Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3971−3976
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Solution Processed Highly Responsive UV Photodetectors from Carbon Nanodot/Silicon Heterojunctions Rishi Maiti,† Subhrajit Mukherjee,† Tamal Dey,‡ and Samit K. Ray*,†,§ †
Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India School of Nanoscience and Technology, Indian Institute of Technology, Kharagpur 721302, India § S. N. Bose National Centre for Basic Sciences, Kolkata 700106, India ‡
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S Supporting Information *
ABSTRACT: Carbon nanostructures technology has recently emerged as a key enabler for next-generation optoelectronic devices working in the deep UV region due to their excitonic absorption. Here, we report the fabrication of “orange juice” derived solution processed carbon nanodots (CNDs)/n-Si heterojunction showing broadband spectral response with a peak responsivity of ∼1.25 A/W in UV (∼300 nm) wavelength. The surface topography and chemical information on synthesized CNDs via a facile synthesis route have been characterized showing the presence of surface chemical states resulting broad optical emission. The CNDs/n-Si photodiode exhibits very low dark current (∼500 pA), excellent rectification ratio (∼5 × 103), and very good photomodulation in UV region. Our device exhibits better responsivity in DUV than state-of-theart GaN based photodetectors. Solution-processability of the devices with superior optical properties of CNDs thus pave the way for future high-performance, low-cost DUV photodetectors. KEYWORDS: carbon nano dots, UV photodetector, solution processed, low cost, high responsivity and detectivity
1. INTRODUCTION Ultraviolet (UV) photodetectors have recently drawn significant attention due to their wide range of applications, e.g., water purification, rocket and jet flame detection, ozone layer monitoring, optical communications,1,2 etc. Contemporary UV detectors based on photomultiplier tubes (PMTs) are bulky and demand large, vacuum based components. UV band-pass filters combined with silicon detectors are popular but suffer from limited response in UV and compromised lifetime due to degradation under UV exposure.3 Crystalline silicon based detectors are being frequently used at visible and near-infrared wavelengths, and the invention of complementary metal oxide semiconductor (CMOS) image sensors has led to a revolution in imaging and detectors.4 Silicon is known to be a good choice for detectors in the visible to near-infrared (vis−NIR) wavelength range5−8 but not so useful for UV detection because it suffers from surface recombination of carriers at UV due to ultrashort absorption lengths. Traditional UV detectors are fabricated using thin films of a wide band gap semiconductor, such as GaN, ZnO, AlGaN, SiC, diamond, etc.9−16 However, the growth of uniform epitaxial films with cost intensive fabrication processes is challenging with Si-CMOS technology. Sheng et al.17 reported a visible-blind, self-powered UV detector that exploits Si based photodetectors with downshifting europium (Eu) complex dispersed in PMMA as luminophores for light trapping and management. Solar-blind deep ultraviolet photodetectors for adverse environmental © 2019 American Chemical Society
condition are not yet fully developed. On the other hand, low dimensional nanostructures2 such as metal nanoparticles and colloidal quantum dots are becoming very popular for photon detection due to enhanced absorption of UV light and its plasmonic enhancement, multiexciton generation, size dependent spectra, and storage of charge carrier in surface and interfacial traps.18 Among them, carbon based nanostructures have gained lot of interest recently for various applications,19 with graphene, being one of the most studied materials over the past decade.20 But graphene has its limitation in optoelectronic applications due to absence of electronic band gap. Recently, carbon nanodots (CNDs)21,22 have attracted much attention owing to their several unique properties, such as UV absorption, excellent photostability, quantum confinement, size tunable photoluminescence (PL), and exceptional multiphoton excitation23,24 characteristics. Carbon dots are better than conventional semiconducting quantum dots because of water solubility, simple low cost synthesis process, chemical inertness, high photostability, low toxicity, and biocompatibility.21 Though CNDs appear potentially attractive for applications in optoelectronics,25−27 photovoltaics,28 energy storage devices,29 sensors,30 and biomedical imaging,31 their integration over large area on mature planar CMOS Received: May 7, 2019 Accepted: May 22, 2019 Published: May 22, 2019 3971
DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
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ACS Applied Nano Materials
Figure 1. Synthesis process and device fabrication: (a) pulp free orange juice as precursor; (b) solution obtained after hydrothermal reaction at 120 °C in a closed Teflon autoclave; (c) washing by dichloromethane to remove organic residues. The top layer contains carbon dots in water, and the bottom layer is dichloromethane with impurities dissolved. The top layer is separated and (d) spin coated on silicon substrates.
