Graphene functionalization and seeding for dielectric deposition and device integration
(Color online) ALD of Al2O3 on pristine graphene. (a) AFM image of graphene on SiO2 before ALD. The height of the triangular shaped graphene is ∼1.7 nm as shown in the height profile along the dashed line cut. Scale bar is 200 nm. (b) AFM image of the same area as (a) after ∼2 nm Al2O3 ALD deposition. The height of the triangular shaped graphene becomes ∼−0.3 nm as shown in the height profile along the dashed line cut. Scale bar is 200 nm. (c) and (d) Schematics of graphene on SiO2 before and after ALD. The Al2O3 grows preferentially on graphene edge and defect sites. Adapted from Ref. 46.
(Color online) SEM images of high-quality nanoribbons are shown in (a), (b), and (d) on HOPG surface, while very rough and discontinuous films are formed out of ALD process window, as shown in (c), (e), and (f); process conditions are contained in each image. Adapted from Ref. 27.
AFM images of ALD aluminum oxide films on HOPG. (a)–(c) 500 cycles TMA/water at different temperatures. Adapted from Ref. 26.
(Color online) (a) Low-T sheet conductance σ(Vbg) of the HfO2-covered side in three partially covered GFETs (samples A, B, and C are black, red, and blue, respectively). The majority of our HfO2-covered samples exhibit Dirac points within ±20 V and μFE > 6000 cm2/V s. (b) Rxx showing well-developed half-integer quantum Hall states on the HfO2-covered side of sample A at a magnetic field B = 8.9 T and T = 1.5 K. Adapted from Ref. 48.
(Color online) (a) HOPG sample with Al2O3 layer deposited from TMA/H2O (200 cycles) process (b) with Al2O3 layer from TMA/O3 process (50 cycles). Adapted from Ref. 25.
(Color online) (a) Cross-sectional TEM image of a device and its schematic diagram. (b) Drain current as a function of the top-gate voltage (channel length: 3 μm, channel width: 2 μm, drain voltage: 1.4 V, HfO2 thickness: 70 nm). Adapted from Ref. 52.
(Color online) AFM images of EG surfaces after (a) 250 cycles ALD at 225 °C and (b) 500 cycles ALD at 300 °C (adapted from Ref. 26). Both samples were deposited with TMA/water based processes.
(Color online) (a) AFM image of EG before H2 anneal and ALD. Many ridges and steps across terraces as well as step bunches are visible. (b) Large area SEM image of the SiC/EG/Al2O3 after H2 anneal and 260 cycles Al2O3 deposition (∼300 Å) at 225 °C. Deposition only occurs on step-bunched edges and ridges.
(a) SEM photograph of 2 × 12 μm graphene FET #1 is shown. The gate length is 2 μm. (b) Measured common-source I–V characteristics of 2 × 12 μm graphene FETs is shown. Adapted from Ref. 58.
(Color online) (a) Al2O3 layer from TMA/O3 process (50 cycles) on the ozone-treated HOPG surface at 200 °C. (b) Cross sectional HR-TEM image of the Al2O3 layer (100 cycles) deposited at 200 °C on the ozone-treated HOPG. The dashed line marks the interface between the Al2O3 layer and HOPG. Adapted from Ref. 61.
(Color online) (a) Schematic diagram of a device structure used in Ref. 67. (b) Optical microscopy image of a graphene device. Black dashed line marks the outline of graphene flake. (c) AFM image of a top-gated graphene device with a 15 nm thick Al2O3 dielectric (scale bar: 0.4 μm). Adapted from Ref. 68.
(Color online) (a) Resistance as a function of top-gate voltage (VTG) at several back-gate voltages (VBG) from −40 to 40 V and at a drain bias of 0.1 V at 25 °C for the device shown in Fig. 11; (b) the hysteresis width of the Al2O3 top-gate dielectric for the device of Fig. 11. Adapted from Ref. 68.
Raman spectra of pristine EG before and after exposure to 20 ozone pulses at 350 °C. Spectra are after the SiC substrate contribution was subtracted, are normalized to the G peak maximum and offset for clarity. Adapted from Ref. 26.
(Color online) (a) Optical image of the device layout with ground signal-ground accesses for the drain and the gate. (b) (False color) scanning electron microscopy image of the graphene channel and contacts. The inset shows the optical image of the as-deposited graphene flake (circled area) prior to the formation of electrodes. (c) Schematic cross section of the graphene transistor. Note that the device consists of two parallel channels controlled by a single gate in order to increase the drive current and device transconductance. Adapted from Ref. 17.
(Color online) Measured output characteristics of the graphene transistor for various top-gate voltages. The inset shows the transfer characteristics at a drain voltage of 100 mV. Adapted from Ref. 17.
(Color online) Measured current gain h 21 as a function of frequency of a GFET with L G ) 150 nm, showing a cutoff frequency at 26 GHz. The dashed line corresponds to the ideal 1/f dependence for h 21. Inset: the maximum f T as a function gate length for the four GFETs measured. All devices are biased at V D = 1.6 V. The solid line corresponds to the dependence for peak f T. Adapted from Ref. 17.
C-V measurement at 1 MHz from a 50-μm-diameter Ti/Au circular capacitor patterned on top of the Al2O3/EG/SiC. From the measured thickness (31 nm) a ∼ 7.6 dielectric constant was extracted. The Dirac voltage shift is ∼ 1 V. Adapted from Ref. 33.
(Color online) SEM (∼240 μm2) and AFM (9 μm2) images of Al2O3 morphology for samples treated with varying amounts of fluorine. (a),(b) 40 s XeF2 exposure results in large areas of no oxide deposition; (c),(d) 120 s XeF2 exposure yields a conformal, uniform film; and (e),(f) 200 s XeF2 exposure exhibits pinholes throughout the oxide.
