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IP Address: 134.153.184.170
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Nonlinear optical absorption in a graphene infrared photodetector
View the table of contents for this issue, or go to the journal homepage for more
Nonlinear optical absorption in a grapheneinfrared photodetector
Prarthana Gowda1, Dipti R Mohapatra2 and Abha Misra1
1Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, India 5600122Department of Physics, Indian Institute of Science, Bangalore, India 560012
Received 24 February 2014, revised 20 May 2014Accepted for publication 22 May 2014Published 30 July 2014
AbstractThe photoresponse of the graphene photodetector elucidated strong dependence on severaloptical parameters, such as the angle of incidence and the incident power of infrared exposure atroom temperature. The sinusoidal dependence of the photoresponse on incidence angle, whichhad not been realized before, has now been revealed. The combined effect of the photo excitedcharge carrier and the photon drag effect explain this nonlinear optical absorption in graphene atlower incident power. The nonlinear dependence of the charge carrier generation on the incidentpower revealed that this process contributed to the nonlinear photoresponse. However, adeviation is observed at a higher incident power due to the induction of thermal effects in thegraphene lattice. This work demonstrates the tunability of the graphene photodetector under asystematic variation that involves both parameters.
Keywords: graphene, photodetector, angle of incidence
(Some figures may appear in colour only in the online journal)
1. Introduction
The discovery of two-dimensional graphene, a nanoscalestructure that is composed of a single layer of carbon atoms,has recently revolutionized material research over a widerange of fields, including electronics, optical and mechanicalfields [1–5]. The unique optical and electronic properties ofgraphene have shown it to be a special material; for example;it exhibits a linear energy dispersion relationship, i.e., carriersare regarded as massless Dirac ferminons with a high Fermivelocity [6] (∼1/300 of the speed of light), which provides anextremely high carrier mobility (15 000 cm2 V−1 s−1) for boththe electrons and the holes [7]. Additionally, despite beingconsidered a zero band gap metal, it exhibits a tunable bandstructure (0–0.25 eV) that provides a frequency independentabsorption coefficient over a broad range of wavelengths,from infrared to terahertz [8, 2]. Graphene shows only a ∼2%absorption of incident light; hence, it offers the potential toreplace conventional metals as transparent and flexible elec-trodes [9]. In graphene the electron hole pairs are generatedby low energy photons ranging between near and mid-infra-red (IR) regimes. Therefore, graphene opens up an avenue asan extremely sensitive material for IR detection because its
sensitivity is not limited by the band gap [10]. Several reportshave demonstrated graphene’s ability to generate photo-currents upon IR exposure under ambient conditions[9, 11, 12]. Thanks to numerous efforts since then, animprovement in device sensitivity for IR detection has beenmade. For instance, a graphene hybrid photodetector exhibitsultrahigh gain when semiconductor quantum dots are incor-porated, where graphene acts as a transport medium [10]. Sunet al have demonstrated that graphene film, with lead sulphidequantum dots, can absorb more IR light with a responsivity of107 AW−1 even at a low intensity of light irradiation [12].Recently, graphene-based photodetectors have demonstratedtwo- and four-fold enhancement in the photoresponse inducedby carbon nanotubes and also in wrinkled graphene, respec-tively, as compared to pristine few-layer graphene [13].Another study on monolayer graphene with an electrontrapping center presented three orders of higher sensitivity of8.12 AW−1, than monolayer graphene (10 mAW−1) [2].Overall, the photoresponsivity of graphene depends mainlyon four different factors, namely intensity, polarization,electrical bias and the wavelength of the incident light [14].Echtermeyer et al demonstrated an optical polarizationdependence on the photoconductivity of single-layer
graphene [15]. So far, direct dependence of the angle of IRincidence (angle of optical incidence) on the photoresponse ofa graphene IR detector has not yet been explored in detail.The present report mainly focuses on elucidating the effect ofthe optical incidence angle, as well as the incident IR lightintensity (flux density), on the sensitivity of a graphene IRdetector under ambient conditions.
