Pierre-Henri Lanoë

Thibault Gallavardin

Patrice L Baldeck

Andraud Mikael Lindgren

Gallavardin

Aurore Dupin

Olivier Maury

Cyrille Monnereau
email: cyrille.monnereau@ens-lyon.fr

Influence of bromine substitution pattern on the singlet oxygen generation efficiency of photon absorbing chromoph

A new molecular engineering strategy absorbing photosensitizers for singlet oxygen chromophore with a five fold enhanced "figure propose a rational explanation on the relation center on the charge-transfer Photodynamic therapy (PDT) is a therapeutic protocol which is currently limited to the treatment of surface diseases, like skin or mucosal cancers. a photosensitizer, 1e,2 ie a molecule possessing an accessible triplet excited state able to generate singlet oxygen ( 1 O 2 ) by an energy transfer to grou oxygen, ultimately leading to cell death sensitized PDT is an emerging approach deeper penetration of light into tissues operates in the biological transparency window nm) and a better control on the irradiation volume than

σTPA .φ φ φ φ∆ ∆ ∆ ∆) at 800nm, compared to a literature benchmark. We propose a rational explanation on the relation that exists between the positioning of the transfer chromophore and its ability to generate singlet hotodynamic therapy (PDT) is a therapeutic protocol the treatment of several cancers. 1 It requires a molecule possessing an excited state able to generate singlet ) by an energy transfer to ground state ultimately leading to cell death. 3 Two-photon PDT is an emerging approach, which allows a (since irradiation operates in the biological transparency window, 700-1100 and a better control on the irradiation volume than its one-photon counterpart envision more in-depth minimizing detrimental side healthy tissues. Ideal two chromophores for PDT must possess two spectroscopic characteristics, absorption cross section wavelength and (ii) a high quantum yield of singlet oxygen generation (φ ∆ ), which of singlet-to-triplet intersystem crossing chromophore in its excited state

Olivier Maury, † and Chantal

5182, Université Claude Bernard, École Normale Supérieure Grenoble, rational variation of the bromine substitution pattern on two-photon is introduced. This approach allowed us to obtain a compared to a literature benchmark. We between the positioning of the Inter-system crossing (ISC) chromophore and its ability to generate singlet oxygen.

photon counterpart. 4 Thus, it is possible to depth treatment of tumors, while side-effects on the neighboring two-photon absorbing (TPA) for PDT must possess two essential characteristics, (i) a large two-photon n (σ TPA ) at the excitation high quantum yield of singlet , which is relevant to the efficiency triplet intersystem crossing (ISC) of the hromophore in its excited state. [START_REF] Mcglynn | Molecular spectroscopy of the triplet state[END_REF] These two parameters can be used to define a "figure of merit" at 800 nm, 6 (σ TPA800 ×φ ∆ ), for the photosensitizer. 4d Two-photon absorbing chromophores have been the subject of various studies during the last two decades, and the associated rules of molecular engineering and structure-property relationship are now well established. 2d,7 In particular, it has been established that chromophore that features electronic transition of the Internal Charge-Transfer (ICT) type are optimal candidates for TPA, especially those with quadripolar symmetry. 7 Comparatively, in spite of the fact that many studies have been focused on improving ISC efficiency and ultimately one-photon PDT activity in chromophores with localized excited states (porphyrin 8 and rhodamine 9 derivatives, mostly) only few authors have systematically studied the relationship that exists between structural parameters and 1 O 2 generation efficiency in the context of ICT chromophores. In most cases, 1 O 2 generation is achieved through substitution of the chromophore with a group responsible of the ISC. This approach, we have also recently explored, 10 has been successful in designing efficient chromophores for two-photon PDT; 4g- i yet, the relations between ISC efficiency (that is, the mechanism by which the delocalized ICT excited singlet state evolves into a localized triplet state) and the position and/or number of ISC promoters(?) remain poorly understood in the context of ICT chromophores. With the aim to more particularly address that point, we initiated a systematic study based on an original engineering approach: it markedly differs form previously published works on the topic, since the chemical nature of the ISC group (two bromine substituents in 2,5-positions on a phenyl moiety) is maintained in all molecules, and the variations we operate concern exclusively its number and positioning along the ICT axis ("p+1" and/or "p+2" position, Figure 1). This allows to vary exclusively the ISC kinetic constant (k ISC ) while keeping all other kinetic constants almost unaffected (vide infra). In this preliminary study, we show that this systematic approach can be used to tune the 1 O 2 quantum yield generation along with other spectroscopic features of interest, to a rather unexpected extent.

Following the aforementioned strategy, and using previously reported compound 1 4i,4m (Figure 1) as a benchmark and model, a series of four quadrupolar chromophores was designed and synthesized. For convenience, those chromophores will be refered to as 0Br, 2Br, 4Br and 6Br in the following. The series was obtained through a convergent "building-blocks" synthesis, based on successive Sonogashira-Hagihara palladium cross-coupling catalyzed reactions from the electron donating group to the molecular core. For [a] 0.80 0.70 0.30 0.53 0.25 λphos [b] -530, 569 594, 637 600, 674 586 φ∆ [c] -0.10 0.50 0.35 0.53 λTPA max(σTPA) 750 ( 460) 760 ( 450) 830 ( 380) 880 ( 260) 550 ( 730) σTPA800 x φ∆ 0 30 175 40 experimental details and characterizations Supporting Information. Absorption and emission spectra were obtained using diluted CH 3 Cl solutions of the compounds (Figure 2 and Table 1). In all cases, a absorption band is observed, centered at 3 and 435 nm, for 0Br, 2Br, 4Br respectively. As expected, this band can be ascribed to a ICT transition, regarding shape, large Stokes shift and positive solvatochromism (Figure S4). The bathochromic shift along the series is in agreement with the electro-deficiency increased number of dibromophenyl moieties conjugated skeleton. All compounds under irradiation in their ICT band solvatochromic ICT emission band centered at 490, 500 and 560 nm, for 0Br, 2Br, 4Br chloroform, respectively. The decay times Figure 4) are in the ns range, which indicates a fluorescence emission process.

Attempts to observe triplet excited state emission of the compounds at room temperature have to date remained unsuccessful. However, at low temperature in g solution, structured phosphorescence emissions were observed, except for 0Br in which ISC does not occur significantly(Table 1, Figure S5).

1 O 2 generation quantum yield (φ ∆ chromophore efficiency as a potential PDT sensitizer. was determined from the characterisic phosphorescence at 1270 nm. All brominated compounds (2Br, 4Br and 6Br) generate singlet quantum yields of 0.10, 0.50 and 0.35, respectively (Table 1, Figure S6). Interrestingly, quantum yields and lifetimes evolve opposite (and almost mirror) to the 1 O 2 generation quantum yield ( This tendency is in excellent agreement with the 1 O 2 quantum yield can be directly connected efficiency, since all kinetic constants other than shown to be unchanged as a result of the structural similarity of all chromophores within the seri for full demonstration). 4Br exhibits a 1 (φ ∆ = 0.50) that is five times higher than its dibrominated analogue 2Br (φ ∆ = 0.10) with a concomitant fluorescence quantum yields and lifetime values 3, Table 1). This trend could potentially be ascribed Interrestingly, fluorescence opposite (and almost generation quantum yield (Figure 3). is in excellent agreement with the fact that, directly connected to ISC other than k ISC are as a result of the structural similarity of all chromophores within the series (see SI

1 O 2 quantum yield five times higher than its dibrominated with a concomitant drop in both fluorescence quantum yields and lifetime values (Figure could potentially be ascribed either to a synergistic enhancement of the ISC efficiency related to the increased number of bromine on the chromophore, or to a positionnal effect, with lateral ("p+1") substitution providing higher ISC efficiency one in this particular series obtained with 6Br clearly rule indeed, in spite of the increased number of heavy atom substituents, 6Br displays a efficiency (φ ∆ = 0.35) and a concomitant increase of the fluorescent quantum yield and lifetime compared to which altogether can be ascri efficiency. Taken together, these that, rather than the number of their detailed position on the carbon backbone is the determining parameter that drives ISC efficiency: positioning of the heavy atoms seems to be on the peripheral phenyls next to the aniline ring ( position, as in 4Br), whereas bromine substitution on the "p+2" (2Br) ring provides in comparison a much lower efficiency. Interestingly, and quite unexpectedly presence of bromine atoms in both positions has a detrimental effect on the ISC, when comparing 4Br and consequence, 4Br is the best photosensitizer of the series, whereas 6Br displays a value that is intermediate respect to the former and that of Finally, TPA cross section photon induced fluorescen biological transparency window compounds show significant values ranging from 250 (Figure 4, Table 1). As for their linear absorption spectra, a significant bathochromic shif the series. 0Br and 2Br g values, but their absorption optimal transparency window. in the NIR but with a moderate cross section value (< GM), whereas 4Br shows section being larger than This shows that, beside having a strong influence on generation, bromine substitution pattern can be used to tune TPA properties (λ TPA max together, this leads 4Br to display the best figure of merit (σ TPA 800 ×φ ∆ , Table 1) among all compounds (including benchmark 1), and makes it structure for future PDT applications parameters (Φ f , Φ ∆ , τ f ) among the chromophore's series (298K, CHCl 3 ). a synergistic enhancement of the ISC efficiency related to the increased number of bromine on the chromophore, a positionnal effect, with lateral ("p+1") substitution providing higher ISC efficiency than the central ("p+2") in this particular series. Examination of the results clearly rules out the first hypothesis : in spite of the increased number of heavy atom displays a loss of 1 O 2 generation and a concomitant increase of the fluorescent quantum yield and lifetime compared to 4Br, which altogether can be ascribed to a lower ISC efficiency. Taken together, these results seem to indicate rather than the number of 1 O 2 generating moieties, their detailed position on the carbon backbone is the determining parameter that drives ISC efficiency: an ideal the heavy atoms seems to be on the peripheral phenyls next to the aniline ring ("p+1"

