Catherine Baroin
email: catherine.baroin@mae.u-paris10.fr

LIVESTOCK PROPERTY MARKS IN AFRICA

Why should we be interested in livestock property marks, in Africa or elsewhere? One reason is that marking is a social signal, a form of message -in a way, a form of language. It usually does not indicate who is the precise owner of the marked animal, but it provides some clues as to the owner's identity and social relationships. Marks are signs of identity, and as such they are often not only used on livestock but on other items as well, such as trees, house poles, wells, bags of dates after harvesting, etc. To understand the meaning of marks, one has to know how they are chosen, combined and used by individuals or groups. This may shed some light on these people's social relationships, and also illustrate their community's ties with other neighbouring ones. The study of property marks may, therefore, be a valuable key for research on the logics and history of a social group, although such a method also has its limits that we shall discuss.

There are two main ways to mark one's livestock in Africa, branding and ear cutting. Herders most often either brand a design on the animal's skin with a hot iron, or they cut its ears in specific shapes. Sometimes however, both types of marking are used simultaneously. Whatever the technique, the aim is the same, i.e. to convey information as to the owner's identity. This is the reason why both brands and earcuts are taken into account in this paper, although from a technical point of view these two ways of marking animals are totally different. This paper will first evoke the scant historical evidence of livestock marking in Africa, before it proceeds to a short review of the existing literature. Then the different components of a proper mark description will be presented and discussed, which is a necessary step as most studies unfortunately are lacking one way or the other in this respect. After that we shall describe the marking of livestock in some of the main African ethnic communities before discussing the logics and the social purposes which underlie property marking in most pastoral societies.

Marking in history

Historical evidence about livestock property marking in Africa is very scarse. Yet it may be guessed, as far as earcuts are concerned, that it has a long lasting history. In the 1920's it was reported in Madagascar [START_REF] Birkeli | Marques de boeufs et traditions de race », documents sur l'ethnographie de la côte occidentale de Madagascar[END_REF], where it was an already long lasting practice. It still is in use there today. On the African continent itself, the same technique (but not the same marks) is mainly documented among the Fulani. These well known cattle herders spread all across the Sahelian zone, South of the Sahara, from Senegal to Sudan. Their historical traditions mention that they originate from the Nile valley, but evidence from the last centuries demonstrates the opposite, i. e. a migration in the reverse direction, from Senegal eastwards. This migration took place from the XIII th century onwards up to now. However, it does not imply that Fulani traditions are mistaken: these herders may indeed have previously migrated from the Nile Valley westwards till Senegal, before migrating back to the East. Anthropological genetics hint to this Eastern origin, as they show that Cameroonian Fulani share up to 27 % of their ancestry with Middle-Eastern people [START_REF] Tishkoff | The genetic structure and history of Africans and African Americans[END_REF]. But whatever the migrations of these herders over the centuries, we lack evidence as to the time when they started marking their cattle the way they do now, i.e. with ear-cuts.

As for livestock branding, it seems more widespread in Africa than ear-cuts. It is documented as a very ancient practice, dating at least as far back as Ancient Egypt. Most african pastoral peoples, both cattle and camel1 herders, brand their livestock today. Branding is common practice not only in Africa but outside this continent as well. It is to be found among many herders, such as cow-boys in the United States of America, Mexico, Spain or Australia. Horses may also be branded. Such is the case in Southern France, in the Camargue region, where branding is called ferrade (from the French word fer which means iron). In former times, branding was not limited to livestock. It was also used for slaves who were, in a way, considered somewhat similar to livestock, being just another type of living property.

Historical evidence about livestock property marking in Africa, on the long run, is indeed scarce. But for a more recent period, let's say over the last century, available information is also very limited. It is, moreover, scattered and difficult to piece together. It consists mainly in marginal remarks given in articles or books which focus on other subjects. Such studies most often lack an index pointing to this topic. We are therefore bound to consider the following review of the existing literature as most likely far from complete. It is nonetheless useful to mention some best known works, before attempting to draw a general outlook of the situation as regards the marking of livestock in Africa today.

Review of existing literature

The French anthropologist Arnold Van Gennep was the first scholar to draw attention to the interest of collecting and studying property marks. He launched a questionnaire on this topic in 1901, and published a short article on Arab brands in 1902, calling for more data to be collected. These brands being some sort of armorial bearings, he observed, they may provide some clues as to the history of Arab migrations.

A much more significant work is H. A. MacMichael's in 1913. It is a detailed description of 133 brands used by the main camel-herding tribes of Kordofan in Northern Sudan, most of them Arabs. For each brand, MacMichael provides the name, drawing, size and position on the animal's body, and the name of users. His short introduction (2 pages) stresses the difficulties of such a research: « a study of the brands used by camel-owners is very confusing owing to the innumerable varieties and exceptions that occur to almost any rule that may be formulated » (p. vi). Yet MacMichael does indicate that some few brands are specific of one tribe or tribal section, and he notes that a common ancestry may be reflected by the use of a common brand, although in some cases the original brand falls into disuse. In spite of the difficulties he faced, MacMichael's very precise study remains a milestone that, for many years, will have no following.

The next worthwhile study is not on camel brands but on cattle ear-marks, on the Western coast of Madagascar. It is a long article published in 1926 by a Norwegian missionary, Emil Birkeli. This author uses ear-marks as evidence to piece together the history of local migrations. His pioneering work was continued in the late 1960's by a French anthropologist, Suzanne Chazan-Gillig. She studies the evolution of the same ear-marks and draws on Birkeli's work to check the changes that took place during the 40 years time gap between his and her stay in this region. Chazan-Gillig's book deals with the history of the Menabe kingdom in Western Madagascar. It only came out in 1991. It entails many descriptions of cattle ear-cuts and their social use by local herders. She observes that cattle ear-marks are associated to a lineage tomb (p. 144), and that these marks are a sign of the king's spacial influence (p. 222). Marks on the right ear refer to the owner's patrilineal affiliation, and marks on the left ear to his matrilineal ties. Marks on the right ear demonstrate the patrilineage unity. When the exogamous patrineal lineage splits -a fact which goes together with the transgression of the exogamy rule-the splitting group creates a new ear-mark for its cattle (p. 222). However, the use of a lineage mark does not necessarily point to a kinship link. It may also translate a political one, since in case of allegiance between two groups, the dominated group will borrow the marks of the dominant one (p. 255). After Birkeli's work in 1926, which is the first detailed study on African cattle ear-cuts, no other significant work is published on livestock marks for nearly three decades. The next important publication is by Marguerite [START_REF] Dupire | Contribution à l'étude des marques de propriété du bétail chez les pasteurs Peuls[END_REF]. It deals with cattle ear-cuts among the above mentionned Fulani. Up to this date, this 20 pages article is the only precise study ever published about cattle ear-marks on the African continent. These cattle ear-marks are cut out during a ritual which the author describes (p. 125 sq.). Their aim is not only to indicate the owner's identity, but also to protect the animal against bad luck, illness or death (p. 130). In case of bad luck indeed, the Fulani herder will switch to a new ear-mark (p. 136). As the above mentionned scholars indicated, Dupire also stresses that a mark is not an individual's, but a primary patrilineage's property. The segmentation of a patrilineage, therefore, automatically leads to the creation of a new mark (p. 132-133). Cattle ear-marks thus demonstrate a tribe's cohesion or diversity (p. 135). As for the individual, his personal situation will be read through his use of patrineal marks on the left ear, and matrineal ones on the right ear (i.e. the opposite from Madagascar). Dupire also observes that marks do not only refer to kinship ties. Another type of ear-cut, known by a different name, marks an animal as part of a borrowing contract, in a type of social exchange that is widespread among, and specific of Fulani herders (p. 131).

Anthropological research among African pastoral people intensifies in the 1960's and brings more material on livestock marking forth. No specific publication, however, is devoted to this topic. In his famous book on Somali camel-herders, A pastoral democracy, published in 1961, Ioan M. Lewis very briefly indicates that camel brands do not belong to individuals, but to lineage-groups. These usually are dia-paying groups (i.e. kinsmen who jointly pay compensation for murder), who are also responsible when camels are stolen by raiders (p. 83). Although Lewis's description is very short, he stresses a particularly important point, namely the relationship between brands and camel theft. This is far from negligible in a society where livestock raiding is so common place.

One year later the French administrative officer Albert Le Rouvreur publishes a book about the Sahelian and Saharan tribes of Chad (1962). It provides much data about the numerous tribes of Northern Chad, and includes 15 tables on camel and cattle brands. Each table indicates the names of the brand users, their main habitat, the design of the brand, its name (sometimes with the translated meaning) and the location of the brand on the body of the animal. The author clearly considers these brands as a type of group signature, but he does not venture into any sociological discussion about their use.

