Aurélie Boisnoir

Yves Pierre

Nicolas Pascal

Rodolphe Chomérat

Lemee

Lemée

Distribution of potentially toxic epiphytic dinoflagellates in Saint Martin Island

Keywords: Dinoflagellates, Caribbean Sea, Ostreopsis, Gambierdiscus, Prorocentrum, Ciguatera Fish Poisoning (CFP)

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Introduction

Some benthic dinoflagellates are a major concern for public health due to their toxins production.

However, few studies focus on the taxonomy of benthic dinoflagellates (Momigliano et al. 2013) and the diversity of the microbenthic communities remains largely unexplored especially in the Caribbean Sea [START_REF] Chomérat | -A taxonomical study of benthic Prorocentrum species (Prorocentrales, Dinophyceae) from Anse Dufour (Martinique Island, eastern Caribbean Sea)[END_REF]).

In the Caribbean Sea, Gambierdiscus species are the causal agent of a worry health issue, the ciguatera fish poisoning (CFP) [START_REF] Bagnis R | Origins of Ciguatera Fish Poisoning: A new dinoflagellate, Gambierdiscus toxicus Adachi and Fukuyo, definitively involved as a causal agent[END_REF][START_REF] Litaker R | Ciguatoxicity of Gambierdiscus and Fukuyoa species from the Caribbean and Gulf of Mexico[END_REF][START_REF] Díaz-Asencio L | Ciguatoxin occurrence in food-web components of a Cuban coral reef ecosystem: Risk-assessment implications[END_REF].

The Caribbean Sea presents a CFP incidence rate fluctuating between 12 and 500 cases / 100,000 habitants and is consequently the second region of the world most affected by CFP after the Pacific Ocean (Chinain et al. 2014). Within the Caribbean Sea, Saint Martin is the most affected island with an estimated CFP incidence rate of 100 cases / 10,000 hab. in 1981 [START_REF] Tester | -Ciguatera fish poisoning and sea surface temperatures in the Caribbean Sea and the West Indies, Ciguatera and related biotoxins workshop[END_REF]. However, only a single survey focused on the abundances of the Gambierdiscus genus on corals in this area [START_REF] Bourdeau | -Facteurs de riques ciguatériques aux Antilles dans la région de Saint Barthléméy, Saint Martin et Anguila[END_REF]. In another study dealing with the distribution of Gambierdiscus spp. in the Gulf of Mexico and the Caribbean Sea, three species have been found in Saint Martin without indications about the colonized substrate [START_REF] Litaker R | Ciguatoxicity of Gambierdiscus and Fukuyoa species from the Caribbean and Gulf of Mexico[END_REF]. In both studies, the presence of Gambierdiscus on macrophytes was overlooked despite their potential contribution on fluxes of toxin in food web. Herbivorous organisms can be contaminated through selective ingestion of the surficial biofilm containing the toxic epiphytes [START_REF] Viallon | Contribution of marine invertebrates to ciguatera poisoning : the case study of French Polynesia[END_REF][START_REF] Boisnoir | The use of stable isotopes to measure the ingestion rate of potentially toxic benthic dinoflagellates by harpacticoid copepods[END_REF] or through ingestion of the whole macrophyte covered with toxic epiphyte dinoflagellates [START_REF] Yasumoto | Toxicity of the surgeonfishes -II Properties of the principal water-soluble toxin[END_REF][START_REF] Bourdeau | -Facteurs de riques ciguatériques aux Antilles dans la région de Saint Barthléméy, Saint Martin et Anguila[END_REF][START_REF] Díaz-Asencio L | Ciguatoxin occurrence in food-web components of a Cuban coral reef ecosystem: Risk-assessment implications[END_REF].

As Ostreopsis and Prorocentrum are associated with Gambierdiscus in tropical areas [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF][START_REF] Besada | Observations on tropical, benthic dinoflagellates fron Ciguatera endemic areas: Coolia, Gambierdiscus, and Ostreopsis[END_REF][START_REF] Faust | Ciguatera-causing dinoflagellates in a coral-reef mangrove ecosystem, Belize[END_REF]) they were wrongly related with CFP; however, they are responsible for specific poisonings. In tropical environments, the toxins synthesized by the genus Ostreopsis are the causal agent of the palytoxicosis [START_REF] Alcala | Human fatality due to ingestion of the crab Demania reynaudii that contained a palytoxin-like toxin[END_REF]) and clupeotoxism [START_REF] Onuma | -Identification of putative palytoxin as the cause of clupeotoxism[END_REF][START_REF] Randall | Review of clupeotoxism, an often fatal illness from the consumption of clupeoid fishes[END_REF]) that occur respectively after the consumption of crustaceans and fish bioaccumulating toxins produced by this genus. The genus Prorocentrum, and specifically the species Prorocentrum lima is responsible for diarrhetic shellfish poisoning (DSP) not limited to tropical areas [START_REF] Tripuraneni | The toxin of diarrheic shellfish poisoning, okadaic acid, increases intestinal epithelial paracellular permeability[END_REF]). Among the Ostreopsis species morphologically described in the Caribbean Sea (Faust & Morton 1995, Faust 1999) several are known to be toxic [START_REF] Ukena | Structure elucidation of ostreocin D, a palytoxin analog isolated from the dinoflagellate Ostreopsis siamensis[END_REF][START_REF] Scalco | Growth and toxicity responses of Mediterranean Ostreopsis cf. ovata to seasonal irradiance and temperature conditions[END_REF][START_REF] Accoroni | -Influence of environmental factors on the toxin production of Ostreopsis cf. ovata during bloom events[END_REF] but the presence of Ostreopsis species in this area has not been confirmed by molecular studies [START_REF] Penna | -A phylogeographical study of the toxic benthic dinoflagellate genus Ostreopsis Schmidt[END_REF]. Concerning the genus Prorocentrum, 15 species sampled in the Western part of the Caribbean Sea have been described morphologically in the 90's by M.A. Faust (Faust 1990a, Faust 1990b[START_REF] Faust | Morphology of ciguatera-causing Prorocentrum lima (pyrrophyta) from widely differing sites[END_REF], Fasut 1993a, Faust 1993[START_REF] Faust | Three new benthic species of Prorocentrum (dinophyceae) from Carrie Bow Cay, Belize: P. sabulosum sp. nov., P. sculptile sp. nov., and P. arenarium sp. nov[END_REF][START_REF] Faust | -Prorocentrum levis, a new benthic species (dinophyceae) from a mangrove Island, Twin Cays, Belize[END_REF] but it is only recently that a taxonomic study conducted in the Eastern Caribbean area confirmed the presence of species known to be toxic [START_REF] Chomérat | -A taxonomical study of benthic Prorocentrum species (Prorocentrales, Dinophyceae) from Anse Dufour (Martinique Island, eastern Caribbean Sea)[END_REF].

Distribution of potentially toxic benthic dinoflagellates in the Caribbean justifies special attention to Saint Martin Island. Although CFP incidence rate has not been recently updated for this Island, CFP is still occurring [START_REF] Boucaud-Maitre D | Incidence and clinical characteristics of ciguatera fish poisoning in Guadeloupe (French West Indies) between 2013 and 2016: a retrospective cases-series[END_REF] despite the regulation of fish consumption by a decree (decree n°2002-1249, https://bit.ly/2OfU8Ut). This decree includes 3 different prohibitions that concern fifteen species. Some species (Caranx bartholomaei, Sphyraena barracuda, Seriola dumerili, Seriola rivoliana) are constantly prohibited from fishing and sale in Saint Martin, Saint Barthelemy and Guadeloupe islands. Other species (Caranx latus, Caranx lugubris, Caranx ruber, Mycteroperca venenosa, Mycteroperca tigris, Alphestes afer, Epinephelus morio, Gymnothorax funebris) are prohibited from fishing and sale beyond the parallel 16°5N. The species Lutjanus buccanella cannot be sold if its weight exceeds 1 Kg whatever the fishing place.

The species Lutjanus jocu adds up the two last prohibitions.

Genera like Coolia, Amphidinium and Sinophysis need to be considered even if their toxicity to human health is unknown [START_REF] Botana | Seafood and freshwater toxins: pharmacology, physiology, and detection[END_REF][START_REF] Holmes | Cooliatoxin, the first toxin from Coolia monotis (Dinophyceae)[END_REF]. In the framework of a program studying the diversity of benthic dinoflagellates present in the Lesser Antilles, the aim of the present study is to describe the distribution of the benthic dinoflagellates community hosted by macrophytes in Saint Martin Island during the cyclonic period, which extends from June to December in the Caribbean area.

