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Photosensitive Graphene P-N Junction Transistors and Ternary Inverters Jun Beom Kim, Jinshu Li, Yongsuk Choi, Dongmok Whang, Euyheon Hwang, and Jeong Ho Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00483 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Applied Materials & Interfaces

Photosensitive Graphene P-N Junction Transistors and Ternary Inverters Jun Beom Kim,1† Jinshu Li,1† Yongsuk Choi,1 Dongmok Whang,1 Euyheon Hwang,1* and Jeong Ho Cho1,2* 1

SKKU Advanced Institute of Nanotechnology (SAINT), 2School of Nano Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Sungkyunkwan University, Suwon 440-746, Korea. *E-mail: [email protected] and [email protected] †

These authors equally contributed to this work.

Abstract We investigate the electric transport in a graphene–organic dye hybrid and the formation of p-n junctions. In the conventional approach, graphene p-n junctions are produced by using multiple electrostatic gates or local chemical doping, which produce different types of carriers in graphene. Instead of using multiple gates or typical chemical doping, a different approach to fabricate p-n junctions is proposed. The approach is based on optical gating of the photosensitive dye molecules; this method can produce a welldefined sharp junction. The potential difference in the proposed p-n junction can be controlled by varying the optical power of incident light. A theoretical calculation based on the effective medium theory is performed to thoroughly explain the experimental data. The characteristic transport behavior of the photosensitive graphene p-n junction opens new possibilities for graphene-based devices, and we use the results to fabricate ternary inverters. Our strategy of building a simple hybrid p-n junction can further offer many opportunities in the near future of tuning it for other optoelectronic functionalities. Keywords: graphene, p-n junction, ternary inverter, organic dye, light

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INTRODUCTION Graphene is a zero-band-gap semiconductor, and its electrical transport characteristics can be tuned between p- and n-type by shifting the Fermi level.1-10 Tuning the Fermi energy of graphene to obtain n-type, in which electrons are the main charge carriers, or p-type, in which holes are the main charge carriers, graphene is straightforward.1-3, 10-17 Thus, a single sheet of graphene can be built as a p-n junction by creating two different areas with positive and negative Fermi levels. A graphene p-n junction is a system where p-type monolayer graphene, which is an area of excess positive charges, is in direct contact with ntype monolayer graphene (an area of excess negative charges) at the edge. In other words, a well-defined p-n junction can be fabricated with p-type and n-type regions between two electrodes. Graphene p-n junctions have recently attracted considerable interest, because they are essential building blocks for various electronic and optoelectronic devices based on graphene.1, 3, 15, 18-20 Graphene p-n junctions are typically fabricated by local chemical doping of graphene or multiple gate modulations.12, 14, 16, 20-25 In the multiple-gate approach, the graphene p-n junction can be formed by using two split gates and biasing them with opposite voltage polarities. An applied positive (negative) voltage causes the Fermi energy level of graphene to exceed (fall below) the Dirac point and leads to n-type (p-type) electrostatic doping. In the chemical doping method, molecules of either a hole acceptor or an electron donor are applied to graphene, leading to p- or n-type characteristics, respectively. In this work, instead of using multiple gates or typical chemical doping, a different approach is proposed to create p-n junctions. We report lateral graphene p-n junctions based on graphene that is partially covered with photosensitive dye molecules. The approach is based on optical gating of the photosensitive dye molecules. In contrast to most other graphene-based p-n junctions, the device is obtained by covering a selected area with dye molecules. The dye molecules are deposited on graphene using a solution process, and they control the carrier density in the covered areas of the sample. Strong π–π interactions between the dye molecules and graphene can facilitate photoexcited charge transfer between them. Upon light illumination, the Fermi energy of graphene covered with dye molecules is increased so that it becomes ntype, but the untreated region remains at the same Fermi level as pristine graphene (p-type). This difference 2


