Broadband high photoresponse from pure monolayer graphene photodetector

Abstract

Graphene has attracted large interest in photonic applications owing to its promising optical properties, especially its ability to absorb light over a broad wavelength range, which has lead to several studies on pure monolayer graphene-based photodetectors. However, the maximum responsivity of these photodetectors is below 10 mA W^-1 , which significantly limits their potential for applications. Here we report high photoresponsivity (with high photoconductive gain) of 8.61 A W^-1 in pure monolayer graphene photodetectors, about three orders of magnitude higher than those reported in the literature, by introducing electron trapping centres and by creating a bandgap in graphene through band structure engineering. In addition, broadband photoresponse with high photoresponsivity from the visible to the mid-infrared is experimentally demonstrated. To the best of our knowledge, this work demonstrates the broadest photoresponse with high photoresponsivity from pure monolayer graphene photodetectors, proving the potential of graphene as a promising material for efficient optoelectronic devices.

ARTICLE

Received 23 Oct 2012 | Accepted 4 Apr 2013 | Published 7 May 2013

By Yongzhe Zhang1,w, Tao Liu1, Bo Meng1, Xiaohui Li1, Guozhen Liang1, Xiaonan Hu1 & Qi Jie Wang1,2,3

Graphene has attracted large interest in photonic applications owing to its promising optical properties, especially its ability to absorb light over a broad wavelength range, which has lead to several studies on pure monolayer graphene-based photodetectors. However, the maximum responsivity of these photodetectors is below 10 mAW 1, which signicantly limits their potential for applications. Here we report high photoresponsivity (with high photoconductive gain) of 8.61 AW 1 in pure monolayer graphene photodetectors, about three orders of magnitude higher than those reported in the literature, by introducing electron trapping centres and by creating a bandgap in graphene through band structure engineering. In addition, broadband photoresponse with high photoresponsivity from the visible to the mid-infrared is experimentally demonstrated. To the best of our knowledge, this work demonstrates the broadest photoresponse with high photoresponsivity from pure monolayer graphene photodetectors, proving the potential of graphene as a promising material for efcient optoelectronic devices.

DOI: 10.1038/ncomms2830

Broadband high photoresponse from pure monolayer graphene photodetector

1 School of Electrical and Electronic Engineering, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798, Singapore. 2 School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore. 3 Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore 637371, Singapore. w Present address: School of Renewable Energy and State Key Laboratory of Alternate Electrical Power

System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China. Correspondence and requests for materials should be addressed to Q.J.W. (email: mailto:[email protected]

Web End [email protected] ).

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Graphene, a two dimensional allotrope of carbon atoms on a honeycomb lattice with unique band structure, has recently attracted signicant interest in photonic applica

tions for realizing, for example, transparent electrodes1, optical modulator2, polarizer3, surface plasmonic devices4,5 and photodetectors69. One of the most impressive advantages is its ability to absorb B2.3% of incident light over a broad wavelength range despite its one-atom layer thickness10. This advantage makes the realization of broadband photodetectors possible, far exceeding the bandwidth of the current photodetectors. In addition, graphene also shows high mobility, which is promising for applications in ultrafast detection. Based on these properties, several works have been reported on the realization of graphene-based photodetectors in a eld-effect transistor (FET) structure69. However, the highest photoresponsivity (photo-generated current per incident optical power) is o10 mAW 1.

Although some strategies such as using surface plasmons11,12 or microcavity13 have been adopted to enhance the performance of the graphene photodetector, the photoresponsivity is still as low as several tens of mAW 1 and the fabrication processes are complex. In conventional FET-based graphene photodetectors, the low photoresponsivity is mainly attributed to the low optical absorption in monolayer graphene and the short recombination lifetime (on the scale of a picosecond) of the photo-generated carriers, leading to a low internal quantum efciency of B616%

(ref. 6). Thus, to achieve high photoresponsivity, one should increase either the absorption efciency of graphene1113 or the lifetime of the photo-excited carriers, while still maintaining the high mobility by modifying the properties of graphene. In addition, although pure monolayer graphene photodetectors have been predicted as having ultra-wideband operation in theory10, all of the reported results based on pure monolayer graphene photodetectors are realized in the visible or the near-infrared ranges69,1115 and not demonstrated in the longer wavelength range.

