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Aluminum-doped zinc oxide films are promising candidates for economic transparent conducting oxide (TCO) applications. To reach high deposition rates (about 7 nm/s at 4.5 W/cm2) in combination with optimum TCO properties by using the reactive mid-frequency sputtering technique, the process window must be precisely controlled, especially for unheated substrates. To overcome the typical hysteresis problem, the process was stabilized by plasma impedance control for ease of use; this enabled the stabilization of the deposition process at any setpoint on the S-curve of the corresponding hysteresis loop. Hence in contrast to former work [B. Szyszka, Thin Solid Films 351, 164 (1999)], wherein a flow control at fixed power restricted the stabilization either to the metallic mode or to the oxide mode, respectively, the samples were prepared at various deposition parameters (setpoint, pressure, and temperature) also within the transition mode. It turns out that for certain process parameters optimum TCO properties were achieved only within this transition mode, which was not accessible without control. Electrical measurements indicated that the optimum resistivity of ZnO:Al films deposited on unheated substrates was about 2.5 times higher than at a substrate temperature of 200 °C. In the latter case, an improved value of 290 μΩ cm at 1.2 mTorr total pressure and of 250 μΩ cm at 13.2 mTorr, respectively, could be reached. Due to band-gap widening, which follows the Burstein–Moss theory, the optimum films showed a neutral color. For those samples, the ellipsometric spectra could be well modeled without using interface layers, indicating the dense structure of the films. The process state on the stabilized S-curves was characterized by partial pressure measurements and optical emission spectroscopy (OES). These investigations show that it is also possible to stabilize the process at fixed flow with a mo- - dified OES control using the process power as the control variable, and using the ratio of an oxygen and a metal emission line as the controlled variable. © 2001 American Vacuum Society.