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in Fermi level creates a junction across the boundary. The position and potential difference of the junction can be controlled by varying the optical power of the incident light. The graphene p-n junction can be tuned effectively using the optical power as well as the geometry of the area covered with the molecules. Thus, the characteristics of the p-n junction can be systematically controlled by varying both the contact area between the photosensitive dye molecules and the graphene, and the optical power of illumination light. A theoretical calculation based on semiclassical Boltzmann transport theory is performed to explain the experimental data thoroughly. In particular, we investigate the transport property in the low-carrier-density regime by using the effective medium theory (EMT), which has been used successfully to describe that of monolayer graphene both quantitatively and qualitatively. We expect that the proposed approach to producing a well-defined p-n junction without extra gates or chemical doping will open a path to new graphene devices. It also has diverse tuning capabilities and does not produce a significant mobility drop arising from substrate- or dopant-induced effects. The graphene p-n junction can be fabricated on both transparent and flexible substrates without the complex fabrication processes used for electrically gated devices. Because covering graphene with dye molecules does not require a complex fabrication process, it is possible to realize all-transparent and flexible p-n junctions. The characteristic transport behavior of the photosensitive graphene p-n junction provides new possibilities for graphene-based devices, and we use the results to fabricate ternary inverters. Our strategy of building a simple hybrid p-n junction can further offer many possibilities for other electronic and optoelectronic devices.

RESULTS AND DISCUSSION The schematic structure of p-n junction transistors prepared using a graphene channel layer treated spatially with photosensitive rhodamine-based organic dye molecules is shown in Figure 1a. Heavily doped Si (the gate electrode) with thermally grown 300-nm-thick SiO2 (the gate dielectric) was used as the substrate. The surface of SiO2 with silanol groups was treated by n-octadecyltrimethoxysilane (ODTS) as a surface passivation layer to minimize electron or hole trapping at the graphene–SiO2 interfaces.26-27 Au was 3



thermally deposited through a shadow mask onto the ODTS-treated SiO2 surface to form source and drain electrodes. The channel length and width were 300 and 1000 µm, respectively. Single-layer graphene synthesized by chemical vapor deposition (CVD) was transferred onto the substrate (Figure S1), and the transistor channel was defined by patterning via conventional photolithography and subsequent O2 plasma etching. Photoresist (PR) was subsequently spin-coated onto the entire substrate and patterned positively to form a PR bank. Finally, photosensitive organic dye molecules (rhodamine 6G; R6G) were then deposited by dip-coating onto the void of the PR bank. The chemical structure of R6G is shown in the lower panel of Figure 1a. The conjugated dye molecules were bound to the graphene surface via strong π–π interactions.2832

Further, the remaining PR bank layer served as a blocking layer to prevent deposition of R6G on the

graphene. The resulting graphene channel was composed of both p-type and n-type regions. The n-type area in the graphene channel was precisely controlled by varying the exposed area of the PR bank (Figure 1b). Figure 1c shows the transfer characteristics [drain current (ID) versus gate voltage (VG)] of the graphene transistors before and after n-type doping using R6G. The exposed PR area was fixed at 50%. The curve for the undoped graphene transistors exhibited a typical V shape with a minimum conductance at the Dirac voltage (VDirac) of +31 V. On the other hand, the p-n junction transistors exhibited double Dirac voltages at VDirac,1 = +31 V and VDirac,2 = -36 V. The n-type doping of graphene induced by deposition of R6G was examined by first-principles density functional theory calculations.33-34 Figure 1d shows side and top views of the electron density difference isosurface for the optimized geometry of the R6G–graphene interface. The effects of the interfacial induced dipole moment on graphene doping were investigated by calculating the variation of the electron density (∆ρ) according to the equation ∆ρ = ρtotal − (ρgraphene + ρR6G), where ρtotal, ρgraphene, and ρR6G are the electron densities of the total system, graphene, and R6G, respectively. The green and blue regions correspond to electron accumulation and depletion regions, respectively. The R6G–graphene system exhibited interfacial induced dipole moments pointing toward the dye molecules, indicating electron donation to the graphene. The arrow indicates that the dipole moment formed at the interface. The direction of the interfacial induced dipole moments supported the formation of an n-doped region by the presence of R6G in the graphene channel. 4


