SUN Chao-Xiang, CHEN Liang, CHANG Yu, TIAN Wei, LI Liang

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A Self-powered UV-visible Photodetector Based on p-Se/Al2O3/n-ZnO Nanorod Array Heterojunction

SUN Chao-Xiang, CHEN Liang, CHANG Yu, TIAN Wei, LI Liang

(Jiangsu Key Laboratory of Thin Films, School of Physical Science and Technology, Soochow University, Suzhou 215006, China)

Self-powered photodetector is capable of transforming radiation signals to electronic signals without assistance of external power supply, which is widely utilized in industry and military. Herein, p-Se/n-ZnO nanorod array heterojunction was successfully synthesized and its application as UV-visible photodetector was explored. Due to the proper built-in electric field at the interface of ZnO and Se, the photogenerated electrons and holes are separated and transported in opposite directions, giving rise to high photocurrent at zero bias. Thus, this heterojunction can work as an independent and wireless self-powered photodetector. Introduction of Al2O3interlayer between Se and ZnO efficiently reduces the dark current. The resulting device achieves high responsivity of 55 μA?W–1and specific detectivity of 5×1010Jones at 500 nm, as well as fast response speed (rise time of 0.9 ms and decay time of 0.3 ms).

ZnO; heterojunction; self-powered; photodetector

Photodetectors, which can convert radiation signals to electronic ones, have attracted enormous attention for a variety of industrial and military applications, including biological and chemical analysis, environmental monitoring, visible light communication, and space exploration[1-7]. Particularly, photodetectors working without consuming external power source, namely self-powered photodetectors, are urgently required to meet the demand of modern optoelectronic devices in the trend of lightweight and energy saving[8-9]. Self-powered photodetectors are usually constructed on hetero or Schottky junction. By exploiting the built-in electric field at the junction interface, photodetectors can detect the light signals at zero bias without consuming any external power[10].

Currently, utilization of low-dimensional wide-bandgap semiconductor nanomaterials, such as ZnO[11], SnO2[12], ZnS[13-14], Nb2O5[15]and GaN[16], has attracted intense attention to develop high performance photodetectors. As a typical II-VI group compound semiconductor, zinc oxide (ZnO), possesses wide direct band gap (3.37 eV), large exciton binding energy (60 meV), high carrier mobility exceeding 100 cm2?V–1?s–1at room temperature, and high resistance to irradiation[17]. These unique features make ZnO a promising candidate for photodetectors. Particularly, one-dimensional ZnO nanorods are usually utilized to construct photodetectors, due to their high crystallinity, large BET area and excellent carrier transport property. However, ZnO photodetectors usually suffer from low responsivity and specific detectivity due to a large number of trap states within ZnO, and slow response originating from the oxygen adsorption and desorption processes. Tremendous efforts have been made to improve the performance of ZnO based photodetectors, such as surface modification or construction of Schottky junctions.

As an intrinsic p-type semiconductor with small ban-dgap (1.6 eV), selenium (Se) is an ideal material to construct UV-visible broadband photodetector[18]. There are many unique physical and optical properties for Se. For instance, its melting point is as low as 217 ℃, and its conductivity is in the range of 10–6–10–5Ω–1?cm–1. In addition, Se possesses a high hole mobility up to 0.63 cm–1·V–1·s–1and large hole carrier concentration of 9.35×1016cm–1[19-22]. Novel piezoelectric, thermoelectric and non-linear optical response were observed in Se. Recently, Se based photodetectors with various structures, such as thin film, nanowire, nanobelt and so on[19,23-24], were fabricated and demonstrated respectful performance in photodetection. To improve the performance, Se was integrated with TiO2and organic semiconductor to form heterojunction[25-26]. For example, Yu,[26]fabricated a self-powered UV-visible photodetector based on composite structure of single Se microtube and conducting polymers. The optimum device exhibited high on/off ratio of 1.1×103, large detectivity (3.78×1011Jones) and high response speed (rise time of 4.5 μs and fall time of 2.84 ms) at zero bias voltage and 610 nm. Hu,[27]fabricated a photodetector by depositing plasmonic metallic nanoparticles on the Se microtube by utilizing both the heterojunction and surface plasmon coupling effect. The device demonstrates broadband photoresponse from 300 to 700 nm with peak responsivity of about 19 mA?W–1at 610 nm and response time of 0.32 ms. Despite these achievements, many associated problems, including interface defects caused by lattice mismatch between hetero-materials, low mobility of organic semiconductor and complex fabrication process, not only bring about obstacles to the fabrication of composite structures but also degrade the photodetecting performances.