2. Experimental section
2.1. Sample preparation
Single-layer graphene was grown on a 25 μm-thick copperfoil with 99.89% purity (Alfa aesar) using chemical vapordeposition (CVD). Before carrying out the graphene growth,the reaction chamber was flushed with argon (Ar) gas flowingat 800 sccm (standard cubic centimeters per minute) for25 min; and during this time, the chamber temperature wasramped up to 980° C. Thereafter, the substrate was annealedfor 1 h in the presence of both Ar and hydrogen gases, whichflowed at rates of 800 and 300 sccm, respectively. Graphenegrowth was conducted for 3 min in the presence of methanegas (20 sccm). As-grown graphene was directly transferredfrom the copper substrate to the silicon dioxide (SiO2) sub-strate (thickness ∼300 nm) by using a wet transfer technique.A large area (1 × 1 cm2) of graphene film can be seen on thesubstrate, as shown in the camera shut image in figure 1(a).The inset shows a magnified optical image (using 100xobjective) that illustrates a uniform contrast without anywrinkling or microscopic defects. Electrical measurementswere conducted on the graphene samples after depositing theCr (5 nm)/Au (50 nm) electrodes. The gap between the elec-trodes was kept at ∼2.5 mm and the incident angle of IR, asdemonstrated in the schematic (figure 1(b)).
2.2. Characterization of graphene
The as-grown graphene sample is characterized by Ramanspectroscopy. Figure 2 shows a Raman spectrum of the gra-phene, where the laser intensity is plotted with the resultingRaman shift. Three major distinct peaks mainly dominate theRaman spectrum. The peaks that are centered at 1345 and1583 cm−1 correspond to the D and G bands, respectively.
The calculated D/G intensity ratio is 0.5, which is char-acteristic of low-defect graphene. The relative intensity of the2D to G band ratio is I2D/IG > 2, which is the signature ofsingle-layer graphene. However, the presence of the small Dpeak could have risen from the structural defects in the large-area graphene, which originated during the transferringprocess.
2.3. Electrical measurements
A constant bias current of 100 μA was applied to the graphenesample using a source-meter (Keithley 2011A), and theresulting change in the electrical response was recorded by adata acquisition system. The IR source, which has a wave-length of 1550 nm (the optical fiber has a diameter of∼125 μm and a ∼8 μm core) of varying power (65, 100, 150and 200 mW), was utilized to inject photons into the graphenelayer at different incidence angles (30°, 60°, 90°, 120° and150°). Under these variable experimental conditions, theexposure of the IR radiation resulted in a potential drop acrossthe graphene device.
The electrical measurements were conducted in twodifferent sets of measurement by (i) varying the angle ofoptical incidence at constant IR power and (ii) by varying theIR power at each angle of optical incidence. The saturationexperiments were conducted with constant exposure to the IRuntil the resistance reached a constant value. In addition, thephotoelastic behavior of the graphene was examined through
Figure 1. (a) Optical image of the graphene transferred onto the SiO2 substrate. (b) Schematic of the experimental setup; Graphene isconnected to a source-meter through the gold electrodes, and variation in the angle of optical incidence is depicted from 30–150°.
Figure 2. Raman spectrum of the single layer graphene.
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Nanotechnology 25 (2014) 335710 P Gowda et al
cyclical exposure of the radiation for a time period of 60 sunder the same experimental conditions as mentioned above.A corresponding change in the resistance during each cyclewas derived as a function of the decay time constant and as afunction of the angle of optical incidence. A change in thedifferential conductivity from the I–V measurements wasobserved before and during the IR exposures under the above-mentioned conditions.
