Highly Sensitive, Encapsulated MoS2 Photodetector with Gate

Subscriber access provided by University of Sydney Library

Communication

Highly sensitive, encapsulated MoS2 photodetector with gate controllable gain and speed Dominik Kufer, and Gerasimos Konstantatos Nano Lett., Just Accepted Manuscript • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Highly sensitive, encapsulated MoS2 photodetector with gate controllable gain and speed. Dominik Kufer and Gerasimos Konstantatos*

D. Kufer and Prof. G. Konstantatos. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain

E-mail: ([email protected])

Keywords: (Molybdenum disulfide, HfO2, TMDC, encapsulation, photodetectors)

Semiconducting, two-dimensional Molybdenum Disulfide (MoS2) is considered a promising new material for highly sensitive photodetection, because of its atomically thin profile and favourable bandgap. However, reported photodetectors to date, show strong variation in performance due to the detrimental and uncontrollable effects of environmental adsorbates on devices due to large surface to volume ratio. Here, we report on highly stable and high performance monolayer and bilayer MoS2 photodetectors encapsulated with atomic layer deposited (ALD) Hafnium oxide (HfO2). The protected devices show enhanced electronic properties by isolating it from the ambience as strong n-type doping, vanishing hysteresis and reduced device resistance. By controlling the gate voltage the responsivity and temporal response can be tuned by several orders of magnitude with R ~ 10 -104 A/W and t ~ 10 ms – 10 s. At strong negative gate voltage the detector is operated at higher speed and simultaneously exhibits a low-bound, record sensitivity of D* ≥ 7.7×1011 Jones. Our results lead the way for future application of ultrathin, flexible and high performance MoS2 detectors and prompt for further investigation in encapsulated transition metal dichalcogenide optoelectronics.

ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Semiconducting, layered 2D materials as the group of transition metal dichalcogenides (TMDCs) are considered candidates for next-generation optoelectronic technology. Especially for optoelectronics they hold great promise due to their direct bandgap in its single layer form and its large absorption coefficient.1,2 Moreover the bandgap allows to operate TMDCs in phototransistor architecture where low dark currents can be achieved by fieldeffect modulation to depletion. These are favourable conditions for highly sensitive photodetectors with strong response and ultra-low noise level.3,4 Up to date, many reports on TMDC photodetectors have been published with focus on Molybdenum Disulfide (MoS2), however with a huge variety on reported performance results.5–18 The devices reported are fabricated as metal-TMD-metal photodetector with applied bias to operate as photoconductor. Most monolayer and multilayer MoS2 show impressively high gain with responsivity up to 103 A/W at the cost of extremely low bandwidth with decay times in the range of several seconds or more.5,6 Other reports have demonstrated much faster response times in the absence of gain and responsivity on the order of mA/W.8,9 However, for their employment as ultrathin detectors in sensing and imaging high sensitivity and decay times of few tens of milliseconds are required.19

Despite its generally beneficial 2D profile, the large surface to volume ratio make those detectors´ performance sensitive to the environment. It has been shown, recently, by Furchi et al. that the MoS2 photoresponse is based on two distinct mechanisms, the photogating and photoconducting effect.10 The photogating effect is dominant and responsible for extremely slow response dynamics and high responsivity values, which originate from long lived charge trapping processes in surface bound water molecules or other adsorbates. In several studies it has been proven that physisorbed gas molecules as O2 and H2O electrically deplete MoS2 and


Page 2 of 20

Page 3 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

MoSe2 by withdrawing electrons from the channel.20,21 Accordingly, a thorough vacuum annealing at temperatures higher than 100 ºC for several hours strongly n-type dope few-layer MoS2 and WS2 field effect transistors and improve significantly resistance and mobility.22–24 Many encapsulation approaches by Al2O3,25,26 HfO227 or Si3N428 deposited at high temperatures or after some pre-annealing demonstrate similar behaviour with strong electron doping, improved electronic performance and reduced hysteresis. Yet, none of them investigated its influence on the responsivity and temporal response time for photodetection.