Band structure engineering of graphene has been widely investigated. A bandgap can be created in graphene for electronic applications, for example, by patterning graphene nanoribbon or nanomesh structures through electron-beam lithography16, nanowire shadow mask etching17, block copolymer lithography18 and/or unzipping of carbon nanotubes19,20. On the other hand, various defect states can also be created in graphene by defect engineering21 for manipulating electrical22,23, chemical23,24 and magnetic25,26 properties. Here we introduce defect midgap states band (MGB)27, as well as a bandgap in graphene through a Titanium (Ti) sacricial layer fabrication method for achieving high photoresponse. Through this scheme, we have demonstrated a high photoresponsivity of 8.61 AW 1, which are about three orders of magnitude higher than those in previous reports from pure monolayer graphene photodetectors. The methodology presented here can provide a new avenue for the development of graphene-based broadband high performance photodetectors and other optoelectronic devices.

In addition to a high photoresponsivity, we have demonstrated the broadest photoresponse from the visible (532 nm) up to the mid-infrared (B10 mm) in a single pure graphene photodetector.

We noted that graphene hybrid photodetector with an ultrahigh photoconductive gain has been recently demonstrated where semiconductor quantum dots were adopted as light absorbers and graphene was used as an ultrafast transport medium28. While, as the photoresponse of these devices are dependent on the properties of the quantum dots, the broadband advantage of the graphene is lost, for example, the hybrid graphene photodetector does not work in the important mid-infrared regime. Compared with the traditional mid-infrared photodetectors such as HgCdTe (HCT) and quantum well

infrared photodetector, the proposed graphene photodetector possesses the advantages of a lower cost and a lower operation power, and it is compatible with the complementary metal-oxide-semiconductor technologies, while still maintaining a high photoresponse.

ResultsConcept of the proposed graphene photodetector. To increase the photoresponsivity of pure graphene photodetectors, apart from the prevailing methods of increasing the absorption efciency of graphene1113, one could also increase the lifetime of the photo-generated electrons. However, owing to the fast interband and intraband recombination rate of a few picoseconds of the photo-generated electrons29 (Fig. 1a), even though a carrier multiexcitation generation (MEG) process has been theoretically predicted30 in a perfect graphene with a multiplication factor of approximately four by the impact ionization process (also called the inverse Auger recombination (AR)), up to now no multiplication effect has been experimentally observed in pure graphene sheet (we noticed that the MEG effect in graphene was observed experimentally and reported almost simultaneously31 at the time of this publication, conrming the high efciency of carrier-carrier scattering in graphene.).

The detailed physical picture on how a high photoconductive gain is achieved in this work is depicted as follows: we rst process the graphene sheet into a graphene quantum dot-like (GQD)-arrays structure; then defect MGB electron trapping centres are formed on the boundary and surface of the GQDs and a bandgap is created due to the quantum connement. The corresponding energy band diagram is shown in Fig. 1b. The sequence of the carrier MEG and trapping in the MGB is depicted in Fig. 1ce. As shown in Fig. 1c, the impact ionization(II) process exists in the GQDs: one photo-excited electron relaxes to a lower-energy state within the conduction band, exciting a valence band electron into the conduction band. Meanwhile, electrons of the conduction band will be involved in the AR process (Fig. 1d): an electron is scattered from the conduction band into the valence band, while simultaneously the released energy is transferred to another electron, which is excited to a higher-energy state in the conduction band. Despite the competing processes between II and AR, II can be more efcient than the AR process in quantum-conned systems32 because of the asymmetry between the II and AR33. Therefore, carrier multiplication (the MEG effect) occurs, which generates more secondary electrons in the conduction band, see Fig. 1c, thus enhancing the electron generation efciency. As a bandgap is formed, the electronphonon scattering rate from the conduction band to valence band decreases34 so that more photo-generated, as well as the secondary-generated electrons can be trapped in the MGB (Fig. 1e). It is noted that holes can also contribute to the carrier MEG process assisted by an external electrical eld, as illustrated in the latter part of the paper. When the photo-generated and the secondary-generated electrons are trapped in the MGB, the related holes can be recirculated many times through hopping within the lifetime of the MGB trapped electrons (Fig. 1f); thus a photoconductive gain is achieved. Owing to the relatively long electron trapping time in the MGB together with the high efciency of the carrier MEG effect in graphene, we expect a high photoresponse can be achieved in GQD array photodetectors.

