**3. Enhanced GFET THz modulator with high-k dielectric layer**

In most GFET devices, ~300-nm SiO2 was used as the gate dielectric on top of a p-type silicon (p-Si) substrate. However, when used as a THz broadband modulator, GFET with graphene/~300 nm SiO2 /p-Si structures has drawbacks such as high switching voltage, small modulation depth, and slow modulation speed [39, 40]. Alternatively, Al2 O3 with a dielectric constant of 7.5 is a high-ĸ material with numerous outstanding dielectric properties in comparison with SiO<sup>2</sup> [41]. In this chapter, we introduce a high-efficiency broadband THz wave modulator with quicker modulation speed and larger modulation depth by using Al2 O3 -based large-area graphene GFET device. The modulator consists of graphene monolayer/~60 nm Al2 O3 /p-Si structures. In our device, an intensity modulation depth of 22% (1% per 1.36 V) and a modulation speed of 170 kHz have been successfully achieved, which are notable improvements from previously reported 15% and 20 kHz, respectively, in the broadband modulators with graphene/~95 nm SiO2 /Si structures [39].

## **3.1. Fabrication and characteristics of the enhanced THz modulator**

The native oxide layer on the (100) p-Si (ρ~1–10 Ω-cm) substrate was dissolved by buffered oxide etching solution to make a naked hydrophobic silicon surface. An Al2 O3 film was deposited on top of the silicon substrate by atomic layer deposition (ALD) technique at 120°C using trimethylaluminum (TMA) and O2 as the source [42]. **Figure 1(a)** shows the X-ray diffraction (XRD) of the Al2 O3 thin film on Si substrate scanned by the Cu *K*α radiation. The strong peak at 69°C is attributed to the silicon substrate, and the sharp peak at ~33°C coincides with the reflection of the α-Al2 O3 phase, which indicates that the Al2 O3 film is a single crystalline. **Figure 1(b)** shows the cross-section of the sample characterized by scanning electron microscope (SEM). As shown in **Figure 1(b)**, the deposited Al2 O3 layer is dense and smooth with a thickness of ~60 nm. Both XRD spectrum and SEM image confirm that Al2 O3 films were well prepared by the ALD system.

results in the observation that narrow ribbon widths down to less than 10 nm are required to open a band gap in graphene and achieve an acceptable level of low off currents. However, fabricating of narrow ribbons of that dimension is a big challenge even with current advanced

The zero-band-gap characteristic means that the density of states (DOS) in graphene is linear with respect to the energy level. Therefore, gate voltage can modulate the DOS linearly to enable modulation of carrier (current) in the channel. In other words, graphene-based fieldeffect transistor (GFET) can be used to tune the carrier concentration in graphene by applying a voltage at the gate, making it possible to modulate the absorption/transmission of THz wave through the devices. Recently, such a graphene-based THz modulator was demonstrated by Sensale-Rodriguez et al. and Maeng et al. [39, 40]. It is reported that the transmission of THz wave through graphene can be controlled by electrically tuning the density of states available for intra-band transitions. Though the modulation depth and speed are limited to 15% and 20 kHz by this electrical device prototype, it opens a new direction for graphene application

**3. Enhanced GFET THz modulator with high-k dielectric layer**

modulation depth, and slow modulation speed [39, 40]. Alternatively, Al2

/Si structures [39].

**3.1. Fabrication and characteristics of the enhanced THz modulator**

etching solution to make a naked hydrophobic silicon surface. An Al2

phase, which indicates that the Al2

XRD spectrum and SEM image confirm that Al2

O3

con (p-Si) substrate. However, when used as a THz broadband modulator, GFET with gra-

constant of 7.5 is a high-ĸ material with numerous outstanding dielectric properties in com-

large-area graphene GFET device. The modulator consists of graphene monolayer/~60 nm

The native oxide layer on the (100) p-Si (ρ~1–10 Ω-cm) substrate was dissolved by buffered oxide

top of the silicon substrate by atomic layer deposition (ALD) technique at 120°C using trimeth-

attributed to the silicon substrate, and the sharp peak at ~33°C coincides with the reflection of

O3

the cross-section of the sample characterized by scanning electron microscope (SEM). As shown

O3

thin film on Si substrate scanned by the Cu *K*α radiation. The strong peak at 69°C is

