When AlN thin films are applied to surface acoustic wave devices, c-axis- and a-axis-oriented films are necessary for longitudinal and transverse waves respectively. However, it is difficult to prepare a-axis-oriented films by sputtering and ion plating. Nevertheless, a-axis-oriented films have been prepared by the electron shower method, and the films can be changed to c-axis orientation by a negative bias of -200 V. The concentration of O in the films decreases with increasing negative bias voltage. These AlN films contain 20 at.% N, which is independent of the bias voltage.