We show that with a simple encapsulation by HfO2 the detector performance can significantly improve. Not only does the oxide prevent from strong current drifting and degradation by environmental effects, but also n-type dopes the TMDCs strongly due to positively fixed oxide charges and the removal of atmospheric adsorbates. The responsivity and speed of the device can be effectively adjusted by changing the gate voltage, from enormous gain and slow decay rates with R ~ 104 A/W and t ~ 10 s at VG = 0 V down to R ~ 10 A/W and t ~ 10 ms in the strong gating regime of VG = -40 V. We report the highest sensitivity, measured up to date for TMDC photodetectors, with D* ≥ 7.7*1011 Jones at which the sensitivity of our measurement unit was reached. Data analysis suggests even higher detectivity in the stronger gating regime. A shot-noise limited detectivity of 8*1013 Jones unveils the huge potential of encapsulated MoS2 devices for application as ultrathin photodetectors in the visible.

Figure 1a shows a 3-dimensional depiction of a MoS2 phototransistor on top of a Silicon substrate covered with SiO2 oxide. The thin flakes with different thicknesses are exfoliated with scotch tape, contacts are fabricated by standard lithography and thickness is determined by Raman spectroscopy as reported in previous work.29 Finally HfO2 film is deposited by atomic

layer

deposition

(ALD)

to

protect

from


the

environment

using

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

tetrakis(dimethylamino)hafnium and H2O precursors. Previous reports on ALD HfO2 growth on TMDCs show non-uniform growth without prior surface functionalization or seedinglayer.30 To avoid this problem and assure full coverage of the surface, relatively thick oxide of around 30 nm was deposited in 300 cycles (see Methods and SOM Figure S1). The electrical performance of the device was measured before and after the ALD process to unveil the effects of encapsulation. In Figure 1b we compare the modulation curve IDS-VG of the as-fabricated MoS2 device with encapsulated MoS2/HfO2. All measurements were performed under ambient conditions. The transfer curves show a vanishing hysteresis after the encapsulation, as a strong indication of removal of positive charges localized at the MoS2 surface which trapped and released electrons during forward and backward sweeps. While the curve of the as-fabricated MoS2 showed a threshold voltage VTh ~ 2 V, the ALD annealing and encapsulation shift its threshold down to VTh ~ -24 V. The strong n-type doping can be explained by the removal of charge trapping adsorbates H2O and O2 and by positively fixed charges inside the top-oxide layer which further induce electrons into the channel.26,31 From the two-terminal measurements estimates of the effective mobility and device resistance can be extracted. The mobility of the device improves clearly from 0.5 cm2/Vs to 12 cm2/Vs. The evident improvement of mobility can be explained by the removal of extrinsic charged impurities due to the annealing process and quenching of homopolar phonon modes due to the dielectric screening.24,32 Additionally the strong doping in the immediate vicinity of the metal contact may also lead to lower contact resistance and increase two-terminal field-effect mobility.33 The device resistance reduces by ~2 orders of magnitude from 5 MΩ to 55 kΩ at VG= +10 V. Despite the strong doping, the device can be fully depleted at strong negative gate voltages, reaching the pico-Ampere limit of the measuring unit.

Moreover, the device shows

extremely high on/off ratios of 108 and ohmic output curves (Figure 1c and 1d), as previously


Page 5 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

reported for HfO2 encapsulated single layer MoS2 field effect transistor (FET).27 The improved mobility and resistance together with the ultralow dark current in depletion form a fundamental basis for highly sensitive phototransistors. Devices were stored inside a nitrogen glovebox and the performance was maintained throughout several weeks after encapsulation. The same fabrication procedure was followed for several MoS2 and few-layer MoSe2 devices with HfO2 encapsulation and on MoS2 devices encapsulated with Al2O3. Results suggest that the encapsulation approach is generalizable to other encapsulant materials and can be successfully applied on other TMDC photodetectors. Measured results can be found in the Supplementary information (SOM T1, S2 and S3).