Material and methods

Abundance of benthic dinoflagellates

Samples were collected between September 1 st and 3 rd , 2015 at 8 sites along the Atlantic and Caribbean coast of Saint Martin Island (fig. 1). All samples were collected between 0.5 and 2 m depth in shore waters.

The most abundant benthic macrophytes, which were locally present all year round, as well as the floating Sargassum spp., were sampled on each site. Approximately 10 g of macrophyte were carefully sampled with surrounding water in a 250 mL plastic flask avoiding the release of microalgae attached to the macrophyte. Acidic Lugol at 1% (vol/vol) was added in all samples to fix the microalgae and 10 seconds agitation allowed benthic dinoflagellates to detach from the macrophyte. Samples were filtered through a 500 µm mesh sieve (Retsch  ,  100 mm) to separate the macrophyte from the release dinoflagellates. Total seawater volume was measured. Microalgae samples were stored in dark at 4 °C. In total, 42 samples of macrophytes were semi-quantitatively analyzed because the fresh weight of the macrophyte was estimated (10 g).

Benthic dinoflagellates were identified at a genus level in order to avoid misidentification based on morphological characters (Hoppenrath et al. 2013, Penna et al. 2005). Benthic dinoflagellates were counted with a 1 mL Sedgewick Rafter © counting cell using a standard light microscope (Leitz, Orthoplan) within one week after sampling. Abundance values and the approximate fresh weight of macrophyte (10 g) allowed the calculation of the number of benthic toxic dinoflagellates per gram of fresh weight of macrophyte (cells g -1 ).

Temperature and salinity

Temperature and salinity were measured in triplicates at each site in 250 mL seawater samples collected close to macrophytes (between 0.5 and 2 m depth).

Temperature was measured using a Hanna © thermometer and salinity by a Master-S/MilliM ATAGO © manual refractometer.

Data analysis

The descriptive analytical values of temperature and salinity are presented as mean ± standard error (SE). Due to a semi-quantitative approach, only the mean is presented for the abundance values of benthic dinoflagellates.

Results

Temperature and salinity

The temperature of the seawater fluctuated between 29.7  0.0 °C and 30.8  0.0 °C (n=3) and the salinity varied between 35.0  0.0 and 40.0  0.0 during the sampling at the different sites at Saint Martin Island. Generally, the average temperature of the seawater was 30.3  0.1 °C and the average salinity was 36.0  0.6 (n=8) during the period study. (Table 1).

Distribution of benthic dinoflagellates

The genera Ostreopsis, Prorocentrum and Coolia were absent at Orient Bay. The genus Amphidinium was absent at Lay Bay, Dawn Beach and Orient Bay. The genus Gambierdiscus was present only at Plum bay and Simpson Lagoon Bay.

The genus Ostreopsis was observed mostly on Phaeophyceae and with decreasing abundances on Ulvophyceae, seagrasses, and Florideophyceae. The highest average abundance was found on benthic Sargassum spp. at Dawn Beach. Indeed, this macrophyte hosted on average 15,000 cells g -1 (fig. 2).

Observed Prorocentrum were mostly on Florideophyceae with an average abundance of 1500 cells g -1 at Simpson Lagoon Bay. This genus was found to a lesser extent on Phaeophyceae, seagrasses, and Ulvophyceae.

The genus Coolia was found preferentially on seagrasses than on Phaeophyceae and Ulvophyceae and Florideophyceae. This genus was hosted with the highest abundance of ca. 1000 cells g -1 on the seagrass Thalassia testudinium Koenig, 1805 collected at Nettle Bay.

The genus Amphidinium was found with the highest average abundance on Phaeophyceae, seagrasses, Florideophyceae and Ulvophyceae. The highest average abundance of Amphidinium cells has been observed at Friar's Bay on the Phaeophyceae, Padina spp., with ca. 60 cells g -1 .

The genus Gambierdiscus was found only at Simpson Bay Lagoon (on average ca. 60 cells g -1 ) and Plum Bay (on average ca. 20 cells g -). This genus was mainly associated with Florideophyceae and to a lower extent to seagrasses. The highest average abundance of this genus was found on the Florideophyceae, Gracilaria spp. collected at Simpson Bay Lagoon (on average ca. 70 cells g -1 ). At Simpson Bay Lagoon up to 125 cells g -1 were found in a sample. None Gambierdiscus cell was observed on Phaeophyceae and Ulvophyceae.

Discussion

Temperature and salinity

The present study was set up during the cyclonic period in the Caribbean Sea. This period is characterized by a high seawater temperature (above 28 °C) and a low salinity (below 35) due to heavy rainfall [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF][START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF][START_REF] Boisnoir | Depth distribution of benthic dinoflagellates in the Caribbean Sea[END_REF], Boisnoir et al. 2019a[START_REF] Arbeláez | Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean[END_REF]. Temperature and salinity measured in Saint Martin Island were above means recorded during ecological studies conducted on benthic dinoflagellates in the Caribbean area [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF][START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF][START_REF] Boisnoir | Depth distribution of benthic dinoflagellates in the Caribbean Sea[END_REF], Boisnoir et al. 2019a[START_REF] Arbeláez | Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean[END_REF]. These results suggest low rainfall and high light irradiance before and during the period sampling. In the Caribbean area, the wet season is watched because this period is supposed to promote the occurrence of high abundance values of benthic dinoflagellates [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF] and to be related with high CFP cases [START_REF] Tosteson | Caribbean ciguatera: a changing paradigm[END_REF]) due to long period with high seawater temperature [START_REF] Tester | -Ciguatera fish poisoning and sea surface temperatures in the Caribbean Sea and the West Indies, Ciguatera and related biotoxins workshop[END_REF][START_REF] Tosteson | Caribbean ciguatera: a changing paradigm[END_REF]). However, these trends were not always found [START_REF] De | Ciguatera fish poisoning: A review[END_REF][START_REF] Carlson R | Distribution and periodicity of toxic dinoflagellates in the Virgin Islands[END_REF]. Furthermore, the relation between abundance values of Gambierdiscus cells and environmental conditions are complex and must consider parameters such as the salinity [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF], light irradiance [START_REF] Morton | Effect of temperature, salinity and light intensity on the growth and seasonality of toxic dinoflagellates associated with ciguatera[END_REF], substrate preferences [START_REF] Lobel | Assessment of ciguatera dinoflagellate populations: Sample variability and algal substrate selection[END_REF], Boisnoir et al. 2019a), and nutrients [START_REF] Morton | Survey of toxic epiphytic dinoflagellates from the Belizean barrier reef ecosystem[END_REF][START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF][START_REF] Irola-Sansores E | Population dynamics of benthic-epiphytic dinoflagellates on two macroalgae from coral reef systems of the Northern Mexican Caribbean[END_REF].

Benthic dinoflagellates community

The species found in Saint Martin Island confirm that the benthic dinoflagellates community composition is in agreement with other studies performed in the Caribbean Sea [START_REF] Morton | Survey of toxic epiphytic dinoflagellates from the Belizean barrier reef ecosystem[END_REF][START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF][START_REF] Irola-Sansores E | Population dynamics of benthic-epiphytic dinoflagellates on two macroalgae from coral reef systems of the Northern Mexican Caribbean[END_REF], Boisnoir et al. 2019a). In the present study, the abundance values of benthic dinoflagellates have to be considered with caution as samples were semi-quantitatively analyzed (the fresh weight of macrophyte was estimated around 10 g). The present study provides order of magnitude and not usual quantitative parameters such as the mean and associated standard error to appreciate the global distribution of benthic dinoflagellates in Saint Martin Island during the cyclonic season.

Potentially toxic epiphytic Ostreopsis, Prorocentrum and Gambierdiscus genera accompanied by the genera Coolia and Amphidinium were found in Saint Martin Island for the first time in the present study. The genus Sinophysis was not found in Saint Martin although this genus was previously observed in the benthic dinoflagellate community of other areas of the Caribbean Sea (Faust 1993b[START_REF] Chomérat N | Studies on the benthic genus Sinophysis (Dinophysales, Dinophyceae): I. a taxonomic investigation from Martinique Island, including two new species and elucidation of the epithecal plate pattern[END_REF], Boisnoir et al. 2019a).

