Structural dynamics of amorphous functional oxides - the role of morphology and electrical stress

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

Abstract

Thin oxide films are critical components in a very wide range of electronic devices, including CMOS transistors in microprocessors and memory, piezoelectric and thermoelectric devices and electroluminescent devices. In most cases we assume that the oxide itself is stable under the levels of electrical stress encountered during normal device operation, and a great deal of work has gone into growing extremely high quality films. Nevertheless, recent developments in devices and materials have led to the growing use of amorphous and polycrystalline sub-stoichiometric oxide thin films (SSOTFs). These materials are fundamentally different to their stoichiometric and crystalline cousins - a fact that can have very important consequences for their use in electronic devices - but it is usually assumed that they behave in the same way. It is increasingly clear that this assumption is incorrect.

Recent studies, some performed by us, have demonstrated that amorphous sub-stoichiometric oxides are surprisingly dynamic under device-level electrical stress. In the case of silicon oxide, for example, we have shown that electrical stress drives the segregation of the oxide into regions with varying oxygen deficiency, and that such changes can be precursors to major changes in the electrical properties of the material. Our initial results suggest that oxide microstructure determines the ease with which oxygen can segregate, and we have seen, in extreme cases, emission of oxygen from the thin films. These changes can be permanent or they can be reversible, enabling cycling between two or more resistance states. Ultimately, such large-scale changes can lead to device failure. Consequently, by understanding how to control their dynamics we can both understand the early stages of oxide failure, and develop exciting new technologies that exploit the dynamic nature of functional oxides.

In this study we propose to investigate these changes using a combination of high resolution experimental characterisation and atomistic modelling of oxygen movement. Studying sub-stoichiometric amorphous oxide thin films is a considerable challenge, both for experiment and for modelling, which is partly why these materials are poorly understood. We will rely on close interaction between experiment and theory to develop, in an iterative process, new models for the structure of substoichiometric amorphous oxides of varying morphology, and their dynamic response to electrical stress. These models will shed light on the physical processes governing electrical changes, and we will use them to generate a set of design rules for material and device optimisation.

We have chosen a representative set of materials to study, each of which has important applications in microelectronics. We will grow the materials in-house, giving us control over their composition and structure and enabling rapid feedback from characterisation and modelling. The majority of our characterisation will also be performed at UCL, but we have long-standing and fruitful collaborations with two leading Transmission Electron Microscopy centres - Forschungszentrum Jülich and the Institute of Materials Research and Engineering in Singapore - which will give us access additional world-leading microscopy techniques to study these challenging materials. Our close collaboration with other leading research and development institutions, including our industrial partners, gives us access to further state-of-the-art facilities and industrially relevant samples.

Planned Impact

Amorphous oxides are of increasing importance across a range of electronics applications, several of which have multi-billion dollar markets (the novel non-volatile memories market is predicted to be $2 billion in 2018 with a Compound Annual Growth Rate of 46%, and it is estimated that 3 trillion Multilayer Ceramic Capacitors /year based on oxides will be produced by 2020). Under normal operating conditions, many of the devices that are so critical to these technologies rely on the application of electric fields strong enough to drive significant changes in the structure of the oxide films, yet not sufficiently strong to cause them to fail catastrophically. However, our understanding of the mechanisms driving such structural changes is patchy, being largely based on studies in crystalline materials or thicker films, and our ability to either mitigate these changes or exploit them in novel technologies is correspondingly limited.

The stakeholders of this research include the academic and industrial semiconductor microelectronics communities, and more broadly those working in solid state materials science. While this is in many ways a fundamental study with a timescale of 5 to 8 years to commercialisation, there are a number of shorter-term impacts beyond the immediate academic benefits, which we hope to begin to realise within the lifetime of the project. The first of these will be the application of our new models to understanding soft and hard dielectric breakdown in transistors. In the medium term, emerging non-volatile resistive RAM (RRAM) memory technologies stand to benefit. This is a rapidly growing industry sector, with several products already in the marketplace from companies such as Panasonic, Adesto and Micron. A concern that currently limits the full exploitation of the potential of RRAM is variable device reliability and endurance. A better understanding of oxygen dynamics in these devices will therefore translate into real commercial benefits. Similarly, our models can be applied to optimise the performance of capacitors that rely on amorphous oxide dielectrics; this can be expected to have the potential for major benefits in a large industrial sector. In the longer term (5 years +), our models and techniques will help in the development of emerging neuromorphic technologies based on amorphous oxides.

Putting our work into the context of EPSRC's four Research Outcomes, our work will contribute most directly to Connected Nation and Productive Nation though the research capability of advanced materials research to drive new processes, products and sustainable solutions, and ambition C3: delivering intelligent technologies and systems.

