J. Henz

Epitaxy of fluorite-structure silicides : metastable cubic FeSi2 on Si(111)

Epitaxial FeSi2 of cubic symmetry has been grown for the first time on Si(111) by solid phase epitaxy (SPE) and molecular beam epitaxy (MBE). Structural investigation by reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) suggest the phase to have the same CaF2 structure as CoSi2 and NiSi2. Ultraviolet photoelectron spectroscopy (UPS) and resistivity measurements show that the phase is metallic and undergoes an irreversible metal/semiconductor transition to the orthorhombic β-FeSi2 phase at a temperature depending on the thickness of the film. Hall effect measurements have revealed holes to be the dominant carrier species. High resolution TEM studies, together with the observation of an anomalous Hall effect as well as theoretical considerations, indicate that this cubic FeSi2 phase is magnetic, in contrast to CoSi2 and NiSi2.

Epitaxy of metal silicides

Abstract Epitaxial growth of the silicon-rich metal silicides NiSi 2 , CoSi 2 and IrSi 3 is reviewed. Codeposition of metals and silicon at low substrate temperatures is shown to alleviate the main problem associated with solid phase epitaxy using metal deposition alone, i.e. the nucleation controlled reaction to the silicide in question. NiSi 2 is an exception since owing to an epitaxial precursor phase the interfacial energies are lowered sufficiently, so that for thin films nucleation is no longer a problem. The growth of high quality CoSi 2 /Si superlattices is shown to be feasible by combining the codeposition technique with silicon MBE. Excellent electrical properties of epitaxial CoSi 2 films is demonstrated, remaining metallic down to a thickness of 10 A. Surface scattering in Si/CoSi 2 /Si heterostructures is nearly eliminated, CoSi 2 thicker than 35 A showing bulk residual resistivities. Such heterostructures have been applied to the fabrication of the permeable base transistor (PBT), using low pressure vapour phase epitaxy for the last overgrowth step after patterning. Device performance is found to be superior to any overgrown silicon PBT reported to date.

Epitaxial silicides with the fluorite structure

Abstract The growth of epitaxial silicides by UHV deposition techniques is reviewed. The discussion is focussed on metallic silicides with fluorite structure, i.e. NiSi2 and CoSi2, and a recently discovered metastable γ-FeSi2 phase, which appears to be isostructural. The role of the microscopic interface structure is emphasized as it may severely influence the growth process itself as well as the structural and physical properties of the films.

On the growth of CoSi2 and CoSi2/Si heterostructures on Si(111)

Abstract Thin epitaxial, perfectly smooth and pinhole-free CoSi 2 layers of type B orientation could be grown on top of Si (111) wafers for the first time, by using a novel solid phase epitaxy (SPE) technique. Due to the high perfection of these layers and a lowering of the substrate temperature during subsequent Si-MBE, single type A Si overgrowth was achieved. This represents the basis for the fabrication of Si/CoSi 2 /Si superstructures.

Growth and Properties of Epitaxial Silicides on Si(III)

The Schottky-barrier heights B of epitaxial NiSi2 and CoSi2 layers on Si(111) have been measured as a function of growth conditions. For type B NiSi2 interfaces both I-V and photoelectric methods yield a high B = 0.75 ± 0.03 eV for layers grown below 400°C, dropping below 0.7 eV for layers annealed above 550°C or grown by MBE above 550°C. B of type A interfaces is always found to be low, i.e., B = 0.62 ± 0.03 eV. Pinhole-free ultrathin (14-70 A) CoSi2 layers could be grown for the first time by using a novel solid phase epitaxy (SPE) technique, in which Co and Si are coevaporated in their stoichiometric ratio near room temperature (R.T.). These layers become perfectly epitaxial with type B orientation upon annealing to only 250°C and remain continuous up to Ts ≈ 600°C. From photoelectric measurements CoSi2 layers grown at Ts 1000 A) requires ramping up of Ts to the usual growth temperatures for Si MBE. Without any optimisation preliminary experiments indicate, that single type growth of Si can be achieved. This is determined by ion channeling of an 1150 A thick Si layer on 30 A of CoSi2 yielding minimum yields of 17% for the [111] and [110] directions and 40% for the [114] direction.

