Communication Chemical Etching
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Chemical Patterning of High-Mobility Semiconducting 2D Bi2O2Se Crystals for Integrated Optoelectronic Devices Jinxiong Wu, Yujing Liu, Zhenjun Tan, Congwei Tan, Jianbo Yin, Tianran Li, Teng Tu, and Hailin Peng* that are detrimental to the performance of integrated devices. Very recently, layered bismuth oxyselenide (Bi2O2Se) was discovered as a promising 2D semiconductor with high electron mobility (≈30 000 cm2 V−1 s−1 at 1.9 K), moderate bandgap (≈0.8 eV), excellent environmental stability, and easy-accessibility to large production via a facile chemical vapor deposition (CVD) method, making it attractive for high-speed and low-power electronic applications.[12,13] However, to date, facile approaches to pattern chemical stable 2D Bi2O2Se crystals into functional units with large-scale integration and excellent controllability remain elusive. Herein, for the first time, we presented a facile wet-chemical etching approach for controlled patterning of CVD-grown 2D Bi2O2Se film using dilute H2O2 and protonic acid as mixed etchants. Centimeter-scale well-ordered 2D Bi2O2Se arrays with tailorable configuration were readily obtained, revealing a high Hall mobility of over 200 cm2 V−1 s−1 at room temperature. Furthermore, our patterned 2D Bi2O2Se crystal arrays have been integrated to air-stable photodetectors, yielding an ultrahigh photoresponsivity of ≈2000 A W−1 at 532 nm. The chemical properties of substances mainly depend on their lattice structures and chemical components. In view of the elemental components, high-mobility semiconducting Bi2O2Se can be derived from its parent compound of Bi2Se3 topological insulator by partially replacing stoichiometric Se atoms with lighter O.[12,13] Bi2O2Se possesses a layered tetragonal crystal structure (I4/mmm, a = 3.88 Å, c = 12.16 Å, and Z = 2), consisting of [Bi2O2] layers and Se square net layers alternately stacked along c-axis.[14] To some extent, Bi2O2Se is an oxide, especially those [Bi2O2] layers might be dissolved in acid solution. On the other hand, the presence of sandwiched Se layers in Bi2O2Se provides effective resistance against nonoxidative acid such as dilute H2SO4 solution. To develop an effective chemical etchant of Bi2O2Se, appropriate oxidants should be involved. Hydrogen peroxide (H2O2), a widely used oxidant with a high chemical potential of 1.77 V, larger than H2SeO3/ Se (0.74 V) and SeO42−/H2SeO3 (1.15 V), is a perfect candidate for its mildness and environmental friendliness in chemical engineering and electronic industry. To this end, we develop a dilute sulfuric peroxide mixture (H2O2/H2SO4) with an appropriate ratio as an effective etchant to pattern 2D Bi2O2Se crystals. As indicated in Figure 1a, the corresponding reactions can
Patterning of high-mobility 2D semiconducting materials with unique layered structures and superb electronic properties offers great potential for batch fabrication and integration of next-generation electronic and optoelectronic devices. Here, a facile approach is used to achieve accurate patterning of 2D high-mobility semiconducting Bi2O2Se crystals using dilute H2O2 and protonic mixture acid as efficient etchants. The 2D Bi2O2Se crystal after chemical etching maintains a high Hall mobility of over 200 cm2 V−1 s−1 at room temperature. Centimeter-scale well-ordered arrays of 2D Bi2O2Se with tailorable configurations are readily obtained. Furthermore, integrated photodetectors based on 2D Bi2O2Se arrays are fabricated, exhibiting excellent air stability and high photoresponsivity of ≈2000 A W−1 at 532 nm. These results are one step towards the practical application of ultrathin 2D integrated digital and optoelectronic circuits. 