Evolution of Wurtzite ZnO Films on Cubic MgO (001) Substrates: A Structural, Optical, and Electronic Investigation of the Misfit Structures

Abstract

We investigated the interface between hexagonal ZnO films and cubic MgO (001) substrates, fabricated via molecular beam epitaxy. X-ray diffraction and (scanning) transmission electron microscopy revealed that growth follows the single [0001] direction when the temperature of the substrate is above 200 °C, while when the substrate temperature is below 150 °C, growth initially is along [0001] and then mainly changes to [0-332] variants beyond a thickness of ∼10 nm. Interestingly, a double-domain feature with a rotational angle of 30° appears during growth along [0001] regardless of the temperature, experimentally demonstrating the theoretical predictions for the occurrence of double rotational domains in such a heteroepitaxy [Grundmann et al., Phys. Rev. Lett. 105, 146102 (2010)]. We also found that the optical properties of the ZnO film are influenced greatly by the mutation of growth directions, stimulated by the bond-length modulations, as we determined from X-ray absorption spectra at Zn K edge. These results also showed the evolution of the 4p(xy) and 4p(z) states in the conduction band with the rise in the temperature for growth. We consider that our findings may well promote the applications of ZnO in advanced optoelectronics for which its integration with other materials of different phases is desirable.

Engineering Ferroelectric Hf0.5Zr0.5O2 Thin Films by Epitaxial Stress

“…Related examples are epitaxial growth of spinel NiFe 2 O 4 (111) films on yttriastabilized zirconia-YSZ(001) 38 or wurtzite ZnO(0001) on MgO(001). 35 XRD pole figures around asymmetrical o-HZO −111 and m-HZO −111 reflections in Figure 3a confirm that both orthorhombic and monoclinic phases, when present in the films, are epitaxial (see φ-scans in Figure S4). The sample on GdScO 3 shows four sets of three high-intensity o-HZO −111 spots, indicating the existence of four crystal variants with 90°r otation in the plane.…”

“…is relatively frequent. 33 Films can present either higher 34 or lower 35 symmetry than the substrate.…”

“…Heteroepitaxy with different symmetry between a top layer and a bottom layer (or the substrate) is relatively frequent. 33 Films can present either higher 34 or lower 35 symmetry than the substrate.…”

Multiferroic h-LuFeO3 Thin Films on (111) and (100) Surfaces of YSZ Substrates: An Experimental and Theoretical Study

“…The growth of h-REFeO 3 phases on the surface with incoherent symmetry is still a challenge. Moreover, there are only few examples of thermodynamically stable hexagonal structures that were successfully deposited on the incoherent surface. − …”

Interfaces between hexagonal and cubic oxides and their structure alternatives

“…However, some issues may arise due to the change of growth directions depending on growth conditions. For example, our group 7 – 9 and others 10 – 14 have demonstrated that the growth direction (either polar (001) or non-polar (100)) of wurtzite ZnO thin films strongly depends on the substrate temperature and growth pressure. Remarkably, it has recently been observed that a change in the orientation of the coupling plane with substrates can improve device properties due to the removal of central symmetry for ZnO.…”