In 1966, Ian Cunnison publishes his anthropological study of an Arab cattle herding tribe, the Humr of Southern Kordofan (Sudan). Right in MacMichael's steps, he provides a list of their brands (p. 205-208), and briefly discusses their social use (p. 99-100). Each of the two Humr tribal sections has its own brand, and « it is quite common for major or minor lineages to have exclusive brands » as well. They are « a help in the search of stray animals », and « a means of protection, in a mystical sense, for their cattle, and they change them when they appear to be no longer effective » (p. 100). This mystical use of brands as a form of protection, which may lead herders to change their marks in case of bad luck, is exactly the same as noted by [START_REF] Dupire | Contribution à l'étude des marques de propriété du bétail chez les pasteurs Peuls[END_REF] regarding Fulani cattle ear-cuts (see above). Cunnison also interestingly mentions in a note (p. 100) that besides livestock brands, « earclips also are used on animals for identification », but that he made no comprehensive collection of them. It is quite unfortunate indeed that he did not go any deeper into the study of these ear-marks, since these Baggara Arabs are an interesting case of herders using both brands and ear-cuts on their animals. Thus the opportunity to study a case of how and why these two types of marking articulate together was missed.

Further South in Uganda, the cattle-herding Karimojong were studied by Neville Dyson-Hudson. His book (1966) provides some information about the Karimojong clan cattlebrands, and precise drawings of 18 brands (fig. 15) as they show on the animal's robe (1966, p. 88-89 & 192-193). Karimojong social structures differ widely from those of the previously mentionned pastoralists, in so far as they belong to this broad cluster of East-African herders who are organised along generation-and age-sets. Such « age societies » spread in Africa from Southern Sudan and Ethiopia through Uganda and Kenya, down to Northern Tanzania. Their age-systems mould political and ritual activities, involving men according to their ageor generation group. Situations vary greatly from one tribe to the other, and wide spans of anthropological literature are devoted to the study of these complex systems. Let us here briefly mention that in such societies a « succession ceremony » usually takes place every fifteen years or so, where the ruling « fathers » hand their responsibilities over to a younger group of « sons ». Because of this ritual succession of age-groups, rituals play a bigger part than they actually do in other societies. As a matter of fact, Dyson-Hudson does mention that « a formal branding of cattle representing the tribal herds » is performed during such succession ceremonies, and that in the following days « all unbranded stock are given appropriate clan brands » (p. 192-193). He does not say, however, if and how cattle is branded during the long time gap between two succession ceremonies.

My book on Tubu brands [START_REF] Baroin | Les Marques de bétail chez les Daza et les Azza du Niger[END_REF]) comes next in this review. The Tubu, called Teda to the North, Daza to the South, herd their livestock over one fourth of the Sahara, from Lake Chad to Southern Libya and from Eastern Niger to Western Sudan. This book provides detailed information on 92 camel brands, collected among the Daza in Niger. Each brand is described through 1) its drawing and position on the animal, 2) its name and signification, 3) its users who are Tubu clans or lineages. These users are either Tubu proper, or endogamous specialists called Azza, which means « smiths » in the Tubu language. Their specific activities and lower social status are similar in many African societies. The Azza of Niger, who were formerly smiths, hunters or musicians, are now mainly herders like dominant Tubu groups who used to « protect » them. They are now reluctant to mention these former allegiance relationships, but they still can be traced through the use of common brands. After this descriptive book, another one published in 1985 entails a fuller analysis of the social use and meaning of brands among the Tubu (Baroin 1985, pp. 87-90).

The sociological interest of studying brands is further discussed in an article by L. G. [START_REF] Hill | On camel brands[END_REF] à propos the Hababin Arabs of Northern Kordofan (Sudan). This area of Sudan, first illustrated by MacMichael (1913), then by [START_REF] Cunnison | Baggara Arabs. Power and Lineage in a Sudanese Nomad Tribe[END_REF], thus appears to be a hot spot for the study of property marks in Africa. This author concentrates a number of interesting sociological remarks which actually apply not only to the Hababin, but to other herding tribes as well. Most of them were already mentionned by previous scholars. Hill also focusses on the methodological limits to the conclusions that can be drawn from the study of marks. He judiciously observes, for example, that the sharing of brands is often, but not necessarily a sign of common descent, since marking is mainly political. It may demonstrate allegiance relationships, or result from an alliance through marriage (p. 10-12). He concludes that « these Hababin brands illustrate some of the dangers that arise from taking the agnatic segmentary model at its face value even though it is commonly used by the people themselves to explain their own brand system » and that the collection of brands « gives the fieldworker some convenient and concrete hooks on which to hang his questions about genealogy, as well as social and political boundaries » (p. 14).

No thorough study of livestock marking in Africa, whether brands or ear-cuts, seems to follow Hill's methodological warnings about the dangers and limits of using these « convenient hooks » to study African herders. In 1972 took place an international symposium on « Pastoralism in Tropical Africa » in Niamey (Niger), which brought together the best specialists. Livestock marks were briefly discussed there, and the papers edited by Théodore Monod in 1975. His lengthy introduction, published both in French and English, devotes only one page to property marks, summing up short comments that were made by the participants during the symposium, mainly about Fulani and Maasai marks (p. 128).

A later contribution to the study of property marks in Africa is F. H. Stewart's article in the Encyclopédie de l'islam, 2003. Wasm is the Arabic term for brands, and Stewart shortly mentions a few studies and sums up some basic remarks about the way brands are used by Arab bedouins, in Africa and in the Middle-East. This review of the existing literature on African livestock marking is probably very incomplete. It should be considered as a first step towards a synthesis on this topic, and unmentionned scholars are kindly asked to point out to their studies, so that a better survey may be achieved in the future. For the time being, let us conclude with the following remark: notwithstanding the few contributions mentionned above, it may seem surprising that, given the wide range of African pastoral societies, so limited interest ever was shown for the study of marks on this continent.

Two methodological reasons might explain this situation. The first is the unescapable complexity of such a study. It was stated by many scholars, first and maybe best by MacMichael in 1913. He insisted that the study of marks is very difficult and confusing, because of the extensive number of marks and users within one tribe -many marks being used by one or several groups of people together. It is also confusing, he observed, because of the many exceptions to any general rule that may be put forward as to the way marks are used. The second reason to the general lack of studies on property marks is the unreliability of the conclusions that one may hope to derive from it. This reason was best stated by [START_REF] Hill | On camel brands[END_REF]. He stresses the fact that the use of common brands indeed may result from many different reasons, whether kinship, alliance, allegiance or other political ties, and the study of brands alone cannot determine which type of relationship is involved. Therefore, the study of brands (or other marks such as ear-cuts) can only be a help (a « hook », in Hill's words) in sorting out the logics of the interplay between groups. It never is enough per se to draw such conclusions. It has to be supported by other data. The question then is the following: is the study of marks really worth the trouble, considering the difficulty of such a study, and the unreliability of its possible results? Obviously a negative answer most often prevailed, and this is why we should be especially thankful to the few scholars who, nevertheless, took the trouble to provide data on this topic. The available information, limited as it is, still enables us to picture out a broad view of livestock marking in Africa. Before doing so however, it is deemed necessary to detail all the main characteristics that a precise description of a brand (or an ear-cut) should entail, in view of future research, because the many loopholes in most collected data make comparisons difficult.

The characteristics of a property mark

Let us first consider the generic word for « mark ». It is interesting to note it down, since a similar word may point to a borrowing of the marks themselves from one community to the other. In Arabic, brands are called wasm, as mentionned above. But this word is unknown to most African herders, who are not Arabs and do not speak Arabic. They speak many different languages, belonging to different linguistic phylae, and each language seems to have its own specific word to designate a property mark. Among the Somali camel herders for example, a brand is called sumad, whereas it is called arbi in the Teda language spoken by the Teda of Tibesti (i.e. the Northern Tubu). A brand is called are among the Daza of Chad and Niger (i.e. the Southern Tubu). As for the Fulani, who cut their cattle's ears, their generic word for an « ear mark » is dyelgol. These few examples suffice to show that, although the custom of marking livestock is widespread across Africa, the generic word for « property mark » often remains specific to each linguistic community. There seem to be little borrowing in this matter, if any. This coincides with another fact: the comparison of marks, when data is available and except for a few exceptions, seems to demonstrate that the borrowing of the marks themselves, from one linguistic community to the other, is also very limited. Each group basically seems to have its own range of specific marks, with their specific characteristics (shape, size, name, location, etc.). Comparing their designs does not seem to point to any close artistic similarity of style either, as opposed to C. Lévi-Strauss and N. Belmont's conclusion regarding two South-American Indian tribes2 (1963, p. 106). The comparison of African property marks would however, undoubtedly, need further research.

