Ab  initio  Study of Hydrogen  Related  Defects in Hydrogen Implanted  Crystalline  Silicon  Stability and Migration Liviu...
Outline <ul><li>Introduction – hydrogen behavior in silicon </li></ul><ul><li>Calculation Details </li></ul><ul><li>Hydrog...
Accumulation of hydrogen into platelets <ul><li>Context:   Accumulation of hydrogen into  2D  extended defects  ( platelet...
CALCULATION DETAILS <ul><li>DFT calculations as implemented in SIESTA . </li></ul><ul><li>Calculation parameters :  </li><...
Hydrogen Related Defects <ul><li>hydrogen interstitials:  </li></ul><ul><ul><li>atomic  H q [X] and  </li></ul></ul><ul><u...
Atomic and Molecular Interstitials Atomic H on different  sites in [111] direction Atomic H interstitals Bi-atomic structu...
Stability of Atomic and Molecular Interstitials
Charged Atomic Hydrogen Diagram Formation energy (with respect to free H atom) in function of Fermi Level for ALL sites an...
Quantitative Evaluation of H ± [X] concentration H + (BC) and H - (AB) are dominant.   <ul><li>Si bulk of volume  V 0   </...
Migration calculations  drag method   <ul><li>Two initial states represented by  I  and  F  containing the 3D positions of...
Migration of H ± /H 2 Migration of H ±  between two equiv. sites Migration of H +   between second NN sites Migration of H...
Migration and accumulation of intrinsic defects <ul><li>In Si self-defects are rapidly migration species </li></ul><ul><li...
Hydrogenated Vacancies q = -2  q = -1  q = 0  q = +1  q = +2
Possible elementary process Elementary processes involving H atoms  Reaching the most stable VH n  type structure, that is...
Decomposition of VH n  type structures V 2-  + H +   -> VH - VH 2   -> VH -  + H + VH 3 +   -> VH 2   + H + VH 4   -> VH 3...
Ejection of H 2  molecule <ul><li>Hydrogen molecule barrier stays unchanged in the presence of a vacancy. </li></ul><ul><l...
Hydrogenated Interstitials <ul><li>Structures found by Random Structure Search; </li></ul><ul><li>IH 2  is the most stable...
Conclusions <ul><li>Hydrogen related defects in silicon have been studied at atomic scale. </li></ul><ul><li>One has ident...
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Hydrogen Related Defects (results)

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Some preliminary results on hydrogen related defects in silicon: stability and migration

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  • Bonjour! Je vais vous présenter l’avancement des travaux liés à ma thèse sur la simulation atomistique de implantation de l’hydrogène en silicium. Cette thèse encadré par Jean-Paul est faite en collaboration avec le LETI de Grenoble, représenté notamment par Mme. Aurélie Tauzin.
  • Voilà le plan de la présentation: Une petite introduction au procédé Smart-Cut suivi par la présentation des quelques résultats à la fois définitifs à la fois préliminaires sur les différentes structures possible de l’hydrogène en silicium c.a.d Les atomes et les structure biatomiques; Les défauts hydrogénés – notamment les lacunes hydrogénées; Les « platelets » - le défauts bidimensionnelles sur la formation desquels on s’intéresse. Finalement, les conclusions après le travail de cette année et si le temps me permettra, la liste des mes activités scientifiques.
  • SMART-CUT est un procédé implémenté principalement par SOITEC, qui est leader européen sur la marché SOI. Les étapes du SMART-CUT sont présentés dans la figure sur ce transparent. Parmi ces multiples étapes nous sommes intéressés a cette étape – ci qui consiste dans l’accumulation de l’hydrogène en platelets pendant le recuit.
  • Au cours de la formation des platelets, les donnés expérimentales montrent que plusieurs défauts (que j’ai listé sur ce transparent) interviennent. Nous ne sommes pas encore capable de dire quels sont les mécanismes entrainant ces défauts qui mènent aux platelets. On peut, par contre, caractériser ses défauts à l’équilibre ce qui nous permets proposer des différents scenarios de formation favorables de point de vue énergétique.
  • Le deuxième type (en ordre de la complexité) sont les défauts biatomiques. Ces défauts biatomiques peuvent être classifié selon leur structure en: Interstitiels qui sont des molécules situées dans les sites T et H. Complexes biatomiques qui sont des structures formées par deux atomes d’hydrogène situés dans deux sites très proches. La dernière catégorie peut être divisés en deux sous-classes: Défauts axiales – les atomes se trouvent sur la même direction crystallographique. Défauts non-axiales – les atomes se trouve sur deux directions crystallographiques différentes. Donc une « zoologie » entière des défauts moléculaire.
