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Sung-Jin Choi, Young-Chul Lee, Myeong-Lok Seol, Jae-Hyuk Ahn, Sungho Kim, Dong-Il Moon, Jin-Woo Han, Stephen Mann, Ji-Won Yang,* and Yang-Kyu Choi* The integration of nanophotonics and electronics can bring about enhanced performance in areas ranging from communication, interconnections, and computing to novel diagnostics.[1–9] A critical challenge for integrating photonics and electronics is the component size mismatch stemming from the micrometer scale of optical elements, i.e., photoconductors (or light detectors), versus the nanometer scale of modern electronic devices.[10,11] Thus, nanometer-scale photoconductors have predominantly been fabricated using II–VI or III–V compound semiconducting nanowires rather than silicon nanowires (SiNWs) to achieve enhanced light absorption,[12–14] although it should be possible to exploit the extensive knowledge base supporting silicon technologies. In complementary metal–oxide–silicon (CMOS) electronics, doping is a powerful technique for controlling the properties of silicon, and dopants can be used to easily and precisely tune the silicon material to act as either an n- or p-type semiconductor.[15] However, in contrast to CMOS electronics, photoconductors in photonics, whether compound or silicon materials, are only “single-type”: the conductance of the photoconductors increases unilaterally because of the photocarriers generated by the incident light. In other words, complementary photoconductors (i.e., those involving light-induced conductance changes of opposite direction) cannot be implemented. Here we show the assembly of p- and n-type SiNWs coated with porphyrins[16] to form hybrid integrated electronic and photonic devices in which the electronic properties and functions are controlled in a predictable manner to provide well-defined SiNWs by a top-down approach.[17–19] The photonic functions are symbiotically supported by the porphyrins to detect incident light and implement complementary photoconductors at the nanometer-scale level. Our results demonstrate that porphyrin-coated SiNWs exhibit high light-sensitivity, and that n- and p-type SiNWs coated with S.-J. Choi, M.-L. Seol, J.-H. Ahn, S. Kim, D.-I. Moon, J.-W. Han, Prof. Y.-K. Choi Department of Electrical Engineering KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Republic of Korea E-mail:
[email protected] Y.-C. Lee, Prof. J.-W. Yang Department of Chemical and Biomolecular Engineering KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Republic of Korea E-mail:
[email protected] Prof. S. Mann School of Chemistry Center for Organized Matter Chemistry University of Bristol, BS8, 1TS, UK
DOI: 10.1002/adma.201101931
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porphyrin molecules form a complementary photoconductor. The resultant complementary photoconductors can be used to form functional logic gates of optical input with remarkably low static power dissipation. Drawing inspiration from biological systems involved with photosynthesis, the physical properties that limit hybrid integration of photonic and electronic devices can be overcome through the use of well-controlled silicon nanostructures. We propose the following design principles: i) different methodologies for detecting light in nanometer-scale photoconductors are achievable by relying on photo-induced charge transfer (PCT) from a porphyrin bound to the surface of a SiNW rather than on the generation of electron-hole pairs (EHPs) inside SiNWs, and ii) complementary photoconductors can be obtained by introducing donor/acceptor (D/A) pairs to utilize porphyrin molecules in combination with corresponding charged species in the silicon lattice. Porphyrins contain extensively conjugated two-dimensional systems and are, therefore, suitable not only for synthetic light-harvesting systems but also for efficient electron transfer because the uptake or release of electrons results in minimal structural and solvation changes upon electron transfer.[20] In addition, the rich and extensive absorption features of porphyrinoid systems guarantee a high absorption cross-section and efficient use of the solar spectrum. The underlying concept of our design is illustrated through a comparison of electrical CMOS structures with hybrid CMOS structures in which electronics and photonics are integrated as inspired by the photosynthetic system (Figure 1a). In general, an electrical CMOS structure is composed of a heterogeneously doped structure between a SiNW and a source/drain (S/D), i.e., n- and p-type SiNW with p+ S/D and n+ S/D for a p- and n-channel MOS field effect transistor (MOSFET), respectively, and the conductance of each SiNW is modulated by an applied gate voltage. In contrast, the proposed device, a hybrid CMOS structure, consists of a homogeneously doped structure between a SiNW and a S/D. Herein the surface of the SiNWs is modified by porphyrins such that it can undergo PCT of electrons or holes in response to incident light. The conductance change of the SiNW, induced by PCT and subsequent field gating effects of the charged porphyrins on the SiNW surfaces (see below), can be chemically and optically modulated. The single-crystal n- and p-type SiNWs used in our studies were prepared by a top-down approach and have widths ranging from 50 to 500 nm and heights of 40 nm (see the Supporting Information for detailed schematic diagrams of the process steps, scanning (SEM) and transmission electron microscopy (TEM) images, and electrical data of the SiNWs, Figure S1 and S2).
