Self-Aligned Nanoforest in Silicon Nanowire for Sensitive

Oct 15, 2012 - S.F.A. Rahman , N.A. Yusof , U. Hashim , R. Hushiarian , M. ... Myeong-Lok Seol , Ji-Min Choi , Jee-Yeon Kim , Jae-Hyuk Ahn , Dong-Il M...
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Letter pubs.acs.org/NanoLett

Self-Aligned Nanoforest in Silicon Nanowire for Sensitive Conductance Modulation Myeong-Lok Seol, Jae-Hyuk Ahn, Ji-Min Choi, Sung-Jin Choi, and Yang-Kyu Choi* Department of Electrical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea S Supporting Information *

ABSTRACT: A self-aligned and localized nanoforest structure is constructed in a top-down fabricated silicon nanowire (SiNW). The surface-to-volume ratio (SVR) of the SiNW is enhanced due to the local nanoforest formation. The conductance modulation property of the SiNWs, which is an important characteristic in sensor and charge transfer based applications, can be largely enhanced. For the selective modification of the channel region, localized Joule-heating and subsequent metal-assisted chemical etching (mac-etch) are employed. The nanoforest is formed only in the channel region without misalignment due to the self-aligned process of Joule-heating. The modified SiNW is applied to a porphyrin-silicon hybrid device to verify the enhanced conductance modulation. The charge transfer efficiency between the porphyrin and the SiNW, which is caused by external optical excitation, is clearly increased compared to the initial SiNW. The effect of the local nanoforest formation is enhanced when longer etching times and larger widths are used. KEYWORDS: Silicon nanowire, localized Joule-heating, metal-assisted chemical etching, nanoforest, organic-silicon hybrid device

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organic-SiNW hybrid devices, stem from the high SVR caused by the extremely small size. With a larger SVR, the external components for charge transfer or sensing can more strongly affect the conductance of the SiNWs, which results in greater sensitivity and transfer efficiency. In this work, a top-down SiNW with a notably enhanced SVR is introduced. A channel region of the SiNW is selectively modified to form a vertical silicon nanoforest structure. Because of the nanoforest morphology, the surface area of the channel is enlarged. A modified domain comprised of SiNW pillars at the top portion of the channel serves as the charge transfer with external components, whereas the nonetched domain at the bottom portion of the channel serves for the electron conduction between the anode and the cathode. Because of the enlarged charge modulation efficiency between the external components and the SiNW channel, the total conductance modulation that stems from the external components becomes more sensitive. There are two challenging issues for the implementation of channel-modified SiNWs. First, selective patterning of the channel region should be conducted. Typical photolithography cannot selectively pattern the channel region because of the resolution limit and highly accurate alignment issues. E-beam lithography can partially solve this problem, but it leads to a level of productivity that is insufficient for wafer-scale

ilicon nanowires (SiNWs) have been widely investigated. The extremely small size and clear semiconducting property of the SiNWs allow them to be used for various applications, such as field-effect transistors (FET),1−3 biosensors,4−6 electrochemical sensors,7−9 solar cells,10,11 and organic-SiNW hybrid devices.12−16 The high sensitivity and various functionalities of SiNWs stem from the stable semiconducting property of silicon and the high surface-tovolume ratio (SVR) of the nanowire structure. To fabricate SiNWs, top-down methods1−4,12,13 or bottomup methods5−11,14−16 can be employed. Top-down methods are based on lithographical patterning, whereas bottom-up methods are based on epitaxial growth. SiNWs produced using bottom-up methods are advantageous in terms of productivity and surface quality; however, monolithic integration of SiNWs with other circuit components is difficult. Various methods for alignment to integrate bottom-up SiNWs have been explored, but the integrability remains too low to compete with that of top-down SiNWs. SiNWs produced using top-down approaches are clearly advantageous from the viewpoint of integration because they can be patterned with an exact size in a precise location at the wafer-scale. Despite the advantage of integrability, top-down SiNWs have a drawback in that it is difficult to reduce their size. Because of the resolution limit of the lithography process, the fabrication of sub-20 nm SiNWs using the top-down approach is challenging. This size limitation is critical because the majority of the interesting characteristics of SiNWs, such as the high sensitivity of SiNW-based biosensors and the large conductance shift of © 2012 American Chemical Society

Received: July 20, 2012 Revised: October 7, 2012 Published: October 15, 2012 5603

