Chapter Ferroelectric gated graphene field effect transistors (GFeFETs) as non-volatile memory devices In this chapter, we will describe experimental studies of ferroelectrically gated charge transport in mechanically exfoliated graphene and its potential applications as nonvolatile memory devices. A pronounced hysteresis of the resistance as a function of gate voltage was observed. This hysteretic resistance change can be reversibly switched by controlling the polarization of the ferroelectric thin film. Based on this, a prototype of a non-volatile memory device was developed. Furthermore, we demonstrate that such non-linear, hysteretic ferroelectric gating can be controlled by the independent background doping of graphene. The results discussed in this chapter have been published in Physics Review Letters [84] and Applied Physics Letters [66]. 38 39 4.1 Introduction and background Nonvolatile memory is a general term for all forms of solid state memory that not need to have their memory contents periodically refreshed. The first non-volatile semiconductor memory was a kB UV-erasable programmable read only memory (PROM), introduced in the 1970s [85]. Later on EEPROM, electrically erasable PROM, was presented which, as its name suggests, could be erased by an electrical pulse [86]. Hence, it is also called flash memory. Flash memory is one of the most powerful and cost-effective solid-state storage technology widely used in various electronics devices and consumer applications. Flash memory works by storing charge in the gate of a transistor. The stored charge changes the threshold voltage (VT ) of the transistor, which can be read out as the stored state. These devices use hot-electron injection for programming and tunneling injection for erasing. Therefore, erasing is generally slower and is accompanied by several bytes at once, typically a sector. There are two major types of flash memory, NAND and NOR [87]. NOR flash memory has a larger cell size, high read speed, slow write speed, slow erase speed, and random data access ability. NAND flash chips are serial access devices with high density, medium read speed, high write speed, high erase speed and an indirect access. In general, the fast “read” times of NOR allow for its usage in code storage, primarily in cell phones and other hand-held devices. On the other hand, the fast “write/erase” times and the significantly lower power requirements of NAND allows for its usage in general storage and transfer of data, such as memory cards, solid state drives, USB flash drives, digital cameras, etc. However, as part of the semiconductor industry, flash memory also faces the fundamental physical barrier of shrinking cell sizes. In addition, flash memory also suffers 40 Control gate Floating gate Dielectrics N D N P Figure 4.1: (a) Structure of a flash memory device. from its relatively long programming time (> 10 µs), and limited cycle endurance. Furthermore, the high programming voltages (> 10 V) complicate its scaling down to nanometer cell sizes [88]. Many of the shortcomings of NAND flash can be addressed by ferroelectric nonvolatile RAM (FeRAM). FeRAM utilizes ferroelectric capacitors for data storage such that “1” and “0” are represented by two opposite polarization states, which can be retained without electrical power. Unlike NAND flash, FeRAM only needs a marginally higher writing voltage than the reading voltage. Moreover, both read and write can be done in a bit-by-bit fashion in FeRAM. These two features allow FeRAM to use less power than NAND flash for devices with more balanced read and write. However, two obstacles prevent FeRAM from being the ideal non-volatile memory. The main disadvantage of FeRAM is that the reading process of “1” state is destructive, and a subsequent writing process is required to return the state to “1”. Furthermore, while 130 nm FeRAM is commercially available, it remains unclear how much further it can be scaled down [89]. Consequently, novel types of non-volatile memory devices are in high demand. 