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Percolating silicon nanowire networks with highly reproducible electrical properties
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Nanotechnology Nanotechnology 26 (2015) 015201 (10pp)
doi:10.1088/0957-4484/26/1/015201
Percolating silicon nanowire networks with highly reproducible electrical properties Pauline Serre1, Massimo Mongillo2, Priyanka Periwal1, Thierry Baron1 and Céline Ternon1,3 1
Univ. Grenoble Alpes, LTM, F-38000 Grenoble, France; CNRS, LTM, F-38000 Grenoble, France Univ. Grenoble Alpes, IMEP-LAHC, F-38000 Grenoble, France; CNRS, IMEP-LAHC, F-38000 Grenoble, France 3 Univ. Grenoble Alpes, LMGP, F-38000 Grenoble, France; CNRS, LMGP, F-38000 Grenoble, France 2
E-mail: [email protected] Received 15 July 2014, revised 27 August 2014 Accepted for publication 11 September 2014 Published 8 December 2014 Abstract
Here, we report the morphological and electrical properties of self-assembled silicon nanowires networks, also called Si nanonets. At the macroscopic scale, the nanonets involve several millions of nanowires. So, the observed properties should result from large scale statistical averaging, minimizing thus the discrepancies that occur from one nanowire to another. Using a standard filtration procedure, the so-obtained Si nanonets are highly reproducible in terms of their morphology, with a Si nanowire density precisely controlled during the nanonet elaboration. In contrast to individual Si nanowires, the electrical properties of Si nanonets are highly consistent, as demonstrated here by the similar electrical properties obtained in hundreds of Si nanonet-based devices. The evolution of the Si nanonet conductance with Si nanowire density demonstrates that Si nanonets behave like standard percolating media despite the presence of numerous nanowire-nanowire intersecting junctions into the nanonets and the native oxide shell surrounding the Si nanowires. Moreover, when silicon oxidation is prevented or controlled, the electrical properties of Si nanonets are stable over many months. As a consequence, Si nanowire-based nanonets constitute a promising flexible material with stable and reproducible electrical properties at the macroscopic scale while being composed of nanoscale components, which confirms the Si nanonet potential for a wide range of applications including flexible electronic, sensing and photovoltaic applications. Keywords: silicon nanowires, nanonets, percolating networks, electrical properties 1. Introduction
catalysts [6] and batteries [7]. However, due to time consuming, complex and expensive technological techniques, the industrialization of devices based on a unique SiNW [8–10] is limited. Moreover, due to a large variability between NW properties, there is an obvious lack of reproducibility from one device to another [11]. As a consequence, it is difficult to exploit the unique properties of NWs in actual devices. In contrast, materials composed of randomly oriented NWs offer good reproducibility and have recently been attracting interest in many applications [12–16]. Also called ‘nanonets,’ for NANOstructured NETworks [17, 18], such materials show several interesting properties that arise either from the individual components, from the NWs or NTs or
One-dimensional (1D) nanostructures, such as nanotubes (NTs) or nanowires (NWs), have attracted much interest in the last few decades. These 1D nanomaterials exhibit interesting electrical, optical, mechanical and chemical properties, thanks to their high aspect ratio and large surface area. Silicon nanowires (SiNWs) are one of the most promising 1D semiconductors, thanks to their notable implementation in the industry and their compatibility with present silicon-based semiconductor technology. Because of their surface properties, SiNWs have been developed in many applications such as solar cells [1, 2], biological and chemical sensors [3–5],
0957-4484/15/015201+10$33.00
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[25, 26], and ZnO nanonets have been integrated into transistors [27] or gas sensors [14]. Nanonets can be prepared by various fabrication techniques, particularly by self-assembly from a solution (e.g. spray coating [28, 29], the Langmuir–Blodgett technique [30–32] or vacuum filtration [23, 33]). The solution-based assembly is particularly attractive since it enables the fabrication of nanonets that have a wide range of thicknesses, from submonolayer coverage to over a 1 μm thickness. Among the solution-based assembly methods, the filtration one, applied in this work, is highly versatile and allows the production of homogeneous 2D nanonets over large surfaces at low costs [13, 23, 34–36]. The fabrication procedure requires that a solution of the nanostructures (NTs or NWs) be filtered using a standard vacuum filtration apparatus. During the filtration, the nanostructures are gradually deposited on the filter surface, forming the nanonet, which can then be transferred on the desired surface by dissolving this filter in an appropriate solvent. Varying the volume or the concentration of the filtered solution allows one to precisely control the nanonet thickness. This filtration process provides a good homogeneity of the nanonet. Indeed, as the nanostructures accumulate on the filter’s surface, the flow resistance in these regions increases so that the solution flow increases toward regions which have accumulated fewer nanostructures. Moreover, owing to this simple and original self-controlled homogenization mechanism, the process can be up-scaled to large sample dimensions in connection with the filter size. Consequently, nanonets are exciting materials that are easy to manufacture. They exhibit interesting electronic, optical, chemical and mechanical properties, with potential for applications in a wide variety of systems including electronics, sensors, solar cells, batteries and super-capacitors. Most importantly, this material system offers an opportunity to explore transport theories that involve percolation. Nevertheless, there is currently a lack of studies focused on semiconducting nanonet properties. Thus, the main focus of this work is to relate some aspects of their electrical behavior in relation with their morphological properties in order to identify their potential applications in diverse areas. Here, we present the results of an investigation on the electrical properties of degenerated n-type Si nanonets made from the vacuum filtration method [15], with a particular focus on the electrical conductance and the reproducibility of the nanonets. First, we demonstrate the high reproducibility of the material in terms of morphological and electrical properties. On one hand, the nanonet density is precisely controlled. On the other hand, the study of hundreds of nanonet-based devices leads to highly reproducible and predictable electrical properties. Then, the conductance dependence with the NW density is investigated, from which three different conduction regimes are evidenced: off-state, percolating regime and bulklike regime. Finally, the electrical conductance stability is studied as a function of the environment, and a way of improvement is explored through the metallic contact postannealing.
from the structural properties of the network itself. These properties include: (i) High surface area/high porosity. Due to the component geometry, the surface area drastically increases in comparison with thin films. As a consequence, the addition of functional materials is favoured due to the high porosity. (ii) Electrical conductance. The so-defined nanonets are above the percolation threshold, and the larger the NW conductivity, the better the network conductance. Factors like NW-NW contacts play a major role and have to be studied and optimized. (iii) Optical transparency. A network of high-aspect-ratio nanostructures has high transparency that approaches 100% when the aspect ratio tends to infinity. However, when dealing with absorbing materials, the transparency decreases when the aspect ratio decreases. (iv) Mechanical robustness and flexibility. A random network of interwoven NWs has significantly higher flexibility than that of a thin film. (v) Reproducibility and fault tolerance. At the macroscopic scale, the nanonet involves several millions of NWs so that the observed properties result from a large-scale statistical averaging on the individual NW properties. Moreover, breaking a conducting path in the network leaves many others still present, and the overall properties of the nanonet are stable. As a consequence, in contrast to individual components, nanonets offer high reproducibility of the physical properties coupled with easy preparation. (vi) High-quality components. It is easy to grow defect-free NWs over a large area so that the electrical and optical properties are improved. (vii) Functionalization ability. By attaching chemical species or nanoscale materials, such as nanoparticles (NPs), with well-defined functionality, it is possible to add properties induced by the synergy between NWs and NPs. When the nanonet thickness is significantly thinner than the length scale of its individual components, the nanonet is considered as two-dimensional (2D) because the percolation properties of such a network show typical 2D characteristics. Carbon nanonets (composed of carbon NTs, functionalized or not) and metallic nanonets (composed of silver or copper NWs) have been studied for a decade and are widely described in the literature, particularly for the fabrication of transparent and conductive thin films for photovoltaic applications [13, 19–24]. In contrast, there are currently very few studies on 2D nanonets based on nanomaterials other than carbon or silver, such as, for example, Si, ZnO and other semiconducting NWs, despite the high potential of such materials. Indeed, according to Grüner et al [18], Si nanonets may be considered as the fourth form of silicon in addition to single crystalline, polycrystalline and amorphous silicon. Nevertheless, germanium and silicon nanonets have been recently studied for use in photodetection applications 2
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Figure 1. (a) SEM image of a Si nanonet obtained from 45 mL of SiNW solution. (b) Schematic of a 2D nanonet. L is the nanostructure
length, D is the diameter, S is the projected surface and J is the surface overlap when two NWs cross. (c) SiNW coverage area as a function of the filtered volume of the SiNW solution. From the Si nanonet SEM images and using ImageJ software, the NW coverage area was determined as a function of the filtered volume of the SiNW solution. The images were captured at different locations on the nanonet, and the error bars match the SiNW coverage area variability from one image to another for a given nanonet. (d) Si NW density as a function of the filtered volume of the SiNW solution. The NW density was calculated from the coverage area when the latter was below 100% and was extrapolated when the coverage area saturated around 100%.
