Si-Nanowire-Based p–n Junction Diode

Jun 16, 2017 - Chemically derived topological insulator Bi2Se3 nanoflake/Si nanowire (SiNWs) heterojunctions were fabricated employing all eco-friendl...
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Topological Insulator Bi2Se3/Si-Nanowire-Based p−n Junction Diode for High-Performance Near-Infrared Photodetector Biswajit Das,† Nirmalya S. Das,†,§ Samrat Sarkar,‡ Biplab K. Chatterjee,‡ and Kalyan K. Chattopadhyay*,†,‡ †

Thin Film and Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700 032, India School of Materials Science & Nanotechnology, Jadavpur University, Kolkata 700 032, India

‡

S Supporting Information *

ABSTRACT: Chemically derived topological insulator Bi2Se3 nanoflake/Si nanowire (SiNWs) heterojunctions were fabricated employing all eco-friendly cost-effective chemical route for the first time. X-ray diffraction studies confirmed proper phase formation of Bi2Se3 nanoflakes. The morphological features of the individual components and time-evolved hybrid structures were studied using field emission scanning electron microscope. High resolution transmission electron microscopic studies were performed to investigate the actual nature of junction whereas elemental distributions at junction, along with overall stoichiometry of the samples were analyzed using energy dispersive Xray studies. Temperature dependent current−voltage characteristics and variation of barrier height and ideality factor was studied between 50 and 300 K. An increase in barrier height and decrease in the ideality factor were observed with increasing temperature for the sample. The rectification ratio (I+/I−) for SiNWs substrate over pristine Si substrate under dark and near-infrared (NIR) irradiation of 890 nm was found to be 3.63 and 10.44, respectively. Furthermore, opto-electrical characterizations were performed for different light power intensities and highest photo responsivity and detectivity were determined to be 934.1 A/W and 2.30 × 1013 Jones, respectively. Those values are appreciably higher than previous reports for topological insulator based devices. Thus, this work establishes a hybrid system based on topological insulator Bi2Se3 nanoflake and Si nanowire as the newest efficient candidate for advanced optoelectronic materials. KEYWORDS: Bi2Se3, topological insulator, silicon nanowire, p−n junction diode, NIR photodetector

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INTRODUCTION With the development of device dependent urban human life, increasing mandate of technologically more advanced tools has lead the scientific community to search for smarter materials and/or composites.1,2 Especially, miniaturization of junctionbased devices has become inevitable in view of financial facility, portability, and ease of application, which successfully is reflected in microchips in almost every smart electronic machines. However, fabricating even smaller dimensional systems are restricted because of unavailability of suitable newer materials and undesired thermal noise occurring parallel to the degree of miniaturization. Tuning materials in nanoregime has been emerged as a potential way out from this challenge which was realized in several hetero junctions fabricated using different nanostructures of ZnO/NiO,3 ZnO/ Cu2O,4,5 etc. In most of the reports, transparent conducting oxides (TCOs) plays the key role in such junction fabrication, which has its own advantages. Nevertheless, it is still not free from limitations. In the very basic step, finding out an appropriate p-type TCO is still under extensive investigations.6,7 Ternary oxides were developed to mitigate this © 2017 American Chemical Society

challenge, but conductivity of such multicomponent oxide is still inferior compared to their n-type semiconductor counterparts.8 The most popular intrinsic semiconductor, silicon is also a commonly used p-type material for junction fabrication.9,10 However, silicon, having a very low band gap,11 hardly favors proper band alignment with traditional n-type oxide semiconductors, which generally exhibit wide band gap.12 Search for proper combination of both p- and n-type counterparts for junction fabrication therefore continues and several new class of ternary oxides, nonoxide materials were developed for smarter p and n type materials. For example, in our early work, we established a new p-type delafossite CuBO2 as a proper match for ZnO nanostructures leading toward efficient junction formation.13 On the other hand, several chalcogenides14,15 were reported to be included into new age smart junctions resulting proper ideality factor and very low knee voltage. Received: January 16, 2017 Accepted: June 16, 2017 Published: June 16, 2017 22788

