Current Controlled Negative Differential Resistance in Niobium Dioxidea

Axe 4 - Dimensionnalité et confinement

Thèse d'Ali Fakih

AAP 2016 - Travail de recherche commencé le 1^er novembre 2016

Soutenance de thèse le 5 novembre 2019 à 14h
Salle de conférence 401 - Couloir 22-23 - Campus Pierre et marie Curie

Laboratoires co-porteurs

• Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC)
  Porteur de projet : Johan Biscaras
  Co-porteur : Abhay Schukla
• Laboratoire de Physique et d'Etude des matériaux (Lpem - ESPCI)
  Co-porteur : Ricardo Lobo
• Institut des NanoSciences de Paris (INSP)
  Co-porteur : Jacek Goniakowski

Abstract

Niobium dioxide NbO₂  has been recently gaining a lot of interest in the fields of solid-state physics and technological nano-devices. On one hand, NbO₂  undergoes a structural distortion accompanied by an electronic phase transition where the material changes from an insulating state at room temperature into a metallic state at temperatures above ∼ 1080 K. On the other hand, NbO₂ exhibits a negative differential resistance phase under the application of electric current, a phenomenon known as current-controlled negative differential resistance CC-NDR. These two characteristics in NbO₂  promote it to be good candidate to develop many functional devices used for variety of applications such as electric switching and memory devices. However, despite the potentialities of NbO₂, the understanding of this material is still incomplete as literature reporting many research work on NbO₂ show contradictory or inconsistent information. One possible reason of this lack of consistency could emerge from the difficulty in the film fabrication of pure phase NbO₂ and the different means in synthesizing them.

In this thesis, we have fabricated thin films of NbO₂  by RF-mangentron sputtering technique on amorphous and crystalline substrates (glass and Si). The deposited films were always amorphous, and annealing treatment of the as-deposited films was necessary to achieve crystallinity. Annealing tests revealed that increasing both the annealing duration and the annealing temperature gives rise to better crystalline films. Optimized amorphous NbO₂  films had a resistivity ρ_amo ∼ 5 Ω.cm while the crystalline films had a ρ_cry ∼ 50 Ω.cm both which are less than the bulk crystalline NbO₂ ρ_bulk ∼ 10⁴ Ω.cm. Upon performing electronic studies on NbO₂, we witnessed CC-NDR with a hysteresis in the V(I) curves. We showed that hysteresis in CC-NDR is due to temperature inhomogeneity. Simultaneous electronic transport and Raman measurements show that CC-NDR is not associated to a phase transition (the phase is always insulating). Moreover, we showed that there is a similar temperature driven change in conductivity in both the amorphous and the crystalline samples, however, the amorphous sample is a better electronic and thermal conductor. Finally, we proved that the CC-NDR may be simply explained by the creation of carriers by temperature in a semiconductor, without the need for invoking more complicated transport mechanisms.

Conférences et séminaires

• GDR MEETICC - Ecole du GDR Matériaux, Etats ElecTroniques, Interactions et Couplages non-Conventionnels
  4-10 février 2018, Banyuls sur mer, France