Responsivity and charge carrier dynamics. Now we turn to the optoelectronic characteristics of the device after HfO2 encapsulation. The spectral responsivity was measured at VG = -32 V over the incident wavelength range of 550–800 nm with a constant illumination power for all wavelength (Figure 2a). The spectrum reveals the expected sharp drop in response at λ ~ 690 nm, corresponding to its bandgap of 1.8 eV, and indicates the photoresponse from the MoS2 flake. Two clear peaks can be resolved which stem from the direct bandgap transition at the K point of the Brillouin zone. These features, denoted as exciton peak A and B, arise from the splitting of the valence band due to spin-orbit coupling. The transfer curve under dark and illuminated conditions in Figure 2b shows the response of the phototransistor depending on its gate voltage and illumination power. The devices are illuminated globally with a spot size of 2 mm at a wavelength of 635 nm. This allows a decrease of the effective light power falling on the device to extremely low values and to acquire the response at lowest measureable powers. Even at very low power density of 25 µW/cm2 a small shift of threshold voltage indicates a sensitive photogating effect.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

To get deeper insights into the charge carrier dynamics under light, the power dependence of responsivity was measured for a wide range of light intensity at several backgates VG ≤ 0 V. At this backgate range the ratio Ilight/Idark reaches its highest value and is therefore most interesting for photodetection. All points were measured with a pulsed laser at 1Hz repetition rate. Figure 2c plots the responsivity, calculated as the photocurrent divided by the impinging power, and Figure 2d sketches simple bandgap models to establish a physical picture behind the power dependence. First we consider the backgate range of -20 V – 0 V which corresponds to VG > VTh in the schematic (left image). The band model used bases on the existence of continuous bandtail and midgap states. These states are introduced in the bandgap due to disorder, defects or sulfur vacancies and can lead to charge trapping and recombination, as previously reported.10,34,35 The right side of the y-axis shows a model with discrete gap-states to simplify the ongoing carrier dynamics, while the left side illustrates a more realistic, continuous energy distribution of the density of states (DOS). Under illumination several mechanisms contribute to the overall photoresponse as charge carrier excitation, separation, carrier trapping and recombination: For VG > VTh, the Fermi level EF is located close to the conduction band and the majority of available electron trap states are filled. Under low light intensity the deep lying, long-lived hole traps fill first and with increasing power the shallower trap states populate. Since the shallow states have higher DOS and shorter lifetimes they dominate the decay dynamics at higher optical intensities. A long lifetime of trap states is essential for high photoconductive gain. Thus the dominating shorter lifetime of shallow states reduces gain and consequently responsivity drops under strong illumination. This behaviour is well known for trap-dominated detectors and leads to sublinear power dependence of the photocurrent.36,37 Similar results were reported earlier for TMDC detectors.5,6,38 The high responsivity of up to 5×104 A/W for lowest powers can be


Page 6 of 20

Page 7 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

attributed to the generally lower contact resistance in the higher gate voltage regime where the device is electrically doped. For VG < VTh, a more complex behaviour is met. A general trend of decreasing responsivity with rising modulation field strength can be seen with two different regimes in its power dependence: a short rise of responsivity at lower power followed by a drop of responsivity at higher power. Moreover, the most sensitive point (responsivity maximum) shifts to higher illumination intensity with increasing gate strength. To understand the mechanism behind this phenomenon one has to carefully distinguish between recombination centers and (sensitizing) trap states.39,40 Recombination centers are located closer to the midgap and favour recombination of electron-hole pairs, while trap states are closer to their corresponding band edge and are likely to trap one charge carrier for certain time (lifetime) depending on the depth of the trap state. At VG < VTh the Fermi level EF,0 (under dark conditions) moves away from the conduction band and progressively more states above are unoccupied, which serve as recombination centres and reduce the responsivity (Figure 2d center). Under illumination the electron and hole density is temporarily augmented (Figure 2d right) expressed with the steady-state or Quasi-Fermi levels EF,n and EF,p, respectively. Therefore, by an initial increase in light intensity the Quasi-Fermi levels move apart and embrace more unoccupied recombination channels, effectively prolonging carrier lifetime of the majority carrier and consequently rising responsivity. Since at stronger field modulation EF,0 consecutively moves further to the midgap and the number of unoccupied recombination centers increases, higher light intensity is needed to fill them again and the responsivity maximum shifts to higher power. After that point, the Quasi-Fermi level EF,n approaches the conduction band and the responsivity drops again due to the mechanism explained in the case of VG > VTh. This is a very simple model to account for the experimental findings of Figure 2c. In reality more complex mechanisms are at play due to possibly different trap energy distribution, trap


