Sylvain Detey

Jacques Durand

Yuji Kawaguchi

David Le

Phonetic input, phonological categories and orthographic representations Phonetic input, phonological categories and orthographic representations: a psycholinguistic perspective on why language education needs oral corpora. ----The case of French-Japanese interphonology development

Phonetic input, phonological categories and orthographic representations: a psycholinguistic perspective on why language education needs oral corpora-The case of French-Japanese interphonology development -

Introduction 1

Using corpora in language education cannot be seen as a brand-new pedagogical technique any more, as far as lexicon, grammar and written corpora are concerned. In the field of Teaching English as a Foreign or a Second Language, concordancing tools have been part of the standard CALL apparatus for more than a decade [START_REF] Flowerdew | Concordancing in language learning[END_REF]. However, the same cannot be said of other languages, such as French, for instance, and the use of oral corpora still seems to be often limited to the use of their written transcriptions.

Since the elaboration of the Français Fondamental in the 1950s [START_REF] Rivenc | Le Français dans le Monde 148 / Pour aider à apprendre à communiquer dans une langue étrangère[END_REF][START_REF] Rivenc | Le Français dans le Monde 148 / Pour aider à apprendre à communiquer dans une langue étrangère[END_REF] 2 , oral corpora have been scarce and often of very limited size, used by French teachers and applied linguists mainly for selection of linguistic forms to be taught, according to frequency of use [START_REF] Biber | What does frequency have to do with grammar teaching?[END_REF] and other criteria. With the development of pedagogical approaches based on so-called 'authentic' documents, 'real' recordings, rather than fabricated dialogues or narratives, have been increasingly used, along with faithful transcriptions, rather than normalized transcriptions [START_REF] Weber | Pourquoi les Français ne parlent-ils pas comme je l'ai appris ?[END_REF]). Yet, these 'real' recordings are often used as any other pedagogical document, and oral corpora are dissected to select and teach lexical and grammatical forms (whether the most frequent ones in French in general or those that are specific to oral French) through their orthographic transcriptions. They are not usually put into use as a source of linguistic substance, i.e. phonetic input that is specifically needed for the development of oral skills, in perception, production and learning tasks.

There are at least two possible explanations for this: 1-The preference accorded to written documents over oral material in traditional formal language education 3 . This is mostly due to practical reasons: lack of oral data and material, stability of written language (useful for metalinguistic analysis -which is linked to the limited capacity of our working memory), etc. Besides, for a long time, there were also ideological reasons (different status for oral and written norms [START_REF] Laks | Description de l'oral et variation : la phonologie et la norme[END_REF]) and scientific motivations (following Saussure's stance, the object of linguistics should be the study of language ('la langue') and not speech ('la parole')) behind this apparent lack of interest in oral data.

2-The lack of scientifically grounded psycholinguistic perspectives on the link between input modality (oral vs. visual) and foreign language learning, which can mislead teachers and learners into thinking that modality does not really matter. However, as psycholinguists know, modality does matter in the perception and learning of language, even though language per se is not tied to any particular sensory modality: not only do the linguistic properties of oral and written stimuli differ, but so does their psycholinguistic processing by language learners, from low-level perceptual process (reading process vs listening process) to higher-level mnesic encoding in the phonological and/or orthographic lexicon [START_REF] Detey | Interphonologie et représentations orthographiques. Du rôle de l'écrit dans l'enseignement/apprentissage du français oral chez des étudiants japonais[END_REF].

Our aim in this contribution is to show that oral corpora must be considered and used in language education not only as a source of formal input but also as a source of substantial input for foreign language (hereafter referred to as L2) teaching and learning. This input can be provided only by oral corpora and not by their written counterparts. The most obvious area of application is L2 phonology learning, but since phonology, lexicon and grammar are crucially linked in language learning, it applies to L2 oral language learning as a whole.

One of the main issues for L2 phonology learning is as follows: what input is available for the optimal development of learners' interphonology (i.e. the new phonological system developed by the learners)? This issue is particularly important when the L2 does not belong to the immediate sociolinguistic environment of the learners, as is the case for Japanese learners of French in Japan, for instance. In this case, there is often an imbalance between orthographic and phonetic input, due to limited linguistic exposure. Given the gap between oral and written French (especially on the morphophonological level [START_REF] Battye | The French Language Today : a linguistic introduction[END_REF]) on the one hand, and between the French and the Japanese writing systems [START_REF] Coulmas | The Writing Systems of the World[END_REF]) on the other hand, it is important to examine in detail the potential role of oral corpora as a source of input for the learner.

In the following paragraphs, we examine the link between speech perception, acoustic cues and phonological learning. We take a closer look at recent studies which show that variability can play an important role in non-native phonemic discrimination training, as is exemplified in the way the English phonemes /r/ and /l/ are processed by Japanese subjects. We then consider the case of French /R/ and /l/ for Japanese learners and point out the confusing role of the orthographic factor in the learning process, which emphasizes the need for more appropriate oral resources for the learners. In the case of French, these can be drawn from the PFC corpus (Phonologie du Français Contemporain -Phonology of Contemporary French) [START_REF] Durand | La phonologie du français contemporain : usages, variétés et structure[END_REF], 2005): the creation of pedagogical resources out of the PFC corpus is precisely the aim of the PFC-EF (Enseignement du Français -French Teaching) project (Detey 2007a;[START_REF] Detey | Voix de la francophonie, éducation langagière et corpus numérisé : PFC-EF, des ressources pour la didactique du français[END_REF][START_REF] Detey | Voix de la francophonie, éducation langagière et corpus numérisé : PFC-EF, des ressources pour la didactique du français[END_REF] Lyche to appear).

On the role of perception and phonetic cues in phonological development

From perceptual process to phonological learning

Although some studies suggest that perception and production can develop independently, the majority of researchers support the idea that perception precedes production in the learning process (for a review, see [START_REF] Escudero | Second Language Phonology: the role of perception[END_REF], as it was initially established by Polivanov andTrubetzkoy (1939/1949). We adopt the same stance, which Escudero (op. cit.: 110) interprets as follows: "the origin of a foreign accent is the use of language-specific perceptual strategies that are entrenched in the learner and that cannot be avoided when encountering the sounds of a second language". We will not give an overview of the main current L2 phonology learning theories here (for a state-of-the-art review, see Eckman 2004, Escudero 2007, andHansen Edwards &Zampini in press), but we will simply mention one of the most recent ones, i.e. the Linguistic Perception model (LP), in its L2 version (L2LP) [START_REF] Escudero | Linguistic Perception and Second Language Acquisition. Explaining the attainment of optimal phonological categorization[END_REF].

Figure 1. The Linguistic Perception model (Escudero 2005: 43) In the L2 Linguistic Perception model, the basis for learning the L2 perceptual system is a full copy of the first language (L1) system (perception grammar and lexical representations), and the learner has full access to the L1 learning mechanism, i.e., according to Boersma's Gradual Learning Algorithm, the creation of data-driven phonetic categories, followed by lexically-driven optimization of these categories: "listeners create categories (e.g. /ɪ/ and /p/) on the basis of distributional information [START_REF] Boersma | Learning abstract phonological from auditory phonetic categories : an integrated model for the acquisition of languagespecific sound categories[END_REF], then use these categories to create phonological forms in their lexicon (e.g. |ʃɪp|) and mapping constraints in their perception grammar (e.g. "74 ms is not /ɪ/"), and finally optimize their constraint rankings by applying the Gradual Learning Algorithm […] to their perception grammar, driven by recognition in the lexicon" (Escudero & Boersma 2004:565). The definitions of the Full Transfer and of the Full Access that this model combines are as follows (op. cit.: 573): -Full Transfer: "the transfer of L1 categories, L1 perceptual mappings, and L1 blank slates to the initial state of the interlanguage perception grammar"; -Full Access: "the access to an L1-like category formation device and to an L1-like constraint reranking device". The difficulty is to establish the relative weight of one and of the other, since Full Transfer predicts phonological deafness (in Trubetzkoy's sense) and Full Access predicts the attainment of a native-like phonological competence (or Full Proficiency as [START_REF] Escudero | Linguistic Perception and Second Language Acquisition. Explaining the attainment of optimal phonological categorization[END_REF] names it). Escudero (2007:127) sums it up as follows: "the L2 perception is handled from the beginning by a separate perceptual system which began as a copy of the L1 system but evolves with experience with the L2. With respect to L2 development, it is proposed that L2 learners have access to the same learning mechanisms […] that were available for L1 learning". As it appears in this short summary of the L2LP model, the role of linguistic perception is crucial in L2 phonological development.