Device fabrication and characterization. We fabricated pure graphene photodetectors in a FET structure through a Ti etching fabrication method, which is shown schematically in Fig. 2ad and discussed in more details in Methods. Three devices were

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a b

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Figure 1 | Energy band diagram and concept of the graphene photodetector. (a) Band structure diagram of pure graphene sheet. The red arrows indicate the intraband and interband electronphonon scattering. (b) Energy band diagram of the GQD including the MGB. Eg is the quantum-conned bandgap.

Shadow region indicates the MGB, and Ef is the Fermi energy level. (c) Generation of an electron-hole pair via photon excitation (PE) followed by the impact ionization (II) process. When the electron is excited into much higher energy of the conduction band, the II cascade of carriers occurs. Each cascade step involves electron-hole pair generation. Owing to the high efciency of the carrier-carrier scattering in graphene, MEG effect exists. It is noted that holes can also contribute to the carrier MEG process assisted by an external electrical eld, to be explained in the latter part of the paper. (d) Excitation of an electron to a higher-energy level in the conduction band via the AR process, followed by recombination of another electron with a hole in the valence band.(e) Trapping/relaxation process of the excited electrons to the MGB. (f) Graphene photoconductive channel consists of GQD arrays. The photo-excited and the secondary electrons are captured by the MGB having a long lifetime. Before the trapped electrons are recombined, the corresponding holes can circulate many times in the photoconductive channel, thus achieving a high photoconductive gain.

fabricated and compared: Sample A is based on a pristine graphene structure (without the Ti layer fabrication); Sample B is a GQD arrays structure fabricated with a B2-nm thick Ti layer;

and Sample C is a GQD arrays structure fabricated with a B20-nm thick Ti layer. Raman spectra measurements were used to characterize the density of defects of these samples35 (see Methods). As shown in Fig. 3a, a higher number of defects were observed in Sample C than in Sample B due to the higher Raman ID/IG ratio36,37 (ID and IG are the D-band and G-band

Raman intensities, respectively). For Sample C, the average crystallite size of the GQDs La is estimated to be B7.5 nm from38

La(nm) (2.4 10 10)l(nm)4(ID/IG) 1, where l 532 nm is

the wavelength of the Raman signal. Similarly, Sample B has an average GQD size of B20.5 nm. The electrical characteristics of

the Samples A and C are shown in Fig. 3b,c, respectively. For Sample A, the transfer curve shows an ambipolar transfer property with the Dirac point at B60 V, while it is at B80 V for

Sample C.

Carrier transport in the processed graphene device. To investigate the carrier transport properties of the processed graphene device, i.e. the GQD arrays device (refer to Sample C in the latter part of the paper, if not specied), we measured the electrical properties of the device at different temperatures (Fig. 4ac). The observations indicate the carrier transport in the graphene FET photodetector is inuenced by the Coulomb blockade of charges39 (see Methods). Figure. 4b shows the current (I) versus

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Figure 2 | Fabrication process of the device. (a) A monolayer graphene was mechanically exfoliated onto a 285 nm SiO2/Si substrate. (b) The graphene photodetector was processed into a FET structure. Two electrodes (that is, the source and the drain terminals) of Ti/Au (20 nm/80 nm) were fabricated on the graphene by photolithography and lift-off processes. The gate terminal was fabricated on the bottom of the Si substrate. (c) A thin nm-scale Ti sacricial layer was deposited onto the graphene by electron-beam evaporation. (d) The Ti sacricial layer was removed via wet etching, and then GQD array structure with various quantum dot (QD) sizes can be formed on the Si substrate depending on the thickness of the Ti layer.

temperature (T) characteristics in the temperature range 8290 K, that is, the temperature dependence of the conductivity. The best t of the temperature-dependent data is obtained with the T 1/3 curve tting (inset, Fig. 4b), but not with the T 1 or T 1/2 tting (see Supplementary Fig. S1 and Supplementary Note 1). These results show that variable range hopping (VRH), frequently observed in disordered systems including carbon-related materials40, is the main carrier transport mechanism (see Methods), which involves consecutive hopping process between two localized states. It is thus conrmed that the processed device is formed by GQD arrays with polydispersed GQDs.

