D. Bolmont

Use of multilayer techniques for XPS identification of various nitrogen environments in the Si/NH3 system

Abstract The experimental ability to reach rapidly, under UHV and on the same substrate holder, the high temperatures (1100 K) required for silicon surface reconstructions and the low temperatures (80 K) needed to condense ammonia molecularly or to adsorb this gas dissociatively on specific silicon surface states, allowed us to elaborate thin multilayers made up of various combinations of the nitrogen chemical environments involved in the Si/NH 3 system. Included in a surface laye ( 3 ), amide (NH 2 ) and imide (NH) fragments and nitrides (Si 3 N) by simultaneously and easily comparable N 1s core level recordings. The results confirm that beside the T s dependence of the degree of dissociation of NH 3 , the silicon surface state configuration (Si(100)-2 × 1. Si(111)-7 × 7 a-Si) is another important parameter explaining the presence of differently adsorbed N, NH or NH 2 groups.

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.

Si-H bond production by NH3 adsorption on Si(111): An UPS study

The adsorption of NH3 on Si(111) has been examined using essentially ultraviolet photoemission spectroscopy (UPS) between room temperature (RT) and 400°C. In this domain NH3 molecules chemisorb dissociatively on some surface sites as deduced from the observation of Si-H monohydride units at 5.4 eV below EF. Other species labeled NHX (X=1, 2 or 3), characterized by two NH3 induced orbitals at 4.9 and 10.6 eV, are also adsorbed at RT with a saturation coverage ≤1/3 monolayer for a 10 L (1 L=10−6 Torr s) exposure. A strong S2 surface state decrease results from adsorptions. With increasing substrate temperature the adsorption of the nitrided NHX species gradually decreases until 300°C where mainly Si-H bonds are observable. No direct conclusive assignment could be given by UPS about the exact nature of the NHX units, but XPS N 1s binding energy (BE) data give arguments for partly dissociated species (NH2 or NH). The nitride formation starts to develop only above 300°C as evidenced by both a rapid XPS nitrogen coverage increase and new Si-N UPS features in this domain.

Thermal nitridation of Si(100)-2 × 1 surface by NH3: XPS results

Abstract Using essentially X-ray photoelectron spectroscopy (XPS), the adsorption of NH 3 on the Si(100) surface is examined in wide ranges of substrate temperature ( T s between room temperature and 800° C), gas pressure ( p NH 3 ) or exposure ( E ). Much effort has been devoted to elucidating: (i) The general T S dependent evolution of the chemisorbed species. Successively, with increasing T S , NH 2 or NH fragments are first bound to the Si surface, which is followed by a hydrogenation stage, and finally, after H desorption above 350 ° C, the start of atomic nitrogen chemisorption is evidenced. (ii) The particular exposure and T S behaviour of the latter nitridation regime, occurring in the 350–800° C range, mostly studied in this article, (a) At high T S (600–800° C) and low NH 3 pressure, growth of Si 3 N 4 islands on the Si(100) surface is deduced from comparative determinations of the nitrogen coverage, of its in-depth repartition (by angular variations) and of local Si 4 environments (by binding energy shifts). Very low nitrogen uptake, governed by N thermal desorption increasing with T S , is found in this domain. For higher NH 3 pressures or exposures ( E > 200 L) complete nitridation in a thin homogeneous Si 3 N 4 overlayer is achieved. Further growth becomes all the more limited (slow nitridation stage) as the reacting Si is more separated from the gaseous nitrogen source by the previously formed nitride diffusion barrier. By giving the empirical laws for the nitride growth we are able to separate clearly for the first time the pressure and time effects during the exposure. We conclude that the exposure only expressed in langmuirs, or in integrated numbers of impinging molecules cannot be a good parameter for the description of Si nitridation. (b) At lower T S (350–600° C) the nitrogen is chemisorbed at the surface in a more homogeneous way and, even if the initial coverage for low exposure is now higher, very slow bulk diffusion confines the interaction within the top layer and leads to a more rapid saturation of the nitrogen uptake with exposure. In this regime mainly intermediate silicon subnitride environments (Si 3 Si) are found. Despite a T S shift, accounting for the room temperature dissociative chemisorption of O 2 on Si, whereas that of atomic nitrogen only starts above 350 ° C, a qualitative similarity can be remarked for the main features involved in the initial, fast silicon nitridation stage with those occurring in oxidation.