41 Among the many candidates, graphene is intriguing because of its ultrahigh carrier mobility and pronounced field effect. However, despite graphene intrinsically having a high resistance state at the Dirac point and a low resistance state when heavily doped, reports on graphene in the context of non-volatile memory information storage are rarely seen. This is due to the difficulty in maintaining the resistance states in graphene without an electric field. Although a chemical modification approach to achieve non-volatile switching with high on-off ratios in graphene has been proposed by Lemme et al. [90], this method alters the unique crystalline structure of graphene and decreases carrier mobility dramatically, making the modified graphene no different than other common semiconductor materials. In the following, we propose a new type of non-volatile memory device in a graphene field-effect-transistor structure using ferroelectric gating. In section 4.2, we present ferroelectric polymer (P(VDF-TrFE)) gated graphene-field effect transistor structure (GFeFET), which is further divided to three parts, asymmetric bit writing, symmetric bit writing and understanding of ferroelectric gating. The summary for this chapter is in section 4.3. 4.2 P(VDF-TrFE) gated GFeFET non-volatile memory This section is divided into three parts. In section 4.2.1, we demonstrate a prototype of non-volatile memory device using a ferroelectric polymer (P(VDF-TrFE)) gated graphene-field effect transistor structure (GFeFET). By solely using ferroelectric gating, the high and low resistance states can only be maintained via the asymmetric 42 polarization of P(VDF-TrFE) thin film. Thus, this strategy is referred to asymmetric bit writing. In section 4.2.2, we show that simple symmetric P(VDF-TrFE) gate sweeping for two distinct resistance states in graphene can be achieved through the introduction of independent background doping (nBG ). We therefore call it symmetric bit writing. In section 4.2.3, we discuss the impact of electron-hole puddles in ferroelectric gated graphene devices. 4.2.1 Asymmetric bit writing using ferroelectric gating The working principle of graphene-P(VDF-TrFE) non-volatile memory for asymmetric bit writing is shown in Fig. 4.2a. The binary bits “1” and “0” are represented by the high resistance state near the charge neutral point and the low resistance state when graphene is heavily doped respectively. The reading of binary information is realized by measuring the device resistance using an excitation current of nA. The switching of binary information between “1” (“0”) and “0” (“1”) is achieved by tuning the polarization of the ferroelectric to be either zero (EC ) or at remnant polarization (Pr ). Correspondingly, the resistance in graphene will be either at the higher resistance value, R1 , or at the low resistance value, R0 , depending on the exact charge density introduced from the ferroelectric thin film. Compared to the chemical modification method, the graphene-ferroelectric memory has two key advantages [90]. First, this hybrid memory structure can in principle retain the high charge carrier mobility of graphene, which is crucial for ultrafast device applications. In fact, the reading speed of an ideal graphene-ferroelectric memory can be as fast as several tens femtoseconds for a device operating at V with a channel length of µm and charge carrier mobility of 200,000 cm2 V −1 s−1 . The second advantage of this memory device 43 is the significantly faster writing speed. It is only limited by the switching time of the ferroelectric thin film and can reach several ten nanoseconds. While the focus here is mainly on graphene, such a graphene-ferroelectric hybrid device is also likely to solve the challenges of an ordinary ferroelectric random access memory (FeRAM). Among these perhaps the most important are the destructive reading process of “1” state and the scalability problem [91]. a “0” to “0” b “1” to “1” D (i) Vmax (i) “0” to “0” (ii) (iii) “0” to “1” (iv) “1” to “1” D (ii) Vmax Pmin E E -Pr “1” to “0” “0” to “1” D (iii) (iv) E Pmin -Pr “1” to “0” D Vmax Vmax Pmin E -Pr Figure 4.2: Working principles and device operations of asymmetric bit writing. (a) Working principle of asymmetric bit writing. (i) Writing from “0” to “0”; (ii) Writing from “1” to “1”; (iii) Writing from “0” to “1”; (iv) Writing from “1” to “0”. (b): (i) Writing “0” into graphene-ferroelectric memory by a full loop sweep of VTG (±85 V). The memory bit is in “0” state before writing. (ii) Writing “1” into grapheneferroelectric memory by an asymmetrical loop sweep of VTG (85 V to -34 V). The memory bit is in “1” state before writing. (iii) Writing “0” into graphene-ferroelectric memory by a full loop sweep of VTG . The memory bit is in “1” state before writing. (iv) Writing “1” into graphene-ferroelectric memory by an asymmetrical loop sweep of VTG . The memory bit is in “0” state before writing. The sample geometry of our graphene-ferroelectric memory devices is shown in Fig. 4.3a. Graphene flakes were sitting on top of Si/SiO2 substrate. Multiple Cr/Au (5/30 nm) contacts were patterned in the edge of graphene flakes by electron-beam lithography. A ferroelectric thin film of poly(vinylidene fluoride-trifluoroethylene) 44 Figure 4.3: (a) Sample geometry of a finished graphene-ferroelectric memory device. (b) Optical image of a graphene samples showing the Hall bar geometry of the bottom electrodes. (c) R vs VBG of the graphene sample before P(VDF-TrFE) coating, measured in two-terminal configuration. (d) AFM image of another graphene sample after P(VDF-TrFE) spin coating. The contrast comes from the slightly different crystallization of P(VDF-TrFE) on SiO2 , graphene, and Au electrodes, respectively. (P(VDF-TrFE)) was then spin-coated (0.5 µm) on top of the GFET structures. From atomic force microscopy (AFM), we conclude that P(VDF-TrFE) forms a continuous thin film on graphene devices (Fig. 4.3d). For all devices, resistance (R) vs the bottom gate voltage (VBG ) has been recorded for reference before P(VDF-TrFE) coating. After thermally evaporating the top gate electrodes, samples were electrically characterized at room temperature in vacuum in a four-contact configuration using a lock-in amplifier with an AC excitation current of 10 nA. The number of graphene layers is confirmed by Raman spectroscopy. In total, we have successfully studied 15 samples. For the representative sample used here, the charge carrier mobility before P(VDF-TrFE) is ∼ 1500 cm2 V−1 s−1 , estimated from the linear slope of the R vs VBG 45 Rmax R (k ) R/R>350% RPr Rmin -1 =700 cm V s -80 -40 V TG -1 40 80 (V) Figure 4.4: Electric hysteresis loop; R as a function of VTG for the grapheneferroelectric sample. The resistance peak at 44 V (-32 V) corresponds to the flipping of electric dipoles in P(VDF-TrFE) from upward (downward) to downward (upward). From the linear part of this curve at high voltage, the charge carrier mobility is estimated to be 700 cm2 V−1 s−1 , taking κPVDF = 10 [93]. curve [92], as shown in Fig. 4.3c. We now present the main experimental observations. As shown in Fig. 4.4, the most important feature for all measured samples is a pronounced hysteresis in resistance measurements when the top gate voltage (VTG ) is swept in a closed loop: V to 85 V, 85 V to -85 V, and finally from -85 V back to V. Similar to the magnetoresistance measurements of a GMR/TMR device, we observe a hysteretic switching between the maximum resistance (Rmax ) and the minimum resistance (Rmin ), but now as a function of the applied electric field, E, instead of a magnetic field, B. For the sample in Fig. 4.4, the resistance change between Rmax and Rmin is larger than 350%. This hysteretic behavior and the double peak structure in R vs VTG curves are closely related to the polarization of P(VDF-TrFE) thin film. As illustrated in the 46 inset of Fig. 4.5, the continuity of electric displacement field, D, at the ferroelectric/graphene interface requires D = ε0 κferro Eferro + P(VTG ) = −n(VTG )e, where Eferro and n(VTG ) are the electric field in ferroelectric and the charge carrier concentration in graphene respectively [94]. Here, the dielectric response of the ferroelectric separates into a linear part (ε0 κferro Eferro ) and a hysteretic part (P(VTG )). While the linear dielectric part induces electrical doping in graphene with an opposite sign to VTG , P(VTG ) can induce electrical doping with either sign. These two components compete with each other such that graphene can remain p-doped (n-doped) even with a positive (negative) VTG until either the doping contribution from the linear part exceeds P(VTG ) or the polarization direction of the ferroelectric is switched. It is this behavior which leads to the hysteretic doping of graphene as a function of VTG . Considering that the conductance of graphene is σ = n(VTG )eµ, the observed resistance hysteresis loop is now directly related to P(VTG ) by D = ε0 κferro Eferro + P(VTG ) = −n(VTG )e = σ/µ. (4.2.1) Using this equation, we convert the R vs VTG curve in Fig. 4.5 into D vs E characteristics, which is further compared with the direct polarization measurement of P(VDF-TrFE) thin film alone, D’. As show in Fig. 4.5, D and D’ have very similar coercive fields (EC ) of ∼ 50 MV/m, consistent with the typical EC reported in literature [95]. This agreement strongly suggests that the hysteresis observed in the transport measurements is indeed caused by the hysteretic polarization of the ferroelectric gate dielectric. From Fig. 4.5, we can also see that the left and right resistance peaks in Fig. 4.4 correspond to the flipping of electric dipoles from upward to downward and from downward to upward, respectively, while Rmin in Fig. 4.4 is related to the maximum polarization point in Fig. 4.5. Another important parameter 47 Figure 4.5: D vs VT G characteristics deduced from the R vs VT G curve in Fig. 4.9. The black