Funding type: COFUND Marie Slodowska Curie Action

This project is under the Marie Skłodowska-Curie Actions (MSCA) program. There are no nationality conditions but the candidates must fulfill the MSCA mobility conditions, which means that she/he must not have stayed more than 1 year in France during the last 3 years immediately before the call deadline (31/05/2019).

Expected start date: 01/10/2020

Contacts:

Pr. Bertrand Vilquin
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Pr. Sharath Sriram, RMIT
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Websites:

• https://www.rmit.edu.au/about/our-locations-and-facilities/facilities/research-facilities/micronano-research-facility
• http://inl.cnrs.fr/en/

Collaborations/External partners :

Domain and scientific context :

After more than 40 years of continuous evolution, our computing systems are reaching their limits. Indeed, the architecture of Von-Neumann, on which our computers are based, physically dissociates the hearts of calculations from the memory. The sequential processing of information is thus confronted with a bottleneck, more commonly known as "Memory Bottleneck". One solution is to draw inspiration from the natural mathematical paradigms of the human brain, in which the data are massively parallel processed with high energy efficiency, realizing the hardware implementation of neuromorphic networks. Currently, we live in a digital and connected world that relies on the rapid transmission and processing of digital data. This is mainly the result of the development of the microelectronics industry, which is the basis of most modern technological devices. This field is currently facing scientific bottlenecks linked to metallic interconnections in microprocessors, preventing further progress in terms of speed and energy efficiency. Electronic/photonic/thermal convergence is currently considered to be one of the most promising solutions for quickly solving this problem. However, silicon, the basic material of the microelectronics industry, is inherently limited in terms of electro-optical properties. In this context, great interest is paid to the heterogeneous integration of new functional materials on silicon substrates. The relevance of this strategy is twofold: on the one hand, it can allow the use of naturally functional materials with electro-optical properties far superior to those of silicon but also with original properties such as changes in the crystallographic phase as well as thermo-optics. And secondly, maintaining silicon - in particular, Silicon On Oxide (SOI) - as a platform allows the integration of electronical, optical and thermal functionalities.

Keywords : functional materials; phase change material (PCM); infrared; electro-optic; neuromorphic network; artificial synapse

Objectives and scientific challenges:

This project aims at developing novel active optoelectronic concepts and integrated devices that exploit the unique switchable properties of using phase-change materials (PCM), in which we can controllably modify the optical properties value by an external signal, leading to control and modulate the infrared wave propagation [1]. PCM are materials whose refractive index can be dynamically modulated through modification of their phase (e.g. transition between two crystallographic phases or between amorphous phase to crystallize one). Two typical examples of PCM are VO₂ and GeSbTe (GST) [2,3]. These two materials show ultra-fast and reversible metal-insulator transition. An optoelectronic device design strategy will enable to fully exploit the unique properties of these materials while limiting their drawbacks. We will design and fabricate original tunable infrared electro-optic devices integrated on silicon for the development, in a first step, of novel ultra-efficient approaches for spiking neural networks. Spiking neural networks (SNNs) offer an event-driven and more-biologically-realistic alternative to standard artificial neural networks based on analog information processing. This can potentially enable energy-efficient hardware implementations of neuromorphic systems that emulate the functional units of the brain; namely, neurons and synapses. Recent demonstrations of ultrafast photonic computing devices based on phase-change materials show promise for addressing the limitations of electrically driven neuromorphic systems. However, scaling these stand-alone computing devices to a parallel in-memory computing primitive is a challenge. In this work, we use the optical properties of the PCMs VO₂ and Ge₂Sb₂Te₅ to propose a photonic SNN computing primitive, comprising a nonvolatile synaptic array integrated neurons. In a second step, the fabrication and characterization of the electro-optic devices will give advices for the developpemnt of original thermotronic devices as like thermal transistor and diode. Thermotronics is concerned with information processing, not by means of electric currents, but of radiative heat flows. The temperature of the various components is the means of control and measurement in the same way as the voltage in electronics. In order to perform information processing, it is necessary to develop equivalents of non-linear electronics such as the transistor or the diode. In thermotronics, it was theoretically shown that this would be possible by coupling the non-linearity of the thermal emission properties of a material, the oxide of vanadium and near-field heat transfers to amplify heat fluxes [4-5]. The purpose of this project is to demonstrate the first bricks of thermotronics thanks. This new area of ​​research will bring a new generation of auto-intelligent thermal sensors and would drastically reduce the energy consumption of current devices by eliminating microelectronic circuits. The final goal is here to propose a MEMS logic circuit making it possible to implement thermal calculation functions in the same way as microelectronics processors. The principal goal of the project is to estimate the possibility to use PCMs in electro-optic neuromorphic network integrated on SOI. A future development would be the fabrication of thermal transistor and diode.