Formation of epitaxial Fe3−xSi1+x (0≤x≤1) silicides on Si(111)

Epitaxial Fe3−xSi1+x films have been grown on Si(111) by codeposition at room temperature. Their structural and electronic properties have been investigated by means of low‐energy electron diffraction (LEED), x‐ray photoelectron diffraction (XPD), and x‐ray photoemission spectroscopy (XPS). These films, with compositions ranging from Fe3Si to FeSi, exhibit a (1×1) LEED pattern. Both XPD and core level binding energy measurements indicate that single Fe3−xSi1+x phases (with 0<x<1), without bulk counterpart, can be stabilized by epitaxy on Si(111). The XPD experiment clearly shows that these Fe3−xSi1+x (0≤x≤1) films adopt the same cubic structure. Furthermore, the Si 2p, Fe 2p3/2, and Fe 3s core levels are slightly shifted to higher binding energies resulting from chemical effects and differences in local coordination when going from Fe3Si (DO3) to FeSi (CsCl). Multiplet splittings ΔE3s are observed in Fe 3s core‐level XPS spectra for all Fe3−xSi1+x compounds except the FeSi (CsCl) one.

Experimental band structure and Fermi surface of a two-dimensional Er silicide on Si(111)

High resolution angle resolved photoemission measurements on a monolayer of Er deposited on Si(111) and annealed at 400°C are presented. A series of two-dimensional energy bands attests to the formation of a surface silicide with a high degree of perfection. In particular, a prominent band with remarkably large hole lifetimes (∼200 meV) and a dispersion of ∼1.65 eV crosses the Fermi level near the Γ point of the surface Brillouin zone. The two-dimensional Fermi surface is typical of a semi-metal and consists of small hole and electron pockets about the Γ and M points respectively.

Reversible process of hydrogen adsorption on Si(111)

Recent studies have revealed interesting properties of the dihydride phase coexisting with the monohydride phase on the hydrogenated Si surface. The adsorption of atomic hydrogen on the Si(111)7×7 surface has been studied for coverages at and below one monolayer at temperatures between 300 and 600 K using photoemission spectroscopy (UPS and XPS). In the very first stages of room temperature (RT) adsorption ( T ≥600 K, only the monohydride phase is present at all exposures. Experimentla evidence that the adsorption sites for the SiH 2 states are not produced by corrosion is presented.

Low temperature NH3 adsorption on clean amorphous silicon and on crystalline silicon surfaces and nitrogen bonding in hydrogenated amorphous silicon nitride films: A comparative X-ray photoelectron spectroscopy study

Abstract Careful X-ray photoelectron spectroscopy studies of the nitrogen core levels were used to compare, in the same substrate temperature T s range (between room temperature and 450 °C), the first interaction stages of amorphous and crystalline silicon surfaces (a-Si and c-Si(111)-(7 × 7)) with ammonia and the uptake of nitrogen on reactive evaporation of silicon in an ammonia ambient, i.e. during the growth of hydrogenated amorphous silicon nitride (a-SiN x :H) films. In all cases we observed strongly correlated behaviours: the N 1s binding energy E B decreased with increasing T s , reflecting the presence of more dissociated species, probably NH 2 or NH and finally nitrogen, either on the silicon surfaces or in the bulk of the a-SiN x :H films. The total N 1s core level intensities corresponding to hydronitride complexes NH X decreased with increasing T S up to 300 °C. Above this temperature the contribution of completely dissociated molecules relevant to nitride environments became prominent. The nitrogen coverage of the silicon surfaces at room temperature as a function of exposure was also compared with the nitrogen content x of a-SiN x :H films as a function of NH 3 pressure during evaporation. Converted into dynamical exposures for a given evaporation rate, this pressure dependence is very well explained in terms of variations in the sticking coefficient deduced from the adsorption studies.

STM investigation of 2- and 3-dimensional Er disilicide grown epitaxially on Si(111)

Abstract The surface atomic structure of 2- and 3-dimensional (D) Er disilicide epitaxially grown on Si(111) has been investigated by scanning tunneling microscopy (STM) and angle-resolved photoemission. The STM images reveal that highly ordered 2D and 3D silicide islands can be grown on the flat Si(111)7 × 7 terraces and atomic resolution scans clearly confirm that both silicides are terminated by a Si bilayer without vacancies. In the 3D case the outermost Si atoms exhibit an additional small buckling with √3 × √3R30° periodicity. The STM data imply a specific registry of the surface Si layer with respect to the vacancy net underneath which is found to be in nice agreement with the symmetry of the dangling bond states at \ gG observed in polarization dependent photoemission.

Non-equivalence of the adatoms in the das model of the Si(111) 7x7 surface, an extended Hückel model calculation

Abstract Electronic structure calculations of the Si(111)7x7 surface using the crystalline extension of the extended Huckel method (E.H.T.) are presented. Good agreement with experimental results has been found for the surface states. Concerning the energy levels, densities of states, charges, adatom backbonds and COOP (Crystal Orbital Overlap Populations). Our results point out the unequivalent contributions of the unfaulted (U) and faulted (F) parts of the unit cell and particularly those of the adatoms. Moreover, it has been shown there are essentially charge transfers between atoms of the two topmost layers.