DOI: 10.1021/acsami.7b00759 ACS Appl. Mater. Interfaces 2017, 9, 22788−22798

Research Article

ACS Applied Materials & Interfaces Although each of the mentioned candidates is capable of exhibiting efficient diode behavior, they must be chosen to serve multiple purposes. For example, a combination of Si bulk/nano structure with n-type TCOs show appropriate junction properties16 but might invite hydrogen bonding due to presence of oxygen atoms9 and hence shall lack longevity when operated in humid ambient. Using chalcogenides, such as ZnS17 or CdS18 as alternatives, can solve this problem but may raise different issues related to resistivity19 or health hazard.20 Moreover, whether to fabricate junctions in nano or bulk area is another topic still to be answered. Nanojunctions are individually efficient21 but may not be suitable for industrial applications where higher values of output current are required. On the other hand fabrication of large area diode in bulk range is not achieved yet due to several experimental drawbacks. This challenge may successfully be answered by fabricating large area diode as a combination of number of individual nanojunctions.10Aiming toward these goals, in this work, we propose a combination of p-Si NW/n-Bi2Se3 NFs junctions as an efficient diode for the very first time. Choosing these two particular components can be advantageous in several ways. Si is a well-known p-type material used for many decades to fabricate p−n junctions. These junctions can provide high output current,13 low leakage current10 and remarkable other related properties.11 On the other hand, Bi2Se3 is one of the most important n-type chalcogenide with many important characteristics like high electrical conductivity,22,23 appreciable thermoelectric properties,24−26 electrochemical properties,27,28 photoconductivity,29 etc. Bi2Se3 has a low band gap of about 0.35 eV,30,31 which is very close to that of Si. This facilitates smooth alignment of energy bands restricting the possibility of undesired band mismatch between the junction components suffered by many other hybrid systems.32 Again, Bi2Se3 is a popular topological insulator,33−35 which shows unique property of conducting surface states and insulating bulk states. Spin−orbit coupling in such materials is another important factor,36 which generates intercross band structure and provides conducting surface states by locking the orientations of spin and momentum of the electrons during their propagation at opposite directions. As a result, forward and reverse bias in a system involving Bi2Se3 can be significantly different resulting in remarkably enhanced diode characteristics. Additionally, it is well-known that both Si and Bi2Se3 have optical energy gaps in low energy region. Easy band alignment is therefore expected in between them. Furthermore, several reports are there demonstrating Bi2Se3 as an important material for IR detection purpose.20 This indicates that a proper junction in between Si and Bi2Se3 might be important for infrared detecting device fabrication. Another very important feature of self-protectiveness could be achieved by using Bi2Se3 NFs as n-type counterpart. Several reports claim that Bi2Se3 exhibit inherent hydrophobic (repel water)32,37 properties which is beneficial for repelling moisture induced degradation of the hybrid system and in turn can enhance the operational lifetime of the device. However, even after choosing proper materials like p-Si NW/ n-Bi2Se3 NFs for device fabrication, employing the proper synthesis route is also important in view of chemical activities of the component materials and financial aspects. Local oxidation of Si NWs37 is the most crucial factor to be taken care of in this regard. In our earlier work,38 we proposed closed pot hydrothermal method to fabricate reduced graphene oxide− bismuth selenide composite. The same technique is however less effective in this work as molecular level growth of Bi2Se3

NFs on Si NW substrate is aimed. Bottom up formation of high aspect ratio Bi2Se3 nanostructure is already reported in high temperature vapor−liquid−solid (VLS) technique39 but will include additional risk of SiO2 formation. Again high-vacuum physical deposition techniques like molecular beam epitaxy (MBE),40 atomic layer deposition (ALD),41 metal organic chemical vapor deposition (MOCVD),42 or pulsed laser deposition (PLD)43 may hinder residual oxygen contamination but will require difficult fabrication set up leading to higher production cost. In our work, we have balanced these issues by employing cost-effective simple wet chemical method for bismuth selenide deposition on Si nanostructures. In this technique, we were able to minimize the possibility of local oxidation of prefabricated Si NWs by carrying out the synthesis fully immersed within a reaction reagent solution in room temperature. This method also caused slow deposition of Bi2Se3 NFs from the very molecular level which strengthens the p−n contact between the Si NWs and Bi2Se3 NFs. The samples were fabricated in different deposition durations. All the samples were thoroughly analyzed by structural, morphological and compositional features using X-ray diffraction, field emission scanning electron microscopy, and energy dispersive X-ray studies, respectively. The samples exhibited excellent diode characteristics with very small leakage current of 9.0 μA and nearly perfect ideality factor of 1.15. Additionally the sample showing best junction properties also showed very efficient infrared detection behavior exhibiting 934.1 A/W responsivity and detectivity of 2.30 × 1013 Jones. Hence this work establishes the novel combination of p-Si NW/n-Bi2Se3 NFs as an efficient system for future optoelectronic applications.