3. Results and discussion
Figure 3(a) illustrates the electrical response of the graphenedevice under IR exposure. The resulting normalized resis-tance (R/Ro, where R and Ro are the resistances of the gra-phene device with and without exposure to the IR radiation,respectively) of the graphene is plotted with the time durationof the IR exposure. The results reveal a decrease in theresistance of the graphene device due to IR exposure; and assoon as the exposure was turned off, the resistance of thedevice reversibly returns to its original value. The electricalresponse of the device shown in figure 3(a) was recorded fordifferent IR powers, while the angle of incidence was keptconstant at 30°. It clearly shows that under a constant expo-sure of IR radiation, all of the electrical responses tend tosaturate after a certain time interval. The IR exposure was
turned off as soon as the resistance reached saturation, and anexponential decay in the response was elucidated. This vol-tage drop during the IR exposure could be attributed to thegeneration of radiation-induced charge carriers, which couldhave resulted in a drop in the initial resistance [16]. This largechange in resistance cannot be solely attributed to the inducedphotocurrent, which is typically of the order of a nanoampere[17], but it may have been due to an induced current becauseof the movement of the excited electrons in the graphene fromthe lower to the higher energy states, which was caused by theincident radiation. These excited electrons decay back to theground state by emitting the excess energy; and in this pro-cess, the decay rate grows shorter than the rate of the energyabsorption in the graphene (due to the scattering of theelectrons). Therefore, this resulted in a saturation state of theelectrical response after IR exposure for a long period of time.The changes in the resistance for the incidence angle of 30° is133, 246, 370 and 437Ω for the different powers of 65, 100,150 and 200 mW, respectively; hence, it is clear that the dropin the resistance increases with the input laser power. It isinteresting to note that the time duration for the saturation alsoincreases monotonically with the incident power (as shown bya dotted straight line in figure 3(b)). At all incident IR powers,a reversible graphene response can be clearly noticed, whichshows the photoelastic nature of the graphene. The saturationphotoresponse time of the graphene device is calculated for
Figure 3. (a) The saturation responses of the resistance plotted with the exposure time at different IR powers for a constant incidence angle of30°. (b) The saturation responses of the resistance plotted with different incident powers at a constant incidence angle of 30°. (c) The rise time(before saturation) is plotted with incidence angles at different IR powers. (d) The cyclical responses for different IR powers at a constantincident angle of 30°.
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all of the incident IR powers, and the results indicate a longerphotoresponse time for the higher incident power. For IRpowers of 65, 100, 150 and 200 mW, the measured responsetimes are 180, 260, 286 and 330 s, respectively. This clearlyindicates that the responsivity of the device decreases with theIR power; in other words, the maximum response time isinversely proportional to the flux of the photons, which alsoaffects the device as it reaches a saturation state, as shown infigure 3(a). The higher power leads to an increase in thenumber of photons, which leads to a greater number of photocarriers (electron–hole pair) and thus enhances carrier scat-tering. Therefore, higher intensity exposure to grapheneresulted in a longer period of time needed to reach saturation,as compared to the lower intensity exposure. In addition, it isworth noting that the resistance rise time, due to the IRexposure, is less than the fall time after turning off the IRradiation (after achieving saturation in the response). Thisinstantaneous rise in the current or drop in the resistancecould be attributed to the combined contribution of photo-generated electrons in the graphene due to IR irradiation, aswell as to the photo-generated carriers due to intrabandtransitions under an applied bias [16].
Similar to an earlier report [2], the change in the resis-tance, due to the IR exposure, was revealed to be a linearfunction of the IR power. In figure 3(b), the electricalresponse of the graphene decreases (change in the resistance)with an increase in the IR power. This decrease in theresponsivity is elucidated by a linear fit (figure 3(b)). There-fore, a linear relation can be drawn according to
α= − ( )P R R/ o , where P is the incident power, R and Ro arethe maximum resistance reached during saturation and initialresistance, respectively, and a constant α depends on thematerial properties. It is obvious that an increase in the IRpower suggests a higher rate of incident photons, whichreveals that the generation of additional photo excited chargecarriers increase the current. This in turn induces a drop in theresistance across the graphene device.
In addition to the above-mentioned IR power dependenceon the electrical response of the graphene device, a remark-able dependence on the angle of optical incidence is presentedin figure 3(c). The time taken by the device to reach asaturation value of resistance (as was revealed in figure 3(a))is calculated for different incident angles at each IR power, asmentioned above. This response time of the device is mea-sured at various angles of incidence and plotted at different IRpowers. A sinusoidal dependence is clearly observed at all ofthe incident powers. An overall lowest magnitude of responseis depicted at the lowest power of 65 mW, and it increaseswith an increase in the IR power. This clearly confirms thatthe generation of photo-induced carriers depend not only onthe incident power, but they also reveal a strong dependenceon the angle of incidence. Similar responses were obtained forthe change in resistance, which is discussed in the latersection of this manuscript.