2D semiconducting materials with high carrier mobility, moderate bandgap, and ambient environment stability are emerging for next-generation electronics and photonics that are compatible with highly mature silicon-based platform.[1–3] To achieve the full potential of high-mobility 2D semiconductors in integrated optoelectronics networks and multipixel readout digital circuits, the prerequisite lies in accurate and efficient patterning of the 2D materials into designed architectures and integrated modules.[4–7] To this end, great efforts have been devoted to developing effective dry-etch approaches for patterning with excellent controllability, such as exemplified by focused ion beam etching, femtosecond laser etching, and reactive ion etching.[8–10] In contrast to these relatively expensive and complicated dry etching techniques, wet chemical etching is a desirable way for its prominent advantages such as simple procedure, scalable production, well controllability, and low cost.[11] However, it is difficult to achieve precision patterning and placement of novel 2D materials without inducing defects Dr. J. X. Wu, Dr. Y. J. Liu, Dr. J. B. Yin, T. R. Li, T. Tu, Prof. H. L. Peng Center for Nanochemistry Beijing Science and Engineering Center for Nanocarbons Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering Peking University Beijing 100871, P. R. China E-mail:
[email protected] Z. J. Tan, C. W. Tan Academy for Advanced Interdisciplinary Studies Peking University Beijing 100871, P. R. China
DOI: 10.1002/adma.201704060
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Figure 1. Wet chemical etching of 2D layered Bi2O2Se crystals. a) Schematic illustration of the chemical etching of 2D Bi2O2Se crystals, utilizing a mixture of dilute oxidative H2O2 and inorganic protonic acid (H3O+) as etchants. b) Typical OM images of 2D Bi2O2Se crystals on mica before (top) and after (bottom) chemical etching process, verifying the high efficiency of etching with H2O2H3O+ as etchants. c) Etching dynamics of 2D Bi2O2Se crystals under treatment of mixed etchants (98% H2SO4, 30% H2O2, and deionized H2O) with different volume ratios.
be represented as follows: Bi2O2Se + 4H2O2 + 4H+ → 2 Bi3+ + SeO42− + 6H2O. We first evaluated the etching efficiency of the dilute H2O2H3O+ etchant. As shown in Figure 1b, CVD-grown Bi2O2Se nanoplates on a mica substrate were totally disappeared after dipping into the dilute H2O2H3O+ solution for ≈20 s, verifying a high etching efficiency of the etchants. Furthermore, to study the etching process, morphological change of Bi2O2Se nanoplates was monitored under the optical micro scopy (OM) using different etchant concentrations. As shown in Figure 1c, change of the Bi2O2Se nanoplates is negligible if only use the sole dilute H2SO4 or H2O2 for >30 min, indicating excellent chemical stability of 2D Bi2O2Se crystals. Remarkably, as the volume ratio of H3O+ to H2O2 was added gradually to 4:4, the dissolving rate increased rapidly. However, excess acid concentration would result in poor controllability over the patterning of fine structures. To balance the etching efficiency and controllability, the optimized volume ratio of H2SO4:H2O2:H2O is 2:4:8 and a typical etching process can be completed in ≈20 s. Interestingly, if the mild pretreatment of oxygen plasma before chemical etching was introduced, it would apparently accelerate
