Chapter 5: UV Photodetection in ZnO Nanowires
5.1. Photodetection Studies of as–grown ZnO Nanowires
5.1.1. Dark Current–Voltage Characteristic
The dark current–voltage (I–V) characteristic of the as–grown ZnO NWs shown in Fig.
5.1 shows a linear behavior up to a certain bias voltage, after that there is a rapid change in current with voltage.The transition from linearity in the I–V characteristic is probably due to presence of defect states which release electrons at higher bias voltage and contributes to the current conduction process.
-12 -8 -4 0 4 8 12
-20 -10 0 10 20
Dark Current (nA)
Bias Voltage (Volt)
Figure 5.1: The dark current–voltage (I–V) characteristics of as–grown ZnO Nanowires.
5.1.2. Spectral Dependence of Photocurrent
Figure 5.2(a) shows the PC spectra of the as–grown ZnO NWs, measured at a bias of 2.5 V, which shows a strong PC peak at 369 nm and two other relatively weak peaks in the visible region. The observed strong peak is due to the band–edge absorption followed by generation of photocarriers (electron–hole pair). The maximum PC obtained at 369 nm is 9.6µA. Therefore in this case, the UV photosensitivity (photo–to–dark current ratio) is
~4.5×103. The other small peaks in the visible region in the PC spectra are fitted with two Gaussian peaks and obtained peak positions are at 496 nm and 620 nm. The observed peaks in the visible region are possibly due to the generation of carriers from different defect states. The observed PC spectra are consistence with the PL spectra of the as–grown NWs, which show similar spectra with strong peak in the UV region and two relatively weak peaks in the visible region. ZnO nucleation layer (Fig. 5.2(b)) shows similar features with three peaks located at 369, 408 and 495 nm. Note that, here the ZnO NWs grown on the Si substrate; therefore observed improvement in the UV PC is the full contribution of ZnO NWs only. In this case, the obtained photosensitivity is quite low for
practical applications. Since the as–grown ZnO nanostructures contains surface defects which are basically trap centres for carriers resulting in a low PC. It is expected that by structural improvement or heterostructure formation with suitable materials, a visible–
blind ZnO NWs based photodetectors with high sensitivity could be made.
300 400 500 600 700
0 2 4 6 8 10
496 620
Photocurrent (A)
Wavelength (nm)
Experimental Gaussian Fitting Component 369
(a)
350 400 450 500 550 600
0 2 4 6 8 10
495 408
Experimental Gaussian Fitting Component
Wavelength (nm)
Photocurrent (
mA)
369
(b)
Figure 5.2: The photocurrent spectrum of the ZnO: (a) nanowires and (b) nucleation layer, measured at 2.5 V bias.
5.1.3. Photoresponse
The photoresponse behaviour of the ZnO NWs measured at air under the excitation of 360 nm UV light is shown in Fig. 5.3. It is seen that PC initially grows very fast and then slowly increases with time and finally saturates. During decay, i.e. when the UV light is turned off, the PC initially decreases rapidly and then slowly reach the initial dark current value after long time. The PC takes more than 20 mins to reach the maximum value and falls to 90% of its maximum value within 11 mins. As expected, the as–grown NWs shows slow photoresponse due to the presence of intrinsic defects/trap centres.
0 900 1800 2700 3600
0 3 6 9
Photocurrent ()
Time (sec)
UV ON UV OFF
Figure 5.3: The photocurrent growth and decay behaviors (photoresponse) of the as–grown ZnO nanowires measured at 2.5V bias.
5.1.4. Mechanism of Photoresponse in ZnO Nanowires
The photoresponse of the ZnO NWs consists of two parts: a rapid process of photogeneration and recombination of electron–hole pairs, and a slow process of surface adsorption and photodesorption of oxygen molecules. The oxygen plays a crucial role in the photoresponse of ZnO. In dark condition, oxygen molecules from the air are easily stuck on the surface of the NWs by adsorption process and trapped electrons [O2(g)+e−→ O2−] available on the surface near the Zn lattice and decreased the conductivity,20 which is shown schematically in Fig. 5.4(a). This process leads to the formation of depletion layer near the surface resulting in the upward band bending of the conduction band (C.B) and the valence band (V.B). The formation of this depletion layer has a prominent effect on the current conduction process when the diameter of the NWs is comparable to the thickness of the depletion layer. Formation of large number of ionized oxygen on the NWs surface enhances the band bending, resulting in a very low conductivity. During the UV illumination with energy greater than the band gap energy of ZnO, electron–hole pairs are generated [hν→e−+h+] by light absorption. Now these electrons/holes easily cross the depletion layers and contribute to the photoconduction process. At the same time, holes take part in the oxidization of ionized oxygen [O2– +h+→O2(g), photodesorption process] and release one oxygen gas molecule by electron–hole recombination process [Fig. 5.4(b)]. Then few of the released oxygen molecules are re–
adsorbed on the surface and decrease the free electron carriers. The energy band diagram during UV illumination is shown in Fig. 5.4(c). After a certain time electron–hole
Figure 5.4: A schematic of photoresponse mechanism in ZnO nanowires: (a) at dark condition and (b) during UV illumination. (c) Schematic energy band diagram of photoresponse process during UV illumination.
generation rate and oxygen re–adsorption rate become constant resulting in a steady photocurrent. It is known that adsorption process is slower than the photodesorption process. Therefore, during UV illumination, not all the holes recombine with the electrons present in the ionized oxygen. As a result, excess holes are available for recombination with the exciton related free electrons. During photocurrent decay, the exciton related electron–hole recombination dominates, which corresponds to the faster decay component, so the photocurrent initially decreases very rapidly. With the surface re–adsorption of oxygen, the photocurrent comes to the initial dark current value very slowly.