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

density and carrier capture probability for electrons and holes and merit further investigation.39,40

Temporal response. One of the major limitations of previously reported MoS2 photodetectors is a frequency response of much lower than 1 Hz for sensitive devices with gain, originating from surface bound H2O molecules underneath and on top of the MoS2 flake. Yet, little efforts have been made to improve TMDC photodetectors in terms of speed while maintaining high sensitivity. Unprotected MoS2 devices usually suffer from drifting after illumination, reveal extremely slow decays back to its previous dark current level or show even persistent photoconductivity,6,41 thus hampering a reliable study of the decay dynamics of MoS2 detectors. Figure 3, panels a and b show the measured IDS during 1 cycle of light modulation for a bilayer MoS2 device before and after encapsulation, respectively. The photocurrent improves by a factor of ~ 40 while the decay time decreases by more than 1 order of magnitude. The power dependent responsivity of another bilayer device is illustrated before and after HfO2 encapsulation in Figure 3c. To fairly compare the device performance with and without encapsulation, the detectors were characterized at a backgate voltage in which the minimum dark current level is reached, that is at VG = -10 V and VG = -40 V for the case of MoS2 and MoS2/HfO2 respectively. Significant improvement of responsivity and speed was observed for all measured devices after the encapsulation process. The strongly gated, encapsulated devices demonstrate stable and constant photoswitching performance throughout many switching cycles (Figure 3d) and relax fast to the initial dark conductivity after switching off the light source. To further prove the impact of H2O molecules on device performance and the improved stability of the encapsulated devices, a series of control experiments were conducted by exposing the devices to harsh and highly humid conditions.


Page 8 of 20

Page 9 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Even after exposure to relative humidity of up to 80% the encapsulated devices exhibited stable electronic and optoelectronic performance (SOM Figure S4). As discussed earlier, the gate voltage has strong influence on the gain of the device by controlling the occupancy of gap states which, in turn, will reflect in its temporal response. Figure 3e shows the normalized photocurrent, measured at the same illumination power of 41 mW/cm2, for different gate bias VG ≤ 0 V. The temporal response strongly depends on applied field modulation and is in agreement with the responsivity behaviour. The larger amount of unoccupied recombination channels at strong gate voltage effectively shorten the majority carrier lifetime, resulting in much faster temporal response down to 7ms. The fall times, measured from 90 % to 10 % of the maximum photocurrent after light is switched off, together with its corresponding responsivity measured at 41 mW/cm2 are plotted versus VG in Figure 3f. The decay of the photocurrent after cessation of excitation depends strongly on the light intensity impinging on the device. At lower intensity, the temporal response will be primarily determined by the long life-time of deep hole traps. With increasing intensity deep trap states get saturated and short-lived, shallow hole traps populate and dominate the decay dynamics. Thus, the decay times get faster with higher illumination intensity.

Noise and sensitivity. As shown in previous sections the responsivity and time response depend severely on backgate voltage and can reach orders of magnitude difference by operating the device either in its depletion or accumulation region. Although the highest responsivity is detected in the accumulating regime, the nearly ohmic contacts lead to lower contact resistance and thus result in high dark current which is not favourable for sensitive photodetection. While many TMDC photodetector reports focus on responsivity - reported at a variety of different VG - a more important figure of merit is the detectivity D* which also takes into account the


