Electrical characterisation of ruthenium Schottky contacts on n-Ge (1 0 0)

Ruthenium (Ru) Schottky contacts were fabricated on n-Ge (1 0 0) by electron beam deposition. Current–voltage (I–V), deep level transient spectroscopy (DLTS), and Laplace-DLTS techniques were used to characterise the as-deposited and annealed Ru/n-Ge (1 0 0) Schottky contacts. The variation of the electrical properties of the Ru samples annealed between 25 °C and 575 °C indicates the formation of two phases of ruthenium germanide. After Ru Schottky contacts fabrication, an electron trap at 0.38 eV below the conduction band with capture cross section of 1.0×10⁻¹⁴ cm⁻² is the only detectable electron trap. The hole traps at 0.09, 0.15, 0.27 and 0.30 eV above the valence band with capture cross sections of 7.8×10⁻¹³ cm⁻², 7.1×10⁻¹³ cm⁻², 2.4×10⁻¹³ cm⁻² and 6.2×10⁻¹³ cm⁻², respectively, were observed in the as-deposited Ru Schottky contacts. The hole trap H(0.30) is the prominent single acceptor level of the E-centre, and H(0.09) is the third charge state of the E-centre. H(0.27) shows some reverse annealing and reaches a maximum concentration at 225 °C and anneals out after 350 °C. This trap is strongly believed to be V–Sb₂ complex formed from the annealing of V–Sb defect centre.     

STM study of acetylene reaction with Si(1 1 1): observation of a carbon-induced Si(1 1 1)√3×√3R30° reconstruction

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C₂H₂ pressure of 2 × 10⁻⁴ Pa (2 × 10⁻² mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si-C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T₄ and/or S₅ sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Growth and characteristics of ZnO nano-aggregates electrodeposited onto p-Si(1 1 1)

In this paper we study nanocrystalline zinc oxide thin films produced by oxidation of electrodeposited zinc nanolayers on a monocrystalline p-Si(1 1 1) substrate.The electrolyte used is ZnCl₂, an aqueous solution of 4 × 10⁻² mol/l concentration. Several deposits were made for various current densities, ranging from 13 mA/cm² to 44 mA/cm², flowing through the solution at room temperature. A parametric study enabled us to assess the effect of the current density on nucleation potential and time as well as zinc films structure. The grazing incidence X-ray diffraction (GIXD) revealed that both Zn and ZnO films are polycrystalline and nanometric. After 1-h oxidation of zinc films at 450 °C in the open air, the structural analyses showed that the obtained ZnO films remained polycrystalline with an average crystal size of about 47 nm and with (1 0 0), (0 0 2) and (1 0 1) as preferential crystallographic orientations.     

Hydrogen adsorption on GaAs(0 0 1)-c(4 × 4)

Exposure of the As-terminated GaAs(0 0 1)-c(4 × 4) reconstructed surface to atomic hydrogen (H) at different substrate temperatures (50-480 °C) has been studied by reflection high-energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM). Hydrogen exposure at low temperatures (∼50 °C) produces a disordered (1 × 1) surface covered with AsH_x clusters. At higher temperatures (150-400 °C) exposure to hydrogen leads to the formation of mixed c(2 × 2) and c(4 × 2) surface domains with H adsorbed on surface Ga atoms that are exposed due to the H induced loss of As from the surface. At the highest temperature (480 °C) a disordered (2 × 4) reconstruction is formed due to thermal desorption of As from the surface. The results are consistent with the loss of As from the surface, either through direct thermal desorption or as a result of the desorption of volatile compounds which form after reaction with H.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

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.     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

Observation of Si(1 0 0) surfaces annealed in hydrogen gas ambient by scanning tunneling microscopy

We investigated the cleaning process of Si(1 0 0) surfaces by annealing in H₂ gas ambient following chemical treatments by scanning tunneling microscopy. We observed the monohydride Si structure: Si(1 0 0):2 × 1-H on the surfaces annealed at 1000 °C in 2.5 × 10⁴ Pa H₂ gas ambient without conspicuous contaminants. On the sample annealed for 10 min or longer times, well-defined Si(1 0 0) structures with alternating S_A and S_B steps were observed, whereas the initial roughness still remained on the surfaces annealed for only 5 min.     

