2.5 Characterization of the graphene/Si Schottky junctions
analyzed by SKPM and CAFM
Scanning Kelvin probe microscopy (SKPM) and conducting atomic force
microscopy (CAFM) were used to investigate the surface potential,
photovoltaic effect, and rectification effect of the graphene/Si
Schottky junctions and to better understand the rapid separation and
transfer of charge carriers in the depletion layer of the junction under
light illumination.[49-50] Figure 6ashows the excellent quality of a two-dimensional AFM surface topographic
image from the graphene/Si Schottky junction. Figures 6b-6cconfirm the surface potential distribution of dark and light states,
respectively. The observed surface potential decrease under the light
compared to the dark states is believed due to the extensive built-in
electric field between graphene and Si.[49] The
built-in electric field rapidly separates the photogenerated carriers,
transfers holes to the graphene, and results in a decrease in the
surface potential and a downshift in the Fermi
level.[49,51] Figure 6d illustrates the
change between graphene and Si under light from the energy band
relationship. Under illumination, the electron-hole pair rapidly
separates under the built-in electric field, extending the life of the
photogenerated carriers.[49,52] Figures
6e-6g , describe the graphene morphology, the current diagram under dark
and light states under zero bias, respectively. It shows that the
current changes significantly under light, as shown in Figure
6g , meaning that the graphene/Si Schottky junction has a noticeable
photovoltaic effect.[53] Figure 6hrepresents the current value corresponding to the white dotted line inFigures 6f- 6g . To determine the rectification efficiency of
the Schottky junction, the current values at varying bias voltages (−3
to +4 V) were measured over a 10×10 μm2 area under
dark states, and the current maps are displayed in Figures
6i-6k . These maps were then utilized to calculate the average
rectification rate of the Schottky junction (Figure 6l ).
Average currents of −0.05 nA at −3 V and 3.55 nA at +3 V means that the
average rectification ratio is approximately 77 at ± 3 V. Hence, our
graphene/Si Schottky junction possesses not only a strong photovoltaic
effect but also an excellent rectification effect.
2.6 Photoelectric response
data was obtained using a near-IR photodetector fabricated using our
graphene/Si Schottky junction.
The photo-response of
the
as-fabricated photodetector is also evaluated using a pulsed photon
source and the results are shown in Figure 7a . The switching
behavior of the photodetector was measured with a 1550 nm pulsed laser
at a reversed bias of -1 V and the laser intensity of 30
mW/cm2. The rising/falling edges of the photocurrents
under the pulsed photon irradiation correspond to the photo-induced
voltages or photocurrents, stemming from the infrared thermionic
emission.[54-55] Our photodetector demonstrated
excellent performance characteristics (e.g. responsivity of 1.1
AW-1 and a detectivity of 1.6×1014cmHz1/2W-1), comparable to that of
the conventionally CVD synthesized and then transferred
graphene.[54-56] Figure 7b displays how
the photocurrent changes while the intensity of the 1550 nm laser
irradiation is varied. The photocurrent steadily increases as the photon
intensity is raised from 5 to 30 mW/cm2, and then
gradually decreases as the photon intensity subsequently decreases back
to 5 mW/cm2. The photocurrent remains unchanged at all
times during the irradiation with a fixed photon intensity, suggesting
an excellent repeatability as summarized in Supplementary Figure
S7 . To examine the response speed of the device, pulsed photon signals
at different pulsing frequencies (from 500 to 2000 Hz) were used to
irradiate the junction and the results are shown in Figure 7c .
