La chimie et la structure électronique d’interface dans les capacitances ferroélectriques,...

Updated: 4 months ago
Job Type: FullTime
Deadline: 13 May 2021

The deployment of 5G technology has raised serious technological issues of energy consumption, reception quality and call failure rate. All three can be minimized by continuously adjusting the antenna impedance as close as possible to 50 ohms. An impedance matching scheme is also necessary for Near Field Communications (NFC) technology to allow reliable and energy efficient contactless communications (e.g. for keyless cars, card or phone payments, Internet of Things…) whatever the ambiant transmission conditions. To answer these 5G and NFC needs, a voltage controllable impedance matching circuit with a highly tunable capacitance (a.k.a. varactors) is required. Working at higher frequencies or lower voltage also requires the reduction of the FE thickness. Unfortunately, two interface-related phenomena, the FE “dead layer” and leakage current, impede this evolution. Recent encouraging ab initio calculations showed the importance of the chemical bonding, polar discontinuity and distortion mismatch at electrode/FE perovskite interfaces for polarization stabilization and Schottky barrier height (SBH) adjustment.

In order to maintain low dielectric losses at microwave frequencies and retain a high dielectric permittivity, FE materials are usually used in the paraelectric phase, i.e. above their Curie temperature, TC . The perovskite solid solution Ba1-x Srx TiO3 (BST) is the most widely used FE in current varactors because of its excellent tunability/losses compromise associated with an easily adjustable TC (via the Ba/Sr ratio), leading to far better quality factors than competing technologies. STMicroelectronics (ST) uses metal/BST/metal parallel plate capacitors to continuously match antenna impedance.

We propose to incorporate a perovskite Interface Control Layer (ICL) consisting of a few nm of conductive or dielectric films between Pt electrodes and BST in STMicroelectronics industrial tunable capacitors.  The isomorphism of the perovskite oxide structure allows to chemically tune electronic and structural properties. Rumpling, polar discontinuity, interfacial B-site cation environment asymmetry, BO6 octahedral rotations and reduction of interfacial defects at BST interfaces are all potential levers to enhance interface polarizability and Schottky barrier height. 

Within the framework of the ANR Be-Polar project bringing together four laboratories (SPEC, Greman-Tours, ST Microelectronics – Tours and Cemes – Toulouse), a systematic interface engineering using Combinatorial Pulsed Laser Deposition (CPLD) will chemically modulate electrode/(Ba,Sr)TiO3 interfaces of industrial capacitors.

The thesis project will focus on the characterization and understanding of the chemical and electronic structure of the ICL between the electrode and the BST layer.

A wide range of thicknesses and stoichiometry is available thanks to the highly versatile CPLD system.

The student will perform high resolution, laboratory based XPS/UPS studies of top and bottom interfaces of samples defined and synthesized by GREMAN and ST-Tours laboratories.

XPS will give access to band line-up while ultra-violet photoemission electron microscopy (PEEM) will map the work function and hence the interface dipole formation of BST on ICL as a function of ICL composition and thickness.

In order to study the correlation between interface chemistry and leakage current, the Schottky barrier height will be measured as a function of applied bias in operando HAXPES experiments using synchrotron radiation in both remanence and in saturation conditions, corresponding to the full operational voltage range of future varactors.

The VO concentration depth profile will be studied using XPS profiling of the valence state signature of reduced B-type cations. Similar studies using operando HAXPES will provide information on changes in the VO depth profile as a function of applied bias and possible field-induced ionic migration. 

Results will be compared with first-principles calculations carried out by the CEMES project partner.

The resulting industrial prototype varactors with the optimized interfaces will be tested against 5G and NFC specifications.

The student will be based in CEA Saclay but a high level of mobility is required with working visits to partner laboratories (GREMAN-Tours, ST Tours and CEMES-Toulouse) and participation in synchrotron radiation beamtimes.

Funding category: Financement public/privé

Projet ANR financement acquis

PHD title: Doctorat de physique

PHD Country: France

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