Resistive switches (RRAM) and memristive behaviour in silicon-rich silicon oxides

Lead Research Organisation: University College London
Department Name: Electronic and Electrical Engineering

Abstract

The main goal of this project is to develop a fundamental understanding and applications of resistive switching in silicon-rich oxide. This may lead to a breakthrough in low-cost on-chip integration of Resistive Random Access Memory (RRAM) devices with Si microelectronics. To achieve that we will carry out detailed experimental studies of switching; develop a physical switching model; apply this model to design and fabricate demonstrator devices; characterise the devices, and develop circuit-level models for systems incorporating Si RRAM and hence extend the capabilities of Si microelectronics into new domains and applications.

RRAM devices are components whose electrical resistance can be varied by applying an appropriate voltage. They are promising candidates for next generation electronic memories, offering a number of significant advantages over conventional Flash memory, including: very high packing density; fast switching; low energy switching; 3D integration to further increase memory capacity; ease of processing. Existing RRAM technologies are primarily based on metal oxide materials. However, Si- based devices have a number of advantages, including ease of integration with silicon CMOS processing technology, along with the possibility to tailor their electrical properties by varying programming voltage pulses.

RRAM devices have potential applications beyond memory: if the device resistance can be continuously varied they may behave in a similar way to neurons, and may therefore be used in novel neural networks or other processing architectures. Also, as resistive switching shares many of the features of oxide failure in CMOS devices, the results from a study of RRAM will yield valuable information that may help reduce device failure, or even recovering damaged devices.

We have recently developed a Si/SiO2 RRAM. Unlike competing technologies, it does not rely on the diffusion of metal ions, can be fabricated only from Si and SiO2, and operates in ambient conditions. Resistance contrast is up to 1,000,000, switching time <90ns, and switching energy 1pJ/bit or lower. Scanning Tunnelling Microscopy suggests individual switching elements as small as 10nm. Devices can be cycled thousands of times and can be operated in either unipolar or bipolar modes, with different characteristics in each: in the former, binary switching between discrete levels can be achieved, while in the latter we are able to continuously vary the device resistance, opening up the possibility of analogue devices such as memristors.

Our devices are an alternative to existing metal oxide-based devices. The Si/SiO2 system is the building block of Si CMOS technology - our devices require no other material. We have found that the externally-set current compliance required for reliable resistive switching in metal oxide systems is not necessary in SiOx devices - asymmetric doping of the structure produces intrinsic self-limiting. In addition, the high degree of nonlinearity inherent in our semiconductor-based RRAM devices mitigates the problem of parasitic leakage currents in arrays of RRAM devices.

Our project will go further than experimental studies of Si/SiO2 RRAM devices. We will also develop comprehensive theoretical models for the resistance switching process, and circuit-level models to investigate the application of our RRAM devices in real systems. Our approach is novel and unique in that it goes all the way from the atomistic modelling and electrical characterization of materials and fundamental electronic and ionic processes involved in resistive switching, through the simulation and fabrication of experimental devices to their optimisation and potential implementation in technology. This can only be achieved via synergy of expertise available at UCL and Glasgow.

Planned Impact

This work has the potential to have significant impact both on the academic community and beyond, dealing as it does with fundamental studies of the physical processes underlying resistive switching and their application to novel silicon microelectronic devices.

Within academia the main beneficiaries will be the emerging Resistive Switching community, the silicon microelectronics community, and the memristor community. The UK is active in each of these (as testified in the latter case by recent EPSRC funding - see the Academic Beneficiaries section). In many of these areas the UK plays a leading role, and our work is likely to stimulate further academic interest both within the UK and internationally.

Those who will most directly benefit beyond academia include: the silicon microelectronics and CMOS industries; those engaged in More than Moore and Beyond Moore activities; the neural networks community; device modellers; solid-state physicists engaged in defect studies; chip manufacturers; memory manufacturers; chip designers, and the space electronics community, thanks to the radiation tolerance of RRAM devices. We are already engaging with industry in our early work, having patented our technology and brought three project partners on board. The partners will provide valuable advice on commercialisation and maximising impact, and will also be closely involved with the scientific aspects of the project.

We anticipate a timescale of 5-10 years for the commercialisation of Si RRAM. Current work on RRAM and novel non-volatile memory concentrates on commercialising transition metal oxide-based technologies as Flash replacements or embedded memory within 18-24 months. Si RRAM is at an early stage in this process, though it may benefit from current transition metal oxide RRAM work. It is not clear at this point what will be the main applications of our early stage technology Si RRAM, though there are several promising options, and our main aim is to lay the ground work in understanding the mechanisms of Si RRAM, simulating potential architectures, and applications and assessing the reliability and variability issues of potential devices. Flash memory and other embedded memories are likely areas of impact, as is device failure and recovery, but one further area is communities concerned with neuromorphic systems and neural networks not requiring high tolerances. Here the short-term impact (3-6 years) will predominantly be academic, but longer-term benefits (10-20 years) are expected as the technology matures.

