Oxide electronics: Transparent success

Atomic layer deposition enables the fabrication of defect-free zinc oxide transistors for high-performance transparent displays.

The performance of organic light emitting diodes (OLEDs) is still not comparable with liquid crystals for producing conventional laptop and flat-screen displays. However, OLEDs show great potential for transparent displays for integration with shop windows or heads-up devices on car windshields. Unfortunately, although there has been steady progress in the development of the diodes themselves, the development of materials for making the peripheral drive-electronics is still lagging behind.

Now, through improvements in the techniques used to grow oxide-based semiconductor and insulator materials, Seongil Im, Sang-Hee Ko Park, and colleagues¹ from the Electronics and Telecommunications Research Institute, Daejon, and Yonsei University, Seoul, demonstrate the fabrication of high-stability transparent thin film transistors and their integration into a fully functional active matrix OLED display.

OLEDs are uniquely suited to fabricating transparent displays because they can be grown at low temperature on glass or even plastic substrates. Switching the devices on and off, however, requires transistors that are capable of delivering large currents at relatively high voltage. Transistors made from amorphous silicon, such as those used in liquid crystal displays, can also be grown at low temperature. They can even be relatively transparent if produced from films that are sufficiently thin. But when used to switch large currents, they become unstable.

A more promising material for making transparent transistors for driving OLED displays is zinc oxide (ZnO)—a wide gap semiconductor. However, until now it has been difficult to grow high quality ZnO and the aluminum oxide (Al₂O₃) dielectrics used for fabricating transistors, at low temperature. Poor quality materials are electrically unstable and produce leak currents when exposed to light — a particularly bad problem for making transparent displays.

Fig. 1: A fully functional transparent active matrix OLED display driven by ZnO-based developed by Seongil Im and colleagues. Such displays may eventually be integrated directly onto the glass of shop-front windows or car windshield displays.

Here, the researchers developed an atomic layer deposition technique for depositing high quality ZnO and Al₂O₃ films at low low-temperature. This enabled them to fabricate ZnO-based TFTs with unprecedented levels of electrical and optical stability, and subsequently the construction of a 2.5 inch active matrix OLED display (Fig. 1).