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EXPERIMENTAL SECTION

Preparation of Silicon Nanowire. Optimized Fabrication of silicon nanowires by chemical etching procedure has been described in detail elsewhere.9 In brief, B-doped p-type Si (111) wafers were cut into 2 cm × 2 cm pieces and HF etching was carried out after Ag seeding. From our previous observation, best morphology and performance was observed from Si NWs obtained by 27 min synthesis duration. The same parameters were used in this work to synthesize Si NW array. The nanostructured substrates were partially masked using Teflon tape and used for Bi2Se3 deposition. Preparation of Bi2Se3 Nanoflakes and p−n Junction. Bi2Se3 nanoflakes were deposited by wet chemical route. In a typical synthesis method, commercially available Bi(NO3)3·5H2O (99.99%) and Na2SeSO3 (99.99%) were used as sources of Bi and Se, respectively. Four milliliters of 0.1 M Bi(NO3)3·5H2O solution and 80 mL of 0.1 M nitrilo-triacetic acid (NTA) solutions were mixed together to prepare a bismuth chelate in chemical bath. Thereafter, reducing solution of 4 mL of 0.5 M ascorbic acid (AA) was added into the above mixture under continuous stirring. Ammonia solution was dropwise added to the solution to maintain the pH near 9.00 and finally the solution turned transparent. 0.1 M Na2SeSO3 solution was then prepared freshly and 6 mL of the same was added to the previous mixture solution. Partially masked Si NW substrates were then fully immersed vertically in this final precursor solution and the same was rested in a hot oil bath at 75 °C with constant stirring. The deposition time was varied from 0.5, 1.0, and 1.5 h and the resulting samples were labeled as BS30, BS60, and BS90, respectively. The samples were then rinsed several times with DI water and ethanol to wash out unreacted precursor (if any) and dried naturally. The as prepared Si NW/Bi2Se3 NF hybrid systems were considered as the target photodetector samples and were subjected to different characterization processes without further treatment. 22789

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CHARACTERIZATION Characterization of p-Si NW/n-Bi2Se3 NF Samples. All prepared samples were characterized to investigate the phase purity and crystallinity using a X-ray diffractometer (Bruker D8 Advanced) with Cu Kα radiation (λ = 1.54056 Å) in a 2θ range from 22° to 60°.The surface morphology of the samples were analyzed by using field emission scanning electron microscopy (FESEM, Hitachi S-4800). The cross sectional analyses were also carried out using FESEM to measure the approximate thicknesses of Bi2Se3 and Si layers. To observe the exact dimension and nature of the contact between p and n layers, high resolution transmission electron microscopy (HRTEM, JEOL JEM 2010) was performed whereas chemical composition and elemental analysis was carried out using energy dispersive X-ray studies using EDX spectrophotometer attached with FESEM (EDS, Thermo Scientific attached with Hitachi S4800). Current voltage characteristics in both forward and reverse bias of the pure components and the hybrid samples were measured using a Keithley nanovoltmeter 2182a, source meter 2400, and 6221 ac/dc current source meter interface with LabVIEW program and all photoresponse properties were performed at room temperature. The dimensions of the probe used in our experiment are 0.9 mm × 0.9 mm. The temperature dependent I−V characteristics were carried out between 50 and 300 K. A Xe lamp (LUXTEL) with tunable output power combined with monochromator (DongWoo, MonoScan ver. 3.1) was used to produce variable monochromatic light from 300 to 1000 nm. The sample showing best transport property was studied for photoresponse investigation. First, the sample was excited with various incident wavelengths and corresponding photocurrent was measured. Best response was obtained for infrared excitation. Hence NIR detection efficiency of different samples was further investigated. An NIR source of 890 nm with tunable output power was used to excite the device and output current in that excited condition was measured.