Figure 3(d) indicates the cyclical responses of the gra-phene device at a constant angle of incidence (30°) when theIR radiation was exposed to the graphene device at a periodicinterval of 60 s for three continuous cycles. The spectra are
plotted for varying IR power, and the change in resistance isshown to follow a similar trend as the trend observed infigure 3(a); that is, the electrical response increases with theincrease in power. A reversible change in the resistance isnoticed for the powers that vary between 65–150 mW; how-ever, at 200 mW, an irreversible recovery can be noticedduring the cyclic change in resistance, which could be relatedto either the induced heating effects or the longer lifetime ofthe excited states.
A maximum change in the resistance of 365Ω wasobserved for a higher power of 200 mW, which is probablydue to the higher thermal energy imparted to the graphene;and hence, thermally activated charge carriers overcame thedefect barriers (induced due to thermal effects), thereby pro-viding a higher photoresponse. Moreover, a longer recoverytime, with a higher radiation power, suggest the longer life-times of the excited states. A recoverable electrical responseagain confirms the photoelastic nature of the graphene, whichis shown to be highly repeatable and reproducible. Theexponential decay and the rise in the resistances are fitted withthe following equations, respectively.
⎛⎝⎜
⎞⎠⎟τ
= = −R At
decay resistance exp (1)1 11
1
⎛⎝⎜
⎞⎠⎟τ
= =R At
rise resistance exp (2)2 22
2
where A1 and A2 are the constants, t1 and t2 are the decay andrise times and τ1 and τ2 are the time constants for the decayand rise responses, respectively. The values of τ1 and τ2,measured from the graphs shown in figure 3(d), lie within27 ± 3 s and 25 ± 5 s, respectively, at different powers. Therespective changes in the resistance are 106, 151 and 238Ω atdifferent powers of 65, 100 and 150 mW, respectively, whichagain depicts an increase in the drop of the resistance thatcorresponds to the input laser power.
A normalized change in the resistance of the graphenewas measured at different angles of incidence (figure 4). Asmentioned earlier, this dependence of the electrical responseshas again been measured in two ways: (i) a measurement wasconducted to evaluate the graphene response during electricalsaturation, and (ii) another measurement was performedduring the cyclic exposure of the IR at various angles ofincidence. Figure 4(a) depicts a change in the normalizedresistance Δ( )R R/ o at all of the incident IR powers, as well asat different angles of optical incidence, where ΔR is thechange in the resistance due to the IR exposure. This changein the response was measured until saturation was reachedunder the IR exposure. Interestingly, and similar to the resultsshown in figure 3(c), the sinusoidal responses were monitoredwith the angle, and it is clear that the amplitude of the sinu-soidal response remains approximately constant, dependingon the incident IR powers. An extrapolation of the curvesprovides the electrical responses at the respective angles.However, a positive phase shift is revealed by a dotted line.An upward shift in the overall change in resistance, whichoccurs with the increase in the IR power, is worth noticing.
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Nanotechnology 25 (2014) 335710 P Gowda et al
The highest change in resistance is depicted at a 90° angle ofincidence. Therefore, the angle of optical incidence is shownto significantly impact the sensitivity of the graphene towardthe IR radiation. A similar behavior was reported in a recentstudy on single-walled carbon nanotubes, which was con-ducted in order to reveal a dependence of the photoelectricsignal on the laser polarization and on the angle of the laserbeam incidence [18]. In this study, a nonlinear dependencewas evaluated, along with the photoelectric conversion effi-ciencies. A relationship between the changes in the electricalresponse Δ( )R R/ o and the angle of incidence (θ) can be drawnas Δ θ Φ= +R R A n/ sin ( ( ))o , where A denotes the amplitudeof the sinusoidal function, n is an integer and Φ represents thephase shift, which depends on the incident power.
This behavior can be explained in the framework of thephoton drag effect, which is defined as the generation ofphotoelectric currents that occur due to the momentumtransfer from the incident photons to the charge carriersduring intraband energy transitions [18]. The contribution ofthe photogalvanic effects is highly unlikely in the present
study due the transparent nature of the graphene. The max-imum change Δ ∼( )R R/ 14o was noticed at 90°, and theminimum change Δ ∼( )R R/ 3o occurred at 120° angles. Thesevalues are very consistent and indicate the effect of theincidence angle in the graphene photodetector.