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In our device, n-type graphene can be produced by covering graphene with photosensitive organic dye molecules. Thus, a well-defined p-n junction can be realized by partial treatment of graphene with the molecules. Figure 2 shows the transfer characteristics of devices with various geometries. Various p-n junctions can be built by changing the ratio of the area covered with R6G molecules to the area of the pristine graphene (i.e., varying the spatially selective coverage of graphene with dye molecules). Several devices with different p-type and n-type area ratios are shown in Figure 2a. The left side of each image indicates the region covered with dye molecules (n-doped region), while the right area represent the region covered with PR (p-doped region), and the numbers in the figure indicate the ratio of the n-doped channel length. The ratio of n-doped region in the graphene channel was deterministically controlled by varying the contact area of the R6G molecules. The graphene transistors with various p- and n-type area ratios were fabricated as shown in Figure 2a. Figure 2b shows the transfer characteristics of the graphene transistors as a function of the n-doped area ratio, which varied from 0 to 0.75 of the graphene channel area. As the ndoped region increased, the VDirac,2 value of the n-doped region shifted gradually toward a negative gate voltage, whereas the VDirac,1 value of the p-doped region remained constant (Figure 2c). The measured p-n junctions all show characteristic double Dirac points, which are fixed at VDirac,1 = ~30 V (p-type) and a VDirac,2 (n-type) value that depends on the covered area. On the other hand, the n-type or undoped (p-type) CVD graphene shows only one Dirac point, which appears at the same location as the observed Dirac voltages in the p-n junctions. We find that the n-type Dirac voltages became more negative when a larger region was covered with the dye molecules. To understand the observed double deep transport characteristics, theoretical calculations were made. The drain current can be expressed as ‫ܫ‬ୈ =

௏ీ ோ౐

, where VD is the drain voltage, and RT is the total resistance

of the system. For our device, RT can be determined as ଵ

ܴ୘ = ∙ ఙ ౤

௅౤

ଵ

ௐ౤

+ఙ ∙ ౦

௅౦ ௐ౦

+ ܴେ ,

(1)

where RC is the contact resistance, which is assumed to be independent of the doped area, and Ln (Lp) and Wn (Wp) are the length and width of the n-type (p-type) graphene layer, respectively. σn (σp) is the conductivity 5



of the n-type (p-type) graphene layer.1,

35-36

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A microscopic calculation was performed to obtain the

conductivities on the basis of the Boltzmann transport theory (see the details in the Supporting information),1, 35 which has been very successful in describing the conductivity of homogeneous (n- or ptype) graphene. However, near the Dirac voltages (i.e., at low carrier densities), the theoretical formalism for a homogeneous system is not valid because of the spatially inhomogeneous potential fluctuations. These potential fluctuations, which cause puddles of electrons and holes in graphene, are known to induce a minimum conductivity near the Dirac voltage.37-38 To calculate the transport properties by considering the electron–hole puddles properly, the EMT was used near the Dirac voltages.38 The form of the twodimensional effective medium conductivity (σ EMC) is defined as ఙሺ௡ሻିఙ

‫ ݊݀ ׬‬ఙሺ௡ሻାఙు౉ి = 0, ు౉ి

(2)

where σ(n) is the conductivity of the electron or hole region.1, 35 Note that at large densities (i.e., large gate voltages away from the Dirac voltages), the effective medium conductivity simply becomes the conductivity of electrons or holes. By solving the above equation, we obtain the conductivity of the p-n junction. The solid lines in Figure 2c show the calculated conductance as a function of the gate voltage using the effective medium conductivity obtained from Eq. (2) for various geometric ratios of the p-n junction. As shown in Figure 2b and 2c, the calculated conductance is in excellent agreement with the experimental results. To fit the experimental data with Eq. (1), the contact resistance between the electrodes and p-n junctions is needed in addition to the effective medium conductivity. In this calculation, we used a contact resistance RC of ~1.4 kΩ for all the geometries of the p-n junctions. Next, the relative doping density of the n-type region in the graphene channel was controlled by light illumination (Figure 3a). The n-type region treated with photosensitive R6G was illuminated by light with various optical powers. Figure 3b shows the UV–vis absorption spectrum of the R6G layer deposited on graphene. The spectrum exhibits a strong absorption peak at 550 nm, which corresponds to the direct transition from the highest occupied molecular orbital to the lowest unoccupied molecular orbital of R6G. A wavelength of 550 nm was selected to control the n-type doping density. Figure 3c shows the transfer characteristics of the graphene p-n junction transistors as a function of the optical power of the light 6