crucial role and result in a strain at the junction region. With increase of growth duration, a discontinuous Bi2Se3 layer is initially deposited on Si NW already. Hence the next layer of Bi2Se3 does not face any structural mismatch and as a result Bi2Se3 NFs are deposited more uniformly. This is reflected in higher number of diffraction planes observed for sample BS60 and sample BS90. The results confirm proper crystalline phase formation of Bi2Se3. To understand the growth and nature of nanoflake deposition, all the samples including the pure components were studied with field emission scanning electron microscopy. Figure S1 depicts the surface morphology of pure Si NWs and Bi2Se3 nanoflakes deposited on bare Si substrates. We can see that the Si NWs have diameter varying between 30 and 50 nm and grown evenly on a large area of the base silicon wafer. The Bi2Se3 nanoflakes, as found in Figure S1c, also grew in a large area on pure Si substrate, the average width of the flakes were observed to be 10−40 nm and a wide variation of dimension in length and breadth of the nanoflakes ranging from few hundred nanometers to 1 μm was found. The surface morphology of resulting junction samples are depicted in Figure 2. It can be clearly seen that a very little amount of Bi2Se3 nanoflakes are grown in sample BS30 (Figure 2a). The surface is full of holes formed between the SiNWs and very small portion of the same is covered with Bi2Se3 nanoflakes (Figure 2b). The cross sectional view of sample BS30 rarely showed any Bi2Se3 nanoflakes as we can find in Figure 2c. Sample BS60 showed higher amount of Bi2Se3 nanoflakes deposited on Si NWs (Figure 2d). It was observed in a closer view (Figure 2e) that the amount of hollow area in between the Si NWs decreased comparatively than sample BS30 and the same was filled with deposited Bi2Se3 nanoflakes. The cross sectional view of the junction also revealed the same fact featuring a distinct junction in between the Si NWs and Bi2Se3 nanoflakes (Figure 2f). However, the morphology was quiet different as observed in case of sample BS90. Figure 2g depicts dense deposition of Bi2Se3 nanoflakes covering the entire Si NW array and any part of Si NWs were hardly visible even at a closer view (Figure 2h). The cross sectional view of the junction presented in Figure 2i also showed Si NWs covered with Bi2Se3 nanoflakes almost up to bases. Further, to study the exact dimension of average individual components and nature of contact in between Si NW and Bi2Se3 nanoflakes, TEM studies were carried out and the results are presented in Figures S2 and 3. Figure S2 shows TEM images of chemically grown pure Si NWs. We can see a bunch of Si NWs having diameter ∼50 nm (Figure S2a) and the lattice image of the same are presented in Figure S2b with SAED image in inset. The lattice spacing observed in HRTEM micrographs was measured to be 0.233 nm, showed presence of (111) plane of cubic Si lattice. Figure S2c represents a single flake of Bi2Se3, where the dimension of the same can be measured to be few microns. The lattice spacing (0.302 nm) of Bi2Se3 nanoflake is depicted in Figure S2d. The fringes show the presence of (015) plane of hexagonal Bi2Se3 lattice. The SAED pattern presented in the inset of the same also indicate proper phase formation of Bi2Se3 and these results fully agree with results obtained in our XRD studies. The exact nature of junction in molecular/atomic level was also investigated via TEM/HRTEM study. We can see in Figure 3a and b that a bunch of Si NWs are properly wrapped with Bi2Se3 nanoflake. Figure 3c shows coexisting lattice

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RESULTS AND DISCUSSION XRD patterns of the samples are presented in Figure 1. Diffraction peaks in the figure are identified to be occurring due

Figure 1. XRD spectra of the sample BS30, BS60, and BS90.

to reflection form various planes of orthorhombic Bi2Se3 lattice (JCPDS card no. 33-0214). It can be clearly seen that only two peaks corresponding to (015) and (0016) are seen for sample BS30, whereas number of diffraction peaks gradually increased in case of sample BS60 and BS90. This result can be accounted for the direct effect of lattice dissimilarity between Bi2Se3 and Si. In case of short growth duration, the mismatch of crystal structure (i.e., rhombohedra for Bi2Se3 and cubic for Si) plays a 22790

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Figure 2. (a) Low-magnification, (b) high-magnification, and (c) cross-sectional FESEM images of sample BS30, (d) low-magnification, (e) highmagnification, and (f) cross-sectional FESEM images of sample BS60, and (g) low-magnification, (h) high-magnification, and (i) cross-sectional FESEM images of sample BS90.