Characterization of single-crystalline In2O3 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

Epitaxial In₂O₃ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates by metalorganic chemical vapor deposition (MOCVD). The films were deposited at different substrate temperatures (450-750 °C). The film deposited at 650 °C has the best crystalline quality, and observation of the interface area shows a clear cube-on-cube epitaxial relationship of In₂O₃(1 0 0)||YSZ(1 0 0) with In₂O₃[0 0 1]||YSZ[0 0 1]. The Hall mobility of the single-crystalline In₂O₃ film deposited at 650 °C is as high as 66.5 cm² V⁻¹ s⁻¹ with carrier concentration of 1.5 × 10¹⁹ cm⁻³ and resistivity of 6.3 × 10⁻³ Ω cm. The absolute average transmittance of the obtained films in the visible range exceeds 95%.     

Solid-state synthesis and magnetic properties of epitaxial FePd3(0 0 1) films

The solid-state synthesis of magnetically soft phase FePd₃ in epitaxial Pd(0 0 1)/Fe(0 0 1)/MgO(0 0 1) film systems was studied experimentally. The system had a Fe to Pd ratio of 1:3. An increase to 450 °C leads to the formation of three variants of ordered L1₀-FePd crystallites. At 500 °C, the solid-state reaction of unreacted Pd with L1₀-FePd crystallites initiates the growth of an ordered epitaxial L1₂-FePd₃(0 0 1) layer. When annealing at 650 °С, a gradual disordering is observed. The magnetic anisotropy (K₁=−2.0×10³ erg/cm³) and the saturation magnetization (M_S=650 emu/cm³) of the disordered FePd₃ phase were determined.     

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.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

Low temperature fabrication of 5-10 nm SiO2/Si structure using advanced nitric acid oxidation of silicon (NAOS) method

We have developed the advanced nitric acid oxidation of Si (NAOS) method to form relatively thick (5-10 nm) SiO₂/Si structure with good electrical characteristics. This method simply involves immersion of Si in 68 wt% nitric acid aqueous solutions at 120 °C with polysilazane films. Fourier transform infrared absorption (FT-IR) measurements show that the atomic density of the NAOS SiO₂ layer is considerably high even without post-oxidation anneal (POA), i.e., 2.28 × 10²² atoms/cm², and it increases by POA at 400 °C in wet-oxygen (2.32 × 10²² atoms/cm²) or dry-oxygen (2.30 × 10²² atoms/cm²). The leakage current density is considerably low (e.g., 10⁻⁵ A/cm² at 8 MV/cm) and it is greatly decreased (10⁻⁸ A/cm² at 8 MV/cm) by POA at 400 °C in wet-oxygen. POA in wet-oxygen increases the atomic density of the SiO₂ layer, and decreases the density of oxide fixed positive charges.     

Surface alloy formation after deposition of Ce on Rh(1 1 0)

Vapour deposition of Ce onto a Rh(1 1 0) single crystal at room temperature is studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). The thicknesses of the deposited Ce layers are estimated to be between 2 and 9 Å. To study the changes in the Ce-Rh surface layer, the samples are annealed at temperatures between 500 and 1000 °C after Ce deposition.After heating, a c(2 × 2) LEED pattern appears for the sample with the thinnest deposited Ce layer (2.4 Å). For samples with thicker Ce-films, the LEED pattern co-exists of a c(2 × 2) structure and a more diffuse 6% contracted (2 × 1) structure. This appears at the same temperature as the Ce 3d and Rh 3d core levels exhibit sharp intensity changes and binding energy shifts.The intensity of the f⁰, f¹ and f² multiplets in the Ce 3d core level spectra change when the annealing temperature is increased. The relative intensity of the Ce 3d f⁰ and f² features compared to the Ce 3d f¹ features is largest after annealing to 500 °C. This is below the temperature at which the ordered surface alloy is formed. When the sample is heated above the formation temperature of the surface alloy, the relative intensity of the Ce 3d f⁰ and f² features decrease.     

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface at 25 °C have been studied by internal reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and low energy electron diffraction. Both molecules form a dative bond to the empty dangling bonds on the In-P heterodimers and the second-layer In-In dimers and vibrate symmetrically at 2319 (2315) and 2285 (2281) cm⁻¹ and asymmetrically at 2339 (2339) and 2327 (2323) cm⁻¹. A fraction of these species dissociate into adsorbed PH₂ with the hydrogen and tertiarybutyl ligands transferring to nearby phosphorus sites. The calculated energy barriers for desorption (<11 kcal/mol) of these molecules is less than that for dissociation (>17 kcal/mol) and explains their low sticking probabilities at elevated temperatures under InP growth conditions.     

