Materials that rapidly switch between amorphous and crystalline states are widely used to manage heat and store data. They now emerge as promising building blocks for ultrahigh-resolution display devices. See Letter p.206
A modern world without electronic display devices is unthinkable. In rich countries, most households have several such devices: television sets, mobile phones and tablets, to name a few. Alongside trends to make large displays (home cinemas and billboards), there is a growing interest in creating ultrasmall displays with high resolution and fast response times — the amount of time a pixel in the display takes to change. Such microdisplays are already used in technologies such as microprojectors and wearable devices. On page 206 of this issue, Hosseini et al.1 report how they take up this trend to go ultrasmall by developing an optoelectronic device that could be used to make ultrathin displays of ultrahigh resolution.
At the heart of Hosseini and colleagues' device is a film of a germanium antimony tellurium alloy (Ge2Sb2Te5; GST), which is classed as a phase-change material. Phase-change materials switch rapidly between a metastable amorphous phase and a stable crystalline phase (Fig. 1a,b) — both of which exist at room temperature2,3 — in response to an external stimulus, such as heat, light or electrical current. This last stimulus, which induces GST to undergo the phase change, is most relevant for display applications. Phase-change materials are mostly known for their use in thermal-energy management; they store or release heat during the phase transition, and so offer good heat-management solutions. But they can also be used as media for rewritable optical data storage, owing to the difference in optical properties between the two phases4. Moreover, they have great potential as non-volatile electronic memories, because the two phases have vastly different electrical conductivities5.
To create their optoelectronic device, Hosseini et al. sandwiched a layer of GST, a few nanometres thick, between two layers of indium tin oxide, which act as transparent electrodes. They found that the optical transmission and reflection properties of this structure are determined by the thickness of the individual layers and their refractive indices. On application of an appropriately shaped pulse of electrical current between the two electrodes, the GST film can switch between the crystalline and the amorphous states, changing the film's refractive index and its colour. The colour of the film in each state can be chosen by tuning the thickness of one of the transparent electrodes (Fig. 1c), and is visible by illuminating the structure with 'white' (broad-band) light.
Colour changes are a crucial element of display technologies, but to make a display, an array of differently coloured pixels must be fabricated in which each pixel's colour is individually switched. It is this point in particular that makes phase-change materials an attractive option with which to build the pixels of microdisplays. By using lithographic techniques, the pixel sizes can be brought down into the submicrometre region, enabling an ultrahigh resolution that exceeds that of most other existing technologies. In current colour displays, each pixel is individually controlled by a type of electrical switch known as a thin-film transistor. A high-resolution technology based on a GST device such as that demonstrated by Hosseini et al. could allow the full incorporation of thin-film transistors, and could be built in a single production line.
Hosseini and colleagues' GST-display principle is different from that of conventional display technologies in many aspects. Their optoelectronic device, from which a future display could be made, is an entirely solid-state system that can be easily manipulated, even on flexible and bendable substrates. Such a GST display would also provide bistability, a property that allows an image to be maintained over a long period even when the power of the display is switched off or is in standby mode. Bistability is useful to reduce power consumption, for example in electronic-book readers. The authors demonstrated a simple, static display that already shows this feature. Power usage could be further improved by operating the GST display in a mode that provides daylight visibility, even under bright outdoor conditions.
Many applications, such as three-dimensional displays, require a high image-refresh rate — the number of times per second that an image is refreshed — to deliver full-motion video content. The GST device described here is intrinsically ultrafast. In solid-state memory applications, GST films are known to switch between the amorphous and the crystalline phases within 100 nanoseconds6. In display applications, this timescale would correspond to an image-refresh rate of 10 megahertz — more than 20,000 times faster than present television technologies.
However, there are several issues that need to be addressed before the proposed technology can enter the display market. First, and as already mentioned, the GST device will need to be pixelated and controlled by thin-film transistors. Second, the device should be able to be connected to electronic components that ensure that each pixel receives the right electrical pulse at the right moment. Third, the range of possible GST-film colours has to be expanded to fill the whole gamut, which is necessary to create vivid colours. Fourth, the display should also be able to give a fully black or a fully white colour, to allow high-contrast colour images to be obtained. Finally, high-end displays are also judged on their ability to provide a large number of grey scales, which is not an obvious property of phase-change films.
If and when these issues are appropriately addressed, this new display concept may find use in applications that are outside the reach of current display technologies. The optoelectronic structure demonstrated by the authors could, in principle, provide functions other than image display, including sensing of incident light, data storage and performing logic operations. The device's image-display capability could also be used in emerging technologies such as smart contact lenses and other wearable devices, or even in implantable displays in human bodies.
Hosseini, P., Wright, C. D. & Bhaskaran, H. Nature 511, 206–211 (2014).
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Phase-Change Superlattice Materials toward Low Power Consumption and High Density Data Storage: Microscopic Picture, Working Principles, and Optimization
Advanced Functional Materials (2018)
Semiconductor Science and Technology (2017)
Optics Communications (2017)
AIP Advances (2017)