Selector for high-density memory
These last years, the demand for high-density and fast-access non-volatile memories has been steadily increasing. However, there is no existing product satisfying this need today, between the fast but volatile DRAM technology and the high-density but slow-access 3D Flash technology. Hence, the 'Storage Class Memory' (SCM) has emerged to fill this speed-density gap in the memory hierarchy, enabling the development of future computing and/or storage systems by providing a fast and cheap memory alternative.
Several emerging memory concepts, like resistive random-access-memory (RRAM), phase-change RAM (PCRAM), or Magnetic RAM (MRAM) concepts hold the promise of showing both speed and density specifications of SCM. On the other hand, in today's memory array cross-point architecture, the main factor limiting high-density integration is the "sneak path" current or capacitive issue during read and write programming of the memory device. To overcome this issue a two-terminal access device (selector) in series with the memory element is required to enable addressing individual memory cells in an array without disturbing the others. This select device must be scalable, have high rectification ability, following the operation mode of the memory cell (typically bipolar) and allow for high drive current densities, required to switch the memory element.
To fulfill these requirements, many selector concepts are currently under investigation. Between them, has recently advanced in the development of the -based Ovonic Threshold Switch (OTS) technology. For high enough voltages, OTS technology relies on the appearance of a negative differential resistance (NDR) branch in the selector I-V characteristic to grand high driving capability and large non-linearity. These performances come at the expense of relatively high voltage and voltage spiking across the memory element. Although excellent potential was demonstrated for this technology [1,2], the switch mechanism is not well understood. Alternative selector concept, generally based on the rectifying behavior of contact are also investigated in view of close integration with MRAM technology. Indeed such selectors, characterized by a diode-like behavior, generally avoids OTS voltage issue which is critical for a low voltage, low power technology such MRAM. has recently started a broad research activity on the topic exploring Mixed Ionic Electronic Conductors (MIEC), Metal-Semiconductor-Metal diode, novel vertical IGZO-based diode.
The purpose of this PhD is to investigate thoroughly the electrical performances and reliability properties of the above-mentioned selector devices by studying the roles of composition, material variations, and integration processes, in order to identify knobs for device improvement. To fulfill these goals state-of-the art measurement tools and data analysis software will be adopted. In addition, a large part of the PhD work will be to develop a physics-based switching model, which should help complement compact, parametric models used at circuit simulation level, in order to enable assessment of the array performance. Overall, an important output of the PhD will be to establish the intrinsic limitations and potentials of the technology.
The PhD work will be carried out in the frame of the Industrial Affiliation Program (IIAP), thus in close relation with industrial partners and within a team consisting of experts in various fields (processing, integration, physical characterization, modeling, reliability...).
References:  B. et al., VLSI 2017,  S. et al., IEDM 2017.
Required background: materials and device physics, electrical engineering.
Type of work: Experimental work (lab fabrication [10%] and electrical characterization) [40%]), and physics-based modeling [~30%], up-to-date literature on selectors [~20%]
Promoter: G. Groeseneken, J. Van Houdt
Daily advisor: A. Fantini, D. Garbin
Required background: materials and device physics, electrical engineering
Type of work: Experimental work (lab fabrication [10%] and electrical characterization [40%]), physics-based modeling [~30%], up-to-date literature on selectors [~20%]
Supervisor: Guido Groeseneken, Jan Van Houdt
Daily advisor: Andrea Fantini
The reference code for this position is 1812-15. Mention this reference code on your application form.