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the etching rate of 2D Bi2O2Se crystals (Figure S1, Supporting Information), presumably due to the improvement of interface hydrophily or the introduction of point defects during the treatment of oxygen plasma.[15,16] With the assistance of microfabrication technology and prepatterned lithography mask, selective-area chemical etching of 2D Bi2O2Se with desired patterns was readily achieved (Figure 2; Figure S2, Supporting Information). Figure 2a shows the photographs of 1 × 1 cm highly ordered arrays of square Bi2O2Se nanoplates etched from CVD-grown continuous film on mica substrate, displaying facile patterning and excellent spatial homogeneity. The typical OM image of large area 2D Bi2O2Se arrays is comprised of about 3000 discrete square nanoplates with an identical feature size and smooth edges—a side length of ≈10 µm and a periodicity of ≈20 µm (Figure 2a). Besides discrete square nanoplate array, more complex 2D Bi2O2Se arrays with different feature sizes and pattern layouts have been acquired with the assistance of standard electronbeam lithography (EBL) technique. As illustrated in Figure 2b,c, the few nanometers thick Bi2O2Se single crystals can be fabricated into different patterns (triangular, circular, and cubic),
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into standard Hall bar geometry with or without etching process on mica substrate free of transfer process. Based on the room temperature Hall measurements, the 2D Bi2O2Se crystal after chemical etching still maintains a really high carrier mobility of 209 cm2 V−1 s−1 (Figure 3b), which is only slightly lower than the typical value of 243 cm2 V−1 s−1 in as-grown 2D Bi2O2Se crystals (Figure 3a). Remarkably, similar value and changing tendency of Hall mobility are observed upon cooling down (Figure S4, Supporting Information). The reason for the fact that the mobility maintains a relatively high value after chemical etching may be attributed to the protection of the polymer mask during the efficient etching with a very short time (typically 20 s). Additionally, the merely unchanged negative transverse Hall voltage slope (Hall coefficient) suggests the chemical etching process brings ignorable doping effect to the 2D Bi2O2Se. Patterning of high-mobility semiconFigure 2. Patterning of 2D Bi2O2Se arrays with well-defined morphologies via a facile selectiveducting 2D Bi2O2Se crystals with tailorable area chemical etching. a) Typical OM image of centimeter-scale 2D Bi2O2Se arrays fabricated configurations facilitates the fabrication of with the help of photolithography, followed by etching of the continuous thin film of Bi2O2Se integrated electronics and optoelectronics. As on a mica substrate, exhibiting clear square shape with smooth edges. The insets show the photographs of Bi2O2Se films before (bottom left) and after (bottom right) chemical etching. shown in Figure 4, based on the controlled etching methodology, 3 × 2 arrays of 2D b) OM image of few nanometers thick Bi2O2Se single crystal embedded with different patterns (triangular, circular, and square). c) The corresponding AFM image of 2D Bi2O2Se crystals after Bi2O2Se crystals were obtained (Figure 4a) chemical etching in the dash line marked area in (b), revealing smooth surface and a thickness and fabricated directly into photodetector of 5.1 nm. d) OM image of 2D Bi2O2Se nanoplate etched into a symbol. arrays with six discrete photosensitive units on mica substrate without a transfer process (Figure 4b). To evaluate photoresponse property of each pixel, even the logo of Peking University with every line smooth Ids–Vds behavior with/without the illumination of 532 nm incicurved (Figure 2d; Figure S3, Supporting Information). We evaluated the electronic quality of 2D Bi2O2Se crystals dent laser was measured, displaying a high photosensitivity with negligible dark current (≈3 nA at Vds = 0.1 V) (Figure 4c). before and after efficient etching. As illustrated in Figure 3, as-grown 2D Bi2O2Se crystal of the same batch was patterned Photoresponsivity of 2D Bi2O2Se patterned arrays, a key metric for photodetector, was deduced as high as ≈2000 A W−1 at incident power of ≈0.1 nW at 532 nm, which is superior to the previously reported pristine graphene and other 2D materials (MoS2 and black phosphorus, etc.).