The microstructure of the Pt silicide formed on clean Si surfaces has been investigated using transmission electron microscopy. Pt up to 200Å was evaporated on atomically clean Si (100) and (111) substrates under an ultrahigh vacuum condition. The silicide was formed by annealing up to 600°C for (100) substrates in a purified He atmosphere and in-situ UHV for (111) substrates. For the (100) substrate, as-deposited Pt showed a fine polycrystalline structure with grain size of about 10tm. Upon annealing at 250° to 300°C, formation of Pt silicide was observed, which was primarily PtSi with only a small amount of Pt2Si. The silicide coverage was incomplete below about 1.5nm. Upon further annealing, the fraction of PtSi increased although Pt2Si persisted until 400°C. At 600°C, PtSi showed an epitaxial relationship with its c axis perpendicular to the Si (100) surface. On the Si (111) surface, PtSi formed epitaxially above 400°C. The silicide structure showed a multidiffraction pattern with three-fold symmetry, reflecting the three equivalent but strained epitaxial orientations. A high resolution lattice image technique was used to investigate the details of the epitaxial structures of PtSi on Si (100) and (111) substrates.

Isothermal annealing, two step annealing and ion beam mixing were performed to induce interfacial reactions between iron thin films and silicon substrate. Both orthorhombic and tetragonal FeSi2 were found to grow epitaxially on (lll)Si with orthorhombic FeSi2 being the predominant phase. No epitaxial growth of FeSi2 on (001)Si was detected. Epitaxial islands as large as 40 μm in size were formed by a scheme combining ion beam mixing and two step annealing.

Epitaxial growth of NiSi2 on (011)Si was investigated by the transmission electron microscopy. Both epitaxial and twin related NiSi2 were formed, with the former being predominant, in samples annealed at 850°C for 1/2 h. The silicides were faceted with {111} interfaces being more prominent than {100} interfaces. Interfacial dislocations were found to be of edge type with 1/6<112> Burgers vectors. The average spacing is about 700 Å which is very close to the theoretically expected value 670 Å.

The absence of twins formed on (111) and (111) planes for (011) samples as well as the results obtained for (001) and (111) specimens suggest that there exists a critical resolved shear stress at the interface for the initiation of the formation of twin related NiSi2.

High quality superlattices can be grown from semiconductor materials with lattice mismatches of several percent if the layers are kept sufficiently thin. The mismatch in these structures is accommodated by coherent layer strains, so that misfit defects are not generated at the superlattice interfaces. These strained-layer superlattices have unique structural, optical, and electrical properties due to both the large layer strains and the flexibility in the choice of lattice mismatched layer materials. Recent results which demonstrate the high crystalline quality of these mismatched heterostructures are reviewed. Further results are presented which illustrate some of the unique material properties of these structures, and potential applications associated with such properties are discussed.

A review is given of the channeling behavior observed in superlattice materials. Elastic strains within the layers have been shown to be the source of the unusual ion dechanneling observed originally in InAs/GaSb and since observed in several other semiconductor superlattices. These strains occur because of the slight mismatch between the lattice constants of the two materials and cause the rows and planes of atoms in the crystal that are inclined to the surface normal to have small directional misalignments at the interfaces between the layers. The effects of these misalignments have been studied by computer simulations of ion trajectories in the crystal and shown to account in a reasonable manner for the observed dechanneling behavior. Calculations have been done as a function of the amount of misalignment to give an indication of how much dechanneling might be expected in other superlattices formed of any pair of materials that have slight mismatches. Various ways of measuring the amount of misalignment have also been studied by simulations and experiments.

Axial ion channeling and backscattering with 2 MeV 4He are used to probe the strain in GaAs. 16P.84/GaP strained-layer superlattices (SLS) by two methods. The tetragonal distortions in the growth direction of SLS's give rise to alternating tilts in inclined crystal directions at each interface. In the first method the large increase in dechanneling along <110> directions containing the angular tilts is measured relative to the negligible increase in dechanneling along the <100> growth direction. This method provides a sensitive depth-dependent measure of strain but requires computer simulation for quantitative analysis. In the second method, channeling angular scans are used to directly measure the lattice strain by the shift in the <110> crystal direction in the top layer relative to the average <110> direction. These results demonstrate the validity of the strain accommodation model for the interpretation of the dechanneling and indicate that GaAsxP1-x/GaP SLS structures of high crystalline quality can be obtained with strains somewhat lower than that predicted theoretically from bulk lattice properties.

