The Physics and Engineering of Oxide Semiconductors for Large-Area CMOS

Lead Research Organisation: University of Cambridge
Department Name: Engineering

Abstract

Electronics and photonics has transformed everyday life over the last twenty years: the silicon microprocessor provides vast processing power in a device that can fit inside a pocket, the liquid crystal display allows us to see information on a high resolution display that can sit on the palm of our hand, and the optic fibre allows us to transmit data at high speeds over long distances. The result is that we are all essentially continuously connected to the internet, and this allows us to communicate with each other and access information instantly. The result has been a profound change in almost every aspect of life including working practices, shopping, healthcare, banking, transport and even relationships. However, whilst we are 'connected', the world of objects that are so much part of our everyday lives are not, and the next big transformation will be to connect these too. This is the vision of the 'Internet of Things'. In the words of Prime Minister David Cameron, 'I see the Internet of Things as a huge transformative development, a way of boosting productivity, of keeping us healthier, making transport more efficient, reducing energy needs, tackling climate change. We are on the brink of a new industrial revolution.'
Advances in technology are the driver for such industrial revolutions, and the Internet of Things needs sensors, rfID, power supplies, logic, displays, lighting and communications to be integrated together onto the everyday objects around us with a form factor that does not adversely affect the prime function of the object, whether that object is our car, our refridgerator, our clothes, our purse or our toothbrush.
This will require a new generation of electronics which can be produced transparently over large areas on almost any substrate, and which is flexible and robust. Such 'large-area electronics' on glass substrates based on amorphous silicon (a technology born in Dundee University in the 1970s) has already been critical for the development of flat panel displays. However, amorphous silicon is not optically transparent and has rather poor electronic properties (most nobably a low electron mobility). Amorphous ionic oxides have emeged as a replacement for amorphous silicon for display applications in recent years as it has superior electronic properties. In particular, amorphous indium gallium zinc oxide (a-IGZO) has been developed to such a point that it will shortly start to be used in commercial products. However, this complex material can only be made as a n-type and not a p-type semiconductor, and so complemetary logic cannot be realised with the result that power consumption is high. Also, it suffers from instabilities which limits its lifetime. As a result, this material is less well suited to the Internet of Things.
This project aims to develop a more simple n-type amorphous ionic oxide semiconductor with an improved stability over a-IGZO, and a complementary p-type amorphous ionic oxiide semiconductor. This will require detailed understanding of the physics of these materials, and in particular the electronic role of impurities. We will subject both the individual materials and devices made from these materials to a wide range of physical tests, including infrared spectroscopy, allowing us to study the device in its applied environment. This is critical as the performance of a thin film device is often dominated by its surfaces. This will enable us to develop both new materials and models for devices which are critical for the design and simulation of circuits and systems. This is critical if the technology is to be applied. We will demonstrate the validity of our materials, processes, models and their application by designing, simulating, fabircating and testing a four-bit rfID tag on a plastic substrate. The cost of producing these devices should end dup being similar to printing, allowing in-line manufacture with the rest of the object they are enabling in the UK.

Planned Impact

Aside from the academic beneficiaries, this project has the potential to benefit both UK and European industry as well as the UK Society, and each will be considered briefly.
The UK is home to a vibrant ecosystem of SMEs working in the large-area electronics field, from substrate suppliers such as DuPont Tejin Films, deposition equipment manufacturers, such as PlasmaQuest, simulation software producers such as Silvaco Data Systems though to systems integrators such as Pragmatic Printing. The UK Plastic Electronics Leadership Group has for some time worked effectively to nurture this cohesive industrial community in the UK. Advancing a high-performance and low-cost thin film materials systems is critical to the success of this community. Whilst organic semiconductors have fulfilled some of the needs of this community, there is a real oportunity for a complementary inorganic thin film material to open up new application areas. It is the challenge that creating all of the components (sensors, rfID, power supplies, logic, displays, lighting and communications) that are necessary to make a product 'electrinocally enabled' that has been a major hurdle to the realisation of the Internet of Things. The inorganic thin film metal oxides are showing real promise as a potential complmentary material system. However, we also recognise that simply providing the commuity with a material is not sufficient. Tools which allow circuits to be designed and therefore systems created are essential too for uptake, which is why this activity is embedded at the heart of this proposal. We believe that the outcome could be a real proposition for manufacture of Internet of Things-enabled objects , and we are sure that the innovation culture in the UK will be quick to pick up on the opportunities that emergy - particularly with the government support for the area that has been recently highlighted.
If we are successful in the first of these impact aims, then the societal benefits will naturally follow. These will include:
- the 'ageing population' with the development of 'smart clothes' that continuously monitor wellbeing;
- the e-commerce sector with embedded person identification in objects;
- connected communities with better monitoring of population behaviours and needs;
- improved transport with enhanced user information;
- households with less food waste through smart packaging and improved energy use through improved moniroting of behaviour and needs.
The potentially ubiquitous nature of this impact is why the Internet of THings is gaining the tag of being the 'New Industrial Revolution'.

