Asymmetry in the excitonic recombinations and impurity incorporation of the two polar faces of homoepitaxially grown ZnO films

Homoepitaxial ZnO layers were grown on O-polar and Zn-polar surfaces of ZnO single crystal substrates by chemical vapor deposition. While the structural properties (surface roughness and rocking curve half width) were identical within experimental error, the optical properties as monitored by photoluminescence (PL) were strikingly different. Four excitonic recombination lines are exclusively found on the O-polar surface. In order to understand the defects involved, secondary ion mass spectrometry was employed which clearly demonstrated that the impurity incorporation is substantially higher on the O-polar surface. Temperature and power dependent PL measurements provide further insight into the initial-final state recombinations. The observed recombinations are caused by excitons bound to a neutral defect complex. In order to account for the thermalization behavior found in the temperature dependent measurements, splittings in the excited as well as in the ground state must be present. Possible defect models are discussed.

Figure 1

Right-hand side photoluminescence spectra of ZnO epitaxial layer with (a) O-face polarity and (b) Zn-face polarity; the observed recombination lines are marked $X_1$ to $X_4$ ($T=4.2\mathrm{K}$, HeCd excitation). Left-hand side PL spectrum of the O-polar film extended to lower energies.

Figure 2

Temperature dependent photoluminescence measurements of the O-polar film from $7\text{to}30\mathrm{K}$ (HeCd excitation).

Figure 3

(a) Intensity ratios of the $X_2$ to $X_1$ lines (squares) as a function of the inverse temperature. (b) Intensity ratio of the $X_3$ to $X_1$ (circles) and $X_4$ to $X_1$ (triangles) lines as a function of inverse temperature. The drawn line is a fit with an activation energy of $0.6\pm 0.1\mathrm{meV}$.

Figure 4

Excitation power dependent photoluminescence spectra reducing the power from 100% to 1%. The inset shows as a comparison of the behavior of the $I_6$ (circles) and $X_2$ (squares) recombination lines.

Figure 5

Secondary ion mass spectra of Zn- and O-polar epitaxial films (a) for negative secondary ions and (b) for positive secondary ions.

Figure 6

Intensity ratio of the normalized secondary ion intensities of the O-polar epitaxial film to the Zn-polar film.

Figure 7

Level scheme for the $X_1$ to $X_4$ transitions (a) for the splitting in the excited state only and (b) the respective splitting in the excited and ground state.