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

bandwidth, geometry and noise of the device. The detectivity is defined as D* = R×Sqrt(AB)/ iN, where R is the responsivity, A the area, B the bandwidth and iN the measured noise current of the device. To get a fast estimation of the sensitivity of devices, D* can be approached by considering the shot noise as main noise source with iN_SN = Sqrt (2eIdark). 2D materials however contain a predominant 1/f-noise component due to non-ohmic contacts and disorder or edge defects which cannot be neglected.42–44 Figure 4a outlines the dark current, the maximum responsivity and the resulting detectivity versus backgate voltage, measured at a bandwidth of 1 Hz. Figure 4b summarizes a full set of measured spectral noise density iN2 at different VG to determine the detectivity. A considerable decrease of the spectral noise density of several orders of magnitude for stronger gating voltages was observed. While the curve at -20 V shows a clear 1/f component, at VG = -40 V (and -35 V) the noise floor of the measuring unit is reached showing a flat spectral noise density line. This is the lowest noise level measureable with our system corresponding to pico-Ampere of dark current. In SOM Figure S5 the magnified noise curves at VG = -20 V and VG =-40 V are plotted. Despite the huge responsivity values at VG ~ -20 – 0 V, the detectivity shows its lowest values due to high noise at such high levels of dark current. The detectivity improves for stronger gating and reaches a maximum measured value at VG = -32 V of 7.7×1011 Jones (1.7×1012 Jones for other device, SOM T1), to the best of our knowledge the highest MoS2 detectivity reported so far. Following the observed trend of noise decreasing faster than responsivity with increasing backgate voltage, we expect that the sensitivity may further improve at stronger gating, where the actual noise of the device cannot be resolved in our measurements. Figure 4c shows the measured noise and the calculated shot noise versus dark current. The measured noise slowly approaches the shot noise limit with increased gating strength. The shot noise limited detectivity of this device reaches 8×1013 Jones, two orders of magnitude higher than the measured sensitivity, showing the huge potential of MoS2 in sensitive photodetection. Owing


Page 10 of 20

Page 11 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

to the 1/f nature of the noise spectral density and the millisecond time response, we propose that even better detectivity can be achieved by measuring at higher frequency.

In conclusion, we have reported on ultrathin MoS2 phototransistors encapsulated with ALD HfO2 preventing the device from degradation and moisture. The transistor performed stable and drift-free under ambient conditions with superior on/off ratio and high n-type doping. The enhanced mobility and resistance due to the encapsulation process lead to improvement of both, responsivity and speed, compared to the unprotected device. By controlling the gate voltage the responsivity and temporal response can be tuned by several orders of magnitude with fast decay times down to a few milliseconds. The favourable operation of the device at very strong negative gate voltage was demonstrated, where a record, low-bound sensitivity of 7.7*1011Jones was measured and fastest operation is possible. Further study of effective encapsulation techniques and prior thorough removal of surface bound adsorbates convince to be a promising approach for high-speed and sensitive MoS2 photodetection in the visible.

Methods: Device Fabrication. MoS2 flakes were micromechanically exfoliated with standard scotch tape method. The flakes were deposited on heavily p-type doped silicon wafer (0.001-0.005 Ohm.cm) covered by SiO2 with thickness of 90 nm or 285 nm. Electrical contacts were fabricated by standard electron beam or laser writer lithography followed by thermal and electron beam evaporation of Ti 2 nm and Au 100 nm. For lift-off the samples were soaked in Acetone during 12 hours followed by Isopropanol rinse. Before first measurements of nonprotected devices the samples were annealed in vacuum for 12 hours at ~ 130 ºC to improve contact conductance. After measurements in ambient conditions devices were covered by HfO2 with atomic layer deposition technique (Savannah 200, Cambridge Nanotech). To


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

assure a full coverage of the nanoflakes by HfO2 the ALD process was performed in 300 cycles leading to approximately 30 nm of oxide thickness. The carrier and purge gas was nitrogen (N2) in which the sample was annealed at 200 ºC for around 1 h prior to deposition to remove surface bound adsorbates. After that the temperature was kept at 200 ºC during deposition. Tetrakis(dimethylamino) hafnium and H2O precursors were used alternating with open valve times of 0.15 s and 0.015 s, respectively, separated by a 25 s pump time. Device Characterization. All measurements were performed in ambient conditions. Currentvoltage characteristics were acquired using an Agilent B1500A semiconducting device analyser. The optoelectronic response was measured under global illumination with a spot size of 2mm. The spectral photoresponse was recorded by illuminating the device with fibercoupled and spectrally filtered light from a supercontinuum light source (SuperKExtreme EXW-4, NKT Photonics). Responsivity and temporal response was measured under shortpulsed illumination at a wavelength of 635nm from a 4-channel LASER controlled with an Agilent A33220A waveform generator. The noise properties of the devices were obtained by analysing the dark current in the conducting channel. We measured several dark current traces with the Agilent system under exactly the same conditions as the optical measurements were performed (same Vg and Vds) at a sampling rate of 125Hz. The frequency dependent noise spectral density at two different Vg is shown in Supplementary Information Fig. S4. Since responsivity was measured at light modulation frequency of 1Hz, we finally extract the noise current at 1Hz to calculate the corresponding detectivity D*(1Hz).