/p-Si structures. In our device, an intensity modulation depth of 22% (1% per 1.36 V) and a modulation speed of 170 kHz have been successfully achieved, which are notable improvements from previously reported 15% and 20 kHz, respectively, in the broadband modulators

modulator with quicker modulation speed and larger modulation depth by using Al2

was used as the gate dielectric on top of a p-type sili-

O3

O3

film is a single crystalline. **Figure 1(b)** shows

films were well prepared by the ALD system.

as the source [42]. **Figure 1(a)** shows the X-ray diffraction (XRD) of

layer is dense and smooth with a thickness of ~60 nm. Both

with a dielectric

O3

film was deposited on


/p-Si structures has drawbacks such as high switching voltage, small

[41]. In this chapter, we introduce a high-efficiency broadband THz wave

lithographic techniques [37, 38].

120 Design, Simulation and Construction of Field Effect Transistors

in a transistor structure [39, 40].

In most GFET devices, ~300-nm SiO2

phene/~300 nm SiO2

parison with SiO<sup>2</sup>

with graphene/~95 nm SiO2

ylaluminum (TMA) and O2

in **Figure 1(b)**, the deposited Al2

O3

Al2 O3

the Al2 O3

the α-Al2

**Figure 1.** (a) θ–2θ scan of Al2 O3 -based GFET by X-ray diffraction with Cu*K*α radiation and (b) SEM image of the crosssection of a single crystalline ~60 nm Al2 O3 film on the Si substrate. The inset shows the surface of graphene/Al2 O3 obtained by optical microscope. The dash lines are a guide to the eye.

Large-area monolayers of graphene films were synthesized by chemical vapor deposition (CVD) on a copper foil, which were then transferred onto the ~60-nm Al2 O3 /p-Si substrate and 300-nm SiO<sup>2</sup> /p-Si substrate, respectively [43, 44]. The SiO<sup>2</sup> /p-Si substrate was used to fabricate the reference sample. The transferred graphene monolayers on the Al2 O3 /p-Si and SiO2 /p-Si substrate were characterized by a Raman spectroscopy with a 514 nm laser in **Figure 2(a)**. The two obvious peaks in the Raman spectra of graphene on the Al2 O3 /p-Si substrate are the G peak at ~1591 cm−1 and two-dimensional peak at ~2687 cm−1. The peak intensity ratio *I G/I2D* is ~0.43, and the full-width at half-maximum (FWHM) of the two-dimensional peak is about 38 cm−1. In addition, the intensity of the D peak at 1341 cm−1 is low with an *I D/IG* ratio of ~0.18, indicating that the transferred graphene is a high-quality monolayer which is similar to the sample grown on the SiO2 /p-Si substrate [45, 46]. The optical microscope shows the top view of the graphene/Al2 O3 in the inset of **Figure 1(b)**, confirming that the device surface is smooth, uniform, and free of pinholes over large areas.

To acquire further understanding of the THz modulation characteristics, we now discuss the carrier properties of the graphene layer under electrical biasing. As graphene can be treated as a thin film, the frequency-dependent amplitude transmission |T(ω)| is given by ∣ *T*(ω) ∣ = <sup>∣</sup> \_\_\_\_\_\_\_\_\_\_\_\_\_\_ <sup>1</sup> <sup>1</sup> <sup>+</sup> *<sup>N</sup>* <sup>Z</sup><sup>0</sup> σ(ω)/(1 <sup>+</sup> *ns* ) <sup>∣</sup>, where *<sup>N</sup>* <sup>=</sup> 1 is the number of graphene layer, Z0 is the vacuum impedance, and ns is the effective refractive index of the substrate. σ (ω) is the complex sheet conductivity of graphene, which can be described by the simple Drude model, namely σ(ω) <sup>=</sup> \_\_\_\_\_\_\_ *iD* π(ω + iΓ) , where Γ is the carrier-scattering rate and *D* is the Drude weight. *D* can be further expressed as *<sup>D</sup>* <sup>=</sup> (*VF <sup>e</sup>*<sup>2</sup> /ℏ) (*π*|*n*|)1/2 , where *VF* is the Fermi velocity, *e* is the electron charge, ℏ is the reduced Planck constant, and *n* is the carrier concentration of graphene. The corresponding variation of the *EF* in graphene extracted from |*EF*<sup>|</sup> <sup>=</sup> <sup>ℏ</sup> *VF* (*π*|*n*|)1/2 clearly shows the relation between *EF* and the carrier concentration *n.* **Figure 2(b)** shows the conical band structures of graphene, whose Fermi level and carrier concentration can be changed by applying different gate biases in a GFET. As a result, the transmittance of the THz wave could be modulated by varying the applied bias voltages at the gate. Ambient atmosphere and processing residual on the surface of graphene often deviates the Dirac point of graphene from the zero voltage point [47]. To tune the transmittance of

per unit area, Cg

phene itself.