Recent studies of the resonance effect between the wavelength of planar channeled ions and the periodicity of strained-layer superlattices are summarized for MeV 4He ions backscattering from GaAsxP1-X/GaP superlattices. When the incident beam energy is adjusted so the half-wavelength matches the thickness of the layers, a catastrophic dechanneling effect is observed. A simple phase rotation analysis has been developed to calculate the dechanneling in each layer and good agreement is obtained with the observed dechanneling per interface. This resonance effect is used to measure the amount of strain in the GaAsxP1-x/GaP strained-layer superlattice.

We have studied the interface and the lattice strain of superlattices by ion channeling technique. The objective in this work is to verify the existence of alternating tensile and compressive strain in the superlattice and to develop a method for measuring the lattice strain directly. Alternating layers of GaSb/Al Sb were grown epitaxially by MBE with 10 periods. The thickness of each individual layer is 30 nm. Channeling measurements and analysis were made using a 1.76 MeV 4He ion beam. The measurements reveal higher dechanneling along the [110] axis than along the [100] axis. This is consistent with the dechanneling results published earlier. The high dechanneling along the [110] axis has been considered due to the lattice strain that occurs in the layers caused by the slight mismatch between the lattice constants of the two materials. The strain effect make [110] axis slightly bent from layer to layer (“zigzag”), but it does not occur in [100] axis. The axial angular scan analyses were made around the [110] direction at the different depths using a movable energy window setting. We have found that the angular position of the best alignment shifts from layer to layer. The oscillation of those angular positions with depth is of a direct evidence of the existence of alternating tension and compression strain layers in the superlattice. The “kink” angle at the interface is given by the difference of the angular position between the first and second layer. This is found to be 0.17° ± 0.03 %. This is in a good agreement with the result calculated from elasticity. Preliminary result of this experiment is recently published.3

We are also investigating the interface and lattice strain by planar angular scan across the (110) plane at a position three degrees from [110] axis.

The similar oscillatory results have been found for {110} planar channeling and the “kink” angle measurement is in a good agreement with the results from axial angular scan.

We believe that the method of ion beam channeling and angular scan is very effective in strain measurements in multi-layered heteroexpitaxy system.

3. W. K. Chu, C. K. Pan and C.-A. Chang, Phys. Rev. Rapid Communication B28, 4033 (1983).

The tetragonal distortion present in both VPE and LPE In1-xGaxAs was measured from X-ray rocking curves of the (511) reflection. These measurements were then used to establish coherency limits. In addition, wafer curvature was measured and compared to that calculated via an expression which incorporates both tetragonal distortion and misfit dislocations. Agreement was found except for a case in which stacking faults were present.

Structure of GexSi1-x alloy films grown on (100) Si by molecular beam epitaxy is analyzed by MeV He+ ion channeling and X-ray diffraction as functions of Ge concentration, film thickness and growth temperature. Critical thicknesses for pseudomorphic growth are determined for x ≤ 0.5, where coherent tetragonally-strained layers are observed. The average strain decreases approximately as the square-root of thickness when the critical thickness is exceeded. At temperatures near the threshold for islanding growth, surface roughness appears as a precursor to degradation of strained-layer epitaxy. No effect on the amount of the tetragonal strain was found in a study of ion-beam damage.

We observe that the formation of low angle grain boundaries (sub-boundaries) depends strongly on the thickness of the recrystallized Si film. The average lateral spacing between adjacent sub-boundaries increases from 40 μm for 4000Å films to 500 μm for unseeded Si films 30 μm thick. For seeded 30 μm Si films on 1.6 mm by 1.6 mm buried oxide islands, areas exceeding 1.0 mm by 1.0 mm have been recrystallized which are free of all sub-boundaries, but which contain dislocations in other configurations.

Lateral solid phase epitaxy (L-SPE) of amorphous Si (a-Si) films vacuum-evaporated on Si substrates with SiO2 patterns has been investigated, in which the film first grows vertically in the regions directly contacted to the Si substrates and then grows laterally onto SiO2 patterns. It has been found from transmission electron microscopy and Nomarski optical microscopy that use of dense a-Si films, which are formed by evaporation on heated substrates and subsequent amorphization by Si+ ion implantation, is essentially important for L-SPE. The maximum L-SPE length of 5–6μm was obtained along the <010> direction after 10hourannealing at 600°C. The kinetics of the L-SPE growth has also been investigated.

The growth of large secondary grains with (112) texture, and solid-state agglomeration to single-crystal islands have been observed by annealing ultra-thin (less than 1000Å) films of Ge. A driving force proportional to surface-energy anisotropy and inversely proportional to film thickness is believed to be responsible for both phenomena. The temperature for agglomeration decreases with film thickness, and is further depressed by the presence of Sn vapor. Patterning Ge into stripes increases secondary grain size and population. Encapsulation with a film of SiO2 suppresses agglomeration and alters crystallographic texture. A surface-relief structure of 0.2μm period and 300Å depth induces a (100) texture in some cases, and alters the morphology of agglomerated islands.