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illuminating the n-type region, which varied from 5 to 500 µW. As the optical power increased, the VDirac value of the n-doped region shifted gradually in the negative gate voltage direction, whereas the p-doped region exhibited no obvious change in VDirac (Figure 3d). The shift in the Dirac voltage of the n-doped region is attributed to the increase in electron density upon light illumination as shown in Figure 3e. The photogenerated extra electrons are equivalent to the electrons produced by an additional gate voltage (δV), which is a function of the optical power and leads to the shift in VDirac on this side. A negative shift of VDirac from 25 to -45 V was observed when the optical power varied from 5 to 500 µW. Thus, the additional gate voltage can be modulated by varying the photo-illumination power. Assuming that the electron density can be obtained from the additional gate voltage δV, we calculated the total conductance of the system using the EMT above and obtained good agreements between our simulation results (solid lines in Figure 3f) and the experimental findings (symbols in Figure 3c). In our theoretical calculation, δV is considered as the effective gating voltage because the transport behavior contributed by the photoexcited carriers shows the same trend as the VDirac shift. The δV value increased dramatically with increasing optical power, but finally saturated at a specific value owing to the limitation of the graphene band structure and density of states, as shown in Figure 3e.39-41 We note that the separation voltage between the two Dirac voltages (|VDirac,1 − VDirac,2|) increased with increasing light power. The separation voltage also corresponds to the Fermi energy difference between p- and n-doped graphene regions.1, 10 A larger |VDirac,1 − VDirac,2| corresponds to a larger Fermi energy difference in the formed p-n junctions. This indicates that by changing the optical power, we can tune the Fermi energy difference of p-n junctions, as the electrical gate voltage produces the difference. The photosensitive double VDirac characteristics of the p-n junction graphene transistors enable the fabrication of ternary inverters, as illustrated schematically in Figure 4a. In the ternary inverters, p-type transistors with an R6G-doped graphene channel were connected to the ground, whereas the p-n junction transistors were connected to the supply electrode. The position of VDirac in the n-doped region of the p-n junction transistors was precisely controlled by light illumination. The graphene transistors shared the same input (VIN) and output terminals (VOUT). The corresponding equivalent circuit diagram of the ternary inverter

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is shown in the right-hand panel of Figure 4a. Figure 4b displays the transfer characteristics of the p-type graphene transistor (plotted as a red dotted line) and the graphene p-n junction transistor (plotted as a black solid line), which were measured separately. The voltage transfer characteristics of the resulting ternary inverter are shown in Figure 4c. VOUT was in logic state 1 when VIN was in logic state 0 at VG < -30 V. This condition was obtained as long as the conductance in the p-n junction transistor was larger than that in the ptype transistor. Further, VOUT was in logic state 0 when VIN was in logic state 1 at VG > 40 V as long as the conductance in the p-type transistor was larger than that in the p-n junction transistor. Between -20 and 20 V, a similar channel conductance region between the two transistors was observed owing to the double Dirac voltage characteristics of the p-n junction transistor. This region yielded a new logic state of 1/2 in the inverters under specific input signals. Although the separation between the logical levels was relatively small because of the near-zero band gap of graphene, the ternary inverter clearly distinguished three logic states over a reasonable voltage range. Furthermore, the control of performance by input light gives rise to the application of our ternary inverter to the photonic electronics.

CONCLUSION In conclusion, we showed that instead of using multiple gates or typical chemical doping, a welldefined graphene p-n junction can be fabricated by partial coverage of graphene with photosensitive dye molecules. The approach is based on optical gating of the photosensitive dye molecules, and a well-defined sharp junction can be produced. In the proposed p-n junction, the potential difference can be controlled by varying the optical power of the incident light. A theoretical calculation based on the EMT was performed to thoroughly explain the experimental data. The characteristic transport behavior of the photosensitive graphene p-n junction opens new possibilities for graphene-based devices, and we used the results to fabricate ternary inverters. Because our proposed chemical doping process does not require a complex fabrication process for metallic gate electrodes, it makes all-transparent and flexible p-n junctions possible. This approach to producing a well-defined p-n junction without extra gates or chemical doping may provide new possibilities for graphene devices. However, it is known that the R6G is undergoing photodegradation 8