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Bio-Inspired Complementary Photoconductor by PorphyrinCoated Silicon Nanowires
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Optical gate
A n+ (D)
hν
p+ (D)
n+ (S)
n+ (D)
p+ (D)
n+ (S) p+ (S)
p+ (S)
V Electrical gate
Electrical CMOS
Hybrid CMOS
B hole electron p-type SiNW
Porphyrin
Charged porphyrin
--- --- ---
VDD
Light on n-type SiNW
--- --- ----- --- ---
VDD
VDD
VDD
--- --- --Figure 1. Symbiotic hybrid integrated electronic and photonic device using SiNW arrays. a) Schematic illustration of the conversion of an electrical CMOS structure for an electrical input system into a hybrid CMOS structure for an optical input system. The SiNWs coated with porphyrins are in contact with two electrodes to measure the conductance. b) Schematic illustration of the observed characteristics (PCT) in SiNWs coated with porphyrins. When light illuminates, the PCT and subsequent charged molecules can gate the SiNWs and control the potential inside the SiNWs.
In the initial state (i.e., in a dark environment), the n- and p-type SiNWs were intentionally designed to have relatively high and low doping concentrations, respectively, which resulted in a controlled current flow (20 nA for n-type and 5–8 pA for p-type SiNWs at voltage of 0.1 V). Therefore, the relatively small and large currents in the p-SiNWs and n-SiNWs, respectively, initially tended to be off and on in the dark state corresponding to the p- and n-channels of a MOSFET in CMOS electronics; p- and n- channel MOSFETs are normally on and off with the logical input 0, where the dark condition represents the logical 0 as an optical input. When white light illuminates (1.22 mW) the porphyrin-coated SiNWs (see the Supporting Information for further details about porphyrin casting and white light, Figure S1, S3, S4, and S5) (i.e., the input state of logical 1), PCT occurs between the chromophore molecules and the SiNWs (Figure 1b), which reduces the net carrier concentration inside the n-SiNWs, while the alternative case where the net carrier concentration is increased occurs in
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the p-SiNW system. Subsequently, the SiNW surface becomes more negatively charged because of PCT, inducing a negative surface potential at the SiNW surface. Therefore, PCT and the resulting charged porphyrins control the net free carriers involved in current flow in the light-on state (logical input 1). The corresponding real-time conductance data (Figure 2a) show that the SiNWs coated with porphyrins clearly form a complementary photoconductor with respect to incident light with a large on/off ratio (over three orders of magnitude). It should be noted that the observed changes in flowing current in the n- and p-SiNWs upon illumination are oppositely directed (i.e., complementary). These properties differ significantly from the oneway changes observed in normal photoconductors upon illumination. For porphyrin-coated n-type SiNWs, the undershoot or overshoot of the conductance at the transition state associated with the switching from light on to off, or off to on, respectively, confirms that the direction of the change in conductance by the incident light (i.e., a decrease) contrasts with the case of EHP generation that increases the conductance of the SiNWs for both nand p-type SiNWs regardless of whether the SiNWs have pristine surfaces or are porphyrincoated. The data demonstrate the high yield and reproducibility of the fabricated complementary photoconductors (Figure 2b) and represent an important and necessary step in the development of logic gates, such as inverters of
Figure 2. a) Real-time measurement of the current flow for p- (top) and n-SiNWs (bottom) with a width of 50 nm as the light changes from off to on. Data were taken at an operating voltage of 0.1 V in air. The doping concentration of the p- and n-SiNWs was purposely designed to be low and high, respectively (implanted dose of 1 × 1013 cm−2 for p-SiNWs and 5 × 1013 cm−2 for n-SiNWs). b) The light sensitivity distribution compiled from more than 50 samples of both p- and n-SiNWs coated with (modified) and without (pristine) porphyrins. The same doping concentration and width for p- and n-SiNWs, as shown in (a), were used. Inset: atomic force microscopy (AFM) image of the SiNWs, width of 50 nm.