From phonotactic information to phonological categories

Therefore, the bridge between the phonological categories and the speech perception process that the LP model establishes strongly relies on the properties of the input, not only at the acoustic level, but also at the phonotactic one: "during the first stage, learning of language specific sound categories by infants is driven by distributional evidence in the linguistic input. This auditory-driven learning leads to a warping of the baby's perceptual space, to discrimination curves, to the perceptual magnet effect [("differences near ambient category centres are less well perceived than differences near ambient category boundaries")], and ultimately to the creation of phonetic categories" [START_REF] Boersma | Learning abstract phonological from auditory phonetic categories : an integrated model for the acquisition of languagespecific sound categories[END_REF]: 1013). The hypothesis that adult learners also perform a distributional analysis when learning phonemic categories has been explored by other researchers. [START_REF] Peperkamp | Allophonic variation and the acquisition of phoneme categories[END_REF] investigate the role of allophonic variation in the perception and acquisition of phonemic distinctions (in their experiment the French allophonic realizations of the French /r/, the voiced [ʁ] (as in 'perde') and the unvoiced [χ] (as in 'perte'), are tested): their results suggest that statistical learning is sensitive to context and to complementary distribution. [START_REF] Chambers | Learning different phonetic distributions in onset and coda position[END_REF] follow a similar hypothesis: they show that adults are able to keep track of phonetic distributions in different phonological contexts (onset and coda) for non-native contrasts, which would support the idea that distributional learning plays an important role in phonological learning. This is also the position taken by [START_REF] Peperkamp | The acquisition of abstract phoneme categories[END_REF] who consider that distributional information can help infants build sound categories prelexically. The results of a series of experiments with artificial language learning tasks and adult subjects suggest that learners can create abstract phoneme categories based on distributional information with or without lexical knowledge.

Perception and phonology in loanword adaptations

In the preceding sections, we examined the role of perception and phonetic information in L1 and L2 phonological development. There is another area from which we can gain insight into the relationship between perception and phonology: loanword phonology [START_REF] Boersma | Phonological perception in loanword adaptation[END_REF]. In recent years a renewed debate concerning the process at work in loanword phonological adaptations has been opposing two camps: a phonological one (LaCharité & Paradis 2005) ("loanwords are adapted according to the rules and/or constraints of the borrowing language targeting aspects of phonological representation" [START_REF] Rose | Vowel epenthesis in loanword adaptation: representational and phonetic considerations[END_REF]: 1112)) and a perceptual / phonetic one, of which Peperkamp and Dupoux (2003) are the most extreme proponents, since they propose that loanword adaptations take place at the perceptual level and are defined as "phonetically minimal transformations" (op. cit.: 367). Other researchers combine the two approaches, such as [START_REF] Rose | Vowel epenthesis in loanword adaptation: representational and phonetic considerations[END_REF], or [START_REF] Dohlus | Phonetics or phonology: Asymmetries in loanword adaptations -French and German mid front rounded vowels in Japanese[END_REF], who examines the asymmetric adaptation of German and French mid front rounded vowels /ø/ and /oe/ in Japanese: "It was shown that the adaptation of German /oe/ and /ø/ as Japanese /e/ is a phonological approximation, but the adaptation of French /oe/ and /ø/ as Japanese /u/ a phonetic approximation. In this context I argue […] that loanword adaptations are basically phonetically grounded, but that a lack of oral input and a large influence of written media trigger phonological approximation" (op. cit.: 133)4 . Whichever stance we decide to adopt, it seems that the perceptual factor, and more precisely the fine-grained phonetic characteristics of the input, cannot be overlooked, as it appears in the study of [START_REF] Vendelin | Evidence for phonetic adaptation of loanwords : an experimental study[END_REF]. The focus of their study is the adaptation of word-final [n] in Japanese loanwords borrowed from French (F) and English (E): in (F)-words, [n] is adapted as a geminate nasal followed by an epenthetic vowel; in (E)-words, [n] is adapted as a moraic nasal consonant5 . The results of their experiment (a forced choice task with non-words uttered by French and American English speakers) show that "French and English stimuli ending in [n] differ with respect to both the length and intensity of the nasal consonant and its release, and that these differences account for the absence versus presence of an epenthetic vowel in the response of [their] Japanese subjects. […] [T]he process of loanword adaptation is guided by perceptual assimilation which exploits the principle of minimal phonetic distance and is sensible to fine phonetic details of foreign speech" (op. cit.: 130).

Overall, these results suggest that fine phonetic details do play a role in L2 speech perception and thus in L2 phonology learning. If we consider that learners need to set up a new perception grammar and that distributional information can help, it seems all the more plausible. When we look into the phonetic input that oral corpora can provide for language learners, we have to examine its features and the effect it can have on the learning process. One of its major and inherent characteristics is its variability: whether within-or between-speakers, the variability of the phonetic input (as opposed to the stability of the written material, or to the one of the familiar voice of the teacher) that oral corpora can offer has to be taken into account. More precisely, what is the effect of phonetic variability on L2 speech perception and L2 phonology learning? To answer that question, we first need to gather some information from L1 speech perception studies.

On the role of phonetic variability in speech perception

Speech perception models: abstractionist versus exemplarist views

In speech perception studies, there are several classical problems [START_REF] Segui | La perception du langage parlé[END_REF][START_REF] Nguyen | La perception de la parole[END_REF], among which the gap between the absence of acoustic invariance of the signal on the one hand and the (general) invariance of linguistic categories on the other hand. As Pisoni and Luce (1987: 23) explain, the "acoustic consequences of coarticulation and other sources of contextually conditioned variability result in the failure of the acoustic signal to meet two important formal conditions, invariance and linearity [(i.e., non-linearity as an overlapping of acoustic cues vs the linearity of the phonemic string in a classical phonological view)], which in turn give rise to the problem of segmentation". When we look at the different speech perception models that have been elaborated in the past decades, we can distinguish two different ways of solving the invariance problem. They correspond to two views of speech perception process (and cognitive representations), which stand at the endpoints of a continuum on which most models can be situated: the abstractionist and the exemplarist viewpoints.

In a very traditional approach, speech perception is viewed as a bottom-up process, in which the listener is filtering the input to recover abstract units and discarding all irrelevant phonetic details, which are considered to be unimportant noise. The lexicon is made up of a set of abstract lexical units, and speech recognition implies mapping together these units and the abstract sequences extracted from the signal (with top-down influences and interactive process in more recent models). However, already in the 70's, researchers like Klatt were exploring other perspectives, since in his LAFS model (Lexical Access From Spectra) (1979) the lexicon is composed of a set of spectral representations of diphone sequences, enabling direct comparison between acousticphonetic input and spectral representations stored in long-term memory [START_REF] Pisoni | Acoustic-phonetic representations in word recognition[END_REF].

In recent years, several researchers have started to pay more attention to what has been termed "Fine Phonetic Detail" (FPD), which encompasses allophonic variation, sociophonetic variation and also important acoustic variation distinguishing male and female voices, for instance (Nguyen, Wauquier & Tuller, in press)6 . As Nguyen, Wauquier & Tuller explain (ibid.): "FPD refers to phonetic properties that are judged non-essential in the identification of speech sounds in a theoretical framework whose limits the exemplar approach endeavors to demonstrate". They sum up the debate as follows (op. cit.: 6) (see also [START_REF] Smith | The role of fine phonetic detail in word segmentation[END_REF][START_REF] Nguyen | La perception de la parole[END_REF]: "Abstractionist models, on the one hand, are based on the assumption that an abstract and speaker-independent phonological representation is associated with each word in the listener's mental lexicon. In exemplar models of speech perception, on the other hand, words and frequently-used grammatical constructions are represented in memory as large sets of exemplars containing fine phonetic information. […] [W]e will argue that both phonetic details and abstract phonological categories are likely to play an important role in speech perception". The last part of the quotation echoes the most recent evolutions in the field, as is suggested by [START_REF] Mcqueen | Phonological abstraction in the mental lexicon[END_REF]: 1113-1114, 1123): "evidence that listeners can show sensitivity to episodic detail should not be taken as evidence against abstract representations. […] However, the data on episodic effects do suggest that extreme abstractionist models are incorrect. […] Hybrid abstractionistepisodic models, therefore hold considerable promise". Given the compatibility between exemplarism and the integration of fine phonetic detail in speech perception models, it comes as no surprise that some phoneticians adopt this framework to examine the role of phonetic variation (especially sociophonetic factors) in their work, as is the case for Foulkes and Docherty (2006: 426): "Currently, the only theoretical framework that embeds indexicality centrally within phonological knowledge is the exemplar-based model of representation […]. [It] intrinsically captures the observation that no natural human utterance offers linguistic information without simultaneously indexing some social factor". Therefore, we can now concentrate on the link between variation and speech perception, first in L1, and then in L2.

