L. Kubler

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.

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si1−xGex morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si1−xGex/Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si1−xGex (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

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.

Ge growth mode modification on carbon-induced Si 001 -c 4 = 4 surfaces

Abstract Strong Ge morphological modifications were observed upon an ordered C-pre-covered Si(001)-c(4×4) reconstructed surface used as a template as compared to the growth on bare Si(001)-(2×1) substrates. While on bare substrates, the Ge wetting layer of the Stranski–Krastanov mode has a critical thickness of approximately 3–4 monolayers (ML), with the c-(4×4) template, island nucleation already occurs after 1 Ge ML, and growth proceeds in a Volmer–Weber mode. This suggests that the C-rich surface derm associated with the c-(4×4) reconstruction is able to strongly affect the Ge wetting.

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.

Influence of silicon X-ray photoelectron diffraction on quantitative surface analysis

Abstract The problem of XPS quantitative measurements on silicon single crystal surfaces is discussed in relation to the X-ray photoelectron diffraction (XPD). This effect consists in electron emission enhancements in some particular space directions with reference to the polar angle θ and the azimuth φ and originates in the preferential electron scattering in the directions of the interatomic axes (forward scattering). The polar scans I ∞ Si (θ) of the Si 2p core level intensities of Si(001) and Si(111) surfaces are given in azimuthal high symmetry directions. The relevant spectra display intensity fluctuations in the emission space up to 40% for angular variations of only 10° around the surface normal. They may induce corresponding inaccuracies in quantitative surface analyses when, by approximate sample positioning, this effect is not taken into account.

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).