Epitaxial Al Films Research Articles

Combined work function and STM study on growth, alloying and oxidation of epitaxial aluminum films on Ru(0001)

We have investigated growth, thermal alloying and initial oxidation stages of thin epitaxial Al films on Ru(0001) by work function measurements and scanning tunnelling microscopy. A characteristic break in the work function versus coverage relation, at 0.15 monolayers, can be related to a structural change in the growing film. After 3.5 layer deposition the work function saturates at the value of clean Al(111). Annealing leads to surface alloying, which is accompanied by a dramatic increase of the work function. The work function changes during oxygen uptake resemble those obtained on bulk Al(111); the stronger absolute decrease and the shift of characteristic break points to lower exposure indicate an earlier onset of oxidation, consistent with the higher defect density on the film as compared to bulk Al surfaces.

Epitaxial growth of Al films on modified AlAs(001) surfaces

In order to understand the physical principles which underlie the formation of metal/semiconductor interfaces, epitaxial Al films are grown by molecular beam epitaxy on As-stabilized and Al-stabilized AlAs(001) surfaces some of which were coated with a single monolayer (1 ML) of Ga or In. Reflection high-energy electron diffraction and x-ray diffraction measurements reveal that the growth direction and epitaxial relationship of the Al films are drastically changed by the surface modification by 1 ML Ga or In. A variety of Al film configurations are obtained, of which the Al(001)/Ga(or In)/AlAs(001) heterostructure is technologically important because it has an ideally lattice matched metal/semiconductor heterostructure. We propose structural models for three Al phases, i.e., Al(001) and two kinds of Al(110), and discuss the physical origin of the variation of Al phases. It is shown that the changes observed in the epitaxial relationships of Al films can be systematically explained in terms of bond strengths at the interface. In the growth of metals on semiconductors, interface modification is shown to be effective for controlling the growth.

Crystallography and electromigration resistance of epitaxial Al films grown on (011)Si substrates by the sputtering method

From transmission electron microscopy observations, Al films sputtered on (011)Si substrates were found to grow epitaxially, similar to those on (001)Si and (111)Si substrates. The condition for single crystallization of the epitaxial Al films was obtained by changing heat treatments and Al film thickness. In addition, a simple acceleration test revealed that the electromigration resistance of Al interconnects on a (011)Si substrate was much higher than that on a SiO2 film.

Interfacial reactions in epitaxial Al/Ti1−xAlxN (0≤x≤0.2) model diffusion-barrier structures

Transmission electron microscopy (TEM), cross-sectional TEM, scanning TEM with energy dispersive x-ray analysis, and Auger electron spectroscopy were used to investigate the nature of rate-controlling interfacial reactions in epitaxial Al/Ti1−xAlxN thin-film couples. TiN and NaCl-structure Ti1−xAlxN layers, 120 nm thick, with compositions x=0.1 and 0.2 were grown on MgO(001) substrates by ultrahigh vacuum reactive magnetron cosputter deposition in N2 discharges. Epitaxial Al films, 200-nm-thick, were then grown in Ar on top of the nitride layers during the same vacuum cycle. The reaction paths for Al/TiN and Al/(Ti,Al)N interactions during anneals at Ta=600 °C for ta up to 150 min was similar, but the extent of reaction was dramatically decreased by the substitution of (Ti,Al)N barrier layers for TiN. The primary mobile species during annealing was Ti which penetrated into the Al layers and reacted to form the ordered tetragonal intermetallic phase Al3Ti while Al segregation resulted in the formation of a narrow metastable zincblende structure AlN layer at the Al/nitride interface. The room-temperature resistivity of 1-μm-thick nitride layers varied from 15 μΩ cm for TiN to 60 μΩ cm for Ti0.8Al0.2N.

