F. Arnaud d’Avitaya

Fabrication and structure of epitaxial Er silicide films on (111) Si

We prepared Er silicide films on (111) Si by (1) deposition of Er and contact reaction at 380 °C or (2) vacuum codeposition of Er and Si maintaining the flux ratio close to 1:2. Subsequent annealing at temperatures up to 900 °C yielded monocrystalline, continuous layers, whose properties were examined by means of low‐energy electron diffraction, Auger spectroscopy (in situ) and (ex situ), x‐ray and high‐energy electron diffraction, and Rutherford backscattering. Method 2 was shown to give better results. The films had a hexagonal AlB2 structure with Si deficiency up to 20%, which is consistent with formerly published results on Si vacancy formation. We showed that the film structure had an additional periodicity of 15 A along the 〈110〉 orientations of Si and of 6 A along the 〈112〉 orientations of Si. We demonstrated a feasibility of Si reepitaxy on Er silicide deposited on (111) Si, thus fabricating a novel semiconductor/metal/semiconductor epitaxial heterostructure.

Study of ballistic transport in Si‐CoSi2‐Si metal base transistors

The hot‐electron transfer ratio from the Schottky emitter diode to the Schottky collector diode through the thin base layer of Si‐CoSi2‐Si metal base transistors has been measured in devices for which the pinhole contribution to the current gain is shown by transconductance measurements to be negligible. The common‐base current gain α of the transistors, measured as a function of temperature and base thickness, exhibits an exponential dependence on CoSi2 film thickness, from which a ballistic mean free path is deduced. This value is in good agreement with the mean free path deduced from conductivity data, both at 77 and 300 K. The behavior of the pre‐exponential coefficient, i.e., the gain extrapolated to zero base thickness, is not completely understood and shows evidence for hot‐electron and space‐charge effects in the epitaxial overgrown Si material of the emitter.

Fabrication and optical properties of Si/CaF2(111) multi‐quantum wells

We have synthesized, by molecular beam epitaxy, Si/CaF2(111) multi‐quantum wells which are photoluminescent at room temperature after ageing in air. In this article, we report on the structural properties and on a detailed optical study of these heterostructures. The photoluminescence spectra for various confinements and the temperature dependence of the lifetimes as a function of emission wavelength are described in comparison with the corresponding characteristics of porous silicon and hydrogenated amorphous silicon. A model based on quantum confinement is proposed to explain the experimental data.

Surface segregation mechanism during two‐dimensional epitaxial growth: The case of dopants in Si and GaAs molecular‐beam epitaxy

In this paper, we present a physical explanation for the dopant surface segregation during two‐dimensional growth in molecular‐beam epitaxy. It is shown that the only way to explain dopant behavior is to assume the climbing of dopant atoms over the steps on the surface, during the growth. An energetical analysis of such phenomenon is presented. Taking into account this mechanism during two‐dimensional growth, two kinetic equations can be written for each layer. Resolution of the system provides analytical solutions, and the growth rate influence on the doping profile is justified. The applicability of this approach to several dopants both in GaAs and Si is discussed. It appears that the origin of this segregation phenomenon lies in the unfavorable energy situation for the incorporation of a dopant instead of a bulk material atom.

Chemical vapor deposition of silicon–germanium heterostructures

We report on the growth of SiGe/Si heterostructures in a commercial chemical vapor deposition cold-wall vertical reactor using SiH4 and GeH4 gas precursors, and we discuss their structural quality. The machine is versatile as it operates in a wide range of temperature and pressure, allowing both the ultra high vacuum and a low-pressure-like regime. In the two growth modes, the Si and SiGe deposition rates and the Ge fraction are calibrated as a function of the temperature and the hydrides partial pressures. The LP-CVD regime is appropriate for the realization of thick gradually relaxed Si0.75Ge0.25/Si pseudo-substrates. The role of the grading rate (5–20% Ge/μm) on the density of threading dislocations and the surface roughness is discussed. Seven-period Si0.70Ge0.30/Si superlattices were grown by UHV-CVD to study the interfacial chemical abruptness and morphological roughness by means of X-ray diffraction, rutherford backscattering and transmission electron microscopy techniques. Results are strongly dependent on the growth temperature in the 575–625°C range.