curve represents the experimental measured D’ of P(VDF-TrFE) thin film with similar thickness. The kinks near EC in D are caused by the assumption of a constant charge carrier mobility, which does not hold near the charge neutral regime. Inset (a): the electric displacement continuity equation at ferroelectric/graphene interface. Inset (b): a polarized P(VDF-TrFE) molecule. Cyan, grey and white atoms represent fluorine, carbon and hydrogen respectively. in Fig. 4.4 is the zero-field resistance RPr , corresponding to a remnant polarization Pr of P(VDF-TrFE) in Fig. 4.5. For device operation, we utilize the maximum resistance peak (R1 ≃ Rmax ) as bit “1”, while bit “0” is represented by RPr . As shown in Fig. 4.2b, a major hysteresis loop, corresponding to a full symmetrical VTG sweep (±Vmax ), can set the memory to “0”, independent of the existing state. In Fig. 4.2b(i), “0” has been rewritten into “0”, while in Fig 4.2b(iv), the binary information has been reset from “1” to “0”. In contrast, writing “1” into graphene-ferroelectric memory requires a minor hysteresis loop with an asymmetrical VTG sweep to minimize the polarization in P(VDFTrFE) thin film when VTG is back to zero. As shown in Fig Fig. 4.2b(ii) and Fig. ′ = −34 V can set the 4.2b(iii), a minor hysteresis loop with Vmax = 85 V and −Vmax 48 resistance state of graphene channel to near Rmax , independent on the initial state of “1” (Fig 4.2b(ii)) or “0” (Fig. 4.2b(iii)). Thus, using major and minor hysteresis loops, we can realize non-volatile switching in graphene-ferroelectric memory. Note that the switching voltage can be reduced by one order of magnitude by simply scaling the thickness of P(VDF-TrFE) to the range of 100 nm. The difference between the two resistance states (∆R/R=(R1 RPr )/RPr ) is determined by the difference between the minimum polarization (Pmin ) and the remnant polarization (Pr ) in P(VDF-TrFE). For the sample discussed here, the resistance change ∆R/R is ≈ 200%. Next, we discuss how to improve the performance of such GFeFET devices. First, ∆R/R can be much increased by removing contaminant residues on graphene surface. For an ideal ferroelectric/graphene interface, ∆R/R can be as large as ∼ ( Pmin − µ′ )/ P1r µ , Pr µ where µ′ is the mobility at Pmin . Another important approach is to increase the remnant polarization by applying larger electric fields. Thus, a better approach would be the preparation of graphene sheets directly on ferroelectric substrates with much higher remnant polarization. Last but least strategy would be to open a band gap in graphene, either by using bilayer graphene or graphene nanoribbons. 4.2.2 Symmetric bit writing using ferroelectric gating and back ground doping Although two distinct resistance states are created by polarizing and depolarizing the P(VDF-TrFE) thin film alternately, the depolarized state (“1”) is not in thermodynamic equilibrium and less stable than the polarized state (“0”) (Fig. 4.6). To solve this problem, we introduce an independent background doping (nBG ) provided 49 Figure 4.6: Free energy as a function of polarization for a ferroelectric material. by a normal dielectric gating. The key idea is use this nBG to control the ferroelectric gating by shifting the hysteretic ferroelectric doping in graphene. Utilizing this electrostatic effect, we demonstrate symmetric bit writing in GFeFET with resistance change over 500 % and reproducible non-volatile switching over 105 cycles. The working principle of GFeFET non-volatile memory for symmetric bit writing is shown in Fig. 4.7. By applying an independent doping nBG , the ferroelectric gating can be effectively controlled by unidirectionally shifting the hysteresis ferroelectric doping in graphene, yielding an asymmetric hysteresis doping loop. Utilizing this effect, the switching from “1” (“0”) to “0” (“1”) can be realized by simply applying an external positive (negative) voltage to the top electrode. After this positive (negative) pulse, the polarization in the ferroelectric will remain at Pr (−Pr ) without the need of an external electric field. Correspondingly, the resistance in graphene with be either at the higher resistance value, R1 , or at the low resistance value, R0 . Thus, the bit 50 -Vmax n n n1 n1 E E n E n0 n n Vmax BG E n0 Figure 4.7: Working principle of symmetric bit writing from ”1” (”0”) to ”0” (”1”). Hysteresis loop shown here is representing the charge density n in graphene as a function of external gate voltage. The corresponding nBG is shown on the right side. writing in GFeFETs can be much simplified and faster bit switching speed can be achieved by releasing the full advantage of