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Letter

Photoelectric Memory Effect in Graphene Heterostructure Field-Effect Transistors Based on Dual Dielectrics Hyun Ho Choi, Jaesung Park, Sung Huh, Seongkyu Lee, Byungho Moon, Sang Woo Han, Chanyong Hwang, and Kilwon Cho ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01132 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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Photoelectric Memory Effect in Graphene Heterostructure Field-Effect Transistors Based on Dual Dielectrics Hyun Ho Choi1,2, Jaesung Park3*, Sung Huh4, Seong Kyu Lee5, Byungho Moon5, Sang Woo Han4, Chanyong Hwang3*, Kilwon Cho1,5*

1

Center for Advanced Soft Electronics, Pohang University of Science and Technology, Pohang

37673, Republic of Korea 2

Department of Physics and Astronomy, Rutgers University, Piscataway 08854, United States

3

Korea Research Institute of Standards and Science, Daejeon, 34113, Republic of Korea

4

Department of Chemistry and KI for the NanoCentury, Korea Advanced Institute of Science and

Technology, Daejeon 34141, Republic of Korea 5

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang

37673, Republic of Korea

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Abstract In this letter, we report a photoelectric memory effect and the photo/field (PF)-induced doping of graphene-heterostructure field-effect transistors (HFETs) based on a narrow-bandgap insulator (NGI) and a wide-bandgap insulator (WGI). Interestingly, we observed PF-induced doping in three types of graphene HFETs with different NGIs (hafnium oxide (HfO2) and silicon nitride (Si3N4), and boron nitride (BN)), which indicates that the doping behavior is a general characteristics of our graphene HFETs with dual dielectrics. We clearly reveal that this doping is a result of charge transfer from defects at the NGI/WGI interface to graphene using the conduction/valence band of NGI as a charge transport pathway. The localized charges at NGI/WGI act as ‘remote dopants’ in the device and offer the flexible switching of the p/n-doping and memory effect by modulation of the gate bias and the photon energy. Doping stability and the scalable pattering of graphene HFETs are successfully demonstrated, enabling these graphene HFET memory devices to be used in scale-up and compatible processes in industrial applications.

Keywords: graphene, heterostructure, field-effect transistor, graphene memory, photoinduced doping

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Efficient charge localization of electronic devices has been widely studied in relation to the electronic memory effect from their storage on devices, which enable their use in digital memory devices as a type of modern semiconductor technology.1 Recently, graphene, a twodimensional honeycomb lattice of carbon atoms, and its memory devices have attracted much interest given that these devices demonstrate high performance levels and the possible introduction of new memory mechanisms.2 For field-effect transistors (FETs), for instance, a typical approach is to use graphene as a floating gate, which works as a charge storage layer.3-6 When graphene is used as a channel (graphene FETs), on the other hand, various storage components have been introduced, such as ferroelectric materials,7 metal nanoparticles,8-10 and multilayer oxides.11, 12 In this case, the memory characteristics can be evaluated by measuring the degree of graphene doping, that is, the shift of the Dirac voltage (VDirac). Recently, it was reported that it is possible to realize charge localization and a consequent memory effect in a graphene-based van der Waals heterostructure.4-6,

13, 14

For

instance, Choi et al. demonstrated graphene memory FETs based on a graphene/hexagonal-boron nitride (h-BN)/molybdenum disulfide (MoS2) heterostructure.6 Electron tunneling and localization were used in this system, and the conduction band of MoS2 was used for electron storage. Moreover, beyond electric field-induced memory devices, a graphene-based heterostructure was shown to be capable of serving as a photo-induced memory device by adopting photo-reactive objects such as pentacene10 and titanium oxide thin films.15 Mechanically exfoliated graphene/hexagonal-boron nitride (h-BN) is a well-known heterostructure with a negligible lattice mismatch and relatively few defects. For this reason, it is 3