  • Nous avons étudié toutes ces structures mono- et di- atomiques dans les états neutres et chargés. Les résultats sont présentés de manière synthétique sur ce transparent. Afin de pouvoir comparer (en déduire la stabilité): nous avons prit pour zéro l’énergie de l’atome dans le site le plus stable. Les conclusions sont les suivantes: Les structures diatomiques chargées ont une énergie très haute, donc elles sont négligeables. Les molécules sont les structure neutres les plus stables en particulière la molécule interstitielle dans le site T. Les structures diatomiques peuvent être classifié de point de vue énergetique en trois catégorie: interstiels, des complexes liés, quas-liées et pas-liés. Dans la série « charge positive »: l’hydrogène en BC est la structure la plus stable; Dans la série « charge négative »: l’hydrogène en AB est la structure la plus stable. Dans cette série les on a trois configurations énergetiquement très proche et trois qui sont à aprox. 0.40 eV de ce « pluton ».
  • Hydrogen Related Defects (results)

    1. 1. Ab initio Study of Hydrogen Related Defects in Hydrogen Implanted Crystalline Silicon Stability and Migration Liviu BÎLTEANU 1 , Jean-Paul CROCOMBETTE 1 , Aurélie TAUZIN 2 1 CEA DEN/DMN/SRMP, Saclay, 91191 Gif-sur-Yvette, France. 2 CEA LETI/DIHS/LTFC, 17, avenue des Martyrs, 38000 Grenoble, France.
    2. 2. Outline <ul><li>Introduction – hydrogen behavior in silicon </li></ul><ul><li>Calculation Details </li></ul><ul><li>Hydrogen Related Defects in Silicon </li></ul><ul><li>Atomic and Molecular Defects Stability </li></ul><ul><li>Migration of Hydrogen Atoms and Molecules </li></ul><ul><li>Migration of Self-defects (vacancies and interstitials) </li></ul><ul><li>Hydrogenated Vacancies </li></ul><ul><li>Mechanisms leading to VH n type defects </li></ul><ul><li>Hydrogenated Interstitials </li></ul><ul><li>Conclusions </li></ul>
    3. 3. Accumulation of hydrogen into platelets <ul><li>Context: Accumulation of hydrogen into 2D extended defects ( platelets ) when silicon is irradiated with highly energetic protons and then the sample is thermally annealed. </li></ul><ul><li>Approach: modelization of hydrogen accumulation via elementary processes involving hydrogen related defects (see the next). </li></ul><ul><li>Method: ab initio atomic scale calculations within the Density Functional Theory (DFT). </li></ul>
    4. 4. CALCULATION DETAILS <ul><li>DFT calculations as implemented in SIESTA . </li></ul><ul><li>Calculation parameters : </li></ul><ul><li>216 Si atoms boxes that is 3  3  3 supercell +1-2 H atoms; </li></ul><ul><li>8 k - p o ints that is 2  2  2 in a Monkhorst-Pack scheme; </li></ul><ul><li>generalized gradient approximation (GGA) of the the exchange-correlation functionals in PBE implementation ; </li></ul><ul><li>Mesh Cut-off 150 Ry; </li></ul><ul><li>Relaxation included (0.04 eV/ Å) </li></ul>
    5. 5. Hydrogen Related Defects <ul><li>hydrogen interstitials: </li></ul><ul><ul><li>atomic H q [X] and </li></ul></ul><ul><ul><li>molecular H 2 [X-Y] , whereas X, Y are label of various sites; </li></ul></ul><ul><li>self-defects: </li></ul><ul><ul><li>vacancies V q and </li></ul></ul><ul><ul><li>interstitials I q ; </li></ul></ul><ul><li>hydrogenated defects: </li></ul><ul><ul><li>hydrogenated vacancies (VH n ) q and </li></ul></ul><ul><ul><li>hydrogenated interstitials IH n . </li></ul></ul>
    6. 6. Atomic and Molecular Interstitials Atomic H on different sites in [111] direction Atomic H interstitals Bi-atomic structures
    7. 7. Stability of Atomic and Molecular Interstitials
    8. 8. Charged Atomic Hydrogen Diagram Formation energy (with respect to free H atom) in function of Fermi Level for ALL sites and charge states. Negative U – 0.32 eV exp. 0.36 eV [1] [1] N. M. Johnson et al., Phys. Rev. Lett. 73 , 130 (1994). NEGATIVE U - EFFECT 2H  H + + H - + 0.32 eV 2H -> H + + H -
    9. 9. Quantitative Evaluation of H ± [X] concentration H + (BC) and H - (AB) are dominant. <ul><li>Si bulk of volume V 0 </li></ul><ul><li>= 5.00  10 22 cm -3 </li></ul><ul><li>C 0 Hydrogen (at %) </li></ul>µ H chemical potential of hydrogen E i q is the form . energy of H q [X] calculated by DFT ; q the charge of hydrogen ( q = 0,  1) ; k B Boltzmann’s constant (0.8617343  10 -4 J  K -1 ) T la temperature (in K) ; Number of particles Mass Conservation Charge Conservation Atomic Fractions of Considered Species