Publications

10 25 50
 
Description We have developed a new silicon-based resistive switching technology that could replace existing flash memories. In the process we have begun to understand the physics of the switching process and have fabricated four generations of devices during the project so far. We have also been able to extend the capabilities of the technology to include neuromorphic (brain-inspired) computation and light-triggered operation.
Exploitation Route We are already looking at commercialising the work, and are in discussion with several industrial partners. We have formed a spin-out company to act as a licensing vehicle to help us with this.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Healthcare,Transport

 
Description Covered in trade press (EE Times) 2016 and 2017 Presented at 2016 SET for Britain awards
First Year Of Impact 2016
Sector Electronics
Impact Types Economic

 
Description Leverhulme Research Grant
Amount £331,470 (GBP)
Funding ID RPG-2016-135 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 09/2016 
End 08/2019
 
Description Leverhulme Visiting Professorship
Amount £29,436 (GBP)
Funding ID VP1-2016-019 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 05/2017 
End 10/2018
 
Description A*STAR 
Organisation Agency for Science, Technology and Research (A*STAR)
Department Institute Of Materials Research And Engineering
Country Singapore 
Sector Academic/University 
PI Contribution Supply of samples for microscopy; exhange of student
Collaborator Contribution Detailed electron microscopy
Impact Several joint papers - Advanced Materials (2016), Scientific Reports (2017), Faraday Discussions (2018). Conference presentations at IPFA 2018 (Singapore) and Faraday Discussion (Aachen 2018).
Start Year 2014
 
Description Forschungszentrum Jülich 
Organisation Julich Research Centre
Country Germany 
Sector Public 
PI Contribution Supplying samples for study by transmission electron microscopy
Collaborator Contribution Detailed electron microscopy studies of resistance switching samples
Impact TBC
Start Year 2014
 
Description Micron 
Organisation Micron Semiconductor
Country United Kingdom 
Sector Private 
PI Contribution Communication about material parameters and device design/application
Collaborator Contribution Advice on device design and potential applications
Impact None yet
Start Year 2013
 
Description Sematech 
Organisation University at Albany
Department International SEMATECH
Country United States 
Sector Charity/Non Profit 
PI Contribution Material specification and design for ReRAM devices
Collaborator Contribution Fabrication of devices and design/application advice
Impact Several sample wafers
Start Year 2013
 
Description Southampton microfabrication facility 
Organisation University of Southampton
Department Primary Care and Population Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution Designs for resistance switching devices and arrays
Collaborator Contribution Fabrication of samples
Impact TBC
Start Year 2013
 
Title A SWITCHING RESISTOR AND METHOD OF MAKING SUCH A DEVICE 
Description Control of resistance switching through tailoring the microstructure of the electrode/oxide interface. 
IP Reference GB1705210.1 
Protection Patent application published
Year Protection Granted 2017
Licensed No
Impact Too early. Being exploted via spin-out.
 
Title A light-activated switching resistor, an optical sensor incorporating a light-activated switching resistor, and methods of using such devices. 
Description Control of resistance switching using light in addition to electrical stimuli. 
IP Reference p111931gb 
Protection Patent application published
Year Protection Granted 2017
Licensed No
Impact Too early. Being exploited by spin-out.
 
Title Oxide memory resistor including semiconductor nanoparticles 
Description This invention relates to memory resistors, arrays of memory resistors and a method of making memory resistors. In particular, this invention relates to memory resistors having an on state and an off state, comprising: (a) a first electrode; (b) a second electrode; (c) a dielectric layer disposed between the first and second electrodes; wherein the dielectric layer comprises nanoparticles of semiconductor material, and wherein in the on state nanoparticles form at least one conductive filament encapsulated by the dielectric layer, thereby providing a conductive pathway between the first electrode and the second electrode. 
IP Reference WO2013005040 
Protection Patent application published
Year Protection Granted 2012
Licensed No
Impact Setting up spin-out company, so we are in discussion with a number of commercial partners and investors
 
Company Name Intrinsic Semiconductor Technologies 
Description Intrinsic is a UCL spinout company, established to commercialise the novel memristive RRAM devices developed by Prof Tony Kenyon and Dr. Adnan Mehonic in UCL Electronic and Electrical Engineering. The research that led to the demonstration of the RRAM devices was supported by EPSRC, UCL Business Proof of Concept funding. The team are also supported by UCL Technology Fund as recipients of funding through their Proof of Concept early stage investment. 
Year Established 2017 
Impact In the process of attracting seed-round investment.
Website https://www.intrinsicst.com
 
Description 6th form work experience 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact week-long visits of 6th form students to experience research in my research group. Working on microscopy of memristors. Over 3 visits, around 15 pupils attended. Some wrote blogs describing their experience; most wrote reports to their schools. The school reported more interest from students in applying for STEM degree courses, and made furtehr requests for pupils to come along to UCL or Faculty open days.
Year(s) Of Engagement Activity 2013,2014,2015
 
Description Inaugural lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact I gave my professorial inaugural lecture in Nov 2016, which was attended by a mixed audience, from school pupils to members of the public, along with university students and colleagues. Attendance was around 110.
Year(s) Of Engagement Activity 2016
 
Description Interviews for EE Times 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Several interviews with EE Times about memristor research
Year(s) Of Engagement Activity 2015,2016
 
Description SET for Britain awards 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Postdoc presented a poster at the 2015 and 2016 SET for Britain awards at the House of Commons
Year(s) Of Engagement Activity 2015,2016