semiconductor interfaces as well as that on the nanowire faces are the origins for the appearance of W segments in NWs, qualitatively consistent with experiments.     

Electronic surface states on the MOVPE-prepared InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface

In this paper, the InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface was studied in detail, which turned out to be the most suitable to develop an InGaAs/GaAsSb interface that is as sharp as possible. In ultra high vacuum the InGaAs surface was investigated with low-energy electron diffraction, scanning tunneling microscopy and UV photoelectron spectroscopy employing synchrotron radiation as light source. Scanning the Γ−Δ−X direction by varying the photon energy between 8.5 eV and 50 eV, two surface states in the photoelectron spectra were observed in addition to the valence band peaks.     

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO₂ on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO₂(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO₂-Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO₂-Ce₂O₃ transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Silicon quantum wires on Ag(1 1 0): Fermi surface and quantum well states

One-dimensional Si quantum wires have been grown on silver single crystals upon deposition of ∼0.25 monolayer of Si on Ag(1 1 0) surfaces. Scanning tunneling microscopy (STM) clearly shows parallel 1D Si chains along the [−1 1 0] Ag crystallographic direction. Low Energy Electron Diffraction (LEED) confirms the massively parallel assembly of these selforganized Nanowires (NWs). We have characterized these nano-objects by measuring the dispersion of the NWs valence band at room temperature using Angle-Resolved PhotoEmission Spectroscopy (ARPES). Also, the Fermi Surface (FS) of the Ag(1 1 0) substrate has been mapped before and after the silicon deposition, trying to put in evidence the metallic or semiconductor character of the NWs silicon's states close to the Fermi level. Our results show the existence of well-defined quantum states associated to the silicon super-structure. Both LEED and ARUPS results confirm that the NWs have typical 1D features, however their metallic or semiconductor character could not be confirmed.     

Theoretical investigation on structural stability of InN thin films on 3C-SiC(0 0 1)

The structural stability of InN thin films on 3C-SiC(0 0 1) substrate is systematically investigated based on an empirical interatomic potential, which incorporates electrostatic energy due to covalent bond charges and ionic charges. The calculated energy differences among coherently grown 3C-InN(0 0 1), 3C-InN(0 0 1) with misfit dislocations (MDs), and 2H-InN(0 0 0 1) imply that the coherently grown 3C-InN(0 0 1) is stable when the film thickness is less than 7 monolayers (MLs) while 2H-InN(0 0 0 1) is stabilized for the thickness beyond 8 MLs. This is because InN layers in 2H-InN(0 0 0 1) are fully relaxed by one MD. The analysis of atomic configuration at the 3C-InN(0 0 1)/3C-SiC(0 0 1) interfaces reveals that the coordination number of interfacial atoms is quite different from that in the bulk region. Thus, 3C-InN(0 0 1) with MDs on 3C-SiC(0 0 1) is always metastable over entire range of film thickness, consistent with the successful fabrication of 2H-InN(0 0 0 1) on 3C-SiC(0 0 1) by the molecular beam epitaxy. These results suggest that the mismatch in atomic arrangements at the interface crucially affects the structural stability of InN thin films on 3C-SiC(0 0 1) substrate.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

Electron affinity study of adamantane on Si(1 1 1)

Recently, tetramantane, a member of diamondoid series (C_4n+6H_4n+12), has shown to exhibit negative-electron-affinity effect which has a potential use for efficient electron emitting devices. Here, we explore the electronic property of adamantane (C₁₀H₁₆), the smallest member of the series. We prepare adamantane films on Si(1 1 1) substrates and then study their electronic structure with photoemission spectroscopy. Photoelectron spectra of adamantane on Si(1 1 1) have shown a peak at low-kinetic energy which could be a generic property of diamondoids. The possibility of the negative-electron-affinity effect in adamantane is further discussed.     

Resistive hysteresis and capacitance effect in NiFe2O4/SrTiO3: Nb(1 wt%) junctions

Epitaxial ultrathin NiFe₂O₄ films were deposited on 1 wt% Nb-doped SrTiO₃ (0 0 1) substrates by reactive cosputtering to form junctions with an area of ∼2 mm², and current-voltage curves show rectifying and asymmetrical hysteresis characteristics. The resistance calculated from the current-voltage curves is strongly voltage dependent, and the hysteretic loops with high and low resistive states were observed. The hysteretic loops are considered to stem from the capacitance effect of the highly resistive NiFe₂O₄ layer, which leads to charge accumulation at the interfaces. The results show that the interfaces of the junctions have a large areal capacitance of ∼100 nF/mm² from 300 to 120 K.     

