Zero-Bias Operation of CVD Graphene Photodetector with Asymmetric

Dec 4, 2017 - The responsivity of a graphene photodetector was substantially enhanced by modulating the potential gradient in a chemical vapor deposit...
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Zero-Bias Operation of CVD Graphene Photodetector with Asymmetric Metal Contacts Tae Jin Yoo, Yun Ji Kim, Sang Kyung Lee, Chang Goo Kang, Kyoung Eun Chang, Hyeon Jun Hwang, Nikam Revannath, and Byoung Hun Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01405 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Photonics

Zero-Bias Operation of CVD Graphene Photodetector with Asymmetric Metal Contacts Tae Jin Yoo,1 Yun Ji Kim,1 Sang Kyung Lee,1 Chang Goo Kang,2 Kyoung Eun Chang,1 Hyeon Jun Hwang,1 Nikam Revannath,1 Byoung Hun Lee1* 1

Center for Emerging Electronic Devices and Systems, School of Material Science and Engineering, Gwangju Institute of Science and Technology,123 Cheomdangwagi-ro, Buk-gu, Gwangju, 61005, South Korea 2 Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, 29 Geumgu-gil, Jeongeup-Si, Jeollabuk-Do, 56212, South Korea KEYWORDS : Graphene, Graphene photodetector, Zero bias operation, Asymmetric structure, Graphene/metal contact. ABSTRACT: The responsivity of graphene photodetector was substantially enhanced by modulating the potential gradient in a chemical vapor deposition-grown graphene channel using asymmetric metal contacts such as Ti, Pd, and Au for the source and drain. The photoresponsivity of a graphene photodetector with asymmetric Au-Ti contact combination increased to 52 mA/W at an illumination power density of 0.25 µW/cm2; this photoresponsivity is ~ 63 times higher than that of a graphene photodetector having a symmetric Au-Au contact combination (0.82 mA/W)

Graphene has been extensively studied for diverse photodetector applications because of its extremely short carrier lifetime and unique carrier multiplication process1–8. Very highspeed photodetectors have been demonstrated in a wide spectral range from UV to terahertz6,9–11. Operation at 65 GHz has been reported with a graphene photodetector integrated with a silicon waveguide9. Uniform absorption rate within visible wavelengths was demonstrated with chemical vapor deposition (CVD) graphene12. Infrared absorption with a double-layer structured graphene photodetector has been reported13. Terahertz absorption was demonstrated with an antenna structure integrated graphene photodetector14. In addition, carrier multiplication by the Auger process can amplify the photocarrier generation by a factor of 5~7 independent of wavelength1–4. Despite the numerous successful demonstrations of graphene-based photodetectors, the small photo absorption volume due to the monolayer thickness and the extremely short carrier lifetime has been a serious challenge in practical applications1,5,15. The efficiency of graphene-based photodetectors has been restricted by the design limit because only the photocarriers generated at the graphene near the metal contact can contribute to the photocurrent because of the short carrier lifetime16,17. Furthermore, when the band structure of a graphene photodetector is symmetric for both the source and drain sides, the net photocurrent does not flow without a drain bias because the photocarriers generated in the source side and drain side have opposite charges and are compensated in the middle of the graphene channel17,18. When a drain bias is applied to the graphene photodetectors, the dark current also increases to the order of microamperes. This is 1,000 times higher than the

