Fabrication and photosensitivity of ZnO/CdS/silica nanopillars based photoresistor

A kind of Zinc oxide (ZnO)/Cadmium sulfide (CdS)/Silica nanopillars structure is fabricated to photoresistor for the first time. The silica wafer with countless nanopillars is used as the substrate for photoresistor. CdS and ZnO film are deposited by frequency (RF) magnetron sputtering onto the silica nanopillars surface to form ZnO/CdS/Silica nanopillars structure. The ZnO nanowires also can grow on the CdS surface by the hydrothermal reaction to ZnO nanowires/CdS/Silica nanopillars structures. The X-ray diffraction curves show that the ZnO and CdS film both on the planar silica and nanopillar silica surface are well-crystallized. The ZnO film deposition can reduce the reflection and increase the absorption of the incoming light, especially for the light with a wavelength less than 350 nm. And the ZnO/CdS heterojunction structure can retard the recombination probability of the photon-generated carries and prolong the lifetime of the photon-generated carriers, which lending to a remarkable photocurrent improvement. The photosensitivity property testing results show that the ZnO nanowires/CdS/Silica nanopillars structure has the best photosensitivity response of 135 for the white light with 10 mW/cm2 illumination.


Introduction
Cadmium sulfide (CdS) with a bandgap of 2.4 eV is an important II-VI semiconductor compounds semiconductor, and it has excellent optoelectronic properties in the visible light region, so it is widely used in electronic and optoelectronic devices such as solar cells [1,2] light emitting diodes [3], photoresistors [4,5], and so on [6,7]. For the CdS based photoresistor, when it is exposed to light with a certain wavelength, the resistance of the photon-sensitive material will be decreased sharply for the photongenerated carriers. When the light is removed, the resistance will be recovered gradually. For the photoresistor, many methods are adopted to increase the sensitivity, for example making the CdS nanostructures to increase the sensitivity surface, such as nanowires, nanorods, nanoflowers, and so on [8][9][10].
And it was proved that the photocurrent to the dark current ratio of the CdS based photodetectors can be further enhanced through the formation of heterojunctions [11,12]. Zinc oxide (ZnO) is a functional semiconductor with the bandgap of 3.37 eV, and the conduction band and valence band of the ZnO are 0.2 and 0.8 eV lower than those of CdS respectively, therefore the ZnO/CdS heterojunctions might be a promising structure for the photosensitive resistor of visible light [13,14]. ZnO nanostructures have excellent optical and physical properties due to high surface-to-volume ratio, therefore, different kinds of one-dimensional ZnO nanostructures are prepared, such as nanowires [15], nanorods [16], nanoflowers [17], and so on. In the previous study, we reported the preparation and photosensitive property of CdS film on silicon nanopillars surface, which proved that the nanopillars with the large high aspect ratio as the substrate can improve the photosensitive property of CdS material [18,19]. We have also fabricated the ZnO nanowires on the Si nanopillars surface successfully [20]. In my opinion, if the ZnO nanowires/ CdS layer fabricate on to the silica nanopillars surface, on one hand, the structure has the largest surface, and on the other hand, the ZnO/CdS heterojunction can improve the photosensitive property of CdS. Therefore, the ZnO nanowires/CdS/ silica nanopillars structure might be a promising structure for the visible light photoresistor application.
In this work, we fabricated the ZnO/CdS heterojunction on the silica nanopillars substrate for photoresistor application. Firstly, we deposited a layer of CdS film on the silica nanopillars surface by radio frequency (RF) magnetron sputtering, and then deposited a layer of ZnO film by RF magnetron sputtering or grew ZnO nanowires on the CdS surface by hydrothermal reaction to form ZnO/CdS heterojunction. This ZnO nanowires/CdS/silica nanopillars structure was used for the photoresistor application for the first time, and the testing results revealed that this structure can improve the photosensitive performance of the CdS layer based photoresistor for white light.