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complementary photoconductors, with optical inputs (see below). Previously, PCT between porphyrins and carbon-based nanostructures, such as C60[21] and single-walled carbon nanotubes (SWNTs),[22,23] was studied as an application for nanostructured photonic devices. However, existing synthetic methods for carbon-based nanostructures have limited their development because of a lack of control over the synthetic mixtures of metallic and semiconducting SWNTs. Moreover, it is difficult to control their polarity type, particularly such as n- and p-type in semiconducting SWNTs, because available dopants are very limited and dopant solubility in the SWNTs is too low. Surprisingly, the observed Figure 3. Plot of the conductance versus geometric width and carrier concentration of the direction of PCT in our system indicates that SiNWs. Conductance versus time for p-SiNWs (a) and n-SiNWs (b) as a function of SiNW the process transfers holes from porphyrins width (50, 100, and 500 nm from bottom to top); a white light (1.22 mW, halogen lamp) was to p-SiNWs, and electrons from n-SiNWs to illuminated and darkened after 120 and 900 s. c) Plot of conductance versus SiNW width (with porphyrins.[21,22,24,25] The changing direction and without coated porphyrins); the symbols are experimental data, and the dashed line is an of charge movement suggests that the por- exponential fit through these data. d) Sensitivity versus controllable parameters (geometrical phyrin molecules can undergo either oxida- width and implanted dose concentration) for p- and n-SiNWs coated with porphyrins. tion or reduction to produce electron donor at a width of 500 nm to more than 103 at a width of 50 nm. or acceptor species in the form of radical cations or radical As we used a doping concentration of ∼1018 cm−3 in n-SiNWs anions, respectively.[26,27] Previous research about SiNWs coated (concentration equivalent to the dose of ∼5 × 1013 cm−2), with porphyrins has been reported based on the structure of a this indicates that the Debye length is only 1–2 nm, and single-type (n-type) SiNW field effect transistor (FET), showing the charged molecules bound to the surface only gate the that the porphyrins coated on the SiNW surface can act as an n-SiNWs to a surface layer thickness of approximately 1–2 nm. electron donor.[28] In our observation, however, we confirmed Therefore, the charged molecules alone cannot play a signifithat the porphyrins play a pivotal role as an electron acceptor cant role in the large sensitivity to illumination; as a result, (or hole donor) during light illumination. We believe that the reducing the net carrier concentration inside the n-SiNWs reason for countertrends may originate from the different porattributable to PCT should simultaneously increase the Debye phyrins. Moreover, we ensure that an origin of signal change length (however, p-SiNWs are sufficiently gated by surface arises not from noise but from the porphyrins with the aid of charged molecules alone because of a low carrier concentraboth n- and p-type porphyrin-coated SiNWs in this work. Theretion). At much lower concentrations of the carrier, the Debye fore, misjudgment can be avoided by our unique and distinclength is expected to be larger than the width of the SiNW as tive complementary behavior. a result of PCT, and the entire SiNW volume is expected to be Although a comprehensive understanding of this new gated. Importantly, in the case of pristine, conventional SiNWs observation requires further investigation, including the use (cSiNWs) without the coated porphyrins, the conductance of other porphyrins and derivatives, the results show that porchange is strongly dependent on the rate of EHP generation phyrin-coated SiNW surfaces operate symbiotically to produce inside the SiNWs, which results in a higher conductance for complementary photoconductive SiNWs. Because the complethe SiNWs. Therefore, the direction of the conductance change mentary change of current for n- and p-SiNWs upon illumifor both p- and n-cSiNWs is the same. Although the change nation is mainly attributed to both PCT and subsequent field in conductance is also larger for narrower cSiNWs because of gating effects of the charged porphyrins on the SiNW surfaces, the surface effect,[30] i.e., phototransistive effects are larger in the sensitivity is expected to be closely related to the dimennarrower cSiNWs, we note that the sensitivity is not as high sions of the nanostructures (i.e., the surface to volume ratio). as that of SiNWs coated with porphyrins. The direction of the It is expected that the highest sensitivity would be achieved conductance change is also different in the case of SiNWs when the entire volume of the SiNW is gated by PCT and surcoated with porphyrins. In the regime of low carrier concenface charged molecules.[29] This scenario can be realized if the tration, the sensitivity can be further enhanced (Figure 3d), carrier screening length is much larger than the width of the as expected, indicating that SiNWs coated with porphyrins SiNWs. In this respect, Figure 3a,b show three sets of data corsuccessfully enable a trans-scale paradigm allowing sophistiresponding to SiNW widths of 50, 100, and 500 nm. Although cated nanometer-scale integration of photonics. Furthermore, the absolute change in conductance (ΔG = G – GO) is larger at it can be expected that a thickness as well as a width of the a width of 500 nm, a more distinct signal is observed at a width SiNW can significantly increase the sensitivity, because the of 50 nm. This feature is more distinct when expressed in scaling of the thickness in SiNWs can also result in a high terms of the relative conductance change (ΔG/G) (Figure 3c). surface-to-volume ratio. As the light is turned from off to on, ΔG/G increases from ∼0.7
