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Méthodes k.p avancées pour la spinorbitronique à base de semi-conducteurs
Duy-Quang TO
Vendredi 11/10/2019, 14:00-17:00
Amphi. Becquerel, École Polytechnique, Palaiseau,

Manuscrit de la thèse


Résumé :

Ce travail de thèse est essentiellement consacré au développement de la théorie de transport tunnel k.p multibandes (14, 30 et 40 bandes) pour une application à la spinorbitronique avec semiconducteur. La spinorbitronique associe généralement les effets de spin et d’orbite, qui par l’intermédiaire du couplage spin-orbite, introduit des propriétés de transport nouvelles comme les effets Hall de spin et les effets tunnel Hall anormal. Celui-ci se caractérise par une déflection de la trajectoire des porteurs polarisés en spin selon la direction transverse de leur flux. D’autres effets caractéristiques concernent i) les mécanismes de transfert de spin (‘spin-transfer’ ou ‘spin-orbit torque’ ) permettant de commuter une aimantation locale par transfert de moment angulaire, généralisant ainsi les processus de transfert de spin ainsi que ii) la conversion spin-charge aux interfaces médiés par les termes Rashba et/ou Dresselhaus.

Dans ce cadre, notre théorie de transport tunnel est adaptable aux hétérostructures semiconductrices, magnétiques ou non, traitant d’une simple interface ou de jonctions tunnel. Elle permet de tenir compte de façon fine des interactions spin-orbite de cœur et d’interface (Rashba, d’interface). Elle utilise de façon générale, l’introduction de bandes hautes supplémentaires, dites fantômes, pour traiter les états spurious inhérents à la théorie k.p multibandes. Outre l’introduction de tels états ‘fantômes’ ne déformant ni la structure électronique, ni le transport polarisé, notre approche utilise la continuité des composantes des fonctions d’onde à chaque interface ainsi que le raccordement des composantes du courant d’onde selon la symétrie particulière des interfaces en considérant soit

  1. la continuité des composantes du courant d’onde (extension de la théorie Ben Daniel Duke),
  2. Les conditions de raccordement correspondant à une symétrie particulière C2v, introduisant un certain mélange trous lourds/trous légers dans la bande de valence (conditions d’Ivchenko)
  3. ou une discontinuité des bandes p ‘hautes’.

Mots clés: k.p, spinorbitronique, semi-conducteurs.

Contact LSI : Henri-Jean Drouhin


Advanced k.p multiband methods for semiconductor-based spinorbitronics

Abstract:

This thesis work is essentially devoted to the development of the tunneling theory k.p 14, 30 and 40 bands for a spinorbitronic application with semiconductor. Spinorbitronic combines the effects of spin and orbit, which via spin-orbit coupling, introduces new transport properties such as spin Hall and anomalous Hall effects. The latter is characterized by a deflection of the trajectory of polarized carriers in the transverse direction of their flow. Other characteristic effects concern i) the spin transfer mechanisms for switching angular momentum transfer magnetization, thus generalizing spin transfer as well as ii) spin-charge conversion mediated by the terms Rashba or Dresselhaus. In this context, our tunnel transport theory is adaptable to semiconductor heterostructures, magnetic or not, dealing with a simple interface or tunnel junctions.

It allows to take into account in a fine way the spin-orbit interactions of heart and interface. It generally uses the introduction of additional high bands, called ghosts, to deal with the spurious states inherent in the multiband k.p theory. Apart from the introduction of the "spurious" states neither deforming the electronic structure nor the polarized transport, our approach uses the continuity of the components of the wave functions at each interface and the connection of the components of wave current according to symmetry interfaces either by

  1. the continuity of the components of wave current (extension of Ben Daniel Duke theory),
  2. the connection conditions obeying a C2v symmetry by mixing heavy holes cd / light in the valence band (conditions from Ivchenko)
  3. or discontinuity of the 'high' bands.

We also demonstrate the equivalence of the continuity conditions for the case of AlAs / GaAs / AlAs III-V quantum wells, which represents a generalization of previous results developed in 14 bands.The whole of this work of analytical and numerical thesis, include several important demonstrations. We show that our theory can be used to describe the spin transport of spin-orbit coupled states in growth axis (100) or (110) semiconductor heterostructures. These results are notably materialized by the calculations of the three components of the spin current in the III-V barriers (GaAs, AlAs) acting as spin phase-shifters. Calculations show that we develop in effect, as provided by the analytic theory, a vector rotation of the spin density matrix in the thickness of the barrier and as provided by the application of an effective spin-orbit field Dresselhaus parallel to the plane of the barrier.

Our theory is also successfully compared to the multiband perturbation calculations using Green's transport functions to address the mechanisms of abnormal Hall tunnel effect in the conduction band and the valence band. The results are remarkable of fidelity which shows the power of the technique used. We also calculate spin current properties in GaMnAs-based ferromagnetic tunnel junctions to derive the spin transfer torque responsible for switching the fine ferromagnetic element. We show, for example, how the transverse spin components of the current are relevant to switch a magnetization. Finally, we adapted our transport theory to III-V structures to compute confined quantum well states in the conduction band and valence band. We successfully compare our state-of-the-art 6-band and 14-band multiband results on the optical anisotropy of absorption between the directions (110 and (1-10) of the electric field when the symmetry of the heterostructure is reduced to C2v symmetry.

Keywords: Spinorbitronics, K.p multiband methods, Semiconductor.

Contact : Elodie DUBOIS

 

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