Density-controlled and seedless growth of laterally bridged ZnO nanorod for UV photodetector applications

This study develops a density-controlled and seedless growth method for laterally bridged ZnO nanorods from Au electrode for use in metal-semiconductor- metal photodetector fabrication. The effect of pre-annealing process on suppressing vertical ZnO nanorods is systematically investigated by atomic force microscopy and scanning electron microscopy. The pre-annealing process is demonstrated to have direct influence on controlling vertical/lateral ZnO nanorod density and morphology. Interlaced and density-controlled ZnO nanorods with approximate single-crystalline structure can be directly grown from the side wall of pre-annealed Au electrode fingers without seed-layer. Through pre-annealing process, dark-current can be decreased from 4.99 × 10 ^-4 to 7.28 × 10^-7 A with an applied voltage of 1 V. Highly dense lateral ZnO nanorod-based photodetectors produce remarkable responsivity of 7.01 × 10³ A/W and UV/visible rejection ratio of 281.21. Moreover, a high internal photoconductive gain (10 ⁴-10⁵) exists in the fabricated photodetectors. For a given bandwidth of 10 kHz and 1 V applied bias, the noise equivalent power of photodetectors with 0, 10, and 20 min pre-annealing periods are estimated to be 3.58 × 10^-13, 6.78 × 10^-13, and 4.86 × 10^-13 W, and correspond to normalized detectivity of 1.85 × 10¹², 1.17 × 10¹², and 1.99 × 10¹² cm Hz^0.5 W^-1, respectively. This result may be attributed to internal photoconductive gain mechanism and high-density bridged ZnO nanorods. Our approach provides a simple and controllable method to fabricate high-performance ultraviolet photodetectors.