Epitaxially Grown Al Electrodes for High-Power Surface Acoustic Wave Devices

A deposition process of epitaxial Al film on a quartz substrate was studied to obtain Al electrodes of surface acoustic wave (SAW) devices having high resistance to stress migration. A vapor deposition system was used under relatively moderate deposition conditions: (1) pressure range of 10-5 Pa; (2) deposition temperature of 150°C; (3) deposition rate in the range of 120∼240 nm/min. The growth mode of epitaxial Al film differed from that of polycrystalline Al film on quartz.

Schottky barrier inhomogeneity caused by grain boundaries in epitaxial Al film formed on Si(111)

Epitaxial Al contacts on Si(111) are fabricated by electron beam evaporation at various substrate temperatures around 250 °C. They are observed by high-resolution transmission electron microscopy. Schottky barrier heights (SBHs) of the contacts are measured using current-voltage and capacitance-voltage methods. In the case of single-crystalline Al film, the SBH does not change and its spatial distribution remains homogeneous up to an annealing temperature of 550 °C. In contrast with this, for an epitaxial Al film containing grain boundaries, the spatial distribution of the SBH becomes inhomogeneous above 400 °C. This is attributed to Si diffusion along the grain boundaries in the Al film.

Single-crystal Al films grown by sputtering on (111)Si substrates

Single-crystal growth of Al films deposited by the sputtering method on (111)Si substrates was investigated in detail by transmission electron microscopy studies. A single-crystal epitaxial Al film 500 nm thick was obtained by conducting an appropriate heat treatment during and after the deposition.

Hydrogen segregation and trapping in the Al/Si(111) interface.

A quantitative measurement of hydrogen trapping in an undisturbed solid-solid interface is reported. Trapping and segregation of hydrogen was found to occur at the semicoherent interface between Si(111) substrates and 1500-\AA{}-thick epitaxial Al films. Ion-beam-analysis techniques were used to measure the depth profile of the trapped hydrogen. Up to 1.7\ifmmode\times\else\texttimes\fi{}${10}^{16}$ H atoms/${\mathrm{cm}}^{2}$ were found to be trapped in the interface. The hydrogen was introduced by 500-eV ion implantation into the surface of the Al layer to avoid any radiation damage at the interface. The amount of trapped hydrogen was found to increase with implantation dose, without saturating. The thickness of the hydrogen-containing layer was less than 13 nm. In ramp annealing at least 70% of the trapped hydrogen was released from the interface in an abrupt stage near 390 K; a smaller release stage occurred at 150 K. The detrapped hydrogen was found to have left the sample. The annealing in the stage at 390 K obeyed first-order kinetics with an activation energy of 0.96\ifmmode\pm\else\textpm\fi{}0.06 eV. Analysis of the annealing data was used to derive the binding enthalpy and entropy of H in the interface relative to solution in the Al as 0.53\ifmmode\pm\else\textpm\fi{}0.06 eV and (-4.0\ifmmode\pm\else\textpm\fi{}1.9)k.

Large-misfit heteroepitaxy of aluminum films by ICB deposition

Epitaxial Al films were deposited on Ge substrates at room temperature by the ionized-cluster beam (ICB) method. On Ge(111), the Al grows with its lattice parallel to the substrate lattice. It is considered that the lattice misfit is accommodated by a lattice coincidence of seven to five match of Al and Ge atoms. On Ge(100), the Al lattice is rotated by 45° from the substrate lattice in the (100) plane. This orientation results in a matching of Al{010} and Ge{011} plane spacings. The same orientation is found for the Al epitaxy on GaAs(100). These results are discussed with reference to the Al epitaxy on Si substrates. The results of Al heteroepitaxy on other substrates are summarized.

Recent progress in depositing epitaxial metal films by an ionized cluster beam

Epitaxial Al films have been deposited by ICB equipment on room temperature Si substrates. The equipment available for forming Al epitaxial and single crystal films has been discussed. Epitaxial Al films can also be deposited on various semiconductor and insulator substrates such as Ge, GaAs, CaF 2 and sapphire at room temperature. Application of epitaxial Al film to VLSI has been proposed. Deposited Al films are thermally and electrically stable. The prospect of VLSI application of epitaxial Al films is shown.