2. Experimental method
single SiNWs with the process described in [43], their resistivity is found in the range 0.3–1.4 mΩ.cm, leading to 0.5 to 2.9 × 1020 phosphorous at.cm-3. In this study, the silicon nanowires used contain the gold catalyst on the top, unless otherwise specified. The 2D Si nanonets were assembled by the vacuum filtration method [23]. First, silicon nanowires were dispersed in deionised water by ultrasonication for 5 min. Then, the NW solution was characterized by absorption spectroscopy, and further dilutions of the solution were done until its absorbance at 400 nm was equal to 0.06. In such a way, the nanowire amount in the solution was reproducible from one solution to another even if the nanowire concentration was not quantitatively determined. After the dilution, the SiNW solution was filtered through a 0.1 μm porous nitrocellulose membrane (47 mm in diameter). As the solvent went through the pores, the nanowires were trapped on the membrane surface, subsequently forming an interconnected random network: the 2D Si nanonet (figure 1(a)). Different volumes of the SiNW solution (0–320 mL) were filtered in order to prepare thin networks of controllable thickness and density. Finally, the networks were transferred onto Si3N4 (200 nm) on Si
2.1. Fabrication of the silicon nanonet
Degenerated n-type silicon nanowires were synthesized by the classical vapor liquid solid mechanism (VLS) proposed by Wagner and Ellis [37]. Gold was chosen as the metal catalyst due to its low temperature eutectic phase with silicon [38, 39]. Silane (SiH4) was used as the gaseous silicon precursor, and phosphine (PH3) was used as the dopant for n-type silicon nanowires. Hydrogen chloride (HCl) was also added during the growth in order to inhibit the gold diffusion and the lateral growth [40, 41]. SiNWs were grown on silicon substrate at 650 °C after the dewetting at 800 °C of a gold thin film (2 nm). This chemical vapor deposition allows epitaxial growth of SiNWs with a diameter monitored by the catalyst size [42]. In accordance with the classical description of the VLS mechanism [37], the gold catalyst droplets stay at the top of the SiNWs after growth. This catalyst can be removed from the nanowires by dipping the samples successively in hydrofluoric acid (HF), iodine potassium iodide (IKI) and HF baths. By separately studying the electrical properties of 3
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Figure 2. Schematics ((a) and (b)) and the colorized SEM top view images ((c) and (d)) of the processed electrical test structure based on the
Si nanonet. The nanonet was transferred onto an insulating 200 nm thick silicon nitride layer. 200 μm diameter metallic contacts Ni (120 nm)/ Al (180 nm)/Au (50 nm) separated by a 50 μm pad pitch were deposited by e-beam evaporation through a shadow mask.
substrate by membrane dissolution, thanks to a treatment with an acetone liquid bath for 30 min.
densities were deduced by analyzing SEM images with the help of ImageJ software. Two-terminal current versus voltage I(V) measurements were performed using a Karl Süss Probe Station controlled by a HP 4155 Analyzer at room temperature in a dark and ambient environment.
2.2. Electrical test structure
In order to perform electrical measurements, and as depicted in figure 2, metallic electrodes with a diameter of 200 μm and a spacing of 50 μm were deposited on the nanonet. First, to remove the native oxide layer, form H-terminated SiNWs and promote a direct contact between the metallic electrodes and the silicon NWs, the nanonets were exposed to a HF vapor flow prior to the metal evaporation during 30 s [44]. Then, using a Plassys e-beam evaporator, a trilayer contact composed of 120 nm of nickel, 180 nm of aluminum and 50 nm of gold was deposited through a shadow mask. Nickel was chosen to obtain a low Schottky barrier with the n-type SiNWs. Moreover, its capacity to form silicide after an appropriate heat treatment offers the opportunity to improve the electrical transport across the contacts [43, 45, 46]. Some devices were annealed at 400 °C during 1 min under a nitrogen flow using a rapid thermal annealing (RTA) furnace from Jet First Company. Above the nickel deposit, an aluminum cover layer was evaporated to avoid the entire consumption of the contacts during post-annealing treatments. In addition, gold was deposited on top of the stack in order to prevent the oxidation and the deterioration of the metallic contacts.
3. Results and discussion 3.1. Si nanonet morphology
The n-degenerated as-grown silicon nanowires vertically stand on the substrate. They are 10 μm long, and their diameters are in the range of 70–100 nm, leading to an aspect ratio in the range of 100–150. These two parameters are kept constant in this study, while the role of the NW length was studied in a previous study [47]. Such a high aspect ratio is essential in order to get a coherent and well-interconnected nanonet [16]. The Si nanonets elaborated by the vacuum filtration method are randomly oriented with a good uniformity over large areas (several cm2). Figure 1(a) shows a coherent, well-interconnected and randomly oriented Si nanonet elaborated from 45 mL of a well-controlled SiNW solution [15]. From the SEM images, the coverage area of a given nanonet is defined as the ratio between the surface covered by NWs and the total surface of the sample. For this purpose, ImageJ software was used to binarize the nanonet SEM images and to calculate the ratio of bright pixels (surface covered by NWs) over the total pixel number (total surface). For each nanonet that corresponds to a given filtered volume, the same image analysis was performed on six pictures that arise from different regions of the samples and from different samples. The obtained coverage area is plotted as a function of the filtered
2.3. Characterization
Morphological studies were performed by scanning electron microscopy (SEM) observations with the help of a Zeiss Ultra Plus Microscope. SiNW aspect ratios were determined by calculating the length over diameter ratio, and the Si nanonet 4
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volume in figure 1(c). The nanonet homogeneity and reproducibility is confirmed by the very small variability represented as the error bars on the graph. As long as the substrate is not totally covered by NWs, the coverage area is proportional to the volume of filtered solution (lower than 160 mL). Above 160 mL, numerous NWs are seen on the surface such that the underlying substrate isn’t more visible, which explains why the coverage area saturates just below 100%. The NW density in the nanonet can be easily deduced from the coverage area as long as the saturation is not reached. By assuming that the NW length (L) and diameter (D) are homogenous, the substrate surface covered by each NW (S) is approximated from S = L × D (figure 1(b)). As a consequence, the NW density is defined as the ratio between the coverage area and the surface covered by each NW. In such a way, the density is slightly underestimated because the overlap between the NWs, defined as J in figure 1(b), is neglected. For volumes larger than 160 mL, the density is extrapolated using the linear dependence between the volume and coverage area. Finally, the NW density is plotted as a function of the filtered volume in figure 1(d). Such a linear dependence of the nanostructure density with the filtered volume was also observed for CNT nanonets [35, 48] using the same vacuum filtration method. It first confirms that the titration of the NW solution is reliable. Then, it guarantees the nanonet large-scale homogeneity. Thus, we first demonstrate that the NW density is well controlled by the filtered volume of the NW solution for a given SiNW amount in the solution. This one is monitored by absorbance spectroscopy. Second, the statistical study of numerous images that arise from different nanonets evidences the reproducibility from one nanonet to another; finally, the nanonet homogeneity is illustrated by the small error bars in figure 1(d). When the NW density is so high that the coverage area reaches 100%, the nanonet adhesion to the substrate is lowered. This lack of adhesion is attributed to the weak Van der Waals interaction that governed the nanonet adhesion. As a conclusion, the NW density into the Si nanonets is monitored with high precision by simply controlling the filtered volume of a controlled SiNW solution through the membrane. Moreover, the so-assembled Si nanonets are above the percolation threshold, allowing conduction across the system [16]. In order to assess the Si nanonet potential as an electrically active material, studies of the electrical properties were performed and detailed in the next part.