Phonetic variability and perceptual learning

Factors of speech variation are well known (Meunier 2005: 360-364): coarticulation and assimilation, talker's characteristics (especially gender and age), speech situations (degree of formality, speech rate, emotions), dialectal and social variations. However, despite this variation, listeners manage to perceive speech adequately. This has been the focus of a series of studies devoted to what has been called "perceptual learning": the idea that listeners adjust their pre-existing phonemic categories through lexical knowledge, to accommodate speakers'pronunciation [START_REF] Norris | Perceptual learning in speech[END_REF]Kraljic & Samuels 2007). To do so, it seems that listeners use both lexical knowledge and acoustic information to tune dynamically their phonemic representations. This partly echoes what Nguyen, Wauquier & Tuller (op. cit.) call "phonetic convergence" (or imitation), i.e. the fact that speakers spontaneously tend to conform to the other speaker's pronunciation to facilitate the interaction: " [it] shows that listeners are sensitive to speaker-dependent phonetic characteristics, which have an influence on both the dynamics of conversational interaction, and across a longer time range the representations associated with words in memory when that interaction has ended". Moreover, these adjustments can also take place when native speakers are faced with non-native accented speech, as is shown by [START_REF] Bradlow | Perceptual adaptation to non-native speech[END_REF] who used Chinese-accented English speakers to test the adaptative capacity of native English listeners : after training with multiple Chinese-accented English speakers, the subjects were able to achieve talker-independent adaptation. Now, if this capacity to adjust to different speakers' pronunciation is part of the native speaker's linguistic skills and is necessary to handle speech perception, it seems reasonable to suggest that language learners should develop a similar capacity. Moreover, if perceptual learning appears here as an end ("being able to adjust quickly to the other's pronunciation"), we can also wonder whether presenting learners with variable stimuli (to train them to perceptual learning) would not also be a means to learn the L2 phonology, if we follow Boersma and Escudero's theoretical assumptions (cf. supra about perception grammar and gradual learning). Therefore, the question is: does variable stimuli training help language learners to build up their phonological competence in L2?

On the role of phonetic variability in L2 phonology learning

Variation and L2 learning

As far as we know, there are only a few studies tackling precisely the effect of phonetic variability on L2 phonology learning. [START_REF] Eisenstein | Target language variation and second language acquisition: learning English in New York city[END_REF] examines the intelligibility of three English dialects (Standard, New York and Black English) for learners, along with learners' attitudes and representations. [START_REF] Major | Testing the effects of regional, ethnic, and international dialects of English on listening comprehension[END_REF] test the impact of dialectal variation on learners'oral comprehension. They suggest that regional (but not ethnic or international) dialects should be inserted in language tests such as the Test of English as a Foreign Language. [START_REF] Sommers | An integrated account of the effects of acoustic variability in first language and second language: evidence from amplitude, fundamental frequency, and speaking rate variability[END_REF] assess the effect of three sources of variability (overall amplitude, fundamental frequency and speaking rate) on L2 vocabulary learning. Interestingly, they show that L2 vocabulary learning can be improved by variable speaking rate. Finally, and more closely related to our own topic, [START_REF] Levy | Perception of French vowels by American English adults with and without French language experience[END_REF] measure how linguistic experience and consonantal context can affect the perception of French vowels /y, oe, u, i/ by American listeners. Their results tend to show that consonantal context can have a strong effect on non-experienced listeners (op. cit.: 155): "Native language expectations of coarticulation affect perception of speech sounds in foreign languages. […] Naïve listeners perceive vowels in an unfamiliar language differently depending on the consonantal context in which they are presented". The importance of coarticulation effects in speech perception and lexical access has also been stressed by [START_REF] Nguyen | Rôle de la coarticulation dans la reconnaissance des mots[END_REF]. This leads us to think that training L2 learners to perceive speech in L2 implies training them to handle coarticulation effects, which, in turn, would imply feeding them with a large amount of highly variable contexts of occurrence for any given (phonemic) unit under scrutiny. Oral corpora data would then be very useful, provided we have suitable data mining tools.

Yet, the question of the impact of variable phonetic input on L2 segmental categories acquisition per se still needs to be addressed in detail. Part of the answer lies in a series of experiments carried out in the 1990s by a team of researchers led by Lively, Pisoni, Magnuson, Yamada and their colleagues (cf. infra).

Training Japanese listeners to identify English /r/ and /l/: 40 years of studies

In the field of L2 speech perception and learning, one discrimination task has been the focus of several enquiries for almost 40 years now [START_REF] Goto | Auditory perception by normal Japanese adults of the sounds "l" and "r[END_REF], "Auditory perception by normal Japanese adults of the sounds "l" and "r"): discriminating minimal pairs such as "rice" and "lice" for instance has been a longstanding challenge for Japanese listeners, for whom the distinction /r/ vs /l/ constitutes a case of negative divergent transfer [START_REF] Tarone | The phonology of interlanguage[END_REF]: 71) (one phonological category in L1 vs two in L2). In these studies, the target language usually under scrutiny is (American) English, and, among the most recent reports, a series of five articles entitled "Training Japanese listeners to identify English /r/ and /l/" [START_REF] Logan | Training Japanese listeners to identify English /r/ and /l/: A first report[END_REF][START_REF] Lively | Training Japanese listeners to identify English /r/ and /l/. II: The role of phonetic environment and talker variability in learning new perceptual categories[END_REF][START_REF] Lively | Training Japanese listeners to identify English /r/ and /l/. III: Long-term retention of new phonetic categories[END_REF][START_REF] Bradlow | Training Japanese listeners to identify English /r/ and /l/: IV. Some effects of perceptual learning on speech production[END_REF][START_REF] Bradlow | Training Japanese listeners to identify English /r/ and /l/: Long-term retention of learning in perception and production[END_REF]) yields a number of interesting conclusions.

First, they show that perceptual training with minimal pairs improves not only perception, but also production of the /r/-/l/ couple (Akahane-Yamada, Tohkura, [START_REF] Akahane-Yamada | Does training in speech perception modify speech production?[END_REF][START_REF] Bradlow | Training Japanese listeners to identify English /r/ and /l/: IV. Some effects of perceptual learning on speech production[END_REF] 7 . Moreover, there is long-term retention of learning in both tasks [START_REF] Bradlow | Training Japanese listeners to identify English /r/ and /l/: Long-term retention of learning in perception and production[END_REF].

Second, they suggest that learning difficulties are linked to perceptual interferences, since Japanese listeners seem to pay more attention to F2 acoustic cues than they should. As Iverson, Kuhl, Akahane-Yamada, Diesch, Tohkura, Kettermann and Siebert (2003: B53) explain: "The perceptual space of Japanese adults are thus mistuned for acquiring the English/r/-/l/ contrast, making acoustic variation that is irrelevant to categorization more salient than the critical differences in F3. These perceptual spaces are hypothesized to interfere with acquisition in at least two ways. First, Japanese adults could be prone to form erroneous category representations for /r/ and /l/, by relying on acoustic cues, such as F2, that are perceptually salient but not reliable or robust for categorization. […] Second, high sensitivity to irrelevant acoustic differences could create problems […], requiring focused attention and longer processing times to detect the critical F3 differences, even for Japanese adults who have formed correct category representations based on F3" (see also Magnuson & Akahane-Yamada 1996).

Finally, it has been demonstrated that English /l/ is perceptually assimilated to Japanese /r/ by native Japanese listeners (even though they can have two distinct mental representations), while English /r/ is perceptually more dissimilar from Japanese /r/ [START_REF] Hattori | Examination of similarity between English /r/, /l/, and Japanese flap: An investigation of best exemplars by English and Japanese speakers[END_REF]. In accordance with this, Aoyama, Flege, Guion, Akahane- [START_REF] Aoyama | Perceived phonetic distance and L2 learning: The case of Japanese /r/ and English /l/ and /r[END_REF] show that training native Japanese children in perception and production leads to better results for /r/ than for /l/. This lends support to the hypothesis of Flege's Speech Learning Model, which states that the more distant an L2 sound (phonetic segment) is from the closest L1 sound, the more learnable it will be.