To further study the Coulomb blockade effect in the processed graphene device, the temperature-dependent transfer curves are also measured (Fig. 4c). Obvious oscillations appear in the transfer curves at low temperatures and these oscillation peaks are gradually broadened with the temperature and disappear when the temperature is higher than 150 K. These observations are clear evidence of the Coulomb blockade effect41,42. To calculate the charge energy of the GQDs, we measured differential conductance (dI/dV) as functions of VG and VD, as shown in

Fig. 4d,e. By calculating the height of the diamond-shape regions16,39, we obtain an energy gap of about 75 meV, consistent with the estimated value of B88 meV calculated from Eg hnF=W, where nF E 106 ms 1 is the Fermi velocity

in graphene, and W is the average size of the GQDs (B7.5 nm).

Broadband photoresponse measurement. We measured the photoresponse characteristics of the processed graphene device at 20-mV source-drain bias and zero gate voltage, with illuminations on the whole device at various wavelengths, and at low temperatures (see Fig. 5). Light sources at the visible (532 nm), the near-infrared (1.47 mm) and the mid-infrared (9.9 mm,10.1 mm and 10.31 mm) were used in the photoresponse measurements. For comparison, the photoresponse of the pristine graphene device was also measured (see Supplementary Fig. S2 and Supplementary Note 2) and no obvious response was observed at a low optical input power density B633 mWcm 2,

which agrees with the previous results (only a low responsivity of about 10 mAW 1 was reported under a high optical pumping of 108 mW cm 2). The photoresponse of the processed graphene device is shown in Fig. 5af, respectively, for the visible, the near-infrared and the mid-infrared illuminations. The processed graphene device shows high photoresponse in the whole tested wavelength range (high photoresponsivity of 1.25 AW 1,0.2 AW 1 and 0.4 AW 1 in the visible, near-infrared and mid-infrared ranges, respectively). These values are about two-three orders higher than those reported in the literature based on pure monolayer graphene photodetectors. More importantly, to the best of our knowledge, this is the broadest range reported for a single graphene-based photodetector device.

The proposed graphene photodetector also possesses high sensitivity capable of nW-level optical power detection, demonstrated in Fig. 5b, d and f. The power dependence of the photo-detector is also shown at different wavelength illuminations (Fig. 5b, d and f). The photoresponse can be expressed by a power law Ipc p Pg, where Ipc is the photocurrent and P is the power of the illumination. For the visible, near-infrared and mid-infrared illuminations, g is 1.07, 0.83 and 0.95, respectively. The non-unity exponentials are the result of complex processes of electron-hole generation, trapping and recombination43, implying complex MGB electron trapping and carrier transport in the processed graphene photodetector.

Dependence on temperature and voltage and GQDs size. The bias dependence of the photoresponse of Sample C is shown in Fig. 6a. As the bias increases from zero to 0.1 V, the photo-responsivity increases up to 8.61 A W 1 correspondingly. The temperature-dependent characteristics of Sample C are shown in

Fig. 6b,c. The results show a strong dependence of the temperature on the photoresponse of the detector. The decrease of the photoresponse with the temperature could be understood as: with the increase of temperature a faster recombination of captured electrons in the MGB states occurs, thus the photoresponse is reduced.

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Figure 3 | Raman spectra and electrical characteristics. (a) Raman spectra of the pristine graphene (Sample A, black curve) and the processed graphene samples fabricated with a 2-nm thick Ti layer (Sample B, red curve) and a 20-nm thick Ti layer (Sample C, blue curve). The Raman spectra were excited at 532 nm with an incident power o1 mW (laser spot diameter of 500 nm). The results show that almost no defect states are in Sample

A as only G-band (1,578 cm 1) and 2D band (2,666 cm 1) are observed, while Sample C has the largest number of defect states as the ratio of the D-band (1,338 cm 1) to the G-band (1,578 cm 1) Raman intensities is the highest. The inset shows the microscope image of Sample C, wherethe size (B20 mm 10 mm) of the monolayer graphene and the source (represented by S) and the drain (represented by D) terminals are indicated.

(b) The electrical characteristics as a function of the gate voltage (transfer curve) of Sample A; the inset shows the currentvoltage (IV) curves of this sample under different gate voltages. The Dirac point occurs at VGB60V. (c) The electrical characteristics as a function of the gate voltage of Sample

C; the inset shows the IV curves of this sample under different gate voltages.

