In situ spectral control of Zn species during helicon-wave-excited-plasma sputtering epitaxy of ZnO

Atomic species in the sputtered plumes from undoped ZnO target were identified by means of emission spectroscopy during helicon-wave-excited-plasma sputtering epitaxy. Luminescent Zn species were found to be excited neutral zinc (Zn*) and zinc cations (Zn+*), and Zn* density was independently controlled by the target bias (Vt) that accelerates the velocity of Ar cations while keeping the plasma density constant. The ZnO film formation seemed to have certain threshold Vt (around −200 V at 600 °C), and the growth rate increased with the magnitude of Vt but decreased with the substrate temperature, indicating that the growth rate is limited by the sticking coefficient of Zn. As a result of the supply mode of Zn that is similar to the case for laser molecular-beam epitaxy, the epitaxial (0001) ZnO on (112̄0) sapphire substrate had ultra-smooth surfaces having atomically flat terraces, and exhibited excitonic reflectance anomalies and luminescence peaks at low temperature.