In order to enable such comparisons, full data has first to be collected as regards all the specific characteristics of any mark (whether brand or ear-cut). This is all the more important as it has to be stressed, quite unfortunately, that most available descriptions of African marks are lacking in this respect, one way or the other, thus making comparisons difficult. Many scholars, for example, do not indicate the size of the mark and its precise position on the animal's robe. How can we then compare such data with other? Sometimes the design of two marks is exactly the same, but their names are different, and the size and position on the animal's robe may be different or similar as well. What conclusion can be drawn from such material? Reliable comparison is only possible with reliable and complete data, and this is why it is worthwhile here to list all the characteristics of any mark that should be recorded during fieldwork, in the hope that researchers in the future may take the trouble to collect comparable information.

A livestock property mark (whether brand or ear-cut) is fully defined through the following characteristics: 1) design, 2) size, 3) name, 4) meaning of this name, 5) location on the robe of the animal (or on its ears in case of ear-cuts), 6) orientation, 7) owner or group of owners (most often clans or lineages). All of these elements need be collected, because any difference on any item implies that one is dealing with a different mark. For example a short straight line on the left cheek cannot be considered the same mark as the same straight line branded on the right cheek, and moreover a long line (same design, different size) on the hind leg (different location) is bound to be a different mark. Such three marks, although they share the same design, are different. They are probably called with different names and used by different groups.

Marking the animal

The branding of an animal is most often done with a simple ironstick with a handle, the other extremity being put to glow in a fire and then applied on the animal's skin. The animal, of course, has to be made still beforehand. It is usually forced to lie on one side and tied up with ropes. Most animals are branded when still young, and if they are later given or sold, the new owner will add his own marks on the animal'skin. This is the reason why some camel wear many different marks, which is a sign of the many owners they had.

Marking an animal most often takes place any time. There is no specific occasion for it, and no ritual is performed. Some herders however choose a propitious day, as defined by the muslim calendar for example if they are muslims. As for rituals, some pastoralists do perform (or used to perform) specific ones when marking their livestock. Dupire describes such a ritual among the Wodaabe Fulani of Niger (1954 p. 124). When his calves reach one year old, as they are tied to the calves' rope near his camp, the Fulani herder cuts his marks in each one's ears with a knife. Then he collects the bits of ears he just cut away, and cuts them further into smaller bits which he throws on the kraal. This ritual is meant to ensure the herd's fertility. Dupire also refers to an older and more elaborate rite reported by I. N. [START_REF] Reed | Notes on some Fulani tribes and customs[END_REF] among the Wodaabe Fulani of Borno (North-Eastern Nigeria). After cutting his calves's ears, the herder there throws the bits of ears in a calabash filled with milk and water, before pouring the mixture on the calves' rope to insure fertility.

In pastoral East Africa, rituals are generally more developped than among West African nomads. The Karimojong of Uganda, for example, do have elaborate rituals when they brand their young cattle. [START_REF]Karimojong politics[END_REF] mentions their collective marking of cattle which takes place during a succession ceremony. At the final stage of this important social event, « in the evening occurs a formal branding of cattle representing the tribal herds. Four calves are selected, each from a particular clan, and from different sections and subsections, and branded with the appropriate clan mark » (p. 192). The rest of the unbranded stock is branded on the following days. But such a succession ceremony only takes place once in many years. Is cattle branded only then, or is it branded meanwhile? Dyson-Hudson does not answer this question, nor does he describes the branding itself.

The physical spot where the mark is branded on the animal's robe varies widely from one tribe to the other, or within one tribe from one mark to the next. Any part of the body may be involved: the flanks, the legs, the neck, the head, etc. Among the Arabs for instance, camels are mainly branded on the thighs but also on the forelegs, neck, cheeks, knees, etc. [START_REF] Van Gennep | Les wasm ou marques de propriété des Arabes[END_REF]. The Humr Baggara of Sudan brand their cattle mainly on their legs, but elsewhere as well (Cunnison 1966, appendix 4). The Tubu of Chad and Eastern Niger, who rarely brand their cattle, brand their camels on any part of the body, depending on the brand itself which is always characterised by its specific location. As for the Karimojong, they brand their cattle on different parts of the body (Dyson-Hudson 1966, fig. 15 p. 88).

The side where the animal is branded also makes sense. Thus in Sudan almost all Kababish camel brands are imprinted on the right side (MacMichael 1913), whereas almost all Humr Baggara cattle brands are on the left side (Hill 1972, p. 5). These details provide the traveller quick information as to what tribe's livestock he is facing. As for the Tubu and Karimojong, they mark their livestock on the right or left, depending on the brand. The side and place of a property mark is also meaningful for cattle ear-cuts. In Madagascar, one distinguishes three main locations, on each ear, where cuts may be seen: they are either at the base of the ear, or in the middle, or at the top. Each one of these three spots may be shaped differently, and each special brand is defined by the combination of cuts on the three of them. These designs are cut out either on the left ear to indicate the owner's matrilineal affiliation, or on the right ear if they refer to his patrilineal ties (see above, and Chazan-Gillig 1991, p. 249). Fulani cattle earcuts are less elaborate, but the cuts on the left or right ear similarly refer to lineage affiliation, although the opposite ones from Madagascar [START_REF] Dupire | Contribution à l'étude des marques de propriété du bétail chez les pasteurs Peuls[END_REF].

Let us now turn to the pattern and size of brands or ear-cuts. Most brands have a simple geometrical pattern, usually made of straight lines or wide curves. These simple designs merely derive from the use of a straight iron-stick for branding, which does not allow elaborate shapes. Some few brands however are made of smaller curves, and then a specifically shaped iron-stick is needed to brand the animal. Such brands are commonly used by chiefs (see for example Baroin 1972, pp. 65-66 and photograph n° 1 p. 237), and their standard size is determined by the iron-tool itself. Not only these, but other marks as well have a specific size, as noted above, and a specific orientation on the animal's skin (horizontal, vertical, oblique, etc.). Indeed, marks that share the same pattern but have a different size or orientation should be considered different.

The general tendency is for camel herders to brand their camels, while ear-marks are specifically developped among some cattle herders. Thus it seems that all desert people brand their camels. Such is the case of the Arabs, the Moors, the Twareg and the Tubu in the Sahara, the Beja in Erythrea (Paul 1954, plate II), and the Somali. The available information provides no evidence of the marking of camels with ear-cuts.

As for cattle, the situation is quite different. Some herders do not mark their cattle at all, or they only do so on a limited number of animals. Such is the case of the Tubu. These herders explain that cows do not need to be marked as much as camels do, because the risk of theft is far less for cattle than for camels. Indeed, cattle cannot travel long distances as fast as camels do, and they have a gregarious instinct that camels lack. Thus cows graze together in the daytime, and come back to camp every evening in need of watering. They are therefore much easier to herd than camels, which graze separately and may go far away from human dwellings or wells for many days before they return for watering. It may thus take several days before a camel's owner finds out that his animal was stolen, when a cow's theft would be discovered at once, giving the thief much less time to flee away with his prey.

Cattle however are indeed most often marked, especially when their owners raise no other type of livestock. These marks are either brands or ear-cuts, or both, depending on tribal habits. Thus the Fulani only mark their cattle by ear-cuts, as do cattle herders in Madagascar. The Karimojong in Uganda only brand their cattle. The Beja in Erythrea brand their camels, but they mark their cattle with ear-cuts. The Tubu brand their camels as well as a limited number of cattle, but do not use ear-cuts. One interesting case is reported of cattle herders who both brand their livestock and cut their ears: the Humr Baggara of Southern Kordofan (Sudan). But their anthropologist, Ian Cunnison, focuses on brands as previous scholars in Kordofan did. He only mentions ear-cuts in passing, and unfortunately does not study the agency of both types of marking. The Maasai, a wide group of East African cattle herders spreading accross Kenya and Tanzania, have elaborate ways to mark their livestock. According to A. H. Jacobs, they use « three types of marking: « (a) ear marks refer to clan and sale-clan identity of original owner ; (b) over-all hide brands, often extensive and elaborate, are generally used for pure decoration or aesthetic purposes ; (c) private family marks are often, but not always, placed on front forelegs of the cattle to reinforce private ownership identity » (Monod (1975, p. 128) who notes down information stemming from A. H. Jacobs). Another anthropologist, Paul Spencer, in his study of the Maasai Matapato, also mentions propitious markings of oxen but he does not give any further detail about it (1988, p. 262).

These few examples hint to the wide range of different situations that may be found in Africa as regards livestock marking. Not only do ways of marking differ widely from one tribe to the other, but they also vary, within one tribe, depending on the type of livestock. Other animals should be considered besides camel and cattle, i. e. small stock (sheep and goats), donkeys and horses. Some herders mark all types of animals, others mark only one or two species. Horses are prestige animals in Africa and they tend not to be branded. Some scholars argue that branding is more commonly used for thick skinned animals such as camel, and ear marking for thinner skinned animals, i.e. mainly cattle. A concern to avoid damaging the animal skin may also be the reason why cattle brands, if any, are often positionned on the legs rather than on other parts of the body. But there are exceptions to this rule, such as the big marks branded on the flanks of some Tubu cattle (Baroin 1972 photographs n° 5 , 7 and 11), and the huge brands of Karimojong cattle sketched by Dyson-Hudson (1966 fig. 15 p. 88). In any case, if marks and ways of marking are specific to each pastoral community, it seems that the social purposes which underlie the marking of livestock are most often similar.

