Crystallization of In2O3 and In2O3:Sn
- Temperature ramp from 20 to 330 °C (5 °C/min) to determine crystallization temperature
- Isothermal annealing to obtain activation energy of crystallization and to understand crystallization mechanisms
- Electric current annealing as a new method
The direct observation of the structure evolution yields a crystallization temperature of the Sn-doped indium oxide (ITO) thin film of 250 °C. The rapid crystallization is accompanied by film roughening and leads to a further decrease of the resistivity due to Sn donor activation .
Isothermal vs electric current annealing: crystallization kinetics
Isothermal annealing is performed at temperatures close to the crystallization temperature, while annealing by direct electric current is carried out at constant power resulting in a stable annealing temperature due to Joule heating. Denoting the integrated XRD intensity at complete crystallization by IC(222), the time dependence of the normalized intensity, I(222)/IC(222) is defined as the degree of crystallization. For both annealing methods it is observed that the resistivity decreases in a single stage in indium oxide (IO) while ITO shows a two-stage behaviour (Figure 1).
The first stage is explained by relaxation of amorphous phase which leads to a formation of oxygen vacancies and their contribution to free charge carriers generation. The second stage coincides with the outset of crystallization and enhancement of the free electron density. It provides a direct evidence of Sn-donor activation (estimated efficiency of 40%) in ITO with crystallization outset [2, 3].
Arrhenius plots of the amorphous-to-crystalline transformation rate for both annealing mode are compared in Figure 2. It shows significantly lowered activation energy of crystallization, Ea, in case of electric current annealing. This means that crystallization sets on at lower temperatures and proceeds faster in this case. As an explanation, electrical inhomogeneity of the films is believed to result in a locally overheated depth region. Due to the lowered activation energy, annealing by electric current is expected to be advantageous, in particular, for ITO films grown on thermally sensitive substrates such as polymers .
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2. A. Rogozin, N. Shevchenko, M. Vinnichenko, M. Seidel, A. Kolitsch, and W. Möller: Annealing of indium tin oxide films by electric current: properties and structure evolution. Appl. Phys. Lett. 89, 061908 (2006).
3. A. Rogozin, M. Vinnichenko, N. Shevchenko, U. Kreissig, A. Kolitsch, and W. Möller: Real-time evolution of electrical properties and structure of indium oxide and indium tin oxide during crystallization. Scr. Mater. 60, 199 (2009).