), whereas bromine substitution on the ring provides in comparison a much lower , and quite unexpectedly, the presence of bromine atoms in both "p+1" and "p+2" detrimental effect on the ISC, as seen and 6Br (Figure 3). As a is the best photosensitizer of the series, a value that is intermediate with the former and that of 2Br. TPA cross sections were determined by twofluorescence measurements (SI). In the window (750 -900 nm), all significant TPA cross sections, 11 with GM (6Br) to 460 GM (0Br) s for their linear absorption spectra, significant bathochromic shift is observed throughout give the best results in absolute ption maxima are slightly below the transparency window. 6Br shows a maximal TPA in the NIR but with a moderate cross section value (< 200 a broad TPA peak, with a cross being larger than 300 GM from 770 to 860 nm.

having a strong influence on 1 O 2 , bromine substitution pattern can be used to max ) of the chromophore. Taken to display the best figure of merit ) among all compounds in the series ), and makes it a relevant applications, provided that some photon absorption spectra in CH 2 Cl 2 at 298 K. modifications are introduced to render it biocompatible, as previously illustrated with related chromophore 1. 4m,12 In summary, this study allowed us, owing to an original molecular engineering approach, to progress significantly in our understanding of some of the relation that exist between the position and number of a given ISC promoter (2,5-dibromophenyl) on ICT chromophores, and their 1 O 2 generation efficiency. In particular, we suggest that there exist "match" ("p+1") and "mismatch" ("p+2") positions for dibromophenyl moieties (Figure 1). This is of particular significance in the context of molecular engineering for two-photon PDT applications, since the position of the heavy atom has only scarcely been discussed in previous published works relying on a similar "heavy atom" strategy for 1 O 2 sensitization in ICT chromophores, 13 and never investigated in such details for this class of chromophores. Besides, we have shown that varying the bromine substitution pattern could be used to tune the two-photon absorption properties of the molecule, as a result of the electron withdrawing effect of the bromine atoms. Altogether, the results of this study suggest that, for works dealing with ISC improvement in ICT chromophores, caution should be taken not only with the choice of the ISC generating moiety, but also with its number and -most importantly-positioning along the chromophore ICT transition axis. Theoretical and experimental investigations are underway to gain deeper understanding on the structure-to-property relationships in this class of brominated two-photon photosensitizers, and in particular to correlate the observed trend to the orbital parameters of the ICT transition.

Figure 2 .

 2 Figure 2. Absorption (full) and emission (dash) spectra in CHCl 3 at 298 K.

Figure 1 .

 1 Figure 1. Compounds of the study and benchmark compound 1 [ref. 4i, 4m]. p+n (n=1,2) refers to the phenyl ring position relative to the terminal aniline

Figure 3 .

 3 Figure 3. Evolution of the photophysical parameters ( among the chromophore's series (298K, CHCl and characterizations, see Absorption and emission spectra were obtained using solutions of the compounds at 298 K broad low-energy centered at 389, 395, 410 4Br and 6Br, his band can be unambigously regarding its characteristic positive solvatochromism bathochromic shift along the series is in deficiency induced by the phenyl moieties along the are luminescent CT band, with a broad, centered at 476, 490, 4Br and 6Br in decay times (Table1, which indicates a excited state emission of the compounds at room temperature have to date remained owever, at low temperature in glassy , structured phosphorescence emissions were in which ISC does not occur ∆ ) quantifies the PDT sensitizer. It characterisic ∆ 1 O 2 Σ 3 O 2 All brominated compounds singlet-oxygen, with quantum yields of 0.10, 0.50 and 0.35, respectivelyInterrestingly, fluorescence opposite (and almost generation quantum yield (Figure3). is in excellent agreement with the fact that, directly connected to ISC other than k ISC are as a result of the structural similarity of all chromophores within the series (see SI 1 O 2 quantum yield five times higher than its dibrominated with a concomitant drop in both fluorescence quantum yields and lifetime values (Figure could potentially be ascribed either

Figure 4 .

 4 Figure 4. Two-photon absorption spectra in CH

Table 1 .

 1 Relevant spectroscopic data for the study.

		0Br	2Br	4Br	6Br	1 [d]
	λmax (ε)	389 (79 x 10 3 )	395 (46,6 x 10 3 )	410 (49,9 x 10 3 )	435 (53,8 x 10 3 )	421 (91,5 x 10 3 )
	λfluo (τf )	476 (1.83)	490 (1.68)	500 (0.96)	560 (1.27)	458 (0.26.)
	φf

Acknowledgment

The authors thank the ANR program (NanoPDT), MACODEV and FINOVI for financial support. J. Bernard is ackowledged for his technical support in TPA measurements.

Supporting Information Available synthesis, analytical characterization, discussion on the excited state kinetics, 1 H and 13 C NMR spectra, HPLC traces, solvatochromism study, 77K time-gated luminescence, fluorescence and 1 O 2 phosphorescence plots. † University of Lyon ‡ University of Grenoble || University of Trondheim


Christian Mougin

Claude Péricaud

Jaqueline Dubroca

Marcel Asther

ENHANCED MINERALIZATION OF LINDANE IN SOILS SUPPLEMENTED WITH THE WHITE ROT BASIDIOMYCETE PHANEROCHAETE CHRYSOSPORIUM

Biotransformation of the insecticide lindane was investigated in sterilized and nonsterilized soils, each supplemented or not by the white rot basidiomycete Phanerochaete chrysosporium. After spore inoculation of the sterilized soil, the fungal biomass increased rapidly during the first week, then proceeded at a lower rate during the following 8-wk. Conversely, only limited fungal growth was detected in non-sterile soils. Lindane mineralization, as a result of indigenous microflora activity, was increased following fungal inoculation. Then, extractable amounts of lindane were quite similar in soils, with or without fungal addition. Mass-balance and SPME analysis revealed the volatilization of degradation intermediates only from non-sterilized and non-inoculated soil. P. chrysosporium seemed to modify lindane degradation pathway by increasing the conversion of volatile intermediates to CO2.

Introduction

Because of an extensive use of lindane worldwide and in France, residual pesticide is commonly detected in food, living organisms, soils and surface waters [START_REF] Belamie | Contamination des eaux de surface par les produits phytosanitaires[END_REF][START_REF] Deo | Biodegradation of hexachlorocyclohexane isomers in soil and food environment[END_REF][START_REF] De Cruz | Residues of chlorinated pesticides in eggs of Grey Heron (Ardea cinerea L.): contribution of capillary gas chromatography ion-trap mass detection[END_REF]. Soil bioremediation is an effective means to reduce the concentration of pesticides in the environment. For many years, various research groups have been interested in the development of white rot fungus technology for th biodegradation of organic environmental pollutants. Among these fungi, Phanerochaete chrysosporium has been extensively studied because of its ability to degrade a wide variety of organic pollutants [START_REF] Barr | Mechanisms white rot fungi use to degrade pollutants[END_REF]. P. chrysosporium was able to mineralize the chlorinated insecticide lindane (1,2,3,4,5,6hexachlorocyclohexane, CAS no. 58-89-9) in liquid cultures (Bumpus et al., 1985;[START_REF] Kennedy | Comparative biodegradation of alkyl halide insecticides by the white rot fungus, Phanerochaete chrysosporium[END_REF]. However, little is known about the mechanisms involved in the lindane breakdown by the fungus. [START_REF] Mougin | Biotransformation of the insecticide lindane by the white rot basidiomycete 7 Phanerochaete chrysosporium[END_REF] shed some light on these mechanisms, by identifying the main metabolites during lindane breakdown in P. chrysosporium liquid cultures. They suggested the involvement of P450s in some reactions leading to the insecticide degradation. In soils, [START_REF] Kennedy | Comparative biodegradation of alkyl halide insecticides by the white rot fungus, Phanerochaete chrysosporium[END_REF] observed a significant mineralization of lindane over 60 d following inoculation with the fungus established on corn-cobs. We have evaluated the effect of bioaugmentation on lindane transformation in sterile and non-sterile soils using P. chrysosporium.

Materials and methods

Soil preparation and incubation

Soil samples were collected in early March 1994, in the 10-20 cm layer in Versailles (France). Soil was a silt loam, comprising 25.5% sand, 55.0% silt and 19.5% clay. The content of organic matte in the soil was 1.65%. The soil pH was 8.1, and the cation exchange capacity was 10.2 meq 100 g -1 The soil was gently air-dried for 4-5 d in the laboratory, sieved (2 mm) and stored at l 5°C for less than 2 months. Before use for experimental work, a portion of the soil sample was sterilized by autoclaving at 121°C for 20 min. Soil incubations were done using 25 g dry soil in 150-ml Erlenmeyer flasks, inoculated with spores of P. chrysosporium BKM-F-1767 (ATCC 24725). An aliquot of 7-8 ml of culture medium [START_REF] Mougin | Biotransformation of the herbicide atrazine by the white rot fungus Phanerochaete chrysosporium[END_REF] was added to achieve 35% (w/w) soil moisture content. The moisture content was kept constant during the experiments by adding sterile water. The medium was supplemented with a mixture of unlabelled and labelled (3.3 kBq) lindane (l191 MBq mmo1 -1 , radiochemical purity > 99.5%, Rhone-Poulenc Agrochimie, Lyon, France) to give a final concentration of 0.8 µg g -1 . The Erlenmeyer flasks were sealed with cotton plugs and incubated in 1-1 sealed flasks with three vials containing 10 ml of I N NaOH (to trap CO2), 10 ml water (to keep moisture constant), and 10 ml n-hexane (to trap volatile organics), respectively. The headspace of the flasks was flushed with air at the begining of the experiments, and then every 7 d. 14 CO2 production and lindane degradation were assessed for 9 wk. Samples were incubated in darkness at 25°C.

