Progress in the growth and characterization of nonpolar ZnO films

Zinc oxide (ZnO) is an important material for its potential applicability to short-wavelength optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs). Nonpolar ZnO materials have been developed in recent years to avoid the strong internal electric fields in active regions of optoelectronic devices and improve luminescence efficiency. The growth and physical properties of nonpolar ZnO films, which are essential for fabricating optoelectronic devices and improving device performance, still remains not well understood. In this review, the technologies for preparation of nonpolar ZnO epitaxial films are summarized, and recent developments are described. Then the main characteristics of nonpolar ZnO films are discussed with the deviations from those of polar ZnO films, including morphology, structural defects, anisotropic strain, optical, and electrical properties. The anisotropic electron transport and strains correlated strongly with the anisotropic surface morphologies of nonpolar ZnO films. Fabricating nonpolar ZnO films with high quality should be further developed to decrease the structural defect densities for substantial improvement of device performance, and intensive studies on their characteristics are especially important for device applications.