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Figure 1. Procedure for fabricating the selectively modified SiNW. (A) Schematic of the initial SiNW device. (B) Schematic of the NW after PMMA spin-coating. (C) Schematic of the NW after localized Joule-heating. (D) Schematic of the NW after Au thin film deposition. (E) Schematic of the NW after metal-induced chemical etching (mac-etch) and subsequent Au removal. (F) Schematic of the NW after PMMA removal.

methacrylate) (PMMA) film (495PMMA A4 resist, MICROCHEM) was subsequently spin-coated onto the SiNWs (Figure 1B). To selectively evaporate the PMMA film on the low-doped channel region, a strong voltage pulse was applied. The power of the pulse was approximately 4.7 mW, and the pulse duration was 266 ms (see Supporting Information for detailed conditions and results). Because the low-doped channel region has a significantly higher resistance compared to that of the highly doped electrode region, the majority of the voltage drop was induced at the channel. The temperature of the channel region was instantaneously increased due to the Joule-heating resulting from the voltage drop, which resulted in the local evaporation of the PMMA film (Figure 1C). Note that the dimensions of the fabricated SiNWs are in a patternable range using state-of-the-art photolithography techniques. However, the localized Joule-heating provides self-alignment when the width of the nanowire is aggressively scaled down to sub-40 nm dimensions. Furthermore, in practical chip-based applications the localized Joule-heating method enables the individual functionalization of each nanowire without the use of individual photomasks. To modify the SiNW and increase the SVR, the mac-etch process was performed on the exposed channel region. To initiate the mac-etch process, 70 Å of gold film was thermally evaporated on the NW. At this level of thickness, gold forms an islandlike morphology rather than a continuous filmlike morphology (Figure 1D).20,21 The thickness condition of the mac-etch process is important because a layer of gold that is too thin cannot induce sufficient space between the silicon pillars, and a layer of gold that is too thick cannot form an island-like morphology.21 During the mac-etch process, gold functions as a catalyst for the local oxidation of silicon; therefore, only the region deposited with gold is etched. The etchant was composed of HF, H2O2, and deionized water, with a respective volume ratio of 2:1:77. The etching time was 10 to 30 s depending on the expected modification ratio. After the macetch process, the remaining gold was removed using a 3:1 gold etchant for 3 min in at 50 °C (Figure 1E). In the final process, the PMMA film was removed with acetone for 30 s at room temperature (Figure 1F). As an organic component of the hybrid device, tetrakis5,10,15,20 (4-sulfonatophenyl) porphine (TPPS, Sigma

fabrication. For precise patterning, the localized Joule-heating method is employed.17,18 When a strong voltage pulse was applied to the polymer-coated NW, the temperature of the highly resistive region is instantaneously increased and the coating polymer is locally evaporated. Therefore, only the channel region with a low doping concentration is exposed, whereas the other highly doped electrode parts remain covered with the polymer. Second, a nanoscale silicon modification method that reliably amplifies the SVR while maintaining stable conductance paths is required. Metal-assisted chemical etching (mac-etch) is employed for this purpose.19,20 The naturally formed nanoisland morphology created by the ultrathin gold deposition becomes the silicon nanostructure because of the catalytic function of the gold. The notable characteristics of the macetch process are its nanoscale profile, constant etching directionality, and etching uniformity. The top part of the silicon is vertically modified to amplify the SVR, whereas the bottom conduction path is unharmed. A bundle of vertically standing silicon nanostructures, which are called a nanoforest, significantly increases the SVR. The nanoforest-formed SiNW was applied to an organicsilicon hybrid device to enhance the electron transfer efficiency of the device and to confirm the validity of the modification. Porphyrin, which is a photoactive organic material in chlorophyll, was employed because of its reliable photoinduced charge transfer (PCT) process.12,13 Porphyrins absorb electrons from silicon and adopt a negatively charged state during illumination with external light.13 The conductance of the SiNW changes because of the negatively charged porphyrins. By comparing the change in the conductance ratio before and after illumination with light, the relative amounts of electrons transferred can be determined. When the NW has a larger surface area, more porphyrins can interact with silicon, and the change of the conductance becomes larger. Figure 1 illustrates the overall fabrication procedure and device schematics. SiNWs with a low-doped channel region and highly doped electrode region were first prepared (detailed information on the initial preparation of the SiNWs is provided in the Supporting Information) (Figure 1A). The length, thickness, and widths of the prepared SiNWs are 500, 40, and 40 to 140 nm, respectively. A 120 nm of poly(methyl 5604