In this work, we report the fabrication of p-Se/n-ZnO nanorod array heterojunction, and its integration into a self-powered and rapid-response UV-visible photodetector. The resulting device exhibits fast response with a rise time of 0.9 ms and decay time of 0.3 ms. Meanwhile, the device achieves the peak responsivity and specific detectivity of about 55 μA?W–1and 5×1010Jones at 500 nm, respectively.

1 Experimental

The fabrication process of p-Se film/n-ZnO nanorod array heterojunction is illustrated in Fig. 1. Firstly, the fluorine doped tin oxide (FTO) glass was cut into 1 cm × 1.5 cm small pieces, cleaned with acetone, ethanol and deionized water in sequence for 10 min each step, and dried with nitrogen. Then the prepared FTO was subject to UV radiation for 20 min to improve its hydrophilic property. Afterward, 40 nm ZnO seed layer was deposited on FTO by atomic layer deposition (ALD) and then annealed at 500 ℃ for 60 min. For the growth of ZnO nanorods, zinc nitrate hexahydrate (Zn(NO3)2) (25 mmol/L) and hexamethylene tetramine (HMTA) (25 mmol/L) were dissolved in deionized water, and followed by stirring for 2 h. After that, the substrates were immersed into the solution with the conductive side facing down. Then the beaker was heated to 90 ℃ and maintained for 6 h in a thermostatic water bath. After the reaction completed, the sample was cleaned by deionized water several times, and then dried in a vacuum oven. To get crystallized ZnO nanorods, the sample was annealed at 500 ℃ for 60 min in a muffle furnace. Then, 2 nm Al2O3film was deposited on the ZnO nanorod array by ALD. For the growth of Se film, a vapor transport and deposition process were performed in a horizontal tube furnace. A ceramic boat containing 3 g Se powder was placed in the middle of the quartz tube. ZnO and Al2O3coated FTO substrate was put 30 cm away from the Se source. To eliminate air in the furnace, pure nitrogen was aerated for 2 h with the speed of 300 sccm. Then the speed was changed to 200 sccm for deposition. During the deposition, the furnace was heated to 380 ℃ and maintained for 4 h. Finally, 100 nm Ag was thermal-evaporated on the surface of Se film as top electrode. The active area of device is 15 mm2.

Fig. 1 Schematic illustration of the fabrication process of the p-Se film/n-ZnO heterojunction

The morphology of the ZnO nanorod array and Se film was characterized using a field emission scanning electron microscope (FE-SEM, Hitachi, SU8010). The crystalline and structural properties of the layers were investigated by X-ray diffraction (XRD, D/MAX–III– B-40 KV, Cu Kα radiation,=0.15418 nm). The absorption spectra of the ZnO and ZnO/Se were measured using a UV-visible spectrometer (Shimadzu, UV-3600). The photoelectric properties of the obtained photodetectors, including current-voltage (-) curves, spectra resp-o-nsivity, and time-dependent photoresponse (-), were characterized using a semiconductor characterization system (Keithley 4200) under 100 mW?cm–2simulated solar light (Newport, 94043A) and monochromatic light produced with monochromator (Zolix, Omni-3009) using order sorting filters. The light intensity was measured by a power meter (Newport, 1936-R). The response speed was recorded by an oscilloscope (Tektronix, MSO 58). The lasers (FU405AL200-GD16 and FU650AD5- GC12) were used as light sources during measurement of the relationship between photocurrent and light intensity. The power intensity of laser can be tuned by changing the DC current.

2 Results and discussion

Fig. 2 shows the top-view and cross-sectional SEM images of the p-Se/n-ZnO nanorod array. As shown in Fig. 2(a), the ZnO nanorod array vertically align on FTO substrate with diameter of 50–150 nm. The length of ZnO nanorod array is about 2.5 μm (Fig. 2(b)). Fig. 2(c) reveals that Se film is composed of large crystals. The cross-sectional image in Fig. 2(d) reveals that the deposited Se film is around 3 μm thick and an intimate contact is formed at the interface between Se film and ZnO nanorod.

