M. Diani

Search for carbon nitride CNx compounds with a high nitrogen content by electron cyclotron resonance plasma deposition

Abstract In order to synthesize and characterize the hypothetically hard compound C 3 N 4 , we used an ultrahigh-vacuum-compatible electron cyclotron resonance plasma source and an in-situ X-ray photoemission spectroscopy (XPS) technique analysis. An N 2 plasma has been used to excite a hydrocarbon gas (CH 4 ) in the vicinity of an Si substrate maintained at a temperature T s . High-nitrogen-containing CN x :H ( x ⩾ 1) coatings have been synthesized for the first time on substrates held at room temperature. However, binding energy analysis of the XPS C 1s and N 1s core level lines revealed a variety of local environments (single and double C-N bonds, in addition to hydrogenated groups) non-compatible with C 3 N 4 . Annealing of samples deposited at room temperature and attempts to synthesize the material at a high substrate temperature (above 500°C) or by nitrogen bombardment of amorphous carbon evidenced the high thermal instability of the C-N bonds and led generally to a nitridation process limited to the surface of the Si substrate.

The Ge Stranski-Krastanov growth mode on Si(001) (2 × 1) tested by X-ray photoelectron and Auger electron diffraction

X-ray photoelectron and Auger electron diffractions have been used here for the first time to identify growth morphology in the earliest stages (0–10 monolayers) of Ge epitaxy on Si(001)(2 × 1) surfaces held at room temperature (RT) and at 400°C. The Ge atomic arrangement in the (110) plane is examined by performing polar angle distribution of the Ge LMM intensities and by comparison with the corresponding Si2p substrate pattern. A detailed plot as a function of the Ge coverage of the forward scattering peak contrasts in the [111] and [001] directions, which correspond to the 1st and 3rd atomic neighbour rows, respectively, yields informations about the layer number distribution and the growth mode. Contrarily to the nearly two-dimensional (2D) growth taking place at RT, we obtain a 3D island formation at 400°C for a critical thickness exceeding 5 ML. Nevertheless, in the coverage domain between 2 and 5 ML for which layer-by-layer growth is normally expected, the observation of a significant up to 4 ML roughness across the surface prefigurates the islanding process and confirms very recent STM reports. Photoelectron scattering results are only consistent with pure 2D formation during the first 2 ML growth.

Selective thermal — as opposed to non-selective plasma — nitridation of SiGe related materials examined by in situ photoemission techniques

Abstract NH 3 thermal and N 2 plasma reactivity with Si(0 0 1), Ge(0 0 1), Si 1- x Ge x (0 0 1) surfaces has been studied by means of in situ X-ray photoelectron spectroscopy (XPS) in a temperature domain ( T ∼ 600°C) compatible with the MBE growth of GeSi-based heterostructures. Si(0 0 1) surfaces present a strong initial thermal reactivity against NH 3 , contrary to Ge(0 0 1) which is totally inert. The selectivity against thermal nitridation, which may be anticipated for thermodynamical reasons, has been verified by nitrogen uptake measurements of N 1s core level intensities as a function of NH 3 exposure, both for Si(0 0 1) and Ge(0 0 1) surfaces. As a consequence of this strong reactivity difference, an exclusive Si 3 N 4 formation and Ge phase separation result from nitridation attempts of Si 1- x Ge x alloys. Thus, an important finding is the indispensable utilization of plasma-assisted nitridation methods in order to achieve low-temperature Ge nitridation, either on clean Ge(0 0 1) surfaces or simultaneously with Si on SiGe alloys. In this paper, the first results relevant to Ge and SiGe alloy nitridation by irradiation of these surfaces by electron cyclotron resonance (ECR) nitrogen (N 2 ) plasmas are presented. These alloys are thermally unstable as nitridation transfer from Ge to Si occurs after annealing, in accordance with thermally favored Si nitridation. In addition, Ge 3 N 4 (Si 3 N 4 ) thick layers were grown using ECR N 2 plasma treatment associated with a concomitant Ge(Si) atom-supply on the substrate, performed by Ge evaporation (SiH 4 reacting gas).

X-ray photoelectron diffraction observation of β-SiC(001) obtained by electron cyclotron resonance plasma assisted growth on Si(001)

Nanometric SiC overlayer synthesis has been performed via a microwave electron cyclotron resonance (ECR) H2 plasma activating CH4 molecules on Si(001) substrates whose temperatures (Ts) expanded from room temperature to 850°C. The films were characterized in situ by angularly resolved photoemission techniques. In addition to the Ts dependence of the growth kinetics obtained from conventional X-ray photoelectron spectroscopy (XPS) measurements, photoelectron diffraction (XPD) is used for the first time to show the appearance of textured growth of β- or 3C-SiC(001)∥Si(001) for Ts above 800°C. To this end the structured polar scans of the C 1s and Si 2p carbide intensities are presented for the two (100) and (110) high-symmetry planes and compared to the corresponding Si substrate signatures. These data are relevant to the first XPD investigation of a binary compound with well differentiated diffusers such as C and Si atoms and identical environments under test in spite of which differentiated C 1s and Si 2p patterns are obtained.