that the Al2

ferent *Vb*<sup>g</sup>

O3

extracted modulation depth of Al2

in turn can be precisely tuned by *Vb*<sup>g</sup>

The dynamic modulation characteristics of Al2

improved by replacing the SiO2

larger than that on SiO2

be 11 times higher than that on SiO2

the carrier density variation for Al2

GFET modulator, respectively.

, is 11.9 n F/cm2

these parameters the carrier mobility of graphene on Al2

for SiO<sup>2</sup>

ence in *g*m indicates a large discrepancy in the back-gate capacitance (Cg

**3.2. Modulation properties of the enhanced THz modulator**

sity through a GFET with graphene/~300 nm SiO2

(10 V)| to be ~2% (1% per 15 V). A thicker SiO2

O3

To further compare the THz modulation properties of devices on SiO2

effect along the transmission direction leading to the weaker modulation [39].

O3

with Al2

O3

**Figure 3(b)** shows the modulation curves measured for Al2

wave at the frequency of 1 THz through the GFET with SiO2

and 546.2 cm2 V−1 s−1, respectively. It can be seen that the carrier mobility of GFET on Al2

. More importantly, the *g*m of GFET on Al2

and SiO<sup>2</sup>

carrier mobility, a larger modulation depth and higher speed can be anticipated in Al2

substrate, we applied to the modulators with back voltages ranging from −20 to 10 V with an increment of 5 V. A blank p-Si substrate was used as a reference sample to eliminate the substrate effect. The THz transmission spectra, we would discussed later, were all normalized to a blank p-Si substrate and the absorption from the substrate was also removed. Therefore, what we analyzed in the following is almost the intrinsic properties of the gra-

We measured the spectral transmission of the modulators by a typical optical-fiber integrated terahertz time-domain spectroscopy (TDS). **Figure 3(a)** shows the normalized THz transmission inten-

back voltage from −20 to 10 V and 0.4–1.5 THz. A weak modulation of the THz transmission indicates a small swing of graphene Fermi level under the back gate bias between 20 and −10 V. A specific frequency of 1 THz was chosen to discuss the modulation behaviors. The gate-dependent transmission curve exhibited a minimum transmission of 88% at −20 V and gradually reached a maximum of 90% at 10 V. The modulation depth was thus calculated by |(T (10 V)–T (−20 V))/T

variation in THz wave transmission was observed at a different gate voltage from −20 to 10 V. At 1 THz, the transmission exhibited a minimum of 71% at −20 V and reached a maximum of 91.3% at 10 V. The total modulation depth was calculated to be 22%. The amplitude of THz transmission is approximately flat from 0.4 to 1.5 THz at each gate voltage, indicating

voltages. It is clear that the maximum modulation depth of 22.5% occurs at 0.85 THz with *Vb*<sup>g</sup> = −20 V. The transmission of THz wave is primarily determined by carrier density, which

O3

homemade setup, in which a Virginia Diodes (VDI) continuous-wave (CW) terahertz source with


. As shown in **Figure 3(d)**, the modulation of THz wave transmission can be greatly

GFET, and 110.625 nF/cm2

O3

and SiO<sup>2</sup>

O3

(~ 6.5 μS) [41]. According to the equation, the huge differ-

 for Al2 O3

Graphene Field-Effect Transistor for Terahertz Modulation

http://dx.doi.org/10.5772/intechopen.76744


/p-Si sandwich structures under each applied

back-gate dielectric layer would increase the cavity

O3


. **Figure 3(d)** compares the transmitted amplitudes of THz

and Al2

as dielectric materials in GFET.