Fig. 2 SEM images of (a–b) ZnO and (c–d) p-Se/n-ZnO heterojunction

(a, c) Top view; (b, d) Cross-sectional view

XRD pattern of the p-Se/n-ZnO nanorod array is plotted in Fig. 3(a). All peaks attained are in well agreement with the characteristic peaks of wurtzite ZnO (JCPDS 36-1451) and Se (JCPDS 06-0362). The sharp and strong peaks demonstrate that both ZnO and Se own high crystallinity. The relative intensity of ZnO (002) peak is much stronger than those of other peaks, indicating ZnO nanorods possess a preferred orientation with-axis perpendicular to the substrate. The comparative UV-visible absorption spectra of pure ZnO and Se/ZnO are shown in Fig. 3(b). The pure ZnO nanorod array exhibits absorption when the light wavelength is below 368 nm, which corresponds to the bandgap of ZnO (3.37 eV)[28-29]. Upon deposition of Se film, there is an apparent absorption enhancement in visible light region. And the absorption edge is red-shifted to 730 nm, which is in consistence with the bandgap of Se (1.7 eV)[30].

Fig. 4(a) and (b) represent the typicalcharacteristics of the device under dark, white light and monochromatic light. The significantly enhanced current under light illumination at zero bias voltage can be obviously detected. And the photocurrent curve does not pass the zero point, which means the device can work in self-powered mode. In comparison,-curve of the device without Al2O3interlayer shows a great dark current under white light illumination in the inset of Fig. 4(a). Fig. 4(c) and (d) represent the-curves of the device under periodic light on-off cycles at zero bias voltage. As shown in Fig. 4(c), the current obviously shows two distinct states when the white light irradiation is on and off, respectively. The dark current is only 2.2 pA; nevertheless, the photocurrent is remarkably boosted to a stable value of 435 pA, delivering a high on-off ratio (photocurrent to dark current ratio) of about 200. Fig. 4(d) shows the photoresponse of the device under 405 nm (1.664 μW?cm–2), 532 nm (1.543 μW?cm–2) and 650 nm (1.093 μW?cm–2) monochromatic light illumination. Reproducible and stable photoresponse is observed for the device. Environmental stability is a key parameter of photodetector, which is crucial to its practical application.-curve of the device under 500 nm illumination for 10 min is shown in Fig. 4(e). Noted that the device is exposed to air in ambient conditions without any encapsulation.

There is no obvious degradation for photocurrent after 10 min measurement, indicating its superior stability. Responsivity and detectivity are two important parameters to characterize the capability of a photodetector. Fig. 4(f) shows the spectral responsivity and detectivity from 300 to 800 nm for the device at zero bias voltage. It is revealed that the device exhibits obvious response to UV-visible light with a decrease at about 730 nm, which corresponds to the band gap of Se. Defects existed in Se film can introduce energy levels in the forbidden band, which allows Se to absorb photon with energy less than its bandgap. Thus, there is still response between 730– 800 nm. The heterojunction device has the peak responsivity and specific detectivity of 55 μA?W–1and 5×1010Jones at 500 nm illumination, respectively. In comparison to the high responsivity in visible light region, the responsivity in UV region is relatively low. The responsivity at 320, 340 and 360 nm is 1.54, 1.15 and 1.57 nA×W-1, respectively. The low responsivity in UV region is possibly because the light illuminates on the device from ZnO side. Due to the wide bandgap of ZnO, UV light is mainly absorbed by ZnO, generating electron- hole pairs. However, the holes cannot transit to the top electrode (Ag) to be collected because of the thick Se film, thus giving rise to low photocurrent and responsivity in UV region.

Fig. 3 (a) XRD pattern of p-Se film and n-ZnO nanorod array, and (b) absorption spectra of pure ZnO and Se/ZnO hybrid structure

Fig. 4 Typical I-V curves for Se-Al2O3-ZnO device under (a) dark, white light and (b) dark, monochromatic lights; I-t curves of the device under (c) dark, white light and (d) dark, monochromatic lights; (e) Long-term response of the device under 500 nm illumination; (f) Spectral responsivity and detectivity of the device at 0 bias voltage

Fig. 5(a) and (c) show the-curves of the Se/Al2O3/ ZnO heterojunction device under illumination of 405 and 650 nm lasers with varying intensities. The photocurrent increases gradually with the light power intensity increasing. The corresponding photocurrents as functions of light intensities are shown in Fig. 5(b) and (d). The photocurrents strongly depend on the light power densities, and the dependence can be described by the power law[10]:

p~P(1)

whereprepresents the photocurrent,is the power density of the incident light, andis an exponent[9]. The fitting curves show that=0.57 and 0.50 for the wavelength of 405 and 650 nm, respectively. The non-unity exponents suggest a complex process of electron-hole generation, recombination, and trapping within the Se/ZnO nanorod array.

