T.A. Nguyen Tan

Heteroepitaxy of metallic and semiconducting silicides on silicon

Abstract Recent developments in the epitaxial growth of metallic and semiconducting silicides on silicon are reviewed. The structural, electronic and electrical properties of these silicide-silicon interfaces are examined with the aid of results obtained with a large variety of in-situ and ex-situ surface techniques. The paper will focus on two topics: (i) recent progress on the epitaxial growth of thin metallic CoSi 2 films on Si(111), (ii) epitaxial growth of thin semiconducting β-FeSi 2 films on Si(111), (100) and (100) vicinal faces.

The influence of steps on the epitaxial growth of iron-silicide on Si(001)

Abstract Iron deposition up to ~ 25 A on planar and stepped Si(001) surfaces was studied by LEED, AES, UPS, and ELS measurements. On both surfaces iron nucleation is of 3D type with notable interdiffusion at room temperature. Annealing of the overlayers up to 800 K enhances the interdiffusion and above 800 K semiconducting FeSi 2 is formed on both surfaces, unstructured on the planar and epitaxially on the stepped one.

Oxidation of thin erbium and erbium silicide overlayers in contact with silicon oxide films thermally grown on silicon

Pure Er and co-evaporated Er and Si layers were deposited near room temperature in UHV on SiO2 films grown on Si(100) wafers and were subsequently annealed at increasing temperature up to 1153 K. The samples were characterized in situ by X-ray photoelectron spectroscopy following deposition and each annealing step. The co-evaporated samples were also post-examined by Rutherford backscattering spectroscopy after the final annealing. The results show that both the Er and the ErSix adlayers react readily with the SiO2 upon increasing temperature to give Er2O3, silicon suboxides and elemental silicon. The erbium oxide remains stable up to 1073 K and then transforms back to erbium silicide with a simultaneous loss of oxygen from the surface via the volatile SiO. This behavior is rationalized in terms of a number of solid phase reactions taking place in the overlayer.

SEMICONDUCTING RHENIUM SILICIDE THIN FILMS ON SI(111)

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 A) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x‐ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 A) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 A) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600 °C. In the Re‐Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2p and Re 4f) are the same in the compound and in the elements. As the ener...

On the formation of molybdenum silicides in MoSi multilayers: the effect of Mo thickness and annealing temperature

Abstract The present work deals with the formation of molybdenum silicides in all-sputtered a-Si/Mo/a-Si/Mo/a-Si/c-Si(100) multilayers. The structure were prepared without breaking the vacuum and the substrates were not intentionally heated during the sputtering. After deposition, the multilayers were annealed at 500 or 700°C. Grazing incidence X-ray diffraction and XPS techniques were used to characterize the Mo/a-Si and a-Si/Mo structures first and then the a-Si/Mo/a-Si/Mo/a-Si/c-Si(100) multilayers. The essential results can be summarized as follows: 1. (1) The nature of the interface formed in the unannealed structures depends strongly on the order of the deposits: if Si is sputtered onto sputtered Mo films, the a-Si/Mo interface is found to be abrupt while in the reverse order the deposit leads to a diffuse Mo/a-Si interface. 2. (2) The thermal treatment has a significant effect only if the temperature is of the order of 700°C. In this case, the result depends on the a-Si and Mo thickness: for thickness lower than ≈200 A only MoSi2 is formed, with predominance of the hexagonal phase, while for thicknesses greater than ≈200°C, we simultaneously observe the tetragonal phases MoSi2 and Mo5Si3.

Interfacial reaction between Ta ultrathin films and Si(111) substrate

We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7×7) surface under ultrahigh vacuum conditions. At room temperature, a dosordered chemisorbed phase is formed at low coverage, θ≲1 monolayer (ML), and is characterized by an UPS Ta 5d peak shifted at about −1.2 eV as compared to bulk Ta. For 1≲θ≲4 ML, although AES and XPS indicate a 1 Ta:2 Si composition, the valence band spectra are still different from the TaSi2 one. At higher coverage, the electronic structure of polycrystalline Ta is progressively recovered. Annealing of the deposits leads to silicide formation by interdiffusion and surface atomic ordering at temperatures from 650°C to 850°C, depending on the thickness. All the deposits in the 0.8–100 ML range, including the smallest ones, are stable until ∼500°C and the silicide formation reaction begins only at higher temperature, reflecting an activation energy barrier. For θ≲5 ML, clustering of the TaSi2 phase occurs. For thicker deposits, continuous TaSi2 overlayers are obtained. The electronic structure of this silicide is discussed in relation with existing models.