[17–19] This may be ascribed to the relatively high optical absorption of 2D Bi2O2Se, as clearly indicated by the observed absorption peak around 500 nm (Figure S5, Supporting Information). 2D Bi2O2Se-based photodetector shows excellent environmental stability, as clearly indicated by the negligible change of Ids–Vds (dark and light) even exposed to air for about 6 months, which is a key metric for Figure 3. Electrical properties of 2D Bi2O2Se crystals before and after the chemical etching practical applications. Besides, photocurprocess. a) Room-temperature Hall measurements of as-grown Bi2O2Se crystal (≈7.2 nm), from rent signals from individual photosensitive which we can extract a Hall mobility of 243 cm2 V−1 s−1. Inset: typical OM image of 2D Bi2O2Se unit as well as their additions were collected device with a Hall bar configuration free of chemical etching; scale bar: 10 µm. b) Hall resistance by selectively illuminating the conducting (Rxy) as a function of magnetic field at 300 K, indicating a Hall mobility (300 K) of 209 cm2 V−1 s−1 and a merely unchanged Hall coefficient. The geometry factor was taken into consideration for channels, showing stable, cooperative operathe extraction of the Hall mobility. Inset: OM image of 2D Bi2O2Se crystal with a thickness of tions between each unit (Figure 4d). The ≈7.1 nm patterned to Hall bar geometry via the H2O2H3O+ assisted process; scale bar: 20 µm. relatively smooth time-dependent on- and
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Figure 4. Patterned 2D Bi2O2Se arrays for integrated optoelectronic devices. a) OM image of ordered 3 × 2 arrays of 2D Bi2O2Se crystal. b) OM image of 2D Bi2O2Se integrated optoelectronic devices on mica, and each channel was labeled with a specific number from 1 to 6. c) Linear Ids–Vds curves of an individual Bi2O2Se photosensitive unit with/without the illumination of 532 nm incident laser, showing excellent environmental stability even exposed to air for 6 months. d) Photoresponse of multiple Bi2O2Se photodetector channels. Source–drain voltage (Vds) = 0.1 V. e,f) Scanning photovoltage images of the 2D Bi2O2Se integrated photodetectors with all pixels on (e), and part of them are on that verified as the capital letter of “T” (f), respectively. The photovoltage here are measured with no Vds applied.
off-state current qualitatively suggests a small noise floating and an excellent signal-to-noise ratio. Based on those spatially well-resolved 2D periodic arrays, these high-performance optoelectronic devices can be functionalized by address and pixel recognition. As shown in Figure 4e,f, scanning photovoltage images of the 2D Bi2O2Se-integrated photodetectors were operated with all pixels on (Figure 4e), and part of them are on that verified as the capital letter of “T” (Figure 4f), respectively. Easy accessibility to large-scale production and integration, excellent photoresponsivity as well as environmental stability make 2D
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Bi2O2Se as a promising candidate for high-performance photodetectors and imaging applications. In conclusion, we first presented the controlled patterning of high-mobility semiconducting 2D Bi2O2Se crystals using a dilute H2O2 and protonic acid as efficient mixed etchant. Centimeter-scale well-ordered 2D Bi2O2Se arrays with tailorable configurations were readily obtained, exhibiting a high Hall mobility at room temperature. Furthermore, patterned 2D Bi2O2Se crystal arrays with an extraordinary photoresponsivity were integrated for stable photodetection. The facile patterning
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and integration of high-mobility 2D semiconducting oxychalcogenide crystals are compatible with mature silicon-based platform, holding promise for innovations for next-generation photodetector arrays, wearable electronic and integrated optoelectronic systems.