Growth of MoSi2 with preferential orientation on (100) silicon

Molybdenum has been evaporated under ultrahigh vacuum conditions on (100) silicon wafers. Molybdenum silicide MoSi2 was obtained either by deposition on substrates at room temperature and subsequent annealing in situ at 650 °C or by metal deposition on substrates heated to 650 °C. Structural characterization was carried out by x‐ray diffraction and transmission electron microscopy. The silicide resulting from the first operating process is a random, polycrystalline material. On the contrary, growth of MoSi2 with well‐defined epitaxial relationship occurs on the heated substrates.

Stark effect modeling in strained n-type Si/Si1−xGex resonant tunneling heterostructures

We present calculations of band discontinuities for Si1−xGex/Si1−yGey strained/relaxed heterointerfaces using the model-solid theory. From the obtained results, we then report a numerical simulation of the conduction-band diagram of a resonant tunneling diode in the Si/Si1−yGey system by solving self-consistently Schrodinger and Poisson equations with and without an applied electric field. An analysis of the Stark effect was made in the investigated heterostructure. Two main features have been extrapolated: (i) two sheets of a two-dimensional electron gas are created, leading to a resonant tunneling through the structure, (ii) a charge transfer can occur due to this tunneling effect. In addition, it is found that this charge transfer is highly sensitive to temperature and tends to saturate as the applied electric field increases.We present calculations of band discontinuities for Si1−xGex/Si1−yGey strained/relaxed heterointerfaces using the model-solid theory. From the obtained results, we then report a numerical simulation of the conduction-band diagram of a resonant tunneling diode in the Si/Si1−yGey system by solving self-consistently Schrodinger and Poisson equations with and without an applied electric field. An analysis of the Stark effect was made in the investigated heterostructure. Two main features have been extrapolated: (i) two sheets of a two-dimensional electron gas are created, leading to a resonant tunneling through the structure, (ii) a charge transfer can occur due to this tunneling effect. In addition, it is found that this charge transfer is highly sensitive to temperature and tends to saturate as the applied electric field increases.

Electroluminescence from silicon nanocrystals in Si/CaF2 superlattices

Electroluminescence (EL) from silicon nanocrystals in Si/CaF2 superlattices grown by molecular-beam epitaxy at room temperature was investigated and compared with that obtained from silicon nanocrystals in Si/SiO2 superlattices. EL spectra exhibited current-tunability, similar to that observed in silicon nanocrystals in SiO2, which was attributed to three main effects: (a) Auger quenching of photoluminescence, which occurs when more than one electron-hole pair is present in the same nanocrystal and which quenches luminescence from relatively larger nanocrystals, (b) size-dependent carrier injection, and (c) the effect of the applied field, when this one is significantly high. In the case of Si/CaF2 superlattices, this last factor did not apply, so the two other factors are mainly at the origin of the effect.

Grain interaction effect in electronic properties of silicon nanosize films

Electronic properties of both nanometer thickness (111) monocrystalline and nanocrystalline free standing silicon films were calculated within a self-consistent linear combination of atomic orbitals method. Grained nature of the nanocrystalline films is found to induce both a direct band gap and its reduction (down to about 2 eV) with respect to an isolated grain of same size.

Nonlinear electrical transport in nc-Si/CaF2 multilayer structures with ultrathin CaF2 layers

We present a study of the electrical transport in (Si/CaF2)n superlattices with n=100 and with Si, CaF2 thickness in each period in the range of 1.2–1.6 nm (Si) and below 1 nm (CaF2), respectively. The results suggest that at gate voltages higher than ±4 V a Poole–Frenkel-type mechanism accounts for the observed electric-field-assisted conduction through the layers.