Figure 3. Typical I(V) characteristics of silicon nanonet devices. The two continuous lines represent the range of I(V) characteristics obtained for a Si nanonet with a 50 μm interelectrode distance and a NW density of 27 × 106 NWs.cm−2. The colorized area matches the fluctuation range (±18%). In-between, the dashed line is allocated to a Si nanonet with the same NW density but without a gold catalyst, proving that the current flows through the SiNWs and not through the metal catalysts. As a reference, the I(V) characteristic without the Si nanonet is also plotted (thick continuous line) in order to insure the electrical insulation between the two metallic electrodes.
shown in figures 2(c) and (d). In the first part, the reproducibility and fault tolerance of the geometry is evidenced. In the second part, the role of the NW density in the nanonet is shown. Then, the electrical properties stability as a function of time is studied; finally, in the last part, an improvement strategy is detailed. 3.2.1. Reproducibility. At the macroscopic scale, the nanonet
involves several millions of NWs so that the observed properties result from large-scale statistics that average the individual NW properties. As a consequence, nanonets should offer high reproducibility of the electrical properties for a given SiNW length, diameter and density. To evidence this reproducibility property, more than 100 devices with a 50 μm interelectrode distance that were based on nanonets with NW density of 27 × 106 NWs.cm−2 were electrically characterized. In figure 3, two typical bidirectional I(V) characteristics are shown (light continuous lines). These two curves are representative of the measured fluctuations (colorized area). Indeed, I(V) characteristics are consistent for different nanonets with the same density within this fluctuation range (±18%), evidencing the high reproducibility of the electrical behavior for a Si nanonet with similar NW density, length and diameter. Two-probe measurements were also done on devices in which contacts were directly evaporated on the substrate without a Si nanonet between the metallic electrodes (thick continuous line in figure 3) to prove that conduction occurs through the nanonet. Two-probe measurements at room temperature for most of these devices show non-linear, symmetric I(V) characteristics with a high current level despite the numerous inter-NW junctions into the nanonet [16]. Moreover, no hysteresis is observed in the I(V) curves, suggesting the presence of a low density of trapped charges inside the structure. The slope of the I(V) curve gradually increases with the voltage for both
3.2. Electrical properties
With this work, we want to evidence that the nanonet geometry is actually an interesting electrically active material. Therefore, in this study, degenerated SiNWs were chosen to get a high conductivity from the NWs so that the electrical behavior of the nanonet is mainly controlled by the internanowire junctions and the contacts between the metal and nanonet. The electrical test structure is displayed in figure 2. The 200 μm circular Ni/Al/Au electrodes were deposited on Si nanonets with a spacing of 50 μm. Representative SEM images of the devices at a low and high magnification are 5
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Figure 4. (a) Silicon nanonet conductance at −5 V as a function of the SiNW density. The studied Si nanonets were elaborated from the same SiNW solution and from different filtered volumes in order to get different SiNW densities. (b) The conductance is plotted in the semilogarithmic scale to evidence the five decades’ increase in the conductance values. Such behavior is typical of the percolating regime, as evidenced by the fit using the power law dependence G ∝ (dNW -dc)t, which is expected from the classical percolation theory [55].
interesting material for further integration into functional devices.
negative and positive applied voltages. This reproducible rectifying response is typical of a resistor addressed via backto-back Schottky contacts that originate from the potential barrier at the metal-NW junctions [49, 50]. Similar electrical behavior was observed in single silicon nanowire devices [51–53]. As shown in the literature [53], conduction in single SiNWs involves 3 mechanisms: (i) Schottky interface junctions at the metallic contacts, (ii) Au-mediated surface conduction along the NW sidewalls and (iii) conduction through the NWs. In the case of nanonets, NW-NW junctions have to be additionally considered. To infer the role of conduction along the gold-rich sidewalls, two-probe measurements were repeated on nanonets with a NW density of 27 × 106 NWs.cm−2 in which gold were removed by immersion in successive baths (HF, IKI and HF), as described in the experimental section. The I(V) characteristic of this nanonet without gold catalysts is shown in figure 3 (dashed line). Changes in the I(V) characteristics before and after chemical treatments for gold removal are not relevant since the I(V) curve after gold removal remains in the fluctuation range of nanonets with a NW density of 27 × 106 NWs.cm−2. Thus, in the case of the n-degenerated NWs, the surface conduction through the Au catalyst residues at the NW surface is negligible. By separately studying the single SiNWs (not shown here), their intrinsic conductance is measured between 15 and 90 μS, whereas the nanonet conductance is equal to 0.34 ± 0.05 μS. As the SiNW conductance is more than one order of magnitude higher than that of the nanonets, the conduction through the NWs is a non-limiting mechanism. As a conclusion, the low bias current regime appears dominated by the Schottky interface junction between the metal and Si nanonet. At high bias, the current is governed by the interplay between the conduction through the NWs and the conduction through NW-NW junctions; the latter should be the limiting mechanism, as the NWs are n-degenerated. Moreover, the large-scale statistics that average the individual NW properties offer highly reproducible electrical properties for nanonets as soon as the SiNW length, density and diameter are fixed, which implies that Si nanonets are an
Role of density. The two-probe measurement configuration was also used to determine the conductance variation with the SiNW density. The conductance (G) is defined as G = I/V. Figure 4 shows the conductance at −5 V as a function of the SiNW density. As can be seen, the conductance values vary by 5 orders of magnitude over the SiNW density range. Such a variation with density was already shown for CNT nanonets [24, 35, 48] and for metallic nanonets such as silver or copper nanonets [13, 19, 54]. In our case, three different regimes are clearly observed. At a very low density, the OFF-state for which no measurable current is observed (first measurable current for 13 × 106 NW.cm−2, figure 4(b)), means that no or very few conduction paths exist between the two electrodes. For densities between 13 × 106 and 35 × 106 NW.cm−2 (figure 4(b)), the four decades’ increase of conductance G evidences the percolating regime. As shown in figure 4(b), this sharp rise can be fitted using the power law dependence G ∝ (dNW -dc)t, which is expected from the classical percolation theory [55]. Here, dNW is the SiNW density; dc is the critical NW density, defined as the lowest density for which current is measurable; and t is the critical exponent related to the dimensionality of the system. The exponents t of 1.3 and 1.9 are expected for 2D and 3D percolation, respectively. From the fit to our data, we find a critical NW density of 15.9 × 106 NWs.cm−2, which is very close to the experimental value (13 × 106 NW.cm−2) and find a critical exponent of 1.29, confirming the 2D geometry. For the highest densities (above 35 × 106 NW. cm−2), the behavior is nearly linear, indicating a scaling that is similar to ‘bulk’ material. This transition from the OFF-state to a bulk-like state was expected on the basis of the percolation theory [55], but it is evidenced for the first time for the Si nanonet. To conclude, Si nanonets based on degenerated SiNWs behave like standard percolating media despite the presence of the numerous NW-NW junctions into the nanonet and the native oxide shell surrounding the SiNWs. 3.2.2.
6
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Figure 5. Conductance evolution over the course of 102 days for Si
Figure 6. I(V) curves before and after the contact annealing
nanonet devices stored in different atmospheres: a nitrogen atmosphere (squares) or an ambient atmosphere (triangles). This plot represents the normalized conductance G/G0 at a potential of −5 V as a function of the time, with G0 as the initial conductance value on the first day. The error bars are allocated to the conductance variability obtained with three different devices. The line without symbols matches the exponential fit for the Si nanonets stored in air with a decay time of 2.2 days.
treatment. This treatment was performed during 1 min at 400 °C in a nitrogen atmosphere. The current value at ±5 V is multiplied by a factor of 2 after the contact annealing.
much slower mechanism. Considering the particular structure of nanonets, and due to the geometry, the silicon oxidation at the NW-NW junctions might be delayed. Thus, by increasing the exposition time to air, the oxide shell present at NW-NW interfaces thickens gradually. As a consequence, the exponential decay of the conductance with time should be directly linked to the oxide thickness at NW-NW junctions. The electron tunneling probability through a potential barrier decreases exponentially with the barrier width, which suggests that conduction through the NW-NW junctions is governed by the electron tunneling across the oxide shell until it reaches barriers that are too large for conduction to occur. To conclude, the electrical properties of silicon nanonets are stable with time, when the devices are protected from oxidation. Working under an inert atmosphere is valuable for experimental study. For further use in functional devices, encapsulating layers are deposited on top of the devices to avoid oxidation. However, such an encapsulation prevents the sensing application. Therefore, passivation treatments are under study, with promising results.
