Fabrication and structural, electrical characterization of i-ZnO/n-ZnO nanorod homojunctions
Well-aligned ZnO nanorods were synthesized by a vapor phase transport method on ZnO buffer layer coated n-Si substrates. 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 and 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 the grainy shape nano-crystallites with the wavy surface morphology. The current-voltage (I-V) characteristics of these structures in the temperature range of 150-300 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 (Φ b0 ) and ideality factor (n) were found to be strongly temperature dependent whereby n decreases while Φ b0 increases with increasing temperature. The ln(I 0 /T 2 ) vs q/kT plot shows a straight line behavior and the values of activation energy (E a = Φ b0 ) and the Richardson constant (A*) determined from the intercept and slope of the plot were 0.926 eV and 2.61 x 10 -8 A cm -2 K -2 , respectively. This value of A* is much lower than the known value of 32 A cm -2 K -2 for ZnO. Thus, a modified ln(I 0 /T 2 )-(σ 0 2 q 2 /2k 2 T 2 ) vs. q/kT plot based on a Gaussian distribution of barrier heights was used which yields a mean barrier height (Φ@? b0 ) and modified effective Richardson (A**) of 1.032 eV and 34.85 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.
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