The gate-bias dependence of photoresponse of Sample C is shown in Fig. 7a,b. As the gate-bias decreases from 20 V to 20 V, the photocurrent increases and the device becomes a more p-type material seen from the transport characteristics in Fig. 3c. This is in contrary to the normal understanding that the electron-hole pair generation by direct photoexcitation would be decreased as the quasi-Fermi level moves below the valence band edge (fewer valence band states are lled with electrons). The observed gate-bias dependence of the photocurrent, we believe, is attributed to the effective electron-hole pair generations by the hole impact ionization assisted by the external electrical eld, which was also observed in the carbon nanotube system44. As shown in Fig. 7b, the rest mass energy of one photo-excited hole in a higher-energy level is combined with the kinetic energy to create a lower-energy hole plus several electron-hole pairs, expressed as:

hk Kh ! hj n e h

1 where Kh is the kinetic energy of holes induced by the external electric eld, hk and hj indicate the higher-energy and lower-energy level holes, respectively, (e h) refer to electron-hole

pairs, and n is an integer. As the gate-bias decreases (the quasi-Fermi level decreases), more holes are involved in the impact ionization process inducing more electron-hole pairs. Thus, there

is a competing effect between the hole impact ionization and the direct photoexcitation for electron-hole pair generation. The former is more efcient than the latter when the material is converted to be more p-type.

Figure 7c shows the photoconductive gain spectra of the Samples B and C with different average GQD sizes. For both devices, the photoconductive gain increases with the photon energy, arising from the increase of the carrier MEG efciency. As the photo-excited electron occupies a higher-energy level in the conduction band under a shorter wavelength illumination (532 nm), it undergoes more exciton excitations through the impact ionization process when scattered into the lower-energy levels in the conduction band. This leads to an increase of the conduction-band electrons thus the captured electrons in the MGB as well, resulting in a higher photoconductive gain. In addition, Fig. 7c shows that the Sample B with a larger average GQD size (B20.5 nm) has a smaller photocurrent gain than the

Sample C with a smaller average GQD size (B7.5 nm). As the size of the GQDs increases, the defect density decreases as evidenced in the Raman measurement results (Fig. 3a), resulting in a fewer number of photo-excited electrons captured by the MGB states.

Theoretically, we can quite accurately calculate the photoconductive gain as a function of bias by the microscopic density matrix method30, but the microscopic analysis requires

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Figure 4 | Carrier transport in the proposed graphene photodetector. (a) The IV characteristics of Sample C at 294 K and 12 K, respectively.

The fact that the IV characteristic is linear at 294 K, while becomes nonlinear and maintains highly symmetric indicates that a carrier transport barrier is formed in the GQD array structure, which is caused by the Coulomb blockade of charges. (b) The temperature dependence of the drain current of Sample C is best tted in a T 1/3 scale. The results show that VRH is the main carrier transport mechanism. The inset shows a good liner tting of the photocurrent in the logarithmic scale with respect to T 1/3. This suggests that the processed graphene device consists of polydispersed GQDs. (c) The transfer curves of Sample C at different temperatures, in which strong temperature dependence is shown. (d) Differential conductance as a function of VG and VD of Sample C at 12 K and (e) its magnication plot. The diamond-shaped region suggests the Coulomb blockade effect inuences the carrier transport.

intensive derivations and accurate prediction of certain parameters, which are beyond the scope of this paper. Here we provide a simple macroscopic estimation on the photoconductive

gain based on the population dynamic analysis. In our calculation, the source-drain bias and gate voltage are set as 20 mV and 0 mV, respectively. Here we assume the electron

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Figure 5 | Broadband photoresponse measurements of the graphene photodetector. (a) Time-dependent photocurrent measurements of Sample C when optically pumped in (a) the visible (VIS) (532 nm), (c) the near-infrared (NIR) (1.47 mm) and (e) the mid-infrared (MIR) (B10 mm) ranges. The GQD photodetector shows high photoresponses: B1.25 AW 1, 0.2 AW 1 and 0.4 AW 1 in the visible, NIR and MIR ranges, respectively. b, d and f show the power dependence of the photocurrents in the three ranges, respectively, where both axes are in the logarithmic scale. The results also demonstrate that the proposed graphene photodetector also possesses a high photosensitivity property, capable of a nW-level input optical power detection. (g) The time-dependent photocurrent measurement over a 20-periods onoff operation under a NIR light (1.47 mm) illumination, demonstrating the reliability of the photodetector.