3.2.3. Si nanonet stability. Based on the results presented in
this study, the Si nanonets appear as an interesting material and structure for functional devices, such as sensors. However, it is necessary to check the stability with the time of such Si nanonets. Therefore, the electrical properties of Si nanonets were studied over a long period (more than 3 months) when stored in nitrogen or in the air between the electrical measurements. Figure 5 shows the time evolution of the normalized conductance G/G0, with G0 as the initial conductance value on the first day. Note that successive electrical measurements on the same device do not alter the electrical properties of the nanonets because the I(V) curves remain identical even after several voltage cycles. As shown in figure 5, the electrical properties of Si nanonets over time are strongly dependent on the storage atmosphere. When stored under nitrogen, the nanonet conductance is stable over many months within a 10% fluctuation range. In contrast, when kept in air, the nanonet conductance decreases exponentially with a characteristic decay time of 2.2 days. Thus, such decay is the consequence of the interaction between the species contained in the atmosphere and the Si nanonet surface. However, as oxygen plasma cleaning or vacuum annealing are unable to restore the electrical properties, adsorption on the surface and pollution by organic species are ruled out. By contrast, when exposed to HF vapor, the Si nanonets retrieve their electrical properties, revealing that oxidation is in fact responsible for the conductance decay. Studies about SiNW oxidation [56–58] have shown that SiNWs terminated with hydrogen atoms are more oxidationresistant than equivalent planar Si(100). However, according to the literature, NWs with a diameter around 85 nm are oxidized within one hour [56, 57]. In our case, the SiNWs are H-terminated with a diameter around 100 nm, but the characteristic decay time is about two days, suggesting a
3.2.4. Si nanonet electrical properties improvement. As
shown above, the low bias current regime appears dominated by the Schottky interface junctions between the metal and nanonet at the contacts. The vacuum work function of Ni is 4.74 eV [59], whereas the electronic affinity of Si is 4.01 eV. By annealing the contact at 400 °C under nitrogen, silicidation occurs, favouring electron transport across the Schottky barrier [43, 60, 61]. The main advantage of this annealing step is the appearance of a low resistive metallic phase of which the interface with SiNWs is abrupt. Figure 6 shows the behavior of a Ni-contacted Si nanonet device with a SiNW density of 27.106 NWs.cm−2 before and after rapid thermal annealing. After the annealing treatment, the current for a bias voltage of ±5 V almost doubles, but the response is still rectifying. Such a comparison is relevant since changes in I(V) characteristics are much larger than the fluctuations recorded on different 7
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nanonets (colorized area in figure 6). This current increase probably results from the formation of the nickel silicidesilicon interface, which enhances electronic transport across the Schottky barrier. However, such annealing does not lead to an ohmic contact as expected. This suggest that the electronic transport is ruled by both the oxide junctions between the crossing nanowires and by the presence of surface states, leading to the Fermi level pinning at the semiconductor surface [49]. The same phenomenon was observed in tens of Si nanonet devices and always leads to a two- or threefold current increase. As a consequence, silicidation is a valuable process that creates reproducible metal-semiconductor interfaces in the Si nanonet device and allows improvement of the electrical properties.
from LTM (Grenoble, France) for his help with the RTP furnace. This work was partly supported by the French RENATECH network. The research that led to these results was funded in part by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement NANOFUNCTION no. 257375.
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4. Conclusion In summary, we have prepared uniform and randomly oriented Si nanonets using the standard and low-cost vacuum filtration method. The NW density is precisely monitored, and the electrical properties of such random architectures are highly reproducible, confirming that the nanonet geometry is an efficient way to average discrepancies between individual NWs. N-degenarated Si nanonets behave like standard percolating media despite the presence of numerous NW-NW junctions into the nanonets and the native oxide shell surrounding the SiNWs. The electrical behavior of the Si nanonets presented in this work is the result of a combination between the Schottky interface junctions and NW-NW junctions, which are in fact tunnel junctions through the native oxide shell. When stored in nitrogen, the electrical properties of Si nanonets are stable over many months. With this work, thanks to the use of ndegenerated NWs, we demonstrate that the NW-NW junctions are not detrimental for the nanonet electrical properties. So, replacing the degenerated NWs by low-doped NWs will produce semiconducting nanonets that behave like thin-film transistors. As a consequence, the Si nanonets constitute a promising material with stable and reproducible electrical properties at the macroscopic scale while being composed of nanoscale components. Therefore, this confirms the Si nanonet potential for applications in a wide variety of systems such as flexible electronic, photovoltaic or sensing systems. Moreover, thanks to their reproducibility and high control over electrical properties, such Si nanonets are an interesting material in which to study complex percolation phenomena in random networks.
Acknowledgments The authors thank the members of the technical staff of the PTA facilities at Grenoble (France) for their technical support. The authors thank X Mescot from IMEP-LAHC (Grenoble, France) and J Grisolia from LPCNO (Toulouse, France) for their help on the electrical measurements; A Gachon from IMEP-LAHC for the shadow mask fabrication; and B Salem 8
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[37] Wagner R S and Ellis W C 1964 Vapor-liquid-solid mechanism of single crystal growth Appl. Phys. Lett. 4 89 [38] Shao M, Ma D D D and Lee S-T 2010 Silicon nanowires— synthesis properties and applications Eur. J. Inorganic Chem. 2010 4264 [39] Schmidt V, Wittemann J V and Gösele U 2010 Growth thermodynamics and electrical properties of silicon nanowires Chem. Rev. 110 361 [40] Gentile P, Solanki A, Pauc N, Oehler F, Salem B, Rosaz G, Baron T, Den Hertog M and Calvo V 2012 Effect of HCl on the doping and shape control of silicon nanowires Nanotechnology 23 215702 [41] Potié A, Baron T, Latu-Romain L, Rosaz G, Salem B, Montès L, Gentile P, Kreisel J and Roussel H 2011 Controlled growth of SiGe nanowires by addition of HCl in the gas phase J. Appl. Phys. 110 024311 [42] Cui Y, Lauhon L J, Gudiksen M S, Wang J and Lieber C M 2001 Diameter-controlled synthesis of single-crystal silicon nanowires Appl. Phys. Lett. 78 2214 [43] Rosaz G, Salem B, Pauc N, Gentile P, Potié A, Solanki A and Baron T 2011 High-performance silicon nanowire fieldeffect transistor with silicided contacts Semicond. Sci. Technol. 26 085020 [44] Morales A M and Lieber C M A 1998 Laser ablation method for the synthesis of crystalline semiconductor nanowires Science 279 208 [45] Coes D J and Rhoderick E H 1976 Silicide formation in Ni-Si Schottky barrier diodes J. Phys. D: Appl. Phys. 9 965 [46] Mongillo M, Spathis P, Katsaros G, Gentile P and De Franceschi S 2012 Multifunctional devices and logic gates with undoped silicon nanowires Nano Lett. 12 3074 [47] Serre P, Chapron P, Durlin Q, Francheteau A, Lantreibecq A and Ternon C 2014 Role of nanowire length in morphological and electrical properties of silicon nanonets 10th Conf. Ph.D Research Microelectronics and Electronics (PRIME) (June-July 2014) 1–4 [48] Zhou Y, Hu L and Grüner G 2006 A method of printing carbon nanotube thin films Appl. Phys. Lett. 88 123109 [49] Chiquito A J, Amorim C A, Berengue O, Araujo M, Bernardo L S and Leite E R E P 2012 Back-to-back Schottky diodes: the generalization of the diode theory in analysis and extraction of electrical parameters of nanodevices J. Phys.: Condens. Matter. 24 225303 [50] Heo Y W, Tien L C, Norton D P, Pearton S J, Kang B S, Ren F and LaRoche J R 2004 Pt∕ZnO nanowire Schottky diodes Appl. Phy. Lett. 85 3107 [51] Chung S-W, Yu J-Y and Heath J R 2000 Pt∕ZnO nanowire schottky diodes Appl. Phy. Lett. 76 2068 [52] Cui Y, Duan X, Hu J and Lieber C M 2000 Doping and electrical transport in silicon nanowires J. Phys. Chem. B 104 5213 [53] Borowik Ł, Florea I, Deresmes D, Ersen O, Hourlier D and Mélin T 2012 Surface and intrinsic conduction properties of Au-catalyzed Si nanowires J. Phys. Chem. C 116 6601 [54] Rathmell A R, Bergin S M, Hua Y-L, Li Z-Y and Wiley B J 2010 The growth mechanism of copper nanowires and their properties in flexible transparent conducting films Adv. Mater. 22 3558 [55] Stauffer D and Aharony A 1992 Introduction to Percolation Theory (London: Taylor & Francis) [56] Bashouti M Y, Sardashti K, Ristein J and Christiansen S H 2012 Early stages of oxide growth in H-terminated silicon nanowires: determination of kinetic behavior and activation energy Phys. Chem. Chem. Phys. 14 11877 [57] Bashouti M Y, Sardashti K, Ristein J and Christiansen S 2013 Kinetic study of H-terminated silicon nanowires oxidation in very first stages Nanoscale Res. Lett. 8 41
