A study of the surface morphological features of the polar faces of ZnO by atomic force microscopy (AFM) methods and AlN thin films deposited on ZnO polar faces by PLD

The effects of both temperature and atmosphere on the resulting morphological features of the polar faces of single crystal ZnO were investigated and characterized by atomic force microscopy (AFM). In studies where ZnO was thermally processed in flowing oxygen at atmospheric conditions within the temperature range of 500 degrees C to 900 degrees C for 30 minutes, the Zn-surface (i.e., (0001)) showed a tendency to reconstruct with increasing temperature until terraces became evident at 900 degrees C. Terrace heights were as small as 0.9 nm. In contrast, the O-surface (i.e., (000 (1) over bar)) was observe to change very little during the O-2-atmoshere, thermal treatment and remained comparatively rougher than the Zn-surface. ZnO samples which were thermally processed under high vacuum (i.e., 5 x 10(-7) Torr) conditions exhibited a more dramatic contrast. The vacuum annealed Zn-surface was observed to develop very smooth surface features (Roughness = 0.09 nm) at annealing temperatures within the 700 - 800 degrees C range. In contrast, and as expected, the O-surface roughness increased due to surface reduction reactions. In addition to these findings, it is noted that AFM measurements may be utilized as a convenient method to distinguish between the two polar surfaces of ZnO. Aluminum nitride was deposited on the Zn- and O- surfaces from 700 to 850 degrees C by pulsed laser evaporation. X-ray diffraction indicated that the AlN was c-axis oriented with no interface reaction products detected between the ZnO substrate and AlN film.