populations in the conduction band and the MGB are n1 and n2, respectively. The recombination rate of electrons is RB1 ps (ref.45), the measured current is I, the capture rate of the MGB is a,

the lifetime of the trapped electrons is t1, the transition time is tt, and the photogeneration rate is b. We assume the number of electrons per absorbed photon due to both electron and hole

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0.22 0.24 0.26

Figure 6 | Temperature and time dependence of the photocurrent. (a) the IV curves of Sample C with and without 532 nm illuminations. A high photoresponsivity of 8.61 AW-1 is achieved at a VD voltage of 0.1 V. (b) Time-dependent photocurrent measurements at different temperatures under the visible light illumination. (c) Temperature dependence of the photocurrent of Sample C. The photoresponse decreases with the temperature, shown in b and c, resulting from a faster recombination of captured electrons in the MGB at a higher temperature, thus the lifetime of these trapped electronsis reduced.

impact ionizations is w. As the electron multiplication factor in perfect graphene sheet is estimated to be around four30, we reasonably assume that this factor in our device is not high neither. The hole impact ionization is not expected to be strong, as a zero gate voltage is applied resulting in a low hole density and a low drain-source voltage is applied. In this case, we assume the factor w induced by both electron and hole impact ionizations is a small value o10. Under illumination, the variation of the number of the electrons in the conduction band originates from photogeneration, carrier multiplication (carrier impact ionization), recombination and relaxation to the MGB. The variation of the electrons on the MGB states results from excitation to the conduction band, relaxation from the conduction band, and recombination with holes in the valance band. Thus, we have

dn1dt wb Rn1 an1 0 2

dn2dt an1

n2 t1

Iewbt1 tt 6

From equations (5) and (6), we obtain a is about 106 and Z is about 10 6 by applying appropriate values, where t1 is about 30 s obtained from Supplementary Fig. S3 and Supplementary Note 3 and tt is about 250 ns achieved from Supplementary Fig. S4. We can then calculate the photoconductive gain of the fabricated graphene photodetector by using43 G Z t1=tt

, which leads to a

photoconductive gain of about 120.

We could also calculate the photoconductive gain by G N

a w a R

electrons/Nphotons where Nelectrons and Nphotons are the

number of generated electrons that measured in the FET and the number of incident photons, respectively. We take the measured photocurrent of 6.961 nA and the visible irradiation of5.57 nW on Sample C for calculation. A photoconductive gain of B127 is obtained at a 20-mV source-drain bias and a zero gate voltage, in good agreement with that obtained from the above population dynamic analysis.

DiscussionWe attribute the high and broadband photoresponse of our devices to the electron trapping in the defect MGB enhanced by the carrier impact ionization. We now discuss additional sets of

0 3

n1 n2

Ie tt 4 From equations (2)(4), the capture rate can be expressed as

a

Iewb ttR 1

t1

I ewb tt

Iewb ttRt1 5

and the quantum efciency Z of the photo-excited and secondary-generated electrons captured by MGB is expressed as

Z

an1

b

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2830 ARTICLE

a b

9

8

7

6

Photocurrent (nA)

ei

ej

hi

5

4

3

hj

hk

+Kh

2 20 10 0

VG (V)

10 20

c

100 Sample C Sample B

10

1

0.0 0.5 1.0Photon energy (eV)

1.5 2.0 2.5

Photoconductive gain

Figure 7 | Photoresponse dependence on the gate voltage and photon energy, and GQD sizes. (a) The gate voltage dependence of the photoresponse characteristics of Sample C. The increase of the photocurrent with a decrease of the gate voltage can be attributed to the hole impact ionization effect assisted by the external electrical eld. As the gate-bias decreases (the quasi-Fermi level decreases, see Fig. 3c), more holes are involved in the impact ionization process to generate more electron-hole pairs. Thus, there is a competing effect between the hole impact ionization and the direct photoexcitation. The former is more efcient than the latter when the material is converted to be more p-type. (b) Hole impact ionization process in the processed graphene photodetector. The rest mass energy of the hole in the high energy level hk is combined with the kinetic energy Kh to create a lower-energy hole and electron-hole pairs as: hk Kh-hj n (e h) where Kh is the kinetic energy of holes induced by the external electric eld. hk and hj