The abundance densities of the genus Ostreopsis were lower in Cuba and Martinique islands than abundances values found in the present study. Indeed, during the cyclonic period less than 1500 cells g -1 were found in both islands [START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF], Boisnoir et al. 2019a) while densities reached 15,000 cells g -1 in the present study. Similar abundances of Ostreopsis spp. were found in Puerto-Rico [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF]) and at Puerto Morelos along the Caribbean coast of Mexico [START_REF] Irola-Sansores E | Population dynamics of benthic-epiphytic dinoflagellates on two macroalgae from coral reef systems of the Northern Mexican Caribbean[END_REF] where average cell densities reached respectively 16,000 cells g -1 and 13,000 cells g -1 during the wet season [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF][START_REF] Irola-Sansores E | Population dynamics of benthic-epiphytic dinoflagellates on two macroalgae from coral reef systems of the Northern Mexican Caribbean[END_REF]. However, up to 100,000 cells g -1 were found at Bois Jolan in Guadeloupe during the wet season (Boisnoir et al. 2019). The dominance of the genus Ostreopsis was reported along the Caribbean coast of Mexico (Irola-Sansores et al. 2018), Puerto-Rico Island [START_REF] Ballantine | Population dynamics and toxicity of natural populations of benthic dinoflagellates in southwestern Puerto Rico[END_REF] and Guadeloupe (Boisnoir et al. 2019a, Boisnoir et al. 2019a). In the present study, this trend must be interpreted with caution.

Indeed, the dominance of the genus Ostreopsis was recorded in Martinique Island between January and February 2014 (Boisnoir et al. 2019a) but it was not observed during a seasonal monitoring settled during 18 months (Boisnoir et al. 2019a) suggesting that the dominance of Ostreopsis can be occasional. In the present study, highest abundances of the genus Ostreopsis were found on benthic Sargassum spp. while none cells were found on pelagic Sargassum spp. In other studies carried out in the Gulf of Mexico and the Caribbean Sea, less than 200 cells g -1 were recorded on pelagic Sargassum spp. [START_REF] Bomber | Epiphytic dinoflagellates of drift algae-another toxigenic community in the ciguatera food chain[END_REF], Boisnoir et al. 2019a). These results suggest that benthic Sargassum spp. should contribute to the potential toxic risk of benthic dinoflagellates contrarily to pelagic Sargassum spp. However, pelagic Sargassum spp. can be an important vector for exchanging microalgae populations [START_REF] Kim | Epibionts associated with floating Sargassum horneri in the Korea Strait[END_REF]. Arrival of toxic benthic dinoflagellates species has been observed in southeast of Australia after long distances on drifting macrophytes and plastic debris [START_REF] Larsson | Hitchhiking in the East Australian Current: rafting as a dispersal mechanism for harmful epibenthic dinoflagellates[END_REF]. The dinoflagellate populations can be harbored by pelagic Sargassum and homogenized between the West Indies, Gulf of Mexico and the African coasts (Boisnoir et al. 2019a).

The genus Prorocentrum is usually dominant in ecological studies settled in the Gulf of Mexico [START_REF] Okolodkov | Seasonal changes of benthic and epiphytic dinoflagellates in the Veracruz reef zone, Gulf of Mexico[END_REF][START_REF] Martínez-Cruz M | Epiphytic dinoflagellates on the seagrass Thalasia testudinium at Dzilam, southeastern Gulf of Mexico[END_REF], and in the Caribbean Sea [START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF][START_REF] Morton | Survey of toxic epiphytic dinoflagellates from the Belizean barrier reef ecosystem[END_REF][START_REF] Arbeláez | Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean[END_REF]) but this dominance was not observed in Saint Martin Island. As in the present study, low abundance values of Prorocentrum were found in Cuba, Guadeloupe and Martinique where less than 2000 cells g -1 where found during the wet season [START_REF] Delgado | Epiphytic dinoflagellates associated with ciguatera in the northwestern coast of Cuba[END_REF], Boisnoir et al. 2019a). However, abundance values reached more than 20,000 along the Caribbean coast of Colombia during the rainy season [START_REF] Arbeláez | Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean[END_REF]. During the rainy season in the Caribbean Sea, the dominance of Prorocentrum genus seems to occur with fewer cells than when the benthic dinoflagellates community is dominated by Ostreopsis.

The presence of the genus Gambierdiscus at only 2 sites in Saint Martin Island is surprising because this Island is located in the high CFP prevalence [START_REF] Olsen | -Ciguatera in the Eastern Caribbean[END_REF]. Furthermore, this genus was previously observed at 8 sites (9 sites in total) on undamaged corals, Acropora palmata [START_REF] Bourdeau | -Facteurs de riques ciguatériques aux Antilles dans la région de Saint Barthléméy, Saint Martin et Anguila[END_REF]. In this previous study, Gambierdiscus would be associated with maximal abundance values close to 20 cells g -1 of macrophyte fresh weight [START_REF] Bourdeau | -Facteurs de riques ciguatériques aux Antilles dans la région de Saint Barthléméy, Saint Martin et Anguila[END_REF] lower than in the present study. The increase of abundance values of Gambierdiscus spp. can be explained with the probable increase of anthropic pressures near the coral reefs of Saint Martin Island [START_REF] Chinain | -Ciguatera risk management in French Polynesia: The case study of Raivavae Island (Australes Archipelago)[END_REF][START_REF]Climate change, coral bleaching and the future of the world's coral reefs[END_REF] between the study of [START_REF] Bourdeau | -Facteurs de riques ciguatériques aux Antilles dans la région de Saint Barthléméy, Saint Martin et Anguila[END_REF] and the present study. Anthropic disturbances such as dredging and construction activities near coral reefs often involve a phase shift from coral reefs ecosystem to macrophytes (Hoegh-Guldberg 1999). Hence, this new ecosystem with abundant macrophytes provides additional and ideal substrate for the attachment of benthic dinoflagellates responsible for the CFP [START_REF] Chinain | -Ciguatera risk management in French Polynesia: The case study of Raivavae Island (Australes Archipelago)[END_REF][START_REF] Rongo | -Ciguatera poisoning in Rarotonga, southern Cook Islands[END_REF][START_REF] Morrison | -Ciguatera Fish Poisoning in La Habana, Cuba: A study of local social-ecological resilience[END_REF].

Usually the bloom formation threshold for this genus is 1000 cells g -1 as at such cell densities the amount of toxins begins to substantially accumulate in the food web [START_REF] Litaker R | Global distribution of ciguatera causing dinoflagellates in the genus Gambierdiscus[END_REF]. However, in some areas as Saint Barthelemy, Guadeloupe, Martinique, Caribbean cost of Colombia and Mexico, abundance values of Gambierdiscus have never been found to exceed 1000 cells g -1 (Boisnoir et al. 2019a[START_REF] Boisnoir | Depth distribution of benthic dinoflagellates in the Caribbean Sea[END_REF], Boisnoir et al. 2019b[START_REF] Arbeláez | Structural variation of potentially toxic epiphytic dinoflagellates on Thalassia testudinum from two coastal systems of Colombian Caribbean[END_REF][START_REF] Irola-Sansores E | Population dynamics of benthic-epiphytic dinoflagellates on two macroalgae from coral reef systems of the Northern Mexican Caribbean[END_REF][START_REF] Lobel | Assessment of ciguatera dinoflagellate populations: Sample variability and algal substrate selection[END_REF]) and CFP cases occurred [START_REF] Boucaud-Maitre D | Incidence and clinical characteristics of ciguatera fish poisoning in Guadeloupe (French West Indies) between 2013 and 2016: a retrospective cases-series[END_REF], Tester et al. 2010[START_REF] Gaitán | -Ictiotoxismo por consumo de barracuda (Sphyraena barracuda) y morena manchada (Gymnothorax moringa) en la comunidad de pescadores artesanales de Tasajera, Caribe colombiano[END_REF][START_REF] Núñez-Vázquez E | -Ciguatera in Mexico (1984-2013)[END_REF] suggesting that the amount of toxins begins to affect the food web at a lower cell densities in some areas. It appears that the bloom threshold needs to be established regionally in the Caribbean Sea (Boisnoir et al. 2019b). Saint Martin could be a part of islands of the Lesser Antilles where abundance values of Gambierdiscus never exceed 1000 cells g -1 and where CFP still occur. The bloom formation threshold of 50 cells g -1 proposed by Boisnoir et al. (2019a) could be suitable for Saint Martin Island. A seasonal monitoring is necessary to know if abundance values of Gambierdiscus can exceed or not 1000 cells g -1 in order to determine if samples were collected during a bloom at Simpson Lagoon Bay.

The genera Coolia and Amphidinium are rarely mentioned in ecological studies carried out in the Caribbean basin. The low abundances found in this present study similar to abundances observed in Guadeloupe and Martinique (Boisnoir et al. 2019a).