A designed circuit diagram for characterizing the response speed is shown in Fig. 6(a). Fig. 6(b) presents the response speed testing, where the device was illuminated with the chopped laser of 100, 200, 300, 400 Hz. A decreased response is shown with increasing frequency of the chopped laser in Fig. 6(c). To characterize the real response speed, one response cycle is enlarged. The rise time (r) and decay time (d) are defined as the values needed for the dark current to reach 90% of the maximum photocurrent and down to 10%, respectively. It is found thatranddof the device are 0.9 and 0.3 ms, respectively, as shown in Fig. 6(d).

Fig. 5 I-t curves of the Se/Al2O3/ZnO heterojunction device under illumination of (a) 405 nm and (c) 650 nm lasers

with varying light intensities (The unit of the light power is mW?cm–2); Relationships between the photocurrent and the light intensity at 0 bias voltage under illumination of (b) 405 and (d) 650 nm lasers

Fig. 6 (a)Diagram of electric circuit to characterize the response speed; (b) Time-dependent voltage curves at different chopper frequency; (c) Normalized voltage-frequency curve; (d) Enlarged one response cycle with rise and decay time

Fig. 7 Energy-band diagrams of the p-Se/Al2O3/n-ZnO nanorod array device (a) before and (b) after contact

To disclose the work mechanism of the photodetector, an energy band diagram of the Se-Al2O3-ZnO is plotted in Figure 7. Before contact, Fermi level of ZnO is higher than that of Se (Fig. 7(a)). Once contact takes place, electrons would drift from ZnO to Se until Fermi levels line up. Upon illumination, well-aligned energy-band structure could separate the photogenerated electron-hole pairs in ZnO and Se at the heterojunction interface and help the charge carriers transport to electrodes, giving rise to large photocurrent.

As shown in Fig. 7(b), due to its particular band structure, Al2O3interlayer prevents electron-hole recombination, which could efficiently decrease the dark current. Thus, the introduction of Al2O3interlayer contributes to higher photo-dark current ratio, responsivity and detectivity.

3 Conclusions

In summary, a self-powered p-Se/Al2O3/n-ZnO UV- visible detector was prepared in this study. The self- powered functionality of the device is attributed to the proper built-in electric field between ZnO and Se arising from the well-aligned energy-band structure at the interface, which enables the photogenerated electrons and hole to separate and transport in opposite directions. Superior optoelectronic performances, including high responsivity (55 μA?W–1) and detectivity (5×1010Jones), fast response speed (rise/decay time of 0.9/0.3 ms) are achieved for this device, which promise its potential application in light communication and image sensors.

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基于p-Se/Al2O3/n-ZnO納米棒陣列異質(zhì)結(jié)的自驅(qū)動紫外–可見光探測器

孫超祥, 陳亮, 常宇, 田維, 李亮

(蘇州大學(xué) 物理科學(xué)與技術(shù)學(xué)院, 江蘇省薄膜重點實驗室, 蘇州 215006)

自驅(qū)動光探測器能夠在無外加偏壓的情況下將光信號轉(zhuǎn)化為電信號, 在工業(yè)和軍事領(lǐng)域有著廣泛的應(yīng)用。本研究報道了p型Se薄膜和n型ZnO納米棒陣列異質(zhì)結(jié)的可控合成以及它們作為自驅(qū)動紫外-可見光探測器的應(yīng)用。由于在ZnO和Se的界面處形成的內(nèi)建電場將光生電子–空穴對分離, 促使它們向相反方向傳輸, 最終被電極收集, 在0偏壓下獲得了較高的光電流(435 pA), 從而實現(xiàn)無線的自驅(qū)動光電探測。并且, 在Se和ZnO界面處沉積的Al2O3層有效降低了暗電流。最終, 此器件在500 nm的單色光下顯示了高響應(yīng)率55 μA?W–1和大比探測率5×1010Jones, 并表現(xiàn)出了極快的響應(yīng)速度(上升時間0.9 ms, 衰減時間0.3 ms)。

氧化鋅; 異質(zhì)結(jié);自驅(qū)動;光探測器

O472

A

2018-12-04;

2018-12-26

National Natural Science Foundation of China (51502184, 51872191); College Students' Innovation and Entrepreneurship Training Program (201710285084X)

SUN Chao-Xiang(1995-), male, undergraduate. E-mail: 2196728627@qq.com

TIAN Wei, associate professor. E-mail: wtian@suda.edu.cn; LI Liang, professor. E-mail: lli@suda.edu.cn

1000-324X(2019)05-0560-07

10.15541/jim20180571