Fungal biomass determination

Fungal biomass in soils was estimated from extractable ergosterol, according to the modified method of [START_REF] Seitz | Ergosterol as an indicator of fungal invasion in grains[END_REF]. Each flask was supplemented with 150 ml methanol, 50 ml ethanol and 20 g KOH, and refluxed for 60 min at 90°C. The cooled saponified mixture was filtered on a Buchner funnel, prior to clean-up. Twenty millilitres of the filtrate were applied on to a glass column packed with a small wad of glass wool and 20 g Extrelut (Merck, Chelles, France). The column was left for 30 min before elution in darkness with 90 ml n-hexane. The organic fraction was evaporated to complete dryness. The residue was dissolved in 20 ml ethanol and sonicated for 1 min.

Ergosterol in the ethanolic solution was quantified by HPLC. Usually, l00 µl of the solution were injected through a 7125 Rheodyne valve onto the analytical column ODS-80TM (25 cm x 4.6 mm i.d.) (Varian). A Varian 9010 pump delivered the mobile phase consisting of the mixture acetonitrile-ethanol-H3PO4 (55/45/0.05; v/v/v) at a rate of 1 ml min -1 . Detection was done in a Varian 9050 detector at 282 nm. A calibration curve was made with authentic ergosterol (Fluka, St. Quentin Fallavier, France), and a correlation curve between dry fungal biomass and culture ergosterol content was established. This gave a linear equation Y (biomass d.w., µg) = 1.9331 + 0.1892X (ergosterol, µg) with a regression coefficient R = 0.992 (data not shown). The ergosterol-to-biomass ratio was around 5.0 mg g -1 , in agreement with the reported value of mg g -1 for P. chrysosporium cultures [START_REF] Davis | Evaluation of methods to extract ergosterol for quantitation of soil fungal biomass[END_REF].

Analytical procedure

The radioactivity in soils samples was extracted by adding 50 ml of methanol-water (80/20; v/v) and shaking for 60 min. The liquid and solid phases were separated by centrifugation, and the solid fraction was extracted a second time. The extracts were pooled, and 80 ml aliquots were diluted with 240 ml of distilled water in a separatory funnel, and then the radioactivity was extracted three times with 100 ml of dichloromethane. After concentration of the organic phase and dissolution in 500 µl methanol, 100 µl aliquots were analysed by HPLC.

Labeled compounds were analysed with the HPLC equipment described above, with a solvent system composed of acetonitrile and water, each acidified with 0.1% H3PO4; it began with 1% acetonitrile for 3 min, followed by a linear increase to l00% acetonitrile over 15 min, and a stationary phase of 10 min. The radioactivity of the column eluate was monitored by an HPLC LB 507 A radioactivity system (EG and G, Evry, France).

Non-extractable radioactivity in the soil samples was determined by combustion in a model 307 oxidizer (Packard Instruments, Rungis, France).

Experimental error

Each experiment was done in duplicate and repeated twice. Results are expressed as means. The standard deviation, excepted in the case of unextractable radioactivity in sterile soils, was less than 10% of the mean. Data presented in Table 1 were submitted to ANOVA to test for differences among soil treatments. In the case of significant differences, means were then compared using the Newmann-Keuls test [START_REF] Snedecor | Statistical Methods[END_REF].

Results and discussion

Figure 1(A) shows the growth curves for P. chrysosporium in soils. No fungal growth was observed in sterile and non-sterile soils supplemented with culture medium alone.

After spore inoculation of a sterilized soil, the fungal biomass increased rapidly during the first week, and then proceeded at a lower but steady rate during the following 8 wk to a final biomass value of 112.4 mg d.w. The biomass in soil was found to be consistent with the proliferation of white mycelium observed throughout the soil. Conversely, only a limited fungal growth (12.l mg d.w.) was detected in non-sterile soils after inoculation with P. chrysosporium. The amount of fungal biomass decreased slowly to 6.8 mg by the end of the experiment. The low fungal biomass in non-sterile soil is due to the competition of the fungus with the indigenous microflora. In general, the fungus is grown on organic substrates, such as corn-cobs, before inoculation to soil. Our results indicate that a liquid inoculum, consisting of culture medium supplemented with fungal spores, allows fungal growth in soils, with a uniform distribution of the mycelium. That might ensure a greater pesticide transformation.

After a short lag phase, the released CO2 resulting from indigenous microflora activity amounted to 21.6% of the initial radioactivity after 9 wk [Fig. l(B)]. This increased to 49. l% following fun-gal inoculation. In contrast, no or negligible mineralization (less than 1.0%) occurred in autoclaved soils, with or without P. chrysosporium.

Effects of sterilization and fungal inoculation on lindane dissipation are shown Fig. l(C).

The amount of extracted lindane was found to be considerably reduced in the nonsterilized soils (8.0 and 7.3% of the initial radioactivity), whether with or without the fungus addition. By contrast, much higher amounts of extractable lindane remained in autoclaved soils (67.6 and 71.1%), and appeared almost unaffected by the presence of P. chrysosporium. No metabolites formed from lindane have been extracted from any of the sterile and non-sterile samples.

Mass-balance analysis of 9-wk P. chrysosporium incubations are shown in Table l. In sterilized soils, total 14 C recovered was 81.0-83.0% of the initial radioactivity. The loss of 14C (around 20.0%) could be partly due to adsorption to the glass flasks. Although a high soil moisture content may favour lindane volatilization [START_REF] Waliszewski | Residues of lindane, HCH isomers and HCB in the soil after lindane application[END_REF], labeled volatiles were not trapped under these incubation conditions. Conversely, in non-sterilized and non-inoculated soil, total 14C recovered represented only 52.4% of the initial radioactivity. An explanation for this loss of radioactive material could be the volatilization of degradation intermediates, as 2.6% of the initial radioactivity were counted during the 9-wk incubations in hexane-containing traps. In agreement with this result, microbial degradation of lindane has been reported to produce volatile transformation products, such as low chlorinated benzenes and phenols, and chlorinefree compounds [START_REF] Deo | Biodegradation of hexachlorocyclohexane isomers in soil and food environment[END_REF]. Headspace solid-phase microextraction (SPME) and g.c. analysis (either with f.i.d. or e.c.d.; [START_REF] De Cruz | Residues of chlorinated pesticides in eggs of Grey Heron (Ardea cinerea L.): contribution of capillary gas chromatography ion-trap mass detection[END_REF] were performed. Nonchlorinated and chlorinated compounds, more polar than lindane, were detected under these conditions. However, their concentrations were too low for identification by mass spectrometry. Fungal in-oculation of non-sterilized soil led to a better 14C recovery, whereas no volatiles were trapped under these conditions. The fungus seems to be involved in the modification of the insecticide degradation pathway, by increasing the conversion of volatile chemical intermediates produced by the indigenous microflora to CO2.

The above results confirm that the bioremediation of soils may be achieved by using a bioaugmentation approach with P. chrysosporium. The fungus appears to be a potential tool to reduce the environmenal contamination by xenobiotics. At least, in the case of lindane, it may establish synergistic relationships with the indigenous microbial community.

Table 1. Mass-balance analysis of 14 C in soil cultures after 9-wk incubations in the presence of labeled lindane.

Fig. 1 .

 1 Fig. 1. Fungal growth (A), lindane mineralization (B) and extraction (C) in soil cultures. Symbols refer to soil treatments: O, sterile-not inoculated; , sterile-inoculated; , not sterile-not inoculated; and , not sterile-inoculated.

Acknowledgements

We gratefully acknowledge Rhone-Poulenc Agrochimie (Lyon, France) for the generous gift of labeled lindane.


P Arurault
email: arurault@chimie.ups-tlse.fr

Gilbert Combes

Laurent Teyssedre
email: gilbert.teyssedre@laplace.univ-tlse.fr

C Aridon

Semprimoschnig

DEVELOPMENT OF WHITE OXIDE-PLASMIC COATINGS WITH IMPROVED THERMO-OPTICAL PROPERTIES FOR SPACE APPLICATIONS

Keywords: Aluminum oxide, MAO process, dielectric and optical properties

published or not. The documents may come

Development of a white oxide-plasmic coating with

improved thermo-optical properties for space applications

INTRODUCTION

Spacecraft and scientific instruments usually contain hardware, including sensitive detectors, which require specified temperatures ranges. The knowledge of the thermal properties (absorptance and emittance) of the used coatings is therefore essential for the thermal design of the spacecraft according to mission specifications. In addition to these basic requirements, it is also necessary to be compliant with other spacecraft requirements such as electrostatic discharges and particulate and molecular contamination. Then, before choosing the proper thermal control coating, the way of degradation of these properties has to be investigated under exposure to UV, high energy protons and electrons and thermal cycling. Coating designed to have specific radiative properties are called thermal control coatings. In the case of very high temperature applications, black coatings, prepared by using a micro-arc oxidation process (MAO), are already qualified to the space constraints. As heat absorbing surface, they provide high absorptivity and emissivity properties. For very specific missions, like solar orbiter, a radiator surface through a white coating with low absorptance, lower than 0.3 and high emissivity could provide significant benefits. The MAO process, also called "plasma electrolytic oxidation", is an electrochemical surface treatment generating oxide coatings on some metallic substrates (Al, Ti or Mg). Such coatings are generally used for their high hardness in tribology applications but the very good adherence on the substrate at high temperature is considered here for a use as an external coating on passive thermal protections. Hence, resulting thermo-optical properties have been investigated.

