Effects of ZnO seed layer annealing temperature on the characteristics of the n–ZnO nanowires/Al2O3/p-Si heterojunction are investigated. Well-aligned ZnO nanowires (NWs) are grown through a simple hydrothermal method. Both the insertion of Al2O3 buffer layer and the annealing treatment of ZnO seed layer are advantageous for the growth of ZnO NWs. This leads to a relatively high rectification ratio of up to 7.8 × 103 at ± 4.0 V in ZnO NWs/Al2O3/p-Si heterojunction photodetectors. The photoelectrical property of n-ZnO/p-Si photodetectors with an enhanced UV/dark current ratio as high as 30 under a reverse bias of 4.0 V is obtained.
© 2015 Optical Society of America
Zinc oxide (ZnO) has been widely studied because of its unique optical and electrical properties . Especially, the ZnO nanostructures including nanoparticles , nanorods  and nanowires  have their own advantages in different applications dependent on their morphology and shape . Zinc oxide nanowires (NWs) are of considerable interest for the optoelectronic applications of ZnO NWs/p-Si heterojunction (HJ) photodetectors.
Among various techniques for the fabrication of ZnO NWs, the hydrothermal method is commonly used due to its simple process and low operation temperature . A ZnO seed layer is usually coated on the substrate before the growth of well-aligned ZnO NWs by the hydrothermal aqueous-solution method. Many kinds of ZnO seed layers are reported, such as spin-coated ZnO seed nanoparticles , sol–gel seed layers  and RF magnetron sputtered ZnO seed layers . It is known that the quality of grown ZnO NWs is very dependent on the preparation conditions of the seed layer. Moreover, the annealing treatment of the seed layer is benefit for the ZnO NW properties including size, shape, morphology, and growth orientation. It was confirmed that the annealing treatment of seed layers cannot only improve the ZnO nanorods/Si interface but also enhance the electrical properties of ZnO/p-Si HJs, especially in an O2 atmosphere . Hwang et al. also reported the effects of pre-annealing conditions on the characteristics of ZnO nanorods and ZnO/p-Si HJ .
Besides, it is demonstrated that the electrical properties of n-ZnO/p-Si HJ were improved by embedding an insulator layer. Recently, Hwang et al. have fabricated a p-Si/n-ZnO HJ photodetector using ZnO as insulator layer, obtaining a UV/dark current ratio of 13 at a reverse-bias voltage of 4.0 V . Our previous work also showed a planar n-ZnO/p-Si photodetector with an ultrathin Al2O3 insulator layer . The Al2O3 layer with optimized thickness exhibits significant advantages in enhancing the crystal quality of ZnO and improving the photoelectrical properties of n-ZnO/Al2O3/p-Si photodetectors by reducing leakage current. However, there is no study of the photodetectors based on n-ZnO NWs/Al2O3/p-Si. In this study, the ZnO NWs are formed by hydrothermal method on a ZnO seed layer, which is deposited through the atomic layer deposition (ALD) method. The effects of ZnO seed layer annealing temperature on the structural, optical, and electrical properties of this photodetector were systematically studied through x-ray diffraction (XRD) (XRD, D8 ADVANCE, Bruker AXS, Inc.) with X-ray source of Cu Kα radiation (40 kV, 40 mA, λ = 1.54056 Å) and scanning electron microscopy (SEM) of JSM6700F operated at 20 KV. Current-voltage (I-V) characteristics of the n-ZnO/Al2O3/p-Si photodetectors were obtained using an Agilent B1500A semiconductor device parameter analyzer.
2. Experimental details
The Al2O3 buffer layer and ZnO seed layer were deposited by ALD at 200 °C using a BENEQ TFS-200 reactor on n-doped Si (100) (ρ = 1-10 Ω.cm) substrates. Before being loaded into the ALD reactor, the silicon substrates were cleaned using a standard Radio Corporation of America (RCA) solution. They were then being dipped into the diluted 5% HF solution for 1 minute to remove the native oxide layer, followed by a rinse with deionization (DI) water and drying in N2. The Al2O3 films were prepared by using trimethylaluminum (TMA) and DI water as precursors, while the ZnO films were deposited by alternating exposures to diethylzinc (DEZn) and DI water. The precursors were alternately introduced to the reactor chamber using high purity N2 (>99.99%) as a carrier gas. The nominal film thicknesses for Al2O3 insulator layer and ZnO seed layer are 10 and 20 nm respectively. After the deposition, the samples were immediately annealed at temperatures from 300 to 600 °C in oxygen ambient for 5 min. Then the ZnO NWs were grown on the seed layer using a simple hydrothermal approach. All the samples were kept upside down in a flask filled with an aqueous solution of zinc acetate (25 mM) and hexamethylenetetramine (HMTA) (25 mM). The growth temperature and time are 80 °C and 20 hours, respectively. After the growth, the wafer was rinsed with DI water and dried in N2 atmosphere. Finally, Au electrodes as ohmic contact with ZnO  were deposited on the upside of the NW array. The area of one electrode pad is about 2.5 × 10−3 cm2. In addition, an Au layer of 100 nm was also deposited on the back side of the substrate to provide an ohmic contact with Si. The schematic diagram of the whole device structure is shown in Fig. 1 . For comparison, the sample without Al2O3 buffer layer was also fabricated.