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frequently used to determine the intrinsic electrical properties of graphene.16-18 However, Ju et al. recently reported unique doping behavior in this system.19 They observed charge doping in graphene when both an incident photon and gate bias were applied to devices. The photon and field-induced doping (PF-induced doping) is erasable and rewritable with high stability, indicating its potential use in memory devices. This fascinating heterostructure system, however, faces a technical hurdle for scalable fabrication due to the difficulty of achieving a large-scale hBN layer of a sufficiently high quality. Furthermore, it requires a clear interpretation of the doping mechanism. In the present paper, we found that the PF-induced doping observed in the graphene/hBN heterostructure is a general characteristic of graphene/narrow-gap insulator (NGI)/wide-gap insulator (WGI) heterostructure FETs, i.e., graphene HFETs. For the scalable patterning of graphene HFETs, this result enables the use of familiar and practical fabrication processes such as chemical vapor deposition (CVD), atomic layer deposition (ALD), plasma-enhanced chemical vapor deposition (PECVD), and photolithography instead of handwork fabrication method such as mechanical exfoliation. Furthermore, this device enables the flexible switching of the doping (p-doping ↔ n-doping), which is a particular advantage when undertaking the arbitrary patterning of doping. The presence of a photon energy threshold for the facile PF-induced doping raises the possibility of graphene HFETs as color sensors. We clearly revealed that the microscopic process of PF-induced doping is a charge transfer across NGI and the consequent polarization of the NGI/WGI interface. This localized charge at the NGI/WGI interface, in other

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words remote dopants, could be erased by illumination at a sufficient photon energy level, and the stability in the dark of these devices is sensitive to the bulk traps in the NGI.

Experimental Methods Preparation of substrates with gate insulators:

In this work, we used highly n-doped Si

substrates (n-Si) or gold-patterned glass (Au) as bottom-gate substrates with various gate insulators on the top: 1. h-BN/SiO2/n-Si, 2. Si3N4/SiO2/n-Si, 3. HfO2/SiO2/n-Si, 4. SiO2/n-Si, 5. h-BN/Au, 6. Si3N4/Au, and 7. HfO2/Au. Initially, a 300 nm-thick SiO2 layer was thermally grown on the top of a Si substrate (capacitance, CSiO2 = 11.5 nF·cm-2). Prior to depositing the NGI layers, the SiO2/n-Si, n-Si, and Au substrates used here were cleaned by sonication in an acetone/IPA solution for 30 min and then given a UV-ozone treatment for 10 min. Next, a thin hBN film was mechanically exfoliated from a bulk crystal and then transferred onto the substrate. Si3N4 and HfO2 were deposited by means of plasma-enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD), respectively. They fully covered the top surface of the substrates. The NGI layers, h-BN, Si3N4, and HfO2 h 30, 25, and 80 nm, respectively, as measured by atomic force microscopy (AFM). The capacitances of NGI/WGI double layers are as follows: 10.5 nF·cm-2 for h-BN/SiO2, 11.0 nF·cm-2 for Si3N4/SiO2, and 11.1 nF·cm-2 for HfO2/SiO2. Synthesis and transfer of the graphene, and device fabrication:

The CVD-grown graphenes

were synthesized and transferred by following methods in the literature.20, 21 Natural graphene was mechanically exfoliated on silicon substrates covered by double-polymer layers of 5


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poly(methylmetacrylate) (PMMA) and water-soluble poly(4-styrenesulfonic acid) (PSS). This graphene-polymer film was delaminated from the substrate and then floated on deionized water. It was then transferred onto the h-BN/SiO2/n-Si and h-BN/Au substrates using a micromanipulator.16 Finally, the source-drain electrodes were patterned by e-beam lithography or by thermal evaporation through a shadow mask. Electrical measurements:

The transfer characteristics (ID–VG) of all FETs were determined at

room temperature in a dark environment under a vacuum (10-4 Torr) using a Keithley 2636B sourcemeter. The system of monochromatic beam irradiation with a gate bias consists of a vacuum chamber (10-4 Torr), a light source with a 500 W Hg(Xe) arc lamp, a grating monochromator covering a spectral range of 254-2000 nm, an optical fiber (core diameter of 2 mm), and an I-V sourcemeter (Keithley 2636B).