Spatial distribution

Due to the high dinoflagellate abundance, the sites of Dawn Beach, Nettle Bay, Friar's Bay, Simpson Lagoon Bay and Plum Bay are interesting to assess the diversity of benthic dinoflagellates in Saint Martin Island. Before to be excluded, Orient Bay needs to explored one more time without pelagic Sargassum grounding.

Conclusion

Potentially toxic epiphytic dinoflagellates including the genus Ostreopsis, Prorocentrum, and Gambierdiscus were found for the first time in Saint Martin Island. These genera were accompanied by the non-toxic genera Coolia and Amphidinium. The species found in Saint Martin Island confirm that the benthic dinoflagellates community composition is in agreement with other studies performed in the Caribbean Sea. The present study revealed different distributions for each genus highlighting stations that will have to be explored to assess the diversity of benthic dinoflagellates species present in Saint Martin Island. In order to study the diversity of benthic dinoflagellate species present in the Caribbean area, it is now necessary to define morpho-genetically the species previously described according to morphological criteria.

Fig. 1 :FiguresFig. 1 :Fig. 2 :

 112 Fig. 1: Distribution of potentially toxic benthic dinoflagellates abundances (cells g -1 ) in Saint

Table 1 :

 1 Temperature (°C), salinity and macrophytes collected in Saint Martin Island.Temperature and salinity were recorded in triplicates and macrophytes were sampled three times at each site.

	Date	Station	Macrophyte	Temperature (°C)	Salinity
	09/01/2015	Grandes Cayes	Thalassia testudinium Koenig, 1805	30.2  0.0	35  0.0
			Benthic Sargassum spp.
	09/01/2015	Friar's Bay	Padina spp.	30.0  0.0	35  0.0
			Laurencia complex
	09/01/2015	Nettle Bay	Thalassia testudinium Koenig, 1805	30.5  0.0	35  0.0
	09/02/2015	Plum Bay	Gracilaria spp.	30.8  0.0	35  0.0
			Halimeda spp.
	09/02/2015	Simpson Lagoon	Halophila stipulaceae Forsskål	30.3  0.0	36  0.0
		Bay	(Ascherson), 1967
			Gracilaria spp.
	09/02/2015	Lay Bay	Dasycladis spp.	30.5  0.0	35  0.0
			Gracilaria spp.
	09/03/2015	Orient Bay	Pelagic Sargassum spp.	29.7  0.0	40  0.0
	09/03/2015	Dawn Beach	Benthic Sargassum spp.	30.4  0.0	37  0.0
			Udotea spp.

Acknowledgments

This study was funded by the "Collectivité Territoriale de Martinique".


Gauthier Lefevre

Tristan Dewolf

Nicolas Guillaume

Serge Blonkowski

Christelle Charpin-Nicolle

Eric Jalaguier

Etienne Nowak

Nicolas Bernier

Tom Blomberg

Marko Tuominen

Hessel Sprey

Guillaume Audoit

Sylvie Schamm-Chardon
email: sylvie.schammchardon@cemes.fr

Nano-analytical investigation of the forming process in an HfO 2 -based resistive switching memory Nano-analytical investigation of the forming process in an HfO 2 -based resistive switching memory

Metal oxide based Resistive Random Access Memory (RRAM) devices are highly attractive candidates for nextgeneration nonvolatile memories, but the resistive switching phenomena remains poorly understood. This article focuses on the microscopic understanding of the initial forming step, which is decisive for the switching process. The integrated resistive switching memory effect in Ti/HfO 2 /TiWN MIM structures is studied. After forming, transmission electron microscopy (TEM) investigations pointed out the presence of a funnel-shaped region, in the ON state of the cell, where slightly oxidized Ti (TiOx) was present within HfO 2 dielectric. Modelling of the measured On state conductance of the cell with the semi-classical approximation is consistent with a conductive nanometric TiOx filament (or a sum of sub-nanometric TiOx filaments) present in the funnel shaped region. The conductive area is likely formed by diffusion after the dielectric breakdown.

I. INTRODUCTION

Resistive switching memory is an emerging technology and highly promising for next generation memory devices and neuromorphic computing [START_REF] Sawa | Resistive switching in transition metal oxides[END_REF][START_REF] Jeong | Emerging memories: resistive switching mechanisms and current status[END_REF][START_REF] Chang | Resistance random access memory[END_REF] . Among these, transitionmetal-oxide-based RRAM is under consideration because of its promising performances in terms of scalability, switching speed, endurance, energy efficiency and NMOS-friendly integration [START_REF] Wong | Metal-oxide RRAM[END_REF][START_REF] Yang | Memristive devices for computing[END_REF] . RRAM is seen as a very attractive solution to design downscaled ultra-low-power devices for new applications such as neuromorphic systems or in-memory computing [START_REF] Park | Nanoscale RRAM-based synaptic electronics: toward a neuromorphic computing device[END_REF][START_REF] Zidan | The future of electronics based on memristive systems[END_REF][START_REF] Benoist | 28nm advanced cmos resistive ram solution as embedded non-volatile memory[END_REF][START_REF] Hirtzlin | Digital biologically plausible implementation of binarized neural networks with differential hafnium oxide resistive memory arrays[END_REF][START_REF] Mahadevaiah | Reliability of cmos integrated memristive HfO 2 arrays with respect to neuromorphic computing[END_REF] .

This technology is based on a resistance transition obtained by applying suitable electrical pulses on metal oxides. The choice of proper materials (electrodes as well as metal oxide) in the Metal Insulator Metal (MIM) configuration is a key issue for fabrication and electrical operation since it leads to various behaviours [START_REF] Pan | Recent progress in resistive random access memories: Materials, switching mechanisms, and performance[END_REF][START_REF] Chen | Resistive random access memory (RRAM) technology: From material, device, selector, 3d integration to bottom-up fabrication[END_REF][START_REF] Cagli | Experimental and theoretical study of electrode effects in HfO 2 based RRAM[END_REF] .

Several models to explain the electrical behaviour of the devices have been proposed. They are based on the assumption that a major role in the switching resistance is played by charged species, such as positively charged O vacancies [START_REF] Waser | Redox-based resistive switching memories -nanoionic mechanisms, prospects, and challenges[END_REF][START_REF] Ielmini | Modeling the universal set/reset characteristics of bipolar RRAM by field-and temperature-driven filament growth[END_REF][START_REF] Larentis | Resistive switching by voltage-driven ion migration in bipolar RRAM-part II: Modeling[END_REF][START_REF] Bocquet | Robust compact model for bipolar oxide-based resistive switching memories[END_REF][START_REF] Xu | A unified physical model of switching behavior in oxide-based RRAM[END_REF] . However, the physics triggering the resistance switching is still uncertain, as experimental evidence not only supports switching due to oxygen vacancies [START_REF] Calka | Chemical and structural properties of conducting nanofilaments in TiN/HfO 2 -based resistive switching structures[END_REF][START_REF] Li | Direct observations of nanofilament evolution in switching processes in HfO 2 -based resistive random access memory by in situ TEM studies[END_REF] but also to metallic migration from the electrodes [START_REF] Privitera | Microscopy study of the con-ductive filament in HfO 2 resistive switching memory devices[END_REF][START_REF] Pey | Real-time analysis of ultra-thin gate dielectric breakdown and recoverya reality[END_REF] and to Schottky emission and/or hopping conduction mechanisms at the electrodedielectric interfaces [START_REF] Zhou | Abnormal high resistive state current mechanism transformation in Ti/HfO 2 /TiN resistive random access memory[END_REF] . To overcome limitations in the development of such memories like switching variability and retention tail losses, a deep understanding of physical phenomena a) currently with Aalto University.

arising across the dielectric solid-state material appears to be essential. If clear evidence of the role of oxygen and vacancies in the oxide-based insulator has been demonstrated, impact of the electrodes nature and electrode-dielectric interface still remains to be understood.

HfO 2 is one of the most suitable dielectric candidate for RRAM technology since it has the advantage of being more mature from a technological point of view, is widely studied and presenting an ease from the integration point of view [START_REF] Lee | Low power and high speed bipolar switching with a thin reactive ti buffer layer in robust HfO 2 based RRAM[END_REF][START_REF] Bersuker | Metal oxide resistive memory switching mechanism based on conductive filament properties[END_REF][START_REF] Govoreanu | 10 × 10 nm 2 Hf/HfO x crossbar resistive ram with excellent performance, reliability and low-energy operation[END_REF] . Concerning electrodes, titanium is commonly used in the current memory manufacturing processes and presents interesting properties (corrosion-resistance and high chemical activity).