ferroelectric. The sample geometry and detailed sample fabrication procedures have been discussed in Chapter 4.2.1. For the samples used in this study, the ferroelectric thin film of poly(vinylidene fluoride-trifluoroethylene 72:28) (P(VDF-TrFE)) is approximately 0.5 µm thick. The GFeFETs were electrically characterized at room temperature in vacuum in a four-contact configuration using lock-in amplifiers with an AC excitation current of 10 − 100 nA. Before polarizing the ferroelectric, we first measured the Hall mobility and the resistance vs SiO2 gate voltage characteristics (R vs VBG ) to determine the sample quality. Most of our samples retain their high charge carrier mobility after P(VDF-TrFE) spin-coating and annealing, as shown in Fig. 4.8. Quantitatively, the ambipolar R vs 51 Figure 4.8: R vs VBG of one sample after P(VDF-TrFE) coating. Red open square and black solid line are the experimental and fitting results (µ = 4,600 cm2 V−1 s−1 [94]) respectively. Inset: Atomic force microscopy of the sample after P(VDF-TrFE). Color scale: to 164 nm. VBG characteristics can be fitted by, R= W eµHall L √ n2res + n2 , (4.2.2) using the Hall mobility µHall [96]. For the sample shown here, the fitting yields a residual carrier concentration nres = 2.77 × 1011 cm−2 . In the following, we present the main results after introducing an independent nBG using the the SiO2 /Si back gate to GFeFETs. This provides a well defined, constant reference for determining the charge carrier concentration induced by ferroelectric doping. To study the effect of nBG on the ferroelectric gating of GFeFETs, it is also important to limit the polarization magnitude in the ferroelectric thin film, since ferroelectric gating is nearly 10 times stronger than the SiO2 gating [66]. Thus, we first introduced very small |Pr | in P(VDF-TrFE) by limiting the maximum top gate voltage (VTG−max ) to ±5 V. Such low VTG only slightly polarizes the ferroelectric, allowing nBG to match or even exceed the |Pr | induced doping in graphene. 52 Figure 4.9: (a) R vs VTG and VBG of the GFeFET with very small Pr and -Pr . (b) Extracted single traces of R vs VTG with different VBG . The blue dotted lines are simulated results. (c) Origin of the resistance peaks and the change in △R/R. 53 In Figure 4.9a, we show the resistance of the representative GFeFET as a function of both VTG and VBG . With VBG ≈ V, the R vs VTG curve shows two symmetrical resistance peaks and nearly negligible △R/R (Fig. 4.9b(iii)). By gradually tuning nBG with VBG , the two resistance peaks become more asymmetrical and shift leftward (rightward) for nBG < (nBG > 0). The shift in peak positions leads to an increase in △R/R, which has a maximum at VBG ≈ −6 V (Fig. 4.9b(ii)) and VBG ≈ 18 V (Fig. 4.9b(iv)), respectively. Crossing these two points, △R/R decreases as |nBG | keeps on increasing. At large enough nBG , the double peak structure eventually disappears in the R vs VTG hysteresis (Fig. 4.9b(i) and Fig. 4.9b(v)). The evolution of the resistance peaks and the change in △R/R can both be explained by two independent but competing doping processes in graphene by polarized ferroelectric dipoles and VBG , respectively. For such a dual-gated system, the interfacial electric displacement continuity equation is expressed by −βP(VTG ) + n∗ = n(VTG , VBG )e, (4.2.3) where βP(VTG ) represents the hysteretic dipole doping by the ferroelectric gating [94], and n∗ = nenv +nBG is the reference doping induced by the dielectric environment and VBG respectively. For n∗ ≈ 0, the doping in graphene is dominated by the ferroelectric gating by n(VTG , n∗ ≈ 0) = −βP(VTG )/e. Using Eq. 4.2.2, it is now straightforward to see that n(VTG , n∗ ≈ 0) will produce a R vs VTG hysteresis with two symmetrical resistance peaks, centering on the two coercive-field points where P(VTG ) crossing zero. Experimentally, this is the R vs VTG curve in Fig. 4.9b(iii) with VBG = V, in which two resistance peaks are centered at VTG = ±2.2 V respectively. By converting each R in Fig. 4.9b(iii) into doping using Eq. 4.2.3, we directly determined the doping curve n(VTG , n∗ ≈ 0). The result is shown in Fig. 4.9c (red curve). As expected, 54 this doping curve is hysteretic and characterized by two zero-field doping levels with equal magnitude, i.e. |n1 | = |n0 | = βPr /e. After acquiring n(VTG , n∗ ≈ 0), we can deduce individual R(VTG , n∗ ) curves for non-zero n∗ by substituting n(VTG , VBG ) = −βP(VTG )/e + nenv + αVBG into Eq. 4.2.2. Here α = 7.2 × 1010 cm−2 V−1 is the doping coefficient of 300 nm SiO2 , and nenv is a fitting parameter [97]. By tuning nenv and matching the resistance peaks of the simulation to the experimental, we