Structural and electrical characterization of rectifying behavior in n-type/intrinsic ZnO-based homojunctions

ZnO nanorods were synthesized on n-Si substrates by a chemical bath deposition method. X-ray diffraction and scanning electron microscopy results showed that the deposited ZnO nanorods crystallize in the wurtzite structure and are highly textured with their c-axes normal to the substrate and show a clearly hexagonal morphology. A heavily compensated, intrinsic ZnO layer (i-ZnO) doped with both Mg and Na was deposited on the nominally undoped ZnO nanorods (which show a natural n-type behavior) to produce an i-ZnO/n-ZnO homojunction. The i-ZnO layer consisted of hexagonal shaped crystallites oriented mainly perpendicular to the substrate surface. The current–voltage (I–V) characteristics of these structures in the temperature range of 100–300 K have been analyzed in the framework of standard thermionic emission (TE) theory with the assumption of a Gaussian distribution of the barrier heights. The values of zero-bias barrier height (Φbo) and ideality factor (n) were found to be strongly temperature dependent whereby n decreases while Φbo increases with increasing temperature. A Richardson plot of our data shows straight line behavior and the values of activation energy (Ea = Φbo) and the Richardson constant (A*) determined from the intercept and slope of the plot were 0.770 eV and 2.61 × 10−8 A cm−2 K−2, respectively. The value of A* is much lower than the known value of 32 A cm−2 K−2 for ZnO. Thus, a modified Richardson plot based on a Gaussian distribution of barrier heights was used which yields a mean barrier height ( Φ ¯ b 0 ) and modified effective Richardson (A**) of 0.782 eV and 39.09 A cm−2 K−2, respectively. This value of A** is much closer to the theoretical value of 32 A cm−2 K−2 for ZnO.