Ionized cluster beam deposition and epitaxy of metal films on large lattice misfit substrates

This paper reviews the metal film formation by ionized cluster beam (ICB) technique on various kinds of substrates. The ICB enables heteroepitaxy of metal films for lattice misfit larger than 25%. The film growth process was studied by in situ MEED, AES and XPS analyses. The films were also examined by ex situ electron diffraction and atomic resolution TEM. On Si(111), single crystal Al film was formed. The Al deposited on Si(100) formed bicrystal structure. The film-substrate interface and the bicrystal grain boundary were very sharp and had little distortion of atomic arrangement. This explains the high thermal stability of the metal films deposited by ICB. Epitaxial Al films were also formed on CaF2, Ge, GaAs, and sapphire substrates. In the film formation of Cu and Au on Si substrates, ICB was very effective to suppress the reaction and interdiffusion of film and substrate. These results indicate that the ICB technique is effective for the formation of high quality metal films and stable metal-semiconductor interfaces. These films are promising for semiconductor metallization, laser mirrors, and x-ray mirrors for advanced applications.

Pinning of Misfit Dislocations in film Growth Studied by Grazing Incidence X-ray Scattering

ABSTRACTUsing grazing incidence x-ray scattering, we have studied incommensurate structures of Pb adlayers on Cu(110) surface and epitaxial Al films grown on Si(111) surface. Similar diffuse scattering profiles were found in both cases, which can be fitted with a Gaussian-plus-Lorentzian lineshape. The results are attributed to a overlayer structure with pinned misfit dislocations at the interface.

Epitaxial Al(110) Films Grown On Heavily-Doped Si(100) by Cluster Beam Deposition.

ABSTRACT:Epitaxial Al films have been grown on heavily doped Si(100) by neutral cluster beam deposition. The Al films were evaluated by RBS, RHEED, optical interference roughness maps and STM imaging. The results were consistent with the formation of epitaxial Al (110) bi-crystals similar to films grown by ionized cluster beam deposition on lightly-doped Si

Epitaxial growth of Al(100) on Si(100) by gas-temperature-controlled chemical vapor deposition

Epitaxial Al(100) film was deposited on Si(100) by low-pressure chemical vapor deposition with the use of tri-isobutylaluminum (TIBA) at the substrate temperature of 380–400 °C with the deposition rate of 400 nm/min. For the epitaxial film growth, it was concluded that preheating the TIBA gas just before the incidence onto the substrate was very important (gas-temperature-controlling). The specular reflectance of the 1-μm-thick aluminum film on silicon substrate was the same level as that of the sputtered film. The x-ray diffraction peak was very narrow and only Al(100) orientation was observed. The transmission electron diffraction (TED) patterns of Al(100) films showed clear spots. Using disilane, we could suppress the diffusion of substrate silicon into the aluminum film.

Growth processes of epitaxial metal films on semiconductor and insulator substrates by ionized cluster beam

Ionized cluster beam (ICB) deposition allows one to grow thermally and electrically stable films on semiconductor substrates. One of the unique characteristics of ICB deposition is that the epitaxial metal films can be grown at room temperature on a substrate with lattice misfits larger than 25%. This is possible because of the flexible control of the film growth process by ICB bombardment. Our investigations on the nucleation and growth processes at the early stage of ICB deposition showed that the ICB bombardment increases the density of the nucleation sites and enhances adatom migration. The special film formation mechanisms by ICB lead to the transformation of the growth morphology from three-dimensional growth to layer-by-layer growth, and enable the formation of a stable metallic film at an early stage of deposition. As a consequence, the epitaxial metal films deposited by ICB have a unique and stable structure and exhibit excellent thermal stability. These features were clarified by detailed RMEED, AES, XPS and TEM analyses of the epitaxial Al, Au and Cu films deposited on Si substrates.

