Influence of substrate surface polarity on homoepitaxial growth of ZnO layers by chemical vapor deposition

The influence of the substrate polarity (Zn polar or O polar) on the structural and optical properties of homoepitaxial ZnO epilayers grown by chemical vapor deposition is investigated. The polarity of the epilayer is controlled by the substrate polarity as shown by high-resolution transmission electron microscopy (TEM) imaging. Changes in stoichiometry in the epilayer are studied by quantitative TEM analysis. A small compressive strain of epsilon(cc)=3x10(-4) is observed in both epilayers and x-ray diffraction measurements indicate a superior structural quality of the epilayers compared to the substrate. Cross-sectional Raman spectroscopy also demonstrates the superior quality of the epilayers, although high strain is present within the substrates. The phonon deformation-potential parameters of the strain sensitive E-2(high) Raman mode are determined to a=-730 cm(-1) and b=-1000 cm(-1). Differences in the excitonic luminescence including the appearance of different emission lines and an increased full width at half maximum in O-face epilayers are observed. It is suggested that the impurity diffusion from the substrate to the layer is affected by the substrate surface polarity with lower impurity concentrations in the Zn-polar film compared to the O-polar epilayer.