Nonetheless, the role of TiN/Ti electrodes is controversial. Ti induces the formation of an interface layer enriched with O vacancies, which could be responsible for the bipolar switching, could reduce forming and set voltages, and improve retention ; anyway, this could also impede the reset, reduce the endurance and smooth the switching procedure. [START_REF] Cagli | Experimental and theoretical study of electrode effects in HfO 2 based RRAM[END_REF][START_REF] Zhou | Abnormal high resistive state current mechanism transformation in Ti/HfO 2 /TiN resistive random access memory[END_REF][START_REF] Gonon | Resistance switching in HfO 2 metal-insulator-metal devices[END_REF] Considerable TEM reports on resistive memories have been proposed, with a range of different techniques, including off-axis electron holography for electrostatic potential mapping [START_REF] Li | Direct observations of nanofilament evolution in switching processes in HfO 2 -based resistive random access memory by in situ TEM studies[END_REF] and energy dispersive X-ray spectroscopy for elemental analysis [START_REF] Niu | Electron holography on HfO 2 /HfO 2 -x bilayer structures with multilevel resistive switching properties[END_REF] . Among these advanced TEM techniques, it was found that Electron Energy Loss Spectroscopy (EELS) was the most appropriate tool for assessing changes during the switching process. [START_REF] Cooper | Anomalous resistance hysteresis in oxide ReRAM: Oxygen evolution and reincorporation revealed by in situ TEM[END_REF] Indeed, to provide relevant information concerning the redox-based ion-migration related switching processes, it's necessary to measure quantitative changes in atomic concentration and electronic states of involved elements with sufficient spatial resolution, which is possible by EELS in the transmission electron microscope. It should be noted that for this purpose, core-loss EELS, directly connected to elemental concentration and electronic structure, is more reliable than low-loss EELS which is indirectly sensitive to chemical composition and more delocalized [START_REF] Egerton | Electron energy-loss spectroscopy in the TEM[END_REF] .

In this article, a chemical investigation of HfO 2 -based Resistive Random Access Memory (RRAM) integrated with a Ti top electrode and a TiWN bottom electrodes is performed based on TEM-EELS. Quantitative physicochemical information is obtained on the observed nanometer-scale conductive paths. Balance between the role played by oxygen and metal (Ti-Hf) atoms is discussed and a two-step interpretation of the forming process is proposed. Results provide appealing details for further improvement.

II. EXPERIMENTAL SECTION

A resistive memory (6400 µm 2 ) comprising a 10 nm Back-End-of-Line thick HfO 2 deposited by Atomic Layer Deposition (ALD), then 10 nm of Physical Vapor Deposited (PVD) Ti and 50 nm of PVD TiN as top electrode, is schematically illustrated in Figure 1. The bottom electrode is grown by ALD and consists of a layer of TiWN [START_REF] Blomberg | Nanocrystalline Ti x W y N z thin films deposited by atomic layer deposition[END_REF] . In this work, studied bitcells were integrated in series with NMOS transistors, enabling a 1 mA current control. 1T-1R single devices were investigated in more detail. Electrical programming and measurements were carried out using a cascade microtech bench working with a Keithley 4200 Source Meter unit, with the bottom electrode being grounded. When switching from the OFF to the ON state, the current was limited to 0.3 mA (compliance current) to avoid the breakdown of the samples. A TEM lamella was extracted at the crossing of Metal 1 and Metal 2, which had a size of 0.09 µm 2 . The TEM lamella was prepared using Focus Ion Beam etching and polishing utilizing an FEI dual-beam Helios 450S. Rough milling was applied using an operating voltage of 30 kV with subsequent reductions to 8 keV, 5 keV and finally 2 keV to obtain a compromise between reducing the surface damage and keeping a high quality parallel-sided lamella. The final thickness of the lamella is 50 to 60 nm over a few micrometers length. Before TEM examination, the sample was cleaned with an oxygen-argon plasma to remove hydrocarbon contamination. The morphological and analytical investigations of the stack were performed using a doubleaberration-corrected FEI Titan Ultimate TEM equipped with a high brightness electron source working at an acceleration voltage of 300 keV, and with a Gatan Quantum energy filter working in the Dual EELS mode (simultaneous acquisition of low-loss and core-loss EELS spectra). High Angle Annular Dark field images (HAADF) were used to localize the area of interest for the analysis. EELS spectra and maps were obtained with an energy resolution of 1.2 eV and a spatial resolution at the nanometer scale. Quantitative elemental maps were obtained using standard EELS signal integration of a denoised signal obtained after a post-acquisition treatment [START_REF] Trebbia | EELS elemental mapping with unconventional methods i. theoretical basis: Image analysis with multivariate statistics and entropy concepts[END_REF] . Intensities in the maps present the relative volumetric concentration of each element (Ti, N, O and Hf), obtained after correction and with reference to stoichiometric TiN and HfO 2 within a pristine stack. Note that the pristine lamella has been extracted from another unstressed RRAM device with the same procedure as the TEM lamella obtained after forming.

III. RESULTS AND DISCUSSION

A. Electrical characterizations

Extensive electrical characterizations were performed on bitcells. The numerous memory cells tested present clear forming, reset and set events. For reliability of results, endurance tests were performed, demonstrating the devices remained functional even after 10 4 cycles (fig. 2, also denoting that the forming step is non-destructive. Characteristics for integration, such as endurance and retention time over long periods, not addressed here, have been presented in previous work. [START_REF] Azzaz | Endurance/retention trade off in HfO x and TaO x based rram[END_REF] FIG. 2. Endurance measurements : clear set, reset (a) and cycling (b) events are observed, indicating proper forming operation.

To initiate switching, a preliminary forming operation is required. In our case, a linear positive voltage ramp was applied on the top electrode, the bottom one being grounded. As it is shown on figure 2(b) and 3, the current increased exponentially until 2 V. At this voltage, the abrupt current increase corresponded to the HfO 2 dielectric breakdown. After this step, the current reached the compliance value, which was low enough to allow the switching observation in the negative part of the current voltage characteristics. Note that the forming step at 2 V was followed by a second breakdown (at about 2.7 V), which was an artifact due to the architecture including a parallel resistance, detailed in [START_REF] Guillaume | Improvement of HRS Variability in OxRRAM by Tailored Metallic Liner[END_REF] . It is worth noting that the current voltage characteristics after forming is linear (insert of figure 3) and this point will be explained in the discussion section.