The social purposes of marking of livestock

Herders in general, and not only African ones, have such an intimate knowledge of their animals that they do not need marks to recognize them. Marking therefore does not answer such a personal purpose, it has a social one. Marks usually belong to groups, such as lineages or clans. Some marks however belong to chiefs, as mentionned above, and a herder who marks his livestock with a chief's mark thus demonstrates that he belongs to this chief's following, or that he puts himself under this chief's protection. Among the Tubu, a newcomer lacking local support from his own kinsmen or clan members can ask a local chief for protection: he asks the chief permission to brand the chief's mark on his livestock. The chief's agreement implies that, in case one of these animals is stolen, he will help the owner find his animal back or take revenge, in the herder's name, against the offender. There is prestige for the chief in helping such a wronged pastoralist, but he will not do it for free. In order to be allowed to brand the chief's mark on his livestock in the first place, the herder does not come to him with bare hands. He has first to give the chief a seemly gift, i.e. an animal.

Not only chiefs, but other herders sometimes create their own brand. Some do it from scratch, when they move to a new place. One such case was mentionned among the Tubu. When he migrated from Bahr-el-Ghazal (North of lake Chad) to Eastern Niger in the late 1800's or early 1900's, a Tubu herder who had no local kinsmen created his own new personal brand which he called keeso, « hard ». Thereby he meant: « beware, I am a tough guy! » in order to scare away potential thieves. From then on, other men from his clan who later came to the area adopted this brand for their own livestock (Baroin 1985, p. 88).

Most often however, people do not create new brands. They merely add a small line to the brand of their group, thus adding a personal touch to it. Such cases have been recorded among the Arabs (Van Gennep 1902, p. 87) or the Tubu. According to W. C. Young, every man does so among the Rasha'ida Arabs of Sudan (Young cited by Stewart 2003, p. 189). The main message conveyed by such marks remains which group the owner belongs to, the small addition being only extra information.

The clear aim of these marks is to show under which person's or group's protection the animal stands. It is a help in proving one's property on stolen or lost animals. But even before any animal is lost or stolen, marks are a warning to potential thieves, as they show them to which group's retaliation they will be confronted if indeed they choose to steal the animal.

These are important signs, because animal theft is extremely widespread in pastoral societies. As L. G. Hill puts it, « animal theft is, and probably always has been, an exciting sport and profitable business in the Sudan » (Hill 1972, p. 2). And not only in the Sudan, one may add. It is an exciting sport because it demonstrates one's audacity, as it exposes the thief to great risk of revenge from the animal's owner and his group. It has indeed to be stressed that, in pastoral societies, revenge against animal theft or any other personal offense never is a mere personal affair, it is always a group concern. The reason for this collective response is that exposition to risk is particularly high for pastoral nomads, and would leave a lone herder defenceless. As a matter of fact, livestock is usually herded in small groups, the more so as pasture is scarce. Herds spread away from each other, with lone individuals to keep an eye on them. Quite often, watching livestock is left to children. One or two children will be left alone all day to keep an eye on cattle or small stock, and will only bring them home to the family camp in the evening. In case of camels, herding is the more arduous as these animals do not pasture in flocks ; each one goes its own way independently, covering long distances in a single day. This situation naturally makes animal theft, especially camel theft, rather easy. And it is the more tempting for young men as it is a risky sport which enables them to demonstrate their shrewdness and fearlessness.

A herder alone would have very little chance of recovering his lost animal. Group solidarity therefore is indispensable in such a case, as it provides a better chance to fall upon the stolen animal and/or upon the thief for retaliation. Group solidarity against theft is also what makes it a risky sport, because such affairs as a rule do not end up peacefully.

It is a tacit rule that members of the same group (whether family, lineage or clan), those very members who should help one another if one of them is stolen, conversely should not steal each other's livestock. Property marks are also meant to avoid doing so. But it does happen sometimes that a young man, by mistake, steals an animal belonging to one of his kinsmen or member of his clan3 . I personally witnessed such a case among the Tubu in Eastern Niger in 1972. Retaliation then cannot take place, since kin solidarity is an indisputable social principle. Retaliation can only be directed towards outsiders. In that case, as the thief had already sold the animal, his close kinsmen had to compensate the lost animal to its owner. They decided merely to pay him back the animal's value, and bade him to forgive this unfortunate mistake.

When there is no kinship or other bound of solidarity between the thief and the animal's owner, which is the usual case, there is no such easy way out. It is a matter of honour that the wronged herder should avenge himself through violent retaliation. This means that he will do all he can, with his group's help, to trace the thief and take revenge by wounding or murdering him. It may take years before such revenge takes place. The Tubu, for example, are known for never giving it up, and for retaliating sometimes decades later. When revenge is taken, and the thief maybe wounded or murdered, his own kin will retaliate in their turn and more wounds or murders on both sides may follow. Such vendettas will only be stopped, sometimes many years later, through the muslim custom of paying compensation for murder, called diya in Arabic. The Tubu, who only became muslims towards the end of the XIX th century, have long been reluctant to adopt this custom, as they deemed it more honourable to avenge a wronged kin through violent reprisal rather than accept a payment in compensation.

Quite clearly, the marking of livestock is related to this logic of violence, which unfolds in the following manner:

1) Property marks are means to direct livestock theft on the proper animals, i.e. on those belonging to people who stand outside one's solidarity network 2) When such a theft occurs, violent revenge is indispensable, because a lack of response would automatically induce potential thieves to steal more of this man's livestock 3) For this revenge to be more effective, the wronged herder needs the help of his kin, or clan-members, or in-laws, or chief, whatever his solidarity network may be.

One may therefore conclude that animal theft, because of the revenge it automatically induces, is an indirect stimulus to kinship solidarity. In this respect, livestock property marking is a center piece in the organisation of violence which, indeed, is an endemic characteristic of these pastoral societies.

Camels in Africa have only one hump and should actually be called dromedaries. Yet we keep the word « camel » here to comply with a long lasting usage.

These authors, comparing clan marks of two Indian tribes, the Guajiro of Colombia and the Caduveo who live across the Brasil-Paraguay border, concluded that in spite of the great distance between them their marks demonstrate the same artistic style.

The stolen animal may be a recent acquisition, that the owner had no time yet to brand with his own marks.


B Negulescu
email: negulescu@lpm.u-nancy.fr

D Lacour

F Montaigne

A Gerken

J Paul

V Spetter

J Marien

C Duret

M Hehn

Wide range and tunable linear TMR sensor using two exchange pinned electrodes

Keywords: magnetic field sensor, TMR, AlOx tunnel junction, sensitivity, linearity range, IrMn, exchange bias, blocking temperature PACS numbers: 73.43.Jn, 75.47.m, 75.50.Ee, 75.60.Nt

A magnetic tunnel junction sensor is proposed, with both the detection and the reference layers pinned by IrMn. Using the differences in the blocking temperatures of the IrMn films with different thicknesses, crossed anisotropies can be induced between the detection and the reference electrodes. The pinning of the sensing electrode ensures a linear and reversible output.

It also allows tuning both the sensitivity and the linear range of the sensor. The authors show that the sensitivity varies linearly with the ferromagnetic thickness of the detection electrode. It is demonstrated that an increased thickness leads to a rise of sensitivity and a reduction of the operating range.

Magnetic field sensors are used in a wide range of applications such as read heads in the magnetic data storage industry, as position, speed and rotation angle detectors in the automotive industry, as electric current detectors or as magnetic flux leakage detectors in the non-destructive testing technology. [1][2][3] Nowadays, the majority of the magnetic sensors developed for industrial applications are based on the Hall effect, the anisotropic or the giant magnetoresistance effects, due to their ease of integration and low production costs. However, the sensitivity can be largely increased by using magnetic tunnel junctions as field detectors. Also, the relatively high resistance of the tunnel magnetoresistance (TMR) devices ensures a low power consumption of the sensor which constitutes a major advantage for mobile applications and green electronics. Besides their use in read heads of hard disk drives, currently the TMR sensors are mainly developed for biological applications, 4 where high sensitivity and low noise are essential characteristics.

The key feature of such sensors is the linear and nonhysteretic electrical response to an external magnetic field.

In the case of a TMR device, the linear characteristic originates from a perpendicular configuration of the electrodes magnetizations at zero applied field, whereas the nonhysteretic behaviour results from the rotation of the detection electrode's magnetization. In order to control the reversal mechanism of the detection layer, our group has previously proposed the use of an exchange coupled ferromagnetic layer. 5 A reversible and linear magnetoresistive response was demonstrated in a device where a hard magnetic CoPt film was used as reference electrode. However, in this initial demonstrator, the two electrodes were switching independently only in a small field range, leading to a reduced operating range.