simulated each experimental R(VTG , VBG ) curve in Fig. 4.9b. As shown by blue dotted lines, the simulation reproduces the evolution of the experimental results very well. Two resulting doping hysteresis for the resistance curves in Fig. 4.9b(i) and Fig. 4.9b(ii) are further compared with n(VTG , n∗ ≈ 0) in Fig. 4.9c. From the comparison, we can see that △R/R approaches the maxima as one zero-filed doping level sits near the Dirac point when |n∗ | ≈ βPr /e (blue hysteresis loop). Further increase in nBG moves both n1 and n0 away from the Dirac point, and △R/R decreases (black hysteresis loop). Thus, we have shown that using a background doping introduced by normal dielectric gating as a reference, the hysteretic behavior of R vs ferroelectric gating in GFeFETs can be quantitatively determined by solving the electric displacement continuity equation. For memory applications, △R/R is of great importance. Following the above discussions, the two zero-field resistance states are R1 = and R0 = W eµ √ L n2res +(βPr /e+n∗ )2 W eµ √ L n2res +(βPr /e−n∗ )2 respectively. Thus, the best strategy to utilize the field- dependent resistance is fully polarizing the ferroelectric and introducing a matching nBG , as demonstrated in Fig. 4.10a. With VTG−max = V, two maxima of ∼250% are present in the △R/R vs VBG curve, which can be also simulated very well by Eq. 4.2.2 and 4.2.3 with βPr /e = 4.2 × 1011 cm−2 . By increasing VTG−max to 30 V, the 55 Figure 4.10: (a) Summary of △R/R as a function of VBG with different VTG−max . Two maxima are observable with VTG−max = V (black open circles). The red solid line shows the simulation with βPr = 4.2 × 1011 cm2 . For VTG−max = 30 V, the maximum △R/R is increased to 500% (VBG = 32 V), while another maximum is out of the VBG measurement range. (b) R (VTG , VBG ) of the GFeFET with higher βPr (∼ × 1012 cm−2 ). Double peak structures dominate over the whole VBG range. maximum △R/R is increased to 500%. The fast increase in Pr not only increases the maximum △R/R, but also increases the separation between the two △R/R maxima, resulting in one maximum out of the VBG measurement range. For this VTG−max , R vs VTG shows a dominant double peak structure over the full VBG range (Fig. 4.10b). However, we can still see the tendency of a transition from double peak structure to single peak structure as VBG exceeding 40 V. Such nBG shifted hysteretic doping in graphene is a ferroelectric analogy to the ferromagnetic exchange bias [98]. Utilizing this electrostatic effect, the bit writing in GFeFETs can be much simplified by switching the ferroelectric polarization between Pr and -Pr , using symmetrical voltage sweeps. With a negative nBG , to write the high resistance “1”, a negative writing voltage (-Vwriting ) is applied to the ferroelectric thin film, setting the dipole polarization to -Pr independent of the initial states in the unit cell (Fig. 4.11a and 4.11b). In contrast, a positive writing voltage with the same 56 Figure 4.11: Symmetrical bit writing in GFeFETs. (a) and (b) Writing “1” using a negative Vwriting . (c) and (d) Writing “0” using a positive Vwriting . Dashed and solid arrows indicate the forward and backward voltage sweep directions respectively. The writing procedures are independent on the initial states before writing. Here, nBG is chosen to match -βPr /e so that one of the resistance peaks is near the Dirac point. (e) and (f) Fatigue test of one GFeFET with symmetrical bit writing. Non-volatile switching cycles exceeding 100k cycles is shown. Inset: Raw data of the fatigue test. nBG ≈ 1.2 × 1012 cm2 is chosen to match -βPr /e. magnitude sets the GFeFET into low resistance “0”, as shown in Fig. 4.11c and 4.11d. Compared to the asymmetrical bit writing by polarizing (P↑ = Pr ) and depolarizing the ferroelectric (P↓ ≈ 0) alternately [66], such symmetrical writing in GFeFETs not only provides simplicity but also takes full advantage of the fast switching speed of ferroelectric. For lead zirconate titanate (PZT) based materials, this can be as fast as 280 ps [99]. Another potential application of this electrostatic effect could be multi-bit-per-cell data storage in GFeFETs utilizing the nBG tunable △R/R. We have also tested the reproducibility of our GFeFETs working with βPr ≈ |n∗ |e. During the fatigue test, a triangular wave of 1k Hz was applied to the P(VDF-TrFE) 57 thin film. Every 12 (24) seconds, the triangular wave was interrupted and one R vs VTG curve was recorded. The corresponding △R/R as a function of switching cycles and the raw data of individual R vs VTG curve are summarized in Fig. 4.11e and 4.11f respectively. The fatigue test clearly demonstrates reproducible non-volatile switching exceeding 100k cycles in the GFeFET. Ultimately, the life span of P(VDF-TrFE)based GFeFETs is only limited by the endurance of P(VDF-TrFE) (107 [95]). Thus, P(VDF-TrFE)-GFeFET memory could provide a cost-effective solution for flexible non-volatile data storage with sub-µs switching speed. On the other hand, inorganic ferroelectric (such as PZT) should be used if fast writing speed (< 10 ns) and ultrahigh endurance (1010 ) are required. 