[18] Zhao Y and Grüner G 2012 Nanonet as a scaffold with targeted functionalities J. Mater. Chem. 22 24983 [19] Madaria A R, Kumar A, Ishikawa F N and Zhou C 2010 Uniform highly conductive and patterned transparent films of a percolating silver nanowire network on rigid and flexible substrates using a dry transfer technique Nano Research 3 564 [20] Scardaci V, Coull R, Lyons P E, Rickard D and Coleman J N 2011 Spray deposition of highly transparent low-resistance networks of silver nanowires over large areas Small 7 2621 [21] Sorel S, Lyons P E, De S, Dickerson J C and Coleman J N 2012 The dependence of the optoelectrical properties of silver nanowire networks on nanowire length and diameter Nanotechnology 23 185201 [22] Langley D, Giusti G, Lagrange M, Collins R, Jiménez C, Bréchet Y and Bellet D 2013 Silver nanowire networks physical properties and potential integration in solar cells So Sol. Energy Mater. Sol. Cells 125 318 [23] Wu Z et al 2004 Transparent conductive carbon nanotube films Science 305 1273 [24] Gruner G 2006 Carbon nanotube films for transparent and plastic electronics J. Mater. Chem. 16 3533 [25] Aksoy B, Coskun S, Kucukyildiz S and Unalan H E 2012 Transparent highly flexible all nanowire network germanium photodetectors Nanotechnology 23 325202 [26] Mulazimoglu E, Coskun S, Gunoven M, Butun B, Ozbay E, Turan R and Unalan H E 2013 Silicon nanowire network metal-semiconductor-metal photodetectors Appl. Phys. Lett. 103 083114 [27] Zhenqing D, Changxin C, Yaozhong Z, Liangming W, Jing Z, Dong X and Yafei Z 2012 ZnO nanowire network transistors based on a self-assembly method J. Semiconduct. 33 084003 [28] Ferrer-Anglada N, Kaempgen M, Skakalova V, Dettlaf-Weglikowska U and Roth S 2004 Synthesis and characterization of carbon nanotube-conducting polymer thin films Diamond Relat. Mater. 13 256 [29] Kaempgen M, Duesberg G S and Roth S 2005 Transparent carbon nanotube coatings Appl. Surf. Sci. 252 425 [30] Acharya S, Panda A B, Belman N, Efrima S and Golan Y A 2006 Semiconductor-nanowire assembly of ultrahigh junction density by the Langmuir–Blodgett technique Adv. Mater. 18 210 [31] Park J, Shin G and Ha J S 2008 Controlling orientation of V2O5 nanowires within micropatterns via microcontact printing combined with the gluing Langmuir–Blodgett technique Nanotechnology 19 395303 [32] Tao A, Kim F, Hess C, Goldberger J, He R, Sun Y, Xia Y and Yang P 2003 Langmuir−Blodgett silver nanowire monolayers for molecular sensing using surface-enhanced raman spectroscopy Nano Lett. 3 1229 [33] Li J, Hu L, Wang L, Zhou Y, Gruner G and Marks T J 2006 Organic light-emitting diodes having carbon nanotube anodes Nano Lett. 6 2472 [34] Dalal S H, Unalan H E, Zhang Y, Hiralal P, Gangloff L, Flewitt A J, Amaratunga G A and Milne W I 2008 Synthesis of ZnO nanowires for thin film network transistors SPIE Carbon Nanotubes and Associated Devices 7037 70370w [35] Hu L, Hecht D S and Gruner G 2004 Percolation in transparent and conducting carbon nanotube networks Nano Lett. 4 2513 [36] Woo C-S, Lim C-H, Cho C-W, Park B, Ju H, Min D-H, Lee C-J and Lee S-B 2007 Fabrication of flexible and transparent single-wall carbon nanotube gas sensors by vacuum filtration and poly(dimethyl siloxane) mold transfer Microelectron. Eng. 84 1610
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Percolating silicon nanowire networks with highly reproducible electrical properties
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Nanotechnology Nanotechnology 26 (2015) 015201 (10pp)
doi:10.1088/0957-4484/26/1/015201
Percolating silicon nanowire networks with highly reproducible electrical properties Pauline Serre1, Massimo Mongillo2, Priyanka Periwal1, Thierry Baron1 and Céline Ternon1,3 1
Univ. Grenoble Alpes, LTM, F-38000 Grenoble, France; CNRS, LTM, F-38000 Grenoble, France Univ. Grenoble Alpes, IMEP-LAHC, F-38000 Grenoble, France; CNRS, IMEP-LAHC, F-38000 Grenoble, France 3 Univ. Grenoble Alpes, LMGP, F-38000 Grenoble, France; CNRS, LMGP, F-38000 Grenoble, France 2
E-mail: [email protected] Received 15 July 2014, revised 27 August 2014 Accepted for publication 11 September 2014 Published 8 December 2014 Abstract
Here, we report the morphological and electrical properties of self-assembled silicon nanowires networks, also called Si nanonets. At the macroscopic scale, the nanonets involve several millions of nanowires. So, the observed properties should result from large scale statistical averaging, minimizing thus the discrepancies that occur from one nanowire to another. Using a standard filtration procedure, the so-obtained Si nanonets are highly reproducible in terms of their morphology, with a Si nanowire density precisely controlled during the nanonet elaboration. In contrast to individual Si nanowires, the electrical properties of Si nanonets are highly consistent, as demonstrated here by the similar electrical properties obtained in hundreds of Si nanonet-based devices. The evolution of the Si nanonet conductance with Si nanowire density demonstrates that Si nanonets behave like standard percolating media despite the presence of numerous nanowire-nanowire intersecting junctions into the nanonets and the native oxide shell surrounding the Si nanowires. Moreover, when silicon oxidation is prevented or controlled, the electrical properties of Si nanonets are stable over many months. As a consequence, Si nanowire-based nanonets constitute a promising flexible material with stable and reproducible electrical properties at the macroscopic scale while being composed of nanoscale components, which confirms the Si nanonet potential for a wide range of applications including flexible electronic, sensing and photovoltaic applications. Keywords: silicon nanowires, nanonets, percolating networks, electrical properties 1. Introduction
catalysts [6] and batteries [7]. However, due to time consuming, complex and expensive technological techniques, the industrialization of devices based on a unique SiNW [8–10] is limited. Moreover, due to a large variability between NW properties, there is an obvious lack of reproducibility from one device to another [11]. As a consequence, it is difficult to exploit the unique properties of NWs in actual devices. In contrast, materials composed of randomly oriented NWs offer good reproducibility and have recently been attracting interest in many applications [12–16]. Also called ‘nanonets,’ for NANOstructured NETworks [17, 18], such materials show several interesting properties that arise either from the individual components, from the NWs or NTs or
One-dimensional (1D) nanostructures, such as nanotubes (NTs) or nanowires (NWs), have attracted much interest in the last few decades. These 1D nanomaterials exhibit interesting electrical, optical, mechanical and chemical properties, thanks to their high aspect ratio and large surface area. Silicon nanowires (SiNWs) are one of the most promising 1D semiconductors, thanks to their notable implementation in the industry and their compatibility with present silicon-based semiconductor technology. Because of their surface properties, SiNWs have been developed in many applications such as solar cells [1, 2], biological and chemical sensors [3–5],
0957-4484/15/015201+10$33.00
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[25, 26], and ZnO nanonets have been integrated into transistors [27] or gas sensors [14]. Nanonets can be prepared by various fabrication techniques, particularly by self-assembly from a solution (e.g. spray coating [28, 29], the Langmuir–Blodgett technique [30–32] or vacuum filtration [23, 33]). The solution-based assembly is particularly attractive since it enables the fabrication of nanonets that have a wide range of thicknesses, from submonolayer coverage to over a 1 μm thickness. Among the solution-based assembly methods, the filtration one, applied in this work, is highly versatile and allows the production of homogeneous 2D nanonets over large surfaces at low costs [13, 23, 34–36]. The fabrication procedure requires that a solution of the nanostructures (NTs or NWs) be filtered using a standard vacuum filtration apparatus. During the filtration, the nanostructures are gradually deposited on the filter surface, forming the nanonet, which can then be transferred on the desired surface by dissolving this filter in an appropriate solvent. Varying the volume or the concentration of the filtered solution allows one to precisely control the nanonet thickness. This filtration process provides a good homogeneity of the nanonet. Indeed, as the nanostructures accumulate on the filter’s surface, the flow resistance in these regions increases so that the solution flow increases toward regions which have accumulated fewer nanostructures. Moreover, owing to this simple and original self-controlled homogenization mechanism, the process can be up-scaled to large sample dimensions in connection with the filter size. Consequently, nanonets are exciting materials that are easy to manufacture. They exhibit interesting electronic, optical, chemical and mechanical properties, with potential for applications in a wide variety of systems including electronics, sensors, solar cells, batteries and super-capacitors. Most importantly, this material system offers an opportunity to explore transport theories that involve percolation. Nevertheless, there is currently a lack of studies focused on semiconducting nanonet properties. Thus, the main focus of this work is to relate some aspects of their electrical behavior in relation with their morphological properties in order to identify their potential applications in diverse areas. Here, we present the results of an investigation on the electrical properties of degenerated n-type Si nanonets made from the vacuum filtration method [15], with a particular focus on the electrical conductance and the reproducibility of the nanonets. First, we demonstrate the high reproducibility of the material in terms of morphological and electrical properties. On one hand, the nanonet density is precisely controlled. On the other hand, the study of hundreds of nanonet-based devices leads to highly reproducible and predictable electrical properties. Then, the conductance dependence with the NW density is investigated, from which three different conduction regimes are evidenced: off-state, percolating regime and bulklike regime. Finally, the electrical conductance stability is studied as a function of the environment, and a way of improvement is explored through the metallic contact postannealing.
