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Atomistic study of structural and electronic transport properties of silicon quantum dots for optoelectronic applications

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[cat]Les nanopartícules de silici (silicon quantum dots, Si QDs, en anglès) són interessants materials que es proposen com a candidats per a la tercera generació de cel•les solars. Degut al confinement quàntic de les càrregues elèctriques dins del QD, el valor de l'energia de gap del material augmenta a mesura que la mida del QD disminueix, donant valors més gran que el Si bulk i fent que els QDs de Si siguin uns bons candidats per a dispositius amb valors de l'energia de gap modificables. En aquesta Tesi Doctoral proposem un marc teòric per estudiar el transport electrònic en nanoestructures aportant una descripció ab initio dels estats electrònics, basant-se en l'ús conjunt de dues tècniques: la Teoria del Funcional de la Densitat (Density Funcional Theory, DFT, en anglès) pel modelatge de la densitat d'estats del dispositiu i el Hamiltonià de Transferència (Transfer Hamiltonian, TH, en anglès) per la descripció del transport electrònic. Les principals conclusions d’aquesta Tesi Doctoral són: • En el cas de QDs de Si de pocs nanometres dins de matrius dielèctriques, la interfície fortament no-planar entre el Si i el SiO2 requereix un tractament diferent de la communtment utilitzada en l'heterojunció planar Si/SiO2. En aquesta Tesi Doctoral hem observat que, per Si QDs de mida petita, el model de partícula-dins-d'una-caixa no descriu les densitats d'estats i les barrers de potencial d'una forma acurada. Això és degut a què aquest model no recull l'efecte de la interfície, propietat que sembla ser essencial en la mida nanomètrica. • Respecte el transport electrònic en QDs de Si, Per una banda, el corrent d'electrons (forats) és més gran per a QDs DE Si de mida més gran (petita), i, per l'altra banda, el corrent d'electrons (forats) és més important per a sistemes amorfs (cristal•lins). • Les principals influències de dopatge tipus p (amb B) i tipus n (amb P) és (1) les configuracions de més baixa energia de formació són dins del QD quan dopem amb P, i a la interfície entre el QD i la primera capa d'oxígens quan dopem amb B, i (2) hi ha un millora en la conductivitat per la posició energètica més favorable pel dopatge amb P però no per la posició pel dopatge amb B.

