PTCDA on Au(1 1 1), Ag(1 1 1) and Cu(1 1 1): Correlation of interface charge transfer to bonding distance

The electronic structure at the interfaces of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and the metal surfaces Au(1 1 1), Ag(1 1 1) and Cu(1 1 1) was investigated using ultraviolet photoelectron spectroscopy (UPS). By combining these results with recent X-ray standing wave data from PTCDA on the same substrates clear correlation between the electronic properties and the interface geometry is found. The charge transfer between the molecule and the metal increases with decreasing average bonding distance along the sequence Au–Ag–Cu. Clear signatures of charge-transfer-induced occupied molecular states were found for PTCDA on Ag(1 1 1) and Cu(1 1 1). As reported previously by Zou et al. [Y. Zou et al., Surf. Sci. 600 (2006) 1240] a new hybrid state was found at the Fermi-level (E_F) for PTCDA/Ag(1 1 1), rendering the monolayer metallic. In contrast, the hybrid state for PTCDA/Cu(1 1 1) was observed well below E_F, indicating even stronger charge transfer and thus a semiconducting chemisorbed molecular monolayer. The hybridisation of molecular and Au electronic states could not be evidenced by UPS.     

Effect of high-temperature annealing on lanthanum aluminate thin films grown by ALD on Si(1 0 0)

Amorphous lanthanum aluminate thin films were deposited by atomic layer deposition on Si(1 0 0) using La(ⁱPrCp)₃, Al(CH₃)₃ and O₃ species. The effects of post-deposition rapid thermal annealing on the physical and electrical properties of the films were investigated. High-temperature annealing at 900 °C in N₂ atmosphere leads to the formation of amorphous La-aluminosilicate due to Si diffusion from the substrate. The annealed oxide exhibits a uniform composition through the film thickness, a large band gap of 7.0 ± 0.1 eV, and relatively high dielectric constant (κ) of 18 ± 1.     

Electroplating of Cu(Ag) thin films for interconnect applications

Electromigration effects in interconnect metallizations cause a need for materials with superior resistance against electromigration failure but with adequate electrical properties. In principle, Cu(Ag) alloys are potential candidates to become an interconnect material of the next generation of microelectronic devices. Therefore, in the following paper the electroplating of such Cu(Ag) alloys from a sulfuric acid electrolyte solution with varying silver content in a home built deposition tool is presented. Besides the general deposition characteristics, the growth mode of the films and the deposition into trenches will be discussed. The investigations show that Cu(Ag) alloys can be deposited with adequate homogeneity of the film thickness by electroplating. Furthermore, the electrical resistivity is low enough to assure a use of these films for interconnect applications. However, distinct island growth and insufficient trench filling capabilities lead to the fact that the additive composition needs to be optimized for Cu(Ag) thin film electroplating.     

Growth temperature dependence of epitaxial Gd2O3 films on Si(1 1 1)

This article reports on the epitaxy of crystalline high κ oxide Gd₂O₃ layers on Si(1 1 1) for CMOS gate application. Epitaxial Gd₂O₃ thin films have been grown by Molecular Beam Epitaxy (MBE) on Si(1 1 1) substrates between 650 and 750 °C. The structural and electrical properties were investigated depending on the growth temperature. The C–V measurements reveal that equivalent oxide thickness (EOT) equals 0.7 nm for the sample deposited at the optimal temperature of 700 °C with a relatively low leakage current of 3.6 × 10^?2 A/cm² at |V_g ? V_FB| = 1 V.     

Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films

CuIn_0.75Al_0.25Se₂ thin films prepared onto glass substrates at T_S=573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient (>10⁴ cm^?1) near the fundamental absorption edge.     

Subband structure and effective mass of relaxed and strained Ge (1 1 0) PMOSFETs

Effective mass and mobility of strained Ge (1 1 0) inversion layer in PMOSFET are studied theoretically in this paper. The strain condition considered in the calculations is the intrinsic strain resulting from growing the Ge layer on the (1 1 0) Si substrate. The quantum confinement effect resulting from the vertical effective electric field is incorporated into the k · p calculation. Various effective masses, such as quantization effective mass, m_z, density of states effective mass, m_DOS, and conductivity mass, m_C, as well as the hole mobility of strained Ge (1 1 0) inversion layer for PMOS under substrate strain and various effective electric field strengths are all investigated.     