MATERIALS AND CHARACTERISATION TECHNIQUES

Rectangular specimens of aluminum 1050 of size 180x150x1 mm 3 were used as substrates. The MAO process was carried out with conventional oxidation equipment consisting of a stirring, cooling system and a power supply. The electric parameters such as voltage, current, frequency can be adjusted. During the experiment, the aluminum specimen and stainless steel plates were used as anode and cathode respectively. The electrolyte was a solution of sodium silicate (concentration between 5 and 30 g/L) and potassium hydroxide (1 g/L). During the oxidation, the temperature of the electrolyte was kept below 25°C. Coatings obtained with different process conditions, were characterized. The main process parameters are summarized in Table 1. The solar absorptance was measured with an Elan Informatique Absorptiometer IR EL520 using an integrated sphere (hemispherical measurement). The normal measurement of emissivity was performed on an Elan Informatique Reflectometer EL510. The absorptance α quantifies the amount of solar rays absorbed by the coating and the emissivity ε the coating capacity to convert its thermal energy into IR ray. The microstructure of the coatings was observed by Field-Emission Gun Scanning Electron Microscopy (FEG-SEM) with a JEOL JSM6700F device. Structural characterization of the coatings was achieved by a set of techniques ranging from XRay diffraction (Bruker D4 Endeavor using a Cu Kα radiation), thermogravimetric analysis -TGA-, differential thermal analysis -DTA-and Electron Probe MicroAnalyser (EPMA).

For DC current measurements, an aluminum top electrode, 12mm in diameter, was deposited on the sample surface by CVD (chemical vapor deposition) while the ground electrode was the aluminum substrate. Current measurements were carried out under vacuum (3.10 -7 mbar) at room temperature by means of a Keithley 617 ammeter. The surface resistivity was measured using the concentric ring probe configuration.

The inner electrode and the guarded ring were deposited by CVD.

RESULTS AND DISCUSSION

Characterisation of the coating

The surface microstructures of the coatings are presented in Figure 2. The surface roughness which can be observed is characteristic of the MAO process. The pores corresponding to the spark channels can also be seen. Figure 3 shows that the coatings are made up of crystalline phases as described by the literature but also by a part of amorphous phases. The crystalline phases of external layer of the coatings are a blend of α and γ alumina and also aluminum silicate. The internal layer is more crystallized even though an amorphous part is still present. The internal layer is constituted by the same crystalline phases than those of the external layer.

#2 #3

The crystallinity of the external surface is dependent of the process. Figure 4 shows typical current transients obtained when applying voltage steps in the range 0 to 100 V followed by short-circuit steps. The polarization time was fixed to 10 minutes and can be considered as leading to a representative conduction current. The volume resistivity values, estimated from the value of the current at the end of the polarization step, and obtained from a linearization of the I(V) data are given in Table 3, along with surface resistivity results. Table 3 highlights that the surface and volume resistivities of sample #1 are more than two orders of magnitude higher when compared to the three others. This result will be commented in the Discussion.

Optical measurements

Thermal treatment and air storage effects on optical and dielectric properties

Thermal effects in space can affect the efficiency of the thermal control coatings through changes in emittance, solar absorptance, loss of adhesion or changes in electrical conductivity. Sample #4 is the most appropriate sample for a space application as cold coating, because of its good optical properties and low conductivity compared to the other tested samples. In the following, the effect of a thermal treatment will be investigated on structural, optical and electrical properties.

For structural properties, the thermal treatment consisted in annealing samples under air atmosphere at 200°C during 5 hours. Figure 5 shows the effect of this treatment on the crystalline structure of the coating. It can be seen that an increase of the temperature increases the crystallinity of the external coatings even though a part of amorphous phases is still present. At 200°C, the amount of γ alumina increases, it can be explained by the crystallization of the hydrated amorphous phases of alumina such as Al(OH) 3 or AlOOH. The thermo-optical properties were measured at room temperature just after the treatment to avoid water absorption along with after storage in air at room temperature. The results do not reveal any significant variation of the optical properties after heating or after storage in air, pointing towards stable optical properties of the coatings.

For dielectric measurements, the thermal treatment consists of an in situ annealing at 200°C for 1 hour.

Figure 6 shows the corresponding results. The peak observed at around 100°C on |Z| for the untreated sample is most probably due to the release of water from the coating. After the thermal treatment, this peak disappears, and the value of the impedance increases, clearly indicating an increase in the resistivity of the material. Voigt and co-authors [START_REF] Voigt | Correlation of chemical composition and electrical properties of rf sputtered alumina films[END_REF] studied the correlation between chemical composition and electrical properties of rf sputtered alumina films. They found that the leakage current increases with the concentration of OH groups, which act as defect sites. As OH groups are presumably located preferentially in the amorphous phase, the way of investigating it consisted in combining TGA and DTA. Table 4 shows the weight losses in the sample interpreted as arising from free water release, and from the known thermodynamic stability of aluminas given, in increasing order of stability, as:

Al(OH) 3 AlOOH γ-Al 2 O 3 δ-Al 2 O 3 θ-Al 2 O 3 α-Al 2 O 3
The lower losses of water associated with OH groups in sample #1 appear to reflect the higher crystalline phase content for this material. It can be seen that electrical resistivity decreases as the concentration of Al(OH) 3 and AlOOH groups increases, being the higher for sample #4. Therefore, the presence of OH groups could explain the lower electrical resistivity of coatings.

Finally, the good thermal stability of the coatings deserves to be mentioned: the weight losses for temperature increase up to 1400°C are in the range 0.6 to 2.3%, about 20% of it corresponding to loss of adsorbed water.

CONCLUSION

We have correlated the operational conditions of the Micro-Arc Oxidation (MAO) process to the structural, dielectric and thermo-optical properties of coatings grown onto Al substrates. The main outputs of the study are as follows: i/ The coatings formed by the MAO process fulfill the target properties such as low solar absorptance (α<0.3) and high IR emissivity (ε>0.8) before exposure to UV, high energy protons and electrons and thermal cycling.

ii/ Strong variation in electrical conductivity of the coatings were obtained depending on process conditions. Relatively low electrical resistivity materials can be processed, which appears promising for space application.

iii/ Low resistivity appears to go with appropriate thermo-optical properties; the low resistivity could be associated to the presence of OH groups in the coatings.

A first set of tests has been performed on the developed white thermal coatings. Studies are in progress to test adhesion, thermo-optical and electrical stability in Earth storage and for space environment, especially thermal cycling, UV and irradiation by charged particles. Results will lead to select the suitable thermal control coating.

Figure 2 :

 2 Figure 2: SEM images of the coatings

Figure 3 :

 3 Figure 3: XRD pattern of the external layer

3. 3 .

 3 Electrical measurements Electrostatic discharges (ESD) can occur in space between various parts of a spacecraft due to differential charging of the spacecraft caused by energetic charged particles. A design rule consists in avoiding resistive materials. Specifications are: o Surface resistivity <10 9 Ω/square o Volume resistivity < 10 13 Ω.m Nevertheless, these values are given as indications only. Indeed they are not sufficient to evaluate the risk of ESD as it also depends on the thickness and on the location on the satellite (internal, external under MLI,…).

Figure 4 :

 4 Figure 4: Transient currents for sample #4 under vacuum at room temperature.

Figure 5 :

 5 Figure 5: Effect of temperature on Xray Diffraction patterns

Figure 6 : 1 Figure 7 :

 617 Figure 6: Impedance (solid symbols) and phase angle (open symbols) measured at 0.1Hz as a function of temperature prior to (triangles) and after (circles) thermal treatment on sample #4

Table 1 : Process parameters for coatings preparation (J=current density, t= process duration, d = substrate thickness). J is given in relative value for confidentiality purpose

 1

	Sample	J (r.u.)	t (min) d (µm)
	#1	J 1	150	200
	#2	2.4xJ 1	50	2000
	#3	1.5xJ 1	30	1000
	#4	2.8xJ 1	30	1000

Table 2

 2

gives the optical parameters of the coatings. Samples #2 and #4 have optical properties complying with the specification of a cold coating. The comparison between table 1 and 2 indicates that high current density and high silicate concentration are required to obtain good optical properties.

Table 2 : Thickness, solar absorbance α and thermal emissivity ε of the coatings. Sample Coating Thickness (µm) α (±0.04) ε (±0.04) Cold coating specification s

 2

	#1	95	0.55	0.77
	#2	100	0.26	0.83	α<0,3
	#3	50	0.38	0.76	ε>0,8
	#4	100	0.30	0.77

Table 3 : DC electrical characteristics of coatings.

 3

	Sample	Volume resistivity (Ω.m)	Surface resistivity (Ω/square)
	#1	4.1.10 12	1.10 16
	#2	6,4.10 9	5.10 11
	#3	8.6.10 9	2.10 12
	#4	2.0.10 9	3.10 11

Table 4 : Weight losses (in weight %) attributed to water of the various samples as obtained by TGA. Temperature intervals were defined according to the range of thermal stability of the corresponding groups.

 4

	Sample	Adsorbed	Al(OH) 3	AlOOH
		water	(100-400°C)	(400-700°C)
		(<=100°C)
	#1	0.12	0.22	0.13
	#2	0.35	0.83	0.54
	#3	0.22	0.50	0.25
	#4	0.57	1.12	0.60

ACKNOWLEDGEMENTS

We are imbedded to Region Midi-Pyrenees for support to this work. GIT and ASTRIUM acknowledge the European Spatial Agency for their support.


domain: spi phys spi.mat

text:
Kremena Makasheva
email: kremena.makasheva@laplace.univ-tlse.fr

Bernard Despax

Laurent Boudou

Christian Laurent

Gilbert Teyssedre

A Discussion on the Likely Mechanisms for Dielectric Charging in AFM

Keywords: AFM, dielectrics, charging mechanisms. I

Charge distribution estimation at the surface and in the bulk of dielectrics is of great interest, both theoretically and experimentally, not only because it brings information on the storage and transport properties of the medium, but also because of various possible applications of thin dielectric layers e.g. in the field of miniaturized components. Scaling down the dimensions of these components to nanoscale level creates difficulties with the application of the commonly used diagnostic methods. With a resolution in the nanometer range, the atomic force microscopy (AFM) can be used to this field. By applying a voltage between the microscope tip and a sample, both conducting and insulating surfaces can be imaged. The AFM can be applied for charging of samples in contact mode or when a tip-to-sample spacing is introduced. The purpose of this work is to discuss about appropriate mechanisms for dielectric charging in AFM that are still controversial in the literature.

