Stabilizing forces acting on ZnO polar surfaces: STM, LEED, and DFT

Abstract

New understanding has been reached on competing forces acting to stabilize the polar surfaces of intrinsic ZnO. To compensate an accumulating dipole moment normal to ionic planes, the Madelung electrostatic force and the bonding ability of undercoordinated Zn ions compete to deplete more Zn atoms from the (0001) face and more O atoms from the (0001) face. In this competition, the former mechanism wins because it provides very low energy binding sites for O ions at face-centered-cubic registries on both surfaces. On the Zn-face, a distorted tetrahedral structure is formed, while on the O-face, a vertical Y structure is formed. In both structures, O ions form the topmost atomic plane. The reconstructed polar surfaces containing these novel structures have cleavage energy of 2.36 J/m 2 , comparable to that of nonpolar surfaces and in agreement with experimental observation. An earlier structure found on the Zn-face annealed at below 1000 K is stabilized mainly by the Madelung electrostatic force and is a metastable structure of that surface.

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

“…The choices of low energy reconstructions without H follow previous investigations [53,57,69,70]. Specifically, on ZnO polar surfaces, although several different surface reconstruction models are proposed by both theoretical calculations and experiments [53,70], the surface energy differences among them are in the order of a few meV/Å 2 [70], suggesting that the (2×2) adatom reconstruction is adequate for our discussions here. For GaN, (2×2) adatom reconstructions were taken into consideration under the condition of N-rich [57].…”

“…The accurate prediction of growth mode will need the absolute energy based on surface reconstructions. In this paper, we adopted surface reconstructions proposed by early STM observations and first principles calculations [53,57,69,70]. In this paper, we follow our cluster and pseudo molecule method [36] strictly.…”

Two-dimensional Electron Gas at Thiol/ZnO Interface

“…This observation contradicts the earlier report [29], in which the authors concluded from the thermal desorption study that, although ethanethiol dissociated on ZnO(0001), it adsorbed molecularly on ZnO(0001 _ ) at room temperature. Dissociative adsorption of ethanethiol on ZnO(0001 _ ) can be understood because this ZnO surface undergoes reconstruction to form hexagonal terraces or saw-teeth shaped steps [30,31], whose edges should be truncated by coordinatively unsaturated Zn atoms. These Zn atoms are chemically active to induce S−H cleavage upon thiol adsorption.…”

Doping dependence of the surface phase stability of polar O-terminated (0001̅) ZnO

“…Theoretical investigations, however, have proposed several other energetically very similar surface structures and predict a rich surface phase diagram [10][11][12][13][14][15][16], indicating that the surface is highly sensitive to the environmental conditions. Also, experimentally, a wide variety of structures has been observed for similar growth conditions [17][18][19][20][21][22]. Although a lot of effort has been made, and the previously conducted studies contributed significantly to a better understanding of ZnO surfaces, the origin of its structural diversity is not satisfactorily understood, and there are still variables whose impact on the surface phase stability have not been considered so far.…”