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Figure 4. Logic gate (inverter) designed with complementary SiNW photoconductors (i.e., SiNWs coated with porphyrins). VOUT versus real-time for pseudo-inverters of p-SiNWs (a) and n-SiNWs (b). The light was illuminated at 120 s. Inset: schematic of the electronic circuit. The load resistor was 300 MΩ, and VDD was varied from 0.2 to 0.5 V. c) A microscope image of a complementary SiNW photoconductor array. d) VOUT versus real-time for a complementary inverter for incident light assembled from n- and p-SiNW photoconductors with coated porphyrins. One end of the n-SiNW was biased at VDD, and one end of the p-SiNW was grounded. Inset: schematic of circuit configuration for complementary photoconductors. e) Current versus real-time measurement for the pseudo-inverters and a complementary inverter made of n- and p-SiNWs.
With high yield, reproducibility, and a large on/off ratio in the presence of incident light, the complementary photoconductor assembly of SiNWs coated with porphyrins enables symbiotic integration of electronics and photonics for the formation of functional devices, such as logic gates with optical inputs. To demonstrate the function of a logic gate fabricated in this manner, we investigate a logic inverter comprising complementary photoconductors. An inverter is a basic logic element that converts a logical 0 into a logical 1 and a logical 1 into a logical 0. We define the light-off state as a logical 0 of an optical input and the light-on state as a logical 1. First, our inverter was constructed from SiNWs coated with porphyrins and an off-chip 300 MΩ load resistor, namely, a pseudoinverter (Figure 4a,b). The differences in the direction of the conductance change in p- and n-SiNWs coated with porphyrins upon illumination suggested that the configurations of the two pseudo-inverters should be different. The output voltage (VOUT) from the device varied from high to zero as the input light source was transitioned from off to on. For both pseudoinverters, the current flowing through the conduction path between the operating voltage (VDD) and ground could undesirably bring about significant static power dissipation in either the light on or off state. However, the inverter made with complementary photoconductors (Figure 4c), i.e., n- and p-SiNWs with coated porphyrins, exhibits a remarkably low static power dissipation (Figure 4d,e). More specifically, the complementary photoconductors yield a static power dissipation of approximately 0.05 nW in either the high or low state, whereas the pseudo-inverters dissipate power at a rate that is 10 to 102 larger, with a VDD of 0.5 V. These properties of complementary photoconductors suggest that the photoconductors may be used in various applications for hybrid integrated electronics 3982
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and photonics with optical inputs, such as modulators (or multiplexer, MUX) or charge shifters of electronic signals, which can be implemented without static power dissipation (for possible applications of complementary photoconductors, see Figure S6, Supporting Information). Overall, our predictable and reproducible assembly of complementary photoconductors, which enables demonstration of all critical logic gates without static power dissipation, represents an important step toward integrated nanophotonics built from well-defined silicon nanoelectronics. Nevertheless, several hurdles remain. For example, we have yet to optimize a fast response. This may be controllable by the choice of different porphyrin derivatives and the employment of bioengineering methods. Although the present study focused on the properties of SiNWbased devices illuminated by a single light source, combinations of these approaches may be used to organize complex hybrid integrated nano-electronics and photonics on silicon technologies.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements S.-J.C. and Y.-C.L contributed equally to this work. The authors thank M.-H. Lim (POSTECH) and Prof. S.-Y. Yoo for providing optical measurements and helpful discussions. Y.-K.C. acknowledges support of this work by the National Research Foundation of Korea funded by the Korean government (Grant number: 2010-0018937), the Nano R&D program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant number: 20090082583), the SNT leading primary research, and partial support by the Advanced Biomass R&D Center (ABC) of Korea Grant funded by the Ministry of Education, Science and Technology (ABC-2010-0029728). Received: May 24, 2011 Revised: June 22, 2011 Published online: July 22, 2011
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