INTRODUCTION

Charge distribution on the surface and in the bulk of dielectric materials is of great interest, both theoretically and experimentally, because of various possible applications. One can find a good review on mechanisms for insulator charging and charge detection methods in the recently published review paper of Rezende et al. [START_REF] Rezende | Detection of charge distributions in insulator surfaces[END_REF]. As it is shown in Ref. [START_REF] Rezende | Detection of charge distributions in insulator surfaces[END_REF], new diagnostic techniques are being developed, in the last two decades, to answer the increasing demand for characterization of miniaturized components used in microelectronics, telecommunications, electrophotography, electrets, etc. Scaling down the dimensions of these components to nanoscale level creates difficulties with the application of commonly used diagnostic methods. The well-known nondestructive diagnostic techniques for measurement of surface charge distribution in insulators like thermal-pulse or pressure pulse methods or probe-based methods have only achieved 10 m of lateral resolution [START_REF] Sessler | Physical principles of electrets[END_REF]. Recently developed diagnostic techniques, like Focused Laser Intensity Modulation Method [START_REF] Petre | Threedimensional space charge cartographies by FLIMM in electron irradiated polymers[END_REF], allow micrometer scale lateral resolution. Unfortunately, it remains inapplicable for charge detection in nanoscale dielectric layers.

Since the scanning force microscope (SFM) [START_REF] Binning | Atomic Force Microscope[END_REF] was developed, it was used for studies of deposition and imaging of charges on insulator surfaces, by means of Kelvin force microscopy mode. By applying a voltage between the force microscope tip and a sample, both conducting and insulating surfaces can be imaged. An interesting review on the aspects of AFM and the probes used in various AFM applications has been recently published [START_REF] Yacoot | Aspects of scanning force microscope probes and their effects on dimensional measurement[END_REF]. The atomic force microscope can be applied for charging of samples in both contact mode [START_REF] Martin | High resolution capacitance measurement and potentiometry by force microscopy[END_REF][START_REF] Terris | Contact Electrification using Force Microscopy[END_REF][START_REF] Schönenberger | Observation of single charge carriers by force microscopy[END_REF] or when a tip-to-sample spacing is introduced [8 -13].

In the literature reporting measurements achieved in AFM, corona discharge is suggested as a mechanism of dielectric charging [8 -11]. We first review this hypothesis. Then we will discuss the possibilities for sustaining gas discharges in small geometries. We will continue with a short discussion on field electron emission. The departure from Fowler-Nordheim theory due to the nanoscale dimensions is discussed next. The last section is about likely mechanisms of dielectric charging in AFM that we propose.

II. CORONA DISCHARGE

Corona discharge is a partial breakdown in gas. It occurs in highly inhomogeneous electric fields [START_REF] Yu | Gas Discharge Physics[END_REF]. Several conditions are required for its ignition: (i) an asymmetric electrode configuration must be present; (ii) a high voltage must be applied to the smaller electrode; (iii) presence of some free electrons is needed; (iv) an avalanche must develop, leaving behind a space charge area. The total current in corona discharges is small, up to hundredths mA, depending on the applied voltage, electrode size and interelectrode distance. It is approximately the same for positive and negative corona discharges in air at atmospheric pressure, as the mobilities of positive and negative ions are close in air. This similarity in the current value of positive and negative corona discharges in air at atmospheric pressure coincides with the similar values of the measured surface potential when the dielectric charging with positive and negative charges is performed in AFM [8 -11]. Perhaps it was a reason for the authors to use the analogy with conventional dielectric charging by corona discharge and to call the mechanism of dielectric charging in AFM corona discharge. Following the general trends of ignition of gas discharges and of ignition of corona discharge, in particular, we can state that the small tip-to-sample distance in the case of AFM dielectric charging does not allow an avalanche build up. Tip-to-sample distances of the order of 100 nm are not much larger than the characteristic size of the small electrode. The electric field can be strong in case of AFM, but not enough strongly inhomogeneous as required for the ignition of corona discharge. A rough estimate of the electric field in AFM gives 10 8 V/m, if we consider a voltage of 20 V and an interelectrode distance of 200 nm (tip-to-sample distance + dielectric thickness). However, tip-to-sample distances of the order of 100 nm are even smaller than the electron mean free path in air at atmospheric pressure = 500 nm. It means that any electron created in the gap will immediately attain the anode without possibility of multiplication, i.e. without an avalanche build up. Corona discharge should thus be excluded from the likely mechanisms for dielectric charging in AFM, but the question of gas breakdown in very small gaps is to be revised.

III. DISCHARGE BREAKDOWN IN SMALL GAPS

The ignition and maintenance of gas discharges in small geometries (micrometer size) change drastically when the gap is in order of only few microns. Papers reporting on discharges in small gaps at high gas pressures (usually atmospheric pressure) [15 -21] show that the breakdown threshold differs from the value estimated from the Paschen's curve when the interelectrode distance is below 5 m. Actually, the deviation from Paschen's law occurs only for those experimental conditions for which the multiplication of electrons is very small. The departure from Paschen's curve was largely discussed first in the work of W. S. Boyle and P. Kisliuk [START_REF] Boyle | Departure from Paschen's Law of Breakdown in Gases[END_REF]: when the multiplication is small, the high electric field is sufficient to draw field emission current from some small irregularities of the cathode. These electrons produce small number of ions, which by space charge effect increase the field on the cathode. This process is able to build up to a breakdown even with an extremely small ionization due to the reduced collisions in small gaps. The mechanisms in place for gas breakdown in very small gaps ( 2-3 m or less, which tend to the typical tip-to-sample distances in AFM dielectric charging) are all connected and primarily depending on the high electric field. They largely differ from the standard mechanisms of ignition and maintenance of gas discharges: ionization in the volume and secondary electron emission. They should be found in the variety of mechanisms solely depending on the electric field. Usually the field dependent mechanisms demand a very high electric field. When dealing with AFM the electric field is of order of 10 8 V/m. The field emission effects, responsible for the departure from Paschen's curve at very small gaps and the theory that describes these effects, given by Fowler and Nordheim, are discussed next.

IV. FIELD ELECTRON EMISSION

Field electron emission is a process of electron emission from solid surfaces (metals, semiconductors and dielectrics) subject of submission to a high electric field, usually larger than 10 8 V/m. It is due to the fact that the high electric field renders the potential barrier at the interface between the emitting surface and the medium (whatever it is: vacuum, gas, solid) so thin (order of tens of Angstrom) that the electrons have chance to tunnel through it. The principles of field electron emission were established by R. H. Fowler and L. Nordheim in their theory based on quantum-mechanical description of tunneling of electrons [START_REF] Fowler | Electron Emission in Intense Electric Fields[END_REF]. In a strict sense, the Fowler-Nordheim theory applies for metals but it can be used for other materials if some additional assumptions that describe the electrode nature are made.

The field electron emission is measurable above a threshold field, which is determined by the work function of the emitting surface. Typical theoretically obtained values of the threshold field are in the range 10 8 V/m -10 9 V/m. We consider here the current density according to the theory of E. L. Murphy and R. H. Good [START_REF] Murphy | Thermionic Emission, Field Emission, and the Transition Regime[END_REF] that describes the combined effect of thermionic and field emission of electrons from metals. The expression (56) from Ref. [START_REF] Murphy | Thermionic Emission, Field Emission, and the Transition Regime[END_REF] represents the Fowler-Nordheim expression for field electron emission current. It gives the zerotemperature approximation. For example, for a field of 2 10 9 V/m when the emitter is a nickel electrode ( = 5.15 eV [23],

is the work function), the emitted electron current density into vacuum is 2.6 10 -3 A/m². For the same electric field, the current density is 2.9 A/m² in case of tungsten electrode ( = 4.55 eV), i.e. higher by 3 orders of magnitude. From these examples, one can notice the important role of the electrode material properties, in particular of the work function.

With the typical values of the work function of pure metals, which are of 4.0 eV or higher, the field emission effect is slightly temperature dependent as shown in Fig. 1 that represents the current density as a function of the electric field when the thermionic and field emissions act together [START_REF] Murphy | Thermionic Emission, Field Emission, and the Transition Regime[END_REF]. This underlines one of the outstanding characteristic of the field emission from pure metals: its almost complete temperature independence up to 600 K. For the logarithmic scales of the electron current density emitted from pure metals, as used experimentally, the temperature effect on field emission currents can be disregarded. We will see later in our analysis that it is not the case when the material is characterized by a low work function ( < 2.0 eV), like for some materials used in the fabrication of AFM tips [START_REF] Yacoot | Aspects of scanning force microscope probes and their effects on dimensional measurement[END_REF][START_REF] Choi | Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique[END_REF][START_REF] Zheng | Modulating the work function of carbon by N or O addition and nanotip fabrication[END_REF]. Fig. 1. Current density as a function of the electric field due to the thermionic and field emissions acting together. The current density is calculated according to expression (56) in Ref. [START_REF] Murphy | Thermionic Emission, Field Emission, and the Transition Regime[END_REF] for Ag ( = 4.64 eV) and Au = 5.47 eV), for three different temperatures T = 0 K (solid line); 300 K (dash line) and 600 K (dot line).