Channeling study of epitaxial Al and Ag films on Si(111) substrates

Al films formed by the ion cluster beam deposition (ICBD) and Ag films formed by molecular beam epitaxy deposition on Si(111) substrates were studied by MeV ion channeling techniques. Both the Al and Ag films were found to be epitaxial despite their large lattice mismatch to Si substrates (both are close to 25%). Both the Al and Ag films were found to contain dislocation loops, which is attributed to the large lattice mismatch. Displaced Al atoms in the Al film but no displaced Ag atoms in the Ag films were found. The existence of displaced Al atoms is attributed to the bombardments of ion or ionized clusters in ICBD. A large step increase in dechanneling at the Al/Si interface was observed, which decreases with the increasing incident energy. The existence of a “semicoherent” interface in which four Al lattice planes match three Si lattice planes almost perfectly, cannot explain the energy dependence of the step increase. The thickness of the Ag films was found to have an obvious effect on the epitaxial growth. A misoriented growth of the Ag film was also observed.

Hydrogen Segregation at the Al/Si Interface Studied Using a Nuclear Resonant Reaction

AbstractHydrogen segregation at the interface between an epitaxial Al film and a Si (111) substrate is studied using the 1H(15N, αγ)12C nuclear resonant reaction. Hydrogen depth profiles show that H atoms diffuse through the 1600 Å thick Al layer during 500 eV H implantation and are trapped at the Al/Si interface. The total amount of interface H is about 2 × 1015 /cm2 after a 1.4 × 1018 H/cm2 implantation, and the H atoms are narrowly distributed in the direction normal to the interface. During an isothermal anneal at 360 K, the amount of interface H decreases exponentially with annealing time; and during ramp annealing from 110 to 500 K, an abrupt release of the interface H is observed at temperature around 380 K. The release rates in both cases are controlled by a first order thermally activated de-trapping process with a binding energy of 0.86 eV/atom.

Epitaxial ordered-alloy formation through surface reactions: Al on Ni{0 0 1}

We have studied the products of vacuum deposition of Al on Ni{0 0 1} to compare with Au on Cu{0 0 1}, which is known to develop a surface alloy. Slow vacuum deposition of Al onto unheated Ni{0 0 1} produces partially-ordered one- and two-layer-thin epitaxial Al films. These films have larger interlayer spacing (by about 15%) than the Ni substrate and a substantial disordered component. Anneals of the films, or slow deposition of Al onto a hot Ni{0 0 1} substrate, produce well-crystallized epitaxial films of Ni 3Al{0 0 1}. The alloy is not confined to just the surface layer, hence it is not a surface alloy. The thickness of the alloy films can be controlled via the deposition rate of Al and/or the temperature of the Ni substrate. Deposition of Ni on Al{0 0 1}; does not result in ordered alloy formation.

Epitaxial growth of Al(111)/Si(111) films using partially ionized beam deposition

We observed the growth of epitaxial Al(111) films on Si(111) at room temperature by the partially ionized beam deposition technique. The films were deposited in a conventional vacuum condition without in situ cleaning. The beam contained 0.3% of Al self-ions and a bias potential of 1 kV was applied to the substrate during deposition. X-ray diffraction (pole figure) revealed that one of the two possible twin structures, with the Al〈1̄10〉∥Si〈1̄10〉 orientation, was preferentially grown on the Si substrate.

Metallization by ionized cluster beam deposition

Preferentially oriented and epitaxial Al films have been deposited by ionized cluster beams (ICB). The thermal stability of these films has been examined by SEM, AES, ion backscattering, and electrical characterization. In preferentially oriented films, long electromigration lifetime and low-temperature contact formation are expected. In epitaxial films on silicon, alloy penetration at the interface, a shift in barrier height, and degradation of crystalline quality were not observed after heat treatment up to 550 °C. Extremely long electromigration lifetime was also confirmed. Epitaxial growth Of Al film on CaF 2 , GaAs, and sapphire was attempted and preliminary results are given.