Supporting Information:


Page 12 of 20

Page 13 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Device performance overview (T1), AFM images of encapsulation (S1), alternative Al2O3 encapsulation (S2), MoSe2/HfO2 device performance (S3), humidity tests (S4) and details on noise measurements (S5).

Acknowledgments: We acknowledge funding support from European Commission's Seventh Framework Programme under Graphene Flagship (contract no. CNECT-ICT-604391) and Fundació Privada Cellex Barcelona. We are also thankful to the Ministerio de Ciencia e Innovación under contract number TEC2011-24744 for financial support. D.K. is supported by an FI fellowship.

(1)

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nat. Nanotechnol. 2012, 7, 699–712.

(2)

Ganatra, R.; Zhang, Q. ACS Nano 2014, 8, 4074–4099.

(3)

Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Nat. Nanotechnol. 2014, 9, 780–793.

(4)

Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Chem. Soc. Rev. 2015.

(5)

Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Nat. Nanotechnol. 2013, 8, 497–501.

(6)

Zhang, W.; Huang, J.-K.; Chen, C.-H.; Chang, Y.-H.; Cheng, Y.-J.; Li, L.-J. Adv. Mater. 2013, 25, 3456–3461.

(7)

Kwon, J.; Hong, Y. K.; Han, G.; Omkaram, I.; Choi, W.; Kim, S.; Yoon, Y. Adv. Mater. 2015, 27, 2224–2230.

(8)

Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G.-B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.; Kim, S. Adv. Mater. 2012, 24, 5832–5836.

(9)

Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. ACS Nano 2012, 6, 74–80.

(10)

Furchi, M. M.; Polyushkin, D. K.; Pospischil, A.; Mueller, T. Nano Lett. 2014, 14, 6165–6170.

(11)

Perea-López, N.; Lin, Z.; Pradhan, N. R.; Iñiguez-Rábago, A.; Laura Elías, A.; McCreary, A.; Lou, J.; Ajayan, P. M.; Terrones, H.; Balicas, L.; Terrones, M. 2D Mater. 2014, 1, 011004.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12)

Perea-López, N.; Elías, A. L.; Berkdemir, A.; Castro-Beltran, A.; Gutiérrez, H. R.; Feng, S.; Lv, R.; Hayashi, T.; López-Urías, F.; Ghosh, S.; Muchharla, B.; Talapatra, S.; Terrones, H.; Terrones, M. Adv. Funct. Mater. 2013, 23, 5511–5517.

(13)

Tsai, D.-S.; Liu, K.-K.; Lien, D.-H.; Tsai, M.-L.; Kang, C.-F.; Lin, C.-A.; Li, L.-J.; He, J.-H. ACS Nano 2013, 7, 3905–3911.

(14)

Chang, Y.-H.; Zhang, W.; Zhu, Y.; Han, Y.; Pu, J.; Chang, J.-K.; Hsu, W.-T.; Huang, J.-K.; Hsu, C.-L.; Chiu, M.-H.; Takenobu, T.; Li, H.; Wu, C.-I.; Chang, W.-H.; Wee, A. T. S.; Li, L.-J. ACS Nano 2014, 8, 8582–8590.

(15)

Zhang, W.; Chiu, M.-H.; Chen, C.-H.; Chen, W.; Li, L.-J.; Wee, A. T. S. ACS Nano 2014, 8, 8653–8661.

(16)

Abderrahmane, A.; Ko, P. J.; Thu, T. V; Ishizawa, S.; Takamura, T.; Sandhu, A. Nanotechnology 2014, 25, 365202.

(17)

Huo, N.; Yang, S.; Wei, Z.; Li, S.-S.; Xia, J.-B.; Li, J. Sci. Rep. 2014, 4, 5209.

(18)

Lee, H. S.; Min, S.-W.; Chang, Y.-G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. Nano Lett. 2012, 12, 3695–3700.

(19)

Konstantatos, G.; Sargent, E. H. Proc. IEEE 2009, 97, 1666–1683.