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of the side wall becomes larger. A larger SVR can be obtained using longer etching times; however, the height of the nanoforest should not exceed the height of the nanowire. The conductance path and the SVR of nanowire have a tradeoff relationship; therefore, the etching time should be carefully decided. This nanoscale vertical morphology is hard to obtain using conventional silicon roughening methods, such as reactive-ion-etching (RIE) or wet-etching with nitric acid. The modified SiNW was applied to the porphyrin-silicon hybrid device (Figure 3A). The effect of the enhanced SVR was verified by analyzing the charge transfer efficiency of the hybrid device. Furthermore, the fabricated hybrid device with the modified SiNWs is important for the development of organicsilicon hybrid devices because the electron transfer efficiency between the organic component and silicon is the crucial property. The modification improves the optical sensitivity when the device is applied to photodetector applications, and the modification improves the programming efficiency when the device is applied to optical memory applications. The mechanism for the electron transfer between the porphyrin and the silicon is based on photoinduced charge transfer (PCT).12,13 In a dark environment, porphyrin is in its inactive state, and the current can flow freely between the two terminals (Figure 3B). In this initial dark state, the conductance is high and constant. During the external optical excitation, porphyrin absorbs electrons from the silicon channel and becomes negatively charged. The negatively charged porphyrins disturb the n-type channel formation because of the repulsive force between the absorbed electrons in the porphyrin and the channel electrons in the SiNW (Figure 3C). Therefore, the conductance decreased during and after the illumination. Figure 4A presents the experimental results of the optical responses. Devices with various mac-etch times were measured. The conductance was observed to have maintained its value during the initial dark state. The initial conductance values for all of the devices were matched by applying appropriate voltages to the devices. A slightly larger voltage was required when a longer modification time was performed due to the increased resistance. The decreased conductance was observed during the external optical excitation. Because the conductance decrease is caused by electron transfer between porphyrin and silicon, the relative charge transfer efficiency can be determined by comparing the amount of change in the conductance. When greater electron transfer occurs, the porphyrin becomes more negatively charged, and a larger change in the conductance is consequently observed. The initial, unmodified SiNW also has optical responsivity; however, the value of this responsivity is small compared with that of the modified SiNWs. A stronger conductance change is observed when longer modification times are applied. After illumination, the reduced conductance slowly returned to its initial value due to the inactivation of PCT and the electron discharge of the porphyrins. Figure 4B provides organized comparison results of the ratio of the initial conductance to the decreased conductance after 200 s of illumination. As shown in the transient results, the conductance change is larger when longer modifications are applied. The conductance ratio was increased approximately 4 times when 30 s of modification was performed. The conductance significantly decreased when more than 30 s was used because the conductance path is completely blocked when the modification is too strong. To maximize the charge modulation performance, the thickness of the SiNW should be carefully determined. When thicker SiNWs are used, a taller

Aldrich) was used. TPPS is a special type of porphyrin that has four sulfur derivatives and no metal center. The derivatives allow better water-solubility in water and stronger chemical binding on the silicon surface. A total of 200 μL of 0.1 mM TPPS with a water solvent was drop-casted onto the initial and modified SiNWs to generate optical responsivity. After 24 h of drying in an air environment, the hybrid devices were measured using a semiconductor parameter analyzer (4156C, Agilent Technologies). Optical microscopy images of the initial SiNWs and the modified SiNWs are shown in Figure 2A,B, respectively. The

Figure 2. (A) Optical microscopy image of the initial SiNW device. (B) Optical microscopy image of the modified SiNW device. The color of the mac-etched silicon becomes black after the mac-etch process. (C) Bird’s eye view SEM image of the selectively modified SiNW. Scale bar represents 1 μm. (D) Magnified SEM image of the selectively modified SiNW. Scale bar represents 200 nm. (E) SEM image of the mac-etched silicon wafer when the etching time is 20 s (H2O/HF/ H2O2 = 7: 2: 1). Scale bar represents 200 nm. (F) SEM image of the mac-etched silicon wafer when the etching time is 30 s. Scale bar represents 200 nm. (G) SEM image of the mac-etched silicon wafer when the etching time is 60 s. Scale bar represents 200 nm.

color of the mac-etched region became dark, which was also observed in previous studies.19,20 Optical microscope analyses are the easiest method to qualitatively verify the results of the mac-etch process. The detailed morphology of the sample after the modification is analyzed using scanning-electron microscopy (SEM) analyses. Clear boundaries between the modified channel region and the unmodified pad region are observed (Figure 2C,D). It is confirmed that the self-aligning process using localized Joule-heating was well conducted. The detailed morphologies with different mac-etching times are shown in Figures 2E−G. Bulk silicon wafers were used to clearly analyze the morphologies. As expected, a nanoforest structure is partially formed in the upper part, whereas the bottom part maintained an unharmed state. The height of the nanoforest was increased with longer etching times; consequently, the area 5605