Results and Discussion The device structure of the graphene HFETs is illustrated with optical microscope images of graphene on a NGI/WGI heterostructure in Figure 1a. On highly doped Si working as a gate electrode, WGI, NGI, and graphene were sequentially stacked, finally forming a van der Waals heterostructure. Three types of devices with different NGI layers were fabricated: silicon nitride (Si3N4), hafnium oxide (HfO2), and hexagonal boron nitride (h-BN). These three NGI layers have slightly different bandgaps, EG (Si3N4: 5.0 eV, HfO2: 5.5 eV, h-BN: 6.1~6.4 eV), but their EG values are much lower than that of SiO2 (EG ~ 9.3 eV), also known as WGI.22-26 Therefore, the charge-neutral level of graphene and the conduction/valence bands of NGI and 6


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WGI exhibit a step-like feature in the corresponding vertical band diagrams (Figure 1b).

Figure 1. Graphene | narrow bandgap insulator | wide bandgap insulator heterostructure field-effect transistors (G|Si3N4|SiO2, G|HfO2|SiO2, and G|h-BN|SiO2) and their PF-induced memory effects: (a) Scheme of the device structure (left) and optical microscopic images of the devices (right). In the case of CVD-grown graphene-based devices, including GCVD|Si3N4|SiO2 and GCVD|HfO2|SiO2, the active channel is in the middle of the 20 x 20 mm2–sized silicon substrate with the channel length/width of 100/1000 µm (top image in panel a). For GExfol|h-BN|SiO2, the channel length/width is 1.5/4 µm. (b) Vertical band diagram. (c–e) Transfer curves of each device. (black) The first sweep representing neutral state. (red and blue) The second sweep right after 1 min of gate bias with monochromatic illumination (wavelength: 365 nm). (gray) Another second sweep right after 1 min of gate bias without illumination. Black and gray curves are overlapped in (e). The applied drain voltage, VD is 10 mV.

Figures 1c-e show the transfer characteristics of graphene HFETs with PF-induced modulation doping. In the first ID(VG) scan in the dark, all devices exhibited the minimum 7


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conductivities with VG at the critical point, specifically VDirac, i.e., nearly zero (black curves). The calculated mobilities for holes (µh) and electrons (µe) are fairly comparable to those in the literature,19-21 which represents the high quality of both CVD-grown and exfoliated graphene with a low density of defects: µh = 5790 cm2V-1s-1 and µe = 5600 cm2V-1s-1 for graphene/hBN/SiO2 FET, µh = 900 cm2V-1s-1 and µe = 730 cm2V-1s-1 for graphene/Si3N4/SiO2 FET, and µh = 1220 cm2V-1s-1 and µe = 1040 cm2V-1s-1 for graphene/HfO2/SiO2 FET. Next, we exposed the devices to light at a fixed value of VG for 1 min and then captured ID (VG) scans with the light off (the red and blue curves). We found that the coupling of the photon and electric field led to a dramatic shift of VDirac toward the VG set point. As a result, a remarkable enhancement of ID was achieved when VG = 0, providing evidence of the doping of graphene. The doping type and majority carrier could be controlled by the sign of VG, with a positive VG for p-doping and a negative VG for n-doping. The doping rate, proportional to ∆VDirac, was dependent on the doping type and the NGI materials used. Nevertheless, the observation of PF-induced doping in all devices is indicative of the generalization of PF-induced doping to the graphene/NGI/WGI heterostructure system. Note that the doping rate was not determined by the photon flux, since the number of incident photons during PF-induced doping process (8.70×1021 cm-2 for 60 s, flux = 1.45×1020 cm-2s-1 when λ = 365 nm, see the Supporting Information) is much larger than the density of carriers accumulated by gate voltage in the channel (up to 4.2×1012 cm-2). It should be noted that this observation wasn’t achieved when we applied a fixed VG without illumination (gray curves) or exposed the devices to light without a gate electric field (black curves). Therefore, the PF-induced doping in this system is due to neither the bias-stress effect nor the photon effect. 8


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Figure 2. The reversibility and stability of the PF-induced doping: (a-c) Shift of Dirac voltages, VDirac under a repetitive sequence of positive, negative, and zero gate bias with illumination in three types of graphene HFETs.

Figure 2 shows the reversibility and stability of the PF-induced doping. Here, VDirac was monitored at a constant interval of time while a combination of positive/negative VG and illumination was applied. We found memorization of PF-induced doping for all devices; the VDirac shift was preserved before the erasing step started. In particular, the graphene/h-BN HFET exhibited ideal stability of its doped status. A portion of the VDirac shift was recovered in the case of graphene HFETs based on Si3N4 and HfO2, but the maintenance of the remnant VDirac shift ensured the presence of the memory effect in all devices. Further, this memorization could be perfectly erased by illumination without any gate bias; VDirac was recovered to its initial value. We repeated the memorization and erasing steps several times and observed a reproducible shift of VDirac and no degradation of the carrier mobility, µ.

