Lateral 2D material heterojunction photodetectors with ultra-high

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Functional Nanostructured Materials (including low-D carbon)

Lateral 2D material heterojunction photodetectors with ultra-high speed and detectivity Ding-Rui Chen, Mario Hofmann, Heming Yao, Sheng-Kuei Chiu, Szu-Hua Chen, Yi-Ru Luo, Chia Chen Hsu, and Ya-Ping Hsieh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19093 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Lateral 2D material heterojunction photodetectors with ultra-high speed and detectivity Ding-Rui Chen,1 Mario Hofmann,2 He-Ming Yao,3 Sheng-Kuei Chiu,4 Szu-Hua Chen,2 Yi-Ru Luo,1 Chia-Chen Hsu,5and Ya-Ping Hsieh*4

1

Institute of Opto-Mechatronics, National Chung Cheng University, Chiayi, Taiwan 168

2

Department of Physics, National Taiwan University, Taipei, Taiwan 106

3

Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong

4

Institute of Atomic and Molecular Sciences, Academia Sinica. Taipei, Taiwan 106

5Department

of Physics, National Chung Cheng University, Taipei, Taiwan 168

*Address correspondence to [email protected]

KEYWORDS: Graphene; 2D material; heterojunction; photodetectors; detectivity; ultra-high speed

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ABSTRACT Lateral heterojunctions in 2D materials have demonstrated the potential for high performance sensors due to the unique electrostatic conditions at the interface. The increased complexity of producing such structures, however, has prevented their widespread use. We here demonstrate the simple and scalable fabrication of heterojunctions by a one-step synthesis process that yield photodetectors with superior device performance. Catalytic conversion of a solid precursor at optimized conditions was found to produce lateral nano-structured junctions between graphene domains and 3nm thin amorphous carbon films. Carrier transport in these heterojunctions was found to proceed by minimizing the path through the amorphous carbon barriers which results in a self-selective Schottky emission process with high uniformity and low emission barriers. We demonstrate the potential of thus produced heterojunctions by realizing a photodetector that combines ultrahigh detectivities of 1013Jones with microsecond response times which represents the highest performance of 2D material heterojunction devices. These attractive features are retained even for millimeter-scale devices and the demonstrated ability to produce transparent, patterned, flexible

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sensors extends lateral heterojunction sensors towards wearable and large-scale electronics.

INTRODUCTION Heterojunctions are at the heart of electronic devices consisting of an abrupt change in electronic properties. Recently, lateral heterojunctions between different twodimensional materials have received significant attention1 due to the quality of their interfaces and their unique electrostatics. Different from vertical and bulk heterojunctions, lateral 2D materials’ heterojunctions exhibit larger depletion regions and higher sensitivity to charge variation2-3 making lateral 2D materials’ heterojunctions ideal for sensor applications.4-5 Unfortunately, the enhanced performance of lateral heterojunction sensors is offset by the inherent complexity in producing them. The fabrication of 2D materials’ heterojunctions by chemical vapor deposition (CVD),6-7 for example, requires the sequential conduction of two optimized CVD processes with limited control over the location or dimension of the produced heterojunctions. Thus, sophisticated top-down fabrication methods need to be employed to form electrical contacts in such structures

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which are not compatible with scalable production methods as envisioned for applications such as wearable sensors and large scale electronics.8 We here report a one-step method to produce lateral heterojunction sensor devices with superior performance and unique scalability. A self-organization process during catalytic conversion of a solid carbon precursor results in nanometer-scale heterojunctions in graphene films. The carrier transport in this film proceeds mainly through graphenic regions but energy filtering is provided by Schottky-emission across nanometer-wide, quasi-2D amorphous carbon barriers. This unique conduction mechanism results in an intrinsic combination of high carrier mobilities and high signal gain as evidenced by the realization of photosensors with excellent performance. Photodetectivities of 1013Jones and sub 100µs response times are achieved which represent the highest reported values for 2D materials’ heterojunctions and are superior even to much more complex photogated detector structures. The observed independence of device performance on dimensions, compatibility with flexible substrates, and the demonstrated ability to produce devices from prepatterned precursor highlight the potential of self-assembled lateral heterojunctions for largescale and wearable electronics.