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[eng]  It is undisputed that the silicon became the material most widely used in electronics in recent decades[1,2]. The qualities of silicon are well known, from its abundance and low cost to its ability to easily combine with oxides, so that the material has become essential in integrated electronic circuits and CMOS technology. A step further, though, is the idea of integrating electronics and photonics on the same silicon-based technology[3]. However, new strategies are needed to overcome the two principal obstacles of a possible bulk Si photonics: the indirect band gap and the band gap amplitude, suitable for operation only in the infrared range. Due to the quantum confinement of electric charges in silicon quantum dots (Si QDs)[4], the value of the energy gap of the material increases as the size of QD decreases, giving values greater than bulk Si and making Si QDs good candidates for tunable-band gap devices. Several applications have been developed in recent years using these new properties, from light-emitting devices [5] to solar cell tandem type [6] or other opto-electronic devices [7]. Objectives of this Thesis This Thesis initiated a collaboration between our group and the group of Prof. Ossicini of the University of Modena and Reggio Emilia, who has been modeling Si QDs for the last five years[8-14]. In this context, we contributed with the capacity to study the transport properties of these models by taking advantage of mixing two different techniques, Transfer Hamiltonian (TH) and Density Functional Theory (DFT). Thus, the aim of this work was to develop an approach to study transport in nanostructures by taking advantage of the atomistic information that ab initio methods can provide. In particular, the transport through a single Si QD embedded in a SiO2 dielectric matrix and the influence of the Si QD size, the amorphization level, and the doping were studied. Results and conclusions About the size of QDs In the case of embedded QDs of few nanometers, the strong non-planar interface between Si and SiO2 require a different treatment with respect to common planar Si/SiO2 devices. In this PhD Thesis, we have shown that, for small QD sizes, the particle-in-a-box model cannot describe accurately DOS and band offset, because of the large contribution of interface states. In this regime an ab initio approach is necessary to take into account the atomistic detail of the interface between the Si QD and the first shell of O atoms surrounding it. Regarding the electronic transport in Si QDs, a correlation between electron (hole) barrier value and electron (hole) current was found, obtaining larger current values for smaller energy barriers. Specifically, a contrary dependence on Si QD size and amorphization level is found for electron and hole current. On one hand, electron (hole) current is higher for large (small) Si QDs, and, on the other hand, it is enhanced for amorphous (crystalline) systems. On the effects of doping Finally, the DFT-TH technique was used to study the influence of impurity atoms, B for p-doping and P for n-doping, in embedded Si QDs. It is remarkable that this study is one of the first attempts to model with DFT the inclusion of impurity atoms in embedded Si QDs, after the wide knowledge of ab initio works on free-standing Si QDs of the last years. The principal features that we found were: • The impurity positions with lower formation energy are inside the dot for P-doping (P-dot) and at the interface for B-doping (B-int). • Relation between shift of the Fermi energy and improvement of conductivity in doped systems (due to the change in energy barriers). • Improvement of the conductivity for the most energetically favorable position of P-doping (P-dot) but not for the position of B-doping (B-int). • Change in the conductivity between doped and undoped is higher for P-doping than B-doping for a given Si QD size and impurity position, and decreases with QD size for a given specie and impurity position. Bibliography [1] M. Segal. Material history: Learning from silicon. Nature, 483, S43 (2012). [2] H. J. Leamy and J. H.Wernick. Semiconductor silicon: the extraordinary made ordinary. MRS Bulletin, 22, 47 (1997). [3] X. Hao, E.-C. Cho, G. Scardera, Y. Shen, E. Bellet-Amalric, D. Bellet, G. Conibeer and M. Green. Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells. Sol. Energ. Mat. Sol. C, 93, 1524 (2009). [4] J. P. Proot, C. Delerue and G. Allan. Electronic structure and optical properties of silicon crystallites: Application to porous silicon. Appl. Phys. Lett., 61, 1948 (1992). [5] Y. Berencen, J. M. Ramirez, O. Jambois, C. Dominguez, J. A. Rodriguez and B. Garrido. Correlation between charge transport and electrolumi-nescence properties of Si-rich oxide/nitride/oxide-based light emitting capacitors. J. Appl. Phys., 112, 033114 (2012). [6] G. Conibeer, I. Perez-Wur, X. Hao, D. Di and D. Lin. Si solid-state quantum dot-based materials for tandem solar cells. Nanoscale Res. Lett., 7, 193 (2012). [7] L. Pavesi, L. D. Negro, C. Mazzoleni, G. Franzo and F. Priolo. Optical gain in silicon nanocrystals. Nature, 408, 440 (2000). [8] M. Luppi and S. Ossicini. Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2: Beyond the quantum confinement effect. Phys. Rev. B, 71, 035340 (2005). [9] R. Guerra, I. Marri, R. Magri, L. Martin-Samos, O. Pulci, E. Degoli and S. Ossicini. Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size. Phys. Rev. B, 79, 155320 (2009). [10] R. Guerra, E. Degoli and S. Ossicini. Size, oxidation, and strain in small Si/SiO2 nanocrystals. Phys. Rev. B, 80, 155332 (2009). [11] R. Guerra and S. Ossicini. High luminescence in small Si/SiO2 nanocrystals: A theoretical study. Phys. Rev. B, 81, 245307 (2010). [12] R. Guerra, E. Degoli, M. Marsili, O. Pulci and S. Ossicini. Local-fields and disorder effects in free-standing and embedded Si nanocrystallites. Phys. Status Solidi B, 247, 2113 (2010). [13] R. Guerra, M. Marsili, O. Pulci and S. Ossicini. Local-field effects in silicon nanoclusters. Phys. Rev. B, 84, 075342 (2011). [14] M. Govoni, I. Marri and S. Ossicini. Auger recombination in Si and GaAs semiconductors: Ab initio results. Phys. Rev. B, 84, 075215 (2011)

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Tesis Doctorals - Departament - Electrònica

# metadata.dc.title[].value
Atomistic study of structural and electronic transport properties of silicon quantum dots for optoelectronic applications