In this letter, we show a fully functional TMR sensor based on a multilayer stack including two IrMn antiferromagnetic (AF) thin films. The sensitivity and the linear range are easily tunable by only varying the thickness of the pinned ferromagnetic detection layer.

The pinning directions of the two AF layers have to be set orthogonal in order to achieve a linear response. If the bottom and the top exchange coupled interfaces have different temperature stabilities, this configuration can be obtained through two successive annealing steps at different temperatures with crossed applied fields. The blocking temperatures (T B ) of the IrMn layers are known to be dependent on the AF's layer thickness as well as on the top or bottom pinned configurations 6 . They rise with increased AF thickness and are higher in the bottom configuration compared to the top one. 7 In the mean time and in the studied range of IrMn thicknesses (above 5nm), the exchange bias field (H ex ) decreases as a function of the IrMn thickness, meaning that a high AF thickness reduces the pinning strength.

Aiming at high temperature stability of the reference electrode, its magnetization is pinned by a thick IrMn layer (15 nm) while a thinner IrMn layer (6 nm) is part of the detection electrode. In order to compensate the decrease of ex H due to the increased thickness of the AF, the reference electrode is placed at the bottom side of the stack 8 and a synthetic antiferromagnetic (SAF) structure is used instead of a simple ferromagnetic layer. The SAF also gives the advantages of reducing the blocking temperature dispersion of the reference electrode, 9 of increasing its rigidity against rotation 10 and of decreasing its stray field. These properties ensure the magnetization stability of the reference electrode and the decoupling between the two electrodes. 11 this value is state of the art. 12 After annealing the samples at 265 °C under 10 kOe the exchange coupling effect is activated and both electrodes magnetizations are pinned parallel to the applied field direction. In this magnetic state, the m(H) loops were measured at different temperatures (starting from room temperature up to 300 °C) in order to calculate the variation of the exchange coupling energy ( ex J ) vs. temperature (presented in FIG. 1). These measurements allow determining the blocking temperature defined as the temperature at which ex J becomes equal to zero. ex J of the detection bilayer is calculated from the measured exchange bias field (

ection ex H det
) as:

ection ex CoFe S ex H t M J det =
, where S M and CoFe t are the saturation magnetization and the thickness of the detection CoFe film respectively. In the case of the reference electrode, ex J is obtained by fitting the measured m(H) curves using a macro spin model of the pinned SAF. 13 From The total sensitivity of a TMR sensor with pinned sensing layer 5

J t M R R H R R H R S × - = × - = ∆ ∆ = 2 1 2 ,
with P R and AP R , the resistances of the tunnel junction with parallel and respectively antiparallel alignments of the electrodes magnetizations enclosing the tunnel barrier.

This variation law was experimentally verified on a set of samples with 3, 5, 7 and 9 nm thick top CoFe layers.

After the first annealing procedure, all MTJs show TMR values of 40 % with the exception of the structure with the thinnest top CoFe layer, where the TMR attains only 27 %. The decrease of the TMR value for devices with such thin electrodes can be related to the loss of spin polarization, as already observed for structures with free sensing layers thinner than 2-3 nm. 14 In these devices the TMR reduction was associated with a decrease of the magnetization in the saturation state, but in our samples the M s values are constant for CoFe layers with thicknesses larger than 2 nm. This rules out the above mentioned explanation for the TMR decrease. Oxygen assisted Mn diffusion towards the barrier during the thermal annealing procedure could also induce TMR losses. 15 This diffusion mechanism is predominant in the top electrode that is generally not as well textured as the bottom one. 7,15 After reorienting the pinning direction of the detection electrode perpendicularly to the reference electrode anisotropy, all the samples show a linear resistance variation (as presented in FIG. 3). The sensitivity varies as a function of the detection CoFe thickness. For the thickest layer, a 10 % TMR linear variation is obtained in a range of ± 40 Oe. The sample with only 3 nm top CoFe has a much larger linearity range, extending between ± 220 Oe. For this sample, the TMR variation on the linear range is 4 %.

The hysteresis evolution with the external field range is presented in the inset of FIG. 3. In fields smaller than 400 Oe, the hysteresis increases linearly with the measuring range. This is the regime where the magnetization of the reference layer is stable and only the detection layer rotates in field. For higher fields, the hysteresis variation slows down and attains a maximum value of 10 Oe corresponding to a complete reversal between the parallel and the antiparallel states of the entire structure.

In FIG. 4 both the linearity range 16 and the sensitivity are plotted as a function of the CoFe detection layer's thickness. As theoretically predicted, we observe a linear variation of S upon thickness. The operating range increases with the decrease of the CoFe thickness, as implied by the theoretical condition for linearity:

) /( CoFe S ex detection ex t M J H H = <<
. Since the SAF structure used in the sensor has a stable antiparallel configuration between -2000 Oe and +500 Oe, the linearity range is not limited by the reference electrode. It is restricted only by the theoretical response of the detection layer, non linear for high fields.

In conclusion, we show a fully functional TMR sensor based on a multilayer stack including two antiferromagnetic (AF) thin films. Using the difference in the blocking temperatures of IrMn layers with different thicknesses, crossed pinning directions can be induced in the top and the bottom electrode of a MTJ device. The linear TMR (H) variation can be tuned by changing the thickness of the sensing ferromagnetic layer. The sensitivity is changed over a decade and the linear field range by a factor of 5 following the theoretical predictions. This sensor architecture appears to be fully tuneable within a wide field range. Furthermore, since the magnetic response of the sensing layer is not driven by the shape of the device, this sensor is fully scalable from millimeter down to nanometer.

This work was financed by the ANR CAMEL project. The authors would like to thank G. Lengaigne for help with the experiments.

Figures caption :

FIG

  The detection electrode consists of a top pinned CoFe x nm / IrMn 6nm bilayer while the reference electrode structure is: IrMn 15nm / CoFe 2.5nm / Ru 0.8nm / CoFe 3nm. The samples are deposited on 5 inch wafers in a DC magnetron sputtering system with a base pressure of 2x10-8 Torr and typical deposition pressures of 2-3 mTorr at the Sensitec research division. An Al 2 O 3 tunneling barrier is obtained by reactive RF oxidation of a 2 nm Al layer at a power of 50 W. The samples are annealed post deposition at chosen temperature (up to 265 °C) and magnetic field values in an N 2 atmosphere oven. The magnetic tunnel junction are then prepared in circular shapes with diameters varying from 40 µm to 100 µm using conventional photolithography and ion mill processes. After optimizing the oxidation conditions, a maximum TMR of 40 ± 3 % and a RxA product of 22 ± 6 MΩ µm2 are measured. Considering the thickness of the Al 2 O 3 layer in our samples (2nm measured by transmission electron microscopy) and the dependence of the TMR values upon the barrier thickness and height,

figure 1 ,

 1 figure 1, the T B value of the detection electrode is about 200 °C, while the reference electrode is pinned up to (275 ± 15) °C. Consequently the unidirectional anisotropies induced by the AF layers in the two ferromagnetic electrodes can be independently tuned by properly choosing the annealing temperatures. A first annealing at high temperature initializes both layers in the same direction. A second annealing at a temperature between 200 °C and 275 °C and with the field applied perpendicular to the first annealing field direction is done afterwards, in order to define an orthogonal pinning for the detection electrode's magnetization. The TMR curves measured on the same wafer after a first annealing at 265

. 1 :FIG. 2 .

 12 FIG. 1: Variation of the exchange coupling constant as a function of temperature for the top detection electrode (filled symbols) and the bottom reference electrode (empty symbols) of a substrate /Ta 5 / Ru 2 / IrMn 15 / CoFe 2.5 / Ru 0.8 / CoFe 3 / AlOx 2 / CoFe 5 / IrMn 6/ Ru 4 MTJ structure (the thicknesses are expressed in nm).

Fig. 3

 3 Fig.3

Fig. 4

 4 Fig.4

  is inversely proportional to

							H	detection ex	when
	H <<	H	detection ex	. The sensitivity can be written as:
			P	AP	P	AP	S	CoFe
				detection
				ex			ex

The linearity range is determined as following: first the TMR (H) curves are fitted with a linear function in different field ranges; for each fit, the non linearity is computed as the maximum deviation from the linear fit divided by the maximum TMR value in the considered field window, expressed in percentage; finally the linearity range is defined by a non linearity smaller than 5%.