Experimental and section
In this work, 2-inches polished (100) monocrystalline silicon wafers with resistivity [ 2000 X cm and 400 lm thickness are chosen as substrates for photoresistors. The fabrication process are recorded in Fig. 1. Firstly, the Si nanopillars are fabricated by cesium chloride (CsCl) self-assembly and dry etching, which is low-cost and suitable for mass production. The fabrication process of the Si nanopillars includes the following four steps: CsCl film deposition, development to nanoislands, ICP etching, and CsCl nanoislands removal, which are recorded in Refs. [21,22] in detail. Secondly, to avoid the effects of silicon on heterojunctions, the Si wafer is heated at 900°C for 1 h in the O 2 atmosphere to form a layer of silica film on the Si nanopillars surface. Thirdly, a 150 nm thickness CdS film as the photosensitive layer is deposited on the planar or nanopillars silica surface by radio frequency (RF) magnetron sputtering. The RF sputtering condition is that: CdS target with 99.99% pure, 1 9 10 -4 Pa base pressure, 0.2 Pa working pressure, 20 sccm Ar, 40 W RF power, and 8 min. Fourthly, two methods are adopted to fabricate the ZnO film on the CdS film surface. The one is that: a 70 nm thickness ZnO film layer is directly deposited by RF magnetron sputtering with 99.99% pure ZnO target, 1 9 10 -4 Pa base pressure, 1 Pa working pressure, 20 sccm Ar, 120 W RF power, and 15 min. The other one is that: 20 nm ZnO film layer is deposited by RF magnetron sputtering as the seed layer, and then the ZnO nanowires grow on the seed layer surface by the hydrothermal reaction method with 0.05 M zinc chloride and 0.05 M hexamethylenetetramine in 80 ml deionized water, and at 95°C keeping 30 min in a water bath [19]. Finally, the Ti/Ag interdigitated electrodes with 100 lm width and 1 lm thickness are deposited onto the surface by the thermal evaporation method to the photoresistor.
The six different kinds of structures are fabricated to CdS based photoresistors for comparison, which are revealed in Fig. 2. Sample A is CdS layer on the planar silica surface, sample B is ZnO/CdS/planar silica structure, sample C is ZnO nanowires/ CdS/planar silica structure, sample D is CdS/ silica nanopillars structure, sample E is ZnO/CdS/ silica nanopillars structure, and sample F is ZnO nanowires /CdS/ silica nanopillars structure. Samples A-C is different structures on the planar silica surface to research the effect of the ZnO/CdS heterojunction, while Samples D-F is the structures on the silica nanopillars surface, which can be compared with the Samples A-C to research the effect of the nanopillars on the optical and electrical properties.
CdS and ZnO film is deposited by a magnetron sputtering system (JGP-450, made by SKY Technology Development Company, Chinese academy of sciences). Ti/Ag electrodes are deposited by a vacuum coating machine (DM-300B).