Strong element dependence of C 1s and Si 2p X-ray photoelectron diffraction profiles for identical C and Si local geometries in β-SiC

Abstract We have measured photoelectron diffraction polar profiles for a β-SiC film grown epitaxially on Si(001). Tha data reveal dominant single domain growth with a good crystallinity but the C 1s and Si 2p profiles exhibit remarkably strong differences in spite of the identical geometries of the sites occupied by these elements in the ZnS-type lattice. Most obvious are the large angular shifts and changes in intensity between expected and measured forward scattering peaks. Our results obtained at low angular resolution (∼ 5°) in the high kinetic energy range (∼ 1000 eV) provide a striking example of the limitations of the often invoked forward scattering picture. The measured profiles are rather well reproduced by single scattering cluster simulations. The observed elemental dependence can be traced back to the marked change in complex scattering amplitude between C and Si along with the very general fact, by no means restricted to the SiC(001) case, that a large number of scatterers, in particular out-of-chain atoms with a low scattering angle, make a substantial contribution to the photoelectron wave around forward scattering directions. The related energy and element dependent interference effects are particularly strong along the low density [001] C chains of the open ZnS-type structure and reflect in a drastic peak splitting due to the strong scattering at lateral Si atoms. In contrast, the [001] Si chains in β-SiC lead to an essentially structureless forward scattering peak due to the lower scattering amplitude of lateral C atoms.

An experimental characterization of Si(111) surfaces by Si 2p X-ray photoelectron diffraction

Abstract X-ray photoelectron diffraction (XPD), which is a by-product of the usual XPS process combined with available angle-resolved electron detection, is used to characterize crystallographically clean Si(111) surfaces. The anisotropic core level electron emission angular dependence, due to the elastic scattering mainly in the direction of internuclear axes, is given by polar and azimuthal scans of the Si 2 p core level intensities I Si (θ) and I Si (φ) in some particular high symmetry space directions. For each 30° azimuthal rotation, one of the three particular polar scan signatures could always be recovered and recognized, in agreement with the azimuthal symmetry of the (111) face. The experimental angles of the core level signal enhancements are compared with the theoretical atomic bulk row-directions which are expected to give XPD features in the frame of the forward focusing theory. This simple approach is sufficient to explain a main part of the XPD patterns suggesting the preponderance of bulk contributions in the emergence of the XPD contrast. Nevertheless the comprehension of the complete fine structure is subjected to the examination of the 7 × 7 reconstruction contribution and of interference features only reached by single scattering or, if needed, multiple scattering calculations.

Surface structure of Si(001) treated by hydrogen and argon electron cyclotron resonance plasmas

Abstract Si(001) surfaces subjected to H 2 or Ar ECR plasma irradiation are studied, in situ, from the standpoints of both impurity removal and induced crystallographic damage. The atomic cleanliness is checked by XPS (X-ray photoelectron spectroscopy) and UPS (ultra-violet photoelectron spectroscopy), while surface crystallographic information given by LEED and XPD (X-ray photoelectron diffraction) experiments. As an H ion-source, the ECR plant appears to be a convenient hydrogenation source, with low damage, able to passivate the surface in the usual hydrogenated LEED phases (dihydride 1 × 1 or monohydride 2 × 1) depending on the employed substrate temperature T . It presents nevertheless poor etching properties concerning the dioxide overlayer in our low plasma pressure domain ( −4 mbar). On the other hand, as an Ar ion source, the ECR plasma is more efficient to etch physically and clean, particularly at low working pressure and aided by a DC negative bias voltage and T s increase but suffers from more crystallographic perturbations checked by the LEED disappearance and quantified by the decrease of the anisotropy factor related to the XPD contrast. Finally, a procedure which combines exposures to the cleaning Ar ions followed by a refinement Si etching of the damaged overlayers using the H plasma allows the attainment of clean reconstructed 2 × 1 surfaces with processing temperatures limited at 500°C and suitable for subsequent epitaxial growths.

X‐ray photoelectron and Auger electron diffraction probing of Ge heteroepitaxy on Si (001) 2×1

Epitaxial molecular beam epitaxy growth of nanometric Ge layers on Si (001) 2×1 has been investigated, in situ, by x‐ray Si 2p photoelectron diffraction and Auger Ge LMM electron diffraction which consist essentially in preferential scattering of electrons in the direction of interatomic axes. Particular attention was paid to measuring the contrasts of this anisotropic emission in the (110) plane as a function of deposition parameters. It can thus be determined how crystalline material quality and epitaxial perfection are affected by the residual pressure below 5×10−8 mbar, the substrate temperature decrease to room temperature, the deposition rate, and the Ge overlayer thickness.

Electron cyclotron resonance plasma ion beam effects on the formation of SiC on Si(001) characterized by in-situ photoemission

Abstract Nanometric SiC overlayer synthesis has been performed via an ultrahigh vacuum-compatible microwave electron cyclotron resonance plasma source. The H2 plasma streaming onto a Si(001) substrate, whose temperature Ts could be varied from room temperature to 850°C, activates and dissociates CH4 molecules. The films are characterized in situ by angle-resolved photoemission techniques. Without the H2 plasma, no surface reaction of CH4 is observed with the Si irrespective of Ts up to 850°C and exposures up to 106L. H2 plasma excitation leads to the rapid formation of a thin SiC overlayer in the whole Ts range. For temperatures below a threshold of about 700°C where thermal interdiffusion between Si and C is negligible, the SiC overlayer thickness rapidly saturates in the nanometric range and the SiC formed is not structured. This thickness is essentially determined by ion penetration in the substrate which can be increased by negative biasing. Above this Ts, SiC growth increases rapidly and the film becomes textured near 800°C, as the growth of β-SiC(001) aligned with Si(001) can be observed. The SiC topmost-layer structure is critically dependent on the plasma conditions with respect to the thermal processing at the film growth interruption. When the plasma is switched off before heating, the surface is essentially Si rich and oxidizable. In the opposite case, the H2 plasma etches the Si-terminated overlayer and passivates the surface.