What is remarkable for us is the integration of the variability factor in several of the experiments that have been realized. The results of replicated experiments [START_REF] Logan | Training Japanese listeners to identify English /r/ and /l/: A first report[END_REF]Lively, Logan & Pison 1993;[START_REF] Pisoni | Training Japanese listeners to identify english /r/ and /l/: A replication and extension[END_REF] totally support the hypothesis we have been developing throughout this paper, i.e., that variable L2 phonetic input can help L2 phonology learning, as is summed up by Iverson, Kuhl, Akahane-Yamada, Diesch, Tohkura, Kettermann and Siebert (2003: B54): "It is telling that the most successful training procedures for teaching English /r/ and /l/ to Japanese adults have involved multi-talker highvariability stimulus sets (e.g. [START_REF] Logan | Training Japanese listeners to identify English /r/ and /l/: A first report[END_REF]. Training procedures involving smaller stimulus sets are easier to learn but do not readily generalize to new stimuli […]. Training with larger stimulus sets may generalize better because the variability provides information about which cues are more robust and trains individuals to ignore irrelevant variation". This has also been positively tested by [START_REF] Iverson | Phonetic training with acoustic cue manipulations: A comparison of methods for teaching English /r/-/l/ to Japanese adults[END_REF], who name this teaching method "High Variability Phonetic Training" (i.e. natural words uttered by multiple speakers), along with three other techniques, proving to be beneficial for Japanese listeners to identify English /r/ and /l/. Magnuson and his colleagues obtain similar results in experiments targeting the role of talker variability in non-native phoneme training, i.e. English /r/ and /l/ for Japanese adults [START_REF] Magnuson | The role of talker variability in nonnative phoneme training[END_REF][START_REF] Magnuson | The effects of talker variability on the perception of English /r/ and /l/ by Japanese listeners: Subject differences and acoustic correlates[END_REF]. Their conclusion is as follows (Magnuson, Yamada, Tohkura, Pisoni, Lively & Bradlow 1995: 394): "while multiple-talker training leads to consistently good results, training with stimuli produced by only one talker may fail to promote generalization to new stimuli and talkers". Overall, a rather clear picture emerges from all these studies, and [START_REF] Bradlow | Training non-native language sound patterns: Lessons from training Japanese adults on the English /r/-/l/ contrast[END_REF] draws several lessons from training Japanese adults on the English /l/-/r/ contrasts. The most important one for us is the following: "Exposure to highly variable training stimuli promotes rather than interferes with non-native contrast acquisition. In particular, exposure to multiple talkers appears to be a highly effective means of ensuring that perceptual learning generalizes to novel talkers. […] This positive effect of the high variability training approach on speech category learning is consistent with exemplar-based models of speech perception […] in which item-specific acoustic-phonetic variability is encoded in the cognitive representation of experienced speech samples". If we take into account the results of these studies, it seems that the natural multiple-talker phonetic input that oral corpora can offer could be particularly useful for L2 phonology learning. This is also compatible with the L2LP model of Escudero (2007:128) who refers to Khul'work: "Khul (2000, p. 11855) argues that […] non-native listeners simply need the "right" kind of perceptual input, i.e., exaggerated acoustic cues, multiple instances of the same sound, and a mass listening experience […]. This suggestion is compatible with the Linguistic Perception model, which proposes that the GLA [(Gradual Learning Algorithm)] will act upon auditory inputs to gradually re-rank constraints […]. Thus L2 perception accuracy would benefit from perception training to enhance L2 input, both its acoustic properties and its frequency".

Phonetic vs. orthographic input: /r/ & /l/ in French for Japanese learners

So far, we have concentrated on the discrimination between /r/ and /l/ in English, and on the distinction between highly-variable multiple-talkers phonetic input versus single-talker phonetic input in pronunciation training. Two issues still need to be tackled: the orthographic factor and the language factor. Until recently, the orthographic factor in L2 speech perception and learning studies has been rather neglected, even though it has often been mentioned as a potential factor of influence on speech processing and linguistic development, whether in perception or in production. In the past few years, it has started to receive more attention in the fields of loanword phonology [START_REF] Vendelin | The influence of orthography on loanword adaptations[END_REF], L2 phonology [START_REF] Bassetti | Orthographic input and phonological representations in learners of Chinese as a Foreign Language[END_REF]) and speech sciences (in production [START_REF] Alario | The role of orthography in speech production revisited[END_REF]) and perception [START_REF] Perre | On-line activation of orthography in spoken word recognition[END_REF]). When we examine in detail the influence of written input (in L1 and/or L2) on oral L2 learning [START_REF] Detey | Interphonologie et représentations orthographiques. Du rôle de l'écrit dans l'enseignement/apprentissage du français oral chez des étudiants japonais[END_REF], and more specifically on L2 phonology learning, either on a segmental [START_REF] Detey | Interphonologie et représentations orthographiques. Le cas des catégories /b/ et /v/ chez des apprenants japonais de Français Langue Etrangère[END_REF] or syllabic level [START_REF] Detey | Can orthography influence L2 syllabic segmentation? Japanese epenthetic vowels and French consonantal clusters[END_REF], we realize that using orthographic transcriptions and/or transliterations (Detey 2007b) of oral data can hinder the development of oral skills, even though written representations can, under certain conditions, facilitate phonemic identification [START_REF] Steele | Assessing the role of orthographic versus uniquely auditory input in acquiring new L2 segments". 7èmes rencontres internationales du réseau français de phonologie[END_REF], help to perform other learning tasks, such as metalinguistic mental operations, and also promote multimodal encoding of lexical units or formulaic sequences. Therefore, when we examine the literature on the interactions between orthography and speech processing, as well as on the Japanese writing system [START_REF] Kess | The Japanese Mental Lexicon : psycholinguistic studies of kana and kanji processing[END_REF], we find it surprising that the studies mentioned above did not pay more heed to the orthographic factor. If the Japanese /r/ (prototypically realized as a phonetic flap [ɾ], and as a lateral [l] in intervocalic position [START_REF] Akamatsu | Japanese Phonetics : theory and practice[END_REF]Labrune 2001a)) has to be considered as the onset of moraic units (Labrune 2001b;[START_REF] Otake | Mora or Syllable ? Speech segmentation in Japanese[END_REF] rather than an autonomous segment, as it appears in the way it is represented in the two kana writing systems (the hiragana series ら、り、る、れ、ろ、and the katakana series ラ、 リ、ル、レ、ロ), it is nonetheless transliterated with an < r > and not an < l > (respectively < ra, ri, ru, re, ro >), since the < l > is not part of the rômaji, i.e. the transliteration system of Japanese. If we add the fact that the ever-growing number of English loanwords in the Japanese language merge, for instance, the English < ra > and < la > into a single Japanese katakana grapheme ラ (Kamiyama 1994), we realise that the orthographic factor cannot be overlooked so easily, especially among young urban Japanese native speakers [START_REF] Detey | Can orthography influence L2 syllabic segmentation? Japanese epenthetic vowels and French consonantal clusters[END_REF]. This points to the fact that the analysis of perceptual confusion between the English /r/ and /l/8 by Japanese listeners should incorporate phonographemic representations.

A possible way of tackling this issue would be to examine the perception by Japanese listeners of the English /r/ and of its French equivalent, since their graphemic implementations are similar on a single-letter level (<r>), whereas their phonetic realizations are quite different (post-alveolar approximant, retroflex ([ɻ]) or not ([ɹ]), for English versus uvular fricative, voiced ([ʁ]) or not ([χ]), for hexagonal French (to mention only the most frequent allophones)). More importantly, we should try to answer the following question: do Japanese listeners misperceive /r/ and /l/ more easily in English than in French, given their increased phonetic dissimilarity in French in comparison with their English equivalents ? This would call for several experiments, which are yet to be realized.

As regards the impact of orthography in French for Japanese learners, we set up an experiment aimed at testing Japanese listeners'perception of the French /r/-/l/ segments (and also the labial /b/ and /v/ (see [START_REF] Detey | Interphonologie et représentations orthographiques. Le cas des catégories /b/ et /v/ chez des apprenants japonais de Français Langue Etrangère[END_REF]) in connection with their orthographic representations. Stimuli consisted of trisyllabic nonwords and varied according to the segment's nature (/l, r, b, v/), position in the nonword (initial, penultimate and final) and vocalic environment (/a/, /i/, /u/, /ø/). For each auditory non-word, subjects had to select an orthographic transcription in a forcedchoice task (e.g. RADEKO versus LADEKO), and 120 Japanese university students in Japan performed the test. The most interesting conclusion of this experiment is the fact that the confusion rates for /l/ and /r/ were similar on average (respectively 23,7% and 23,5%), even though the vocalic environment had a differential effect on the performances (for instance with /i/, the confusion rates were 28% for /r/ and 13,6% for /l/) (cf. supra, the effects of coarticulation). According to Flege's Speech Learning Model, if we ignore the orthographic dimension of the task, we should have found more confusion for /l/ than for the French /r/ (since the latter is phonetically more different from the Japanese /r/ than /l/). This sheds light on the role that orthography can play in L2 learning, and calls for a better integration of phonetic and phonographemic training in L2 phonology learning.

Available resources for French: the PFC corpus

In the preceding paragraphs, we pointed out the advantages of using phonetic data from oral corpora by comparing the results yielded by single versus multiple talkers input in discrimination training. In the last paragraph we contrasted phonetic and orthographic input, and we emphasized the possible interferences of the orthographic representations in the learning process. Therefore, three types of input have been considered so far, from the most 'stable' to the most 'variable': orthography, singletalker, multiple-talker. Of course, a lot more experimental work needs to be accomplished to deepen our knowledge of the process at stake, and these three types of input have been, and will be, coexisting in language classrooms for a long time and for different reasons. However, if orthographic ('the teacher's writing') and single-speaker ('the teacher's voice') input have almost always been used in language education, the same cannot be said of multiple-speakers and oral corpora data. This is where the novelty lies, because these corpora are relatively new.