B. TEM characterizations

After the forming operation, a lamella has been extracted from the region in between metal 1 and 2 electrodes as indicated in figure 1. The HAADF image of the thinned lamella is shown in figure 4. In such an image, intensity is proportional to the atomic number of the elements so that the heavy elements appear brighter than the light ones. In particular, the HfO 2 layer is brighter than the TiWN one, which is brighter than the Ti, TiN, metal 1 and metal 2. Two localized irregularities in the intensity were observed mainly within the HfO 2 brighter layer at the positions defined by the dotted rectangles (noted area 1 and 2 in figure 4), which suggests a possible modification of element distribution. Outside these areas, the stack is still homogeneous, with well-defined and planar interfaces. EELS elemental maps of N, Ti, O and Hf in area 1 were compared with those obtained in a pristine stack as shown in figure 5 (Hf map not registered in the pristine stack). Interfaces were relatively abrupt compositionally in the pristine stack, which was not the case in the central part of area 1, where diffusion of elements is clearly evidenced. An oxygentitanium interdiffusion is observed at the top of the dielectric layer, whereas hafnium migrated in the TiWN electrode. Elemental profiles were taken perpendicularly to the layers in the switched stack at the border as well as in its central part, providing complementary information (fig. 6). In addition to the diffusions already mentioned in the maps, two important aspects need to be discussed. First, O is also present with Hf in the TiWN bottom electrode but with a low concentration that leads to a Hf-rich place. Thus, O profile tends to diffuse mainly toward the top electrode as Hf seems to diffuse toward the bottom electrode as also evidenced in the sections obtained for different depths in the stack as shown on figure 7 (g and h cases). Moreover, Ti is present not only in the upper part of the dielectric but it also connects to the bottom electrode. Places where this occurs can be found by looking at the rainbow map of Ti where yellow traces are seen within the red dielectric forming a funnel-shaped envelope. More quantitatively, this is evidenced with Ti profiles (green) extracted from sections obtained for different depths in the stack as shown on figure 7 (c to f cases). Thus, Ti diffuses from the top electrode to the bottom electrode consistently with conductive atomic force microscopy (C-AFM) observations [START_REF] Singh | Resistive switching study in HfO 2 based resistive memories by conductive atomic force microscopy in vacuum[END_REF] . The way Ti is connected within the HfO 2 layer should be approached studying the Energy-Loss Near-Edge Structures (ELNES) at the onset of the Ti -L 2,3 and O-K edges obtained from the funnel-shaped area. If the Ti -L 2,3 fine structures of Ti and oxides can be of help interpreting fingerprint, this is not the case for O-K ones in HfO 2 . Indeed, a thorough investigation of the published work showed that these are very sensitive to the crystallisation state of HfO 2 (amorphous or not) [START_REF] Wilk | Correlation of annealing effects on local electronic structure and macroscopic electrical properties for HfO 2 deposited by atomic layer deposition[END_REF] , to the phase in which it crystallizes (monoclinic or tetragonal) [START_REF] Sharath | Control of switching modes and conductance quantization in oxygen engineered hfo x based memristive devices[END_REF] and to the presence of dopant and O vacancies [START_REF] Wang | Effect of al and y incorporation on the structure of HfO 2[END_REF] . It is not possible to distinguish between the different contributions, the major modifications being gathered over 10 eV within the same small range of energy. On the Ti -L 2,3 ELNES side, the fine structures are quite simpler to investigate. Two main peaks describe the first 10 eV of the Ti -L 2,3 ELNES arising through the transition of 2p core electrons to the narrow unoccupied transition metal d states. The L 3 transition from the 2 p 3/2 level is the lowest in energy, followed by the L 2 transition from the 2 p 1/2 state. The shape and the position of these 2 peaks are dependent on the Ti coordination (site geometry) and valence states. For example, the L 2,3 onset shifts to higher energy with increasing oxidation state in Ti oxides [START_REF] Yoshiya | First principles calculation of chemical shifts in ELNES/NEXAFS of titanium oxides[END_REF][START_REF] Stoyanov | The effect of valence state and site geometry on Ti -L 3,2 and O -K electron energy-loss spectra of Ti x O y phases[END_REF] and nitrides [START_REF] Terada | Chemical shift of electron energy-loss near-edge structure on the nitrogen k-edge and titanium l3-edge at TiN/Ti interface[END_REF] . Moreover, for Ti oxides with Ti valences III and above, like in Ti 2 O 3 (III) and TiO 2 (IV), the ligand-field due to the octahedral coordination of Ti causes the separation in 2 peaks of each L 3 and L 2 signatures due to the split of the 5d states in the conduction band in the two-fold e g and the threefold t 2g states [START_REF] Stoyanov | The effect of valence state and site geometry on Ti -L 3,2 and O -K electron energy-loss spectra of Ti x O y phases[END_REF][START_REF] Mitterbauer | Comparative electron energy-loss near-edge fine structure investigations of titanium oxides[END_REF] . In figure 8 (b), ELNES of the Ti -L edge are shown for different positions in the stack as indicated by numbers on the Ti rainbow map. They correspond to a 3 × 5 nm 2 area analyzed around each position. Looking at the Ti state in the top electrode, outside (pos. 1) and above the funnel shaped area (pos. 2 and 3), one can see that the position of the maximum of the L 3 peak is shifted towards higher energies by 0.7 and 1 eV compared to the one of pure Ti (obtained with similar conditions and added on the figure for reference). This corresponds to O/Ti ratios of 0.1 and 0.3 respectively, thus suggesting a slight oxidation, which is more pronounced above the funnel shaped as already observed in the O map of fig. 5. Within the funnel (pos. 4 to 7), Ti -L 2,3 ELNES shapes are not modified and are similar whatever the position considered with 2 main peaks but further shifted towards higher energies with respect to pure Ti (1.4 eV). However, the peak shift is less than for a Ti with valence III like in TiN (obtained with similar conditions and added to the figure for reference). On the basis of these comparisons, one can estimate the valence of Ti to be less than III when present within HfO 2 . The Low-Loss (LL) spectra corresponding to the Ti -L 2,3 ELNES signatures are also shown in figure 8 (c). Signatures at positions 1 and 2/3 are consistent with a slightly oxidized Ti with, respectively, a small shift of the main peak by 0.3 eV with respect to the value for pure Ti (pos. 1) [START_REF] Kihn | EELS studies of Ti-bearing materials and ab initio calculations[END_REF] and the beginning of the splitting of this peak (pos.2/3) as observed but in a much more pronounced way for titanium oxides TiO (valence II) to TiO 2 (valence IV) [START_REF] Bosman | Mapping chemical and bonding information using multivariate analysis of electron energy-loss spectrum images[END_REF] . Going inside the funnel area (pos. 4 to 7), the LL signatures can be described at a first approximation as the weighted sum of the LL signatures of HfO 2 (pos. 8) and partially oxidized Ti ( as depicted in pos. 2/3). Note that the HfO 2 characteristic signature preserved there excludes the presence of O-deficient HfO 2 or Hf, these having different low-loss signatures (basically one large main peak located around 22 eV) [START_REF] Privitera | Microscopy study of the con-ductive filament in HfO 2 resistive switching memory devices[END_REF][START_REF] Sharath | Control of switching modes and conductance quantization in oxygen engineered hfo x based memristive devices[END_REF][START_REF] Jang | Investigation of oxygen-related defects and the electrical properties of atomic layer deposited HfO 2 films using electron energy-loss spectroscopy[END_REF] .

C. Discussion

Advanced TEM analysis presented here highlights numerous interesting observations. After forming operation, two damaged areas appeared in the TEM lamella extracted from a 1T-1R stack. The memory cell was modified at these places and for one of them (area 1), a funnel-shaped area containing partially oxidized Ti and HfO 2 was observed. Moreover, O diffusion towards the top electrode and O depletion of HfO 2 at the bottom electrode were also evidenced. Similar phenomena were observed for area 2 but with a much limited extent. These observations are in favor of a conduction path connecting the top and bottom electrodes made of a conductive TiO x phase within a HfO 2 matrix, relayed by an O deficient HfO y phase at the bottom electrode.

The TEM observation of a funnel-shaped area has a consequence on the conduction. The smallest part of the funnel near the bottom electrode constitutes a constriction. The area of this constriction limits the current. Since this constriction has a small size (around 10 nm in diameter from figure 7) compared to an electronic mean free path, it is reasonable to consider the semi-classical approximation of quantum conductance instead of the classic Ohm's law. The temperature independence of the conductance demonstrated in similar OxRRAMs (4 K to 300 K range [START_REF] Blonkowski | Bipolar resistive switching from liquid helium to room temperature[END_REF] ) and the linear current voltage characteristic as seen in the insert of figure 3 (I on = G on .V with G on = 1.5 × 10 -4 S) support the use of the Sharvin conductance given by [START_REF] Brandbyge | Quantized conductance in atom-sized wires between two metals[END_REF][START_REF] De | Transition from sharvin to drude resistance in highmobility wires[END_REF] :

G on = 2q 2 h k 2 F S c 4π (1)
In equation ( 1), q is the electron charge, h is the Planck constant, k F the Fermi wave vector and S c the constriction section (supposed to be a disk of diameter d). The first fraction in equation ( 1) represents the quantum conductance and the second approximates the number of conduction channels. As deduced previously TiO x is likely at the origin of the OxRRAM On state conduction after the forming process. Let us consider if this is reasonable. If TiO x had the Ti properties, the corresponding Fermi wave vector k F = 1.52 × 10 10 m -1 would correspond to a constriction section diameter of 5 Å. Now, considering TiO x with a much lower electronic density than Ti, n ∼ 1 × 10 21 cm -3 , in the framework of free electron model k F is now equal to k F = (3π 2 n) 1/3 = 3 × 10 9 m -1 , which leads to a section diameter of 1.8 nm. If we remind that the volume of the funnel contains both TiO x and HfO 2 , the constriction area 10 nm in size could be considered as discontinuous thus Sc could be the sum of several smaller conductive areas, resulting in the same conductance.