Subsurface dielectric layers in silicon formed by oxygen ion implantation have been the subject of various investigations, as an attractive alternative to building devices and integrated circuits on insulating substrates. More recently, the methods of high resolution analytical electron microscopy (HRAEM) have been applied to the analysi off these buried dielectric layers with significant results.(1,2 When oxygen is implanted into silicon, the interface between the amorphous oxide and the silicon is sharp on an atomic scale, but islands of silicon exist in the oxide for some distance from the interface. The effect is ion implantation condition dependent. Dielectric layers formed by nitrogen ion implantations are expected to be similar to those formed by oxygen implantationbut offer advantages from the materials science viewpoint in terms of thermodynamic stability and mechanical strength.(3,4In this paper we report observations of the structure of silicon after two different implantations conditions which are the first part of an investigation to determine effect of implantation variables on microstructure.

A structure of interface between crystalline Si (c-Si) and underlying SiO2 film formed by Si-on-Insulator technique has been analyzed by modeling of interface atomic arrangement. A ball-and-spoke model of a stoichiometrically abrupt c-Si/SiO2 interface has been constructed by connecting a (100) c-Si lattice and a continuous random network model of amorphous SiO2 . A Keating-type potential has been used for the interatomic interactions. The bond bending distortion energy of both Si and O atoms increases at the interface, while the bond stretching energy is negligibly small. The amount of interface energy due to bond distortion is 0.20J/m2.

Buried oxide layers, formed by high dose ion implantation, have been examined using Auger depth profiling. The phase distribution of oxygen in the wings of the implant profile and effects of high temperature annealing, have been investigated. Ion beam induced cascade mixing, which occurs during sputter etching, limits the minimum detectable size of SiO2 precipitates. However, it is possible to minimise this effect by reducing the ion beam energy. At very small precipitate sizes (<100Å) silicon atoms at the edge of the precipitate particles will make a major contribution to the Si KLL Auger spectrum. A previously reported theoretical model has been expanded to take account of this phenomenon.

An integral part of very large scale integrated (VLSI) circuits is the multilayer structures for electrical interconnection and insulation. Many conducting materials are used for interconnection including polysilicon, silicon, silicides, polycides and metals. An important point in considering these materials is the interconnection between them and the corresponding characterization of the interface by way of the specific contact resistance, which directly affects the interfacial contact resistance.

For a planar ohmic contact formed between a metal and any layer with a much larger sheet resistance (for example single crystal silicon) a technique based on the transmission line model provides a method of characterizing these contacts. However, for planar contacts between layers with comparable sheet resistivities for example polysilicon to single crystal silicon this technique must be modified. In this paper we review the transmission line approach used to obtain the specific contact resistance between such layers and provide initial results of measurements made on the poly to single crystal interface. We also present a series of test structures, currently under fabrication that will provide more detailed experimental data.

Scanning electron microscopy and energy dispersive x-ray analysis have been used to study the formation of AuGe ohmic contacts to GaAs. Both sintering and alloying of the metallization result in deep trenches and spikes. Lateral growth of the metallization at the perimeter of the contact is also reported. In situ analysis during the formation of the ohmic contact has demonstrated the formation of a solid phase complex of Au-Ge-As and the rapid loss of As at 450°C. The influence of the Au-Ge-As complex and the loss of As on the specific contact resistivity is discussed.

Ohmic contacts to GaAs have been studied using a high resolution TEM, an SEM and a STEM with an energy dispersive x-ray attachment. Two different deposition sequences of the constituent Au-Ge-Ni metals yielded specific contact resistivities that varied by one order of magnitude. Crosssection images of the interface between the GaAs and the metal Au-Ge-Ni layers following alloying showed protrusions at the interface. Contacts with low specific resistivities showed deeper protrusion and also significantly more Ge and Ni in the GaAs.

The rf glow discharge decomposition of dilute mixtures (∼1%) of SiH4 in H2, with suitable doping agents such as PH3 or B2 H6 added, produces n+ and p+ microcrystalline (μc) silicon films with resistivities (ρ) in the 10−1 Ω.cm range at exceedingly low substrate temperatures Ts (150–250°C). The μc-Si films consist of two phases with crystalline clusters embedded in an amorphous matrix. The films are stable when temperature-cycled in the 30–400°C range and the resistivity decreases when annealed above the original deposition temperature. A model of microcrystalline film growth involving the reactions between the gas phase radicals and the growing film surface is proposed.