from the structural properties of the network itself. These properties include: (i) High surface area/high porosity. Due to the component geometry, the surface area drastically increases in comparison with thin films. As a consequence, the addition of functional materials is favoured due to the high porosity. (ii) Electrical conductance. The so-defined nanonets are above the percolation threshold, and the larger the NW conductivity, the better the network conductance. Factors like NW-NW contacts play a major role and have to be studied and optimized. (iii) Optical transparency. A network of high-aspect-ratio nanostructures has high transparency that approaches 100% when the aspect ratio tends to infinity. However, when dealing with absorbing materials, the transparency decreases when the aspect ratio decreases. (iv) Mechanical robustness and flexibility. A random network of interwoven NWs has significantly higher flexibility than that of a thin film. (v) Reproducibility and fault tolerance. At the macroscopic scale, the nanonet involves several millions of NWs so that the observed properties result from a large-scale statistical averaging on the individual NW properties. Moreover, breaking a conducting path in the network leaves many others still present, and the overall properties of the nanonet are stable. As a consequence, in contrast to individual components, nanonets offer high reproducibility of the physical properties coupled with easy preparation. (vi) High-quality components. It is easy to grow defect-free NWs over a large area so that the electrical and optical properties are improved. (vii) Functionalization ability. By attaching chemical species or nanoscale materials, such as nanoparticles (NPs), with well-defined functionality, it is possible to add properties induced by the synergy between NWs and NPs. When the nanonet thickness is significantly thinner than the length scale of its individual components, the nanonet is considered as two-dimensional (2D) because the percolation properties of such a network show typical 2D characteristics. Carbon nanonets (composed of carbon NTs, functionalized or not) and metallic nanonets (composed of silver or copper NWs) have been studied for a decade and are widely described in the literature, particularly for the fabrication of transparent and conductive thin films for photovoltaic applications [13, 19–24]. In contrast, there are currently very few studies on 2D nanonets based on nanomaterials other than carbon or silver, such as, for example, Si, ZnO and other semiconducting NWs, despite the high potential of such materials. Indeed, according to Grüner et al [18], Si nanonets may be considered as the fourth form of silicon in addition to single crystalline, polycrystalline and amorphous silicon. Nevertheless, germanium and silicon nanonets have been recently studied for use in photodetection applications 2
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Figure 1. (a) SEM image of a Si nanonet obtained from 45 mL of SiNW solution. (b) Schematic of a 2D nanonet. L is the nanostructure
length, D is the diameter, S is the projected surface and J is the surface overlap when two NWs cross. (c) SiNW coverage area as a function of the filtered volume of the SiNW solution. From the Si nanonet SEM images and using ImageJ software, the NW coverage area was determined as a function of the filtered volume of the SiNW solution. The images were captured at different locations on the nanonet, and the error bars match the SiNW coverage area variability from one image to another for a given nanonet. (d) Si NW density as a function of the filtered volume of the SiNW solution. The NW density was calculated from the coverage area when the latter was below 100% and was extrapolated when the coverage area saturated around 100%.
2. Experimental method
single SiNWs with the process described in [43], their resistivity is found in the range 0.3–1.4 mΩ.cm, leading to 0.5 to 2.9 × 1020 phosphorous at.cm-3. In this study, the silicon nanowires used contain the gold catalyst on the top, unless otherwise specified. The 2D Si nanonets were assembled by the vacuum filtration method [23]. First, silicon nanowires were dispersed in deionised water by ultrasonication for 5 min. Then, the NW solution was characterized by absorption spectroscopy, and further dilutions of the solution were done until its absorbance at 400 nm was equal to 0.06. In such a way, the nanowire amount in the solution was reproducible from one solution to another even if the nanowire concentration was not quantitatively determined. After the dilution, the SiNW solution was filtered through a 0.1 μm porous nitrocellulose membrane (47 mm in diameter). As the solvent went through the pores, the nanowires were trapped on the membrane surface, subsequently forming an interconnected random network: the 2D Si nanonet (figure 1(a)). Different volumes of the SiNW solution (0–320 mL) were filtered in order to prepare thin networks of controllable thickness and density. Finally, the networks were transferred onto Si3N4 (200 nm) on Si
2.1. Fabrication of the silicon nanonet
Degenerated n-type silicon nanowires were synthesized by the classical vapor liquid solid mechanism (VLS) proposed by Wagner and Ellis [37]. Gold was chosen as the metal catalyst due to its low temperature eutectic phase with silicon [38, 39]. Silane (SiH4) was used as the gaseous silicon precursor, and phosphine (PH3) was used as the dopant for n-type silicon nanowires. Hydrogen chloride (HCl) was also added during the growth in order to inhibit the gold diffusion and the lateral growth [40, 41]. SiNWs were grown on silicon substrate at 650 °C after the dewetting at 800 °C of a gold thin film (2 nm). This chemical vapor deposition allows epitaxial growth of SiNWs with a diameter monitored by the catalyst size [42]. In accordance with the classical description of the VLS mechanism [37], the gold catalyst droplets stay at the top of the SiNWs after growth. This catalyst can be removed from the nanowires by dipping the samples successively in hydrofluoric acid (HF), iodine potassium iodide (IKI) and HF baths. By separately studying the electrical properties of 3
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Figure 2. Schematics ((a) and (b)) and the colorized SEM top view images ((c) and (d)) of the processed electrical test structure based on the
Si nanonet. The nanonet was transferred onto an insulating 200 nm thick silicon nitride layer. 200 μm diameter metallic contacts Ni (120 nm)/ Al (180 nm)/Au (50 nm) separated by a 50 μm pad pitch were deposited by e-beam evaporation through a shadow mask.
substrate by membrane dissolution, thanks to a treatment with an acetone liquid bath for 30 min.
densities were deduced by analyzing SEM images with the help of ImageJ software. Two-terminal current versus voltage I(V) measurements were performed using a Karl Süss Probe Station controlled by a HP 4155 Analyzer at room temperature in a dark and ambient environment.
2.2. Electrical test structure
In order to perform electrical measurements, and as depicted in figure 2, metallic electrodes with a diameter of 200 μm and a spacing of 50 μm were deposited on the nanonet. First, to remove the native oxide layer, form H-terminated SiNWs and promote a direct contact between the metallic electrodes and the silicon NWs, the nanonets were exposed to a HF vapor flow prior to the metal evaporation during 30 s [44]. Then, using a Plassys e-beam evaporator, a trilayer contact composed of 120 nm of nickel, 180 nm of aluminum and 50 nm of gold was deposited through a shadow mask. Nickel was chosen to obtain a low Schottky barrier with the n-type SiNWs. Moreover, its capacity to form silicide after an appropriate heat treatment offers the opportunity to improve the electrical transport across the contacts [43, 45, 46]. Some devices were annealed at 400 °C during 1 min under a nitrogen flow using a rapid thermal annealing (RTA) furnace from Jet First Company. Above the nickel deposit, an aluminum cover layer was evaporated to avoid the entire consumption of the contacts during post-annealing treatments. In addition, gold was deposited on top of the stack in order to prevent the oxidation and the deterioration of the metallic contacts.