Results and discussion
The morphologies of ZnO/CdS/Silica structures are examined by scanning electron microscope (SEM, Hitachi-S4800), shown in Fig. 3. Fig. 3a, c, e, g and i are the samples on the polished wafer surface, while Fig. 3b, d, f, h and j are that on the nanopillars wafer surface. Figure 3a and b are the polished Si wafer and Si nanopillars after oxidation, and the nanopillars are about 200 nm average diameter and 1 lm height. After RF magnetron sputtering, the CdS layer is about 150 nm thickness on the planar silica surface and the thickness is uniform shown in Fig. 3c. From the Fig. 3d, the CdS layer fabricated by RF magnetron sputtering deposition can cover both the top and side-wall of the silica nanopillars tightly, and the thickness is slightly lower than that on the planar surface. After the ZnO film deposition, the film on the planar silica surface increases to 220 nm shown in Fig. 3e, which reveals that the ZnO layer is about 70 nm. Fig. 3g, h are the ZnO nanowires grow on the CdS surface, the nanowires on the planar silica surface is about 50 nm width and 500 nm high, while the ZnO nanowires growing on the silica nanopillars surface are about 20 nm width and 200 nm high. The growing rate of the ZnO nanowires on the planar surface is much higher than that on the silica nanopillars surface [22]. The reasons are that on the nanopillars surface, the seed layer is thinner than that on the planar surface, and the cramped space between the silica nanopillars limits the growth of the ZnO. Form Fig. 3g-j, the ZnO nanowires are thick both on the planar and nanopillars silica surface.
The crystal structure of the ZnO nanowires/CdS/ silica nanopillars structure is determined by X-ray diffraction (XRD) using a Rigaku diffractometer with CuKa X-rays from 20°to 65°and a scan step of 0.02°, which are recorded in Fig. 4 The reflectivity of the ZnO nanowires/CdS/silica nanopillars structures is measured by an ultravioletvisible-near-infrared spectrophotometer (Agilent Cary 5000) from 200 to 800 nm. For the planar silica surface in Fig. 5a, the bare silica wafer has a high reflectance above 45% for the whole wavelength. While after the CdS layer deposition, the reflectivity has a sharp reduction at the wavelength below 600 nm for the CdS absorption. With the ZnO film covering, the reflectivity for wavelength from 200 to 380 nm becomes much lower, which is absorbed by the ZnO. After the ZnO nanowires growing, the reflectivity is suppressed to below 15% for wavelength from 200 to 600 nm. On the silica nanopillars surface in Fig. 5b, the reflectivity for the whole wavelength is below 15%. For the bare silica nanopillars surface, the reflectivity for wavelength from 300 to 800 nm is lower than 8%, for wavelength 200-300 nm, the reflectivity is much high. After the CdS layer covering, there is a slight increase in the reflectivity of wavelength 300-800 nm, and there is a low reflectivity for wavelength from 200 to 300 nm. With the ZnO nanowires growing, the reflectivity is lower than that without ZnO nanowires, especially for the wavelength below 400 nm. Compared with Fig. 5a and b, the reflectivity of the structure on the nanopillars based wafers is much lower than that on the planar silica wafer surface. The lower reflection property of nanopillar arrays can be attributed to that the highly rough surface of the nanopillars lead to strong light scattering and most of the incoming light could be absorbed after multiple reflections in the forest of nanopillar arrays. The CdS material can absorb some incoming light of energy more than 2.4 eV (corresponding to 517 nm wavelength), and the ZnO film can absorb some incoming light of energy more than 3.37 eV (corresponding to 368 nm wavelength). The ZnO nanowires with large surface can reduce the reflection further, therefore, the ZnO nanowires/CdS/silica nanopillars structure has low reflection.
The photosensitive property of based ZnO/CdS/ silica photoresistors is tested by a homemade instrument, including a dark box, a white light source, and a resistance measurer [23]. The illumination of the testing environment in dark is 1 lW/ cm 2 , while that of in white light is 10 mW/cm 2 . When the light incomes, the CdS material absorb the photons and the electron-hole pairs appear, and then the resistance declines instantaneously. When the light shuts off, the electron-hole pairs recombine gradually and the resistance increases gradually. The photosensitivity response (S) is defined as the resistance of the photoresistor in dark dividing the resistance exposed to light. The photosensitivity performance is recorded in Fig. 6; Table 1, and we can see that all the ZnO/CdS/silica based photoresistors have obvious photosensitivity properties. When the light turns on or off, the resistance declines or increases sharply without hesitating for all the photoresistors. Take the photoresistors with only the CdS layer as the example, on the planar silica surface, the resistance under 10 mW/cm 2 white light is 18 KX, when the illumination of environment change to 1 lW/cm 2 , the resistance restores to 1.35 MX, so the response is 1.35 MX / 18 KX = 75. With the same calculation method, the response for CdS layer on silica nanopillars surface is 2.1 MX / 23 KX = 91, which is obviously higher than that of the planar based one. After 70 nm ZnO covering the CdS film, the S increase to 105 and 125 respectively, when the ZnO nanowires grown, the photosensitivity response further improves to 112 and 135, which indicates that the ZnO film covering can increase the photosensitivity response of the CdS based photoresistor both on the planar silica and silica nanopillars surface. On one hand, the ZnO film can reduce the reflection and increase the absorption of the incoming light, especially for the light with the wavelength of less than 350 nm. On the other hand, the conduction band and valence band of the ZnO are 0.2 and 0.8 eV lower than those of CdS respectively, when the light incoming, the photon-generated electrons of the CdS layer will be injected from the conduction band of CdS into the conduction band of ZnO, so the holes will transport along with CdS film, while the electrons will move along ZnO layer. As a result, the heterojunction structure between CdS and ZnO can prevent the recombination probability of the excess photon-generated carries and prolong the lifetime of the photon-generated excess carriers, lending to a remarkable photocurrent improvement. Compared with the photosensitivity performance of sample A to F, we can see that the nanopillars based photoresistors have the higher response than that of the polished silica based one. The advantages of the nanopillars based wafer are that: (1) the nanopillars substrate has a larger surface radio, which is covered by the more sensitive material surface than the planar one. (2) The nanopillar substrate has a larger surface

Conclusions
In this work, it is the first time that ZnO/CdS heterojunction on the silica nanopillars substrate are fabricated for photoresistor application. We fabricated the ZnO film and ZnO nanowires\CdS heterojunction structure on both the polished silica wafer and nanopillars silica surface for comparison.  Both the CdS and ZnO structures on the polished silica and silica nanopillars surface are well-crystallized gotten from the XRD curves. After the ZnO covering, the reflectivity can be reduced, especially for the wavelength less than 350 nm. With the ZnO nanowires growing, the reflectivity is much lower than that without ZnO nanowires. No matter with or without ZnO or CdS film, the reflectivity of the silica nanopillars wafer is much lower than that of the planar surface. After the ZnO/CdS/Silica nanopillars photoresistor fabrication, the photosensitive property is tested and the results show that the ZnO/CdS structure can improve the photosensitive performance and the silica nanopillars can increase the photosensitivity response. The ZnO nanowires/CdS/ Silica nanopillars structure has the best photosensitivity response of 135 for the white light with 10 mW/cm 2 illumination, which proved that this structure can be used for the photoresistor with high response.