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Figure 1. Process characterization (a) Schematic of fabrication process, (b) Raman spectra of polymer film after annealing at various temperatures indicating transition between amorphous carbon and graphene, (c) X-ray photoelectron spectra of films annealed at different temperature with indication of decreasing peak intensity corresponding to O-C=O bonds, (d) photograph of photosensor array on flexible substrate produced by pre-patterning the solid precursor, (inset) microscope image of graphenic film on flexible substrate. RESULTS

In order to obtain high quality graphene devices with lateral heterojunctions, we carried out the catalytic conversion of a solid precursor as illustrated in Figure 1 (a). Previous reports had observed the formation of homogeneous graphene films at high temperatures9 which are not suitable for our purpose. Instead, we tried to identify a conversion condition that yields a film with spatially varying composition. Therefore, the effect of annealing temperature on the film properties was investigated. Raman

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spectroscopy shows that at low annealing temperatures only amorphous carbon films are formed (Figure 1(b)). This result is in agreement with previous reports that PMMA would carbonize at temperatures in excess of 400°C.10 At high annealing temperatures, on the other hand, Raman spectra show the signature of high quality graphene with a negligible D-Band intensity and a high 2D to G ratio (Figure 1(b)). The largest change in the character of the Raman spectrum can be observed between 500°C and 700°C where the width of the D-Band and G-Band are significantly reduced and a 2D band is developing, which suggests the formation of graphene. This transition was investigated by X-ray photoelectron spectroscopy (Figure 1(c)). Deconvolution of the C 1s peak shows a decrease in the peak at around 288.5eV occurs between 600°C and 700°C which indicates the removal of the oxygen containing acrylate group and suggests total conversion of PMMA within this temperature range. This observation can explain the observed trend in Raman spectra and indicates that the temperature range between 600°C and 700°C is sufficient to produce graphene while retaining a portion of amorphous carbon. The advantages of the employed solid-precursor based conversion approach are illustrated in Figure 1 (d) where arrays of strips where produced by pre-patterning the

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precursor prior to growth. This was accomplished by mechanically ruling groves in the PMMA precursor that had been spin coated on a copper substrate using a diamond tool on a microscope stage. The pre-pattering approach is compatible with a wide array of deposition methods, such as ink-jetting, screen printing, and imprint lithography. Moreover, the here employed solid precursor, PMMA, is also compatible with lithographical and e-beam patterning strategies enabling ultrahigh-resolution device fabrication. The produced patterns are virtually invisible and flexible and are thus ideal for wearable electronic devices.

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Figure 2. Characterization of film annealed at 700°C (a) Optical micrograph, (b) AFM image, (c) Atomic force micrograph of amorphous region (d) absorption spectrum, (e) TEM image, (f,g) SAED of amorphous and crystalline regions.

The morphology of thus produced films was subsequently investigated. Optical microscopy (Figure 2(a)) and atomic force microscopy (Figure 2(b)) indicate that continuous films are formed after annealing at 700°C. Step-height measurements at the edge between the substrate and the edge reveal that the films are approximately 3nm thick (Figure 2(c)). Surprisingly, absorption spectra of the film are virtually identical to high quality CVD-grown graphene11 (Figure 2(d)) but are in fact more transparent. This behavior makes our films promising for transparent and visible-blind photodetectors.12 To understand the origin of the ultrahigh transparency we carry out transmission electron microscopy (Figure 2(e)). We observe an island-like microstructure of dark and bright regions. TEM selected area diffraction (SAED) shows sharp diffraction spots with hexagonal symmetry in the island region Figure 2(f) while nano-sized ring-like patterns suggest the absence of long range ordering in the surrounding material (Figure 2(g)). These results imply that graphene regions that are interspersed in a matrix of amorphous carbon. The relative concentration of both phases can be

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distinguished from the transmittance spectra (Figure 2(d))). We do not observe a pronounced absorption around 400nm as expected for amorphous carbon13 which suggests that the majority of the sample is covered by single- or bilayer graphene films.

Figure 3. (a) Current-voltage-characteristics of device with fit to Schottky-barrier emission model(inset) schematic of barrier transmission, (b) Temperature dependence of series resistance, (c)Richardson’s plot, (d) current distribution (on log-scale) in film with low resistance islands (1Ω) in high resistance matrix (1000Ω) calculated by solving Kirchhoff’s laws in 180x100 pixel matrix, (e) Schottky barrier height under variable illumination.