I-H Choi

P Weisbecker

S Barrat

E Bauer-Grosse

Growth of highly oriented diamond films by the MPCVD technique using CO-H 2 , CH 4 -H 2 and CH 4 -N 2 -H 2 gas mixtures

Highly oriented diamond films were deposited on Si (001) substrates by the microwave plasma assisted chemical vapor deposition technique (MPCVD) using the bias enhanced nucleation (BEN) step. The formation of highly oriented diamond (HOD) films is often linked to the presence of few ppm of nitrogen which stabilizes the texture but deteriorates the chemical quality of the film. In this study, we show that carbon monoxide can be used without nitrogen addition to conserve the <001> texture and to promote the {100} facet extension on the top of the diamond film. After a nucleation step carried out by the BEN technique, and after a subsequent growth time limited to 30 min in a CH 4 /H 2 gas mixture avoiding the coalescence step, HOD films were synthesized using CH 4 /H 2 , CO/H 2 and N 2 /CH 4 /H 2 gas mixtures for a growth time fixed to 10~14 hrs. The effect of carbon monoxide or nitrogen on the morphologies and the texture of the diamond films is discussed from the results of scanning electron microscopy (SEM) and the texture measurements performed by X-ray diffraction.

Introduction

Chemical Vapor Deposition (CVD) is a promising technology for the production of low-cost and high quality diamond films. The heteroepitaxy of diamond is a key challenge for future applications as electronic devices or ionising radiation detectors. These applications require a significant increase in the electrical and transport properties such as carrier velocity and lifetime or doping-induced defect formation [1][2][3][4][5]. However the polycrystalline nature of diamond coatings synthesized on silicon substrates induces the formation of chemical and structural defects like grain boundaries, stacking faults and non carbon atom insertion which damage electronic performances. As reported by M.G. Donato et al. [3] and V.I. Polyakov et al. [5], the grain boundaries constitute the main locations where impurities and defects are concentrated. This effect inevitably induces a dramatic deterioration of properties like the free carrier lifetime. One way to limit it is to increase the density of epitaxial diamond particles at the first stage of nucleation to further diminish the grain boundary misorientation.

In our previous studies, we obtained 26% epitaxial diamond crystals for total nucleation densities above 10 9 cm -2 without specific carburization step and for a low bias enhanced nucleation step duration [6].

Since HOD films have been synthesized from a limited epitaxial ratio, the natural evolutionary selection mechanism initially described by Van der Drift [7] and applicable to the diamond thickening step is still necessary to eliminate the ill-oriented diamond crystals and to favour the epitaxied ones which progressively constitute the diamond surface (texturing process). As reported by some authors [8][9][10], the addition of nitrogen seems to be often unavoidable to correctly carry out the texturing in the case of MPCVD processes and to obtain the {100}{111}<100> microstructure according to the terminology introduced by Wild et al. [11]. Nevertheless, two opposite phenomena occur when nitrogen is added: a beneficial effect on the growth rate and on the growth parameter α (α = 3

    111 001  
), but a detrimental effect due to the incorporation of nitrogen atoms mainly into {111} growth sectors [8,[12][13].

A few ppm of nitrogen in a CH 4 /H 2 gas mixture drastically increases the growth rate and seems to stabilize the growth parameter  close to 3 leading to the rapid formation of the {100}{111}<100> structure. However, the remaining {111} facets are generally rough and incorporate nitrogen. Except for K. Janischowsky et al. [14] who obtain HOD films without N 2 addition in the case of a hot filament assisted CVD process, the absence of nitrogen leads to the formation of near <110> texture [10] which is not compatible with the HOD film formation. The possibility to use other gases like CO, O 2 or CO 2 has been envisaged previously [15][16][17][18][19]. Brunsteiner et al. [18] observed that the addition of CO in a CH 4 /H 2 mixture can extend the range of crystal-faceted deposition with a higher deposition rate and a higher quality of diamond. Initial growth of heteroepitaxial diamond was successfully performed by Suesada et al. [16] by using a mixture of CH 4 and CO where well octahedrashaped crystals were obtained for a high carbon proportion (C/H 2 =10vol%). Finally, some authors have reported the possibility to obtain high quality of diamond coating at low temperature by using CO gas [17,19]. The low temperature favours the development of octahedral-shaped crystals which improves the formation of the <100> fibre axis.

In this paper, we tried to optimise the synthesis of HOD films by using a multi-step growth process using different gas compositions: CO-H 2 , CH 4 -H 2 and CH 4 -N 2 -H 2 gas mixtures. The objective is to reduce the nitrogen pollution inside the film and then to rapidly enlarge the {100} facets by using CO while maintaining the diamond quality and the growth rate.

Experimental procedure

Details of the experimental system have been described previously [6,20]. All samples were prepared by using a modified ASTeX MPCVD reactor equipped with a 2.45-GH Z generator.

The multi-step process consisted to perform a nucleation/growth step (step 1) whose conditions are constant for all samples and the duration limited to 30 min avoiding the coalescence (see table 1). Then, two successive (step 2 and 3) growth steps were carried out using CO-H 2 , CH 4 -H 2 or CH 4 -N 2 -H 2 gas mixtures (see table 2 for the five presented samples).

The other experimental conditions were similar to the growth of the step 1. Feedstock gas flows of H 2 (99.999%), CH 4 (99.99%) and CO(99.97%) were regulated by mass flow controllers. Nitrogen (99.999%) was fed into the chamber via a micro leak for a concentration of about 180 ppm.

The morphology and crystallographic orientation of the films were evaluated by scanning electron microscopy (Philips XL30S FEG) coupled with an image analysis software (APHELION, ADCIS S.A.) and X-ray diffraction pole figure measurements. The films have been characterized by a 4-circle X-ray diffraction goniometer using Co Kα radiation. {111} X-ray diamond pole figures were measured on each specimen with a step size of 1 in  and 2 in φ. The maximum tilt value was =80, the azimuth angle varied from 0 to 360 with a continuous scan. {111} silicon pole figures have been recorded with the same parameters to check the instrumental characteristics and the crystal morphology was characterized using the growth parameter α.

Results and discussion

Morphological characterization of nucleation/growth deposit

Fig. 1 shows the SEM micrograph of the typical deposit after the step 1 (table 1). Welldefined cubo-octahedral diamond crystals have grown from a CH 4 /H 2 gas mixture with   1.8 as simulated in the inset. We can observe the presence of multiply-twinned particles and a lot of epitaxied crystals which present a square {100} facet parallel to the substrate surface (see for example, the crystal pointed by an arrow). In all samples, the epitaxial ratio was close to 26% and the total nucleation density is about 10 9 cm -2 .

Structural et morphological characterizations of HOD diamond films

Growth in a CH 4 /H 2 gas mixture

Fig. 2a shows the surface morphology of the sample A obtained after a total growth sequence of 10 hrs (step 2) as reported in table 2. This morphology is characteristic of nitrogen-free gas mixtures with the presence of well-shaped grains whose facets look like rooftops [10]. The corresponding {111} X-ray pole figures is reproduced in Fig. 2b. The presence of a central peak fairly broad and a ring approximately located at X70° can be associated to a fibre axis between <110> and <111>. The obtained texture is in agreement with morphological observations (Fig. 2a and2c) where we observe large (111) facets tilted in relation to the sample surface. Some emerging crystals exhibit a little {100} facet surrounded by large {111} facets which is characteristic of a high value of the growth parameter .

Therefore, the obtained texture is not in agreement with the evolutionary selection model which predicts that single crystals having a high growth parameter ( close to 1.8 in this case) lead to the formation of a <001> fibre axis. Consequently, as illustrated in Fig. 2a, epitaxied crystals do not survive during the thickening. A careful observation of sample A at higher magnification (Fig. 2c) shows that they are not single crystals but rather lamellar-twinned particles with a surprising crystallographic relationship which seems to be close to {100}//{111} and <110>//<110>. The occurrence a such relations, between three octahedral crystals (labelled 1,2 and 3 in Fig. 2d), leads to the formation of rooftop morphology as schematised in Fig. 2d (grazing and normal views from top to bottom). This geometrical configuration, that we call afterwards abnormal twinning, induces that <110> (1) direction of the octahedron (1) is parallel to the <112> (2) direction of the octahedron (2). Similar considerations can be made between octahedra (1) and (3). If we consider that the film microstructure is mainly defined by these crystallographic relationships, the corresponding fibre axis must have two components <110> and <112>. This is almost confirmed by Ayres et al. that observe the main <110> orientation with a secondary component near <113> [10].

The difference between the reported <113> direction and the deduced <112> from our geometrical considerations can be due to the rough evaluation of texture component by Ayres et al. that specify a near <112> direction, or by a more complicated twinning as {100}//{111}. The supposed crystallographic relationship can be envisaged if we consider (111) diamond reconstruction such as C(111)2x1:H whose symmetry is compatible with a (001)2x1:H [21,22]. Further investigations are necessary to precise this microstructure

Effect of nitrogen

Fig. 3a shows the surface morphology of the sample B prepared with the same conditions as sample A except the addition of 180 ppm nitrogen during the step 2 (see table 2). A large portion of grains are <001>-oriented with an azimuthal orientation characteristic of epitaxied crystals. This structure is in agreement with the growth parameter  which is still close to 1.8 for this experiment. A small portion of grains are in-plane misoriented due to the incomplete selection during the thickening. The corresponding {111}X-ray pole figure is reported in Fig. 3b. This one is clearly dominated by the four {111} peaks of diamond as expected for the highly oriented diamond (HOD) films on silicon substrates.