indicate the higher-energy and the lower-energy level holes, respectively, (e h) refer to electron-hole pairs and n is an integer. Thus, with an external

electrical eld, more electron-hole pairs can be generated through hole impact ionization. (c) The photoconductive gain spectra of Sample B and Sample C, respectively, at different photon excitation energies. The photoconductive gain rises as the photon energy increases, which can be attributed to the increase of the carrier MEG effect. With a higher photon energy excitation (532 nm), a photo-generated electron occupies a higher-energy level and generates multiple secondary electrons when scattering to the lower-energy levels in the conduction band through the MEG process, thus leading to a larger number of the captured electrons in the defect MGB.

reasons that address potential explanations of these ndings. One is based on the built-in-eld scenario6,7,14,46. In this scenario, the slope of the IV curve should keep a constant with and without illuminations, and the device should have a certain output photocurrent even when no bias is applied owing to the presence of the built-in-eld which separates the generated electron-hole pairs. In addition, the photoresponsivity should be at the level of several mAW 1. These are in contrast to our observations. Another possible reason proposed in the literature is relied on the photo-thermoelectric effect8,9,47. This effect only occurs in asymmetric situations, either with asymmetric materials such as graphene pn junctions or with asymmetric illuminations. In our case, the channel material is built by just a single layer graphene sheet (no junction exists) and the illumination is uniform. Therefore, none of the above explanations proposed in the literature can explain the observed high photoresponsivity.

Our device covers a broad wavelength range, including the mid-infrared. The mid-infrared detector has wide applications ranging from medicine48, remote sensing49, environment monitoring50, to telecommunications51. The traditional mid-infrared HgCdTe and quantum well infrared photodetector photodetectors require complicated fabrication and growth of single-crystalline alloys using expensive molecular beam epitaxy or metalorganic chemical vapour deposition, thus the fabrication process is complex and has a high cost. The proposed graphene photodetector has a much lower cost and a lower operation voltage, and is compatible with the complementary metal-oxide-semiconductor technology.

Despite the reported high photoresponsivity of our graphene photodetector over a broadband range, it needs to be mentioned that one drawback of the current device is that the operation speed is not high, which is mainly limited by the relatively long trapped electron lifetime in the defect states, caused by the

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present chemical fabrication/etching method introducing extremely complex deep surface and edge defects. The lifetime of the trapped electrons in the MGB is inuenced by many factors, for example, defect types, trap-state density and defect trap depths. It has been demonstrated that by varying the types and the densities of defects one can signicantly reduce the electron trap lifetime in various material systems5255 from tens of seconds down to a few milliseconds or even shorter. Thus we reasonably believe that the trapped electron lifetime in graphene can also be decreased by engineering the defect types and defect densities, for example, by achieving mechanically fabricated quantum dot edge defects with electron-beam lithography16 or well-dened extended defects22 using nickel supporting layer, or atomic-level nanoribbons with edge defects by a surface-assisted coupling method32. Certainly, by decreasing the lifetime of the trapped electrons, the photoresponsivity of the photodetector is sacriced. This, however, can be compensated by increasing the low carrier mobility (caused by the VRH effect and the high defect scattering) in our highly disordered GQD array devices. This can be achieved by using more accurate fabrication techniques, for example, electron-beam lithography, helium ion lithography56, scanning tunnelling microscope lithography57 and focused ions beam58 to reduce the disorders and defect density, and by exploring other nanostructures such as nanoribbons and nanomeshs (without relying on the VRH effect), which are now under investigations. Therefore, we could conduct further band structure engineering of graphene to improve the operation speed as well as the temperature performance while still maintaining a high photoresponsivity of the proposed photodetector.

In summary, we have demonstrated a new type of broadband pure monolayer graphene-based photodetector through band structure engineering. The demonstrated pure graphene photo-detector shows a high photoresponsivity, which are about twothree orders of magnitude higher than those in the former reports. Compared with other band structure engineering approaches, the proposed fabrication method is simpler and a larger scale of device can be easily fabricated. Further device performance improvement can also be achieved through defect engineering and nanostructure engineering. The proposed scheme opens up exciting opportunities for band structure engineering of graphene for future graphene-based optoelectronic applications.