3. Results and discussion
The influence of annealing temperature on the thickness of ZnO seed layers are investigated firstly. Table 1 lists the thicknesses of ZnO seed layers annealed at different temperature, which are measured by spectroscopy ellipsometry. As can be seen, the thickness of ZnO seed layer increases from 21.1 to 23.9 nm with increasing the annealing temperature, which can be explained by the growing sizes of ZnO crystals in the prepared thin film .
Figure 2(a) gives the XRD patterns of ZnO seed layers annealed at different temperatures in O2 ambient for 5 min with/without an Al2O3 buffer layer. As can be seen, the XRD peaks of thin ZnO seed layer grown directly on Si substrate could hardly be observed, indicating it is poorly crystalized. This can be explained by the lattice mismatch between Si and ZnO. It is well known that the crystal lattice parameters of Si (diamond, a = 0.543 nm ) and ZnO (wurtzite, a = 0.324 nm and c = 0.520 nm ) are not particularly compatible. However, a peak appears at 2θ = 34.4° after inserting an Al2O3 buffer layer, which is consistent with the standard values of the bulk ZnO crystal (JCPDS 36-1451). This peak is assigned to the spacing in (002) directions of ZnO. It suggests that inserting an ultrathin Al2O3 buffer layer between ZnO and Si substrate is an effective method to reduce defects and improve the interface quality . Under high temperature annealing treatments, excessive thermal energy causes interdiffusion between ZnO layer and Si substrate. The diffused constituent from Si to ZnO acts as impurities in the ZnO thin film which will degrade the optical property of film. Since Al-O bond energy (511 kJ/mol) is much larger than the Zn-O bond energy (211 kJ/mol) and the Si-O bond energy (452 kJ/mol), it is much more difficult for Si-O and Zn-O bonds to break or to react with each other across an Al2O3 layer . Hence the inserted Al2O3 thin film can work as a buffer layer and interrupt the elemental interdiffusion during annealing, which was helpful to decreasing defects and enhancing crystallization of the ZnO seed layers . With increasing annealing temperature of the ZnO seed layer, not only the (002) peak intensity increases regularly but also the width at half maximum of (002) peak also decreases. It demonstrates that the crystallinity of the film is effectively improved by supplying sufficient thermal energy under the annealing treatment. Generally, ZnO seed layer with better crystal quality is propitious to the growth of high-quality NWs .
Figure 2(b) presents the corresponding XRD patterns of ZnO NWs grown on seed layers shown in Fig. 2(a). These ZnO NWs have a wurtzite structure. The sharp and strong ZnO (002) peak reveals its highly c-axis-oriented structure with excellent single crystallinity. The variation of peak intensity of ZnO NWs is consistent with the corresponding annealed seed layers. It can be observed that the ZnO NWs grown with seed layer annealed at 500 °C show the strongest intensity of (002) peak. However, the intensity of (002) peak for the ZnO NWs grown with seed layer annealed at 600 °C is slightly lowered due to the oxygen atoms evaporation . The morphology of ZnO NWs conducted by SEM is presented in Fig. 3 . It can be obtained from the SEM images that the as-grown ZnO NWs are well vertically aligned and densely cover the entire wafer with a single crystal structure. The diameter of the hexagonal NWs is approximately 40 nm with a length of about 1.2 um.
