Fabrication and Characterization of Rare-Earth Silicide Thin Films

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Fabrication and Characterization of Rare-Earth Silicide Thin Films UNIVERSITÉ CATHOLIQUE DE LOUVAIN Ecole Polytechnique de Louvain ICTEAM Fabrication and characterization of rare-earth silicide thin films Dissertation présentée en vue de l’obtention du grade de Docteur en Sciences de l’Ingénieur par Nicolas Reckinger Promoteurs: Prof. Vincent Bayot Prof. Jean-Pierre Raskin Février 2011 UNIVERSITÉ CATHOLIQUE DE LOUVAIN Ecole Polytechnique de Louvain ICTEAM Fabrication and characterization of rare-earth silicide thin films Dissertation présentée en vue de l’obtention du grade de Docteur en Sciences de l’Ingénieur par Nicolas Reckinger Membres du jury: Prof. Vincent Bayot, promoteur Prof. Jean-Pierre Raskin, promoteur Prof. Emmanuel Dubois Prof. Denis Flandre Dr. Thierry Baron Prof. Siegfried Mantl Dr. Xiaohui Tang Prof. Danielle Vanhoenacker-Janvier, président Février 2011 Abstract With the continuous reduction of the dimensions of metal-oxide- semiconductor field-effect transistors (MOSFET), issues related to the formation of source and drain contacts by ion implantation appear. A new source and drain architecture based on metallic contacts over silicon was proposed to replace conventional highly doped extensions. To real- ize such a device, the so-called Schottky barrier (SB) MOSFET, requires working with materials presenting low Schottky barrier heights (SBH) to silicon. For n-type MOSFETs, rare-earth silicides, alloys between silicon and a rare-earth metal, are the best candidates. For Schottky barrier MOSFETs to compete with conventional ones in terms of on- and off- currents, the SBH of rare-earth silicides is intrinsically still too high and must accordingly be further reduced. Such a barrier decrease can be achieved with the dopant segregation technique, where a thin dopant layer is interposed between the silicide and silicon. In the first two chapters of this thesis, we expose the theoretical aspects of metal-semiconductor (MS) contacts. Chapter 1 starts with a recall of the main steps of the long history of MS contacts. Next, the simplified energy band structure of metals and semiconductors is briefly described and applied to establish the energy band diagram of MS contacts. Then, the principal models of SBH formation are developed. Finally, ohmic contacts and the notion of specific contact resistance are tackled. The purpose of chapter 2 is to present the electrical extraction techniques of SBHs and the methods of material characterization that are used extensively in the two following chapters. Chapter 3, devoted to the growth and the characterization of er- bium (Er) silicide contacts on n-type silicon, is the central part of this work. Different techniques of metal deposition and thermal treatment are successively considered. Evaporation and growth in ultrahigh vac- uum conditions leads to reference Er silicide thin films with excellent morphological and electrical characteristics. Ex situ growth with a pro- tective titanium capping layer produces Er silicide with a state-of-the-art SBH of 0.28 eV at the optimal annealing temperature. It is also inferred i ii Abstract that the observed SBH drop with increasing annealing temperature can be associated to the progressive transformation of amorphous Er sili- cide into crystalline Er silicide. However, these films are plagued by superficial oxidation due to oxygen penetration through the titanium cap. By optimizing the process conditions (annealing atmosphere and cap thickness), oxygen-free Er silicide films can be fabricated. Finally, the last chapter of this text deals with the SBH modulation of Er silicide contacts on n-type silicon by dopant segregation. The extraction of an effective SBH of about 0.1 eV and the dopant redistribution in the sili- cide both testify to efficient segregation. The SBH reduction turns out to be all the more efficient that the dopant concentration at the interface is elevated. In the end, through quantum simulations, low temperature deviations of the current-voltage characteristics of silicide/silicon con- tacts are attributed to the nanoscale modulation of the SB profile by the applied voltage. Remerciements Tout le processus de la thèse (et de la vie en général d’ailleurs) con- siste, au fur et à mesure du temps, à prendre confiance en soi et en ses capacités, sans pour autant tomber dans les pièges faciles de l’arrogance, de la suffisance et l’autosatisfaction, scléroses de l’esprit. Comme me le répète le paternel depuis ma plus tendre enfance: « Il ne faut jamais se reposer sur ses lauriers »! Avec un peu de recul, je me rends mieux compte combien on peut évoluer en l’espace de seulement quelques an- nées (et encore heureux qu’il en soit ainsi!), tout en restant foncièrement la même personne. Ce processus progressif de construction et de décou- verte de soi ne peut se faire sans un environnement humain valorisant et