Photoemission studies of initial oxidation for ultra-thin zinc film on 6H-SiC(0 0 0 1) surface with synchrotron radiation

The thermal oxidation process of metallic zinc on 6H-SiC(0 0 0 1) surface has been investigated by using atomic force microscopy (AFM), synchrotron radiation photoelectron spectroscopy (SRPES) and XPS methods. The AFM images characterize the surface morphology of ZnO film formed during the thermal oxidation and SRPES record the valence band, Si 2p and Zn 3d spectra at different stages. The O 1s peak is recorded by XPS because of the energy limit of the synchrotron radiation. Our results reveal that the silicon oxides layer of SiC substrate can be reduce by hot metallic zinc atom deposition. The oxygen atoms in the silicon oxides are captured by the zinc atoms to form ZnO_x at the initial stage and as a result, the oxidized SiC surface are deoxidized. After the zinc deposition with the final thickness of 2.5 nm, the sample is exposed in oxygen atmosphere and annealed at different temperatures. According to the evolution of peaks integrated intensities, it is considered that the Zn/SiC system will lose zinc atoms during the annealing in oxygen flux at high temperature due to the low evaporation temperature of pure zinc. After further annealing in oxygen flux at higher temperature, the substrate is also oxidized and finally the interface becomes a stable SiC-SiO_x-ZnO sandwich structure.     

Elaboration of (1 1 1)-oriented La-doped PZT thin films on platinized silicon substrates

PLZT thin films with different thickness were deposited in situ on platinum coated silicon substrates using a multi-target sputtering system. The purpose was to grow (1 1 1)-textured PLZT films on Pt (1 1 1). To this aim, the role of some key parameters on both crystalline quality and electrical properties was investigated. An ultra-thin TiO₂ seeding layer was deposited, prior to PLZT, which strongly affected the crystallographic orientation of the films. The relation between temperature deposition and film crystallinity is analysed. TEM observations show the presence of some very small grains of Zr_0.9La_0.1O_1.95 at the film bottom interface. In the range of thickness investigated, the plot of the inverse capacitance as a function of the film thickness split up into two different curves, each with a linear shape, which however allows determination of a single value of interface capacitance. Above a thickness of 400-500 nm a saturation of the dielectric properties seems to be reached.     

Comparative study of the GaAs(1 0 0) surface cleaned by atomic hydrogen

In attempt to correlate electronic properties and chemical composition of atomic hydrogen cleaned GaAs(1 0 0) surface, high-resolution photoemission yield spectroscopy (PYS) combined with Auger electron spectroscopy (AES) and mass spectrometry has been used. Our room temperature investigation clearly shows that the variations of surface composition and the electronic properties of a space charge layer as a function of atomic hydrogen dose display three successive interaction stages. There exists a contamination etching stage which is observed up to around 250 L of atomic hydrogen dose followed by a transition stage and a degradation stage which is observed beyond 700 L of exposure. In the first stage, a linear shift in the surface Fermi level is observed towards the conduction band by 0.14 eV, in agreement to the observed restoration of the surface stoichiometry and contamination removal. The next stage is characterized by a drop in ionization energy and work function, which quantitatively agrees with the observed Ga-enrichment as well as the tail of the electronic states attributed to the breaking As-dimers. As a result of the strong hydrogenation, the interface Fermi level E_F − E_v has been pinned at the value of 0.75 eV what corresponds to the degradation stage of the GaAs(1 0 0) surface that exhibits metallic density of states associated with Ga_As antisites defects. The results are discussed quantitatively in terms of the surface molecule approach and compared to those obtained by other groups.     

CF interaction with Si(1 0 0)-(2 × 1): Molecular dynamics simulation

In this study, the interaction of CF with the clean Si(1 0 0)-(2 × 1) surface at normal incidence and room temperature was investigated using molecular dynamics simulation. Incident energies of 2, 12 and 50 eV were simulated. C atoms, arising from dissociation, preferentially react with Si to form Si-C bonds. A Si_xC_yF_z interfacial layer is formed, but no etching is observed. The interfacial layer thickness increases with increasing incident energy, mainly through enhanced penetration of the silicon lattice. Silicon carbide and fluorosilyl species are formed at 50 eV, which is in good agreement with available experimental data. The level of agreement between the simulated and experimental results is discussed.     

Effect of modification of S-terminated Ge(1 0 0) surface on ALD HfO2 gate stack

When S-termination on a Ge(1 0 0) surface was desorbed at an elevated temperature and an atomic layer deposition (ALD) HfO₂ film was deposited, interfacial thickness was less than 1 nm. As a result, the equivalent oxide thickness (EOT) of the stack on the initially S-terminated surface was thinner than that deposited on the O₃-oxidized surface, while HfO₂ film thickness was almost identical on both surfaces. Nevertheless, the HfO₂ stack on the initially S-terminated surface exhibited improved leakage current characteristics due to an increase in barrier height. Its thinner but robust interface will contribute to the scaling down of gate oxide integrity.