Intrinsic photocurrent characteristics of graphene ... - OSA Publishing

Intrinsic photocurrent characteristics of graphene photodetectors passivated with Al2O3 Chang Goo Kang,1 Sang Kyung Lee,1 Sunhee Choe,1 Young Gon Lee,1 Chang-Lyoul Lee,2 and Byoung Hun Lee 1* 1

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School of Materials Science and Engineering, Department of Nanobio Materials and Electronics Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea * [email protected]

Abstract: The intrinsic photo-response of chemical vapor deposited (CVD) graphene photodetectors were investigated after eliminating the influence of photodesorption using an atomic layer deposited (ALD) Al2O3 passivation layer. A general model describing the intrinsic photocurrent generation in a graphene is developed using the relationship between the device dimensions and the level of intrinsic photocurrent under UV illumination. ©2013 Optical Society of America OCIS codes: (040.5160) Photodetectors; (160.4236) Nanomaterials.

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1. Introduction Graphene, a single layer of carbon atoms, has attracted much attention as a candidate material for photodetectors because graphene can absorb 2.3% of incident light in the infrared to visible wavelength range, even at a one atomic layer thickness [1]. This is remarkably high absorption efficiency because a 15 nm layer of Si or 20 nm layer of GaAs is required to absorb the same amount of incident light [2–4]. The photon-induced carrier multiplication in graphene is another advantage for high efficiency photonic device applications [5,6]. The combination of high carrier mobility and strong interaction with light at a wide range of wavelengths make graphene an excellent candidate material for novel photonic devices [1,7– 10]. In particular, a two-dimensional structure of graphene is another merit for scaled #193500 - $15.00 USD Received 8 Jul 2013; revised 16 Sep 2013; accepted 17 Sep 2013; published 25 Sep 2013 (C) 2013 OSA 7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.023391 | OPTICS EXPRESS 23392

optoelectronic device integration, especially in high speed monolithic optical interconnect systems [3,11]. Thus, the photo-electrical response characteristics of carbon-based materials including carbon nanotubes (CNTs) and graphene have been investigated by several research groups [12–16], which have uncovered several challenges associated with photonic applications of graphene. For example, Chen et al. [15] reported that an oxygen photodesorption lowered the conductance of p-type single-walled carbon nanotubes (SWNTs) and a gradual re-adsorption onto the SWNTs leads to the recovery of sample conductance when the light is turned off. Shi et al. [12] showed a negative shift of Id-Vg curves and gradual decrease in drain current due to photon-induced oxygen desorption which dedopes p-type graphene. Sun et al. [13] reported a change in a resistance due to a photo-induced molecular desorption in a graphene film under UV illumination. A rapid decrease in photocurrent due to the photodesorption in the visible to UV wavelength range has also been reported [12]. These are serious problems for photonic device applications because the photodesorption shifts the baseline of photocurrent, which results in significant scattering in the sensitivity of graphene photodetectors. These issues are related to the high surface-to-volume ratio of graphene, which makes graphene especially sensitive to environmental factors, thereby incurring unintentional doping, unstable device operation, and poor performance. In particular, the slow change in resistance under illumination caused by the photodesorption limits the speed of photo-response from a few seconds to hundreds of seconds. Furthermore, since this slow response often dominates the photo-response characteristics, it has been difficult to investigate the intrinsic photocurrent generation in graphene photodetectors [17]. In this work, the intrinsic photo-response of chemical vapor deposited (CVD) graphene photodetectors were investigated after eliminating the photodesorption using an atomic layer deposited (ALD) Al2O3 passivation layer. A general model describing the intrinsic photocurrent generation in a graphene was also developed using the relationship between the device dimensions and intrinsic photocurrent under UV illumination. 2. Experiments A single layer CVD graphene grown on Cu foil [18] was transferred to a SiO2 (90 nm)/Si substrate [19]. The device was fabricated as follows. Au (100 nm) source/drain electrodes were patterned using i-line photolithography. At this stage, the entire layer of graphene was under the electrode contacts the source/drain metal, minimizing a contact resistance. After the metal patterning, the graphene channel (W = 6 μm ~15 μm, L = 4 μm ~7 μm) was patterned using photolithography and reactive ion etching (RIE) in an oxygen. The RIE was performed for 50 seconds at 50 W at room temperature in O2 ambient (200 mTorr). After the RIE step, photoresist was stripped by dipping the sample in a photoresist stripper solution (AZ400T) at 50 °C for 5 minutes. Then, the samples were annealed in high vacuum (~10−7 Torr) at 200 °C for 2 hour to minimize the residue of photoresist on the graphene surface. Afterwards, the graphene channel was passivated using 30 nm of Al2O3 deposited at 130 °C by a low temperature ALD process. For ALD, trimethylaluminium (TMA) and a H2O precursor were used with N2 carrier gas. Then, the Al2O3 was patterned using a lift-off process to open a part of the source/drain electrodes for an electrical contact. Finally, a PDA was performed at 200 þC for 30 minutes in a high vacuum of ~10−7 Torr to densify the film and drive out residual H2O molecules near the graphene. The final structure of the graphene channel photodetector consisted of a source/drain electrode, a highly doped Si substrate as a back gate, 90 nm of SiO2 gate dielectric, and 30 nm of an Al2O3 passivation layer. Electrical and photonic properties of the graphene photodetector were characterized using a semiconductor parameter analyzer (Keithley 4200) and 365 nm wavelength LED lamp with 200 µW/cm2 intensity. The drain current was measured with various drain biases ranging from 1 to 100 mV while modulating the illumination intensity of the UV lamp. All the measurements reported here were carried out in air ambient.


