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 ${ϵ}_{cc}=3\times {10}^{-4}$ 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 $E_2\left(\text{high}\right)$ Raman mode are determined to $a=-730{\text{cm}}^{-1}$ and $b=-1000{\text{cm}}^{-1}$. 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.

Figure 1

(Color online) Phase of the reconstructed exit wave of a ZnO substrate. The sample gets thicker from right to left. An atomic overlay shows the $\left[11\overline{2}0\right]$ zone axis of ZnO as used for image simulations. The simulations are compared with the experiment for two thicknesses (squares). The inset shows a line profile over the Zn-O dumbbell.

Figure 2

(Color online) Phase of the exit waves of substrate and epilayer of the O-face (left) and Zn-face (right) samples. The overlay sketches the atoms in $\left[11\overline{2}0\right]$ zone axis. Again rings represent Zn columns; dots represent O columns. The polarity of the epilayers is the same as in the substrate.

Figure 3

(Color online) Phase of Zn and O columns in the epilayer of the Zn-face (upper graph) and O-face (lower graph) samples. Thin lines with data points represent experimental data; straight lines result from simulations.

Figure 4

(Color online) Depth-dependent shift of the $E_2\left(\text{high}\right)$ mode in cross-sectional micro-Raman measurements. Circles and squares indicate the peak position in O-face- and Zn-face-terminated samples at room temperature.

Figure 5

(Color online) Depth-resolved intensity ratio of the $E_1\left(\text{TO}\right)$ to $E_2\left(\text{high}\right)$ Raman modes for Zn-face and O-face ZnO at a depth of $0–7\mu \text{m}$.

Figure 6

(Color online) PL spectra of homoepitaxial grown ZnO layers on Zn- and O-face substrate surfaces; $(T=1.6\text{K})$.