Structural and optical properties of r.f.-sputtered SixC1 − x:O films

Abstract Oxygenated silicon-carbon alloy (Si x C 1 − x :O) films have been prepared by reactive r.f. sputtering. These films were deposited, in a gas mixture of argon and oxygen, from a composite target consisting of a silicon disc on which graphite chips had been placed. The structural and optical properties have been investigated using infrared (IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and visible-near-infrared optical absorption. XPS as well as IR measurements show that the films are essentially composed of silicon, carbon and oxygen as expected. The atomic fractions of these elements, deduced from quantitative XPS analysis, are of the order of 50%, 28% and 22% for Si, C and O atoms respectively. A study of the chemical shifts of C 1 s and Si 2p core level spectra reveals that the films contain not only Si atoms bound to C and O but also free silicon and free carbon clusters. The Raman spectroscopy results confirm the presence of free silicon and indicate that the films are amorphous. The optical gap of these films is around 1.3 eV and the refractive index is found to follow the Sellmeier law with a value of about 2.05 at λ = 2 μm.

The influence of growth techniques on the structure of epitaxial ErSi1.7 on Si(111)

Abstract The very low lattice mismatch between the ErSi1.7(0001) and Si(111) allows a convenient epitaxial growth of the former on silicon substrates. However, the morphology of the films formed is found to depend greatly on the epitaxy techniques used. We have thus undertaken a study of the different possible techniques (single metal deposition, alternate evaporation and co-evaporation) so as to determine the procedure which gives silicide thin films of good quality. Best results are obtained by co-evaporation: the films are free of pinholes and present a flat surface. The formation of the silicide films was followed in-situ by LEED and XPS. The ErSi1.7 silicide is characterized by a sharp ( 3 × 3 )R30° LEED pattern which always appears after annealing at temperatures ⩾ 650°C whatever deposition technique is used. The Si KLL spectra, on the other hand, are found very appropriate to detect the presence of holes. The texture of the films was studied ex-situ by SEM. Glancing incidence X-ray diffraction and SIMS have also been used to get insights into the crystalline parameters and the in-depth composition of the silicide layers. The lattice of the epitaxial film is laterally extended to adapt to the Si(111) lattice and compressed axially. No evidence of ordered Si vacancies in the silicide volume is detected. The Er : Si ratio is the same throughout the film. The formation of ErSi1.7 is discussed in relation to the nucleation controlled reaction and the atomic structure of the compound.

Interface formation of W evaporated on Si(111) (7 × 7)

Abstract XPS, XAES, UPS and LEED have been used to study the interface formation of W evaporated onto Si(111) (7×7) surfaces, with samples maintained at room temperature and after subsequent annealing of the deposits. The intensity variations of the core levels (Si 2p and W 4f) and of the Auger signals (Si KLL, Si LVV and W NOO) as a function of surface coverage θ correspond to an abrupt junction and a layer-by-layer growth mode. This hypothesis is confirmed by work function measurements. The W 4 f 7 2 undergoes an energy shift of − 0.4 eV (towards lower binding energy) at low coverage, θ ⪷ 1 monolayer (ML), and reaches its bulk metal value at θ ≈ 10 ML; this shows in addition a rather short transition region. Meanwhile, the only variations observed with Si core levels are energy shifts of −0.2 eV for Si 2p and +0.2 eV for Si KLL, which take place at the very first deposition of a submonolayer amount of W. These shifts are attributed to a surface band bending variation. When the metal deposit attains a surface coverage θ ≈ 0.1 ML the Si characteristic surface states disappear from the UPS spectra, while the (7×7) LEED pattern is replaced by a (1×1) diagram which afterwards vanishes in the background for θ⪸1 ML . UPS indicates a rapid predominance of d-metal emission, and a W valence band is fully developed at θ ⪸ 4 ML . Upon annealing of 5–100 ML deposits at temperatures above 750°C, XPS and UPS testify the completion of WSi2 formation and LEED patterns indicate an epitaxial overlayer.

Interaction of oxygen with (Er + Si): formation of erbium pyrosilicate Er2Si2O7

Abstract Silicon oxide (SiOx) and erbium oxide (ErOx) layers in the form of SiOx/ErOx/SiOx/Si structures were sequentially deposited onto silicon substrates by reactive RF-sputtering without breaking the vacuum. The structures were subsequently heat treated at 800°C under an argon pressure of 10 −3 mbar. XPS measurements revealed that the layers thus obtained are homogeneous. The relative intensities of the Si 2p, Er 4d and O 1s core level peaks suggest a Er:Si:O composition ratio equal to 2:2:7. Furthermore, the chemical shifts observed for the Si 2p and Er 4d peaks showed the formation of a compound in which silicon (Si) and erbium (Er) are, respectively, in tetrahedral and octahedral oxygen environments. XRD measurements showed the formation of erbium pyrosilicate (Er 2 Si 2 O 7 ) which is consistent with the XPS results.