Publications

10 25 50
 
Description 1) We have produced stable n-type amorphous oxide semiconductors based on zinc tin oxide and indium gallium zinc tin oxide. We have shown that the stability of these devices can be quantitatively controlled through the addition of impurities - most notably nitrogen. We have also produced a high quality p-type oxide semiconductor based on cuprous oxide and have investigated the physics of this material in detail.
2) We have investigated the physcis of the band structure of these materials and in particular the physical nature of defect states in real thin film transistor devices. We have applied a thermalisation energy-based analysis technique to quantitatively assess the degradation mechanisms. This is allowing us to see patterns in the behaviour of these materials with different stoichiometries and therefore to develop an understanding of the fundamnetal physical processes underlying bias stressing. This is enabling us to develop new metal oxide materials with significantly improved stability to gate bias stress, in particular through the incorporation of nitrogen which appears to eliminate the effect of the most unstable oxygen vacancy defects.
3) We have published device models for thin film tranistsors based on our understanding of the device physics.
4) We have investigated novel transistor device structures for testing and have integrated n- and p-type transistors into CMOS inverter circuits.
Exploitation Route Our findings are being taken forward by Pragmatic Printing Ltd. through our Knowledge Transfer Partnership.
Sectors Electronics

 
Description The work on thin film transistor physics and modelling has been used as a key element in the award of a Knowledge Transfer Partnership (KTP) project between my research group and Pragmatic Printing Ltd., who are an SME developing metal oxide-based logic. This KTP has had strong links between this EPSRC project and the KTP. In particular, we have incorporated materials that have been developed in the EPSRC project into Pragmatic Printing's devices for evaluation. The basic theory of the operation of these devices has also been transferred through the KTP. Collaboration has continued since the end of the KTP as well with ongoing close links between the research group and the company.
First Year Of Impact 2017
Sector Electronics
Impact Types Economic

 
Description Knowledge Transfer Partnership
Amount £101,539 (GBP)
Funding ID KTP010131 
Organisation Innovate UK 
Sector Public
Country United Kingdom
Start 05/2016 
End 04/2018
 
Title Research data supporting "Analysis of the Conduction Mechanism and Copper Vacancy Density in p-type Cu2O Thin Films" 
Description 1) Hall_asdeposit.csv, Hall_500.csv, Hall_600.csv and Hall_700.csv: These are the Hall measurement data including the Hall mobility and carrier density of as-deposited and annealed films (500, 600 and 700 ¡ÆC) in Table 1. This raw data was used for calculating trapped hole densities (i.e. ptrap(Hall) and ptrap(DOS)) in Fig. 2(a), a GLC coefficient in Fig. 2(b), the mobilities in Fig. 3 and the density of copper vacancies in Fig. 5. 2) Eu.csv: This is used for extracting the Urbach energy (Eu) in Table 1. 3) DOS.csv: This is the raw data (extracted subgab density of states) used to create Fig. 1. 4) SEM_asdeposit.tif, SEM_500.tif, SEM_600.tif and SEM_700.tif: These are SEM images of as-deposited and annealed films (500, 600 and 700 ¡ÆC) in Fig. 4(a) 5) XRD_asdeposit.uxd, XRD_500.uxd, XRD_600.uxd and XRD_700.uxd: These are the raw data of the XRD patterns of as-deposited and annealed films (500, 600 and 700 ¡ÆC). This data was used for estimating the grain size in Fig. 4(b). 6) Data for Figure 4c and 4d.csv: This is the raw data of Fig. 4(c) (i.e. extracted densities of free holes (pfree), trapped holes (ptrap) and total holes (ptotal), and the ratio of pfree to ptotal (beta_TLC) in the case of the as-deposited film) and of Fig. 4(d) (i.e. extracted beta_TLC of all samples). 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Research data supporting "Highly Stable Amorphous Zinc Tin Oxynitride Thin Film Transistors under Positive Bias Stress" 
Description Data for publication in raw form 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Research data supporting "The Origin of the High Off-State Current in p-Type Cu2O Thin Film Transistors" 
Description Orginal data files relating to understanding the The Origin of the High Off-State Current in p-Type Cu2O Thin Film Transistors 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Description School Visit (South London) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Gave a talk on 'Large-Area Electronics:Enabling the Internet of Things' at a Science Fair held at St Olave's Grammar School on 21 March 2018. This was attended by >300 Year 12 students from across the London Borough of Bromley.
Year(s) Of Engagement Activity 2018