3. Results and discussion 3.1. Si nanonet morphology
The n-degenerated as-grown silicon nanowires vertically stand on the substrate. They are 10 μm long, and their diameters are in the range of 70–100 nm, leading to an aspect ratio in the range of 100–150. These two parameters are kept constant in this study, while the role of the NW length was studied in a previous study [47]. Such a high aspect ratio is essential in order to get a coherent and well-interconnected nanonet [16]. The Si nanonets elaborated by the vacuum filtration method are randomly oriented with a good uniformity over large areas (several cm2). Figure 1(a) shows a coherent, well-interconnected and randomly oriented Si nanonet elaborated from 45 mL of a well-controlled SiNW solution [15]. From the SEM images, the coverage area of a given nanonet is defined as the ratio between the surface covered by NWs and the total surface of the sample. For this purpose, ImageJ software was used to binarize the nanonet SEM images and to calculate the ratio of bright pixels (surface covered by NWs) over the total pixel number (total surface). For each nanonet that corresponds to a given filtered volume, the same image analysis was performed on six pictures that arise from different regions of the samples and from different samples. The obtained coverage area is plotted as a function of the filtered
2.3. Characterization
Morphological studies were performed by scanning electron microscopy (SEM) observations with the help of a Zeiss Ultra Plus Microscope. SiNW aspect ratios were determined by calculating the length over diameter ratio, and the Si nanonet 4
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volume in figure 1(c). The nanonet homogeneity and reproducibility is confirmed by the very small variability represented as the error bars on the graph. As long as the substrate is not totally covered by NWs, the coverage area is proportional to the volume of filtered solution (lower than 160 mL). Above 160 mL, numerous NWs are seen on the surface such that the underlying substrate isn’t more visible, which explains why the coverage area saturates just below 100%. The NW density in the nanonet can be easily deduced from the coverage area as long as the saturation is not reached. By assuming that the NW length (L) and diameter (D) are homogenous, the substrate surface covered by each NW (S) is approximated from S = L × D (figure 1(b)). As a consequence, the NW density is defined as the ratio between the coverage area and the surface covered by each NW. In such a way, the density is slightly underestimated because the overlap between the NWs, defined as J in figure 1(b), is neglected. For volumes larger than 160 mL, the density is extrapolated using the linear dependence between the volume and coverage area. Finally, the NW density is plotted as a function of the filtered volume in figure 1(d). Such a linear dependence of the nanostructure density with the filtered volume was also observed for CNT nanonets [35, 48] using the same vacuum filtration method. It first confirms that the titration of the NW solution is reliable. Then, it guarantees the nanonet large-scale homogeneity. Thus, we first demonstrate that the NW density is well controlled by the filtered volume of the NW solution for a given SiNW amount in the solution. This one is monitored by absorbance spectroscopy. Second, the statistical study of numerous images that arise from different nanonets evidences the reproducibility from one nanonet to another; finally, the nanonet homogeneity is illustrated by the small error bars in figure 1(d). When the NW density is so high that the coverage area reaches 100%, the nanonet adhesion to the substrate is lowered. This lack of adhesion is attributed to the weak Van der Waals interaction that governed the nanonet adhesion. As a conclusion, the NW density into the Si nanonets is monitored with high precision by simply controlling the filtered volume of a controlled SiNW solution through the membrane. Moreover, the so-assembled Si nanonets are above the percolation threshold, allowing conduction across the system [16]. In order to assess the Si nanonet potential as an electrically active material, studies of the electrical properties were performed and detailed in the next part.
Figure 3. Typical I(V) characteristics of silicon nanonet devices. The two continuous lines represent the range of I(V) characteristics obtained for a Si nanonet with a 50 μm interelectrode distance and a NW density of 27 × 106 NWs.cm−2. The colorized area matches the fluctuation range (±18%). In-between, the dashed line is allocated to a Si nanonet with the same NW density but without a gold catalyst, proving that the current flows through the SiNWs and not through the metal catalysts. As a reference, the I(V) characteristic without the Si nanonet is also plotted (thick continuous line) in order to insure the electrical insulation between the two metallic electrodes.
shown in figures 2(c) and (d). In the first part, the reproducibility and fault tolerance of the geometry is evidenced. In the second part, the role of the NW density in the nanonet is shown. Then, the electrical properties stability as a function of time is studied; finally, in the last part, an improvement strategy is detailed. 3.2.1. Reproducibility. At the macroscopic scale, the nanonet
involves several millions of NWs so that the observed properties result from large-scale statistics that average the individual NW properties. As a consequence, nanonets should offer high reproducibility of the electrical properties for a given SiNW length, diameter and density. To evidence this reproducibility property, more than 100 devices with a 50 μm interelectrode distance that were based on nanonets with NW density of 27 × 106 NWs.cm−2 were electrically characterized. In figure 3, two typical bidirectional I(V) characteristics are shown (light continuous lines). These two curves are representative of the measured fluctuations (colorized area). Indeed, I(V) characteristics are consistent for different nanonets with the same density within this fluctuation range (±18%), evidencing the high reproducibility of the electrical behavior for a Si nanonet with similar NW density, length and diameter. Two-probe measurements were also done on devices in which contacts were directly evaporated on the substrate without a Si nanonet between the metallic electrodes (thick continuous line in figure 3) to prove that conduction occurs through the nanonet. Two-probe measurements at room temperature for most of these devices show non-linear, symmetric I(V) characteristics with a high current level despite the numerous inter-NW junctions into the nanonet [16]. Moreover, no hysteresis is observed in the I(V) curves, suggesting the presence of a low density of trapped charges inside the structure. The slope of the I(V) curve gradually increases with the voltage for both
3.2. Electrical properties
With this work, we want to evidence that the nanonet geometry is actually an interesting electrically active material. Therefore, in this study, degenerated SiNWs were chosen to get a high conductivity from the NWs so that the electrical behavior of the nanonet is mainly controlled by the internanowire junctions and the contacts between the metal and nanonet. The electrical test structure is displayed in figure 2. The 200 μm circular Ni/Al/Au electrodes were deposited on Si nanonets with a spacing of 50 μm. Representative SEM images of the devices at a low and high magnification are 5
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Figure 4. (a) Silicon nanonet conductance at −5 V as a function of the SiNW density. The studied Si nanonets were elaborated from the same SiNW solution and from different filtered volumes in order to get different SiNW densities. (b) The conductance is plotted in the semilogarithmic scale to evidence the five decades’ increase in the conductance values. Such behavior is typical of the percolating regime, as evidenced by the fit using the power law dependence G ∝ (dNW -dc)t, which is expected from the classical percolation theory [55].
interesting material for further integration into functional devices.