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Figure 3. Four types of graphene (G) | single-bandgap insulator field-effect transistors (G|SiO2, G|Si3N4, G|HfO2, and G|h-BN) and their electrical performances: (a) Scheme of the device structure (top) and optical microscopic images of the G|h-BN device (bottom). (b–e) Transfer curves of each device. (black) The first sweep. (red and blue) The second sweep immediately after 1 min of gate bias with monochromatic illumination (wavelength: 365 nm). (green and purple) The third sweep directly after the second sweep. The applied drain voltage, VD is 10 mV. The nonzero initial VDirac might be originated from a dangling bonds of Si3N4 and SiO2 surfaces27 or dipole moments induced by hydroxylated SiO2 surface28 and h-BN/Au interface.29 10


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Although this PF-induced doping under different illumination conditions is quite complicated, its memory/storage characteristics can be understood through the concept of carrier localization at metastable states in the gate insulator. The coupling of the incident photon and the gate electric field generates the driving force for charge carriers to be transferred and localized to metastable states, completely screening the applied VG. When the light and VG are switched off, the charged metastable states, specifically remote dopants, become fixed due to a he lack of activation energy to overcome ng the localized states, which finally induces the electric field with charge carriers accumulating at the graphene. The negative and positive remote dopants result in the p-doping and n-doping of graphene, respectively. With regard to our device structure, the plausible locations of the charged remote dopants are the defects in the gate insulator: acceptor-like defects for negative dopants and donor-like defects for positive dopants. In the gate insulator, we can categorize the defects according to their position, as follows: 1) NGI bulk, 2) NGI/WGI interface, and 3) WGI bulk. Even though nitrogen vacancy and carbon vacancy in h-BN were previously proposed as a doping mechanism,19, 30 these are not applicable to the devices based on Si3N4 and HfO2, and clear experimental evidences to support them are still lack. To clearly reveal the doping mechanism, we conducted a simple yet powerful systematic experiment. We fabricated graphene FETs with a single-layer gate insulator (SiO2, Si3N4, HfO2, or h-BN) and checked for a PFinduced VDirac shift. Figure 3a shows the structure of graphene FETs with four different single insulating layers and an optical microscopic image of a graphene/h-BN FET. The transfer characteristics of each device before and after the PF-induced doping step are presented in Figures 3b-e. No significant change of VDirac in the transfer curves was observed from all devices. 11


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A slight shift of VDirac was observed immediately after the PF-induced memorization step in the graphene/Si3N4 and graphene/HfO2 FETs, but it mostly vanished and was close to the initial value at the next VG sweep. These results clearly reveal that the charged traps causing the PFinduced doping in our graphene HFETs are located at not the bulk of the NGI and WGI but at the NGI/WGI interface. More precisely, trapping and detrapping resulting in PF-induced doping are much more predominant in NGI/WGI interface than in the bulk of the NGI and WGI. The gate insulator SiO2 is commonly known to contain a number of defects on its surface due to the presence of dangling bonds and hydroxylated groups, which have been reported to generate in-gap states with various energy levels.28 Further, when the surface of SiO2 is fully hydroxylated, for instance, the density of the hydroxyl group is ~1015 cm-2, a value which is much higher than the density of the accumulated charge carriers (~1012 cm-2). Although our SiO2 surface may not be fully hydroxylated, it is likely that the NGI/WGI interface contains sufficient density of the in-gap states completely to screen the applied VG when it is polarized.

