Properties of aluminum-doped zinc oxide films deposited by high rate mid-frequency reactive magnetron sputtering

Abstract

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 µOmega cm at 1.2 mTorr total pressure and 250 µOmega 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 modified OES control using the process power at the control varible, and using the ratio of an oxygen and a metal emission line as the controlled variable

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

“…1 O 12 , are promising as reduced-indium TCO thin-film materials. New developments of high-rate and large-area deposition techniques for these multicomponent oxide thin films have been recently reported using magnetron sputtering deposition (MSD) and vacuum arc plasma evaporation (VAPE) methods [6][7][8][9][10][11].…”

Artificial control of ZnO nanodots by ion-beam nanopatterning

“…͑3͒. Experimental results reported from some ZnO research groups have shown various exponent values ͑2/3, 1/3, and 3/5͒ [31][32][33] for Eq. ͑3͒.…”

Preparation of transparent conducting B-doped ZnO films by vacuum arc plasma evaporation

“…12 Recently, newly developed deposition methods have been reported that may be able to resolve these problems. [13][14][15][16][17][18] For example, a newly developed vacuum arc plasma evaporation ͑VAPE͒ method that can deposit transparent conducting oxide films with uniform resistivity on large area substrates with a high deposition rate may be useful for preparing AZO or BZO thin-film transparent electrodes on thin-film solar cells, because the plasma energy generated during VAPE depositions is lower than that generated during dc-MSD. [14][15][16] However, we have previously reported that doping Al into ZnO films is very difficult in VAPE depositions because of the large difference in decomposition energy ͑or vapor pressure͒ between Al 2 O 3 and ZnO.…”