Evidence of metastable zinc blende phase and its influence in nanocrystalline ZnO film growth

Abstract

ZnO is a wide band gap semiconductor used in a broad range of technological applications such as gas sensing, piezoelectronics or transparent conductive oxides. Many II‐VI compounds typically crystallize in a rock salt (RS), a wurtzite (WZ) or a zinc blende (ZB) phase. For ZnO it is well known that the most stable form under ambient conditions is the WZ crystal structure. The WZ structure can be described as an AaBbAaBb stacking sequence of close‐packed Zn and O planes along the [0 0 0 1] axis, where the upper‐case letters stand for Zn planes and the lower‐case letters for O planes. The metastable RS phase forms under large hydrostatic pressure, while so far ZB has only been found during hetero‐epitaxial growth on cubic substrates or in some free‐standing nanostructures [1,2]. Here, using a combination of transmission electron microscopy (TEM) based automated crystal orientation mapping (ACOM) and high‐resolution TEM (HRTEM), we present evidence of a metastable zinc blende phase in chemical vapor deposited (CVD) nanocrystalline ZnO films. We further show how this ZB significantly affects the growth in these films. The vapor deposition growth of polycrystalline films usually begins with the formation of densely spaced nuclei on the substrate. During further film growth neighboring nuclei impinge on each other, leading to a growth competition between grains. Grains with their fastest growth direction normal to the substrate (i.e. [0 0 0 1]) overgrow otherwise oriented grains, resulting in a classical columnar growth morphology, as shown in Figure 1 a. Unexpected for such columnar film growth, however, is the observation that the growth of many columnar grains appears to stop, followed by a renucleation to form smaller grains as marked by circles in Figure 1 a. We investigated this behavior by ACOM (Figure 1 b), revealing a special epitaxial relationship of the new renucleating grains with the underlying columnar grains. The orientation data shows that the renucleating grains share a common [2 ‐1 ‐1 0] axis with their neighboring grains (Figure 1 c & d) and that the misorientation between the grains is close to 70°. This epitaxial relation has been further investigated by HRTEM, which reveals that the origin of this epitaxial orientation relationship can be attributed to a few nm sized core of ZB phase (Figure 2 a‐d). This ZB region can form on top of the columnar grains by a simple change in stacking sequence to AaBbCcAaBbCc (Figure 2 e). We believe that the new WZ grains then nucleate on the {1 1 1} facets of this ZB core, producing the orientation relationship observed by ACOM. Furthermore, our analysis of the fast growth direction of the renucleating WZ grains using ACOM, proves that, unlike the initial columnar grains, the fast growth direction is no longer parallel to the [0 0 0 1] axis. We argue that this is due to a change from having a Zn‐terminated (0 0 0 1) polar facet (as for WZ1 in Figure 2 e) to having an O‐terminated (0 0 0 ‐1) facet (as for WZ2 in Figure 2 e) exposed to new adatoms. Polarity determination of the columnar and renucleating grains by convergent beam electron diffraction (CBED) confirms this hypothesis. This nucleation of WZ on top of {1 1 1} ZB facets bears a strong resemblance with the formation free‐standing ZnO tetrapod nanostructures [1]; indeed it is remarkable how a similar mechanism appears to play an important role in these compact thin films, even if the driving force for the formation of ZB on top of WZ ZnO remains still unclear [2]. We have further investigated [2 ‐1 ‐1 0] fiber textured CVD ZnO films, for which the same growth mechanism appears to be active and could explain the abundance of (1 0 ‐1 3) twin boundaries previously found in these films [3].