Experimental Section CVD Growth and Characterizations of 2D Bi2O2Se Crystals: The CVD growth process was conducted in a homemade low pressure CVD system equipped with an 1 inch diameter quartz tube inside a horizontal tubular furnace (Lindberg/Blue M). Bi2Se3 and Bi2O3 (purchased from Alfa Aesar) with a certain ratio were then located at the center of the furnace, and the freshly cleaved fluorophlogopite mica substrates ([KMg3(AlSi3O10)F2], Tiancheng Fluorphlogopite Mica Company Ltd., China) were placed downstream as substrates. The furnace was programmed to reach set temperature of 640 °C with elevation rate of 30 °C min−1 and kept for 5–60 min with an argon gas flow rate of 50–200 standard cubic centimeters per minute and a growth pressure of 50–200 Torr, finally cooled down to room temperature naturally. Morphology of 2D Bi2O2Se crystals before and after chemical etching was characterized by optical microscopy with reflection mode (OM, Olympus DX51) and atomic force microscope (AFM, Bruker Dimension Icon). Region-Selective Wet-Chemical Etching Process: The atomically thin 2D Bi2O2Se crystals grown on mica substrates could be patterned to predesigned geometrical shape by our region-selective chemical etching process with the assistance of photolithography or EBL. On one hand, the Bi2O2Se continuous film could be effectively etched into the designed arrays by the standard photolithography, in which the positive photoresist of AR-P-5350 was spin coated onto the Bi2O2Se/mica, then followed by the UV illumination and exposed-area development by using the developer of the mixture of AR-P-300-26 and H2O (volume ratio 1:7). On the other hand, the standard EBL was employed to achieve the patterning of the discrete Bi2O2Se nanoplates. First, poly(methyl methacrylate) (PMMA, MicroChem, 950 kg mol−1, ≈4 wt% in anisole) was spin coated onto Bi2O2Se/mica as lithography mask. The patterns were designed based on the specific location and contour of the Bi2O2Se in the software of Autocad, then standard EBL exposure and development were applied. As mentioned in the article, the oxygen plasma pretreatment could improve the hydrophilia of PMMA/Bi2O2Se/ mica surface and facilitate the diffusion of aqueous etchant to 2D Bi2O2Se surface. The optimized component ratio of H3O+:H2O2:H2O was 2:4:8. After steeping into the mixed etchant together with regular shaking for 10–60 s, 2D Bi2O2Se was quickly transferred to large quantities of water to terminate the etching reaction and avoid endcutting effect. And then, 2D Bi2O2Se crystal on mica was rinsed with water for several times to remove some residues. Finally, the PMMA film was dissolved by acetone vapor and 2D Bi2O2Se with different patterns was blow-dried with nitrogen for subsequent characterizations. Device Fabrications: The insulating mica substrates were compatible with device fabrication, which facilitated the transfer-free fabrication of Hall bar device and photodetector arrays of patterned 2D Bi2O2Se crystals. Standard photolithography was used to define alignment markers first. Then, the device electrodes were designed by standard EBL process. Ohmic contact was made by chromium/gold (Cr/Au, 8 nm/50 nm) via thermal evaporation deposition. Note that, in order to prevent the charge accumulation on mica substrate during EBL process, conductive polymer photoresist (SX AR-PC-5000) was spin coated on mica prior to the EBL process. Finally, silver paste was used to bond the devices for subsequent Hall and photovoltage mapping measurements. Electrical Property, Microarea Absorption, and Photovoltage Mapping Measurements: Ids–Vds curves were carried out using a Keithley 4200SCS semiconductor system at room temperature. Hall bar devices were mounted in the Quantum Design physical property measurement system (PPMS-9 T) with magnetic field applied perpendicularly.
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Microarea absorption was measured on a homemade spatially resolved spectrograph. White light illuminated through the 2D sample and a confocal optical system successively before coupling into the spectrograph. The sapphire 532 nm laser spot was used as excitation light source, having a full-width at half-maximum of 0.7–1.0 µm through a ×100 objective. Under this matched laser, the net photovoltage without source–drain bias was measured by scanning photovoltage microscopy, in which the voltage was recorded by lock-in amplifier, while scanning a focused 532 nm laser with ≈1 µm spot size on the device.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements J.X.W., Y.J.L., and Z.J.T. contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (No. 21525310), the National Basic Research Program of China (No. 2014CB932500), China Postdoctoral Science Foundation Funded Project, and National Program for Support of Top-Notch Young Professionals.
Conflict of Interest The authors declare no conflict of interest.
Keywords 2D Bi2O2Se crystals, chemical etching, high-mobility, integrated devices Received: July 20, 2017 Revised: September 11, 2017 Published online:
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