Coordination
Dr. Florian Ruske
Dr. Florian Ruske
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High-Mobility ZnO:Al

We could show that ZnO:Al capped with a thin silicon layer is temperature-stable against SPC- and RTP- temperature annealing procedures. In fact, the electrical properties are even improving upon annealing under a protective cap [1,2]. Without any protective capping layer ZnO:Al will usually degrade electrically upon high temperature treatments. Very high charge carrier mobilities for capped and annealed films up to 67 cm2/Vs at a carrier concentration of about 7 *10 20 cm-3 could be achieved for 900 nm thick films. This leads to a resistivity of 140 µΩcm. Furthermore we could show the feasibility of thermal post depositions treatments for improving the electrical properties of ZnO:Al produced by a large industrial sputter coater on commercial float glass. Especially, high mobilities could be achieved [3]. The combination of two annealing procedures allows systematic influence on the electro-optical properties of ZnO:Al.The reasons for the improved charge carrier mobility upon annealing under a protective layer are a focus of our current research. Limiting factors for the charge carrier mobility in ZnO:Al are mainly the grain boundaries of the polycrystalline material and scattering at the charged donor atoms. Our results strongly suggest a decreased defect density at the grain boundaries as a reason for the increased charge carrier mobility.

The relation between the charge carrier concentration and the transparency in the near-infrared region (Fig. 1) indicates the trade-off between electrical conductivity and optical transparency in transparent conducting oxides. Achieving high electron mobilities is a key technological challenge in order to maximize both electrical conductivity and optical transparency.

Fig.1 Transmission and absorption of two ZnO:Al films of 900 nm thickness shown in black and grey. The two films differ in their charge carrier concentration and their charge carrier mobility. The higher the carrier concentration, the smaller is the so called “optical window”, in which the film has the highest transparency.


References

[1] K. Y. Lee, C. Becker, M. Muske, F. Ruske, S. Gall, B. Rech, M. Berginski, J. Hüpkes, Appl. Phys. Lett. 91, 241911 (2007).

[2] F. Ruske, M. Roczen, K. Lee, M. Wimmer, S. Gall, J. Hüpkes, D. Hrunski, B. Rech, J. Appl. Phys. 107 (2010).

[3] M. Wimmer, F. Ruske, S. Scherf, B. Rech, submitted to Thin Solid Films