Once we are convinced that oral corpora can actually become pedagogical tools for L2 phonology learning, we need to find the available resources. As far as French language is concerned, oral corpora are relatively scarce, and it is only recently (February 2008) that the French Ministry of Culture and Communication opened an official website (www.corpusdelaparole.culture.fr) on which all main French oral corpora are connected and freely accessible. Among these, the most important one, and also more directly connected to our topic, is the PFC corpus, an achievement of the PFC project (Phonologie du Français Contemporain: usages, variétés et structure) coordinated by Jacques Durand (University of Toulouse), Bernard Laks (University of Paris X and Chantal Lyche (University of Oslo). The project's initial aim was to record, partially transcribe orthographically and analyze over 500 speakers from approximately 50 different locations in the francophone world, from France to Belgium, Burkina Faso, Canada, Ivory Coast, Louisiana, Switzerland, and other French speaking areas.

The project has been extensively described in other publications [START_REF] Durand | Le projet « Phonologie du français contemporain » et sa méthodologie[END_REF], but two interesting aspects of the project need to be underlined here. First, one of the distinctive qualities of the corpus is the use of the exact same protocol for all the recordings, including a wordlist (encompassing the segmental inventory of French) and a text (containing liaisons and schwas contexts) that subjects had to read aloud. Therefore, the words "rat" ("rat"), "lierre" ("ivy") and "extraordinaire" ("extraordinary"), for instance, were recorded by hundreds of different speakers, sometimes with noticeable allophonic variation. According to the studies we reviewed earlier, using these data for training Japanese learners to perceive and produce the French /r/ may prove to be useful, provided they are used adequately. Moreover, the PFC protocol also includes two conversations (one formal, one informal) and any user (teacher or learner) can look for a word or a structure in the corpus thanks to a freely accessible search engine on Internet (www.projet-pfc.net) that provides contextualized oral tokens of the item and corresponding standard orthographic transcriptions.

The second point we wish to mention is that such resources need ad hoc adaptation for language education contexts and teachers/learners' requests. The PFC-EF (Enseignement du français) sub-project, launched in 2006, aims at exploiting the PFC corpus for pedagogical purposes, mainly through a more user-friendly interface and tools on its website (www.projet-pfc.net/?pfc-ef) (for more information see [START_REF] Detey | Voix de la francophonie, éducation langagière et corpus numérisé : PFC-EF, des ressources pour la didactique du français[END_REF], but also through pedagogical publications (Detey, Durand, Laks & Lyche to appear). Therefore, we can hope that in a near future the PFC corpus will be used by French teachers and learners around the globe to develop oral skills in French, and more specifically master the French sound system.

Conclusion

In this contribution we tried to show that oral corpora could be used in foreign language education to help learners develop oral skills. More specifically, we concentrated on the role of oral corpora data as phonetic input for L2 phonology learning. After a brief presentation of the growing number of experiments devoted to what has been called 'Fine Phonetic Details' in the speech processing literature, we reviewed a number of studies which focus on the beneficial effect of high-variability stimuli training for Japanese learners of English struggling with the /r/-/l/ distinction. We pointed out the relatively neglected impact of the orthographic factor on L2 speech perception and learning, which we illustrated with Japanese learners of French and the French liquids /r/ and /l/. Finally, we briefly presented the PFC (Phonologie du Français Contemporain) corpus and its pedagogical exploitation in the PFC-EF (Enseignement du Français) sub-project. Even though we need further experimental work to test more accurately our hypotheses concerning the use of variable phonetic input with Japanese learners of French, it is thanks to the recent constitution of largesized corpora like PFC that we will be able to set up some of the necessary tests. Meanwhile, the oral data are freely available and can be used in many different pedagogical contexts, from oral comprehension tasks to Francophonie awarenessraising language observation activities.

This last quotation is of particular relevance for us, since it mentions two facts on which our general argument relies, as far as Japanese learners of French are concerned: the lack of oral input and the influence of the written medium.

Their study is based onShinohara's work (1997) on French and English on-line adaptations and integrated loanwords in Japanese: a French word like piscine ([pisin] 'swimming pool') is adapted as[pisinːɯ] (therefore what they call "geminate" refers to[nː]; for more discussion on the nasal geminates in Japanese see for instance[START_REF] Otaka | Can't the nasal sequence [nn] be regarded as a geminate in Japanese ?[END_REF]) and Shirai (1999)), while an English word like 'napkin' is adapted as[napɯkiN]. However, these two examples are not well chosen, since they are not prosodically symmetrical: in French, primary stress is always on the final syllable, while it is a lexical stress in English (on the first syllable, in the case of 'napkin'). Prosodic factors should be more systematically taken into account in the study of lowanword adaptations, even at the segmental level. Moreover, the adaptation of the French word [pisin] should rather be transcribed as[piʃinnɯ] or [piɕinnɯ], as the authors themselves suggest in(Peperkamp, Vendelin & Nakamura 2008: 130) with the following transliteration: "pishinnu".

Even though the authors (ibid.) point out that allophonic and between-speakers phonetic variations might actually be processed differently by listeners. See also[START_REF] Kraljic | Accommodating variation: dialects, idiolects, and speech processing[END_REF] for differences between context-independent (idiolect) and context-conditioned (dialect) variation processing by the perceptual system.

To which we must add the /w/ category (see[START_REF] Yamada | The intelligibility of Japanese speakers' productions of American English /r/, /l/ and /w/, as evaluated by native speakers of American English[END_REF][START_REF] Hardison | Bimodal speech perception by native and nonnative speakers of English : Factors influencing the McGurk effect[END_REF]).


L Prazakova

E Nolot

E Martinez

D Rouchon

F Fillot

N Bernier

R Elizalde

M Bernard

G Navarro

The Effect of Ge Content on Structural Evolution of Ge-rich GeSbTe Alloys at Increasing Temperature

Keywords: Phase change memories, Structural evolution, Raman spectroscopy, X-Ray diffraction

The material engineering of GeSbTe alloys has led to the significant improvements of thermal stability, necessary to ensure the data retention of the Phase-Change Memory device (PCM) over extended temperature range. However, despite the proven benefits of Ge enrichment of GeSbTe alloys, the effect of Ge content on the structure and its evolution as a function of annealing temperature remains unclear. In this paper, we present the structural analyses of as-deposited and annealed Ge-rich GeSbTe, considering the Ge enrichment (15 -55 at. %) of Ge2Sb2Te5 reference alloy. Based on the combination of Raman spectroscopy and X-ray diffraction techniques, we describe the progressive reorganization of the main structural features common to all investigated compositions. Therefore, we present a model describing the overall crystallization dynamics in Ge-rich GeSbTe. We highlight several competing phenomena in the system at increasing temperature, such as Ge diffusion and segregation as well as nucleation and growth of GeSbTe and Ge phases, which depends on Ge content. Our results contribute to the understanding of the crystallization mechanisms in Ge-rich GeSbTe alloys, unveiling the primary structural reorganization and crystallization of the GeSbTe phase followed by the crystallization of the Ge phase.