    10. 10. Migration calculations drag method <ul><li>Two initial states represented by I and F containing the 3D positions of all atoms in the calculation box. </li></ul><ul><li>The direct (hyper)line IF is sampled and the system is placed in various intermediary points on this (hyper)line R λ = I + λ F with λ  [0, 1]. Usually 10 – 25 points are used. </li></ul><ul><li>Then the system is allowed to relax in the (hyper)plan perpendicular on the (hyper)line in the respective intermediary points. </li></ul><ul><li>The collection of energy values of the system relaxed in this way describes the migration barrier. </li></ul>R λ I F
    11. 11. Migration of H ± /H 2 Migration of H ± between two equiv. sites Migration of H + between second NN sites Migration of H 2 molecule between two T first NN sites <ul><li>Atomic hydrogen migration barrier </li></ul><ul><ul><li>is charge independent </li></ul></ul><ul><ul><li>values 0.46 eV compared with exp. 0.48 eV [ Van Wieringen A., and Warmoltz N., Physica 22 , 849 (1956)]. </li></ul></ul><ul><ul><li>the migration barrier (of H + ) between two second NN BC sites is decomposed into two elementary barriers. </li></ul></ul><ul><li>Molecular hydrogen migration barrier is 5 times higher (~2.5 eV), hence H 2 is immobile when compared to H ± ) </li></ul>
    12. 12. Migration and accumulation of intrinsic defects <ul><li>In Si self-defects are rapidly migration species </li></ul><ul><li>Vacancies tend to accumulate. </li></ul><ul><li>The migration and accumulation barriers are less than the H ± migration barrier  vacancies diffuse and accumulate quicker. </li></ul>Wright and Wixom, J. Appl. Phys. 106 (2008) Migration of silicon self-interstitial
    13. 13. Hydrogenated Vacancies q = -2 q = -1 q = 0 q = +1 q = +2
    14. 14. Possible elementary process Elementary processes involving H atoms Reaching the most stable VH n type structure, that is VH 4 Elementary processes involving also H 2 molecules
    15. 15. Decomposition of VH n type structures V 2- + H + -> VH - VH 2 -> VH - + H + VH 3 + -> VH 2 + H + VH 4 -> VH 3 - + H + <ul><li>H + migrates via a hoping like mechanism. </li></ul><ul><li>The interm. barriers are ~0.5 eV or less . </li></ul><ul><li>H - does not have any « barrier » to reach VH n defects. </li></ul>VH 3 - -> VH 2 2- + H +
    16. 16. Ejection of H 2 molecule <ul><li>Hydrogen molecule barrier stays unchanged in the presence of a vacancy. </li></ul><ul><li>The migration can be associated to a hoping-like behavior. </li></ul><ul><li>The H 2 barrier is ~5 times higher than that of an atomic hydrogen which might explain its lack of mobility. </li></ul>VH 2 2- -> V 2- + H 2
    17. 17. Hydrogenated Interstitials <ul><li>Structures found by Random Structure Search; </li></ul><ul><li>IH 2 is the most stable structure. </li></ul><ul><li>Possible mechanism to reach IH 2 structure: </li></ul>Morris et al. PRB 78 (2008) {I,H} {I,H 2 } {I,H 2 } * {I,H 3 } {I,H 4 }
    18. 18. Conclusions <ul><li>Hydrogen related defects in silicon have been studied at atomic scale. </li></ul><ul><li>One has identified the main migrating species (atomic hydrogen) that have been revealed to be charged. </li></ul><ul><li>The calculated negative-U effect leading to the formation of charged atomic defects is in perfect agreement with the experimenal values DLTS. </li></ul><ul><li>The migration energy of both H ± species is charge independent being almost equal to the experimental value obtained via permeation experiments. </li></ul><ul><li>Several reaction-type mechanisms that can be elementary processes in hydrogen accumulation have been investigated. </li></ul><ul><li>Close to vacancy dominated region, the migration of H + is done through a hoping-like mechanism, while the H - seems to have no barrier. </li></ul><ul><li>Hydrogen molecule has a ~2.2 eV migration barrier, which indicates that the molecule is highly immobile with respect to the hydrogen atoms, a fact that has been implied previously from the experimental measurements. </li></ul>

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