Light emitting diodes based on organic materials are a promising technology for making high-performance active-matrix displays. But although the light emitting components of an active-matrix organic light emitting diode (AMOLED) display are a mature technology, they are limited by the poor performance of the transistors used to drive them.

Jae Kyeong Jeong and colleagues from Samsung Mobile Display Corporation, Korea1 may have found a solution in a ZnO-based semiconductor that exhibits both good electrical characteristics and high stability.

Amorphous silicon transistors used in conventional liquid crystal displays are reliable, inexpensive, and easy to grow at low-temperature over large areas. However, after many hours of operation, electrical stress can cause their switching-voltage to fluctuate, which seriously degrades image quality and lifetime of displays.

Semiconducting oxides based on ZnO offer a promising alternative to amorphous silicon for making the transistors of AMOLED. ZnO-based transistors are able to deliver the high currents needed to drive organic LEDs and can be grown over large areas. However, even transistors made from the most well studied of these materials, ZnO and InGaZnO exhibit bias-induced changes in their switching characteristics that cause problems. To address this, Jae Kyeong Jeong have been looking for a more stable oxide semiconductor, and believe they have found one in the form of ZrInZnO.

“According to our research, transistors made from ZrInZnO are at least over 10 times more stable than amorphous silicon transistors under the same bias stress conditions,” says Dr Jeong. “And better than those reported in the literature for InGaZnO and ZnO.”

Fig. 1: Transmission electron microscope image of the authors' ZrInZnO thin films suggests that as grown, the microstructure of these films consists of nonuniform distribution of spherical crystallites embedded in an amorphous matrix. Annealing of the films at 350 ºC increases the size and uniformity of the grains, reduces the volume of the surrounding amorphous phase, and substantially improves the electrical characteristics and stability of resulting transistors.

The ion sputtering technique with which the material is grown is easily scalable to large areas and microstructure consisting of nonuniform distribution of spherical crystallites embedded in an amorphous matrix (Fig. 1). Annealing at 350 ºC increases the size and uniformity of the crystals and substantially improves the electrical characteristics and stability of resulting transistors. Notably, the authors say that with better control of the interface between the oxide and gate dielectric of their devices, the stability of devices could be improved still further.