Growth of epitaxial ultrathin continuous CoSi2 layers on Si(111)

Perfectly smooth, pinhole-free and epitaxial CoSi 2 layers of type B orientation have been grown on Si(111) with a thickness ranging from 14 to 70 a. The outstanding perfection of these ultrathin films was achieved by a new evaporation method, which consists in the coevaporation of Co and Si at room temperature and in a succeeding anneal. If the evaporation rates are exactly stoichiometric, an ordered structure can be detected by RHEED already at room temperature, whereas upon annealing to only 250°C a remarkable sharp Kikuchi pattern is obtained. XPS measurements of the, valence band confirm that the phase is indeed CoSi 2 . Films grown in such a way, retain their quality up to a temperature of 600°C. To let thicker films grow, we use MBE at substrate temperatures which are as low as 300°C.

Investigation of the defect structure of thin single‐crystalline CoSi2 (B) films on Si(111) by transmission electron microscopy

Thin epitaxial single‐crystalline B‐type CoSi2 films (twin‐oriented) have been grown in ultrahigh vacuum by stoichiometric codeposition of Co and Si on slightly misoriented (0.1°–0.3°) Si(111) substrates. The microstructure as well as the nature of interfacial defects has been investigated in detail by transmission electron microscopy. The defect structure is found to depend closely on the initial deposition parameters, annealing temperature, and the topography of the Si substrate. It will be shown that even during the early stages of layer growth, loss of coherence is obtained and lattice strain already starts to occur with the introduction of misfit dislocations with Burgers vector b=a/2〈110〉 inclined to the interface or with Burgers vector b=a/6〈112〉 parallel to it. It is demonstrated that ultrathin CoSi2 films with thickness of about 1 nm grown on slightly misoriented substrates with parallel surface steps, exhibit quite different defect structures at annealing temperatures between 300 °C and 550 °C. ...

Near‐infrared optical properties of CoSi2 thin films

Transmission and reflection measurements in the energy range 0.5–1 eV have been performed on thin CoSi2 films grown by molecular‐beam epitaxy on Si(111). For film thicknesses above 200 A, the transmission factor decreases exponentially with film thickness, with an optical attenuation length of 180 A nearly independent of photon energy. Deviations from this law for film thicknesses below 200 A are explained by reflection effects. These data, supplemented by reflection measurements, can be fitted by theoretical calculations of the transmission and reflection factors, thus leading to the determination of the CoSi2 optical indexes. The energy dependence of the real and imaginary indexes is found to be consistent with the Drude model. Some interesting aspects of this intraband absorption either in ultrathin films or at low temperature are presented.

2p-core level spectroscopy of La1.85Sr0.15CuO4, CuO and NaCuO2: Mixed valency in the superconductor

Abstract We have measured with a VG ESCALAB machine the 2p-core level XPS spectra of the high T c superconductor La 1.85 Sr 0.15 CuO 4 , of divalent CuO and of trivalent NaCuO 2 reference substances. The half width of the 2p 3/2 line is in the superconductor with 3.6 eV appreciably larger than in CuO with 3.1 eV. Together with the 2p 3/2 line of NaCuO 2 a degree of valence mixing of about 15% Cu 3+ has been derived for the superconductor.

On the stability of thin epitaxial NiSi2 layers on Si (111)

Abstract By repeated deposition of several A of Ni below 100 °C and subsequent annealing to typically 350 °C, thin continuous NiSi 2 -layers have been grown epitaxially on Si (111). Thicknesses exceeding ∼- 70 A require a different procedure due to the increasing importance of lateral growth, spoiling the layer quality. We show that MBE at substrate temperatures above 500 °C is not a viable technique to increase the thickness of the ultrathin layers. The reason is found to lie in the insufficient stability of the NiSi 2 templates, disintegrating into islands at temperatures above 500 °C. Perfectly smooth layers up to 1000 A have, however, been grown by a new method in which alternate layers of Ni and Si (typically 1 A and 4 A respectively) are deposited onto the initial template at substrate temperatures between 350 °C and 380 °C.