O3


GFET [48]. With

O3 is 123


were calculated to be 682.4

(~75.5 μS) is calculated to

/ p-Si and Al2


gate dielectric at dif-

) and consequently in

O3

O3 /p-Si

**Figure 2.** (a) Raman spectra of the monolayer graphene on the 300 nm SiO2 /p-Si and 60 nm Al2 O3 /p-Si substrate. (b) conical band structure of graphene and the sweeping of the Fermi level. (c) the scheme of GFET and THz transmission(*W* = 5 mm, *L* = 5 mm). (d) Total resistance *Rtotal* as a function of back gate voltage *Vb*<sup>g</sup> in the GFET.

THz wave through the graphene, a bias at gate was applied to deviate EF further from the charge neutrality point (CNP) where carrier density of state and thus σDC(EF ) and σ(ω) are minimized due to the conical band structure.

In addition, to evaluate the CNP of monolayer graphene, a graphene-based FET based on Al2 O3 /*p*-Si was fabricated with silver paste as the source and drain electrodes, respectively. Meanwhile, *p*-Si was used as the back gate to vary the carrier concentration *n* and Fermi level EF of graphene as shown in **Figure 2(c)**. Based on the transfer characteristic measurement, the resistance between source and drain (Rtotal)-dependent back gate voltage (*Vb*<sup>g</sup> ) is shown in **Figure 2(d)**. The maximum resistance of the graphene FET occurs at 18 V, where Fermi level is located at the CNP for this device. The Fermi level of graphene would tend to remain with CNP with the back-gate voltage closing to the Dirac voltage. Carrier concentration got the minimum and the THz transmittance got the maximum at the CNP, respectively. The fabrication of SiO<sup>2</sup> -based GFET is similar to that of the Al2 O3 -based devices. The CNP of SiO2 -based GFET is ~15 V, which is very close to Al2 O3 -based devices.

The carrier mobility (*μ*) in GFET can be calculated by *μ = gmL/(WCg Vds),* where *g*<sup>m</sup> = dIds / dVbg|Vds <sup>=</sup> constant is obtained from the *Rtotal* -*Vbg* curve shown in **Figure 2(d)**; both of the length (*L*) and width (*W*) of graphene channel are 5 mm; the constant *Vds* is 1 V; the back-gate capacitance per unit area, Cg , is 11.9 n F/cm2 for SiO<sup>2</sup> GFET, and 110.625 nF/cm2 for Al2 O3 GFET [48]. With these parameters the carrier mobility of graphene on Al2 O3 and SiO<sup>2</sup> were calculated to be 682.4 and 546.2 cm2 V−1 s−1, respectively. It can be seen that the carrier mobility of GFET on Al2 O3 is larger than that on SiO2 . More importantly, the *g*m of GFET on Al2 O3 (~75.5 μS) is calculated to be 11 times higher than that on SiO2 (~ 6.5 μS) [41]. According to the equation, the huge difference in *g*m indicates a large discrepancy in the back-gate capacitance (Cg ) and consequently in the carrier density variation for Al2 O3 and SiO<sup>2</sup> -based GFET. Due to the larger *g*m and higher carrier mobility, a larger modulation depth and higher speed can be anticipated in Al2 O3 -based GFET modulator, respectively.

### **3.2. Modulation properties of the enhanced THz modulator**

THz wave through the graphene, a bias at gate was applied to deviate EF

In addition, to evaluate the CNP of monolayer graphene, a graphene-based FET based on

band structure of graphene and the sweeping of the Fermi level. (c) the scheme of GFET and THz transmission(*W* = 5 mm,

of graphene as shown in **Figure 2(c)**. Based on the transfer characteristic measurement,

**Figure 2(d)**. The maximum resistance of the graphene FET occurs at 18 V, where Fermi level is located at the CNP for this device. The Fermi level of graphene would tend to remain with CNP with the back-gate voltage closing to the Dirac voltage. Carrier concentration got the minimum and the THz transmittance got the maximum at the CNP, respectively. The fabrica-

dVbg|Vds <sup>=</sup> constant is obtained from the *Rtotal* -*Vbg* curve shown in **Figure 2(d)**; both of the length (*L*) and width (*W*) of graphene channel are 5 mm; the constant *Vds* is 1 V; the back-gate capacitance

the resistance between source and drain (Rtotal)-dependent back gate voltage (*Vb*<sup>g</sup>

O3

/*p*-Si was fabricated with silver paste as the source and drain electrodes, respectively. Meanwhile, *p*-Si was used as the back gate to vary the carrier concentration *n* and Fermi level

O3



/p-Si and 60 nm Al2

in the GFET.

