Amorphous metal oxide electronics: Flexible futures

Think of display screens that can fold like paper. Think of see-through electronics that can convert your car windshield into a GPS tracking display. Think of stretchable biosensors that are transparent and sit on a contact lens. Think no further as this is the realm of flexible electronics and can soon be reality.

Figure 1. Dawn of flexible and transparent electronics.

Metal oxides (MOs) are a prime candidate for enabling the above and much more. The prime reason behind this expectation is the mechanism of charge transport in MOs, i.e. the manner in which electrons flow through them. The MO charge transport mechanism is briefly explained below and will convince you as to why we have to think beyond silicon (Si) when thinking flexible. When trying to understand materials compatibility for flexible electronics, it is important to appreciate the general rule that amorphous is flexible. Crystalline materials are characterized by long-range arrangement of their constituent atoms which gets disrupted when these materials are mechanically stressed and break. Crystalline materials are therefore not compatible with flexibility. On the other hand, amorphous materials lack long-range order. These are characterized by disorder in the way their atoms are positioned, and so are compatible with applications that require mechanical flexibility.

Consider the case of chemical bond formation in Si. Flow of electrons in Si occurs through the sp³ orbitals (think about the classical, textbook-example of carbon). These orbitals have a characteristic geometry that is ‘fully-developed’ for crystalline Si, as shown in the Figure 2. Single-crystals of Si are one of the purest electronic materials humans have ever made, and aptly have transformed the world we live in today, primarily due to their excellent charge transport. Crystalline Si however requires high temperatures and ultra-clean environments for fabrication. Besides, it is mechanically brittle. These factors put a high price tag on crystalline Si. It is possible to get a relatively lower-cost amorphous form of this material. As the Figure 1 highlights, this is characterized by a significant disorder in the crystal structure. Since the flow of electrons through a mesh of sp³-orbitals requires a seamless, long-ranged 3D network, amorphous Si expectedly shows a significantly poorer charge transport. Si therefore offers a way to flexibility, but with a large compromise on charge transport. This is not quite the case for MOs. Charge transport in these materials occurs through s-orbitals of the metal, which are geometrically spherical and therefore ‘squishy’. Due to this attribute, charge transport through them does not care for long-range order and is similar for both the crystalline and amorphous states. Amorphous form of MOs requires significantly lower fabrication temperatures making them both low-cost and compatible with plastic substrates. Besides, these MOs have wide band-gaps (so, transparent), can be processed as semiconductor inks via simple chemical synthesis protocols, and are well-suited for printing and large-area coating.

Figure 2. Charge transport mechanism in Si versus metal oxides. Adapted from: Nomura, K, et al, Nature, 432, 488-492, 2004

Indium oxide (In₂O₃) is one such MO that has been explored in detail over the last several years for application in flexible electronics. Electron mobilities (a measure of charge transport) for In₂O₃ have been found to be significantly better than amorphous Si. Flow of electrons through In₂O₃ is, however, difficult to control. Devices made out of this material, such as the thin-film transistors (TFTs) used to power pixels in a display screen, are not reliable. Researchers have circumvented this problem by doping the host In₂O₃ with gallium (Ga) and Zinc (Zn). The resulting material, IGZO, is being actively pursued by Sharp, LG and Samsung as a TFT material for displays in mobile phones, as the AMOLED and newer technologies continue to require higher driving currents, and therefore higher mobility TFTs.

Although IGZO TFTs exhibit good charge mobilities, finding ways to improve this metric would help further improve switching speeds (for example, refresh rate of pixels in a monitor screen). A new technology aimed precisely at achieving this is currently in research phase. The idea is to form two layers of dissimilar MOs in contact with each other. These two MOs can be In₂O₃ and zinc oxide (ZnO). The dissimilarity between these two in terms of how electrons are arranged in them, leads to an interesting phenomenon at their interface where the two layers meet. Excess electrons from ZnO migrate to this interfacial region and get ‘locked’ there forming a thin, highly conductive region. Since this region is a flat, 2-dimensional region of electrons, it is usually called a 2D electron gas (2DEG). These complex bilayer MO structures have been demonstrated as TFTs with electron mobilities better than In₂O₃ and IGZO. Importantly, this exceptional charge transport has been achieved by simply spray-painting the MO inks, similar to the way cars are painted, demonstrating the potential for industrial scalability.

Materials such as hybrid perovskites and small molecules that currently form a major research trend in electronics tend to behave wonderfully only in their crystalline state. Efforts therefore rely on increasing crystallinity in these materials. However, they are not good choices for flexible electronics and are outplayed by MOs, precisely for reasons outlined in this article. An important characteristic of MO-electronics is mechanical flexibility owing to their amorphous nature. MOs marry excellent charge transport with low-cost manufacturability and are expected to be a key component in the internet-of-things network in near future.