This observed morphology differs from previously reported solid-precursor based catalytic graphene growth9 where the formation of continuous films was reported. This variation is due to the kinetically hindered growth at the lower employed temperature in our experiments. Simulations have indicated that the formation of planar graphene -9-



is energetically most favorable but the presence of larger energy barriers could stabilize complex three-dimensional carbon networks at lower temperatures.14 The conversion of such defective regions into graphene was found to be more efficient in the presence of crystalline edges15 yielding two phases, a self-perfecting crystalline phase and a stable amorphous phase, in agreement with our observations. To investigate the carrier transport in this novel nanostructured material, we measure the current-voltage behavior. We observe a non-linear IV curve that indicates nonohmic conduction (Figure 3(a)). The behavior can be well fitted by a Schottky emission model where carriers with sufficient energy can traverse a barrier.16 The fit reveals an ideality factor of 1.2 which is significantly lower than previously reported Schottky barriers for graphene heterojunctions and indicates the high quality of the interfaces.17 One significant difference to ideal Schottky diodes is the presence of a large series resistance in the MΩ range for mm-scale devices that has to be considered in the fitting. Temperature-dependent transport measurements were conducted to extract quantitative information about the mechanism of carrier transport. From individual fits of the Schottky-diode model to IV sweeps at different temperatures, we extract device

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parameters such as series resistance and reverse bias saturation current. The analysis of these parameters with temperature reveals two important mechanisms. First, the series resistance decreases with temperature (Figure 3(b)) and a temperature coefficient of resistance of ~-3%/K is extracted which is comparable to values expected from graphene 18 and much larger than values for amorphous carbon.19 Therefore, we can conclude that conduction proceeds mainly through the crystalline domains of the film. From a Richardson’s plot of the reverse saturation current, we identify a barrier height of 60 meV (Figure 3(c)). This value is close to the reported values for barrier heights of graphene flakes in an insulating matrix.20 These two observations suggest that conduction proceeds by maximizing conduction in the graphenic regions and minimization of the hopping between islands akin to percolation. This hypothesis is supported by simulations of the current pathways in a model system consisting of randomly distributed conductive islands in a highresistance matrix (Figure 3(d)). It can be seen that the current is very inhomogeneously distributed with the highest current densities localized in a small number of regions that exhibit narrow gaps between conductive islands. The percolation picture can explain the unique features observed in our film. First, the existence of a small number of

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junctions critical for the conduction process results in a high homogeneity of barrier properties and thus a high ideality factor.21 Moreover, the low extracted barrier height is not representative for the amorphous carbon phase but instead is the lowest barrier in the film that needs to be surmounted to form a conduction pathway. Thus, the film morphology yields an inherently ideal device structure since defects and wide amorphous carbon gaps will exhibit larger barriers and does not contribute to the conduction.

Figure 4. (a) IV with and without illumination, (b) detectivity vs. power, (c) current-time response with switching illumination for two devices with 300um and 3mm channel length, (d) comparison of device performance to state-of-the-art graphene heterojunction devices (details in supplementary material) - 12 -


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The self-selective conduction mechanism should be well suited for sensor applications since the barrier is providing an energy filtering mechanism that is sensitive to changes in the graphene carrier concentration.16 We illustrate this potential by realizing a photodetector due to the anticipated broad-band absorption characteriztics of graphene. For this purpose, devices are uniformly illuminated with a 532nm laser whose intensity was varied by a neutral density filter. It is found that the barrier height is decreased with increasing illumination power (Figure 3(e)) supporting the proposed sensing mechanism. Additionally, broad-band sensitivity of the device was observed (Suppl. Figure S1) that corroborates the presence of barriers in the sample as the origin of the photoresponse. Due to the barrier lowering more electrons can contribute to conduction resulting in a significant variation in the current-voltage behavior upon illumination (Figure 4(a)) by 6 orders of magnitude. This behavior is due to the peculiar carrier transport in the hybrid film and no photosensitivity was observed for films annealed at lower temperatures, since they only consist of the amorphous carbon phase (Suppl. Figure S2).