Figure 2. Physical characterization of as-synthesized CNDs. (a) TEM image of prepared CNDs showing spherical in shape. (Inset) HRTEM micrograph of synthesized CNDs showing well resolved lattice fringes. Average diameter of CNDs is found to be ∼2 nm. (b) Typical C 1s XPS spectra showing presence of different surface oxidation states. (c) Absorption spectra with strong UV absorption showing the potential of demonstrating UV detector and broad PL spectrum excited by 325 nm He−Ne laser (inset, Pl emission photograph). (d) Photoluminescence spectrum of as-synthesized CNDs sample with different excitation power showing linear variation (inset). In short, pure ethanol was poured in filtered, pulp free orange juice solution in 1:1 volumetric ratio and was mixed by magnetic stirring at 600 rpm (Figure 1a). The solution was heated in a Teflon autoclave for 2 h at 120 °C for solvothermal reaction. After the completion of reaction, the system was allowed to naturally cool down to room temperature. As shown in Figure 1b, the obtained dark yellowish solution was repeatedly washed by centrifuge in dichloromethane to get rid of organic impurities. The washing procedure is shown in Figure 1c. The solution on top was collected and the comparatively larger sized particles were removed by addition of acetone followed by centrifugation at 5000 rpm. The supernatant was separated and centrifuged at a higher speed to obtain smaller sized CNDs. The final product was spin-coated on n-silicon substrates as shown in Figure 1d. 2.2. Characterization. Structural aspects of as-synthesized CNDs were examined by a high-resolution transmission electron microscope (JEOL JEM-2100F). X-ray photoelectron spectroscopy (PHI 5000 Versa Probe II, INC, Japan) was used to explore the compositional properties and the functional groups attached to CNDs. UV−visible absorption spectroscopy of CNDs was performed in a fiber probe based Avaspec-3648 spectrometer. Photoluminescence spectroscopy was performed with a He−Cd laser emitting UV radiation of 325 nm
platform is yet to be explored. Low-temperature solutionprocessed semiconductors21 are attractive for various reasons: they can be deposited over large areas using spin coating, rollto-roll printing, and spraying processes, etc. They can be processed at both low temperatures and ambient conditions, and they are compatible with flexible substrates. Here, fabrication and characteristics of Si-CMOS compatible CNDs heterojunctions are reported. These photodetectors exhibit very low dark current (∼500 pA) and excellent rectification (∼5 × 103) behavior with a high on−off ratio (∼104) at reverse bias. Fabricated p-CNDs/n-Si heterojunctions demonstrate high photoresponsivity and detectivity in the UV region (300 nm), making it useful for realizing UV photodetectors with low cost solution processability with enhanced performance.
2. EXPERIMENTAL SECTION 2.1. Synthesis. The detailed synthesis process of CNDs has been reported elsewhere.32,33 The whole procedure is depicted in Figure 1. 3972
DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
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ACS Applied Nano Materials
Figure 3. Electrical characteristics of heterojunction using a Newport solar simulator having broadband emission with irradiance of 1 SUN, which is equivalent to 100 mW/cm2 under air mass (AM) 1.5G condition for photoresponse measurements: (a) device schematic of the proposed Au/ CNDs/n-Si UV photodetector; (b) false colored cross-sectional view of the fabricated device using FESEM; (c) typical I−V curves of the device (plotted on semilogarithmic scale) measured under the dark and illumination. (d) Typical rectification characteristics of p-CNDs/n-Si diode for 75 Hz of ac input signal. equipped with TRIAX-320 monochromator and a Hamamatsu R928 PMT detector. 2.3. Device Fabrication. As-synthesized CNDs, which are p-type in nature attributed to the presence of electron withdrawing oxygenous functional groups on the surface,34were spin coated on n-Si substrates to fabricate p−n heterojunction diodes. For electrical contacts, thin Au dots (device area ∼0.2 mm2) were deposited on CNDs as top electrodes by thermal evaporation. To achieve ohmic back contact on n-Si, large area aluminum was also deposited by thermal evaporation and then annealing at 200 °C for 5 min. To get an understanding of the role played by CNDs, a control device was fabricated on n-silicon without CNDs layer. 2.4. Electrical Measurements. All electrical performance of the devices was characterized using a Keithley semiconductor parameter analyzer (4200-SCS) and a probe station as shown in Figure 1d. Performance of the UV photodetector devices was studied under illumination of a He−Cd laser of 325 nm emission.