(0.302 nm). This result directly indicates that in our synthesized samples, the junction fabrication has been extended down to the nano domain which is expected to facilitate easier carrier transfer in between the two components. Although XRD patterns and SAED images clearly showed that counterparts of Si/Bi2Se3 hybrid system were synthesized in proper phases, we also carried out energy dispersive X-ray studies to obtain compositional information on all the hybrid samples. The results are depicted in Figure 4. We can see that with increasing synthesis duration, the distribution of Bi and Se gradually increases with lesser amount of exposed silicon. This result clearly supports the proper experimental optimization employed in our synthesis procedure where the content of Bi2Se3 was varied with synthesis time and the morphology was tuned accordingly. On the basis of the results obtained from EDX and FESEM studies, the growth mechanism of the devices were proposed later and transport properties were explained accordingly (discussed later). Electrical Studies. Temperature-dependent current−voltage (I−V) characteristics of the Bi2Se3 samples grown on Si nanowires (SiNWs) were studied and the results are presented in Figure 5. The sample exhibits nonsymmetric diode behavior between the temperature ranges 50−300 K under dark condition, as shown in Figure 5a. According to the thermionic emission (TE) theory, the current across the junction at forward bias voltage can be fitted by the equation44

Figure 3. (a) Low- and (b) high-magnification TEM image of sample BS60, (c) lattice image of sample BS60; (d) lattice image of junction of pure Bi2Se3 NFs, and Si NW. Inset showing intercalated lattice of the components.

patterns of Bi2Se3 and Si whereas in the magnified image (Figure 3d), we can see that four atomic planes Si (0.233 nm) crystal is perfectly merging with three atomic planes of Bi2Se3

I = Io[exp(qV /ηkT ) − 1] 22791

(1) DOI: 10.1021/acsami.7b00759 ACS Appl. Mater. Interfaces 2017, 9, 22788−22798

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Figure 4. (a) Elemental distribution and EDX spectra of sample BS30. (b) Cross-sectional view and elemental distribution of (c) Si, (d) Bi, and (e) Se in sample BS60 (left column) and sample BS90 (right column).

where I is the net current flowing through the device, V is the applied voltage across the terminals of the diode, q is the absolute value of electron charge, k is the Boltzmann’s constant, η is the ideality factor, and T is the absolute temperature (K). The dimensionless parameter, ideality factor (η), can be determined from the slope of the linear region of the semilog I−V curves q η= [dV /d(ln I )] kT

I0 = AA*T 2 exp( −qϕIV /kT )

(3)

where A is the diode area, A* the effective Richardson constant (32 A/cm2 K2 for p-Si),46 and ϕIV is the Schottky barrier height (SBH). SBH as a function of temperature can be obtained from the extrapolation of the I0 in the semilog I−V curves (Figure 5b), which can further be expressed by eq 4 ϕIV =

(2)

kT ln(AA*T 2/I0) q

(4)

The values of ideality factor and SBH with respect to temperature were calculated by eqs 2 and 4 and are presented

Again, the reverse saturation current, I0, can be expressed by45 22792

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Figure 5. (a) Temperature-dependent I−V characteristics, (b) corresponding semilog plot, (c) variation of ideality factor, and (d) barrier height variation from I−V and C−V measurements.

values of temperature coefficient (α) were estimated. It was determined to be 3.90 and −3.4 meV/K, for ϕIV and ϕCV, respectively. These results are in good agreement with other reported values of temperature coefficients studied for different Schottky diodes.51,52 The obtained current−voltage (I−V) characteristics for all the samples in both forward and reverse bias under NIR irradiation are depicted comparatively in Figure 6. It can be seen that all the samples exhibit proper junction behavior with different turn on voltage, reverse saturation current and other diode parameters. Figure 6(a) (inset) shows the junction behavior under dark condition for the Topological insulator Bi2Se3 grown on Si substrate and Si nanowires (SiNWs) substrate. Figure 6(a) shows the I−V characteristics under irradiation of a NIR 890 nm light with intensity 8 mW/cm2. The photo current increased for both the cases where Si NWs and pure Si were used as the base material. Under the dark as well as NIR irradiation the rectifying ratio (I+/I−) was measured from the above curves at ±0.5 V. The rectification ratio for SiNWs substrate over the Si substrate under dark and NIR irradiation was measured to be 3.63 and 10.44, respectively. Above results indicate that the Si nanowires (SiNWs) are more preferable as substrate than pure Si to deposit Bi2Se3 for opto-electric applications. In Figure 6b, we compared the I−V characteristics of the three different samples under NIR irradiation. The schematic representation, as shown in Figure 6c, demonstrates the possible growth of the hybrid system. In case of sample BS30, the short growth duration did not allow completion of synthesis of a compact hybrid system and each individual Si NW-Bi 2Se3 NF combination work as independent nanoheterojunctions instead