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To quantify the performance of this novel hybrid material as a photosensor, we calculate a detectivity 𝐷 ∗ = 2 × 1013𝐽𝑜𝑛𝑒𝑠 following the approach by Chang et al.22 (Figure 4(b)) The reported values are higher than most previously reported graphenebased heterojunctions23 and en-par with complex phototransistors.24 This improvement over traditional graphene-based photodetectors highlights the potential of lateral heterojunction, where the two subphases are not overlapping but are sharing only an atomically thin junction area. Due to the different geometry, band bending can extend deep into the subphases resulting in a high sensitivity of carrier transport to photoexciation.25 Based on this explanation, further increase in detectivity would be expected if the extend of the extent of the depletion region could be increased, i.e. by adjustment of the carrier concentration in the barrier. The doping of the barrier, however, seems to be complicated, as evidenced by the small change in performance upon exposure to air, indicating the high stability of the photodetection mechanism. Another important observation is that the device exhibits a very high response speed. When toggling between light and dark state the rise time is approximately 75µs (Figure 4(c)). The observed combination of high detectivity and ultrafast response time is superior to previous graphene-based heterojunction photodetectors. As shown in

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Figure 4(d)) our device outperforms both graphene/carbon heterojunctions, such as graphene/nanotube junctions and graphene/inorganic heterojunctions, such as graphene/WS2 or graphene/GaAs (see supplementary information for more details). Moreover, the presented lateral heterojunction device’s characteristics are even better than graphene-based phototransistors, which rely on a different gain mechanism. The favorable comparison of detectivity and response time to state-of-the-art devices highlights the potential of our approach which combines high performance with the ability to produce large scale devices in a single processing step. The presented lateral heterojunction device is particularly well suited for large-scale wearable or printed devices due to the surprising observation that the response time is similar for devices with 250um and 3mm channel length. This independence on device dimensions can be understood from the previously discussed conduction mechanism. The percolative carrier transport process focuses conduction through pathways with the least number and the lowest width of barriers. Under these conditions the extent of the film and the distribution of the barrier widths have little effect on the response time which is instead controlled by the transition time through the barrier and the photoexcited minority carrier lifetime.26

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When focusing on the transition between illuminated and dark conditions, we observe decay time of several hundreds of microseconds. (Figure 4(c)) Different from the rise time this time constant is found to be controlled by the applied voltages (Supp. Figure S3) which indicates that the delay is due to the drift velocity of carriers within the device. Based on this data, we can extract an apparent carrier mobility of 133cm2/Vs which is comparable to mobilities of discontinuous graphene films27 and significantly higher than mobilities in many organic28 or nanoparticle-based sensors.29 Moreover, the small delay times in Figure S2 for high applied bias suggests the absence of considerable trapping effects which corroborates the high quality of the produced interfaces.

CONCLUSION In conclusion, we have demonstrated a facile method to produce high-performance graphene-barrier photodetectors. Using a one-step conversion process, a selforganized, nanostructured film is produced that exhibits lateral heterojunctions. In this material, carrier conduction proceeds by percolation between crystalline graphene regions in an amorphous carbon matrix as determined by temperature-dependent transport measurements. The resulting film has a very high photosensitivity (1013 - 16 -


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Jones) and fast response speed in the micro-second range that are independent of device dimensions. The presented improvements in performance are based on the attractive features of lateral heterojunctions and could be applied to other sensors types, such as gas sensors or biosensors, in the future. Our approach has great potential for achieving high device performance while simplifying the fabrication process enabling the scalable production of novel electronic devices.

EXPERIMENTAL SECTION Poly(methyl metacrylate) (PMMA) (A9, microchem) were first spin coated onto copper foil (Alfa Aesar 46365, purity 99.8%). Graphene was then produced by annealing the PMMA/Cu foil under an inert atmosphere (Hydrogen, 20sccm) at a pressure of 1 Torr for 37 min. After growth, the furnace was opened for cooling until to room temperature (Hydrogen, 10sccm). The resulting samples were transferred using established procedures[28] by destructively removing Cu and deposition on a Silicon wafer with 90nm thermal oxide or TEM grids for future characterization. Raman spectroscopy was carried out in a home-built confocal micro-Raman system at an excitation wavelength of 532nm. Electrical characteristics were measured using

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a Keysight B2912A semiconductor analyzer. Electron microscopy was conducted in a LVEM 5 SEM/TEM.

ACKNOWLEDGEMENTS This research was supported by Ministry of Science and Technology, R.O.C. (No.104-2112M-002-026-MY3 and104-2112-M-194-002-MY3) and Academia Sinica in Taiwan.

ASSOCIATED CONTENT

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Lateral 2D material heterojunction photodetectors with ultra-high

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