After the growth of ZnO NWs, ZnO NW/p-Si HJs were fabricated to form the ultraviolet photodetector. The effects of seed layer annealing temperature on the electrical properties of ZnO NW/p-Si HJs can be analyzed using I-V curves, as plotted in Fig. 4 . All samples exhibit a clear rectifying behavior. The rectification ratios for the various samples are listed in Table 1. Here the rectification ratio is defined as the forward current at 4 V divided by the reverse current at −4.0 V. As can be seen, the n-ZnO NW/p-Si HJ without Al2O3 buffer layer and annealing treatment has the lowest rectification ratio of 67.3. When an Al2O3 buffer layer is inserted between ZnO NWs and Si substrate, the rectification ratio of HJ is greatly increased due to an obvious reduction of leakage current under reverse bias voltage. While the ZnO seed layers are annealed in O2 with increasing temperatures, the rectification ratios can be further increased. The maximum value of rectification ratio for Sample 5 with ZnO seed layer annealed at 500 °C can reach up to 7791.5, which is approximately 112 times higher than the one without seed layer annealing. The value is also much lower than those obtained from ZnO p-n junction photodetectors [23, 24 ].
It is proposed that rapid thermal annealing treatment can integrate small grains to form larger ones and improve the crystallinity. The leakage current of the corresponding ZnO NW/p-Si HJs can be then greatly reduced due to the improvement of the interface between the ZnO NWs and Si substrate and the decrease of the defects in ZnO seed layers. Thus the annealing treatment strengthens the rectifying behavior significantly.
Figure 5 shows the I-V curves of the n-ZnO NWs/p-Si and n-ZnO NWs/Al2O3/p-Si photodetectors in the dark and under 365 nm UV illumination. As shown in Fig. 5(a), there is little difference between dark current and photocurrent for the n-ZnO NW/p-Si HJ, which is not suitable for a UV photodetector. However, for the n-ZnO NW/Al2O3/p-Si photodetectors, the samples exhibit completely different I-V behaviors, as shown in Figs. 5(b)-5(f).
To gain a better understanding of the way of the Al2O3 layer working in the n-ZnO NW/p-Si HJ, the energy-band diagram of n-ZnO/Al2O3/p-Si HJ under reverse bias and in UV illumination is illustrated in Fig. 6 . The potential barrier for electrons in p-Si is Ve-Si = 2.8 V in the p-Si/i-Al2O3 interface. As a result, it is difficult for the intrinsic electrons to cross over the interface between p-Si and Al2O3. Moreover, the electron injection from p-Si to n-ZnO is effectively suppressed at a reverse bias in the dark. However, it could be fully absorbed in the depletion region of n-ZnO and generate electron-hole pairs when UV light with a wavelength shorter than 378 nm is applied on the device. The photo-generated electrons drift toward Au electrodes while holes do toward the p-Si side under the electric field. On the other hand, there is a barrier height of only 0.4 V for holes of n-ZnO in the n-ZnO/i-Al2O3 interface, which is much smaller than the one of Ve-Si. A large photocurrent can be then generated due to the photo-generated holes can transmit through the Al2O3 layer easily. However, the influence of Al2O3 thickness on the UV photocurrent is relatively less than dark current, which results in a large UV/dark current ratio. Comparing the I-V characteristics of these samples, it is noticed that the UV photocurrent is almost the same level, while the background dark current level is dramatically different under the reverse bias. The max UV/dark current ratio at an applied bias of −4.0 V is 30.3 when the seed layer of sample is annealed at 500 oC. However, a great number of holes hindering the growth of ZnO NWs will be created at 600 °C due to severe Zn dissipation. Thus the UV/dark current ratio for the sample with seed layer annealed at 600 oC decreases slightly, which is in consistent with the XRD results.
In summary, n-ZnO NWs/p-Si and n-ZnO NWs/Al2O3/p-Si HJs were fabricated using ALD and hydrothermal method. The ZnO NWs with seed layers annealed at various temperatures grow dominantly in wurtzite structure with a preferential (002) orientation. A thin Al2O3 layer works as both a buffer layer for the high quality growth of ZnO seed layer and a barrier layer for the realization of visible-blind UV detection. The n-ZnO NW/Al2O3/p-Si HJ with seed layer annealed at 500 °C exhibits the best electrical and optical properties, achieving an extremely high rectification ratio of up to 7791. A UV/dark current ratio as high as 30 under a reverse bias of 4V is also obtained. These results indicate an enhanced UV photodetecting ability for n-ZnO NW/Al2O3/p-Si HJs with annealed ZnO seed layers at an appropriate temperature.
This work is supported by the National Natural Science Foundation of China (no. 51102048, 61376008), the SRFDP (no. 20110071120017), Innovation Program of Shanghai Municipal Education Commission (14ZZ004), Hui Chun Chin and Tsung Dao Lee Chinese Undergraduate Research Endowment (CURE), and the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201404). We are grateful to Prof. Xiao-Sheng Fang from Department of Materials Science, Fudan University for the experimental assistance.
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