Effect of carbon monoxide

The sample C has been prepared to investigate the influence of CO addition on the growth morphology and the microstructure. As reported in table 2, we have chosen 10% CO in H 2 to ensure that well-faceted crystals develop as reported by Brunsteiner et al. [18]. As seen in Fig. 4, the obtained layer showed well-shaped crystallites, but the presence of in-plane well oriented crystals is difficult to detect. A careful observation of the crystal morphology shows that the abnormal twinning observed for the sample A in the absence of nitrogen does not appear in this case. On the other hand, the growth parameter α is decreased from 1.8 to 1.6 by using CO. This is now too low to obtain a <100> fibre axis, and the thickening induces the disappearance of epitaxied crystals.

The multi-growth technique

From the previous results, it appears that CO can be used to avoid the abnormal twinning while maintaining a good crystal morphology. The decreasing in the growth parameter  from 1.8 (with CH4) to 1.6 (with CO) can be used to extend the {100} diamond facets but is detrimental to the formation of the <100> fibre axis necessary to select the epitaxied crystals.

In a first approach, we propose to carry out HOD films by using two steps with different deposition parameters. The first step is to establish the <001> fibre axis by using a mixture of CH 4 /H 2 /N 2 , and a second step with a mixture of CO/H 2 to develop the (001) facets without nitrogen pollution. Fig. 5a shows the morphology of the sample D obtained with a CH 4 /H 2 /N 2 mixture during 3 hrs (step 2) followed by 7 hrs (step 3) with a CO/H 2 mixture. Epitaxied crystals are visible on the top of the diamond surface but a significant proportion of azimuthally misoriented crystals are still present. A careful observation indicates that these crystals exhibit (100) faces which are not strictly parallel to the surface. For this sample, the establishment of the <001> fibre axis is determined by the second step whose duration is limited to 3 hrs. This one is probably not sufficient to correctly define the fibre axis. The third step probably favours the development of misoriented crystals by decreasing the growth parameter . The {111} X-ray pole figure presented in Fig. 5b shows the dissymmetry of the four {111} diamond peaks which could be attributed to the preferential development of these misoriented crystals. Fig. 5c shows the surface morphology of the sample E whose conditions are reported in table 2. We have strongly raised the duration of the second step to better develop the <001> fibre axis. Very few crystals are azimuthally misoriented on the top of the surface and the {001} diamond facets are large and parallel to the sample surface. The {111} X-ray pole figure reported in Fig. 5d confirms this structure. To underline the beneficial effect of CO during the third step, we have calculated by image analysis the surface ratio (S (001)d /S total ) as the ratio of the {001} diamond surface constituted by all {001} facets on the top of the film to the total surface of the surface sample S total . Fig. 6 shows the results of the image analysis for the samples B (single step with nitrogen), D and E (double steps). If we consider that the surface ratio does not depend on the thickness for a constant value of , it is clear that the surface ratio strongly increases when CO is used without deterioration of the fibre axis as confirmed in the case of sample E (Fig. 5c). In the case of the sample D (Fig. 5a), the presence of non epitaxied crystals for which {100} facets are tilted in relation to the surface, hides the effect of CO and induces a decreasing in the surface ratio. In our experiments, it is still necessary to call up a second step with the presence of nitrogen which is too long (10 hrs in our case). This limitation will be probably overcome by using experimental conditions for which CO will favour near-octahedral crystal shapes (higher growth parameter) and by increasing the nucleation density to more rapidly promote the <001> fibre axis.

Conclusion

Highly oriented diamond films have been elaborated by using different gas mixtures like CH 4 /H 2 , CO/H 2 and N 2 /CH 4 /H 2 . The nitrogen gas is detrimental to the electronic properties but a few quantity allows the <001> fibre axis to be created by avoiding the formation of abnormal twinning. This one seems to be defined by the crystallographic relationships {100}//{111} and <110>//<110>, and induces a fibre axis with two components close to <110> and <112> irrespective to the growth parameter ( = 1.8 in our case). The use of alternative carbonaceous gases like CO permits the growth of diamond without nitrogen, but the morphology change (growth parameter decreasing in our conditions) complicates the creation of the <001> fibre axis. We have presented HOD films elaborated by a double step process where the fibre axis is firstly created by using a N 2 /CH 4 /H 2 mixture, followed by a growth with a CO/H 2 mixture which promotes the extension of the (001) facets while maintaining the fibre axis. Complementary studies are in progress to find experimental conditions where CO could be used during the formation of the <001> fibre axis.


S Barrat

S Saada

J M Thiebaut

E Bauer-Grosse

Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Introduction

Highly oriented diamond films deposited by chemical vapor deposition (CVD) processes have focused interest of many research groups ten years ago (see for this purpose the review paper [1]).

This interest is mainly due to the perspective of large industrial developments of deposited heteroepitaxial diamond coatings whose crystalline quality must approach the natural diamond quality. The nature of the substrate mainly depends on the type of envisaged applications, however the single silicon wafer constitutes unquestionably the largest material used because large single crystals are available at low cost and present some similitude with diamond. The CVD synthesis of heteroepitaxial diamond deposits is mainly due to the development of an in situ substrate pretreatment so called Bias Enhanced Nucleation (BEN) initially presented by Yugo et al. [2], Jiang et al. [3],and Stoner et al.[4]. This procedure is carried out before the diamond growth step with close experimental conditions, leads to a very important increase in the nucleation density and then, by optimizing the bias conditions, to an epitaxial relation between the diamond crystals and the silicon substrate. The final goal of many studies is to induce highly oriented films mainly constituted by epitaxied crystals. The morphological characterization of the film surface does not allow to correctly evaluate this state since evolutionary selection rule leads to the elimination of misoriented crystals during thickening. It is necessary to determine the epitaxial ratio which can be defined as the percentage of epitaxied crystals in relation to the total crystal number before the texture formation. Classically, highly oriented diamond films are obtained from an epitaxial ratio of few per cent [4]. The higher it will be, the better the subsequent diamond film quality will be.

However, the epitaxial ratio is difficult to increase because of the extreme complexity of the -3-physical and chemical substrate environment during the pretreatment step. If we restrict this discussion to the most used CVD process, i.e. the microwave plasma assisted chemical vapor deposition (MPCVD) technique, the BEN step consists in superimposing a microwave discharge (which is in contact with the substrate) with a d.c glow discharge created by negative biasing of the substrate relative to the reactor wall (or relative to an additional anode located above the substrate).

As studied previously [6][7][8][9], the variety of device configurations (size and nature of reactor, shape and size of substrate-holder, electric-field distribution of microwave plasma and d.c bias) and the large synthesis parameter variations (nature and composition of the plasma-producing gas mixture, substrate temperature, microwave power, pressure and bias conditions) lead to a high dispersion of results in terms of diamond quality, epitaxial ratio, deposit homogeneity and sometimes in term of synthesis reproducibility [6]. This lack of result uniformity does not permit to clearly determine the main mechanisms of diamond formation during the bias step, and the multiple nucleation models presented in the literature illustrate this diversity [4][5][6][7][10][11][12][13][14][15][16]. Two main aspects can be considered in regard to the BEN step in the case of MPCVD synthesis on silicon substrates: the chemical and electrical behavior of the double-discharge which has been strongly studied [6,[8][9]11,14], and the substrate modifications submitted to these discharges that interest many research groups [4][5][6][7]10,[12][13][14][15][16][17][18][19] and whose knowledge constitutes today a real stake for the optimization of this process. The later aspect is particularly sensitive because the substrate transformation during the bias is mainly linked to the temperature that generally varies between 800 K and 1200 K, and to the duration comprised between 20 s and 2 hrs. For shortest time nucleation homogeneity is poor [20] and to preserve a high nucleation density, the BEN step is often preceded by an in situ carburation step.

Therefore in some cases, silicon substrates undergo large transformations whose sequence is difficult to analyze. The study of these transformations is all the more complicated than the nucleation process often occurs heterogeneously with time and location. Consequently some studies are based on the variations of the diamond deposit quality along the substrate surface [6,8,16,19--4-26].

In this context, to facilitate the understanding of the physical and chemical mechanisms which occur during the bias step, our main goal is to find experimental conditions for which deposits are uniform and reproducible in order to easier correlate the macroscopic parameters of the process to microscopic phenomena related to the nucleation mechanisms independently of the location on the substrate. Our second objective is to determine the pretreatment the simplest as possible to eliminate steps of the process which seem not necessary for nucleation and orientation of the diamond crystals. This two points are discussed below then characterizations of two deposits are presented.