O3

neutrality point (CNP) where carrier density of state and thus σDC(EF

**Figure 2.** (a) Raman spectra of the monolayer graphene on the 300 nm SiO2

122 Design, Simulation and Construction of Field Effect Transistors

*L* = 5 mm). (d) Total resistance *Rtotal* as a function of back gate voltage *Vb*<sup>g</sup>


The carrier mobility (*μ*) in GFET can be calculated by *μ = gmL/(WCg*

due to the conical band structure.

GFET is ~15 V, which is very close to Al2

Al2 O3

EF

tion of SiO<sup>2</sup>

further from the charge

/p-Si substrate. (b) conical

) is shown in

*Vds),* where *g*<sup>m</sup> = dIds /


) and σ(ω) are minimized

To further compare the THz modulation properties of devices on SiO2 / p-Si and Al2 O3 /p-Si substrate, we applied to the modulators with back voltages ranging from −20 to 10 V with an increment of 5 V. A blank p-Si substrate was used as a reference sample to eliminate the substrate effect. The THz transmission spectra, we would discussed later, were all normalized to a blank p-Si substrate and the absorption from the substrate was also removed. Therefore, what we analyzed in the following is almost the intrinsic properties of the graphene itself.

We measured the spectral transmission of the modulators by a typical optical-fiber integrated terahertz time-domain spectroscopy (TDS). **Figure 3(a)** shows the normalized THz transmission intensity through a GFET with graphene/~300 nm SiO2 /p-Si sandwich structures under each applied back voltage from −20 to 10 V and 0.4–1.5 THz. A weak modulation of the THz transmission indicates a small swing of graphene Fermi level under the back gate bias between 20 and −10 V. A specific frequency of 1 THz was chosen to discuss the modulation behaviors. The gate-dependent transmission curve exhibited a minimum transmission of 88% at −20 V and gradually reached a maximum of 90% at 10 V. The modulation depth was thus calculated by |(T (10 V)–T (−20 V))/T (10 V)| to be ~2% (1% per 15 V). A thicker SiO2 back-gate dielectric layer would increase the cavity effect along the transmission direction leading to the weaker modulation [39].

**Figure 3(b)** shows the modulation curves measured for Al2 O3 -based GFET. A distinctive variation in THz wave transmission was observed at a different gate voltage from −20 to 10 V. At 1 THz, the transmission exhibited a minimum of 71% at −20 V and reached a maximum of 91.3% at 10 V. The total modulation depth was calculated to be 22%. The amplitude of THz transmission is approximately flat from 0.4 to 1.5 THz at each gate voltage, indicating that the Al2 O3 -based GFET is an efficient broadband THz modulator. **Figure 3(c)** shows the extracted modulation depth of Al2 O3 -based GFET from 0.4 to 1.5 THz at different applied gate voltages. It is clear that the maximum modulation depth of 22.5% occurs at 0.85 THz with *Vb*<sup>g</sup> = −20 V. The transmission of THz wave is primarily determined by carrier density, which in turn can be precisely tuned by *Vb*<sup>g</sup> . **Figure 3(d)** compares the transmitted amplitudes of THz wave at the frequency of 1 THz through the GFET with SiO2 and Al2 O3 gate dielectric at different *Vb*<sup>g</sup> . As shown in **Figure 3(d)**, the modulation of THz wave transmission can be greatly improved by replacing the SiO2 with Al2 O3 as dielectric materials in GFET.