Methods

Device fabrication. We fabricated pure graphene photodetectors in a FET structure (inset, Fig. 3a) for characterizations. First, a monolayer graphene was fabricated by using the micromechanical exfoliation technique on a 285-nm SiO2/Si substrate where the Si substrate is highly doped. Two electrode terminals (Ti/Au: 20 nm/80 nm), that is, the source and the drain terminals, were then formed by standard photolithography and lift-off techniques on top of the graphene. To clean the photoresist, Ar/H2 annealing was applied at 400 degree for 2 h. Then, the gate terminal was fabricated on the bottom of the Si substrate. After that, a thin Ti sacricial layer with different thicknesses was deposited by e-beam evaporation. Finally, the GQD array structures were formed by removing the Ti sacricial layer using hydrouoride, hydrodioxide (H2O2) and deionized water with a volume ratio of 1:1:200 (the etching rate was around 20 nm min 1). The thicker the deposited

Ti layer, the smaller the average size of the GQDs after etching. Thus the Ti sacricial layer with various thicknesses can be deposited to control the average size of the GQDs.

Device characterization. Raman spectra measurements were used to characterize the density of defects of the fabricated devices (Fig. 3a). The Raman spectra were excited at 532 nm with an incident power o1 mW (laser spot diameter of 500 nm).

For the pristine graphene sample (Sample A), the G-band and the 2D band are at 1,578 cm 1 and 2,666 cm 1, respectively. For the processed graphene samples B and C, three defect-related Raman peaks are excited, namely D, D0 and D D0

located around 1,338 cm 1, 1,620 cm 1 and 2,934 cm 1, respectively. The D and D0 peaks originate from double-resonance processes at the K point in the presence of defects, involving, respectively, intervalley (D) and intravally (D0) phonons37. A higher number of defects were observed in Sample C than in Sample B due to the

higher Raman ID/IG ratio. As shown in Fig. 3b,c, after defects are introduced, the on-state electric conductance decreases to 0.14G0.

Carrier transport in the device. We measured the electrical properties of the devices at different temperatures (Fig. 4ac). It is observed that the IV characteristics of the processed graphene is linear at 294 K for VD changes from 1 V

to 1 V, while it become remarkably nonlinear and highly symmetric, and the conductance reduces at low biases, at 12 K. These phenomena indicate the Coulomb blockade effect exists as carriers transport in the processed graphene sample. It is noted that the Schottky barrier formed between the contact and the graphene layer could also generate a decreased conductance at a lower bias, but this effect is excluded because it would also introduce a nonlinear effect at room temperature and an asymmetric behaviour at low temperatures41, which are not observed in our measurements. This means the observed phenomena comes from the graphene FET channel.

Photoresponse measurement. The photoresponsivity measurement was performed in a digital deep level transient spectroscopy (BIORAD) system with a compressor (CTI-CRYOGENICS, 8200). Three light sources (visible: 532 nm semiconductor laser; near-infrared: 1470 nm bre laser; mid-infrared: tunable quantum cascade laser from 9.75 to 10.48 mm) were used to illuminate the whole device. The electrical characteristics were collected by a semiconductor device analyser (Agilent, B1500A).

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Acknowledgements

Y.Z.Z would like to thank Dr Hong Li for discussions. This work is supported by the

grant (M58040017) from Nanyang Technological University (NTU), and partially by the

Ministry of Education, Singapore (MOE2011-T2-2-147 and MOE2011-T3-1-005).

Author contributions

Y.Z.Z. and Q.J.W. designed the experiment. Y.Z.Z., T.L. and Q.J.W. wrote the

paper. Y.Z.Z., B.M., X.H.L. and G.Z.L. performed device fabrication and optical

experiments. Y.Z.Z., T.L. and X.N.H. performed theoretical analyses and simulations.

Q.J.W. supervised the project. All authors discussed the results and commented

on the manuscript.

Additional information

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How to cite this article: Zhang, Y. et al. Broadband high photoresponse from pure

monolayer graphene photodetector. Nat. Commun. 4:1811 doi: 10.1038/ncomms2830

(2013).

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