Introduction

GeSbTe alloys (GST) have been drawing attention over the last decades for their unique phase-change properties, in particular once integrated in Phase-Change Memory devices (PCM). Indeed, GeSbTe chalcogenides feature a reversible transition in temperature between the amorphous and the crystalline phase, used respectively as the high resistance state and the low resistance state in a memory device [START_REF] Raoux | Phase Change Materials and Their Application to Nonvolatile Memories[END_REF]. The Ge2Sb2Te5 compound (GST225) is considered a reference alloy, since already used in optical memory applications [START_REF] Yamada | Rapid-phase transitions of GeTe-Sb 2 Te 3 pseudobinary amorphous thin films for an optical disk memory[END_REF][START_REF] Redaelli | Phase Change Memory[END_REF] As regards the structure and crystallization mechanism, GST225 has been largely studied, pointing out its crystallization to metastable cubic structure (fcc) at ~160 °C and further to stable hexagonal structure (hex) at ~350 °C [START_REF] Friedrich | Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements[END_REF]. In recent years, the quest to improve the thermal stability of PCM led to the engineering of the alloy stoichiometry. Consequently, Ge-rich GST alloys were developed, demonstrating the ability to ensure the PCM device data retention in extended temperature range [START_REF] Cheng | A thermally robust phase change memory by engineering the Ge/N concentration in (Ge, N)xSbyTe z phase change material[END_REF][START_REF] Privitera | Atomic diffusion in laser irradiated Ge rich GeSbTe thin films for phase change memory applications[END_REF][START_REF] Palumbo | Forming operation in Ge-rich Ge x Sb y Te z phase change memories[END_REF][START_REF] Navarro | Phase-Change Memory: Performance, Roles and Challenges[END_REF][START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized ${\rm Ge}_{\rm x}{\rm Sb}_{\rm y}{\rm Te}_{\rm z}[END_REF]. Previous works evidenced the increase of the crystallization temperature with increasing Ge content, as well as the GST and Ge phase segregation, which occurs at high temperatures and during the device operations [START_REF] Zuliani | Overcoming Temperature Limitations in Phase Change Memories With Optimized ${\rm Ge}_{\rm x}{\rm Sb}_{\rm y}{\rm Te}_{\rm z}[END_REF][START_REF] Sousa | Operation fundamentals in 12Mb Phase Change Memory based on innovative Ge-rich GST materials featuring high reliability performance[END_REF]. Transmission electron microscopy (TEM) analyses were used to study the dynamics of the segregation process for a specific Ge-rich GST alloy, detecting the formation of the Ge crystalline phase prior to a detectable GST phase [START_REF] Agati | In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials[END_REF][START_REF] Agati | Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys[END_REF]. However, in order to appreciate the fine structural evolutions of the material, a methodology based on the combination of X-ray diffraction (XRD) analysis and Raman spectroscopy can be applied. The combination of these techniques was used to investigate the phase segregation and to demonstrate the GST phase preferential crystallization in Ge-rich GeTe/SbTe-based alloys [START_REF] Di Biagio | Crystallization Study of Ge-Rich (GeTe) m (Sb 2 Te 3 ) n Using Two-Step Annealing Process[END_REF]. Moreover, the methodology was also used to describe the dynamics of the structural evolution of undoped and N-doped Ge-rich GST alloys as a function of temperature, showing the sequence of the main structural reorganization (i.e. Ge-Te, Sb-Te, a-Ge, etc.) [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF]. The Ge enrichment of GST alloys is known to be beneficial to the thermal stability of the amorphous layer; however, a clear understanding of the impact of the Ge content on the structure of the alloys and its evolution in temperature is still missing. In this paper, we investigate the impact of Ge content in Ge-rich GST alloys by the combination of XRD and Raman spectroscopy analyses, further supported by complementary Transmission Electron Microscopy with Energy Dispersive X-ray spectrometry (TEM-EDX) measurements. Based on our findings, we elaborate a model describing the dynamics of the structural reorganization, phase segregation and crystallization in Ge-rich GST alloys in temperature, and its dependence on Ge content.

Experimental Methods

Ge-rich GST layers were deposited on 200 nm Si (100) wafers by magnetron co-sputtering from Ge2Sb2Te5 and Ge targets in Ar atmosphere and at room temperature. The thickness of deposited layers is 100 nm. The layers are protected by 3.5 nm-thick carbon capping layer deposited in-situ without air exposure to prevent layer surface oxidation. The target Ge enrichment in the deposited layers ranges from 15 at. % to 55 at. %. In the following, we will address the layers as GexGST with x representing the Ge enrichment. The composition of the layers was measured by Wavelength Dispersive X-Ray Fluorescence Spectroscopy (WDXRF Rigaku AZX 400) using the methodology described in [START_REF] Jeynes | Quantifying nitrogen in GeSbTe:N alloys[END_REF]. The as-deposited samples were annealed at different temperatures ranging from 200 °C to 450 °C. The ex-situ annealing was performed with a ramp-up of 10 °C/min and the samples were annealed for the duration of 5 minutes at the target temperature under inert N2 atmosphere. Raman spectra of as-deposited and annealed samples were measured at room temperature with a 532 nm laser diode excitation source (Renishaw InVia). The measurements were performed in the range of the Raman shift from 100 cm -1 to 1000 cm -1 . The laser intensity was kept at low level to avoid the heating and hence the induced modification of the layer morphology. X-ray diffraction (XRD) data were obtained at room temperature with a two-circle diffractometer (PANanalytical Empyrean). The Cu Kα radiation (λ = 1.5406 Å) was used as X-ray source. The XRD measurements were performed in Bragg-Brentano geometry within the 2θ range 22°-56° (with a step size of 0.01°). The in-situ ellipsometry (Woollam M2000) measurements on as-deposited samples were performed using the incident light in the visible range. The same ramp as in ex-situ annealing (10 °C/min) was used for the measurements. Transmission Electron Microscopy with Energy Dispersive X-ray spectrometry (TEM-EDX) measurements were performed on the Ge-rich GST layers annealed at 400 °C. The samples were prepared by Ga + Focused Ion Beam (FIB) milling (FEI Helios 450). Silicon oxide and tungsten protection layers were deposited to protect the regions of interest from the tails of the ion beam. The samples were observed at 200 kV using a probe corrected Titan Themis FEI microscope equipped with the Super-X detector system for EDX. The EDX mappings were obtained with the resolution of 320x900 pixels using a dwell time of 50 µs. The quantification was performed using the k factors implemented in the Bruker Esprit software.

Results

In this part, we first report on the as-deposited samples studied by WDXRF, ellipsometry and Raman spectroscopy. Next, we introduce the main structural features and their evolution as a function of temperature in Ge45GST alloy, which is used as an example. In the third part of the results, we extend the analyses to all the studied Ge-rich GST compositions.

As-deposited Ge-rich GST layers

WDXRF quantification of the Ge, Sb and Te content in the studied Ge-rich GST layers is shown in Fig. 1. The measured values are in good agreement with the nominal ones, with the Sb over Te ratio of 0.4, as expected from the Ge2Sb2Te5 target used for the deposition. The global Ge content in the studied alloys increases from about 20 at. % up to more than 60 at. %. In-situ ellipsometry was performed in order to observe the change in optical behavior of Ge-rich GST samples with increasing temperature. Fig. 2 shows the angle psi (ψ), related to the ratio between reflection coefficient for p-and s-polarization, as a function of the annealing temperature. The GST225 sample shows two sharp increases of ψ at around ~165 °C and ~350 °C corresponding to the amorphous-to-fcc and fcc-to-hex transition, respectively. Increasing the Ge content in the layers leads to the delay of the first transition to higher temperature, whereas the fcc-to-hex transition is not observed up to 400 °C. Moreover, the global difference in ψ is reduced as the Ge content increases. In Ge35GST and Ge45GST, the first transition is separated into two steps, evidencing the growth of an intermediary phase in the system, which will be explained later in the text. A similar double step transition in Ge-rich GST alloys was already observed in Resistivity vs Temperature (R vs T) measurements [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF]. A second sharp increase of ψ can be observed in Ge-rich GST at higher temperatures and may be attributed to the growth of the GST phase once Ge is completely segregated and starts to crystallize. Raman spectra of amorphous as-deposited (AD) layers are reported in Fig. 3. The spectra were normalized with regard to the intensity of the peak at 154 cm -1 . The spectra can be divided into two main frequency regions:

-from 100 cm -1 to 190 cm -1 , we observe several overlapping bands at ~110 cm -1 , 124 cm -1 and 154 cm -1 , corresponding to the different vibration modes of the GST phase [START_REF] Andrikopoulos | Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition[END_REF][START_REF] Andrikopoulos | Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials[END_REF][START_REF] Němec | Ge-Sb-Te thin films deposited by pulsed laser: An ellipsometry and Raman scattering spectroscopy study[END_REF][START_REF] Sosso | Raman spectra of cubic and amorphous Ge 2 Sb 2 Te 5 from first principles[END_REF][START_REF] Caravati | Coexistence of tetrahedral-and octahedral-like sites in amorphous phase change materials[END_REF][START_REF] Mazzarello | Signature of Tetrahedral Ge in the Raman Spectrum of Amorphous Phase-Change Materials[END_REF][START_REF] Bo | Raman spectra and XPS studies of phase changes in Ge 2 Sb 2 Te 5 films[END_REF]. In particular, the band at 154 cm -1 is assigned to the Sb-Te vibrations of SbTe3 pyramidal units, the band at 124 cm -1 is assigned to the Ge-Te vibrations in GeTe4-nGen (n = 1, 2) mixed tetrahedra units. The band at ~110 cm -1 corresponds to Eg 2 vibration of Sb-Te bonds (See Supplementary material). -from 190 cm -1 to 300 cm -1 , two overlapping bands can be distinguished at about 270 cm -1 and 220 cm -1 . These bands are assigned to the TO and LO vibrational modes of amorphous Ge (a-Ge), respectively [START_REF] Chambouleyron | Optical properties of non-stoichiometric germanium nitride compounds (a-GeNx)[END_REF][START_REF] Kazimierski | Transition from Amorphous Semiconductor to Amorphous Insulator in Hydrogenated Carbon-Germanium Films Investigated by Raman Spectroscopy[END_REF][START_REF] Jamali | Evaluation of chemical and structural properties of germaniumcarbon coatings deposited by plasma enhanced chemical vapor deposition[END_REF][START_REF] Lannin | Raman scattering and short range order in amorphous germanium[END_REF][START_REF] Jamali | The effect of carbon content on the phase structure of amorphous/nanocrystalline Ge 1-x C x films prepared by PECVD[END_REF].