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ACS Photonics

Figure 4. Photo- and field-induced memory effect in G|h-BN|SiO2 field-effect transistors: (a) The dependency of the incident photon energy. (b) In-situ shifts of the Dirac voltage and (c) charge transfer rates under positive (red) or negative (blue) gate voltage with 365 nm monochromatic light illumination. 13


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There are two plausible charge transfer pathways for defects at the NGI/WGI interface to be charged: from graphene to the defects, and vice versa. For instance, the n-doping of graphene can be achieved by a hole transfer from graphene to the NGI/WGI interface or an electron transfer from the NGI/WGI interface to graphene. To understand the mechanism of charge transfer in our graphene HFETs, we focused on the PF-induced doping behavior of the graphene/h-BN HFET with various levels of incident photon energy (Figure 4a). We found that there is a threshold of the incident photon energy (hνTh) for a facile PF-induced doping (shift of VDirac), and p-doping and n-doping require different values of hνTh (hνTh = 2.85 eV and 2.21 eV, respectively). With a constant VG but with the incident photon energy below hνTh, the device exhibited a negligible shift of VDirac. At an incident photon energy level above hνTh, the doping rate was increased with the photon energy, but the saturation voltages of VDirac were independent of the photon energy. The presence of hνTh is relevant, as electrons or holes should reach the conduction or the valence band of h-BN for a facile charge transfer across h-BN, and hνTh can be thought of as the minimum activation energy for carriers to reach the band edges of h-BN. These experimental results cannot be understood in terms of a charge transfer from graphene to the defects because the sum of hνTh for p-doing and n-doping (4.96 eV) should be close to the value of h-BN bandgap (6.0~6.4 eV) and the saturation of VDirac should be dependent on the incident photon energy. Therefore, the charge transfer from the defects at h-BN/SiO2 to graphene is the proper microscopic process for PF-induced doping. The hνTh values equal to 2.85 eV for pdoping and 2.21 eV for n-doping are indicative of the energetic depths of the acceptor-like defect state from the conduction band of h-BN and the donor-like defect state from the valence band of h-BN, respectively. 14


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The in-situ VDirac shift and charge transfer rates were evaluated under the simultaneous application of VG (±60 V) and 365 nm monochromatic light illumination (Figures 4b and c). The charge transfer rate, dn/dt was determined from the VDirac shift, ∆VDirac for a (short) time interval, ∆t: dn/dt = (Ci/e)(∆VDirac /∆t), where Ci is the capacitance of gate insulator.31 While a relatively gentle slope was shown with a positive VG, a very sharp slope of the VDirac shift with a negative VG was observed during the doping process, indicating a higher doping rate during n-doping as opposed to that during p-doping. Therefore, the charge transfer rate across h-BN is higher for electrons than for holes. This is presumably due to the higher density of donor-like defects.19, 32

Figure 5. Time-dependent band structure of graphene (G) | narrow-bandgap insulator (NGI) | widebandgap insulator (WGI) heterostructure FETs during (a) positive memorization/erasing and (b) negative memorization/erasing

Figure 5 shows the doping and dedoping dynamics in the graphene/NGI/WGI 15


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heterostructure. Under optical illumination with a photon energy level overcoming the energy barrier, holes of acceptor-like defects and electrons of donor-like defects at the NGI/WGI interface are excited by photons to the valence band and conduction band of NGI, respectively. These carriers can be mobile, but they require driving force for movement towards the graphene across the NGI (STEP P-I). The gate electric field can selectively assign this driving force to one between the holes and electrons. Under positive gate voltage, the band banding induced by the gate electric field allows the excited holes at the NGI/WGI interface to transfer into the graphene while the excited electrons remain confined at the NGI/WGI interface. Consequently, the interface becomes negatively charged, and it screens the back gate effectively (STEP P-II). This process continues until the charged NGI/WGI interface perfectly screens the back gate, that is, until the disappearance of the electric field applied to NGI, and graphene becomes charge neutral, as was observed experimentally (STEP P-III). After turning off both the light and the gate bias, the excited electrons are stabilized to acceptor-like defect states, preserving the polarity of the NGI/WGI interface. This negatively charged interface accumulates hole carriers at the graphene and finally results in a shift of EF downward from the Dirac point, i.e., hole doping (STEP P-IV). For dedoping, we expose a photon with sufficient energy to this device without a gate bias. The holes at the graphene overcome the energy barrier and then reach the valence band of NGI. The preexisting electric field at the NGI enforces the drift of these carriers toward the NGI/WGI interface (STEP P-V). Finally, these carriers reaching the NGI/WGI interface fall into acceptorlike defect states and then neutralize the NGI/WGI interface. This process continues until the charged NGI/WGI interface is perfectly neutralized and there is no electric field in the NGI layer (STEP P-VI). The electron doping and dedoping qualitatively follow dynamics identical to those 16


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of hole doping and dedoping, except for the signs of the gate bias and charge carrier.