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under light illumination, which may decrease the device performance.43 Therefore, encapsulating and isolating from ambient oxygen or water for the R6G-graphene channel are required to secure the photostability.44

METHODS Single-layer graphene was synthesized using a previously reported thermal CVD method on a Cu foil (Alpha Aesar, thickness = 25 µm).42 A poly(methyl methacrylate) (PMMA, Aldrich Co., MW = 350,000) supporting layer was spin-coated onto the single-layer graphene layer on Cu. A graphene layer synthesized on the opposite side of the Cu foil was removed by reactive ion etching (RIE) with O2 plasma (70 mTorr, 100 W, 2 s). The Cu layer was subsequently removed using a 0.2 M aqueous solution of ammonium persulfate. The graphene–PMMA film was then transferred onto an ODTS-treated SiO2/Si substrate containing pre-patterned source/drain electrodes (Cr/Au = 3/30 nm). The PMMA supporting layer was removed by acetone (60 °C, 30 min). The graphene channels were patterned by photolithography (AZ Electronic Materials: AZ 5214-E PR and AZ 500 MIF developer) and subsequent RIE for 2 s. The AZ 5214E PR was subsequently spin-coated onto the substrate with the graphene transistors (30 s and 4000 rpm) and patterned to form a PR bank. The substrate was immersed in a 10 µM aqueous solution containing an n-type dopant (R6G, Aldrich Co.) for n-type doping of the graphene channel in the void of the PR bank. The thickness of R6G film onto the graphene surface was around 19 nm. Finally, the samples were rinsed with distilled water for over 10 min and then dried in a vacuum chamber. The photoresponse of the graphene transistors was measured using a home-made probe station connected to a Keithley 4200 semiconductor parameter analyzer and a laser diode with wavelength of 550 nm. The monochromatic laser (Susemicon) with wavelength of 550 nm and optical power of 4 mW (beam radius ~3 µm) was utilized as a light source. The optical power was controlled from 5 to 500 µW using an optical attenuator (Thorlabs NDC-50C-4M), which was calibrated using an optical power meter (Thorlabs PM 100D).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications 9



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website at DOI: Details about simulation; Raman spectrum and optical image of CVD-grown graphene transferred onto the substrate.

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

Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFCMA1402-00.

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Figure 1. (a) Schematic of device structure of a graphene p-n junction transistor based on photosensitive organic dye molecules. Lower panel shows the chemical structure of the organic dye (R6G) used in this study. (b) Optical top-view image of a graphene p-n junction transistor. (c) Representative transfer characteristics of the graphene transistor and p-n junction transistor. (d) Side and top views of the electron density difference isosurface of the graphene–R6G interface. Green and blue indicate electron accumulation and depletion regions, respectively. Arrow indicates the induced dipole moment formed at the R6G– graphene interface.

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Figure 2. (a) Optical top-view images of graphene p-n junction transistors with different p-type and n-type area ratios. (b) Transfer characteristics of the graphene p-n junction transistors as a function of the n-doped area ratio, which varied from 0 to 0.75 times the graphene channel area. (c) Calculated results for transfer characteristics in (b). (d) Variation of Dirac voltages of the devices as a function of the n-doped area ratio.

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Figure 3. (a) Schematic device structure of p-n junction transistors where n-doped region was illuminated under different optical powers at a fixed incident wavelength of 550 nm. (b) UV–vis absorption spectrum of the R6G–graphene hybrid film. (c) Transfer characteristics of the p-n junction transistors where n-doped region was illuminated under different optical powers at a fixed incident wavelength of 550 nm. (d) Variation of Dirac voltages of the devices as a function of the illumination power. (e) Photogenerated electron density versus optical power of the p-n junction transistors where n-doped region was illuminated under different optical powers. (f) Calculated results of transfer characteristics in (c).

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Figure 4. (a) Schematic device structure of the ternary inverter based on the n-type graphene transistor and p-n junction transistor. The n-doped region of the p-n junction transistor was illuminated using 550-nm 50µW light. Right-hand panel shows a schematic top-view image and equivalent circuit diagram of the ternary inverter. (b) Transfer characteristics of both transistors in the ternary inverter. (c) Voltage transfer characteristics of the ternary inverter.

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Photosensitive Graphene PN Junction Transistors ... - ACS Publications

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