(20)

Yue, Q.; Shao, Z.; Chang, S.; Li, J. Nanoscale Res. Lett. 2013, 8, 425.

(21)

Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; You, L.; Li, J.; Grossman, J. C.; Wu, J. Nano Lett. 2013, 13, 2831–2836.

(22)

Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Nano Lett. 2013, 13, 4212–4216.

(23)

Ovchinnikov, D.; Allain, A.; Huang, Y.-S.; Dumcenco, D.; Kis, A. ACS Nano 2014, 8, 8174–8181.

(24)

Lembke, D.; Allain, A.; Kis, A. Nanoscale 2015, 7, 6255–6260.

(25)

Das, S.; Chen, H.-Y.; Penumatcha, A. V.; Appenzeller, J. Nano Lett. 2013, 13, 100– 105.

(26)

Na, J.; Joo, M.-K.; Shin, M.; Huh, J.; Kim, J.-S.; Piao, M.; Jin, J.-E.; Jang, H.-K.; Choi, H. J.; Shim, J. H.; Kim, G.-T. Nanoscale 2014, 6, 433–441.

(27)

Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nat. Nanotechnol. 2011, 6, 147–150.

(28)

Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. ACS Nano 2012, 6, 5635–5641.

(29)

Kufer, D.; Nikitskiy, I.; Lasanta, T.; Navickaite, G.; Koppens, F. H. L.; Konstantatos, G. Adv. Mater. 2015, 27, 176–180.

(30)

Cheng, L.; Qin, X.; Lucero, A. T.; Azcatl, A.; Huang, J.; Wallace, R. M.; Cho, K.; Kim, J. ACS Appl. Mater. Interfaces 2014, 6, 11834–11838.

(31)

Liu, H.; Ye, P. D. IEEE Electron Device Lett. 2012, 33, 546–548.

(32)

Radisavljevic, B.; Kis, A. Nat. Mater. 2013, 12, 815–820.

(33)

Fang, H.; Tosun, M.; Seol, G.; Chang, T. C.; Takei, K.; Guo, J.; Javey, A. Nano Lett. 2013, 13, 1991–1995.

(34)

Ghatak, S.; Pal, A. N.; Ghosh, A. ACS Nano 2011, 5, 7707–7712.

(35)

Ghatak, S.; Ghosh, A. Appl. Phys. Lett. 2013, 103, 122103.


Page 14 of 20

Page 15 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(36)

Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. H. Nat. Photonics 2007, 1, 531– 534.

(37)

Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Nature 2006, 442, 180–183.

(38)

Wu, C.-C.; Jariwala, D.; Sangwan, V. K.; Marks, T. J.; Hersam, M. C.; Lauhon, L. J. J. Phys. Chem. Lett. 2013, 4, 2508–2513.

(39)

Klee, V.; Preciado, E.; Barroso, D.; Nguyen, A. E.; Lee, C.; Erickson, K. J.; Triplett, M.; Davis, B.; Lu, I.-H.; Bobek, S.; McKinley, J.; Martinez, J. P.; Mann, J.; Talin, A. A.; Bartels, L.; Léonard, F. Nano Lett. 2015, 15, 2612–2619.

(40)

Rose, A. Phys. Rev. 1955, 97, 322–333.

(41)

Zhang, Y.; Li, H.; Wang, L.; Wang, H.; Xie, X.; Zhang, S.-L.; Liu, R.; Qiu, Z.-J. Sci. Rep. 2015, 5, 7938.

(42)

Renteria, J.; Samnakay, R.; Rumyantsev, S. L.; Jiang, C.; Goli, P.; Shur, M. S.; Balandin, A. A. Appl. Phys. Lett. 2014, 104, 153104.

(43)

Kwon, H.-J.; Kang, H.; Jang, J.; Kim, S.; Grigoropoulos, C. P. Appl. Phys. Lett. 2014, 104, 083110.

