Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.

Adviser: Eric I. Altman.

Polar materials are non-centrosymmetric creating a net dipole within each repeating unit along certain crystallographic axis. The propagation of these dipoles will diverge without charge compensation at their surfaces. As a consequence, oppositely poled surfaces usually demonstrate distinct electronic, structural or chemical properties. The potential to induce unique, and possibly switchable, electronic, magnetic, and chemical properties at interfaces between polar and non-polar materials has motivated a great deal of research in recent years. Ferroelectric materials are an important subset of polar materials whose polarization direction can be switched by applying an external electric field. This effect allows one to envision materials with switchable surface properties.

To determine how polar substrates can influence the properties of non-polar and polar films, three major systems were exploited in this thesis: (i) non-polar chromium oxide on polar zinc oxide (Cr2O3/ZnO), (ii) polar zinc oxide on non-polar chromium oxide on polar zinc oxide (ZnO/Cr 2O3/ZnO) and (iii) polar zinc oxide on ferroelectric lithium niobate (ZnO/LiNbO3).

Ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS), high resolution transmission electron microscopy (HRTEM), reflection high energy and low energy electron diffraction (RHEED and LEED), and X-ray diffraction and reflectivity (XRD and XRR) were employed to characterize the growth mode, film quality and interfacial electronic properties. Temperature programmed desorption (TPD) was used to study the surface chemistry.

First, the Cr2O3 growth on ZnO {0001} was layer-by-layer with initial disorder followed by the formation of epitaxial Cr2O 3 (0001). Despite the initial disorder, HRTEM and XRD/XRR measurements on thicker films revealed an abrupt interface with the Cr2O 3 lattice extending all the way to the interface. The polar interfaces showed a small band offset that decayed with increasing film thickness, suggesting that the compensating charges at the interface may partially migrate to the film surface. Moreover, statistical analyses of UPS valence band spectra revealed an enhanced density of states near the valence band edge for Cr2O 3 on ZnO (0001), consistent with stabilization of the positive interface by filling surface electronic states. In contrast, no significant valence band differences were observed between bulk Cr2O3 and thin Cr2O3 layers on ZnO (0001¯).

Second, the ability to affect the surface properties of non-polar Cr 2O3 films through polar ZnO (0001) and (0001¯) supports was investigated by two means: the characterization the polarity of ZnO films grown on top of the Cr2O3 and the study of 2-propanpl chemistry upon the Cr2O3 surfaces. From the first perspective, the ZnO growth mode was determined to be StranskiKrastanov (2D to 3D), which can be attributed to the ZnO layers initially adopting a non-polar structure with a lower surface tension before transitioning to the polar bulk structure with a higher surface energy. Thick Cr2O3 layers supported ZnO (0001¯) growth regardless of the underlying ZnO substrate polarization; however, the polarization direction of ZnO films grown on Cr2O 3 films less than three repeat units thick follows the direction of the underlying substrate polarization. From the second perspective, acetone was the only major product seen for < 3 Cr2O3 repeat unit thick films supported by both positively and negatively poled ZnO substrates while for thicker Cr2O3 films, enhanced propene desorption indicative of alcohol dehydration was observed for positively poled ZnO substrates. Atomic force microscopy revealed that the Cr2O3 films grown on positively poled ZnO substrates were rougher despite diffraction results that indicated epitaxial Cr2O3 growth independent of the substrate polarization direction. Thus these results suggest that the substrate polarization impacts the film only within a very short range or indirectly impacts the reactivity of the non-polar layer through the growth process which determines the density of defects in the Cr2O 3 film.

Finally, the growth of ZnO on LiNbO3 (0001) and (0001¯) polar surfaces was determined to be Stranski-Krastanov, also with a small degree of roughness at the interface. The film maintained the substrate surface crystallography initially and then transitioned to an ordered ZnO {0001} phase after passing through a disordered region. A band offset revealed by XPS indicates different charge screening mechanisms at oppositely poled interfaces and potentially charge transfer near the film-substrate interface. In addition, the reaction of 2-propanol was used as a probe to identify the polarity of thick ZnO films. The results indicate that ZnO film grown on either LiNbO3 (0001) or (0001¯) polar surfaces ultimately develops a negative polarization. Therefore, it is concluded that the LiNbO3 polar substrate has a more obvious impact over a short range near the ZnO/LiNbO3 interface but this does not translate into directing the polarization direction of thicker ZnO films.