in the Figure 5c and d, respectively. It is found that the SBH increased from 0.174 to 1.153 eV and ideality factor decreased from 2.68 to 1.16 with increase in temperature from 50 to 300 K. A similar trend is also reported for other Schottky devices.44,45 It can be seen (Figure 5d) that the barrier height (ϕIV) is minimum at low temperature and increased gradually with temperature. This positive temperature coefficient, contrary to features of semiconductors, can be explained considering the interface inhomogeneity.44,45,47 However, the barrier height (ϕCV) calculated from capacitance measurements is not influenced by such interface inhomogeneity. To investigate this, variation of barrier height with temperature was also studied from C−V measurements (Figure S3). It was observed (Figure 5d) that the barrier height gradually decreased with temperature in this case. The distinction between the two processes occurs from method of biasing. Barrier height ϕIV measured from I−V method, involves forward bias range of the current−voltage characteristic, whereas ϕCV is determined from the reverse bias section of capacitance−voltage plot.47,48 The increase of ϕIV with temperature is attributed to inhomogeneous barrier at the interfaces. As the temperature increases, spreading of electric field distribution across all the points of inhomogeneity in turn increases the effective barrier height.49 However, ϕCV, as mentioned earlier, is not much sensitive to interface inhomogeneity, rather dependent on the width of the depletion region.50 Higher temperature cause appreciable shrinkage in that depletion layer which in turn lowers the barrier height. Similar observations were reported for several other systems47−49 involving semiconductor junctions. The barrier height, in both the cases, varies linearly with temperature. From the linear fitting of the corresponding plots, 22793

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duration. Here, multiple nano junctions of Bi2Se3 NF-Si NW are formed which are connected by common layer of Bi2Se3. This optimized morphology result in enhanced photocurrent as can be observed from Figure 6b. Moreover, sample BS60 is composed by integrating multiple nano junctions under common Bi2Se3 NF thin layer with large effective surface area. As a result, higher contribution of conducting surface states than that of insulating inner states is enabled in the system and lesser influence from contact resistance occurs as reflected in the experimental results. However, because of much higher growth time, sample BS90 hardly belong to nano domain, the junction area between Bi2Se3 NFs and Si NWs increases in this case and near-bulk feature of the Bi2Se3 layers enhances the possibility of direct contact of base Si substrate with Bi2Se3 than formation of proper Si NW/Bi2Se3 nanoheterojunction. The sample BS60 was subjected to wavelength-dependent photocurrent studies. The obtained result is summarized in Figure 7a. We can clearly see the sample responding to a large extent upon excitation by electromagnetic radiation having wavelength ∼890 nm. This clearly indicated the NIR detection capability by the sample. Bi2Se3 is well-known for this application as reported by several other research groups.53 Silicon, on the other hand is also known for its band gap lying in the IR region. Recently, Luo et al. and their group has extensively work on Si NW-based junction devices for NIR detection in the range around ∼850 nm.54,55 In our device, the combination of Bi2Se3 NF/Si NW shows much enhanced infrared response at room temperature around ∼890 nm which is close to previous reports. Inspired by this concept, we studied the IR response activity of both pure Bi2Se3 NFs and the hybrid system present in sample BS30, BS60 and BS90. The results are depicted in Figure 7b and c. The high resolution photoresponse curves presented in Figure 7d was further used to measure the response time (rise and decay) of the detector and corresponding results are shown in Table 1 and Supporting Information (Figure S4). We can clearly observe that both the responsivity and detectivity were superior for the n-Bi2Se3 NF/p-Si NW heterojunctions fabricated in sample BS60 compared to sample BS30 and BS90. Sample BS30, having poor continuity of Bi2Se3 NFs do not show good photoresponse and distinct on−off phenomenon is absent. Sample BS90 having comparatively higher thickness of Bi2Se3 NFs includes higher resistance and path length for the photogenerated carriers and showed inferior IR response. Bi2Se3 NF samples deposited on pure silicon substrates also showed much poor response in IR detection compared to sample BS60. The results are directly related to optimization of thickness of Bi2Se3 NF layer on Si NW substrates as achieved in sample BS60. First, because of the presence of built in electromotive field at the interface of Si−Bi2Se3, the photogenerated electron−hole pair is easily separated−electron moves via Si path and holes find their way through Bi2Se3. This decreases the probability of e−−h+ recombination leading to an effective photodetection device. The well-known “topological insulating” feature of Bi2Se3 NFs has also contributed in the observed results. In an identical system (with bulk Si) Zhang et al.56 reported good photodetection behavior and identified that the surface states of Bi2Se3 nanostructures on silicon substrates is maintained on atomically flat interfaces. The band alignment between Si and nano Bi2Se3 was also established as a positive factor contributing to the good photoresponse property of the hybrid. At thermal