The dynamic modulation characteristics of Al2 O3 -based modulator were further studied with a homemade setup, in which a Virginia Diodes (VDI) continuous-wave (CW) terahertz source with

Obviously, the Al2

replacing 300 nm SiO2

O3

**Figure 4.** (a) Modulated THz waveform from the Al2

film is superior over SiO2

O3

O3

frequency of 5 kHz. (b) the dependence of the normalized modulation magnitude on the modulation frequency.

for many THz technology applications as well as for fundamental research.

was observed. Furthermore, the modulation speed increased from 20 to 170 kHz as well. One reason for this significant improvement is that the high-ĸ dielectrics can remarkably reduce the Coulomb impurity scattering [41] to achieve a high *g*m and consequently a larger THz modulation depth [49]. This work provides an effective method to fabricate high quality GFET THz wave modulator with large modulation depth and fast switch speed, which is vital

In contrast to rigid THz modulators, flexible THz modulators are expected to be used in application fields with complicate surfaces [50]. A typical type of flexible modulator is a fieldgrating device, with which the intensity or phase of THz wave can be modulated by electrical gating or laser, but its properties remain unchanged under device deformation. This device is highly desired in nonplanar applications. Graphene is a highly flexibility material where its electronic structure can be maintained under deformation. Therefore, it is promising to develop flexible THz modulators based on graphene FET. Here, we give a typical example.

The schematic diagram and photograph of the flexible THz modulator are presented in **Figure 5(a)** and **(b)**, respectively. The whole device is fabricated on a flexible commercial PET substrate. First, monolayer graphene was synthesized by typical chemical vapor deposition (CVD) on the copper foil and then was transferred onto the PET substrate [44–46]. The silver pastes were brushed at the two sides of graphene strip as the source and drain electrodes. The effective length and width of the channel of graphene FET was defined to be 2 and

with 60 nm Al2

**4. Flexible THz modulator based on graphene FET**

**4.1. Device fabrication of the flexible THz modulator**

as a gate dielectric in GFET modulator. By

, an enhancement of 11 times in modulation depth


Graphene Field-Effect Transistor for Terahertz Modulation

http://dx.doi.org/10.5772/intechopen.76744

125

**Figure 3.** Normalized intensity of transmitted THz wave through the (a) SiO2 - and (b) Al2 O3 -based GFET at different back gate voltage. The modulation depth of Al2 O3 -based GFET as a function of applied gate voltage is shown in (c), and (d) the comparison of the amplitudes of the THz wave transmission through the GFET modulators with SiO2 and Al2 O3 dielectric at 1 THz.

a central output in the 340 and a 240–400 GHz zero-bias Schottky diode intensity detector are included. In the measurement, we applied a square biasing voltage to the device and the output modulated THz waveform was recorded by an oscilloscope. The applied voltage pulse is −10 V at the minimum and 10 V at the maximum with various modulation frequencies. **Figure 4(a)** shows the recorded waveform of Al2 O3 -based modulator at a carrier frequency of 340 GHz. **Figure 4(b)** shows the dependence of the normalized modulation magnitude on the modulation frequency, which gives rise to a 3-dB bandwidth (*f* c ) of 170 kHz. As we know, the RC time constant of a transistor is an important parameter to determine the switch speed. The device resistance (R) was estimated to be 261 Ω by extracting the average resistance as the back voltage sweeping from −10 to 10 V. The capacitance (C) can be expressed by *C* = Aεε0 /d, where ε is the relative dielectric constant of the ALD-deposited Al2 O3 film (~7.5), *ε*<sup>0</sup> is the permittivity of free space, *A* is the effective area of active graphene device of 5 × 5 mm2 , *d* is thickness of the Al2 O3 film of 60 nm. The capacitance C is then calculated to be ~27.7 nF. As a result, the calculated *RC* time constant is ~138.7 kHz, which is very close to the directly measured 3-dB bandwidth (170 kHz). These results confirm that Al2 O3 -based GFET possesses a higher modulation speed than the conventional silicon-based graphene THz modulators [39].

**Figure 4.** (a) Modulated THz waveform from the Al2 O3 -based modulator under a square voltage pulse at a modulation frequency of 5 kHz. (b) the dependence of the normalized modulation magnitude on the modulation frequency.

Obviously, the Al2 O3 film is superior over SiO2 as a gate dielectric in GFET modulator. By replacing 300 nm SiO2 with 60 nm Al2 O3 , an enhancement of 11 times in modulation depth was observed. Furthermore, the modulation speed increased from 20 to 170 kHz as well. One reason for this significant improvement is that the high-ĸ dielectrics can remarkably reduce the Coulomb impurity scattering [41] to achieve a high *g*m and consequently a larger THz modulation depth [49]. This work provides an effective method to fabricate high quality GFET THz wave modulator with large modulation depth and fast switch speed, which is vital for many THz technology applications as well as for fundamental research.