An ab initio-based approach to phase diagram calculations for GaN(0 0 0 1) surfaces

Surface phase diagrams of GaN(0 0 0 1)-(2 × 2) and pseudo-(1 × 1) surfaces are systematically investigated by using our ab initio-based approach. The phase diagrams are obtained as functions of temperature T and Ga beam equivalent pressure p_Ga by comparing chemical potentials of Ga atom in the vapor phase with that on the surface. The calculated results imply that the (2 × 2) surface is stable in the temperature range of 700-1000 K at 10⁻⁸ Torr and 900-1400 K at 10⁻² Torr. This is consistent with experimental stable temperature range for the (2 × 2). On the other hand, the pseudo-(1 × 1) phase is stable in the temperature range less than 700 K at 10⁻⁸ Torr and less than 1000 K at 10⁻² Torr. Furthermore, the stable region of the pseudo-(1 × 1) phase almost coincides with that of the (2 × 2) with excess Ga adatom. This suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1 × 1) to the (2 × 2) with Ga adatom and vice versa.     

Surface treatments toward obtaining clean GaN(0 0 0 1) from commercial hydride vapor phase epitaxy and metal-organic chemical vapor deposition substrates in ultrahigh vacuum

We studied processes of cleaning GaN(0 0 0 1) surfaces on four different types of wafers: two types were hydride vapor phase epitaxy (HVPE) free-standing substrates and two types were metal-organic chemical vapor deposition (MOCVD) films grown on these HVPE substrates and prepared by annealing and/or Ar ion sputtering in ultra high vacuum. We observed the surfaces through treatments using in situ low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy, and also using ex situ temperature programmed desorption, X-ray photoelectron spectroscopy, X-ray diffraction, and secondary ion mass spectrometry. For HVPE samples, we obtained relatively clean surfaces under optimized three-step annealing conditions (200 °C for 12 h + 400 °C for 1 h + 500 °C for 5 min) without sputtering, after which the surface contamination of oxide and carbide was reduced to ∼20% of that before annealing. Clear GaN(0 0 0 1)1×1 patterns were obtained by LEED and RHEED. STM images showed flat terraces of ∼10 nm size and steps of ∼0.5 nm height. Upon annealing the HVPE-GaN samples at a much higher temperature (C), three-dimensional (3D) islands with facets were formed and the surface stoichiometry was broken down with the desorption of nitrogen in the form of ammonia, since the samples include hydrogen as an impurity. Ar⁺ sputtering was effective for removing surface contamination, however, postannealing could not recover the surface roughness but promoted the formation of 3D islands on the surface. For MOCVD/HVPE homoepitaxial samples, the surfaces are terminated by hydrogen and the as-introduced samples showed a clear 1×1 structure. Upon annealing at 500-600 °C, the surface hydrogen was removed and a 3×3 reconstruction structure partially appeared, although a 1×1 structure was dominant. We summarize the structure differences among the samples under the same treatment and clarify the effect of crystal quality, such as dislocations, the concentration of hydrogen impurities, and the residual reactant molecules in GaN films, on the surface structure.     

Addimer chain structures: Metastable precursors to island formation on Ge-Si(0 0 1)-(2 × n) alloyed surface

We have identified addimer chain structures as metastable precursors to compact epitaxial islands on the (2 × n) reconstructed SiGe wetting layer, using polarity-switching scanning tunneling microscopy (STM). These chain structures are comprised of 2-12 addimers residing in the troughs of neighboring substrate dimer rows. The chain structures extend along equivalent 〈1 3 0〉 directions across the substrate dimer rows in a zigzag fashion, giving rise to kinked and straight segments. We measure a kink-to-straight ratio of nearly 2:1. This ratio corresponds to a free energy difference of 17 ± 4 meV, favoring the formation of kinked segments. The chain structures convert to compact epitaxial islands at elevated temperatures (?90 °C). This conversion suggests that the chain structures are a precursor for compact island formation on the SiGe wetting layer. We digitally process filled- and empty-state STM images to distinguish chain structures from compact islands. By monitoring the populations of both species over time, the chain-to-island conversion rates are measured at substrate temperatures ranging from 90 to 150 °C. The activation energy for the conversion process is measured to be 0.7 ± 0.2 eV with a corresponding pre-exponential factor of 5 × 10^4±2 s⁻¹.