Figure 6. (a) Time-dependent Dirac voltage shifts for the negatively memorized G|h-BN|SiO2, G|HfO2|SiO2, and G|Si3N4|SiO2 transistors. Negative memorization: VG = -50 V and λ = 365 nm for 1 min. (b) Digital camera and optical microscope images of the patterned graphene-based memory devices (substrate size: 2×2 cm2).

We found that the doping stability as a function of time is highly dependent on the fabrication method when developing the NGI. The time-dependent VDirac shifts for p-doped 17


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graphene HFETs as a function of time are presented in Figure 6a. To develop the NGI, three different fabrication methods were used: exfoliation (h-BN), ALD (HfO2), and PECVD (Si3N4). As shown in Figure 6a, the graphene HFET using the exfoliated h-BN exhibited perfect storage of the doped characteristics for 28 hours. In contrast, the doped characteristic was rapidly erased within 28 hours for the graphene HFET using PECVD-grown Si3N4. This rapid dedoping indicates, according to our PF-induced doping mechanism, that the charge carriers spontaneously diffused into the charged NGI/WGI interface without using delocalized energy bands (the conduction or valence band) and then neutralized the charged traps. We consider that this diffusive charge transfer occurred through NGI bulk traps, whose density is sensitive to the fabrication method. Because PECVD-grown Si3N4 and ALD-grown HfO2 layers, in principle, cannot have in-plane crystallinity to a high degree, in contrast to exfoliated h-BN, they offer more defects, such as voids, dangling bonds, dislocations, and grain boundaries, which can provide in-gap trap states. The generalization of PF-induced doping in graphene/NGI/WGI HFETs can actualize the scalable fabrication of high-mobility graphene FETs with arbitrary doping patterns. Figure 6b shows 6×6×3 graphene/HfO2/SiO2 HFETs patterned on a 2×2 cm2 Si substrate. We can achieve arbitrary doping patterns by controlling the position of the illumination spot and gate bias. Although this is a simple demonstration, more complicated logic is possible through a combination with a proper circuit design.

Conclusion 18


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In

summary,

we

studied

the

photoelectric

response

of

graphene/NGI/WGI

heterostructure FETs (graphene HFETs) and found that PF-induced doping is a general characteristic in this system. This doping process can be understood in terms of the charge transfer from defects at the NGI/WGI interface to the graphene using the conduction/valence band of NGI as a charge transport pathway. As a result, the charged defects serve as a modulated remote dopant, consequently shifting the graphene Fermi level. The PF-induced doping is highly stable in the dark and can be perfectly erased by photon irradiation. Further, this device system enables the flexible switching of the doping (p-doping ↔ n-doping), which is a particular advantage for the arbitrary patterning of doping. We finally demonstrated an array of graphene HFETs using scalable fabrication techniques, thus suggesting the commercial use of graphene HFETs in photoelectric memory applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] (JP), [email protected] (CH), [email protected] (KC) Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS This work was supported by the following programs: Basic Science Research Program (2015R1A3A2033469),

Nano

Material

Technology

Development

Program

(2012M3A7B4049888) through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning (MSIP), Realization of Quantum Metrology Triangle funded by Korea Research Institute of Standards and Science (KRISS-2017-GP20170034) and the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2011-0031628).

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The table of contents entry Manuscript title: Photoelectric Memory Effect in Graphene Heterostructure Field-Effect Transistors Based on Dual Dielectrics Authors: Hyun Ho Choi, Jaesung Park*, Sung Huh, Seong Kyu Lee, Byungho Moon, Sang Woo Han, Chanyong Hwang*, Kilwon Cho*

In this work, we report a photoelectric memory effect and the photo/field-induced doping of graphene-heterostructure field-effect (HFET) transistors based on a narrow-bandgap insulator (hafnium oxide (HfO2) and silicon nitride (Si3N4), and boron nitride (BN)) and a wide-bandgap insulator (SiO2), which is a general characteristics of our graphene HFETs with dual dielectrics. The left schematic shows the device structure of the graphene HFET and the right figure presents the shift of Dirac voltages under a repetitive sequence of positive, negative, and zero gate bias with illumination.

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