Figure 4(b) shows a change in the resistance with dif-ferent angles of optical incidence during the cyclic exposureof the IR at different powers. Again, a sinusoidal response canbe observed with the angle of incidence. However, interest-ingly unlike the saturation experiments, the magnitude of thechange in the resistance does not remain constant with the IRpower, but it gradually increases with the IR power alongwith a phase shift. The lesser magnitude of the change inresistance resulted due to a short exposure time of 60 s, as wasseen earlier in figure 3(d). The incidence angle of 90° showedthe highest change in response, as compared to any otherangle varying between 30°–120°.
Figure 5 shows the measured photoresponses of the gra-phene for varying levels of IR power at different angles (themeasurement is shown in figure 4). It is interesting to note that
Figure 4. (a) The changes in the normalized resistance during saturation are plotted with different angles of incidence at various IR powers.(b) The changes in the normalized resistance during cyclical measurements are plotted with different angles of incidences at different IRpowers.
Figure 5. (a) The normalized change in resistance is plotted with different incident powers at various incidence angles during the saturationexperiments. (b) During cyclical experiments, the normalized change in the resistance is plotted with different incident powers at variousincidence angles.
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Nanotechnology 25 (2014) 335710 P Gowda et al
for both saturation cases (figure 5(a)), as well as the cyclicexperiments (figure 5(b)), a linear dependence of the photo-response on the IR power is elucidated. The slope of the fittedline depicts the photoresponse per unit flux of the photons,which indicates that under electrical saturation, the highest fluxwas received when the incidence angle was kept 90°, and it wasat minimum flux for an angle of 120°. Figure 5(b) illustrates asimilar relationship between the photoresponse and the num-bers of photons irradiated on the sample per unit time at adifferent incidence angle for the cyclical experiments. Similarto the saturation experiments, a linear dependence is revealedfor all of the angles at various incident IR powers. Again, amaximum change is indicated at 90°, and the minimum changeis at 120°. Therefore, the change in resistance is directly pro-portional to the photon that was irradiated in per unit time ontothe graphene.
Figure 6(a) shows a plot of differential conductivity (dI/dV) with the applied bias, V, which is measured from the I-Vmeasurements that were conducted before and during the IRexposure to the graphene device. An enhanced differentialconductivity on the IR exposure is illustrated, which verifiesan increase in the carrier generation during the IR exposure.Figure 6(b) illustrates a plot of charge carrier concentration(Δη), which was generated using different IR powers with theangle of optical incidence. The charge carrier concentration iscalculated using the relationship given below [16]
Δμ Δη
= * *RL
W e
1 1,
where, ΔR is the change in the resistance, L and W are therespective length and width of the graphene, e is the electroncharge, μ is the mobility and Δη is the change in the chargecarrier concentration due to IR illumination. The resultingcurve is fitted with a cosine relationship given byΔη θ Φ= +A ncos ( ( )), where A denotes the amplitude, n isan integer and Φ represents the phase shift, depending on theincident power. Figure 6(b) shows a 90° phase shift from theresults obtained for a change in the resistance, and it alsoconfirms the earlier observations in figure 4.
4. Conclusion
The response of the graphene IR detector has been presentedunder variable conditions of incidence angles, as well asincident powers. The electrical response is found to be line-arly dependent on the IR power; however, a nonlinear relationis deduced from the angle of optical incidence. A large var-iation in the angle, from 30° to 120°, was applied in order toexplain this interesting behavior from the photoresponse ofthe graphene. The observed phenomenon is explained on thebasis of the photon drag effect, which is defined as the gen-eration of charge carriers upon IR exposure and thermaleffects.
Acknowledgements
AM would like to acknowledge partial funding support fromthe Center for Infrastructure, Sustainable Transportation andUrban Planning (CiSTUP), Indian Institute of Science. Theauthors would also like to acknowledge Mr Ashish Suri forhis contribution to the experimental setup.
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