The described bands can be distinguished in the spectra of all Ge-rich GST samples. It can be noted that the band at 154 cm -1 assigned to the Sb-Te vibrations is stable in position and width and represents the dominant peak in the spectra, independently from the Ge content in the layer. On the contrary, the decrease in intensity of Ge-Te modes at 124 cm -1 and the clearer separation of the peak at 110 cm -1 can be observed in the alloys with higher Ge content. The intensity of the a-Ge modes increases with the Ge content in the layer, as expected.

Evolution of Ge45GST as a function of temperature

Based on the previous investigations on Ge45GST alloy [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF], we initially focus on this composition in order to describe the evolution of the main structural features with increasing temperature. This description will further allow to highlight the main structural changes common to all the investigated Ge-rich GST samples. The Raman spectra in Fig. 4a evidence the stability of Sb-Te features (154 cm -1 ) in temperature.

On the contrary, the intensity of Ge-Te vibrations (124 cm -1 ) first decreases at 300 °C and later increases at 350 °C. These changes suggest a primary reorganization of Ge-Te modes (i.e. decrease of disorder) at low annealing temperature and a further progressive rearrangement of the Ge-Te bonds around stable Sb-Te units in a GST phase. The evolution of Ge-Te features observed at 300 °C cannot be related to the appearance of a crystalline phase, as confirmed by the XRD pattern of Fig. 4b. Starting from 350 °C, the band at 100-190 cm -1 of the Raman spectra can be attributed to the cubic crystalline GST225 phase (GST-fcc) [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF], as confirmed by XRD. At the same temperature, the a-Ge modes (190-300 cm -1 ) are gradually reduced, turning into a narrow peak at ~300 cm -1 , which is assigned to the crystalline Ge mode (c-Ge) [START_REF] Kazimierski | Transition from Amorphous Semiconductor to Amorphous Insulator in Hydrogenated Carbon-Germanium Films Investigated by Raman Spectroscopy[END_REF][START_REF] Jamali | The effect of carbon content on the phase structure of amorphous/nanocrystalline Ge 1-x C x films prepared by PECVD[END_REF]. During the transition from a-Ge to c-Ge, an intermediary peak at ~280 cm -1 can be observed, which later disappears at higher temperatures. The formation of the cubic crystalline Ge phase starting from 350 °C is also confirmed by XRD. Hence, GST-fcc and Ge crystalline phases separate and further grow independently. The feature at ~30.5° in the XRD pattern at 350 °C is assigned to the formation of a transient GeSbTe phase (called G + GST) with higher Ge content with respect to GST225. This transient phase represents a mandatory step between the starting amorphous composition, which is homogeneous and highly enriched in Ge content, and the completely segregated GST-fcc and Ge phases [START_REF] Prazakova | Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping[END_REF].

Structural evolution of the GexGST as a function of Ge enrichment and temperature

Fig. 5 shows the Raman spectra of the annealed GexGST layers. The intensity of Raman spectra was normalized with regard to the intensity of the peak at 154 cm -1 , as previously shown for Ge45GST. Indeed, this well-defined peak assigned to the Sb-Te vibrations is present in all the spectra at all different annealing temperatures.

In the spectra of Ge15GST, Ge25GST and Ge35GST at 300 °C, the broad band below 190 cm -1 (Sb-Te and Ge-Te vibrations of a GST phase) already corresponds to that of cubic GST225. This likely indicates that the Ge-Te bond rearrangement around Sb-Te units in the layer structure is highly advanced or even completed. On the contrary, the intensity of Ge-Te modes in Ge45GST and Ge55GST is significantly lower, indicating that the structure is still disordered. The a-Ge modes (190-300 cm -1 ) present in all compositions imply that the Ge in the layer still remains in the amorphous phase.

At the annealing temperature of 350 °C, the Ge-Te modes reorganization can already be observed in all compositions. Moreover, the peak at 110 cm -1 , observed at lower temperatures, completely disappears. The most significant changes occur in the region of a-Ge modes, where the c-Ge mode appears in all the spectra except for Ge35GST. The intensity of c-Ge mode varies with the Ge content in the layer. In Ge35GST, however, the intermediary peak at 280 cm -1 can be found, corresponding to the one previously observed in Ge45GST. Layers annealed at 400 °C and 450 °C show a good match between the spectra of all compositions. The intensity of c-Ge peak increases with the Ge content. Slight asymmetry and hence the position shift of the GST phase broad band (100-190 cm -1 ) in the spectra at 450 °C is due to the presence of an additional low intense contribution at ~170 cm -1 . This contribution could be related to the rising transition of the GST225 cubic phase towards the hexagonal phase [START_REF] Bo | Raman spectra and XPS studies of phase changes in Ge 2 Sb 2 Te 5 films[END_REF][START_REF] Xu | Optical constants acquisition and phase change properties of Ge 2 Sb 2 Te 5 thin films based on spectroscopy[END_REF]. The XRD patterns of the studied layers are presented in Fig. 6. The peak intensity in all patterns was normalized with regard to the dominant peak of the GST phase (i.e. GST[200] peak). The XRD patterns of the layers annealed at 300 °C (Fig. 6a) show the presence of the cubic GST225 (GST-fcc) crystalline phase for Ge15GST, Ge25GST and Ge35GST, while the XRD patterns of Ge45GST and Ge55GST show only a broad feature indicating a still disordered structure. The peaks in XRD patterns at 350 °C (Fig. 6b) evidence the already developed GST-fcc phase in all the compositions. Moreover, the second set of peaks, assigned to the Ge cubic crystalline phase, can be observed in all the patterns except for the Ge35GST composition. These observations are in good agreement with the Raman analyses. Furthermore, the broad band at ~30.5°, alongside the dominant GST[200] peak, is observed in the pattern of Ge35GST at 300 °C. This band is assigned to G + GST transient phase and disappears at higher temperature. The same phenomenon was observed in Ge45GST at 350 °C. These findings are compatible with the ellipsometry results, evidencing an intermediary transition along the increasing ψ values (Fig. 2). The XRD patterns at 400 °C and 450 °C show the continuous intensity increase and narrowing of the GST-fcc and Ge cubic peaks indicating the crystal growth of the separated GST-fcc and Ge phases. In Ge15GST and Ge25GST, high annealing temperature further induces the intensity increase of the peaks corresponding to the GST[111] and GST[222] planes, which indicates the preferential crystallization of the GST-fcc phase in these planes. XRD pattern of Ge15GST at 450 °C also evidences the significant position shift of the GST[200] and GST[220] peaks and the appearance of several new broad bands. These changes may indicate the advancing fcc-to-hex phase evolution of GST225 (GST225-hex: PDF No. 04-008-1147), or much more likely the vacancies reordering in the cubic phase, which occurs well before the fcc-to-hex transition [START_REF] Wang | A Review on Disorder-Driven Metal-Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials[END_REF].

