Chapter 5: UV Photodetection in ZnO Nanowires
5.3. Photoresponse Behaviour of Al doped ZnO Nanowires
and the last exponential term represents the oxygen adsorption process. The decay time constants are calculated to be 25.7s and 347.9s for the as–grown NWs, 12.3s and 298.5s for the RTA treated at 700°C, 13.6s and 118.4s for the RTA treated at 800°C, respectively. Therefore, the PC growth as well as decay becomes faster after the RTA treatment. The calculations of individual time constants show that electron–hole recombination as well as generation rates become double after RTA at 700°C and do not change significantly with higher temperature annealing. On the other hand, oxygen adsorption rates during the photocurrent growth as well as decay are systematically decreased. Similar bi–exponential decay behavior with time constants of several seconds has been reported by several groups for the ZnO nanobelts, NWs and thin film.18,106,186,191,240 Figure 5.7(d) shows the photoresponse of the RTA treated NWs under periodic UV illumination. It is observed that the maximum photocurrent in the next cycle is slightly increased compared to the previous cycle because of the incomplete growth and decay of the PC during the measurement cycle. Second and third cycle of photocurrent growth and decay show exactly the replica of first cycle, indicating a reproducible PC response of the RTA treated ZnO NWs, which is important for the real time application in photodetectors.
As the RTA processing significantly reduces the surface defect related trap centres and modified the surface of the ZnO NWs, the band bending is less here compared to the as–
grown NWs case resulting in a comparatively higher conductivity, as revealed in the dark I–V characteristics. As a consequence, the photocurrent reached the saturation value very fast. Therefore, the structural improvement caused faster photocurrent growth and decay from the RTA treated ZnO NWs. This is consistent with the PL results discussed earlier.
Al:ZnO NWs network. As each of the NWs is connected with the nearest neighbour, current carriers can easily flow from one NW to the other by tunnelling process through the junction between the NWs. The I–V characteristics show nearly linear behaviour with comparatively higher dark current than the undoped NWs. Earlier we found that the dark current of the undoped ZnO NWs varies in the range 2–10 nA. However in the present case, the obtained dark current is one order of magnitude higher and it increases with the increase in doping concentration. The dark current increases from 110 to 180 nA at a bias of 3V with increase in Al concentration from 3% to 6%. The higher dark current could be attributed to the substitutional doping of Al atoms at the Zn site of the ZnO lattice. Here trivalent Al+3 ions contribute to the enhanced current conduction process by increasing the free electron density in the doped NWs.
-10 -5 0 5 10
-0.8 -0.4 0.0 0.4 0.8
Dark Current (A)
Bias Voltage (V) Al:ZnO NWs(3%) Al:ZnO NWs(6%)
Figure 5.8: The dark current–voltage characteristics of the 3% and 6% Al doped ZnO nanowires.
5.3.2. Photoresponse
The photoresponse of the Al:ZnO NWs measured under the excitation of 365 nm UV light is shown in Fig. 5.9. The obtained PC and photoresponse parameters are tabulated in Table 5.2. The response and reset times are <1s, which is far lower than the response time of the undoped ZnO NWs (several seconds). For the 3% and 6% doped Al:ZnO NWs, the maximum PC is about 5.2–5.6 µA. Although these NWs show a high increment in the PC under the UV excitation, the photosensitivity values are very low due to the higher dark current. This behaviour is consistent with the previous studies on Al:ZnO NWs.184,241,242 Such a low photosensitivity from the Al:ZnO NWs hinders the fabrication and development of highly efficient UV photodetectors. However in the
0 400 800 1200 1600 0
2 4 6
0 400 800 1200 1600 0
2 4 6
(b)
Photocurrent (A)
Time (sec)
Photocurrent (A)
Time (sec)
(a)
Figure 5.9: Reproducible photoresponse behaviors (at 365 nm) of the: (a) 3% and (b) 6% Al doped ZnO nanowires, measured at 3 V bias.
present case, the photoresponse and reset times are very fast, compared to the as–grown undoped ZnO NWs. The photoresponse and reset times of the Al:ZnO NWs show a dependence on Al concentration. At low doping level of Al, the photoresponse and reset times (defined as the time required to reach 1–1/e (63%) and recovery to 1/e (37%) of maximum PC) are significantly reduced. The 3% doped Al:ZnO NWs shows photoresponse and reset times of 0.9 and 1.1s, respectively. Additional doping shows a marginal reduction in the photoresponse and reset times. Therefore, the presence of Al accelerates the photocurrent saturation rate, which indicates that the oxygen re–
adsorption rate reached equilibrium instantly. Earlier, similar improvements in the photoresponse and reset times are obtained by Ag doping in the ZnO NWs.194 Therefore, the Al:ZnO NWs show a fast response and comparatively low photosensitivity. The low photosensitivity could be considerably improved by making metal/semiconductor heterostructures with the NWs, which is discussed in the next chapter.
Table 5.2: Photocurrent and photoresponse parameters of the 3% and 6% Al doped ZnO nanowires.
Sample Name
Dark current
(nA)
Photocurrent (µA)
Photosensitivity Response time (s)
Reset time
(s)
3%Al:ZnO 110 5.2 47 0.9 1.1
6%Al:ZnO 180 5.6 31 0.8 1.0
The obtained ultrafast photoresponse from the Al:ZnO NWs can be explained as follows.
It is known that the photoresponse of ZnO depends on electron–hole generation, surface
adsorption and photodesorption processes. It is also known that fast or slow response strongly depends on the time taken to reach the equilibrium rate of oxygen desorption and re–adsorption process. In the present case of Al:ZnO NWs, Al doping possibly change the equilibrium defect concentration. Due to this, surface defects in the Al:ZnO NWs gradually increases with the increase in Al concentration. This is supported by the optical absorption and PL results as explained in the previous chapter. The optical absorption and PL data show gradual increment in the surface defects related absorption and emission in the green region. For this reason, the rates of oxygen desorption and re–
adsorption processes are significantly improved and it instantly reach the equilibrium rate. This process results in a much faster photoresponse in the Al:ZnO NWs.