As far as the geometry of the emitter is considered, the Fowler-Nordheim representation is verified for electron emitter, which is a sharply pointed wire, of apex radius of the order of m [START_REF] Dyke | Field Emission : Large Current Densities, Space Charge, and the Vacuum Arc[END_REF]. It was found that the experimental plot ln I = f (1/V) is linear for a considerable range of currents below a critical value I c . For currents higher than I c , which corresponds to very high electric field intensities, a departure from linearity towards lower currents for a given voltage occurs. This departure is due to the space charge limited current (SCLC) [START_REF] Dyke | Field Emission : Large Current Densities, Space Charge, and the Vacuum Arc[END_REF]. However, we should bear in mind that the hypotheses of Fowler-Nordheim theory do not apply for all geometrical configurations and electric field distributions. The Fowler-Nordheim theory describes a planar cathode-anode gap with uniform electric field. It is assumed that the potential barrier is exact or rounded triangular barrier. This assumption is valid for potential barriers having width significantly smaller than the emitting surface, i.e. for bulk metals (dimensions larger than 100 nm). Under the above assumptions the free-electron model is correct and the WKB approximation is proper to estimate the barrier transmission coefficient, which prevents from solving the strict Schrödinger's equation. When using divergent field configuration, as the one in AFM, and emitters with radius of only few nanometers, a departure from the Fowler-Nordheim theory should be expected. In particular, it has been established that field emission current-densities from nanometer-size emitters can be several orders of magnitude higher than traditional field-emitting objects. The main reason could be that the traditional Fowler-Nordheim theory of field emission becomes doubtful and the strict treatment of the problem requires a revision of some of the hypotheses of the theory.

V. LIKELY MECHANISMS OF DIELECTRIC CHARGING IN AFM

Due to the peculiar features of field emission observed with nanometer size field emitters leading to higher current density than that obtained with traditional field emitters, charging of the dielectric surface in tip-to-sample mode in AFM through field electron emission can be envisaged as the operating mechanism. Quickly after reaching the dielectric surface the electrons will be trapped in electron acceptors levels (electron traps) that are due to the typical physical and chemical disorder existing at any dielectric surface [28 -30].

To substantiate our hypothesis about the primary role of the field electron emission in the dielectric charging in AFM, we will present numerical results for the emitted electron current density from different materials used for the fabrication of AFM tips [START_REF] Yacoot | Aspects of scanning force microscope probes and their effects on dimensional measurement[END_REF][START_REF] Choi | Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique[END_REF][START_REF] Zheng | Modulating the work function of carbon by N or O addition and nanotip fabrication[END_REF]. These results are obtained according to the theory of E. L. Murphy and R. H. Good (the expressions for the current density given in Ref. [START_REF] Murphy | Thermionic Emission, Field Emission, and the Transition Regime[END_REF]) that describes different type of electron emission: the thermionic emission, field electron emission and the intermediate region.

Figure 2 shows results for the current density as a function of the electric field in the case of thermionic and field emissions acting together at T = 300 K (solid lines) and in the the zero-temperature approximation when only the field electron emission proceeds, for the materials studied in [START_REF] Choi | Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique[END_REF], cf. Table 1. [START_REF] Reagan | Electric charging and nanostructure formation in polymeric films using combined amplitude-modulated force microscopy-assisted electrostatic nanolithography and electric force microscopy[END_REF]. Results for the current density, presented in Fig. 2, clearly show the impact of temperature increase when the work function is below 2 eV, i.e. the role of thermionic emission being important even for a temperature T = 300 K. As expected this effect is more pronounced at low electric field (E < 1.8 10 8 V/m) for all the cases considered here. The low limit of field emission decreases with the decrease of the work function and in the case of a work function as low as = 0.47 eV, the thermionic and the field emission act together even for temperatures below T = 300 K. The high limit of field emission, which represents the applicability of Fowler-Nordheim formula, shifts towards higher electric fields with the increase of the work function and for Silicon ( = 4.3 eV) this limit at T = 300 K is E = 1.0 10 10 V/m.

Following the above presented results and their analysis one can suggest that the field electron emission acting simultaneously with the thermionic emission can be considered as the main mechanisms of dielectric charging in AFM. However, other mechanisms of charge generation should be considered as well. Because the electric field is high at the dielectric/metallic substrate interface (back side of the dielectric layer in the considered configuration), injection of positive charges can occur with a rate depending on the nature of the interface. This would lead to internal positive charging with a possible transportation of these charges towards the surface. Such injection process has been observed in dielectrics above field value of about 10 7 V/m, one order of magnitude lower than the average field considered here. As a result, the AFM measurement should "probe" the negative charge deposited by the tip minus the influence of the positive charge injected in the dielectric from the back electrode. Whether this occurs or not is beyond the scope of our current discussion but it certainly must be considered in future studies.

Nevertheless, there is still to explain the positive charging of the dielectric surface when the AFM tip is at a positive potential. This can be discussed taking into account that the tip electrode at a positive voltage would act as a collector for electrons extracted from the dielectric surface. In this scheme, the dielectric surface is the electron emitter and the tip is the collector, leading to a positive charging of the dielectric. This process can be envisaged taking into account the high field value and the nature of the dielectric surface layer characterized by surface states with lower ionization energy than the bulk material. This means the extracted electrons are not those from the dielectric valence band. Surface states have been evidenced experimentally and could provide adequate centres with a low ionization potential, without the need to remove valence band electron, a mechanism energetically more demanding.

VI. CONCLUSION

Measurements of dielectric charging using AFM tips with different work function but the same curvature radius are under way to substantiate our analysis both qualitatively and quantitatively.

Fig. 2 .

 2 Fig.2. Current density as a function of the electric field: (solid lines) the thermionic and field emissions acting simultaneously, T = 300 K; (dash lines) the zero-temperature approximation when only the field electron emission proceeds, for the materials studied in[START_REF] Choi | Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique[END_REF], cf. Table1.

Table 1 .

 1 Materials investigated for the use as AFM tip[START_REF] Choi | Field-emission characteristics of nitrogen-doped diamond-like carbon film deposited by filtered cathodic vacuum arc technique[END_REF]. DLC= diamond-

	like carbon.
	Material	N-doping for DLC	(eV)
	Silicon	--	4.30
	DLC coated silicon	--	1.86
	DLC coated silicon	5 sccm	1.43
	DLC coated silicon	7 sccm	1.27
	DLC coated silicon	10 sccm	0.47
	zero-temperature approximation, i.e. when only the field
	emission proceeds (dash lines). The presented curves are for
	the materials listed in

Table 1 .

 1 We can notice from the data presented in Table1that the work function is strongly modified after surface treatment of the silicon emitters. The DLC coating leads to a decrease of the work function with more than a factor of 2. Doping of the DLC-coated Si-emitters with N atoms has an additional impact on the work function. With the increase of nitrogen concentration, the work function decreases due to a displacement of the Fermi level towards the conduction band. Because of the decrease of the work function of material, the current density increases by several decades for the same applied electric field. For typical values of the electric field in the AFM configuration (10 8 V/m) the measured currents can be quite significant. An estimate for 20 nm radius of apex of the AFM tip gives current values of about tens to hundreds of pA, which correspond to the current values measured in AFM experiment

ACKNOWLEDGMENTS

This work was supported by the RTRA STAE in France under project SYMIAE.


text:
Kremena Makasheva
email: kremena.makasheva@laplace.univ-tlse.fr

H Wang

Bernard Despax

Laurent Boudou

F Coccetti

Gilbert Teyssedre

New materials versus new design: study on the physico-chemical and electrical properties of thin SiO x N y layers for their use in RF-MEMS switches

come

New materials versus new design: study on the physico-chemical and electrical properties of thin SiOxNy layers for their use in RF-MEMS switches

INTRODUCTION

Dielectric materials are regularly exploited in a large variety of solutions in electrical engineering. Quite often they make part of these systems as components on different scale levels: from millimeter to nanometer size. The insulating materials in use require a deep knowledge on their composition, physical and electrical properties [START_REF] Rezende | Detection of charge distribution in insulator surfaces[END_REF]. For example, the dielectric charging phenomenon is one of the challenges to meet because of the necessity to control or to avoid it. The dielectric charging is an obstacle for many electrical engineering solutions as it modifies the control parameters and in certain cases it can even lead to complete loss of command of these systems. This is also the case with the performance of MicroElectroMechanical Structures -RF-MEMS capacitive switches. Currently, the dielectric charging of RF-MEMS switches is the main cause of failure and low reliability of capacitive switches with electrostatic actuation [START_REF] Lamhamdi | Voltage and temperature effect on dielectric charging for RF-MEMS capacitive switches reliability investigation[END_REF]. In general, in our work, we are interested in revealing the mechanisms at the origin of dielectric charging phenomenon and in this case, our effort is directed to an improvement of the reliability of technological devices containing thin insulator layers. The purpose of this work is to elaborate thin dielectric layers with adequate performance for their use in RF-MEMS capacitive switches. To achieve this, instead of elaborating new materials, we choose to check a new concept: we initiated a study on the properties of a system of dielectric materials that will gain from design rather than from composition. Our research effort is directed towards the implementation of thin dielectric layers either constructed as multi-layer systems or with gradual variation of their properties. Both steps the deposition and the characterization (structural and dielectric ones) of those dielectric systems are envisaged in our work. Before going to the assembly, we should know well and to be in command of the properties of the dielectric mono-layers. Thus we first characterized mono-layers with different chemical compositions, deposited singly on a substrate before evaluation of the performance and reliability of the multi-layer systems. In this work we present results from experimental study on the physico-chemical and electrical properties of thin dielectric silicon nitride-oxide layers (SiO x N y ). These dielectric layers are deposited by using pulsed plasma-enhanced chemical vapor deposition (PPECVD). Details on the deposition technique and the experimental conditions are given in the next section. A brief description of the applied diagnostic methods is then presented. Later on in our paper, we will report the obtained results, which will be immediately followed by a discussion. Finally, we will give our concluding remarks.

