Oxide semiconductors - where do they fit?
Changing requirement landscape
Several major trends have been driving technological innovation in the display industry since its early days. These trends include image quality, screen size, portability and form factor. These have determined the direction of technological innovation and shaped entire value chains. The trends have in many cases resulted in a step change in technology, altering the value chain. This has, in turn, pushed many out of business while opening the door to others. The winners have been those who had bet on the right trend at the right time.
While these trends still remain strong undercurrents, new drivers are being introduced that will play a more prominent role in shaping the industry going forward. These new drivers will open up new frontiers, both on the technology and the market side. Indeed, they enable displays to both expand their existing markets and to diversify into new spaces.
These major drivers that are set to change the display landscape and technology include product differentiation, flexibility, 3D, transparency, system-on-panel, power savings, etc. As before, these trends have the potential to result in radical changes in technology, which would create opportunities and risks to all those exposed to the value chain of the display industry.
We will assess how these market trends will affect the choice of backplane technology and which backplane technology is likely to emerge as the winner.
Backplane technologies must adopt too
Interestingly, there are already many different backplane technologies that are mature and available, or are fast emerging. These include amorphous silicon, nanocrystalline silicon, low-temperature poly-silicon, solution-processed or evaporated organic semiconductor and various metal oxide thin film transistor technologies. Add to this multiplicity of solutions a range of emerging nano-systems (e.g., various nanowires, graphene, carbon nanotubes) and you will find a decision-making nightmare.
This is because each thin film transistor technology offers a different set of characteristics, suitable for different needs. And yet none offers a one-size-fits-all-solution for all needs. This suggests that, at least initially, many different technologies will co-exist, each rising to satisfy a fragment of the emerging spectrum of needs and thus each occupying a different niche.
The table below outlines key parameters of several thin film deposition techniques. It assesses them on parameters such as deposition technique, mobility, spatial uniformity and temporal stability. These attributes will determine how each technology will ultimately fit within the changing display industry.
Table 1. Key parameters of thin-film deposition techniques
Source: IDTechEx report Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts
Amorphous silicon backplane is a winner when it comes to large-sized LCDs. It is amorphous therefore spatial uniformity is not an issue. The industry, as a whole, also has significant experience in using PECVDs for large-area deposition. The synergy with the a-Si photovoltaic technology also helps. The material can also be made thin and can also be transferred off glass and onto flexible substrate, introducing flexibility. The technology however fails when it comes to 3D, higher on-board processing power, and large-sized OLEDs. The limiting factor in all these aspects is the limited field-effect mobility.
Polycrystalline silicon is a winner in many small size displays. Its position is unchallenged in small- to medium-sized OLED displays. It is also the best in offering 3D, CMOS capability and high on-board processing power. This is because it has the highest mobility. It however falls short on increasing display size and increasing flexibility. In the former case, the limiting factor is partially inherent to the presence of grain boundaries, which create spatial non-uniformity. It is also partially because the economies of scale are not there over large-area screens. In the latter case, it fails because the annealing temperature can be high, making it incompatible with plastic substrates. This is despite the fact that its higher processing would enable the integration of the gate drivers on-board. The flexibility picture will change if lasing techniques (either sequential lateral solidification or excimer laser annealing) are developed so that only the surface will absorb the heat while the underlying substrate remains at near room temperatures.
Organic thin film transistors are strong on flexibility, but fail on other parameters such as 3D, resolution, size, etc. We note that they perform better on size when it comes to LCDs because spatial uniformity requirements are not as stringent. We maintain this assessment despite the fact the ultra-large size printing was a major driver for investment in organics. Current technological status and recent lab-to-fab transfer experiences suggest that reproducibility and spatial uniformity are critical issues. Plastic Logic is a good case study. We note however that the industry is young and further scale-up experience will help the technology move along the learning curve. The main question however will be whether investment will flow in or not.
Where do oxides sit in this emerging landscape?