typical dark current of silicon-based photodetectors in nanoampere order. Thus, a graphene photodetector having different metal contacts such as palladium (Pd) and titanium (Ti) for source and drain was proposed by Mueller et al. to overcome these challenges18. Non-zero net photocurrent could be obtained in this device because the difference in metal work function used in the source and drain generates a gradient in the Fermi level of the exfoliated graphene channel. In this device, the external photoresponsivity was 6.1 mA/W at 1550 nm, but the dark current was on the order of milliamperes because 0.4 V of drain bias was applied to achieve high photoresponsivity. The actual value of the difference in the effective work functions of the source/drain metals was not reported, but a rough estimate based on the material parameters reported in other works is ~0.8 eV, and the corresponding internal potential is ~ 50 mV. Singh et al. also reported another epitaxially grown graphenebased photodetector having asymmetric metal contacts with gold (Au) and aluminum (Al)19. The external photoresponsivity was 4.9 mA/W at 632.8 nm, and the dark current of this device was ~2 µA at 0 V. The active channel area of this device was 0.1 mm2, which is much larger than that of a graphene flake-based photodetector. The difference in the work function of the source/drain metals was 1.41 eV. Although these prior works paved the way to show the advantage of using an asymmetric metal contact structure, they used exfoliated graphene and epitaxial graphene to ensure the maximum carrier lifetime and defect-free contact between the graphene and metal. Thus, there is no guarantee that this structure would work for CVD graphene, which has a much higher defect density than epitaxial graphene. Because the CVD-

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grown graphene inevitably has various intrinsic defects20,21, it is necessary to examine whether the Fermi level pinning between metal and graphene may hinder the use of an asymmetric work function scheme for a large area and practical implementation of a CVD graphene-based photodetector22. In this work, CVD graphene-based photodetectors were fabricated with an asymmetric metal contact structure with various metal combinations (Au, Pd, Ti), and the actual differences in the effective work functions of the contact metals were measured. With a 0.34 V difference in the work function between the Ti and Au contacts, a maximum external photoresponsivity up to 52 mA/W was achieved, which is 10 times higher than that reported for epitaxial graphene photodetectors, even though the active area was smaller than those of the epitaxial-grown graphene photodetectors and exfoliated graphene flake photodetectors.

Figure 1: Fabrication process of graphene photodetectors having symmetric and asymmetric metal contacts.

METHOD Device fabrication. A 1cm × 1cm graphene sheet grown on copper (Cu) foil using a thermal CVD process was used. The monolayer graphene was transferred to a silicon dioxide (SiO2) (90 nm)/silicon substrate using a poly(methyl methacrylate) (PMMA)-assisted wet transfer method23,24. The area ratio of Raman spectrum, A(2D)/A(G), measured after the transfer process, was 5.45, and the D peak was negligible, indicating the graphene transfer was successfully performed (see Figure S1)25. Figure 1 illustrates the fabrication process for the graphene photodetector having asymmetric metal contacts. The graphene channel was patterned using an i-line lithography process, metal wet etch, and oxygen ashing process in sequence. The metal wet etch step was used because a 30 nm Au hardmask layer was used to minimize the polymer contamination from the photoresist. Then, 100 nm Au was deposited and patterned. In this step, the Au hardmask layer on the graphene channel was etched together. In the case of an asymmetric metal-contact structure, Pd and Ti drain contacts were patterned with an image-reverse lithography process and lift-off process after patterning the source metal (Au) contact. Finally, 30-nm aluminum oxide (Al2O3) was deposited as a passivation layer, followed by a final passivation anneal performed in high vacuum (~ 10−7 Torr) at 300 °C for 1 h26 to ensure a stable operational condition for the graphene photodetector. Electrical and optical characterization. The electrical characteristics of the base graphene field effect transistors (GFETs) were measured using a semiconductor parameter

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analyzer (Keithley 4200), and the optical characteristics of the graphene photodetectors were analyzed using five light emitting diode (LED) sources (470, 530, 625, 850, 980 nm) and halogen lamp. The power of the illuminated light was measured with a thermopile sensor (Newport 919P Thermopile sensors). To measure the devices at the zero-bias operating condition, a low-noise current preamplifier (SR 570) and a dynamic signal analyzer (Agilent 35670A) were used.

