Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires
We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 103 for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs.
KeywordsZnO Nanowires rapid thermal annealing photocurrent photoresponse
rapid thermal annealing
field emission scanning electron microscopy
high-resolution transmission electron microscopy.
ZnO nanostructures (NS) such as nanowires (NWs), nanorods, thinfilm, etc. are extensively studied for their applications in various optoelectronic devices, e.g., UV photodetectors, UV light emitting diodes, phototransistors etc. [1, 2, 3, 4, 5, 6]. However, photodetection and photoconductivity of the ZnO NWs depends on the surface condition, structural quality, and the growth methods. For the as-grown ZnO NS, the UV photosensitivity and photoresponse are below the required level for real-time device application [7, 8, 9]. Efforts are being made to improve the UV photosensitivity and photoresponse of the ZnO NS. It is known that the photoconduction in the ZnO NS is controlled by oxygen adsorption and desorption on the surface of the ZnO NS [4, 10, 11]. Therefore, surface modification or structural improvement can improve the photosensitivity as well as photoresponse of ZnO NWs. Various groups have put efforts to enhance the photoresponse and photosensitivity by using appropriate dopant , surface passivation using ZnS coating , polyacrylonitrile/L-lysine treatment [1, 14], integrating ultrathin metal nanoparticles layer [5, 15], and making ZnO nanorod/graphene heterostructure . Since the as-grown ZnO NS contains surface defects which are basically trap centers for carriers, during UV excitation, the trap centers easily trap the photocarriers resulting in low photocurrent (PC) as well as weak photoluminescence (PL). In a recent study, we found that defect-related trap centers on the ZnO NWs surface can be considerably removed by employing RTA processing and an enhanced band-edge PL could be obtained . As a consequence, it is expected that RTA-treated ZnO NWs can show enhanced UV photosensitivity. Here, we report on the effect of RTA on the enhanced photoconduction and photoresponse behavior of the vertically aligned ZnO NWs and the origin of the enhanced PC is explained based on the experimental results. The bi-exponential growth and decay time constants of the PC are carefully analyzed and the mechanism of the observed faster growth and decay after RTA are explained through a suitable model.
ZnO NWs were synthesized by a vapor-liquid-solid method using gold catalyst. Details of the experimental process and growth parameters are discussed elsewhere . In brief, commercial ZnO nanopowder (Sigma-Aldrich, USA, purity 99.999%, average size 50-70 nm) was used as source material and vapor deposition was carried out in a horizontal muffle furnace. The ZnO vapor was formed at 950°C and was deposited on the ultrathin gold coated (approximately 2 nm thick) n- type Si (100) substrate, which was placed downstream at 750°C. The Si(100) substrate was pre-cleaned by standard method followed by HF etching to remove the native oxide layer. After the ZnO deposition, morphology and structure of the samples were analyzed using field emission scanning electron microscopy (FESEM, Sigma, Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM, JEM2100, JEOL, Tokyo, Japan) with selected area electron diffraction (SAED). The SEM and TEM images confirm the vertical growth of ZnO NWs on the Si substrate. Subsequently, the as-grown ZnO NWs was subjected to RTA processing at 700°C and 800°C for 120 s in Ar gas ambient using a commercial RTA system (Mila3000, Ulvac, Yokohama, Japan). During RTA process, the heating and cooling rate was kept at 30°C/s. For the PC measurements, a 25-μm-thick gold wire was used for electrical contact on the top of the ZnO NWs array using good quality silver paste with diameter of the circular contact area approximately 400 μm. The distance between the two electrodes was 1.5 mm. A good Ohmic contact was obtained after annealing at 200°C for 10 min in Ar ambient. For the current measurement on RTA-treated NWs, two new contacts with similar specifications were made near the old contacts on top of the ZnO NWs. The photoresponse was measured using a picoammeter (Model 6487, Keithley, Aurora Road, Cleveland, Ohio) under the illumination of monochromated UV light (wavelength 360 nm) from a 150 W xenon lamp at a light intensity of approximately 0.5 mW/cm2 in ON and OFF conditions. The UV light is tightly focused onto the sample making sure that the region between the two electrodes is only illuminated. The PC spectra were recorded in the excitation range of 300 to 700 nm. The specular reflectance of the as-grown and RTA-treated ZnO NWs was measured using a UV-Vis spectrometer (Varian Carry 50, Varian Inc., Palo Alto, CA, USA). The PL spectra of the as-grown and RTA-treated samples were recorded with a 325 nm He-Cd laser excitation using a high-resolution commercial PL spectrometer (FS 920P, Edinburg Instruments, Kingston, UK). For comparative analysis, PL measurements on all samples were done under identical experimental conditions. All the measurements were carried out at room temperature and atmospheric pressure.