Principal routes to commercial impact will be via IP generation, exploitation via licensing opportunities, and opportunities for start-up companies in areas of disruptive advances. This approach ensures UK impact even in areas in which the UK does not have a strong manufacturing presence - existing highly successful examples of such fab-less models include ARM and Imagination Technologies. In maximising the impact of our work on the industrial community we will rely on advice from our project partners, in particular the Aerospace Corporation and Imec, and from UCL Business, who are currently supporting the commercialisation of our amorphous oxide-based ReRAM technology via a spin-out company.

Publications

10 25 50
 
Title Beyond Sight Within Grasp (Red, Yellow and Blue) 
Description 'Beyond Sight Within Grasp' is a collaborative project developed by Tony Kenyon, Professor of Nanoelectronic & Nanophotonic Materials at UCL, and the artist David Rickard for 'Trellis Public Art'. Nanotechnology is a relatively new field of research that was first established in the 1980's following advances in microscope technologies. With one nanometre equal to one billionth of a metre, approximately 1/100,000 the width of a human hair, this remarkably small scale is considerably smaller than the wavelength of visible light. Therefore, things at a nanoscale are literally impossible to see with any optical microscope. Instead scientists build up images using a variety of tools such as the Atomic Force Microscopes (AFM) which measure movements in a very fine tip that gently hovers above, touches or taps the sample surface. Visualising matter at nanoscale is much like seeing in the dark, as we are all blind at this scale. For Trellis Public Art we worked in collaboration with London Vision East to engage with the local visually impaired community to create a new art work which considers our relationship to nanotechnology, whilst inverting the visual dominance typically found within public art by engaging touch as a primary mode of sensory perception. 
Type Of Art Artwork 
Year Produced 2019 
Impact Over the course of the week that the pieces were exhibited, several hundred members of the public engaged with the works - by viewing and touching them, and questioning the curators. Sine then, we have started to develop works to form part of a more permanent exhibition on the UCL campus. 
URL https://www.youtube.com/watch?v=tnA2rxKZErE
 
Description We have established that the microstructure of oxides is a key determinant of the electrical characteristics of devices during resistance switching. Other contributing factors, such as stoichiometry, have much less effect.
Exploitation Route We are commercialising this through our spin-out company, Intrinsic Semiconductor Technologies.
Sectors Digital/Communication/Information Technologies (including Software),Electronics

 
Description Our experimental work has contributed to new patent filings, which are being exploited by our spin-out company, Intrinsic Semiconductor Technologies.
First Year Of Impact 2017
Sector Electronics
Impact Types Economic

 
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
 
Title A LIGHT-ACTIVATED SWITCHING RESISTOR, AN OPTICAL SENSOR INCORPORATING A LIGHT-ACTIVATED SWITCHING RESISTOR, AND METHODS OF USING SUCH DEVICES 
Description A switching resistor comprises a dielectric layer disposed between a first electrode layer and a second electrode layer, the switching resistor having a high resistance state and a low resistance state. The switching resistor is responsive to a voltage bias, applied between the first electrode layer and the second electrode layer, wherein the voltage bias exceeds a threshold to switch from the high resistance state to the low resistance state. The switching resistor is sensitive to photo-illumination to reduce said threshold. 
IP Reference WO2019016539 
Protection Patent granted
Year Protection Granted 2019
Licensed No
Impact This patent is being commercialised by our spin-out company, Intrinsic Semiconductor Technologies.
 
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 A switching resistor comprises a dielectric layer (e.g silica SiO2) disposed between a first electrode layer (e.g Silicon) and a second electrode layer (e.g ITO), the switching resistor having a high resistance state and a low resistance state. The switching resistor is responsive to a voltage bias (24 figure 2) exceeding a threshold which is applied between the electrodes, such that the bias enables a switch from a high to a low resistance state. Crucially, the switching resistor is sensitive to photo illumination (light irradiation) of the device which reduces the threshold. The first electrode may be a p-type silicon substrate absorbing photo generated free electron (free carriers) which create Frenkel pairs in the adjacent dielectric, which may be silicon oxide (SiOx). The Frenkel pair represent an oxygen vacancy and oxygen interstitial ion, the oxygen vacancy creating a conductive filament ion the dielectric layer. The second electrode is preferably comprised of Indium Tin Oxide ITO. The photo illumination is within the wavelength 300nm to 1500nm. The silica dielectric and ITO are transparent to light. The reduction in threshold due to light (photo) illumination varies from 0.1 to 0.5 V. The first electrode may comprise multiple strips. The switching resistor may be incorporated into an optical sensor. A method of operation is also included. 
IP Reference GB2564844 
Protection Patent granted
Year Protection Granted 2019
Licensed No
Impact This patent is being commercialised by our spin-out company, Intrinsic Semiconductor Technologies.
 
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.
 
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