Results and discussion

Control of the double-discharge

As predicted by previous studies [24,27], increasing homogeneity requires a strict control of the double-discharge. We have recently described the interaction between the microwave plasma and the d.c discharge [9] and demonstrated that the d.c discharge homogeneity was directly linked to the microwave location over the substrate/substrate-holder system and to the electric-field distribution of the d.c voltage. Concerning the last point, a graphite electrode located above the substrate has been added to minimize the d.c electric-field gradient near the substrate-holder. It is important to point out that the distance between the substrate and the electrode must be chosen to avoid microwave plasma disturbance. This condition can be qualitatively foreseen by calculating the electric-field distribution of the microwave wave inside the reactor when the microwave plasma is not initiated yet. This distribution is shown in fig. 1 where the vertical axis corresponds to the electric-field intensity and the horizontal axes (r,z) define a half-plane of the microwave cavity limited by the substrate-holder (lower part of the plane) and by the microwave antenna (upper part of the plane). For a system with a symmetry of revolution (r,,z), the calculation of the -5-electromagnetic field in TM modes (Transverse magnetic modes) is carried out from the following magnetic excitation relationship:

      0 ) H . r ( rc 2 r H . r r . 1 r z H . r r . 1 z 2 2                                  (1)
with  the relative permittivity of the environment,  the eigen frequency of the cavity and c the light velocity. The eigenvalue resolution is performed by a finite element method and gives after derivation the electric-field module at each node of the cavity. This one consists to oscillations with the presence of antinodes. We experimentally observed that the introduction of an anode inside these antinodes does not disturb the subsequent microwave plasma initiation and decreases the d.c electric-field gradient near the substrate-holder when the bias is applied. The presence of a high microwave electric-field at the extremities of the substrate-holder is also particularly visible in figure 1. That induces very strong point effects when the microwave plasma is switched on and corresponds to the catching areas usually observed when the microwave plasma is initiated. Its homogeneity is all the more difficult than point effects exist. As presented in a next paper, simulations of the microwave electric-field distribution with various substrate-holder geometries showed that point effects can be strongly decreased for particular shapes. Fig. 2 reports the final configuration of the system substrate/substrate-holder that minimizes the microwave plasma heterogeneity and favors the d.c. discharge spreading over the substrate surface. As described by previous studies [6,9,16], the nature of materials submitted to the bias has an influence on the d.c discharge extension. So it is well known that the bias plasma often starts on the uncovered substrate-holder (molybdenum or graphite in the most cases), then spreads over the silicon substrate with diamond formation. Moreover, the extension of the bias plasma on the surrounding materials constituting the uncovered substrate-holder is detrimental to the BEN reproducibility since successive syntheses lead to a modification of the chemical nature of the substrate-holder by formation of carbides or diamond. To avoid these transformations, we have deposited a thin ceramic coating on the substrate-holder surface (see figure 2) which can be easily replaced, and that prevents the extension of the bias plasma beyond the silicon surface.

From this new experimental configuration, and for a total pressure comprised between 1 and 40 hPa, the substrate constituted by a square of 1x1 cm 2 of silicon is well located inside the microwave discharge that appears very homogeneous without point effects. When the voltage is comprised between 80 and 170 volts, the bias plasma spreads instantaneously over the all substrate surface without any visual microwave plasma disturbances. This leads to very homogeneous deposits over the all substrate surface that we have characterized by image analysis [36].

Simplification of the BEN process

Two approaches can be considered for the simplification of the BEN process. The first one consists to limit the number and the duration of the steps preceding the bias. The second one is to reduce the duration and the intensity of ion bombardment during the bias in order to minimize the modification of the substrate surface while maintaining a sufficiently high nucleation density with an acceptable epitaxial ratio and a good homogeneity. As mentioned by Wolter et al. [23], standard process parameters used in the most BEN steps (medium pressure, hydrocarbon/hydrogen ratio below 5%) must allow to deposit an uniform diamond film with a high nucleation density by using short bias durations. This statement is worth developing since some studies pinpoint that longer bias time leads to an epitaxy loss [7,16,17,28] and a decreasing in the diamond quality [12]. High bias voltages are reported to raise the nucleation density but also the growth of poorly oriented crystallites [20,29].

Concerning the in situ pretreatment steps, we have chosen to only keep the hydrogen etching a few minutes before the bias step without preliminary carburation step. This last one is actually very controversial since some studies associated it to the formation of a -SiC conversion layer [12,17,21,22,30] while others suggested the formation of non diamond phases like amorphous carbon [18] or a combination of both [19,29,31]. Moreover, some studies reported that the bias step led to the formation of this conversion layer [4][5][6][7]10,12,15]. Whatever the substrate transformations during the carburation step, it is experimentally demonstrated that diamond epitaxy can be achieved without this one [8,10,11,13,[27][28][29][30].

We conserve the hydrogen etching for two main reasons. Firstly the bias step on silicon substrates cannot be started at temperature below 900 K since the efficiency of the bias plasma seems linked to the silicon conductivity. Indeed we observed that a too low temperature induces a wrong extension on the bias plasma and consequently a large heterogeneity of the subsequent diamond deposit.

Nevertheless to limit the hydrogen etching duration, we mounted a tungsten coil inside the molybdenum substrate-holder (see fig. 2) which is electrically insulated from this one and heated by Joule effect. This heating device allows us to quickly increase the substrate temperature up to 813 K before the hydrogen etching step. The second reason of maintaining the etching step is the supposed cleaning effect of the hydrogen plasma which is reported to remove the native oxide layer of the silicon [7,11,28,32]. This one is thought to inhibit the nucleation as it is used as masks in selected area nucleation applications [33]. However, this cleaning notion must be taken carefully since the reduction of oxides in a hydrogen plasma seems counterbalanced by the oxidation from residual oxygen present in the classical MPCVD reactor [34]. This aspect is presented in a next paper [35]

Synthesis and Characterizations of the deposits

From previous considerations and few preliminary attempts, the final synthesis conditions are reported in table 1. Before introducing the silicon substrate into the reactor, it is cleaned in acetone, simmered in trichloromethane and then simultaneously etched and passivated in 40% hydrofluoric acid for 60 seconds. The preheating step is carried out in pure hydrogen and kept until the substrate temperature reaches 813 K. Then, the subsequent hydrogen plasma etching quickly increases the substrate temperature up to 1073 K. This temperature has been chosen because it insures a good bias plasma spreading as soon as its start, but the absolute value of the temperature does not seem to have a strong effect on the epitaxial nucleation [20]. The bias step is performed for 30 seconds with moderate voltage and methane/hydrogen ratio. The consequence is a very high and uniform nucleation density for a short polarization time so called USBEN for Ultra Short Bias Enhanced Nucleation. We have evaluated the nucleation density to about 4.7 10 9 cm -2 by performing SEM characterizations when the sample has been submitted to a 20 minutes growth step after biasing (fig. 3a). Some epitaxial crystals are visible and pointed out by open white circles. Statistical image analysis treatment developed in a previous work [36] shows that the epitaxial ratio is equal to 6% corresponding to an epitaxial crystal density equal to 2.8 10 8 cm -2 . There are few epitaxial crystals in comparison with the total number of crystals, however it is high enough to quickly produce <100> highly oriented diamond films via the evolutionary selection rule [36]. Figure 3b shows the morphology ofa highly oriented film obtained for identical synthesis conditions of the deposit seen fig. 3a, but for a growth step duration of 20 hrs. We clearly observe the selection of epitaxial crystals at the expense of others leading to a homogeneous surface mainly constituted by epitaxied crystals.

Concerning the possible mechanism of diamond nucleation during the bias step, we can advance that the very short bias duration with low bias voltage does not strongly modify the silicon substrate but sufficiently to have nucleation and orientation. This is supported by the fact that the average energies of the main hydrocarbon ions during the bias is reported to be comprised between 50 and 100 eV for pressure about 25 hPa [37,38]. In our case where the pressure is fixed at 13 hPa, the ion energy is slightly higher but does not exceed 120 eV. Therefore, the number of displaced silicon atoms due to the impinging ions is about 8.5 [40], that approximately corresponds to a penetration depth of about 1 nm [40,41].

Conclusions

We improve the homogeneity of the highly oriented diamond films by increasing the localization and the stability of the double-discharge (d.c and microwave discharges). This improvement is carried out by modifying the nature and the geometry of the substrate-holder and by increasing the electric-field homogeneity associated to the d.c polarization. The consequence is a drastic reduction

of the polarization time while maintaining a high nucleation density with an usual epitaxial ratio [5][6][7][8][9][10][11][12][13][14][15][16][17] and with a very good homogeneity. This one leads to the synthesis of highly oriented diamond films when a thickening is performed. The weak duration of the ion bombardment with ions having a quite low energy seems to mean that the bias step probably consists to a slight modification of the substrate surface.