Figure 6. (a) IR response of sample BS60 and Bi2Se3 NF-pristine Si system. Inset shows corresponding data in dark condition. (b) IR response of sample BS30, BS60, and BS90. (c) Schematic of growth of Bi2Se3 NF/Si NW heterojunction.

of resulting into a single device. The obtained comparative results and the fitting parameters are presented in Table S1. Considering the above-mentioned factors, the diode characteristics of the samples in our work can be explained properly under the presence of NIR light. In case of sample BS30, morphological inferiority, as well as the contact resistance results in improper diode behavior. As seen from Figure 6b, to achieve highest current we required more applied voltage than the other samples. Considering the morphological effect of sample BS30, Bi2Se3 NFs are not fully grown and not evenly distributed over the entire Si nanowire thin film. Thus, the device in sample BS30 shows inferior response. A special role of Bi2Se3 as a topological insulator must also be taken into account here, as we know that topological insulators are wellknown for their intrinsic feature of insulating inner states and conducting surface states. The situation improves a lot in case of sample BS60 as an effect of optimization of synthesis 22794

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Figure 7. (a) Wavelength vs current plot for sample BS60; (b) room-temperature photoresponse properties for samples BS30, BS60, and BS90; (c) photoresponse properties of Bi2Se3 NFs grown on pure Si substrate and Si NWs. (d) High-resolution photo response properties of sample BS60.

Table 1. Comparison of Different Parameters of Photoresponse Activity sample

wavelength (nm)

responsivity (A/W)

Ip/Id

Bi2Se3/ Si NWs Bi2Se3/ Si sub graphene/Si Bi-layer graphene/CH3-SiNH Bi2Se3 NW/Si AuNPs@graphene/CH3-SiNWs individual Si NW/MLG Sb2Te3 a-C/Si Bi2Te3/Si polycrystalline Bi2Te3

890 890 850 808 850 850 980 980 visible 1064

938.4 287.7 0.225 0.328 924.2 1.5 8.7 21.7 0.292 1 3.03 × 105

2.5 1.6 ∼104 ∼107 10 ∼106 107 2.36 50 1.0

τr/τf (ms) 41/79 110/112 1.2/3 22/56 μs 45/47 181/233 8.3/33.1 μs 100/100

detectivity (Jones) 2.35 7.2 ∼1012 ∼1013 2.38 ∼1014 7.94 1.22 ∼2.9 2.5

× 1013 × 1012

× 1012 × × × ×

1012 1011 1013 1011

references this work this work 44 45 53 54 55 57 58 59 59

showed that Bi2Se3 NFs grown on Si NWs exhibit much enhanced detection compared to pristine Si counterpart with 2.7 times and 1.4 times faster response in light ON and OFF conditions, respectively. To study the photo responsivity and detectivity of sample BS60, the same was illuminated with different intensities of incident IR radiation ranging from 1.0 to 12.3 mW/cm2. From the results, it is clearly observed that the photo current of the sample BS60 increased gradually with increasing light intensity. The obtained results were fitted with a general power law Ip ≈ Pθ, where photocurrent (Ip) is a direct function of light intensity (P) and θ is a constant. Figure 8b shows the power law variation of the photocurrent as a function of light intensity and θ was determined to be 0.86 by fitting the curve. This value differed from the theoretical value of 1.0, which indicates the

equilibrium, upward bending of electronic states of Si will occur for proper alignment of its Fermi level to that of Bi2Se3 as a result of negligible band gap mismatch between the two counterparts. This in turn facilitates easy carrier transport between the two in the presence of sufficient junction area. We extended this idea on enhancing the contact properties between Bi2Se3 NFs and Si NWs as Si NWs are facilitated with larger effective surface area than bulk Si. Efficiency of any photodetector is closely related to the career mobility. Due to proper optimization of growth time in our work, the as grown Bi2Se3 NFs possess proper thickness. Hence the average mobility of photo generated careers is not exhausted before reaching the electrode or device circuit and neither is affected by any possibility of “short circuit”. Analysis from high-magnification image of the current response characteristics in Figure S4 22795

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Figure 8. (a) Photoresponse switching, (b) photocurrent variation, (c) reponsivity, and (d) detectivity of sample BS60 under different NIR intensities.