ment for as-synthesized CNDs by varying the incident power from 4 to 32 mW (λexc = 325 nm), as shown in Figure 2d. The power dependent integrated intensity plot (inset of Figure 2d) shows linear variation with the exponent value of 0.95 for different excitation power which is as expected for the emission of excitonic recombination.36 A schematic diagram is shown in Figure 3a and a crosssectional SEM image (false colored) of the fabricated device in Figure 3b. It is evident that the CNDs film has good uniform thickness of ∼95 nm. Figure 3c shows the current−voltage (I− V) characteristics in semilog scale under dark and illuminated conditions with a Newport solar simulator having broadband emission with irradiance of 1 SUN, which is equivalent to 100 mW/cm2 under air mass (AM) 1.5G conditions and studied using semiconductor parameter analyzer (4200-SCS). The dark current is found to be extremely low and saturates at around ∼1 × 10−9 A at −2 V bias, achieving photocurrent to dark current ratio of nearly 104 at −1 V. The asymmetric nature of the I−V curves indicates the formation of the heterojunction between n-type silicon and p-type CNDs. The diode equation I = I0(eeVA /(ηk BT ) − 1) has been used to fit the I−V characteristic for a small range of positive voltage (η is the ideality factor), kB is the Boltzmann constant, I0 is the saturation current in dark condition, VA is the applied bias voltage, T is the room temperature). The relatively high value of ideality factor (1.9) may be attributed to the surface defects formed in CNDs/n-Si interface and the high density of trap states, which are inherent in the CNDs due to the colloidal synthesis route.5,34 The typical rectification behavior of fabricated heterojunction diode depicting half wave rectification for 75 Hz ac input signal is shown in Figure 3d. The rectification ratio at the same bias voltage of this heterojunction is found to be ∼104 at 2 V.
3. RESULTS AND DISCUSSION Figure 2a shows the HRTEM micrograph exhibiting the formation of spherical carbon nanodots with an average size of ∼2 nm. XPS spectra (Figure 2b) depicts the binding energy of C 1s electrons present in CNDs which can be fitted by Gaussian function, CC or C−C bonds at 284.6 eV, hydroxyl (C−OH) or epoxy bonds at 286.4 eV, and carboxyl (−COOH) bonds at 288.8 eV, which agrees well with previous reports.35 As-synthesized CNDs display a strong absorption peak at 270 nm due to the π−π* transition of aromatic >CC< bonds and extends to NIR region where the extinction coefficient of CNDs was found to be ∼45 mL mg−1 mm−1 at 300 nm as shown in red in Figure 2c. The broad photoluminescence (PL) spectra (380−750 nm) for asdeposited sample is arising due to the combination of the sp2 hybridized core state and sp3 hybridized surface state emission33 as shown in blue in Figure 2c. We have carried out excitation power dependent photoluminescence (PL) measure3973
DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
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Figure 4. (a) Typical switching characteristics of the heterojunction device with fixed bias voltage (−3 V) with different illuminated powers under 325 nm laser excitation. (b) Spectral responsivity of CNDs based UV photodetector under different bias voltages. (inset: responsivity zoomed near 1000 nm). (c) Spectral detectivity of the UV detector for different bias voltages. (d) Schematic energy level diagram for the fabricated CNDs heterojunction diode.
mechanism has been reported previously by Manna et al.39 in CdS/silicon heterostructure and by Cosentino et al. in amorphous Ge-QDs.40 The spectral responsivity also shows a small hump at 1000 nm due to the absorption of silicon substrate, shown in the inset of Figure 4b. Though visible−NIR radiation which passes through CNDS is absorbed by n-Si, the generated minority carries (holes) in Si need to be transported efficiently to reach the Au electrode (cathode) under reverse bias condition. The equilibrium valence band offset between Si and CNDs shows the creation of an energy barrier for hole transport. In addition, the hole mobility in Si is quite low and the heterojunction formed with solution processed CNDs contains high density of interfacial traps acting as recombination centers. The combinatorial effects of the above result in a much lower responsivity in the vis−NIR region compared to a Si homojunction photodiode. On the other hand, the responsivity of the heterostructure is strongly dominated by the minority carriers (electrons) generated in direct band gap CNDs due to strong UV absorption, which are transported efficiently due to favorable conduction band offset in the junction to the anode. The responsivity in the UV wavelength is also affected by the recombination centers at the heterointerface and the value would have been much higher in the absence of interfacial trap density. A photodetector’s detectivity can be approximated as7 R(λ) D(λ) = 2qJ , where q is the electronic charge, R(λ) is the