One can now formulate assumptions on the forming scenario leading to this funnel. The first step is the dielectric breakdown that consists in the appearance of a conduction path in the dielectric. This step, common to all dielectrics and semiconductors [START_REF] Lombardo | Dielectric breakdown mechanisms in gate oxides[END_REF] , arises in our case at around 2 V (figures 2b and 3). The conduction path may be attributed to several phenomena in the dielectric submitted to a high electric field (Zener effect, electron avalanche, chemical bond breaking). In this conduction path, the current density is very high. For the 300 µA in figure 2(b) the current density is 3 × 10 -10 A cm -2 for a 1 nm filament section. Note that several conduction paths can be initiated. However, all the current flow only in one of them limiting the effect on the others. In this conduction path and its vicinity, because of Joule heating, the temperature can increase considerably. Therefore, a thermally activated diffusion becomes likely and the diffusion force which have the direction opposite to the atom concentration gradient (figure 5 b &c) pushes Ti atoms from top to bottom electrode. It is this region, which is observable with TEM and not the initial conduction path. Note that in the second region (area 2 in figure 4), the effect is drastically reduced, likely because the current flows mostly in the area 1, expected to be the place of the first breakdown. In the case of oxygen, the concentration spread consistently with diffusion law in the region where the temperature is high. Note also that an electrical field induced drift of Ti positive ions is also possible. During the forming, the top electrode is positively biased, thus the force acting on Ti atoms from top electrode that could be ionized is directed toward the bottom electrode. However, according to [START_REF] Singh | Resistive switching study in HfO 2 based resistive memories by conductive atomic force microscopy in vacuum[END_REF] the switching characteristics remains the same whatever the forming polarization. For this reason it seems that diffusion, in the case discussed here, is more likely than drift.

It's worth noting that switching mechanisms arising and observed in this work are intimately linked to the experimental procedure and the memory cell studied (i.e. Ti/HfO 2 (10 nm)/TiWN MIM configuration with its proper shape, dimensions and crystalline state) where each characteristic plays a relevant role.

IV. CONCLUSIONS

In this paper, a large set of TEM analyses (Z-contrast imaging, spatially-resolved EELS elemental maps, ELNES fingerprint) coupled with electrical measurements have been used and the results thoroughly discussed in order to elucidate the forming mechanism in an NMOS integrated HfO 2 -based resistive switching memory with Ti/TiWN electrodes. After forming, for the first time demonstrated using core-loss EELS elemental maps, a Ti-rich funnel-like region appears, which can be linked to a conductive filament within the HfO 2 dielectric. The measured On state conductance was compared in the framework of a semi classical approximation and is consistent with the smallest size of the funnel in the vicinity of the bottom electrode. A two-step forming scenario consisting in dielectric breakdown followed by diffusion may explain the funnel aspect of the conductive region. Such a new insight is essential to a deeper understanding of the physics of the subsequent switching processes. Also, it helps improving the device reliability, which is desirable for technological implementation of HfO 2 -based RRAM, a highly attractive candidate for next generation nonvolatile memories.

CONFLICTS OF INTEREST

There are no conflicts to declare.

FIG. 1 .

 1 FIG. 1. Schematic top view and cross-section (along the dotted line) of the 1T-1R stack, the TEM lamella was extracted within the crossing area of Metal 1 and Metal 2.

FIG. 3 .

 3 FIG.3. I-V characteristics for quasi-static forming step up to 4 V (forming step with linear scale in inset).

FIG. 4 .

 4 FIG. 4. HAADF image of the device after the forming step : two modified regions are within dotted rectangles (a) and shown enlarged in (b) and (c).

FIG. 5 .

 5 FIG. 5. EELS elemental maps of N (a) and (d), Ti (b) and (e) and O (c) and (f) in a pristine stack and in area 1 respectively. Hf map in area 1 (g).

FIG. 6 .

 6 FIG. 6. EELS rainbow map of Ti in the switched stack (a) and two elemental profiles obtained perpendicular to the layers, outside the center (b) and in the center (c) along the dashed lines drawn on (a).

FIG. 8 .

 8 FIG. 8. EELS rainbow map of Ti in the switched stack where analyzed positions are indicated (1 to 8) (a), Ti-L2,3 ELNES for the positions 1 to 7 with added ELNES of Ti and TiN as references (b) and corresponding Low-Loss spectra, with the one of pure HfO 2 (position 8) (c).

ACKNOWLEDGEMENTS

This work was partially performed at the Platform For NanoCharacterization (PFNC) of CEA-Leti with support from the Recherche Technologique de Base (RTB) of the french ministery of research. This work has been partially supported by European commission, French State and Auvergne-Rhône Alpes region through the funding of ECSEL 621217 PANACHE and 783176 WAKeMeUP Projects part of IPCEI (Important Project of Common European Interest) on microelectronics and French Nano2022 program.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.


domain: sde phys

text:
Charles Caer

Xavier Le Roux

Khanh Van

Delphine Do

Nicolas Marris-Morini

Laurent Izard

Dingshan Vivien

Eric Gao

Cassan

Dispersion engineering of wide slot photonic crystal waveguides by Bragg-like corrugation of the slot



Abstract-A comb slot photonic crystal waveguide geometry allowing flexible dispersion engineering of wide slot photonic crystal waveguides is proposed. Additionally, a figure of merit combining confinement within the slot and high group index is introduced to quantify the electromagnetic density within slow wave slot waveguides, and is applied to the proposed waveguide geometries. Group indices exceeding 130 and 90 over bandwidths of 1.5 and 2 nm are achieved for slots wider than 300 nm. The figure of merit exhibits sevenfold and fortyfold enhancements in comparison with classical slot waveguides with slot widths around 50 nm and 100 nm, respectively. Index Terms-Slot waveguides, slow wave, silicon photonics

I. INTRODUCTION
lanar Photonics Crystal Waveguides (PCW) offer a promising route to investigate light-matter interactions thanks to the slowing down of light. The strong reduction of group velocity at the edge of the first Brillouin zone induces an enhancement of the electromagnetic field, raising opportunities for several applications in integrated photonic devices [START_REF] Baba | Slow light in photonic crsystals[END_REF][START_REF] Krauss | Slow light in photonic crsystal waveguides[END_REF], particularly relying on non linear optical effects [START_REF] Soljačić | Enhancement of nonlinear effects using photonic crystals[END_REF][START_REF] Leuthold | Non linear silicon photonics[END_REF][START_REF] Monat | Slow light enhanced nonlinear optics in periodic structures[END_REF].

On the other hand, the study of slot waveguides [START_REF] Almeida | Guiding and confining light in void nanostructure[END_REF] revealed that Total Internal Reflection (TIR) allows one to strongly confine light within the low index material, while creating strong discontinuities of the electric field at the slot inner interfaces. By adequately filling slots with non-linear polymers, for example, ultrafast optical modulation was demonstrated [START_REF] Koos | All-optical high-speed signal processing with silicon-organic hybrid slot waveguides[END_REF][START_REF] Wülbern | 40 GHz electro-optic modulation in hybrid siliconorganic slotted photonic crystal waveguides[END_REF], paving a way for Silicon Organic Hybrid Photonics. The concept of slot waveguide has been further extended to photonic crystals (PCs). A guided mode located into the Photonic Bandgap (PBG) with a positive slope [START_REF] Di Falco | Photonic crystal slotted slab waveguides[END_REF][START_REF] Gao | Demonstration of an air-slot mode-gap confined photonic crystal slab nanocavity with ultrasmall mode volumes[END_REF][START_REF] Wu | Light Localization in Slot Photonic Crystal Waveguide[END_REF] was especially exploited. Slow light in such Slotted Photonic Crystal Waveguides (SPCW) with group indices exceeding 100 has already been experimentally demonstrated [START_REF] Di Falco | Dispersion control and slow light in slotted photonic crystal waveguides[END_REF] and it has also been applied to optical modulation by polymer infiltration [START_REF] Wülbern | 40 GHz electro-optic modulation in hybrid siliconorganic slotted photonic crystal waveguides[END_REF][START_REF] Lin | Electro-optic polymer infiltrated silicon photonic crystal slot waveguide modulator with 23 dB slow light enahncement[END_REF]. Nevertheless, most of the previous works focused on d e v i c es with a nar r o w slo t [ 9 -1 4 ] ( less o r aro und 100 nm) in order to enhance optical field density. Indeed, a strong decline of the optical intensity has been reported when the slot is enlarged over 200 nm [START_REF] Almeida | Guiding and confining light in void nanostructure[END_REF]. Narrow slots provide very high optical field intensity [START_REF] Almeida | Guiding and confining light in void nanostructure[END_REF], but represent a drawback for filling by low index materials.

II. MODEL

This letter aims to address this issue by proposing a novel design of wide slot photonic crystal waveguide. Our approach is based on the use of the symmetric guided mode of SPCWs having a positive slope in the ω(k) dispersion diagram, and tailoring it by a Bragg-like corrugation of the slot itself, as shown on fig. 1. Such comb has been first studied in Ref [START_REF] Riboli | Band gap characterization and slow light effects in one dimensional photonic crystals based on silicon slotwaveguides[END_REF], but here we introduce it within a 2D Photonic crystal.

