6.1.1 General Properties of ZnO

ZnO is an oxide of the group II metal zinc, and belongs to the P63mc space group [1, 2]. ZnO is on the borderline between a semiconductor and an ionic material. Under most growth conditions, ZnO is an n-type semiconductor, though p-type conductivity of ZnO has also been reported for growth under certain conditions [3, 4]. ZnO exhibits a wurzite structure (hexagonal symmetry) or rock salt structure (cubic symmetry). However, ZnO crystals most commonly stabilize with the wurzite structure (hexagonal symmetry), whereas the crystals exhibit the rock salt phase (cubic symmetry) at high pressure. The wurzite crystal structure and the projection along the [0001] direction are shown in Figure 6.1. Even though it is tetrahedrally bonded, the bonds have a partial ionic character. The lattice parameters of ZnO are a = 0.32495 nm and c = 0.52069 nm at 300K, with a cla ratio of 1.602, which is close to the 1.633 ratio of an ideal hexagonal close-packed structure. In the direction parallel to the c-axis, the Zn-O distance is 0.1992 nm, and it is 0.1973 nm in all other three directions of the tetrahedral arrangement of nearest neighbors.

Figure 6.1: (a) Crystal structure of ZnO (wurzite). (b) Projection along the [0001] direction (pointing out of the plane of paper, denoted by the "X" mark at the center).

The lattice consists of interpenetrating hexagonal close-packed lattices, separated along the c-axis by u...