(Color online) ALD of Al2O3 on PTCA-coated graphene. (a) AFM image of graphene on SiO2 before ALD. The height of the triangular shaped graphene is ∼1.6 nm as shown in the height profile along the dashed line cut. Scale bar is 500 nm. (b) AFM image of the same area as (a) after ∼2 nm Al2O3 ALD deposition. The height of the triangular shaped graphene becomes ∼3.0 nm as shown in the height profile along the dashed line cut. Scale bar is 500 nm. (c) and (d) Schematics of graphene on SiO2 before and after ALD. The Al2O3 grows uniformly on noncovalently PTCA-coated graphene. Adapted from Ref. 46.
(Color online) (a) Representative AFM image of an epitaxial graphene surface prepared by UHV graphitization. (b),(c) AFM images of a bare epitaxial graphene surface after ALD of (b) 25 cycles of Al2O3 and (c) 25 cycles of HfO2. (d),(f) AFM images of a PTCDA functionalized epitaxial graphene surface (d) immediately after PTCDA deposition, and following ALD of (e) 25 cycles of Al2O3 and (f) 25 cycles of HfO2. All AFM images were taken in intermittent contact mode. Adapted from Ref. 86.
(Color online) (a) Current_voltage characterization and (b) capacitance_voltage characterization of the dielectric stack (10 nm HfO2, 3 nm Al2O3) grown on PTCDA-functionalized graphene. Adapted from Ref. 86.
(Color online) Two-point back-gated measurements of graphene flakes. Transfer characteristics and corresponding transconductances (inset) after the different stages of buffered dielectric processing: before processing (left most pair), after NFC polymer deposition (right most pair), after HfO2 deposition (second pair from right), and after 50 W O2 plasma treatment for 30 s (second pair from left). The schematic shows the completed device configuration. Adapted from Ref. 11.
(Color online) Four-point and two-point top-gated measurements of a graphene transistor that incorporates the buffered dielectric. Four-point transfer characteristics at different back-gate voltages. The inset plots V Dirac as a function of V TG and V BG. Adapted from Ref. 11.
(Color online) Field-effect mobility (5000 cm2 V−1 s−1 at 300 K) of a device that was exfoliated onto NFC and then capped with the buffered dielectric (L = 0.9 μm, W = 1.1 μm). The graphene flake that comprises this device was exfoliated from Kish graphite. The inset shows the normalized current of the Dirac point from 5 to 300 K. V D = 10 mV for all measurements. Adapted from Ref. 11.
(Color online) Measured current gain (h 21) of a graphene transistor built on a flake that incorporates the buffered dielectric as the top-gate dielectric (L = 1.1 μm, W = 16.2 μm). The gray line shows the ideal 1/f dependence, and the intersection of the black dashed lines show f T = 9 GHz. The inset shows the top-gated transconductance exhibited by this device, and the red circle indicates the point at which the high-frequency measurement was made. V TG = 0.8 V, V D = 0.5 V, and I D = 4.59 mA in this measurement. Adapted from Ref. 11.
(Color online) (a) Image of devices fabricated on a 2-inch graphene wafer and schematic cross-sectional view of a top-gated graphene FET. SEM image of (b) a top-gated Hall bar device and (c) a top-gated field-effect transistor fabricated on the epitaxial graphene wafer. The scale bar in (c) is 2 μm. Adapted from Ref. 10.
(Color online) Measured current gain |h21| as a function of frequency for (A) nine 550-nm-gate length graphene FETs and (B) three 240-nm-gate-length graphene FETs. The cut-off frequency ranges between 20 and 53 GHz for 550-nm-gate graphene FETs, and the f T increases to 60–100 GHz for 240-nm-gate devices. The drain biases were 2 V for all devices except for the device 1 in (B), which was measured at VD = 2.5 V. Adapted from Ref. 10.
(Color online) AFM image of epitaxial graphene grown on SiC, showing the terrace structure and steps. The scanning area is 21 μm × 21 μm. The solid lines sketch the outline of graphene channels of different devices. Channels I and III both lie within a single terrace, whereas the channel II contains one step along the transport direction. Adapted from Ref. 72.
(Color online) AFM image and feature heights showing comparison of (a) 1 nm Al2O3 and (b) 1 nm HfO2 on natural graphite. Large (10 nm) clusters form after deposition of Al and oxidation at 200 °C in dry O2, in contrast to HfO2 which is substantially more uniform. Adapted from Ref. 30.
(Color online) Film uniformity and coverage on seeded graphene is dependent on deposition temperature. AFM micrographs of seeded ALD Al2O3, TiO2, and Ta2O5 films. Films deposited on graphene via ALD indicates that Al2O3 requires temperatures >150 °C while complete coverage of TiO2 and Ta2O5 occurs at ≤ 150 °C. The use of Ta2O5 as a dielectric material; however, may result in an unacceptable surface roughness for subsequent device fabrication. Adapted from Ref. 31.
(Color online) Comparison of a self-aligned device to a device in which L A = 1 μm. (a) Transfer characteristics reveal a dramatic improvement in ION/IOFF and transconductance (inset) when self-alignment is employed. (b) The output characteristics of the self-aligned device are also superior. Here, VTG is swept from −4 to 4 V in 2 V steps. (c) The measured resistance profiles (solid lines) show a 2.1 kΩ reduction of the parasitic resistance in the self-aligned device. Adapted from Ref. 93.
(Color online) Evaluation of μHall and ns (a) indicates that deposition of high-κ dielectrics on EG by O-ALD and EBPVD can lead to improved μHall through dielectric screening. Adapted from Ref. 34.
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