ZnO is an oxide semiconductor that has been shown to be a useful optoelectronic material due to the wide tunability of the direct band gap from 3 to 4.4 eV by Mg or Cd doping. In practical device applications, the device characteristics are strongly influenced by the polarity direction of the crystal, i.e., whether the wurtzite-type crystal is terminated by a zinc face or an oxygen face. For perfect epitaxial thin film layers, the crystal polarity is determined by the termination of the underlying layer. In practical films, however, the film termination can change, depending on the film growth conditions. When grown on an Al₂O₃(0001) surface, ZnO films tend to be terminated by the zinc face when grown at a slow rate at high temperature. The termination switches to the oxygen face at high growth rates or low growth temperatures.

In this work [1], we have used co-axial impact-collision ion scattering spectroscopy (CAICISS) to determine the termination type of homoepitaxial ZnO films grown by Nd:YAG laser ablation at various growth conditions. CAICISS can be used to determine the crystal polarity by measuring the angle dependence of backscattered He ions from a crystal surface. The measurement geometry and the crystal polarity models are shown in Fig. 1. For ZnO structure analysis, the time-of-flight window is set at He ion backscattering from Zn atoms. The scattering is angle dependent, mostly due to He ion focusing and shadowing that occurs in the two topmost atomic layers. High backscattering intensity is observed at slightly different polar angles for the Zn- and O-face crystal terminations.

Homoepitaxial Mg:ZnO films were grown at several temperatures to verify if a termination change occurs similarly to growth on Al₂O₃(0001). Although there were significant surface morphology differences, the CAICISS polar scans were essentially unchanged. The optimal growth temperature for surface flatness was found to be 550 °C at an oxygen pressure of 10^-5 Torr. The films were grown by pulsed laser deposition (PLD) with a fourth-harmonic Nd:YAG laser operating at 10 Hz and an ablation fluence of 2.3 J/cm².

The polar angle ion scattering scan is shown in Fig. 2 together with an atomic force microscope (AFM) topography image. The double peak structure in the ion scattering intensity between 70 and 80 degrees indicates the presence of the ZnO oxygen face, same as the substrate surface.

These results show that it is possible to use solid-state laser pulsed laser deposition to fabricate ZnO films with a clear step-and-terrace morphology while maintaining a constant ZnO layer termination.

Reference

• [1] T. Masuda, T. Sato, M. Lippmaa, T. Dazai, N. Sekine, I. Hosako, H. Koinuma and R. Takahashi, J. Appl. Phys. 136, 095303 (2024).

Authors

• T. Masuda^a, T. Sato^b, M. Lippmaa, T. Dazai^c, N. Sekine^d, I. Hosako^d, H. Koinuma^a, and R. Takahashi^c
• ^aSmart Combinatorial Technology, Inc.
• ^bVacuum Products, Inc.
• ^cNihon University
• ^dNICT