Fig. 1 Schematic structure of the Pd electrodes on the ZnO photodetector

Fig. 2 XRD patterns of the ZnO nanostructures deposited at different substrate temperatures: a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 3 Crystalline size of the ZnO nanostructure deposited at different substrate temperatures

Fig. 4 AFM images of the ZnO nanostructure deposited at different substrate temperatures (dimension of the specimens is 5 μm × 5 μm): a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 5 R rms of the ZnO nanostructure deposited at different substrate temperatures

Fig. 6 Different magnifications of the FESEM images of the ZnO nanostructure at different deposition temperatures: a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 7 Room-temperature PL spectra of the ZnO films at different deposition temperatures: a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 8 Energy bandgap of ZnO at different deposition temperatures

Fig. 9 The I-V characteristics of the Pd/ZnO/Pd photodetector deposited at different substrate temperatures: a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 10 Schottky properties of the ZnO photodetectors as a function of deposition temperatures: a saturation current, b barrier height, c ideality factors

Fig. 11 Spectral response as a function of wavelength at a bias of 5 V deposited at different temperatures: a as deposited, b after annealing at 500 °C for 2 h under N2 flow

Fig. 12 The I-V characteristics of the Pd/ZnO/Pd photodetectors before and after annealing at 500 °C in darkness and under UV illumination (385 nm, 1.12 mW/cm2) deposited at different temperatures: a RT, b 200 °C, c 300 °C, d 400 °C

Fig. 13 Photosensitivity as a function of deposited temperature before and after annealing

Fig. 14 Response-time spectra of the as-deposited ZnO photodetectors with UV light illumination (385 nm, 1.12 mW/cm2) turned on and off repeatedly at different deposition temperatures with a bias voltage of 5 V

Fig. 15 Response-time spectra of the ZnO photodetectors with UV illumination turned on and off repeatedly under different conditions after annealing at 500 °C for 2 h: a \( T_{\text{s}} = {\text{RT}},\;\lambda_{\text{UV}} = 40 5\, {\text{nm}},\;U_{\text{bia}} = 5\, {\text{V}} \); b \( T_{\text{s}} = 200\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 3 8 5\, {\text{nm}},\;U_{\text{bia}} = 5 \,{\text{V}} \); c \( T_{\text{s}} = 200\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 40 5\, {\text{nm}},\;U_{\text{bia}} = 5 \,{\text{V}} \); d \( T_{\text{s}} = 300 \,^\circ {\text{C}},\;\lambda_{\text{UV}} = 3 8 5\, {\text{nm}},\;U_{\text{bia}} = 5 \,{\text{V}} \); e \( T_{\text{s}} = 300\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 40 5 \,{\text{nm}},\;U_{\text{bia}} = 5\, {\text{V}} \); f \( T_{\text{s}} = 400\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 3 8 5\, {\text{nm}},\;U_{\text{bia}} = 5\, {\text{V}} \); g \( T_{\text{s}} = 400\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 40 5\, {\text{nm}},\;U_{\text{bia}} = 5\, {\text{V}} \); h \( T_{\text{s}} = 200,{ 3}00{\text{ and 4}}00\, ^\circ {\text{C}},\;\lambda_{\text{UV}} = 40 5\, {\text{nm}},\;U_{\text{bia}} = 2\, {\text{V}} \)

Fig. 16 Schematic diagram of the photoconductive mechanism of the ZnO crystallite: a darkness; b, c under light illumination