4.2.3 Understanding of ferroelectric gating In the following, we show a quantitative understanding of graphene devices under ferroelectric gating. This is because although GFeFETs have potential applications for non-volatile memory and data storage [66], a comprehensive understanding in the hysteretic ferroelectric gating is still missing. In contrast to linear doping with normal gate dielectrics, n = αV g [21], the non-linear dielectric behavior of ferroelectrics introduces a pronounced hysteresis in the charge carrier doping. Furthermore, ferroelectric gating introduces strong electron-hole puddles in graphene even far away from the Dirac point. Therefore, Hall measurements alone can not be used to determine the induced charge carrier concentration (Fig. 4.12). Last but not least, ferroelectric gating is characterized by two symmetrical remnant polarizations, i.e., P↑ = Pr and P↓ = −Pr for upwards and downwards dipole configurations, respectively. Consequently, P↑ and P↓ induce two identical zero-field resistance states in graphene [66], 58 which is not practical for devices applications. d Figure 4.12: (a) R vs VTG and VBG of GFeFET. (b) Top: Single trace of R vs VTG with VBG = 30 V; Bottom: the corresponding VHall vs VTG characteristic showing all negative Hall signals, independent of ferroelectric dipole flipping. (c) Top: Single trace of R vs VTG with VBG = −6 V; Bottom: the corresponding VHall vs VTG characteristic showing all positive Hall signals. (d) The corresponding ferroelectric and non-ferroelectric phase diagram for the explanation of the Hall measurement results. When P(VDF-TrFE) is polarized, gate-tunable electron-hole puddles exist in graphene over a wide range away from the Dirac point. Thus, Hall signals can not be directly used to deduce the mobility. Fig. 4.12a shows the resistance vs VTG and VBG characteristics of a GFeFET after fully polarizing P(VDF-TrFE). VTG is swept in a close loop (0 → 30 → −30 → V) with different set points of VBG . For single 59 traces of R vs VTG , Hall signals also show pronounced hysteresis due to ferroelectric gating. However, Hall signals not change the signs over the whole VTG sweeping range, indicating the existence of minority charge carriers which are not controlled by ferroelectric gating (Figs. 4.12b and 4.12c). The origin of all positive/negative Hall signals is illustrated in Fig. 4.12d. Around 5-20% area of P(VDF-TrFE) thin films is non-ferroelectric, which cannot be polarized by external electric field [95]. Thus, ferroelectric gating introduces strong electronhole puddles in GFeFETs since graphene is inhomogeneously doped by ferroelectric and non-ferroelectric phases respectively. As shown in Fig. 4.12, the sign of the Hall signal dependents on the minority charge carriers in the non-ferroelectric doped areas, which is controlled by the SiO2 gating. 4.3 Summary and conclusion In summary, we have demonstrated the working principle and real device operation of a novel hybrid non-volatile memory device using graphene based on ferroelectric substrates. By tuning the ferroelectric dipoles to be either highly order or highly disorder, a reversible non-volatile switching between the high and low resistance states in graphene have been realized. Using an independent linear dielectric gating (nBG ) as a reference, we show that ferroelectric gating can be quantitatively determined by the electric displacement continuity equation. nBG can also be used to control ferroelectric gating by introducing a unidirectional shift in the hysteretic ferroelectric doping in GFeFETs. This effect allows symmetrical bit writing in non-volatile GFeFETs with resistance change ratio over 500% and switching cycles exceeding 105 cycles. These make this new memory structure a promising candidate for the next generation of 60 ultra-fast non-volatile memory. To realize precise control of nBG , it will be crucial to prepare graphene on ferroelectric substrates and introduce fixed molecular doping by donor/acceptor molecules. An alternative way to introduce a fixed strong nBG in graphene can be realized by using epitaxial graphene on SiC wafers. [...].. .48 resistance state of graphene channel to near Rmax , independent on the initial state of “1” (Fig 4. 