Low-Cost Dopants for High Performance and Aqueous-Derived ZnO Thin-Film Transistors

Transparency and flexibility are new terms of innovation associated with advanced display applications such as head-up displays for cars, smart windows, display tables, smart cards and rollable smart phones. Hence, thin film transistors (TFTs) as a driving element in display devices are required not only semiconductors that are capable of high level of performance but also unique materials properties such as transparency.¹ Moreover, for the low-cost and large scale device applications, the advanced process conditions are required, such as a low-temperature sintering, solution-processiblity and so on. Solution-processed metal-alloy oxides such as indium zinc oxide (IZO), indium gallium zinc oxide (IGZO) has been extensively researched due to their high electron mobility, environmental stability, optical transparency, and solution-processibility.² In spite of their excellent material properties, however, there remains a challenging problem for utilizing IZO or IGZO in electronic devices: the supply shortage of indium (In). The cost of indium is high, what is more, indium is becoming more expensive and scarce and thus strategically important. Therefore, developing an alternative route to improve carrier mobility of solution-processable ZnO is critical and essential.³

Generally, alkali earth metal dopant, especially magnesium, is well known as a carrier suppressor of zinc interstitials or oxygen vacancies. However, we found the doping effect of alkali earth metals in ZnO matrix on the contrary. We reported alkali earth metal as new N-type dopants and discussed the alkali earth metal doping effects on ZnO semiconductors in terms of its crystallinity, morphology, interface charge trapping, XRD, and Hall measurement.⁴Using these alkali earth metal dopants, we demonstrated the magnesium or calcium doped ZnO TFTs with the dramatically improved the field effect mobility, on/off current ratio and bias stability. In addition, we proved that the increase in the carrier concentration was the dominant reason for the enhancement in the electron mobility of the alkali earth metal-doped ZnO TFTs.

Herein, we introduce magnesium and calcium as low-cost dopants for high performance and low temperature and aqueous-precursor ZnO thin film transistors (TFTs). Ca-doped ZnO TFTs exhibited excellent electrical performance with the field effect mobility of 5.02cm²/V∙s, on/off current ratio of 10⁷and negligible hysteresis and highly stable TFTs were demonstrated. We believe that in-depth study of the morphology, electrical properties of alkali earth metal-doped ZnO films make a great contribution to next-generation high-performance TFTs for flexible, printed and transparent electronics.

Figure caption

(a) Mechanism of formation of aqueous ZnO, (b) transfer curves of TFTs based on ZnO doped with various alkali earth metals (Mg 0.002 mol%, Ca 0.1 mol%, Ca 0.2 mol%).

Reference

1. M. Grundmann, H. Frenzel, A. Lajn, M. Lorenz, F. Schein and H. von Wenckstern, Phys Status Solidi A, 2010, 207, 1437.

2. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature, 2004, 432, 488.

3. S. Y. Park, B. J. Kim, K. Kim, M. S. Kang, K. H. Lim, T. I. Lee, J. M. Myoung, H. K. Baik, J. H. Cho and Y. S. Kim, Adv. Mater., 2012, 24, 834.

4. S. Y. Park, K. Kim, K.H. Lim, E. Lee, S. Kim, H. Kim and Y. S. Kim, RSC advances, 2013, 3, 211339.