RESULTS AND DISCUSSION Figure 2 (a)–(c) show the representative Id-Vg curves of the graphene photodetectors with different source-drain metal combinations: Au-Au, Au-Pd, and Au-Ti, respectively. The Au-Au case shows a typical symmetric I-V curve. Because the different metals have different contact resistances, the I-V curves shown in Figure 2 were compensated for the contact resistance obtained from a constant mobility model27. Even after the compensation, there is a slight difference in the on current or off current. This can be attributed to the difference in the fabrication process, because the Au-Pd case and Au-Ti case went through one more lithography step for the lift-off process. At the minimum point of the I-V curve at the Dirac voltage, the majority carrier type changes from hole to electron as the Fermi level of the graphene passes through the center of the upper and lower Dirac cones. When different metals are used, there is a hump in the I-V curve. The hump at the second Dirac voltage is generated because a p-n junction is formed in the graphene by the work function difference as illustrated in Figure 2 (d) and (e)28. The first Dirac voltage was near 0 V for both the Au-Pd and Au-Ti cases, indicating that the additional lift-off process did not seriously affect the intrinsic properties of the graphene channel. The distance between the original Dirac voltage and the second Dirac voltage is determined by the difference in the work function. Thus, from the I-V curve, we can see that the Au-Ti combination generated a more asymmetric contact than the Au-Pd combination. The exact value of the two Dirac voltages can be extracted by fitting the experimental I-V curves with a modified mobility calculation model (see Figure S3)27,28. The second Dirac voltages of the Au-Pd and Au-Ti cases were −5.5 V and −27 V, respectively. Because the difference between the Dirac voltages is proportional to the difference in the effective work function of the source and drain metals, the effective difference in the work function can be extracted by28 фeff = фG (фAu) + ΔE = фG + h vF [π(C/e) | VDirac,0−VDirac,1|]1/2 where фG is the work function of graphene and ∆E is the amount of Fermi-level shift in reference to the Fermi level of the source metal, which can be denoted as фM1 (фM2). The difference in the effective work function of Au-Pd was 0.15 eV and that of Au-Ti was 0.34 eV. The difference in the experimentally extracted effective work function is quite different from the differences in the vacuum work function of the metals used in this work, which are 0.12 eV for Au-Pd and 0.95 eV for Au-Ti. This kind of discrepancy has often been explained with the Fermi level pinning or the interface dipole at the metal-graphene interface20,22. If the difference in the effective workfunction of Au-Ti and Au-Au on CVD graphene are translated into the pinning factor S, the pinning factor is ~ 0.34. Since no pinning state corresponds to S=1, this result means

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ACS Photonics that the CVD graphene indeed resulted in the considerable amount of Fermi level pinning. However, further study is necessary to understand the general mechanism for the Fermi level pinning, because the role of graphene defects or other factors has not been fully understood. Yet, using the electrical characterization approach, we have confirmed that the different level of potential gradient has been generated in the graphene channel using different contact metals. The differences in the electrical characteristics are reflected in the optical responses of graphene photodetectors as shown

in Figure 3. Id-Vg curves measured with halogen lamp illumination showed a distinct shift from the VDirac,1 proportional to the effective work function difference. The amounts of VDirac shift were +0.3 V, +0.8 V, and +2 V for the Au-Au, Au-Pd, and Au-Ti cases, respectively. This shift has been explained with the photo gating effect29. Because the photo gating effect is enhanced by stronger photo carrier separation in a device with a higher potential gradient, the enhanced photo gating effect is an additional evidence supporting the presence of an internal potential gradient.

Figure 2: Transfer characteristics (Id-Vg) of graphene FETs with metal contacts having (a) symmetric work functions and (b), (c) asymmetric work functions. VDirac,0 is the intrinsic Dirac voltage of graphene, and VDirac,1 represents the charge neutrality point generated by the p-n junction formed in a graphene channel, which is due to the asymmetric work function of the metals used for the source and drain. (d)–(f) Schematic diagrams of graphene photodetectors with different source/drain metal combinations. ф1 is the work function of Au and ф 2 is the work function of the asymmetric metal. E0 is the vacuum level and EF is the Fermi level of the metal and graphene. ∆EG represents the doping state of the graphene channel. The black-filled (empty) circle is an electron (hole) that is generated by the light absorption. ∆Eeff is the difference in the effective work functions of the metals used for source and drain. Because of the internal potential generated by the work function difference of both metal contacts, the photocarriers can be more efficiently collected with the asymmetric metal contacts. The ∆Eeff for the Au -Ti case is 0.34 eV and that for the Au-Pd case is 0.15 eV.

