Chapter 4 BP Thin Film Transistors
4.4 Anisotropic Properties of Black Phosphorus
As mentioned before, BP thin films with rectangular shape can be fabricated by using the technique introduced in chapter 2. The crystal orientation of the rectangular BP film can be immediately verified by optical microscope, as the long and short sides of the rectangular BP corresponds to ZZ and AC directions, respectively. Thus, the current transport direction can be determined without complicate and tedious techniques to determine the lattice orientation of BP like Raman measurement, and the fabrication time can be reduced to prevent BP from severe oxidation. Besides, the calculation of the field-effect mobility will be more precise since it can be extracted by the transconductance and the geometric factor L/W of the thin film when the device operates in linear region. However, the L/W for an irregularly shaped BP film is so indefinite that it inevitably leads to a finite error in the calculation of mobility.
Thanks to the geometric certainty of rectangular BP, a more accurate mobility can be acquired.
To investigate the anisotropic properties of the rectangular BP, two rectangular BP flakes were mechanically exfoliated onto the Si/SiO2 substrate ,and they were fabricated along ZZ and AC direction, respectively. The contact metal is AuGe alloy.
Table 4.5 lists some important electrical properties of these two devices. Fig. 4.11 (b) and Fig. 4.12 (b) show the thickness of these two devices which is about 11.5 nm and 20 nm, respectively, suggesting that both devices can demonstrate good on/off current ratio (ZZ = 1 × 104 and AC = 2.3 × 103), as shown in Fig. 4.11 (e) and Fig. 4.12 (e). Output characteristics show that AuGe alloy forms a good ohmic contact with BP.
Notice that the peak hole mobility along AC direction can be up to 298 cm2/V-sec, which is comparable to other studies [93, 100, 104, 105]. In addition, the hole (electron) mobility ratio of AC/ZZ is about 1.88 (2.78), which is consistent with some previous researches [28, 30]. In conclusion, the using of contact and direction engineering can contribute to a high hole mobility for BP, revealing the potential of BP in the application of field-effect transistors.
(a)
(b)
(c) (d)
(e)
ZZ AC
Thickness = 11.5 nm. Output characteristics for (c) hole and (d) electron transport.
(e) Transfer characteristics.
(a) (b)
(c)
(d)
(e)
ZZ AC
Thickness = 20 nm. Output characteristics for (c) hole and (d) electron transport (e) Transfer characteristics.
Table 4.5 Electrical properties of BP TFTs along two transport directions.
Orientation Carrier Mobility(cm
2/V*s) On/Off Subthreshold Swing(V/dec)
different directions were fabricated on the same BP flake, as shown in Fig 4.13. The thickness of this BP film is about 35 nm. The transfer characteristics of these two BP TFTs along AC and ZZ directions are shown in Fig. 4.13 (c) and (d), respectively.Table 4.6 shows the electrical properties of these two devices. Note that the hole mobility ratio between AC and ZZ is reduced to only about 1.52 in this case, which is lower than the theoretical results. The reason is that when there are two pairs of source/drain electrodes corresponding to AC and ZZ directions on the same BP flake, the drain current cannot be completely parallel to the AC (ZZ) orientation owing to
mobility along AC (ZZ) direction. Fig. 4.14 illustrates the current spreading effect
when measuring the mobility ratio μAC/μZZ.
(a)
(b)
(c) (d)
Fig. 4.13 (a) AFM image of the BP TFT sample, W/L = 4.35um/3.5um for ZZ, and 1um/1.7um for AC, respectively. (b) Thickness = 35 nm. Transfer characteristics
for (c) ZZ direction and (d) AC direction.
Table 4.6 Electrical properties of BP TFTs along two transport directions on the same flake.
Orientation Carrier Mobility(cm2/V*s) On/Off Subthreshold Swing(V/dec)
ZigZag Hole 140 𝟐 × 𝟏𝟎𝟐 13.31
ZZ
AC
Electron 12 𝟏 × 𝟏𝟎𝟏 20.96
ArmChair Hole 213 𝟏. 𝟓 × 𝟏𝟎𝟐 14.21
Electron 15 𝟖 × 𝟏𝟎𝟎 34.35
Yao Hsiao, Master Thesis, Black phosphorus with unique rectangular shape and its anisotropy for electronics and optoelectronics (2018).
Fig. 4.14 Illustration of the current spreading effect in BP TFT.
In fact, the mobility is not a fixed value because of the various scattering mechanisms, such as surface roughness scattering, phonon scattering and Coulomb scattering. Among these scattering mechanisms, the surface roughness scattering is the dominant mechanism in nano-scaled 2D materials. Therefore, the mobility in 2D materials is not a constant but a function of back gate voltage. Fig 4.15 shows the hole mobility and the mobility ratio between AC/ZZ versus back gate bias according to the
seems to gradually decline as the back gate bias sweeps from 0V to -100V. The μAC/μZZ ratio is found to be in the interval of 1.8 to 2.8. The big ratio (>2.4) after Vg is
less than -40V can be attributed to the fact that the thickness of the ZZ sample (thickness = 11.5 nm) is thinner than the AC sample (thickness = 20 nm), as mentioned in Fig. 4.11 (b) and Fig. 4.12 (b). As a result, when the back gate voltage sweeps to a large negative bias, a thinner film will suffer the surface roughness scattering at the BP/SiO2 interface worse than a thicker film, resulting in a faster drop of the mobility and the increasing μAC/μZZ ratio.
Fig. 4.15 The mobilities along AC (black line) and ZZ (blue line) directions and the μAC/μZZ ratio versus back gate bias (red line), the ZZ and AC samples are
mentioned in Fig 4.11 and Fig. 4.12, respectively.
On the other hand, Fig. 4.16 displays the hole mobility and the μAC/μZZ ratio versus back gate bias based on the sample in Fig. 4.13. Notice that the peak mobility occurs at Vg = 25V for both AC and ZZ directions. The drop of mobility is due to the surface roughness scattering. Since these two samples share the same BP film and thus the same thickness, the impact of surface scattering on these two devices can also be seemed as the same. Originally, the μAC/μZZ ratio = 1.52 at Vg = 25V, and then gradually decreases as the back gate bias sweeps to -100V, implying that at large negative bias, the scattering mechanism will become the dominant factor to affect the mobility, while the influence of different transport directions will become less important.
Fig. 4.16 The mobilities along AC (black line) and ZZ (blue line) directions and
the μAC/μZZ ratio versus back gate bias (red line), the AC and ZZ samples are mentioned in Fig. 4.13.