Influence of substrate temperature to prepare (1 0 3) oriented AlN films

(1 0 3) Oriented AlN films is an attractive piezoelectric material for the applications on surface acoustic wave (SAW) and film bulk acoustic wave (FBAR) devices. As regards the SAW properties of the (1 0 3) oriented AlN films, the electromechanical coupling constant (K²) is larger than the (0 0 2) oriented AlN films. As regards the bulk acoustic wave (BAW) properties of (1 0 3) oriented AlN films, it can excite a quasi-shear mode (velocity = 5957 m/s, K² = 3.8%) that can be used for FBAR liquid sensor. In this research, the (1 0 3) oriented AlN films were successfully prepared on the silicon substrate by rf magnetron sputtering. Different temperatures (100 °C, 200 °C, 300 °C, and 400 °C) were used in this experiment process. The crystalline structure of films was determined by X-ray diffraction (XRD) and the surface microstructure was investigated by the atomic force microscope (AFM). The result exhibited the optimal substrate temperature is 300 °C. The optimal (1 0 3) oriented AlN films have the strongest XRD intensity, the smallest full width at half maximum (FWHM) value (0.6°), the largest grain size (15.8 nm) and the smoothest surface (Ra = 3.259 nm).     

Dissociation of sexithiophene on Al(1 1 1) surface

The growth of sexithiophene ultra-thin films on an atomically clean Al(1 1 1) surface has been investigated by angle-resolved ultraviolet photoemission spectroscopy. Although sexithiophene on Al(1 1 1) represents a weakly interacting system, sexithiophene molecules were found to dissociate even below room temperature, presumably via reactive sites. The mechanism is distinctly different to that claimed on the reverse system, i.e. aluminum evaporated on thiophene oligomers films, where isolated aluminum atoms were reported to covalently bond to thiophene rings and thus dramatically affect the molecule geometry and the conjugation. The effect of the insertion of an ultra-thin separator film of sexiphenyl, on the dissociation of sexithiophene was also examined.     

Comparison of the agglomeration behavior of Ag(Al) films and Ag(Au) films

It is essential to suppress agglomeration of Ag films caused by thermal treatment for their successful application as new metallization materials. Co-sputtered Ag(Al) and Ag(Au) films were investigated, with regard to their change in morphology and electrical resistivity after vacuum annealing. As a result, agglomeration of the Ag(Al) film (Al: 4.3 at.%) was not recognized even after annealing at 600 °C. However, void formation followed by de-wetting was observed for the Ag(Au) film after annealing, similar to that for a pure Ag film. The morphological change was accompanied by an increase in the resistivity of the Ag(Au) films with annealing temperature. On the other hand, the resistivity of the Ag(Al) films did not increase by annealing at temperatures from 400 to 600 °C. However, the film with the highest Al content, which was most resistive to agglomeration, had too high resistivity for use as a metallization material. By analysis of the Auger depth profile, the presence of very thin oxide layers at the surface of the film and at the interface with the substrate was confirmed for Ag(Al) films after annealing. This was considered to be the reason for the large difference in agglomeration behavior between the Ag(Au) and Ag(Al) films.     

Structural characterization of ultra-thin (0 0 1) silicon films bonded onto (0 0 1) silicon wafers:: a transmission electron microscopy study

Ultra-thin (0 0 1) silicon films (thickness less than 25 nm) directly bonded onto (0 0 1) silicon wafers have been investigated by transmission electron microscopy. Twist interfacial dislocations accommodate the twist between the two crystals, whereas tilt interfacial dislocations accommodate the tilt resulting from the residual vicinality of the initial surfaces. In low-twist angle grain boundaries, twist interfacial dislocations are dissociated and no precipitates are detected. In high-twist angle grain boundaries, there is no dissociation and a high density of silicon oxide precipitates is observed at the interface. Tilt interfacial dislocations are pinned by these precipitates, they are more mobile than precipitates. Without precipitates, their lines are straighter than those with precipitates, and this is especially when the bonded wafers are annealed at a high temperature. When no precipitates are present, tilt interfacial dislocations are associated by pairs, and we demonstrate that each tilt interfacial dislocations introduce a diatomic interfacial step at the interface.     

Synthesis of GaN nanowires by ammoniation of Ga2O3 films on Nb layer deposited on Si(1 1 1) substrates

Single-crystalline GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniation of Ga₂O₃/Nb films at 900 °C in a quartz tube. The as-synthesized GaN nanowires are confirmed to be single-crystalline GaN with wurtzite structure by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and field-emission transmission electron microscopy (FETEM); scanning electron microscopy (SEM) shows that the GaN nanowires are smooth, with diameters of about 50 nm and lengths typically up to several microns, which could provide an attractive potential for incorporation in future GaN electronic devices into Si-based large-scale integrated circuits. Finally, the growth mechanism of GaN nanowires is also briefly discussed.     