3. Results and discussion

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Fig. 1. (a) Top-down SEM image of a graphene photodetector. (b) Raman spectra of the graphene channel. The inset is a SEM image of the graphene channel that magnifies the white circle in (a). (c) The DC Id-Vg curve of graphene photodetector before the passivation. The graphene channel is exposed to air ambient. (d) The DC Id-Vg curve of the graphene photodetector after the passivation and 200 °C PDA. The graphene channel is passivated with Al2O3.

Figure 1(a) shows a top-down scanning electron microscope (SEM) image of a graphene photodetectors. After the source/drain patterning, the quality of the graphene channel was measured using Raman spectroscopy at 0.514 nm as shown in Fig. 1(b). Raman spectra showed a small D peak and a high 2D/G ratio of 2.67, the signature of high quality monolayer graphene. The inset of Fig. 1(b) is a SEM image of a graphene channel in which the circled area shown in Fig. 1(a) is magnified. The Al2O3 passivation layer is deposited at 130 °C on the graphene channel region using a lift-off photoresist patterning method. After the Al2O3 deposition, the devices are annealed to densify the Al2O3 layer and to drive H2O molecules out of the graphene/Al2O3 interface. Some devices did not undergo Al2O3 passivation so that the role of the dielectric passivation could be explored. Before investigating the photo characteristics, the quality of the graphene channel in back gate graphene photodetectors was examined using basic electrical characterization methods. The drain current-gate voltage (Id-Vg) characteristics of these photodetectors were measured at Vd = 10 mV with a Vg sweep range of −40 V to 40 V before and after the Al2O3 passivation. The Id-Vg curves were asymmetric before the Al2O3 passivation, and the Dirac point was observed at Vg = 15 V (30 V for reverse voltage sweep). Hysteresis due to a tunneling-induced charge trapping and an electrochemical reaction between the graphene and ambient species was around 15 V, equivalent to 3.7 x 1012 charge traps/cm2 [20–23] (Fig. 1(c)). These are typical behaviors of air-exposed graphene photodetector or CNT based photodetectors on SiO2 substrates [22,24–27]. The apparent interaction of graphene with the ambient species, causing asymmetric carrier transport and a large hysteresis, is primarily attributed to the oxygen/water redox couple-induced charge transfer reaction [23,28].


On the other hand, after the Al2O3 passivation followed by a 200 þC post-deposition anneals (PDA), Id-Vg curves became almost symmetric with significantly less hysteresis (Fig. 1(d)). The Dirac point moved to Vg = −6 V ( + 4 V for reverse voltage sweep) due to the selfcleaning effect of the ALD Al2O3 process [29]. As expected, the Al2O3 passivation was effective in protecting the graphene channel from interacting with the ambient species [23]. λ=365 nm, Vd=10 mV

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Fig. 2. Photo-response of (a) an air-exposed graphene photodetector and (b) a Al2O3-passivated graphene photodetector with a UV on/off cycle of 20 seconds (200 μW/cm2 power and 365 nm wavelength UV lamp was used and the graphene photodetectors were biased with Vd = 10 mV and Vg = 0 V).

Finally, the photonic characteristics of the graphene photodetectors were measured by detecting the current change under illumination from a light-emitting diode (LED) lamp (power = 200 μW/cm2 at 365 nm wavelength). Figure 2(a) shows the conductance modulation of air-exposed graphene under UV illumination. When the UV lamp was turned on, the drain current steeply increased by ~100 nA with a ~0.3 sec rise time due to the generation of a photocurrent, but gradually decreased even below the dark current level with a tens or hundreds of seconds time constant [12,13]. As the illumination cycle was repeated, similar responses were observed, but the baseline of the drain current continued to decrease. The steep current increase (ΔIP) at the beginning of the illumination cycle is due to the electron hole pair (EHP) generated in the graphene channel [17,30]. The gradual decrease of the drain current under illumination is attributed to the gradual shift of the charge neutrality level caused by photodesorption (ΔIPD) and re-adsorption (ΔIR) of molecular species on the surface of graphene channel [12]. When the graphene channel region was passivated with Al2O3, however, a photo-response became very abrupt ~200 ms of falling time and a baseline shift was negligible as shown in Fig. 2(b). Interestingly, the drain current decreased when the UV lamp was turned on. This behavior is opposite of the photo-response of the graphene exposed to air ambient. The mechanism changing the direction of the photocurrent before and after Al2O3 passivation is explained below. To explain the origin and direction of the photocurrent (ΔIP) in our devices, band diagrams of graphene photodetectors with and without an Al2O3 passivation layer are presented in Figs. 3(a)-3(c). The black dashed line represents the Fermi level, EF, while the solid black line denotes the charge neutrality level. ΔΦ represents the EF of graphene pinned by the contact metal [30]. ΔΦ is ~0.2 eV at the graphene in contact with Au electrode [31]. The band alignment and the direction of band bending are determined by the difference between EF of the metal and bulk graphene, ΔΦ [32,33].


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