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Figure 3. (A) Schematic of the porphyrin-SiNW hybrid device. (B) Schematic showing the electron distribution of the modified SiNW in a dark environment. (C) Schematic showing the PCT behavior and resulting electron distribution of the modified SiNW during light illumination.

significantly reduced. Before the modification, the difference in the optical responsivities between the SiNWs with 40 and 140 nm widths was ∼52%; however, this value decreased to ∼23% after the modification. From this result, the selective mac-etch method is demonstrated to be an effective method to improve the top-down SiNW with a large width. The unique advantages of the top-down fabrication method are maintained while the drawback caused by the small SVR is significantly improved. In summary, a top-down SiNW with a self-aligned nanoforest channel was produced and analyzed. The critical disadvantage of the top-down SiNW, which is an insufficient SVR caused by the size reduction limitations, can be improved through the localized formation of a nanoforest. Functionalization of the external components can more effectively modulate the conductance of the nanowire due to the enhanced SVR, which results in higher charge transfer efficiency and sensitivity. To selectively pattern the channel region without any misalignment, a localized Joule-heating method was employed. The instant temperature increase induced by the strong voltage pulse naturally caused localized evaporation of the PMMA in the channel region. The mac-etch process, which is an effective SVR amplification method, was subsequently employed for the nanoscale modification of the exposed channel. The modified SiNW was applied to a porphyrin-SiNW hybrid device to enhance the charge transfer efficiency and to verify the effect of the selective nanoforest formation. The change in the conductance after the optical exposure was increased after the modification, and the change was greater when longer etching times and wider SiNWs were used. The increase of the charge transfer efficiency is advantageous when applied for photodetecting or charge retaining purposes. Furthermore, the

nanoforest can be obtained, which results in a larger SVR. However, when the nanoforest is too tall, molecules such as porphyrins cannot uniformly cover the entire structure. Furthermore, porphyrins that are placed too far from the conduction path cannot strongly influence the channel conductance. When thinner SiNWs are used, better coating conformality can be obtained but the SVR is limited. Analyses of the effects of modification of SiNW with different widths were performed to verify the effect of the modification area (Figure 5). All of the modification conditions were the same for all devices. Regardless of the modification, the SiNW with the smallest width exhibited the strongest optical response. This result makes sense because a smaller width indicates a higher SVR when the thicknesses and the lengths of the SiNWs are identical.13 After modification with the mac-etch process, the optical response of the SiNWs was enhanced. The clear trend observed in this experiment was that the effect of the modification is stronger for the SiNWs with larger widths. The enhancement ratio of the SiNW with a 40 nm width was ∼14%, and the enhancement was ∼43% for the SiNW with a 140 nm width. Because the mac-etch process was conducted only in the vertical direction, the width of the device dominantly determined the modification area. When the width of the device is smaller, the mac-etch process is performed over a narrower area, which results in a weaker enhancement of the SVR. In contrast, when the width of the device is larger, the mac-etch process can be performed over a wide area, which results in a stronger enhancement of the SVR. Although the SiNW with the smaller width shows better conductance modulation even after the modification, the difference between the narrower-width nanowire and the wider-width nanowire is 5606

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selective formation of the nanoforest will be useful for improving every type of conductance-modulating application, such as biosensors, electrochemical detectors, and organic hybrid optical devices.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed procedures for the initial preparation of the SiNWs, microscopy and AFM images of the devices after the localized Joule-heating, and quantitative comparison results are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored through grants from the National Research and Development Program under Grant 20120001131 for the development of biomedical function monitoring biosensors sponsored by the Korea Ministry of Education, Science and Technology (MEST). It was also supported by the Smart IT Convergence System Research Center funded by the MEST (SIRC-2011-0031845).

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Figure 4. (A) Transient characteristics of the conductance of the modified NWs. Black (top), green, blue, and red (bottom) curves are from the initial NW, 10 s etched NW, 20 s etched NW, and 30 s etched NW, respectively. The voltages applied for the devices were 1.0, 1.05, 1.12, and 1.22 V, respectively. (B) Conductance reduction ratio of the modified NWs. Etchant composed of H2O, HF, and H2O2 with volume ratio of 77:2:1.

Figure 5. Conductance reduction ratio of initial NWs and modified NWs with various NW widths. The length and thickness of the NWs were 500 and 40 nm, respectively. Hollow circle data are from the initial NWs, and the filled circle data are from the 20 s modified NWs.

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