EXPERIMENTAL DETAILS

Materials

To elaborate our dielectric layers, we use pulsed plasma-enhanced chemical vapor deposition (PPECVD) [START_REF] Bedjaoui | Physico-chemical, Structural and Physical Properties of Hydrogenated Silicon Oxinitride Films Elaborated by Pulsed Radiofrequence Discharge[END_REF][START_REF] Perret-Tran-Van | Controlled fabrication of Si-nanocrystals embedded in thin SiON layers by PPECVD followed by oxidizing annealing[END_REF] as a deposition technique. PPECVD is known to be technologically compatible, highly efficient, performing, reproducible and quite flexible so as to elaborate materials with different composition and high quality. Thin sub-stoichiometric silicon nitride-oxide layers (SiO x N y , x < 2, y < 1) are deposited on Au/Ti (200nm/50nm) metalized Si (100) wafers in the plasma of radiofrequency (f = 13.56 MHz) capacitively coupled discharge sustained with power of 5 W. The CW (continuous wave) signal is modulated by square pulses with period T = 400 ms. The pulse duration is 100 ms (duty cycle = 0.25). The gas mixture contains SiH 4, N 2 O and He and the total gas pressure in the discharge reactor is p = 65 Pa. The total gas flow is 210 sccm. Helium component in the mixture is fixed to 40%. The gas flow ratio =N 2 O/SiH 4 allows adjusting the silicon concentration in the film. In our case, it varies between = 100 and = 2. The lower the gamma ratio, the richer in silicon the dielectric film is. High value ( = 100) means that the dielectric layer owns properties close to that of a SiO 2 layer. The metalized silicon substrates are clamped on the grounded electrode. The substrate temperature is kept to T s = 573 K during the deposition process. Different times for deposition have been used according to the envisaged layer composition and thickness. Single layers with different compositions (different values; = 100, 25, 10 and = 2) have been deposited separately on Au/Ti metalized Si wafers. A multi-layer structure (4 layer structure) containing layers with the same values as for the single layers, was deposited on an intrinsic silicon (100) substrate for simplicity of the following material characterization.

Applied diagnostic methods

Several diagnostic methods have been applied to determine the physico-chemical and electrical properties of the deposited SiON layers. The results presented in next section are from diagnostic of these layers in ellipsometry, transmission electron microscopy TEM, X-ray photoelectron spectroscopy as well as from electrical measurements consisting of transient DC charging/discharging current measurements. Details on each diagnostic technique are specified when the results obtained from that type of diagnostic are discussed.

RESULTS AND DISCUSSIONS

Some of the results from our study on thin SiON layers have already been disclosed elsewhere [START_REF] Makasheva | Dielectric layers with gradual properties[END_REF]. In this paper we will complete the study with new results. We have revealed in Ref. 5 that the as-deposited SiON layers, singly or arranged in a multi-layer system, are in an amorphous phase. A significant concentration of amorphous Si-nanoparticles (a-Si = 26%) have been found in the SiON matrix of the layer with = 2 compared to the composition of the layer with = 25, where the a-Si nanoparticles are less than 1%. There is not a-Si nanoparticles in the = 100 thin layer. The SiON layer in that case is close as composition to a SiO 2 layer. These results are consistent with the results for the volume resistivity obtained from electrical characterization. The variation of the calculated volume resistivity as a function of shows a significant decrease of resistivity when is decreased. Furthermore for values under 10, the decrease of volume resistivity is sharp, and the value for = 2 is much than 6 orders of magnitude lower compared to that for = 100. Before going to the presentation of our further results on the study of silicon nitride-oxide thin layers we will pay attention to some peculiarities in the layer's characterization when metalized substrates are utilized.

Key steps to do when using metalized substrates

This part is especially devoted to the characterization of dielectric layers deposited on metalized substrates. It is necessary step because of some particular points to survey during the diagnostic that can influence the procedure and induce uncertainty in the obtained results. As the reliability of RF MEMS capacitive switches is targeted in our study we use Au/Ti metalized substrates. In the current RF MEMS technology the transmission lines are golden made [START_REF] Melle | Reliability Modeling of Capacitive RF MEMS[END_REF]. The reasons for using gold are common: low resistivity, which allows minimum transmission losses and compatibility with planar microfabrication processes. The Ti layer is to guarantee adherence between the Si substrate and the Au layer. The PPECVD process that we use for deposition of SiON layers demands certain time for evacuation of the discharge reactor. We have estimated that an overnight pumping with simultaneous heating (T s = 300°C) assures well enough high vacuum (p = 2 10 -4 Pa) as initial conditions for the deposition process. Moreover it implements annealing of the golden layer. To perform a good diagnostic of the deposited dielectric layers we should first study the modifications that the deposition technique induces in the Au/Ti layers deposited on the substrate and then adapt the diagnostic protocol.

Spectroscopic ellipsometry

Fig. 1 shows light penetration depth in the Au layer for as-deposited Au-layer and one after annealing. It is obtained from spectroscopic ellipsometry measurements. We have used a SOPRA GES-5 spectrometer in the spectral range 250-850 nm at incident angle of 69° degrees. One can see that the long time pumping of the plasma reactor not only assures good initial conditions for the deposition process but improves the transmission properties of the golden layer. To adapt the procedure in ellipsometry we had first to determine the angle of incidence, which is the most appropriate one to perform the measurements. In general, it is the Brewster's angle. The Brewster's angle for Au varies strongly as a function of the wavelength and this variation determines the need of such assessment. In ellipsometry the measured quantities are: tan( ), which represents the ratio between modulus of the reflection coefficients and cos( ) that gives the phase difference induced by the reflection. Cos( ) is extremely sensitive to the angle of incidence. To decide at what angle the measurements must be done it is important to have a good sensitivity all along the wavelength range, which is equivalent to a zero value of cos( ). Figure 2 presents cos( ) measurements as a function of the wavelength for different incidence angles of the golden layer after annealing. It can be seen in Fig. 2 that the best choice of incident angle lays in the range 68° -70°. Our choice was 69°. After the spectra acquisition, it is the simulation procedure of the recorded spectra to be performed. We have used WinElli, software provided by SOPRA LAB [7]. The small light penetration depth (18 nm is the upper limit) in the annealed Au-layer (Fig. 1), compared to the Au-layer thickness (200 nm), means that we can assume the Au-layer as bulk material, consequently as substrate in the simulation of the recorded spectra. Because of the modifications in the properties of Au-layer induced by annealing during the pumping time, the optical indices n and (nrefractive index andextinction coefficient) proper to the annealed Au-layer must be used in the regression of the measured spectra of SiON layers. To extract the needed n, values that will be further used in the regressions we apply the bulk calculation, which assumes that the sample (the Au-layer in this case) is a bulk material.

Transmission Electron Microscopy

Another step to do before characterization of the SiON layers is to verify that there is not diffusion of Au atoms into the deposited layer. This additional check comes out from the deposition conditions as the substrate temperature is kept to 300°C during the pumping time and the deposition one; a value close to the eutectic temperature for Au, which is 363°C [START_REF] Lani | Gold metallizations for eutectic bonding of silicon wafers[END_REF].

We have used transmission electron microscopy (TEM) to observe the SiON layer deposited on Au/Ti Si-substrate in a cross sectional view. To achieve the observation we have prepared specimens transparent to electrons in accordance with the standard procedure: mechanical polishing and Ar + ion milling. The observation in bright field mode [START_REF] Schamm | Imaging Si nanoparticles embedded in SiO2 layers by (S)TEM-EELS[END_REF] is performed on a field emission TEM, PHILIPS CM30. A Bright field TEM image of SiON layer deposited with = 100 is shown in Fig. 3. It can clearly be seen that there is not Au-clusters penetrating in the deposit. Hence the substrate temperature used in the deposition does not provoke contamination of the SiON layers with Au-atoms. A layer thickness of 78.0 1.0 nm was obtained from this technique. In ellipsometry, following the procedure described above and applying the Forouhi-Bloomer approximation for the optical dispersion relation [START_REF] Forouhi | Optical dispersion relation for amorphous semiconductors and amorphous dielectrics[END_REF] in the modeling, we have obtained a value of 79.6 1.1 nm for the same layer, which is close to the value obtained in TEM. Thus, it justifies and confirms all the steps in the procedure of ellipsometry characterization of dielectric layers deposited on metalized substrates. To observe the multi-layer structure we have used a field emission TEM, FEI Tecnai TM F20. This microscope is equipped with a corrector for spherical aberration and the Gatan Imaging Filter (GIF) TRIDIEM. The operation voltage is 200 kV. For EFTEM, we have detected the Plasmon signal (at 17 eV) associated with the Si nanoparticles [START_REF] Schamm | Imaging Si nanoparticles embedded in SiO2 layers by (S)TEM-EELS[END_REF]. All the Si-nanoparticles with diameter larger than 1 nm, either amorphous or crystalline, are visible and appear as bright in a dark background (the SiON matrix). EFTEM image of the multi-layer structure is shown in Fig. 4. The Si substrate appears in bright. We can easily distinguish the 4 layers ( = 100, 25, 10 and 2) deposited on it, starting with the layer that does not contain a-Si. The silicon rich layer ( = 2), is on top of the structure. The assembly of the multi-layer structure containing layers with different conductivities (Si-concentration) implies the option to be in command of the dielectric charging in RF-MEMS switches.