Oxide semiconductor TFT technology is an emerging option which ticks many of the right boxes. They have high mobility, which makes them suitable for OLED, 3D and on-board processing. They have wide-bandgaps therefore they can also be transparent. Their high mobility enables lower aspect ratio and smaller pixels, and therefore lower consumption and higher resolution. They can be processed at low temperatures, making them amenable to flexible substrates, although metal oxides (similar to ITO) are not expected to be fully flexible and their properties will likely change if bent too much and/or too many times. They are amorphous; therefore they can be put down over large areas. Their main deposition technique is sputtering, although progress has been made in terms of printing it. It is easy to achieve high-throughput using sputtering (e.g., CIGS thin film photovoltaics are sputtered).
They are the leading technology that can step in to enable large-sized OLED or/3D displays. This is a large market segment in which they have a strong value proposition. The challenges that must be overcome include stability under illumination and large-area manufacturing using sputtering. The former challenge becomes important in transparent displays, whereas the latter is critical if oxide TFTs are to be used for large-sized displays.
Table 2. Assessing the merits of oxide thin film transistors for enabling different market drivers in the display industry and comparing with its closest rival technology.
Source: IDTechEx report Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts
In conclusion, there will be no one-size-fits-all solution. The market drivers are changing the requirements on the backplane and in doing so they will fragment the market along technical capabilities. This means that each technology, existing or emerging, will occupy different parts of the market and will address different needs. This makes the investments and R&D decisions of the future challenging as all efforts cannot be consolidated on one technology for all markets. This fragmented picture will persist until one technology sufficiently improves to outperform all others in every aspects. We are not there yet but metal oxides are the most promising candidate.
A key report by IDTechEx
Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts makes sense of this changing and fragmented space. It analyzes major drivers that are shaping the display industry, examines the trends and assess how these trends create new functional needs on the technology side. It provides an in-depth review of existing and emerging thin film transistor solutions and critically assesses the pros and cons of each.
In terms of metal oxides, it assesses the different material systems available (IGZO, HIZO, IZO, ZNO, TZO, ZnO, etc) and the merits of each. In doing so, it outlines and discusses the leading research frontiers. The report also discusses various requirements of dielectrics for emerging displays and explores the material options for use as dielectrics on wide-bandgap metal-oxide semiconductors.
The report links material properties of all thin film transistor technologies to device figures-of-merit, including mobility, sub-threshold voltage, threshold voltage, stability, contact resistance, etc. These figures-of-merit are then connected to attributes of backplanes and thereby to the emerging functional needs of the display industry as a whole. Linking the mega trends with micro-level technological details, we are able to map out how the fragmented display backplane technology will look going forward.
In our assessment, we also provide a detailed outline of activities in the OLED display segment, including:
- An analysis of announced production capacity
- Number of units sold by manufacturer
- Which backplane technologies are used by which manufacturers
- A timeline of venture/partnerships activities taking place across the world in the OLED space.
- Product development cycle for oxide semiconductors
Who should buy this report?
Major display manufacturers: This report will help you understand how the drivers and the functional needs of the industry are changing, learn which technologies will win in which market segments and why. It will enable you to ensure that you do not lose out when the landscape alters and when the value chain become disrupted.
Thin film transistor technology licensors and researchers: It will help you identify how the changing display industry will benefit from which thin film transistor technology, pinpoint key research frontiers and questions, identify target markets and players for licensing and discover more about your competitors.
Material suppliers to all thin film transistor technologies: Helps you understand which thin film transistors (and their associated material system) will win in which markets and why. It will help you devise your strategies by backing the right technologies in the right time frames and for the right markets.
Equipment suppliers: Understand which new technologies will be required and why, and therefore which new equipment systems will be needed, helping you plan ahead and form the right partnerships or relationships.
Circuit designers: See how oxide thin film transistors require new compensation techniques, why and for which market segments (this determines the required performance specification). The report highlights a new area of circuit design for companies.
Top image Amorphous oxide - Source: K. Nomura et al., Nature, 2004