negative and positive applied voltages. This reproducible rectifying response is typical of a resistor addressed via backto-back Schottky contacts that originate from the potential barrier at the metal-NW junctions [49, 50]. Similar electrical behavior was observed in single silicon nanowire devices [51–53]. As shown in the literature [53], conduction in single SiNWs involves 3 mechanisms: (i) Schottky interface junctions at the metallic contacts, (ii) Au-mediated surface conduction along the NW sidewalls and (iii) conduction through the NWs. In the case of nanonets, NW-NW junctions have to be additionally considered. To infer the role of conduction along the gold-rich sidewalls, two-probe measurements were repeated on nanonets with a NW density of 27 × 106 NWs.cm−2 in which gold were removed by immersion in successive baths (HF, IKI and HF), as described in the experimental section. The I(V) characteristic of this nanonet without gold catalysts is shown in figure 3 (dashed line). Changes in the I(V) characteristics before and after chemical treatments for gold removal are not relevant since the I(V) curve after gold removal remains in the fluctuation range of nanonets with a NW density of 27 × 106 NWs.cm−2. Thus, in the case of the n-degenerated NWs, the surface conduction through the Au catalyst residues at the NW surface is negligible. By separately studying the single SiNWs (not shown here), their intrinsic conductance is measured between 15 and 90 μS, whereas the nanonet conductance is equal to 0.34 ± 0.05 μS. As the SiNW conductance is more than one order of magnitude higher than that of the nanonets, the conduction through the NWs is a non-limiting mechanism. As a conclusion, the low bias current regime appears dominated by the Schottky interface junction between the metal and Si nanonet. At high bias, the current is governed by the interplay between the conduction through the NWs and the conduction through NW-NW junctions; the latter should be the limiting mechanism, as the NWs are n-degenerated. Moreover, the large-scale statistics that average the individual NW properties offer highly reproducible electrical properties for nanonets as soon as the SiNW length, density and diameter are fixed, which implies that Si nanonets are an
Role of density. The two-probe measurement configuration was also used to determine the conductance variation with the SiNW density. The conductance (G) is defined as G = I/V. Figure 4 shows the conductance at −5 V as a function of the SiNW density. As can be seen, the conductance values vary by 5 orders of magnitude over the SiNW density range. Such a variation with density was already shown for CNT nanonets [24, 35, 48] and for metallic nanonets such as silver or copper nanonets [13, 19, 54]. In our case, three different regimes are clearly observed. At a very low density, the OFF-state for which no measurable current is observed (first measurable current for 13 × 106 NW.cm−2, figure 4(b)), means that no or very few conduction paths exist between the two electrodes. For densities between 13 × 106 and 35 × 106 NW.cm−2 (figure 4(b)), the four decades’ increase of conductance G evidences the percolating regime. As shown in figure 4(b), this sharp rise can be fitted using the power law dependence G ∝ (dNW -dc)t, which is expected from the classical percolation theory [55]. Here, dNW is the SiNW density; dc is the critical NW density, defined as the lowest density for which current is measurable; and t is the critical exponent related to the dimensionality of the system. The exponents t of 1.3 and 1.9 are expected for 2D and 3D percolation, respectively. From the fit to our data, we find a critical NW density of 15.9 × 106 NWs.cm−2, which is very close to the experimental value (13 × 106 NW.cm−2) and find a critical exponent of 1.29, confirming the 2D geometry. For the highest densities (above 35 × 106 NW. cm−2), the behavior is nearly linear, indicating a scaling that is similar to ‘bulk’ material. This transition from the OFF-state to a bulk-like state was expected on the basis of the percolation theory [55], but it is evidenced for the first time for the Si nanonet. To conclude, Si nanonets based on degenerated SiNWs behave like standard percolating media despite the presence of the numerous NW-NW junctions into the nanonet and the native oxide shell surrounding the SiNWs. 3.2.2.
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Figure 5. Conductance evolution over the course of 102 days for Si
Figure 6. I(V) curves before and after the contact annealing
nanonet devices stored in different atmospheres: a nitrogen atmosphere (squares) or an ambient atmosphere (triangles). This plot represents the normalized conductance G/G0 at a potential of −5 V as a function of the time, with G0 as the initial conductance value on the first day. The error bars are allocated to the conductance variability obtained with three different devices. The line without symbols matches the exponential fit for the Si nanonets stored in air with a decay time of 2.2 days.
treatment. This treatment was performed during 1 min at 400 °C in a nitrogen atmosphere. The current value at ±5 V is multiplied by a factor of 2 after the contact annealing.
much slower mechanism. Considering the particular structure of nanonets, and due to the geometry, the silicon oxidation at the NW-NW junctions might be delayed. Thus, by increasing the exposition time to air, the oxide shell present at NW-NW interfaces thickens gradually. As a consequence, the exponential decay of the conductance with time should be directly linked to the oxide thickness at NW-NW junctions. The electron tunneling probability through a potential barrier decreases exponentially with the barrier width, which suggests that conduction through the NW-NW junctions is governed by the electron tunneling across the oxide shell until it reaches barriers that are too large for conduction to occur. To conclude, the electrical properties of silicon nanonets are stable with time, when the devices are protected from oxidation. Working under an inert atmosphere is valuable for experimental study. For further use in functional devices, encapsulating layers are deposited on top of the devices to avoid oxidation. However, such an encapsulation prevents the sensing application. Therefore, passivation treatments are under study, with promising results.
3.2.3. Si nanonet stability. Based on the results presented in
this study, the Si nanonets appear as an interesting material and structure for functional devices, such as sensors. However, it is necessary to check the stability with the time of such Si nanonets. Therefore, the electrical properties of Si nanonets were studied over a long period (more than 3 months) when stored in nitrogen or in the air between the electrical measurements. Figure 5 shows the time evolution of the normalized conductance G/G0, with G0 as the initial conductance value on the first day. Note that successive electrical measurements on the same device do not alter the electrical properties of the nanonets because the I(V) curves remain identical even after several voltage cycles. As shown in figure 5, the electrical properties of Si nanonets over time are strongly dependent on the storage atmosphere. When stored under nitrogen, the nanonet conductance is stable over many months within a 10% fluctuation range. In contrast, when kept in air, the nanonet conductance decreases exponentially with a characteristic decay time of 2.2 days. Thus, such decay is the consequence of the interaction between the species contained in the atmosphere and the Si nanonet surface. However, as oxygen plasma cleaning or vacuum annealing are unable to restore the electrical properties, adsorption on the surface and pollution by organic species are ruled out. By contrast, when exposed to HF vapor, the Si nanonets retrieve their electrical properties, revealing that oxidation is in fact responsible for the conductance decay. Studies about SiNW oxidation [56–58] have shown that SiNWs terminated with hydrogen atoms are more oxidationresistant than equivalent planar Si(100). However, according to the literature, NWs with a diameter around 85 nm are oxidized within one hour [56, 57]. In our case, the SiNWs are H-terminated with a diameter around 100 nm, but the characteristic decay time is about two days, suggesting a
3.2.4. Si nanonet electrical properties improvement. As
shown above, the low bias current regime appears dominated by the Schottky interface junctions between the metal and nanonet at the contacts. The vacuum work function of Ni is 4.74 eV [59], whereas the electronic affinity of Si is 4.01 eV. By annealing the contact at 400 °C under nitrogen, silicidation occurs, favouring electron transport across the Schottky barrier [43, 60, 61]. The main advantage of this annealing step is the appearance of a low resistive metallic phase of which the interface with SiNWs is abrupt. Figure 6 shows the behavior of a Ni-contacted Si nanonet device with a SiNW density of 27.106 NWs.cm−2 before and after rapid thermal annealing. After the annealing treatment, the current for a bias voltage of ±5 V almost doubles, but the response is still rectifying. Such a comparison is relevant since changes in I(V) characteristics are much larger than the fluctuations recorded on different 7
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nanonets (colorized area in figure 6). This current increase probably results from the formation of the nickel silicidesilicon interface, which enhances electronic transport across the Schottky barrier. However, such annealing does not lead to an ohmic contact as expected. This suggest that the electronic transport is ruled by both the oxide junctions between the crossing nanowires and by the presence of surface states, leading to the Fermi level pinning at the semiconductor surface [49]. The same phenomenon was observed in tens of Si nanonet devices and always leads to a two- or threefold current increase. As a consequence, silicidation is a valuable process that creates reproducible metal-semiconductor interfaces in the Si nanonet device and allows improvement of the electrical properties.
from LTM (Grenoble, France) for his help with the RTP furnace. This work was partly supported by the French RENATECH network. The research that led to these results was funded in part by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement NANOFUNCTION no. 257375.
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4. Conclusion In summary, we have prepared uniform and randomly oriented Si nanonets using the standard and low-cost vacuum filtration method. The NW density is precisely monitored, and the electrical properties of such random architectures are highly reproducible, confirming that the nanonet geometry is an efficient way to average discrepancies between individual NWs. N-degenarated Si nanonets behave like standard percolating media despite the presence of numerous NW-NW junctions into the nanonets and the native oxide shell surrounding the SiNWs. The electrical behavior of the Si nanonets presented in this work is the result of a combination between the Schottky interface junctions and NW-NW junctions, which are in fact tunnel junctions through the native oxide shell. When stored in nitrogen, the electrical properties of Si nanonets are stable over many months. With this work, thanks to the use of ndegenerated NWs, we demonstrate that the NW-NW junctions are not detrimental for the nanonet electrical properties. So, replacing the degenerated NWs by low-doped NWs will produce semiconducting nanonets that behave like thin-film transistors. As a consequence, the Si nanonets constitute a promising material with stable and reproducible electrical properties at the macroscopic scale while being composed of nanoscale components. Therefore, this confirms the Si nanonet potential for applications in a wide variety of systems such as flexible electronic, photovoltaic or sensing systems. Moreover, thanks to their reproducibility and high control over electrical properties, such Si nanonets are an interesting material in which to study complex percolation phenomena in random networks.
Acknowledgments The authors thank the members of the technical staff of the PTA facilities at Grenoble (France) for their technical support. The authors thank X Mescot from IMEP-LAHC (Grenoble, France) and J Grisolia from LPCNO (Toulouse, France) for their help on the electrical measurements; A Gachon from IMEP-LAHC for the shadow mask fabrication; and B Salem 8
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