The fabricated CNDs/n-Si heterojunction exhibits fast switching behavior when illuminating with a broadband light source. Figure 4a shows the variation of photocurrent (Iph) for different laser powers (Popt) at a fixed bias voltage of −3 V. With increase of the incident laser power from 0.5 mW to 3 mW, the current on/off ratio is seen to increase steadily due to the enhancement of photocurrent. Standard lock-in method and a broadband xenon arc lamp are used to measure the spectral photocurrent response. The responsivity (R) of a photodetector can be defined as the ratio of photocurrent density (Jph) to the power of incident light (Popt) for a particular wavelength incident on the detector. The spectral responsivity (for different bias voltages) of our Au/CNDs/n-Si photodetector is shown in Figure 4b. The peak responsivity occurs at 300 nm and is observed to be 1.25, 0.77, and 0.50 A/ W for a bias voltages −4 V, −2 V, and −1 V, respectively. This may be attributed to the presence of strong absorption peak of CNDs in the UV wavelength range as earlier shown in Figure 2b. A control device (Au/n-Si/Al) was fabricated to observe the effects of CNDs. As shown in Figure S1 of the Supporting Information, the control device does not show any significant responsivity in the UV region. Hence, it is evident that CNDs are responsible for UV photoresponse. We observed a maximum EQE of ∼520% (not shown) due to photoconductive gain by the samples. Photoconductive gain has been observed in many 2D materials.37,38The large photoconductive gain originates from different mobilities of the majority and minority carriers. As is clear from the band alignment shown in Figure 4d, electrons with a higher carrier mobility originating from the higher energy conduction band of CNDs are efficiently collected at the anode, while the hole transport toward the cathode is quite inefficient. This leads to additional charge injection from the cathode to maintain the electrical neutrality, resulting in EQE of more than 100%. This
d
responsivity, and Jd is the photocurrent density. The detectivity value for the CNDs heterojunction device is found to be highest at 300 nm and is 2.06 × 1014, 1.63 × 1014, and 1.37 × 1014 cm·Hz0.5/W at a bias voltages of −4 V, −2 V, and −1 V, respectively, as shown in Figure 4c. Different figures of merit of CNDs based photodetectors which are already present in the literature are summarized in Table 1, showing superior 3974
DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
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ACS Applied Nano Materials Table 1. Comparison of Different Parameters for Several CNDs/GQDs UV Photodetectors material
device type
spectral range
graphene quantum dots (GQDs) 2-terminal MSM diode Ag/GQD/Au n-doped graphene quantum dots(n-GQDs) 2-terminal MSM diode on IDE Au/n-GQD/Au graphene quantum dots (GQDs) 2-terminal tunneling diode graphene/ GQD/graphene graphite quantum dots on 2D graphene 2-terminal device silicon nanowire/carbon dot core−shell 2-terminal device heterojunction carbon nanodots (CNDs) 2- terminal p−n junction n-Si/p-CNDs heterojunction
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performance in terms of spectral responsivity and detectivity mainly due to the strong UV absorption shown by CNDs despite being solution processed device fabrication. Comparisons to other materials and device structures have been shown in Table S1 in Supporting Information. To explain the observed photodiode behavior in detail, an energy band diagram (schematic) of the CNDs/n-Si device is depicted in Figure 4d with previously reported material parameters from the literature.6,41,42 In thermal equilibrium, a built-in electric field exists in the depletion region due to the band offset resulting in sweeping of the photogenerated carriers when illuminated, giving rise to a photocurrent. With the increase of reverse bias, higher values of responsivity lead to the better utilization of photocarriers. From a microscopic point of view, one type of photocarrier is trapped by the localized states resulting in increase of the recombination lifetime (τ0) of another type. If the lifetime of a charge carrier is significantly greater than its transit time, it will make several transitions through the material between the contacts. If the contacts are able to compensate the carriers drawn from the opposite side by the injection of an equivalent carrier density, which is required for the charge neutrality condition, then the free carrier will continue to circulate until it is annihilated by recombination, leading to a large photoconductive gain. This enhanced photoresponse in UV region indicates that solution processed CNDs based devices are attractive for next flexible optoelectronic devices sensitive to UV light.
detectivity (Jones)
ref
DUV (1011
44
UV−vis UV−vis−NIR
4 × 107 A/W 353 mA/W
300−1100 nm
0.5A/W
45 46 ∼1014
our present work
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tamal Dey: 0000-0003-4862-8107 Samit K. Ray: 0000-0002-8099-6690 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.M. acknowledges a research fellowship from MHRD, India. S.M. gratefully acknowledges IMPRINT USR project (Grant IIT/SRIC/R/USR/2018/102) for his financial assistantship. T.D. acknowledges DST, India, for providing an INSPIRE fellowship (Grant IF-160592). The XPS facility at IIT Kharagpur is gratefully acknowledged.
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4. CONCLUSION We demonstrate a CNDs/n-Si heterojunction broadband photodiode that shows enhanced UV sensitivity. The synthesized CNDs by facile chemical route from orange juice display strong UV absorption with a broadband PL emission. The CNDs based photodiode exhibits superior performance in terms of very low dark current density, high rectification ratio, stable switching behavior, and high responsivity at the UV region. The unavailability of the deep UV source limits our measurement up to 300 nm. This work sheds light on great opportunities of using CNDs in planar CMOS technology.
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peak responsivity
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00860. Figure S1 showing responsivity curve for Au/n-Si/Al control device (without CNDs) and Table S1 showing comparison of performance of the reported device with other materials and device structures (PDF) 3975
DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
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DOI: 10.1021/acsanm.9b00860 ACS Appl. Nano Mater. 2019, 2, 3971−3976