presence of the trap density at the interface.57,58 Figure 8c and 8d shows the photoresponsivity (R) and detectivity (D) of the sample as a function of different light intensity calculated by the following equations, namely, eq 5 and eq 6 as R=

observed that the hybrid device shows appreciably high detectivity of 2.3 × 1013 Jones at excitation intensity as low as 1.0 mW/cm2. Such a high value of detectivity has rarely been reported for identical systems involving topological insulators.53,54 Table 1 also represents comparative values of parameters like R, D, τr/τf, and Ip/Id of other reports and those obtained in this work. The relatively higher detectivity and responsivity at an intensity of 1.0 mW/cm2 is related to the operational mechanism. The enhancement of photo responsivity and detectivity can be explained taking into account the factors like easy band alignment and carrier transfer between Bi2Se3−Si NW at their nanoheterojunctions.53,59 We can clearly observe that the hybrid structure fabricated by deposition time optimization exhibits comparable or better efficiency in all aspects compared to traditional and similar systems. In view of the above performances showed by the samples, we can infer thickness optimized n-Bi2Se3 NFs grown on p-Si NW as one of the most efficient multipurpose hybrid systems for future optoelectronic devices.

Ip − Id Pin

(5)

and D = A1/2 R /(2qId)1/2

(6)

where Ip, Id, Pin, A, and q are photocurrent, dark current, incident light power, effective area, and elemental charge, respectively. The maximum responsivity of 934.1 A/W was achieved at low light intensity. The higher value of responsivity in low intensity excitation was attributed to low concentration of photogenerated e−−h+ pair. With increase in incident excitation power, concentration of photogenerated e−−h+ pair enhances. This in turn increases the scattering of photogenerated carriers leading to generation of local heat. This can effectively reduce the responsivity of the system. The value of responsivity obtained at 1 mW/cm2 is higher than the previous reports of Bi2Se3-based devices52 and other topological insulator-based photodetectors as can be found in comparative results shown in Table 1. The photoresponsivity of Bi2Se3/ pristine Si and that of Bi2Se3/Si NWs are also compared (Figure S5), it was found that the latter shows an enhancement of 2.3 times than in case of pristine Si counterpart. To detect the weak optical signals, detectivity is another important parameter for any detector device. The detectivity of the samples was measured as a function of incident light intensity and the results are presented in Figure 8d. It was

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CONCLUSION In this present work, n-Bi2Se3 NF/p-Si NW heterojunction thin film devices were fabricated by a cost-effective chemical technique and synthesis duration was properly optimized for the first time. The synthesized samples were properly characterized for structural, compositional and morphological features. The intrinsic property of topological insulator Bi2Se3, which was modulated as a consequence of variation of growth duration, affected the junction behavior of the device. The nature of junction and hence the transport properties were explained to be a direct function of the synthesis time and 22796

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morphology. Variation of barrier height and ideality factor with temperature was analyzed from temperature dependent current−voltage characteristics applying TE theory. The temperature dependence of barrier height was also determined from C−V measurements. The rectification ratio (I+/I−) for SiNW substrate over pristine Si substrate under dark and nearinfrared (NIR) irradiation was found to be 3.63 and 10.44, respectively. Furthermore, opto-electrical characterizations were performed for different light intensity and highest photoresponsivity and detectivity were determined to be 934.1 A/W and 2.30 × 1013 Jones, respectively. Those values are appreciably higher than previous reports for topological insulator based devices. Thus, this work establishes a hybrid system based on topological insulator Bi2Se3 nanoflake and Si nanowire as the newest efficient candidate for advanced optoelectronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00759. FESEM images of pure Si NWs and Bi2Se3 NFs; TEM micrographs of pure Si NWs and Bi2Se3 NFs; table of comparison of different fitting parameters of I−V curves; detail calculation of C−V measurements and corresponding plots; photoresponse behavior of Bi2Se3 NFs deposited on pristine Si and SiNWs during light ON and OFF switching; and responsivity and detectivity of Bi2Se3 NFs deposited on pristine Si and SiNWs at 8 mW/cm2 light power PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 33 2413 8917. Fax: +91 33 2414 6007. E-mail: [email protected]. ORCID

Kalyan K. Chattopadhyay: 0000-0002-4576-2434 Present Address §

N.S.D.: Department of Basic Science and Humanities, Techno India, Batanagar, Kolkata 700141. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University Grants Commission, the Govt. of India (UGC) for “University with potential for excellence” (UPE II) scheme for financial support.

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REFERENCES

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