The relative balance between the maximum and the minimum
((Imax - Imin) / Imax)
was found to merely decline by no more than 10% even at 2,000 Hz, as
shown in Supplementary Figure S8 . The effect of the residual
photocurrent storage in the device is practically negligible, as
demonstrated in Figure 7d , where the photocurrent measurement
was repeated on the same device during a three-week time span. By
expanding the photo-response curve at the time axis, the rise time
(tr) and fall time (tf) for the device
are estimated to be 60 μs and
63
μs, as shown in Figures 7e . These results are also comparable
to the photo-response recorded from the conventionally CVD synthesized
and then transferred graphene.[56]To obtain the specific
detectivity, the dark current noise must be obtained. Generally, it can
be measured directly using the Fast Fourier Transform (FFT) algorithm
method. Using a preamplifier with a sampling rate of 10 kHz to measure
the dark current and the data were analyzed with FFT software. As shown
in Figure 7g , based on the dark current of the device at 0 V,
the measured dark current noise is 3.2 fA Hz-1/2 at 1
Hz, which is again comparable to that obtained from a
CVD-synthesized and then transferred
photodetector.[55,57-58]
Besides the excellent photodetection demonstration, we also fabricated
two other devices from our layer-tunable and transfer-free graphene
films. A field-effect transistor (FET) was successfully fabricated from
our monolayer graphene directly grown on a SiO2substrate and its performance characteristics are shown in SupplementaryFigure S9 . A heating device was successfully fabricated from
our monolayer graphene directly grown on a transparent glass substrate
and its performance characteristics are shown in SupplementaryFigures S10-S12 . Like our graphene-Si Schottky junction
photodetector, our field-effect transistor and heating device also
demonstrate excellent performance properties as compared to similar
devices fabricated through other methods but without the need for any
graphene layer transfer.[59-61]
3. Conclusion
A novel versatile strategy for the direct synthesis of layer-tunable and
transfer-free graphene on arbitrary substrates via ion implantation
technology has been demonstrated. The stochastic nature of the
ion-implantation in terms of carbon atom distribution as well as
uncertainties during their post-implantation diffusion and precipitation
into graphene structure are eliminated by using smart Janus bi-metal
substrates with drastically different carbon solubility. Through this
approach, high-quality of mono- and bi-layer graphene films have been
synthesized directly on a variety of device-relevant substrates (Si,
SiO2, glass, sapphire etc.) without the need for any
graphene layer transfer. The layer-tunability is precisely controlled by
the equivalent fluences (atoms/cm2) during the carbon
implantation. The graphene/Si-based near-infrared photodetectors,
graphene/SiO2-based field-effect transistors, and
graphene/glass-based heating devices have been successfully fabricated
without the graphene layer transfer, and the comparable performance
characteristics of our devices to the current state of the art
approaches using the graphene layer transfer have been demonstrated.
Since ion implantation is a mature technology widely used in the current
integrated circuit manufacturing, our work has a potential to promote
layer-tunable graphene-based devices in a wide range of applications.
4. Experimental Section
Graphene Synthesis : A 100 nm thickness of Ni layer was evaporated
on various substrates (Si, SiO2, glass, sapphire etc.)
using an electron beam evaporator. These substrates were implanted with
70 keV carbon ions at room temperature to fluences of
4×1015 ions/cm2 and
8×1015 ions/cm2 to achieve uniform
mono- and bi-layer graphene structures, respectively. Following the
C-ion implantation, 150 nm Cu film was placed on top of the Ni layer at
room temperature by magnetron sputtering. Sandwiched samples were then
annealed at 1000 ℃ for 10 min under 10-5 mbar. After
annealing, the furnace was cooled to room temperature under
10-5 mbar. Further experimental details are described
in the Supplemental Material.
Graphene Transfer: After synthesis, the Cu-Ni alloys are easily
removed using a thermal release tape, leaving the graphene on the
arbitrary substrate for detailed characterization and direct device
fabrication. Thus, there is no additional graphene transfer step needed.
Characterization: SIMS (Cameca IMS-4F, Paris, France) was
performed to obtain the elemental depth profiles using a 7.5 keV
Cs+ beam oriented 70.2° from the sample normal and the
detector was positioned 140.4° from the incident beam. Raman scattering
(SENTERRA) at the wavelength of 532 nm was used to determine the
thickness, quality, and uniformity of the graphene films.
Crystallographic information and the number of graphene layers were
determined by TEM (JEOL JEM-ARM 300F). AFM (Multimode 8) was used to
determine the uniformity and thickness of the graphene films and STM
(Multimode 8) was used to assess the atomic structure of the graphene
films under ambient conditions. The structure of back-gated GFETs was
examined by SEM (HITAGHT S-3400N). The microstructure and electrical
quantity were measured by AFM, SKPM, and CAFM (Oxford Instruments,
Cypher S). By using an Agilent semiconductor parameter analyzer
(B1500A), photoelectric parameters were measured at ambient conditions,
by combining with a semiconductor characterization system (Keithley
4200).
Devices Manufacturing: The photodetectors, FETs, and heating
devices were fabricated using monolayer graphene directly synthesized on
Si, SiO2, and glass substrates, respectively. Electrodes
were prepared by silver (100 nm) electron beam evaporation.
Theoretical calculation: Molecular dynamics (MD) simulations were
performed using the ReaxFF,[62-64] which has been
confirmed to provide accurate descriptions of bond breaking and bond
forming for Ni−C systems. The initial models of the Ni-C system were
built by randomly replacing Ni atoms with C atoms. The size of the
initial system is 50 × 50 × 3.47 Å. Then these initial structures were
optimized to reach a stable configuration for 20 ps in the NVT ensemble.
Finally, the optimized model was heated from 300K to 1800K at 10K/ps,
and then the system was annealed to 300K with a constant cooling rate of
-1K/ps. All the MD simulations were conducted in the canonical NPT
ensemble using the Lammps MD package.[65]