2b(ii)) or “0” (Fig 4. 2b(iii)) Thus, using major and minor hysteresis loops, we can realize non-volatile switching in graphene -ferroelectric memory Note that the switching voltage can be reduced by one order of magnitude by simply scaling the thickness of P(VDF-TrFE) to the range of 100 nm... points of VBG For single 59 traces of R vs VTG , Hall signals also show pronounced hysteresis due to ferroelectric gating However, Hall signals do not change the signs over the whole VTG sweeping range, indicating the existence of minority charge carriers which are not controlled by ferroelectric gating (Figs 4. 12b and 4. 12c) The origin of all positive/negative Hall signals is illustrated in Fig 4. 12d... stable than the polarized state (“0”) (Fig 4. 6) To solve this problem, we introduce an independent background doping (nBG ) provided 49 Figure 4. 6: Free energy as a function of polarization for a ferroelectric material by a normal dielectric gating The key idea is use this nBG to control the ferroelectric gating by shifting the hysteretic ferroelectric doping in graphene Utilizing this electrostatic effect,... 5-20% area of P(VDF-TrFE) thin films is non -ferroelectric, which cannot be polarized by external electric field [95] Thus, ferroelectric gating introduces strong electronhole puddles in GFeFETs since graphene is inhomogeneously doped by ferroelectric and non -ferroelectric phases respectively As shown in Fig 4. 12, the sign of the Hall signal dependents on the minority charge carriers in the non -ferroelectric. .. constant reference for determining the charge carrier concentration induced by ferroelectric doping To study the effect of nBG on the ferroelectric gating of GFeFETs, it is also important to limit the polarization magnitude in the ferroelectric thin film, since ferroelectric gating is nearly 10 times stronger than the SiO2 gating [66] Thus, we first introduced very small |Pr | in P(VDF-TrFE) by limiting... resistance is fully polarizing the ferroelectric and introducing a matching nBG , as demonstrated in Fig 4. 10a With VTG−max = 5 V, two maxima of ∼250% are present in the △R/R vs VBG curve, which can be also simulated very well by Eq 4. 2.2 and 4. 2.3 with βPr /e = 4. 2 × 1011 cm−2 By increasing VTG−max to 30 V, the 55 Figure 4. 10: (a) Summary of △R/R as a function of VBG with different VTG−max Two maxima... larger electric fields Thus, a better approach would be the preparation of graphene sheets directly on ferroelectric substrates with much higher remnant polarization Last but least strategy would be to open a band gap in graphene, either by using bilayer graphene or graphene nanoribbons 4. 2.2 Symmetric bit writing using ferroelectric gating and back ground doping Although two distinct resistance states... non-linear dielectric behavior of ferroelectrics introduces a pronounced hysteresis in the charge carrier doping Furthermore, ferroelectric gating introduces strong electron-hole puddles in graphene even far away from the Dirac point Therefore, Hall measurements alone can not be used to determine the induced charge carrier concentration (Fig 4. 12) Last but not least, ferroelectric gating is characterized... polarizes the ferroelectric, allowing nBG to match or even exceed the |Pr | induced doping in graphene 52 Figure 4. 9: (a) R vs VTG and VBG of the GFeFET with very small Pr and -Pr (b) Extracted single traces of R vs VTG with different VBG The blue dotted lines are simulated results (c) Origin of the resistance peaks and the change in △R/R 53 In Figure 4. 9a, we show the resistance of the representative... expressed by −βP(VTG ) + n∗ = n(VTG , VBG )e, (4. 2.3) where βP(VTG ) represents the hysteretic dipole doping by the ferroelectric gating [ 94] , and n∗ = nenv +nBG is the reference doping induced by the dielectric environment and VBG respectively For n∗ ≈ 0, the doping in graphene is dominated by the ferroelectric gating by n(VTG , n∗ ≈ 0) = −βP(VTG )/e Using Eq 4. 2.2, it is now straightforward to see that . slope of the R vs V BG 45 Figure 4. 4: Electric hysteresis loop; R as a function of V TG for the graphene- ferroelectric sample. The resistance peak at 44 V (-32 V) corresponds to the flipping of. ferroelectric thin film of poly(vinylidene fluoride-trifluoroethylene) 44 Figure 4. 3: (a) Sample geometry of a finished graphene -ferroelectric memory device. (b) Optical image of a graphene samples showing. new type of non-volatile memory device in a graphene field-effect-transistor structure using ferroelectric gating. In section 4. 2, we present ferroelectric polymer (P(VDF-TrFE)) gated graphene- field