Results and discussion
The room temperature PL spectra of the as-grown and RTA-treated ZnO NWs are shown in Figure 2b. As-grown and RTA-treated ZnO NWs exhibit strong near band-edge (NBE) UV emission at 380 nm and a broad green emission band. Gaussian multipeak fitting shows the existence of two green emission bands, one at 500 nm and another at 545 nm . The observed NBE emission is due to bound excitonic recombination, and the green emission at approximately 500 nm is due to the presence of oxygen vacancy states on the surface of ZnO NWs and second green emission band is due to presence of deep interstitial oxygen states inside the NWs . More details of these results are reported elsewhere . In an earlier study on SnO2 NWs, Kar et al.  showed that post-rapid thermal annealing improves the crystalline quality of the NWs due to the decrease of oxygen vacancy states. In our case, we also observed similar structural improvement which results in enhanced band-edge emission and reduced vacancy-related emission intensities. The RTA-treated NWs at 700°C show threefold enhancement of the UV emission peak intensity compared to the as-grown case and the intensity of the green emission is considerably reduced. The NWs, that are RTA treated at 800°C show slight enhancement of UV emission peak intensity and significant reduction of green emission intensity. The intensity ratio of UV-to-visible emission is increased from 2.0 to approximately 4.85 for the RTA at 700°C. Whereas, a five times enhancement of this ratio is observed for the NWs treated at 800°C, compared to the as-grown case. Further analysis shown that after RTA, only first green emission band is survives, while the 2nd green emission band is fully removed. However, one new emission peak is observed at 394 nm after RTA, which corresponds to the recombination at band-tail states . Details of the peak fitting and peak parameters can be found in the Additional file 1.
The comparison of performances of the ZnO nanostructures-based photodetectors
Light of detection (nm)
Photosensitivity enhancement factor from unmodified photodetector
104 - 106
where A3 and A4 are positive constants and Iph(∞) refers to the photocurrent after infinitely long time of the decay experiment, which essentially is the dark current. The decay time constants are calculated to be 25.7 and 347.9 s for the as-grown NWs, 12.3 and 298.5 s for the RTA treated at 700°C, 13.6 and 118.4 s for the RTA treated at 800°C, respectively. Therefore, the photocurrent 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 for further annealing. On the other hand, oxygen adsorption rates during the photocurrent growth as well as decay are systematically decreases. 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 [1, 5, 11, 14, 30]. Figure 5b shows the photoresponse of the RTA-treated NWs under periodic UV illumination. 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 cycles of photocurrent growth and decay show exactly the replica of first cycle, indicating a PC response of the RTA-treated ZnO NWs, which is important for the real-time application in photodetctors.
We have shown a fivefold enhancement of photosensitivity in the UV region and faster photoresponse in RTA treated vertically aligned ZnO NWs, as compared to the as-grown NWs case. The photocurrent growth and decay rates (photoresponse) from RTA-treated NWs are improved by a factor of approximately 2. The dark current-voltage characteristics clearly indicate the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing substantially removes the surface defect-related trap centers and modified the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. The obtained results demonstrated that the RTA processing is an effective and simple way to achieve higher photosensitivity and relatively fast photoresponse, which is significant for the fabrication of ZnO NW based UV photodetectors.
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