X-ray photoelectron spectroscopy

To enlarge the structural analysis of the SiON dielectric layers and complete the results [START_REF] Makasheva | Dielectric layers with gradual properties[END_REF] obtained from Fourier Transform InfraRed Spectroscopy (FTIR), we have performed X-ray photoelectron spectroscopy (XPS) measurements by using a VG ESCALAB 220-iXL spectrometer. Here it should be pointed out that the XPS technique is sensitive to elements except hydrogen or helium. The layer surfaces of the deposits were excited by energetic Bremsstrahlung radiation produced in the MgK anode (1253.6 eV). The kinetic energies of the electrons emitted from the surface were collected at a constant energy resolution of 1 eV. The energy step used to record the spectra was 0.2 eV. Two SiON layers ( = 100 and = 2) have been characterized. Since the thin film with = 100 is rather nonconductive, electron compensation was used during the measurement. First the surfaces of as-deposited layers have been analyzed. Then they were sputtered with accelerated argon ions (Ar + ) under a pressure of p Ar = 2.5 10 -5 Pa at theoretical rate of about v = 0.1 nm/s. Under the low-energy Ar + sputtering in a zone of about 150 m in diameter and depth of 3 -5 nm, the surface contaminant species were completely removed and the individual atomic concentrations in the bulk were obtained from the integrated peaks after calibration of the background level. We have used the C1s photoelectron peak area as reference. The deconvolution was done by AVANTAGE of ThermoFisher Scientific with Gaussien/Lorentzien functions (G/L ratio: 25% to 70%). The composition of each layer was evaluated from the intensity ratio of the Si2p, N1s and O1s photoelectron lines by using their sensitivity factors. The chemical composition of the as-deposited SiON layers with γ=100 and γ=2 are summarized in Table 1. Then, we provide information about the different chemical bonds and valence states on the SiON surfaces (Fig. 5) and in the bulk (Figs. 6,7) after sputtering. The thicknesses of the SiON layers were found to be 86.7 nm and 82 nm for γ=100 and γ=2, respectively. The results in Table 1 show that the surfaces of both SiON layers were carbon and oxygen contaminated due to their exposure to air. Furthermore, the composition of the layer with γ=100 is close to SiO 2 , while the one with γ = 2 is silicon rich. These results coincide with the results for the same layers obtained from FTIR [START_REF] Makasheva | Dielectric layers with gradual properties[END_REF]. It is now generally accepted for a SiO x layer (0 < x < 2) that the Si2p spectra can be interpreted by five oxidation states Si 0 , Si 1+ , Si 2+ , Si 3+ and Si 4+ [START_REF] Chen | Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition[END_REF]. Thus the SiO x matrix is formed by Si-(Si 4-n -O n ) tetrahedras with n = 0-4. In this scheme the transition from a pure amorphous Si (a-Si, n = 0) where the Si-atom is bounded to four adjacent Si-atoms to a pure a-SiO 2 (n = 4) where the Si-atom is bounded to 4 oxygen atoms is done by consequent replacement of Si-atom by O-atom for each oxidation state. The binding energy range of the transition Si 0 Si 4+ is between 99.3 eV to 103.3 eV with a shift of approximately 1 eV per oxidation state [START_REF] Chen | Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition[END_REF]. Table 1. Summary of XPS measurements (at.%) for SiON thin layers deposited on a Au/Ti Si substrates.

Figure 6 presents the results for the SiON layer with γ = 100. It can be seen (Fig. 6(a)) that the Si2p photoelectron peak includes only two components: the two highest oxidation states, the peak at 102.6 eV with FWHM = 2.0 eV and the one at 103.6 eV FWHM = 2.2 eV. Both peaks are probably slightly perturbed because of the small amount of N-atoms present in the layer composition. This perturbation can be imposed by replacement of an oxygen atom in the Si-(Si 4-n -O n ) tetrahedral structure by a nitrogen atom [START_REF] Chen | Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition[END_REF].

The large FWHM, in this case, confirms that the layer with γ = 100 is close as structure to a stoichiometric SiO 2 layer for which the FWHM is always 2.3 eV. The spectra in Fig. 6(b) is about N1s core level and indicate that there is more than one type of chemical environment for nitrogen [START_REF] Viard | XPS and FTIR study of silicon oxynitride thin films[END_REF]. However, the presence of N-atoms in the SiON network remains minor. The Si2p and N1s core levels of the two samples (γ=100 and γ=2) appear very different. Figure 7 presents the results for the SiON layer with γ = 2. Figure 7(a) reveals the existence of Si 0 at 99.5 eV with FWHM = 1.33 eV. Thus we can confirm the silicon richness in this layer. The splitting of the Si2p photoelectron peak in three other components validates the presence of oxidation states. The first two components are to be attributed to the presence spin orbital splitting Si2p3/2. They can be perturbed by the availability of the small amount of nitrogen (Table 1). The third component, which has a small contribution to the Si2p photoelectron peak, is at 103.6 eV (FWHM 1.67eV). It characterizes the Si-O 4 state; the silicon highest oxidation state. Figure 7(b) shows the N1s core level (fitted to two Gaussian components) and suggests different environment for the N-atoms [START_REF] Viard | XPS and FTIR study of silicon oxynitride thin films[END_REF]. Similarly to the layer with γ = 100, the contribution of N-atoms to the layer network is small. From the above observations we can conclude that Si-Si Si chemical bonds are significantly present in the Si-rich film (γ = 2) whereas in the layer with γ = 100 the highest oxidation states Si-(Si-O 3 ) and Si-O 4 are the only present ones.

Electrical characterization

For electrical a circular golden top electrode of 4 mm in diameter was deposited on the SiO x N y thin film surface by sputtering, while the ground electrode was the Au/Ti layer, deposited on the Si substrate. The conduction current measurements were carried out under secondary vacuum (10 -5 -10 -6 mb) at room temperature. The current flowing through the volume was recorded at different electric fields by means of a Keithley 617 electrometer. The polarization time was fixed to 1000 s and can be considered as leading to a representative conduction current. After each polarization step, the sample was short-circuited for 1000 s and the transient current was recorded. Fig. 8 shows an example of the current transients obtained for SiON layer with =25. This gamma value is representative for the layers with high volume resistivity ( = 100, 25 and 10). The layer thickness was 193 nm as deduced from spectroscopic ellipsometry [START_REF] Makasheva | Dielectric layers with gradual properties[END_REF]. Except for the lowest applied voltages, the charging current reaches a steady state within less than 1000 s. However, in the depolarization stage, much longer time would be necessary to obtain a complete discharge of the samples. Besides, the transient contribution to the charging current is not symmetrical to the discharging current, all the more that the field is high. Finally, it can be seen that the shape of the depolarization current depends on the applied field; meaning that non linear phenomena are at play. Indeed, considering the transient discharging current corresponding to 20V polarization voltage, the current decays with a slower rate at long time. The feature is even more clearly revealed considering depolarization after 30V application that was recorded for 1 hour. All of these features demonstrate that transient processes are controlled by charge accumulation within the material.

The discharge process depends both on the field-induced space charge and the nature of trapping centers that may be dependent on the position in the sample. Further understanding would require identification of the nature of carriers and localization of trapped charges which are actually not available. 9 shows the current vs. field characteristics obtained on the monolayer samples for =2 to 100. As commented earlier, the conductivity significantly increases as is decreased. The response appears linear up to fields of the order of 40kV/mm, without clear effect of the composition.

CONCLUSION

The structural, physico-chemical and electrical characterizations of silicon nitrite-oxide layers deposited singly or in a multi-layer structure show the possibility to assembly a structure with gradual properties and to be in command of the conductive properties of as-deposited multi-layer system. Results from different diagnostic techniques are consistent with each other. The results from XPS measurements not only confirm those from FTIR diagnostic presented earlier but go further in the description of the deposited materials. The electrical measurements demonstrate that transient processes are controlled by charge accumulation within the material.

Figure 1 .

 1 Figure 1. Light penetration depth in the golden layer for the same wavelength range as used in the ellipsometry measurements: as-deposited golden layer (open symbols); golden layer after annealing at 300°C during 16 hours (closed symbols).

Figure 2 .

 2 Figure 2. Cos( )-measurements as a function of the wavelength for different incidence angles of the golden layer after annealing.

Figure 3 .

 3 Figure 3. Bright field TEM image of = 100 layer.

Figure 4 .

 4 Figure 4. EFTEM image of the multi-layer structure.

Figure 5 .

 5 Figure 5. XPS spectra of the layer surfaces: in SiON with = 100 and = 2.

Figure 6 .

 6 Figure 6. Binding energy of SiON layer with γ = 100 after sputtering: (a) Si2p spectrum, (b) N1s spectrum.

Figure 7 .

 7 Figure 7. Binding energy of SiON layer with γ = 2 after sputtering: (a) Si2p spectrum, (b) N1s spectrum.

Figure 8 .

 8 Figure 8. Current transients obtained on SiON with =25 during (a) charging and (b) discharging for applied fields in the range 5 to 150 kV/mm.

Fig.

  Fig.9shows the current vs. field characteristics obtained on the monolayer samples for =2 to 100. As commented earlier, the conductivity significantly increases as is decreased. The response appears linear up to fields of the order of 40kV/mm, without clear effect of the composition.

Figure 9 .

 9 Figure 9. Current density versus electric field for different -values. For =2, the measured current density has been divided by 10 4 for clarity.

ACKNOWLEDGMENT

The authors would like to acknowledge the invaluable help of Dr. Caroline Bonafos (CEMES-CNRS, Toulouse, France) and S. Perret-Tran-Van (LAPLACE, Toulouse, France) for the observations and interpretation of the results obtained by TEM. The work was supported by the RTRA STAE in France under project SYMIAE.