Eigenmodes were calculated using the Plane Wave Expansion method with the MPB software [16] and based on the approximation of the slab-effective index, in which we considered a silicon on insulator slab with height h of 260 nm, with silicon and silica refractive indices of 3.48 and 1.44, respectively. The low index material is a top cladding layer of 1.6 refractive index filling holes and slot. A normalized hole radius of 0.3a was considered to ensure a wide TE bandgap.

First, we fixed the waveguide width to larger values to match with the objective of slotted waveguides with a large slot inside. The waveguide width was thus fixed to a value of 1.25√3a, with a the PC triangular lattice parameter typically around 400nm. Opening a slot within the channel introduces a "slot" mode, whose origin has been discussed in Refs. [START_REF] Di Falco | Photonic crystal slotted slab waveguides[END_REF][START_REF] Gao | Demonstration of an air-slot mode-gap confined photonic crystal slab nanocavity with ultrasmall mode volumes[END_REF][START_REF] Wu | Light Localization in Slot Photonic Crystal Waveguide[END_REF]. Our calculations showed that increasing the slot width allows adjusting the slope of the exploited waveguide mode. In the considered waveguide geometry, we found an optimum slot width value around Ws=0.71a. Smaller slot widths lead to a negative slope of the dispersion curve and multimode configurations, while larger slot widths lead to positive slope and group indices below 25. The already dispersionengineered chosen configuration characterized by W1=1.25√3a and Ws=0.71a offers a flat band with a constant group index around 50 in an nearly 6 nm bandwidth centred at 1550 nm. In this configuration, the confinement of the optical power is about 25% within the slot and the band reaches the edge of the FBZ at a midgap frequency. It constitues the starting point for further engineering of the dispersion curve. Figure 1 shows a scheme of the proposed modifications of the waveguide geometry relying on the introduction of a Bragglike corrugation of the slot width with the same lattice period Dispersion engineering of wide slot photonic crystal waveguides by Bragg-like corrugation of the slot Charles Caer, Xavier Le Roux, Van Khanh Do, Delphine Marris-Morini, Nicolas Izard, Laurent Vivien, Dingshan Gao, and Eric Cassan P as the one of the photonic crystal. In addition with W2 and W3, which control the shift of the two first rows of holes of the neighbouring lattice, three parameters were introduced to describe that comb slot itself: l depicts the comb duty cycle, dy the enlargement of the channel and dx the displacement of the center of the Bragg teeth with respect to the center of the first row holes. The influence of W2 and W3 was first studied. PWE simulations revealed that these two parameters mainly allow controlling the slope of the dispersion curve at the limit of the light line crossing. W2 and W3 above the nominal value of 0.5√3a lead to a positive slope of the dispersion curve, while values below lead to a negative slope. Due to the influence of the comb slot paramters (dy, dx, l) responsible for an increase of the ω(k) curve slope in most situations, we choose to precompensate the dispersion curve by fixing W2 and W3 values below 0.5√3a to prepare a first ω(k) negative slope and enable flat band configurations as a whole. Then, we tuned the comb slot parameters to study their influence. The main effect of dy is adjusting the mode slope, increasing values of dy being responsible for an increase of the dispersion curve slope. The main effects of dx are shifting the mode dispersion curve and adjusting the field localization in the waveguide longitudinal direction. Finally, the main effect of l is adjusting the slope of the dispersion curve and making an inflection point appear in the ω(k) curve.

To illustrate the possible richness of the dispersion engineering approach by tailoring the slot itself of slotted photonic crystals, we provide now results obtained in two given configurations. The first one consists in a sharp comb aligned with the first hole array (fig 2).The second is a wide slot with a sharp narrowing facing the first hole array (fig. 3). The parameters in the two considered cases are l=0.25a, dy=0.4a, dx=0.5a. and l=0.725a, dy=0.3a, dx=0, respectively. The two designs possess a guided mode with a strictly monotonous slope. Sets of W2=0.4√3a and W3=0.5√3a, W2=W3=0.45√3a were found to be suitable for the first and the second design, respectively. An interesting point is that in both designs, there is an inflection point just before the mode exits the light cone (i.e. near k=0.4). Therefore, there is a possibility to slightly tune the band in order to have negative Group Velocity Dispersion (GVD). In this zone, the group index can reach extremely high values (>200), but we have chosen parameters in order to get exploitable values in terms of practical applications, i.e. bandwidth and losses.

Here, we achieved an average group index of 140 over a bandwidth of 1.5 nm for the first design (fig. 2) and an average group index of 95 over a bandwidth of 2 nm for the second design (fig. 3). These values are comparable with those of previous studies [START_REF] Baba | Slow light in photonic crsystals[END_REF][START_REF] Brosi | High-speed-low voltage electro-optic modulator with a polymer infiltrated silicon photonic crystal waveguide[END_REF][START_REF] Hao | Improvement of delay-bandwidth product in photonic crystal slow-light waveguides[END_REF][START_REF] Schulz | Dispersion engineered slow light in photonic crystals : a comparison[END_REF]. It is noticeable to consider that they were obtained with a cladding layer in a very wide comb, i.e. 325 nm on average for the first design and 370 nm for the second, which should considerably facilitate the filling of the slot by polymers or any other materials. For the first structure, we found that the fraction of the optical power at k=0.43 within the comb is near 25%, whereas it reaches 35 % for the second waveguide. Another interesting fact is that negative GVD occurs over almost 1 nm in both cases, opening a perspective for dispersion compensation devices and alloptical signal processing.

To express the confinement of the optical field, we revisited the calculation reported in Ref [START_REF] Almeida | Guiding and confining light in void nanostructure[END_REF], which defined the ratio η of optical power within the slot over the total power, and the optical intensity I as η divided by the cross-section of the slot, i.e. Ws*h with h the height of the slab. To include the effect of electromagnetic density enhancement due to slow light propagation, we modified this definition of the optical intensity and calculated it as ng*I instead, i.e. we included the light group index into the optical intensity definition. In case of standard slot waveguides, taken as a function of the slot width Ws, the ratio saturates around 30% for Ws higher than 200 nm, and I decreases from 30 to 3 µm -2 when Ws ranges from 10 to 200 nm. There, the confinement is done only by the index contrast and the group index remains around 3 in the wavelength ranger of interest.

In case of slot photonic crystal waveguides, group index can be significantly higher. In the two configurations reported in figs. 2 and 3, for example, ng is around 140 and 90, respectively. Values of optical density as high as 540 and 470 µm -2 are thus obtained. These optical densities represent an enhancement by a factor 7 if compared with typical values around 70 µm -2 just reported above for classical TIR slot waveguides with narrow slots of 50 nm width, and a factor 40 if compared with more feasible slot width around 100 nm, whose optical density is around 12 µm -2 . Higher localizations of the electric field may be also obtained within a small volume of the comb as seen in figure 4. Moreover, adjusting the dx parameter of the proposed waveguide geometry (shift of the comb with respect to the photonic crystal lattice) raises an additional degree of freedom for field localization.

III. CONCLUSION

In conclusion, we propose an hybrid structure of slotted photonic crystal waveguides relying on a wide comb slot inserted into a two dimensional photonic crystal lattice. In this configuration, the role of the photonic crystal lattice is essential, and the introduced Bragg-like corrugation of the wide slot only acts as an additional degree of freedom for engineering of the waveguide dispersion curve. For illustration, two structures have been presented, operating in slow light regime with group index of 140 and 90 over bandwidths of 1.5 nm and 2 nm, respectively, with slot widths above 300 nm. In-slot optical confinements of 25% and 35 % are reported. We show that the slot wideness is not realized at the expense of low optical intensity, as the proposed configurations exhibit a fortyfold optical intensity enhancement if compared with 100 nm slot TIR slot waveguides.

Due to the easiness of wide slot filling and simultaneously ultra-high optical intensity, the proposed structure may be useful for the realization of devices requiring strong interactions between light and an optically non-linear low index material, a biochemical fluid, or any other material like magnetooptic ones.

Fig. 1 .Fig. 2 (Fig. 3 (

 123 Fig. 1. (a) Schematic picture of a 2D SPCW with a wide comb. (b) Fraction of the unit cell implemented for PWE method calculation. The different parameters providing degrees of freedom for tailoring the mode dispersion curve are represented.

Fig. 4

 4 Fig. 4 From left to right : 3D surface plot of |E|² (dielectric map is superposed for visualization), mode field patterns of |E|² and |H|² for the first structure (a) and the second structure (b). All mode field patterns are taken at k=0.43. These profiles show that a significant fraction of the total power is concentrated within the comb.