Discussion

Raman spectra of as-deposited amorphous Ge-rich GST layers feature the vibrational modes of the GST phase and the a-Ge phase. In particular, the main band of all spectra is assigned to the vibrations of Sb-Te units characteristic for the GST phase and remains stable up to high annealing temperatures (Fig. 5). On the contrary, we can observe changes in the intensity of Ge-Te modes with temperature, indicating the ongoing Ge-Te bonds rearrangement around Sb-Te units. In Ge15GST, Ge25GST and Ge35GST such rearrangement leads to the formation of the crystalline GST-fcc phase at 300 °C (Fig. 6). In Ge45GST and Ge55GST, however, the formation of the crystalline GST-fcc phase is delayed to 350 °C. At 350 °C, the coexistence of both GST-fcc and Ge crystalline phases is observed (Fig. 6), which demonstrates that the Ge phase segregation already started. Ge35GST is the only sample not showing the Ge crystalline phase at 350 °C, however, we can recognize the already advanced evolution of a-Ge modes (i.e. ordering) through the appearance of the intermediary peak at 280 cm -1 (Fig. 5). These observations highlight the existing competition/concurrence of the following phenomena, which depend on Ge content in the layer and on temperature: a) the nucleation and growth of a GST phase (i.e. GST225-fcc); b) the Ge segregation out of the formed GST phase, which is delayed by increasing Ge content (i.e. lower diffusivity), but favored by increasing temperature (i.e. higher diffusivity of Ge); c) the backward chemical diffusion of Ge (hinders the GST phase formation), which increases with Ge content; d) the nucleation and growth of Ge crystalline phase (favors the Ge segregation). This model, presenting the several phenomena, shows that the segregation of the different phases is initiated by the reorganization driven by the stable Sb-Te units in GST phase and by the consequent nucleation of a GST phase. In Ge35GST, the balance between Ge (~50 at. %) and Sb/Te content likely delays the formation of the Ge crystalline phase due to the competition between delayed Ge segregation and enhanced Ge backward diffusion. Indeed, the Ge segregation is greatly disfavored by high Ge content in Ge45GST and Ge55GST, which can explain the observed delay of the GST phase crystallization to 350 °C. Fig. 7 shows the values of peak broadening at full width at half maximum intensity (FWHM) for GST [220] and Ge[111] peaks and the calculated crystallites sizes of each phase. The FWHM parameter here represents the line broadening of the peak after subtracting the instrumental contribution. The crystallites size of the GST-fcc and Ge phases was calculated using the Scherer equation. For both GST-fcc and Ge phases we can observe the decrease of FWHM, i.e. increase of crystallites size with temperature, thus indicating a strong correlation between the phases' crystalline growth. On the contrary, there is no evident dependence on Ge content. It is interesting to notice that the GST-fcc crystallites size is larger than the equivalent Ge crystal size at all compositions. The crystallites size of both phases reaches similar values at 450 °C, independently from the Ge content in the layer. This result supports previous discussions about primary nucleation and crystalline growth of GST phase and consequent crystalline growth of the Ge phase, which is very fast at high temperatures. Fig. 7 also highlights the crystallization temperature of a-Ge reduced below 350 °C, whereas it is generally reported to be in the range of 480 °C -530 °C [START_REF] Khan | Effect of annealing on structural, optical and electrical properties of nanostructured Ge thin films[END_REF][START_REF] Tao | Characterization and mechanism of crystallization of Ge films on silicon substrate with graphite buffer layer[END_REF][START_REF] Katsuki | Crystallization of amorphous germanium in an Al/a-Ge bilayer film deposited on a SiO2 substrate[END_REF][START_REF] Tsao | Low-temperature growth of polycrystalline Ge thin film on glass by in situ deposition and ex situ solid-phase crystallization for photovoltaic applications[END_REF]. Indeed, the heterogeneous nucleation of Ge can be likely observed in our samples, which is induced at the interfaces of the already existing GST crystalline seeds [START_REF] Carria | Amorphous-Crystal Phase Transitions in Ge x Te 1-x Alloys[END_REF][START_REF] Debunne | Evidence of Crystallization-Induced Segregation in the Phase Change Material Te-Rich GST[END_REF][START_REF] Knaepen | In situ x-ray diffraction study of metal induced crystallization of amorphous germanium[END_REF]. The progression of GST crystallization followed by Ge crystallization is well observable in particular in the GexGST layers with low Ge enrichment, in which the two phases appear at different annealing temperatures (Fig. 6). On the contrary, the GST and Ge crystalline phases appear at the same annealing temperature in the Ge-rich GST layers with higher Ge enrichment, which results from two concurrent phenomena: delayed GST phase crystallization and high crystalline growth speed of Ge phase. The dynamics of the GST crystallization and Ge segregation processes can be well observed in the case of Ge35GST. XRD patterns of Ge35GST at 300 °C, 350 °C and 400 °C without normalization are shown in Fig. 8. The measurement parameters were kept the same during the acquisition, which allows us to compare the peak intensity of the emerging GST-fcc phase. The XRD pattern at 300 °C shows the peak GST [200] together with the broad band of the transient phase G + GST at ~30.5°. The low intense but narrow peak of GST[200] indicates the formation of high quality grains though present in small quantity, which are enclosed in a disordered Ge enriched G + GST phase. High Ge content and relatively low temperature do not yet allow the system to expulse abundant Ge and therefore completely separate the Ge and GST-fcc phases. Increasing the annealing temperature up to 350 °C, the Ge diffusivity and hence the Ge expulsion from G + GST are enhanced. This process leads to the gradual disappearance of the transient G + GST phase and to the progressive definition of GST polycrystalline and Ge crystalline regions. This is evidenced in the XRD pattern at 350 °C as the intensity of the GST[200] peak increases and the peak becomes slightly broader than at 300 °C. The broad feature assigned to G + GST is then reduced in intensity. XRD pattern at 400 °C further confirms the complete disappearance of G + GST phase, whilst the separated Ge phase appears (Ge[111] peak), enhanced by the heterogeneous nucleation and high growth speed at the interface with the already formed GST-fcc phase. The model is represented schematically in Fig. 9. The difficulty to observe such transient phenomena in GexGST layers with lower Ge contents is likely related to the reduced formation of the region enriched in Ge, which surrounds the GST-fcc nuclei. Lowering the Ge content likely reduces the time scale for observing the broad band as well as its intensity. In order to follow the progress of the demixing process in the alloy, TEM images with EDX analyses were performed on GexGST layers annealed at 400 °C. The Ge and Te cartographies in Fig. 10a show the formation of Ge-rich and Te-rich regions indicating the complete segregation of GST and Ge phases. This findings confirm the XRD results (Fig. 6), where the already developed GST-fcc and Ge cubic crystalline phase are observed. Moreover, the cartographies show the dominance of the Ge phase with increasing Ge content, as expected.

Based on the data extracted from EDX analyses, we reconstructed the compositional distribution of phases present along the tie line between GST225 phase and pure Ge phase. The cartographies shown in Fig. 10b demonstrate the distribution from highly occurred phase to less occurred phase (i.e. transition from dark to light zones, respectively) for the studied alloys. We can clearly observe the gradual transition of the distribution towards the pure Ge phase as the Ge content in the layers increases. This transition suggests the progressive tendency of the pure Ge phase to dominate the system dynamics, which is in good agreement with our XRD results.

Conclusions

The crystallization dynamics of the Ge-rich GST alloys with a large range of Ge enrichment were investigated by Raman spectroscopy and XRD. Combining the results from the two characterization techniques we observe that the structural evolution process in temperature begins with the Ge-Te bond rearrangement around stable Sb-Te units characteristic of GST phase, independently from the Ge content in the alloy. Our results further support the primary crystallization of GST-fcc phase, followed by the heterogeneous nucleation and consequent growth of Ge phase. We show that the crystallites sizes of the GST-fcc and Ge phases are strongly correlated and increase with annealing temperature, independently from Ge content in the alloy. We present a model taking into account several phenomena, which occur during the structural evolution of the alloys and which are delayed to higher temperature as the Ge content increases. Besides, the model includes the experimentally observed transient G + GST phase. The GST phase crystallization followed by the Ge phase crystallization are well observed in layers with low Ge content since they appear at different temperatures. On the contrary, these two processes appear simultaneously at high temperature in the layers with high Ge content, resulting from delayed GST phase crystallization and fast Ge crystal growth. These findings contribute to the long-lasting discussions on the crystallization mechanism of Ge-rich GST alloys.

Fig. 1 :

 1 Fig. 1: Ge, Sb and Te content in studied Ge-rich GST alloys measured by WDXRF.

Fig. 2 :

 2 Fig.2: In-situ ellipsometry measurements showing the delay the amorphous-to-fcc transition of GST phase and Ge phase crystallization for studied Ge-rich GST alloys.

Fig. 3 :

 3 Fig. 3: Raman spectra of as-deposited GexGST layers.

Fig. 4 :

 4 Fig. 4: Raman spectra (a) and XRD patterns (b) of as-deposited (AD) and annealed Ge45GST. Indexation of XRD patterns is performed based on database data for cubic crystalline Ge and GST225-fcc phases (ICDD PDF Nos. 00-054-0484 and 00-004-0545).

Fig. 5 :

 5 Fig. 5: Raman spectra of GexGST layers annealed at 350 °C (a), 350 °C (b), 400 °C (c) and 450 °C (d).

Fig. 6 :

 6 Fig. 6: XRD patterns of GexGST layers annealed at 300 °C (a), 350 °C (b), 400 °C (c), and 450 °C (d).

Fig. 7 :

 7 Fig.7: Correlation of the FWHM and crystallites sizes for the GST225-fcc phase and Ge crystalline phases (dashed line). The values were taken from the GST[220] and Ge[111] peaks. The solid line represents the 1:1 ratio between the crystallites size of both phases.

Fig. 8 :

 8 Fig.8: XRD patterns (range 26°-34°) for Ge35GST alloy at 300 °C, 350 °C and 400 °C highlighting the intensity increase of theGST[200] peak related to the GST225-fcc phase and the reduction of the broad band assigned to transient G + GST phase.

Fig. 9 :

 9 Fig. 9: Scheme representing the progress of GST225-fcc phase crystallization, disappearance of transient G + GST phase with the expulsion of Ge atoms and following Ge phase crystallization with increasing annealing temperature.

Fig. 10 :

 10 Fig. 10 : TEM-EDX analyses on GexGST layers annealed at 400 °C. a) Ge an Te cartographies highlighting the elemental migration; b) Ge-Sb-Te ternary diagram showing the compositional distribution along the tie line

Acknowledgments

This work has been performed with the help of the NanoCaracterization Platform of Minatec with the financial support of the "Recherche Technologique de Base" program. It has been partially supported by the European 783176 WAKeMeUP Project, financed by European commission, French government and Auvergne-Rhône Alpes Region.

Data Availability

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

Supplementary materials

See the Supplementary material for the extended analyses of Raman spectra of as-deposited and annealed (350 °C) Ge15GST, Ge35GST and Ge55GST alloys.