X-ray microdiffraction investigation of crystallinity and strain relaxation in Ge thin lines selectively grown on Si(0 0 1) substrates

We used X-ray microdiffraction (XRMD) to investigate the crystallinity and strain relaxation of Ge thin lines with widths of 100, 200, 500 and 1000 nm selectively grown on Si(0 0 1) substrates using a patterned SiO₂ mask by chemical vapor deposition. The variations of the strain relaxation in the line and width directions were also investigated in Ge thin lines with a width of 100 nm. After growth, crystal domains with very small tilt angles were detected in Ge lines with all four line widths. The tilt angle range was larger in thinner Ge lines. After annealing at 700 °C, the formation of a single, large domain with a specific tilt angle was detected by XRMD for Ge thin lines with widths of 100 and 200 nm. These experimental results reflect the effects of SiO₂ side walls around the Ge thin lines on crystallinity and strain relaxation of Ge.     

Atomistic model structure of the Ge(1 0 0)–GeO2 interface

An atomistic model of the Ge–GeO₂ interface has been generated through first-principle methods based on density functional theory. The interface model consists of amorphous GeO₂ connected to crystalline Ge through a substoichiometric oxide region showing regular structural parameters. Structural and electronic properties are compared to available experimental data and studied as they evolve across the Ge–GeO₂ interface.     

Electrical characterization of thiols self-assembled layers on GaP (1 1 1) structures

The experimental studies on III–V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol—CH₃(CH₂)₁₁SH) and aromatic thiol (4, 4′ tiobisbenzenthiol-S (C₆H₄SH)₂ have been deposited on the surface of GaP (1 1 1) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current–voltage (I–V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in “reverse bias” way, the I–V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and “in forward bias” way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes’ ideality factors determined from I–V characteristics are unusually high (n2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation.     

Spin dynamics in (1 1 0)-oriented quantum wells

Quantum structures of III–V semiconductors grown on (1 1 0)-oriented substrates are promising for spintronic applications because they allow us to engineer and control spin dynamics of electrons. We summarise the theoretical ideas, which are the basis for this claim and review experiments to investigate them.     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Evaluation of PZT thin films on Ag coated Si substrates

Fabrication characteristics of hybrid thin film components are investigated. Lead zirconate titanate (PZT) films, thickness 10 μm, are fabricated by using laser ablation on the Ag electrode (about 1 μm thick) which is deposited on 200 μm Si substrates by evaporation. Composition close to the target material is obtained in PZT films even in air and without substrate heating. Low surface energy in the Ag−Si system causes spheroidization of the Ag layer on the fresh Si substrate, but the surface can be modified by grinding and oxidization. Only some cavities exist at the interface. The interface between the Ag electrode and PZT layer is physically continuous, as revealed by electron microscopy. After annealing at 750°C for 2 h, the PZT layer consists of the rhombohedral perovskite phase with a fraction of the pyrochlore phase. Detrimental interdiffusion between Pb and Si occurs during annealing if the PZT thin film is directly on the Si substrate. This is retarded by the presence of the Ag layer.     

Strained single-crystal GOI (Ge on Insulator) arrays by rapid-melting growth from Si (1 1 1) micro-seeds

Liquid-phase epitaxial growth of Ge islands on insulator (GOI) using Ni-imprint-induced Si (1 1 1) micro-crystal seeds (∼1 μm?) is proposed. As a result, single-crystalline GOI (1 1 1) structures with large area (∼10 μm?) are realized. The transmission electron microscopy observations reveal no dislocation or stacking fault in the laterally grown regions. Moreover, the Raman measurements show that the tensile strain (∼0.2%) which enhances the carrier mobility is induced in the growth regions. This new method can be employed to realize the multi-functional SiGe large scale integrated circuits.     

Hydrogen bond directed assembly of oligothiophene/fullerene superstructures on Au(1 1 1)

We previously reported that a branched quaterthiophene donor chromophore functionalized with a phthalhydrazide hydrogen bonding (H-bonding) unit (MeBQPH) gives twofold more efficient bulk heterojunction organic solar cells (with C₆₀ acceptors) compared to a nearly identical donor incapable of H-bonding (MeBQPME). Here, scanning tunneling microscopy (STM) studies confirm the formation of MeBQPH trimer rosettes on Au(1 1 1) through phthalhydrazide H-bonding interactions that are sufficiently strong to compete with adsorbate/substrate interactions. The MeBQPME comparator molecule void of hydrogen bonding functionality does not similarly assemble on the metal surface. Complementary density functional theory (DFT) calculations facilitate a structural understanding of the MeBQPH donor assemblies and their strong stabilization through formation of six hydrogen bonds. STM studies also reveal the templated growth of C₆₀ on ordered MeBQPH monolayers with C₆₀ molecules preferentially occupying the threefold interstitial sites of the MeBQPH monolayer. This work supports the idea that H-bonding interactions can be used to control the morphology of organic donor–acceptor blends to potentially create efficient and stable bulk heterojunction photovoltaic devices.