Figure 3: Electrical characteristics of graphene photodetectors with (a) Au-Au contact, (b) Au-Pd contact, and (c) Au-Ti contact. The illumination power of the halogen lamp was 0.25 µW/cm2, and 0.1 V of drain voltage was applied.

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Figure 4: (a) Channel length dependence of photocurrent generation for different metal combinations under illumination of 530 nm LED. The Vg-VDirac voltage was −20 V, to induce p-type doping in a graphene. (b) Dark current for different metal combinations measured at 2µm channel length.

Figure 5: (a) The wavelength dependence of photocurrent generation for different metal combinations. (b) The wavelength dependence of dark current measured for different metal combinations. Five LEDS with wavelengths from 470, 530, 625, 850, to 980 nm at an illumination power density of 1mW/cm2 were used. Channel length was 4 µm.

Similar trend was observed at the photocurrent measured while varying the channel length to see the lateral field dependence as shown in Figure 4(a). During the photocurrent measurement, the overdrive gate bias equal to the Vg-VDirac was fixed at −20 V, and a small drain bias of ~ +0.1 V was applied to enhance the stability of the measurement. In the case of a graphene photodetector, only the region within ~ 0.2 µm from each side of the source and drain can contribute to the photocurrent because of the extremely short carrier lifetime16,17. However, as the channel length decreases, the total resistance of the device decreases and the effective drain voltage applied to the channel increases. As the band bending in a channel region increased, the photocurrent increased by ~ 200% as expected. In particular, the graphene photodetector with a Au-Ti contact showed the highest photocurrent, ~ 1.7 µA. The impacts of ∆Eeff on the dark current do not seem to be more significant than the photocurrent as shown in Figure. 4 (b). Compared to the symmetric Au-Au contact (∆Eeff = 0 eV), the average dark current of the Au-Ti contact (∆Eeff = 0.34 eV) increased by 159%, but it slightly decreased in the case of the Au-Pd contact (∆Eeff = 0.15 eV). Then, the wavelength dependence of photocurrent was analyzed using five different LED sources (470, 530, 625, 850, 980 nm), as shown in Figure 5(a). Same bias condition used for the channel length dependence experiment was applied to 4µm channel length devices. In all wavelengths, relatively weak wavelength dependence was observed for all metal combinations. The amount of photocurrent was also propor-

tional to the difference in the effective workfunction of metal contacts. In case of Au-Ti contact, 0.71 µA of photocurrent was generated which is 251% increase compared to that of Au-Au contact. Au-Pd contact showed 140 %. Similar trend was observed for the dark current as shown in Figure 5(b). Comparing to the symmetric metal contact graphene photodetector (Au-Au), the dark current increased by 174% for Au-Ti contact and 121% for Au-Pd contact. Overall, weak wavelength dependence is a typical characteristic of graphene photodetector.

Figure 6: The amount of photocurrent as a function of work function difference of the source and drain metal contact under the zero-bias operation (Vd = 0 V, Vg-VDirac= 0 V) under a halogen lamp at an illumination power density of 0.25 µW/cm2. The red square dots are the dark current under the zero-bias operation.