formateur. Dans le cadre plus particulier de la recherche scientifique, on peut disposer du meilleur équipement du monde, sans le facteur humain, on ne peut aller bien loin! C’est pourquoi l’expression des remerciements revêt à mes yeux une importance toute spéciale. J’espère sincèrement n’oublier personne, je ne voudrais attirer sur moi les foudres de quiconque :-). Tout d’abord, je tiens à exprimer ma reconnaissance à mes deux pro- moteurs, qui se sont successivement préoccupés de ma petite personne. En premier, Vincent, qui m’a permis de m’introduire au monde de la recherche à l’UCL et m’a encadré pendant mes premières années et qui, à sa façon, m’a appris à travailler de façon autonome et indépendante. Sa perspicacité et son sens critique aigu m’ont amenés à acquérir ou affûter le jugement que je porte sur mon propre travail. Ensuite Jean- Pierre, qui a repris le flambeau, pendant mes presque trois années de thèse. Je tiens vraiment à le remercier pour sa disponibilité presque permanente (combien de fois n’ai-je pas pu constater qu’il répondait à mes courriels à des heures indues, où la plupart des gens dorment ou en tous cas ne travaillent plus), son ouverture d’esprit, sa bonhomie, son optimisme, son enthousiasme communicatif parfois déconcertant ;-), son sens de l’empathie, j’en passe et des meilleures. Ensuite, s’il y a bien quelqu’un qui mérite ma reconnaissance éter- nelle, c’est Xiaohui. Elle m’a tout d’abord encadré pendant mon mémoire iii iv Remerciements de fin d’études, avec Vincent, et m’a tout appris des arcanes des chambres propres. Au début, la communication « franglais-anglinoise » (« pimimi » et autres florilèges) n’a pas été des plus simple, mais après une période d’adaptation, nous avons vite fini par nous comprendre, ne dit-on pas que les grands esprits se rencontrent ;-). En outre, il a fallu un peu de temps pour installer la confiance du fait de cette réserve toute asiatique, mais une fois établie, elle est et demeure solide. L’apprentissage du chi- nois m’a également permis de mieux comprendre aussi bien la langue que la culture chinoises. Xiaohui a aussi parfois dû supporter mon impulsiv- ité à l’énervement (qui l’eût cru!), genre soupe au lait, et mon penchant pour la taquinerie, j’espère ne pas l’avoir trop traumatisée :-). Je ne peux pas non plus oublier mon deuxième collègue de bureau qui vient compléter la fine équipe, Alex, la grande brute1 moldave, à la curiosité insatiable. Derrière ses dehors un peu bourrus et laconiques se cache en réalité un coeur en or (il ne va pas aimer lire cela mais c’est fait exprès :-)). Il a égayé l’ambiance du bureau de ses blagues et commen- taires scatologiques, et autres borborygmes et météores. Plus sérieuse- ment, sa persévérance et son intelligence pourraient servir d’exemple à plus d’un. Je dois aussi y associer Dana avec laquelle j’ai pu discuter de tout et de rien à de nombreuses reprises. Avec Xiaohui, elle a apporté une touche féminine indispensable dans ce monde très (trop) masculin de la recherche en sciences appliquées. Au cours des projets européens auxquels j’ai pu participer, j’ai eu la chance d’entrer en contact et de travailler avec des collègues d’autres pays. Notamment, je voudrais remercier Emmanuel, Dmitri, Sylvie et Guilhem pour l’aide substantielle qu’ils m’ont apportée au cours de mes séjours à l’IEMN. Sans leur coopération, je peux vraiment affirmer que cette thèse n’aurait pas pu s’accomplir. Plus particulièrement Em- manuel, dont les commentaires toujours très avisés et constructifs et le perfectionnisme m’ont beaucoup aidé au cours de mon travail. Je pense aussi aux collègues polonais, Jacek et Adam, qui m’ont préparé de nom- breux échantillons TEM. Maintenant, parmi mes collègues proches à l’UCL, chacun avec ses particularités appréciables, je voudrais faire une mention spéciale à: - Benoît H, qui sans peut-être s’en rendre compte, m’a gratifié à plusieurs reprises de conseils vraiment judicieux, et dont j’envie le calme et le sens de la diplomatie, - Pierre-Olivier, le bricoleur inventif, dans lequel j’ai trouvé mon maître du point de vue de la distraction, et ce n’est pas peu dire, aussi cueilleur 1Prononcer brrrroute. Remerciements v de champignons à ses heures, - Augustin, tout en discrétion, efficacité et pragmatisme, - Nicolas, qui comprend à merveille le second degré, grand amateur de blagues de m... et féru de digressions métaphysiques, - Romain, avec qui j’ai apprécié les discussions ouvertes sur tout sujet imaginable, - Sébastien F, au sens pratique inégalable, toujours prompt à la taquinerie envers Alex (qui, soit dit en passant, le mérite bien), - Aryan et son imperturbable « zénitude », sans oublier mes autres collègues des défunts DICE et EMIC, Fred, Loïk, Cédric, Benoît O, Umesh, Vikram, Khairuddin, Khaled, Valeria, Ce- sar, Mohamed, Rémi, Luis, Gilles, Ester, Bertrand, David B, David L, Gaël, Arnaud, Catherine, Olivier, Joaquin, El Hafed, Guillaume, Gef- froy, Sylvain etc.
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