(44)

Ghatak, S.; Mukherjee, S.; Jain, M.; Sarma, D. D.; Ghosh, A. APL Mater. 2014, 2, 092515.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Device schematic, transfer and output characteristics of MoS2 and MoS2/HfO2 field effect transistor. (a) A 3D sketch of two typical TMDC FETs, the front device exposed to air and all kind of molecules and the back device protected with HfO2. (b) Transfer curve IDS-VG before and after ALD encapsulation. The protected MoS2 FET shows strong n-type doping, vanishing hysteresis and strong reduction of device resistance. (c) The transfer characteristics of MoS2/HfO2 for a range of bias voltages VDS with on/off ratios of up to 108. The inset shows the curves in logarithmic scale. At very strong negative backgate voltage the device can be switched off leading to dark currents of pico-Ampere (at the limit of the measuring unit). (d) At VG = - 40 V the device maintains switched off even up to high bias of 5 V while the output curves are nearly ohmic at positive VG. The inset shows a linear plot of the IDS-VDS curves. Channel dimensions: W = 5 um, L = 3 um.


Page 16 of 20

Page 17 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. Optoelectronic response and charge carrier dynamics. VDS = 5 V. (a) Spectral responsivity of the monolayer MoS2 flake measured at VG = -32 V with an illumination intensity of 3.2 mW/cm2. The two exciton peaks A and B, together with a sharp drop at 690 nm can clearly be resolved. (b) Measured IDS-VG curves in the dark (black) and under illumination with different light intensity (coloured). (c) Power dependent responsivity for a wide dynamic range at VG = -40 V until 0 V. The responsivity was measured with a pulsed Laser at 635 nm with 1 Hz frequency. Two trends can be observed namely a general reduction of responsivity with gate bias strength and a drop of responsivity at lower illumination power in the regime of strongest gate voltage VG. (d) Schematic to explain the behaviour of power dependent responsivity. On the left side of the Energy axis a bandgap model with continuous bandtail states for electrons and holes is shown. On the right side a discrete model is used to depict the carrier excitation and trapping dynamics. The left image shows the case of VG > VTH. The centred and right images show VG < VTH under dark and illuminated conditions, repectively. EF0 is the Fermilevel, EFn and EFp the quasi Fermilevels of electrons and holes under illumination.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Encapsulation effect on photoresponse and speed of mono- and bilayer devices: Drain Current during 1 cycle of light modulation for a bilayer MoS2 device (a) and its corresponding protected device MoS2/HfO2 (b). The photocurrent improves by a factor of ~ 40 and at the same time the decay time decreases by more than 1 order of magnitude. VDS = 1 V. (c) Power dependent responsivity before and after HfO2 encapsulation of a bilayer MoS2 device. Responsivity were measured at backgate voltages where the device was fully depleted, at VG = -10 V and VG = -40 V for MoS2 and MoS2/HfO2, respectively. The responsivity is improved by more than an order of magnitude and the device is sensitive to even lower illumination intensity than before. VDS = 1 V. (d) Logarithmic scale photoresponse to light pulses of 5 mW/cm2 with 1 Hz repetition rate. The pulses are stable, the drain current IDS doesn´t drift and returns to the initial pico-Ampere level after switching off the light. (e) Normalized photocurrent pulses measured at 100 mHz and 41 mW/cm2 for several backgate voltages. With increasing gate bias the decay times (90% – 10%) reduce significantly. (f) The responsivity of a single layer MoS2 device and its corresponding decay time as a function of backgate voltage plotted for 41 mW/cm2 illumination intensity. VDS = 5 V.


Page 18 of 20

Page 19 of 20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. Noise and sensitivity. VDS = 5 V (a) Drain current IDS (dark), responsivity R and measured detectivity D* as a function of back-gate voltage VG. The responsivity values correspond to the maximum values of Fig. 2c for each backgate voltage. Despite a drop of responsivity for more negative VG the faster decreasing noise leads to increase in sensitivity with maximum measured D* = 7.7×1011 Jones at VG = -32 V. At stronger VG the measuring unit reaches its noise floor. (b) Noise power spectral density for the same range of backgate voltages. The curves show clear 1/f component for low frequency which is not visible within the systems noise floor at -35 V and -40 V. (c) Measured versus theoretical shot-noise limited noise current density. For strong gating bias the dark current reduces and the measured noise current approaches slowly the shot-noise limit.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Figure 561x212mm (96 x 96 DPI)


Page 20 of 20

Highly Sensitive, Encapsulated MoS2 Photodetector with Gate

Recommend Documents