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ACS Photonics Finally, the photocurrent was measured at the zero-bias operating condition (Vd = 0 V, Vg-VDirac = 0 V). Because there is no external bias, the photocurrent can be modulated only by the potential gradient generated by the asymmetric metal contact. The graphene photodetectors with 2 µm channel length were used to maximize the photocurrent. The photocurrent increased linearly, proportional to the difference in the effective work function of the source and drain metals as shown in Figure. 6. The slope of the fitting curve shown as a dashed line in Figure. was 44.7 nA/eV. The photoresponsivity of the graphene photodetector with a Au-Ti contact was 52 mA/W, and the dark current was 70 nA. The dark current also increased by 125% as the contact structure changed from a symmetric (Au-Au) to an asymmetric (Au-Ti) structure. Table 1 shows the performance comparison between the graphene photodetectors. The photocurrent of the graphene photodetector increased by more than 6300% using the asymmetric contact compared to the results reported in the literature, even without any drain bias. On the other hand, the dark current could be drastically reduced to the order of nanoamperes from the microampere to milliampere range because of the zero bias operation. Table 1. Characteristics of symmetric graphene photodetector and silicon photodiodes compared with asymmetric graphene photodetector (this work). Silicon photodetector

Symmetric graphene photodetector

Asymmetric graphene photodetector

Contact metal

-

Au-Au

Au-Ti

Dark current

20 pA (10 mV)

~ 30 µA (0.1 V)

70 nA (0 V)

−2

2

−7

2

−7

function asymmetry of each metal contact; characteristics of the light source used in this experiment

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was partially supported by Creative Materials Discovery Program (2015M3D1A1068062) and Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Korea

REFERENCES (1) Brida, D.; Tomadin, A.; Manzoni, C.; Kim, Y. J.; Lombardo, A.; Milana, S.; Nair, R. R.; Novoselov, K. S.; Ferrari, A. C.; Cerullo, G.; Polini, M. Ultrafast Collinear Scattering and Carrier Multiplication in Graphene. Nat. Commun. 2013, 4, 1987. (2) Winzer, T.; Knorr, A.; Malic, E. Carrier Multiplication in Graphene. Nano Lett. 2010, 10, 4839–4843. (3) Rana, F. Electron-Hole Generation and Recombination Rates for Coulomb Scattering in Graphene. Phys. Rev. B 2007, 76, 155431. (4) Tielrooij, K. J.; Song, J. C. W.; Jensen, S. A.; Centeno, A.; Pesquera, A.; Zurutuza Elorza, A.; Bonn, M.; Levitov, L. S.; Koppens, F. H. L. Photoexcitation Cascade and Multiple Hot-Carrier Generation in Graphene. Nat. Phys. 2013, 9, 248–252. (5) Urich, A.; Unterrainer, K.; Mueller, T. Intrinsic Response Time

2

Active area

10 cm

~ 10 cm

10 cm

Responsivity

0.1~0.6 A/W

< 10 mA/W

52 mA/W

Spectral range

300~1100 nm

470~980 nm (LED)

CONCLUSION We have demonstrated that the zero-bias operation of a CVD graphene photodetector is feasible with asymmetric source and drain metal contacts. 52 mA/W of photoresponsivity at 70 nA of dark current were achieved. Our work indicates that the performance of graphene photodetector without any external bias can be further improved by optimizing the channel length and increasing the difference in the effective work function of the contact metals.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Raman spectrum of graphene channel and SEM image of a Au-Ti device; total resistance fitting with experimental data to extract the internal potential by work

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Zero-Bias Operation of CVD Graphene Photodetector with Asymmetric Metal Contacts Tae Jin Yoo, Yun Ji Kim, Sang Kyung Lee, Chang Goo Kang, Kyoung Eun Chang, Hyeon Jun Hwang, Nikam Revannath, Byoung Hun Lee1*

The responsivity of graphene photodetectors was substantially enhanced by modulating the potential gradient in a chemical vapor deposition-grown graphene channel using asymmetric metal contacts such as Ti, Pd, and Au for the source and drain. The photoresponsivity of a graphene photodetector with asymmetric Au-Ti contact combination increased to 52 mA/W at an illumination power density of 0.25 µW/cm2; this photoresponsivity is ~ 63 times higher than that of a graphene photodetector having a symmetric Au contact combination (0.82 mA/W)

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