Quantifying the barrier lowering of ZnO Schottky nanodevices under UV light

Abstract

In this study we measured the degrees to which the Schottky barrier heights (SBHs) are lowered in ZnO nanowire (NW) devices under illumination with UV light. We measured the I-V characteristics of ZnO nanowire devices to confirm that ZnO is an n-type semiconductor and that the on/off ratio is approximately 10(4). From temperature-dependent I-V measurements we obtained a SBH of 0.661 eV for a ZnO NW Schottky device in the dark. The photosensitivity of Schottky devices under UV illumination at a power density of 3 μW/cm(2) was 9186%. Variations in the SBH account for the superior characteristics of n-type Schottky devices under illumination with UV light. The SBH variations were due to the coupled mechanism of adsorption and desorption of O2 and the increase in the carrier density. Furthermore, through temperature-dependent I-V measurements, we determined the SBHs in the dark and under illumination with UV light at power densities of 0.5, 1, 2, and 3 μW/cm(2) to be 0.661, 0.216, 0.178, 0.125, and 0.068 eV, respectively. These findings should be applicable in the design of highly sensitive nanoscale optoelectronic devices.

Figure 1

(a) SEM image of ZnO NWs grown on a Si substrate. (b) Low-magnification TEM image of a single ZnO NW. (c) SAED pattern of a single ZnO NW. (d) High-resolution TEM image of a single ZnO NW.

Figure 2

(a) Schematic representation of the ZnO NW device; inset: SEM image of a single–ZnO NW device. (The figure was drawn by Shuen-Jium You) (b) I_d–V_d curves measured at various gate voltages. (c) I_d–V_g curve recorded at a value of V_d of 0.5 V.

Figure 3

(a) I–V curve (black) and fitted data (red) of a ZnO NW Schottky device. The inset shows the circuitry of the back-to-back Schottky device. (b) I–V curves of the Schottky device measured at temperatures ranging from 298 to 398 K. (c) Plots of ln(I/T²) versus 1000/T at various applied voltages; the data had been extracted from (b). (d) Values of φ_b plotted with respect to the applied voltage.

Figure 4

(a) Schematic representation of a ZnO device illuminated with UV light. (The figure was drawn by Shuen-Jium You) (b) I–V characteristics of a Schottky device in the dark (black) and under 365-nm UV light (red); inset: the same data plotted on a logarithmic scale. (c) I–V characteristics of an ohmic device in the dark (black) and under 365-nm UV light (red). (d) Photoresponses of the ZnO NW Schottky (black) and ohmic (red) devices under illumination with UV light at a power density of 1 μW/cm².

Figure 5

(a) Photoresponse of a ZnO Schottky device measured under illumination with UV light at power densities from 0.5 to 3 μW/cm². (b) I–V curves of the Schottky device in the dark and under illumination with UV light (365 nm) at power densities from 0.5 to 3 μW/cm².

Figure 6

(a) Plot of the values of φ_b with respect to applied voltage under illumination with UV light at various power densities. (b) Values of φ_b0 plotted with respect to the illumination power density; data were obtained from (a); inset: plot of Δφ_b0 (the difference between the values of φ_b0 under UV light and in the dark) with respect to the power density of the UV light.