High free electron mobility in Al-doped ZnO
Significant improvement of the carrier mobility up to 46 cm2 V-1 s-1 and resistivity down to 2.3x10-4 Ωcm has been recently demonstrated for AZO films grown by reactive pulsed magnetron sputtering (RPMS) at FZD. It was achieved by a fine tuning of the oxygen partial pressure using gettering of O2 by sputtered metal atoms during RPMS at substrate temperature of 350 °C. The value of 46 cm2 V-1 s-1 is significantly higher than typical mobilities reported for AZO films grown by RPMS. It is comparable to the highest values achieved at comparable free electron density in AZO films grown by pulsed laser deposition or radio frequency magnetron sputtering.
The figure sorts all results of the films grown by RPMS for different target compositions and deposition conditions in a plot of the free electron mobility versus the free electron density. The mobility is limited by an interplay of two electron scattering mechanisms, one is grain boundary limited transport (denoted as “Seto”), which dominates at low free electron density (Ne<5x1020 cm-2), and another is ionized impurity scattering and impurity clustering (denoted as “Masetti”), dominating at high (Ne>6x1020 cm-3) free electron densities. It has been observed that the lower is the Al concentration in the magnetron target (cTAl is in the range 1.7-8.7 at%) the higher are optimum µe values and the better is film crystallinity. This has been explained assuming that at cAl>3 at.% there is an excess of electrically inactive Al which might locally drive formation of new phases deteriorating the film crystallinity and thereby enhancing electron scattering.
S. Cornelius, M. Vinnichenko, N. Shevchenko, A. Rogozin, A. Kolitsch, and W. Möller: Achieving high free electron mobility in ZnO:Al thin films grown by reactive pulsed magnetron sputtering. Appl. Phys. Lett. 94, 042103 (2009).