There are no man-made optical artificial synapses in the microelectronics industry to date. The aim of this thesis is to achieve the fabrication of such a device from transferable materials and processes in the semiconductor industry.

• Theoretical, optical and thermal models have to be developped and adapted to PCMs.
• Demonstrate and predict the behavior of neuromorphic network, thermal memories, transistors and diodes.
• Demonstrate the possibility of integrating phase change materials on silicon with respect to the CMOS processes, in particular at low thermal budget.

Expected original contributions :

• Development and demonstration of artificial synapses based on Phase Change Materials on SOI using CMOS compatible methods.
• Identification of the optimal deposition conditions (i.e. thickness, stoichiometry, temperature, etc.) of the PCM for the targeted applications.
• Realization of MEMS for thermal modulation evaluation.
• Realization of integrated synaptic matrices of "Crossbar" type.
• Prototyping and testing the learning capabilities of the synaptic network.

Research program and methodology :

The thesis aims to develop and integrate very thin layers of functional oxides to develop a new class of nanodevices, transferable in an industrial environment. To do this, the thesis will take place between INL and RMIT. The work will be based on the following tasks:

T1 – Integration on silicon of PCM (RMIT & ECL). This task will be devoted to the development of phase change materials, VO₂ and GST in the form of thin films and their integration on SOI. These materials will be used for their optical non-linearities as a function of their thermal and electrical excitations. It will be necessary to fully understand the characteristics of these thin films in order to offer a good design of MEMS systems. The objective will push the project towards a more practical design in which the change of phase in PCM is controlled electrically. We will first fabricate crossbar arrays in which the PCM is sandwiched between two orthogonal transparent conducting oxide stripes. This will enable us to selectively switch synapse dots of PCM as a first demonstration of electrically-controlled reconfiguration of the phase change.

T2 - Simulation and design of devices (ECL). Once the optical properties of materials have been determined, simulations of the modification of the optical properties and thermal radiation will make it possible to obtain the characteristics of the proposed devices. Multiphysics and optical simulations at INL will optimize the geometry of new M(O)EMS components (basic bricks and networks).

T3 - Manufacturing and characterization of the devices (RMIT & ECL). The means of the laboratories involved in micro and nano manufacturing will ensure the realization of optical neuromorphic networks in silicon and novel thermotropic devices. This task will lead to the experimental proof of neuromorphic learning and thermal logic gates.

Scientific supervision:

• Description of the supervision committee :

• Integration inside the laboratories (percentage of working time inside these laboratories) : 67% at INL, 33% at RMIT

PhD funding : Co-Fund Marie Sladowska Curie Action (MSCA) ECL/RMIT (ECLAUsion program)

Profile of the candidate :

S/he should work towards his/her Masters/honours or Engineering degree in a field apposite to one of these areas: Physics ; Electronic Engineering; Material Science and Engineering. An experience in clean-room fabrication, material deposition or electronic characterization will be strongly appreciated.

Objectives for the valorization of the research work:

The results obtained can be valorized through communications in the field of materials science (Advanced Materials, ACS Nano ...), and nano-electronic devices (IEEE, AIP, Elsevier ...). Furthermore, dissemination of the work at international conferences dedicated to materials and devices for electronic / photonic / thermal engineering will also be carried out (eMRS, IEEE-ESSDERC, IEEE-IMW, ...). The targeted valorisation objectives are from 2 to 5 peer-reviewed international journals and at least 3 papers in international conferences.

Skills that will be developed during the PhD :

The candidate will be trained in the fabrication of materials in the form of thin layers. He / She will also receive training in clean room steps for the realization of simple electrodes and devices. His working environment will allow him to acquire knowledge in electrical characterization of materials (current - voltage, capacitance - voltage measurements). He / she will be integrated into the international nanoelectronics and photonics communities, the national functional oxide (OXYFUN) and international communities.

Professional opportunities after the PhD:

The training and formation received during the thesis will allow the PhD student to apply for an academic position in all the laboratories involved in this field or to seek a private job in research and development departments of large companies that take advantage of this type of materials (Thales , STM, IBM ...).

Bibliographic references about the PhD topic :

1. Wuttig et al. Nat. Photon. 11, 465 (2017).
2. Z. Yang et al. Annu. Rev. Mater. Res. 41, 337 (2011).
3. Raoux, Annu. Rev. Mater. Res. 39, 25 (2009).
4. Ben-Abdallah et al, Appl. Phys. Lett. 103, 19 (2013).
5. Ben-Abdallah et al, Phys. Rev. Lett. 112, 4 (2014).