Introduction
Operation principle and technical limit of three-dimensional structure FET
In addition to the channel length, another factor affecting device performance is the thickness of the oxide layer. As shown in Fig.1.6, as the oxide layer is made thinner, the capacitance of the channel interface increases, and device performance improves.
FET Based on a 2-D Material
Fig. 3.3 shows representative transfer curves for BP units as a function of exposure time. On the other hand, as shown in fig. 4.4 (c), most of the BP surface was covered with large bubbles (dark area) due to severe degradation on the perfluorooctanephosphonic acid substrate.
Fabrication method
Exfoliation method of 2D materials
As shown in Figure 2.1, a layered material such as graphite is attached to a tape, and when the tape is overlapped and peeled off, the layer is separated. As shown in Figure 2.2, when a tape covered with thin pieces of graphite is brought into close contact with a substrate such as a SiO2/Si wafer and then slowly peeled off, some of the flakes are transferred to a target substrate. The mechanical exfoliation can be easily applied to obtain high-quality two-dimensional materials, even though this method is impractical for industrial applications.
After transferring the flakes to a target substrate and we monitor the samples with an optical microscope. In the case of graphene, the results of a previous research showed that monolayer graphene can be easily identified on 300 nm thick SiO2 grown on Si [2]. Since this identification method relies on the optical interference effect between the substrate of the transferred 2D materials, we may need to use different Si-grown SiO2 thicknesses for optimal BP identification.
Mask align process for electrode patterning
On the other hand, with prolonged exposure (over 70 hours), the transfer curve begins to show a significant change in shape as shown in Figure 3.3(b). Our observation is consistent with previous experimental results that the degradation rate is faster on hydrophobic substrate [1]. Device transfer curves as a function of exposure time (b) on the PEI substrate and (d) on the perfluorooctane phosphonic acid substrate.
On the other hand, a very large bubble was observed on the perfluorooctanephosphonic acid substrates. We confirmed that the degradation behavior differs significantly depending on the hydrophobicity of the substrates by TEM cross-sectional observation. We observed the strong substrate-dependent BP degradation, mainly depending on the hydrophobicity of the substrate.
Black phosphorus field effect transistor
BP crystal and device structure
Because BP is a two-dimensional material like graphene, graphene research in the past has provided much inspiration for BP research. On the other hand, BP is more suitable for device applications because it is a semiconductor material [1, 3]. The band gap of monolayer BP is about 2 eV (direct gap) while the bulk counterpart has a band gap of about 0.3 eV.
Therefore, BP is a material with very high potential in optical and thermal devices as well as electrical devices. Monolayer black phosphorus (phosphorene) can be obtained by exfoliation and the optical properties of phosphorene have been determined experimentally, but most of the electrical performance is measured using a few-layer phosphor [7]. The isolation of monolayer BP is difficult compared to graphene due to the stronger layer-layer interaction in BP.
Electrical properties
With the exposure to ambient conditions, BP devices showed the gradual shift of the minimum conduction point to a higher gate bias, indicating the p-do effect (Figure 3.3). We can attribute the observed transfer curve shape change to the irreversible BP degradation due to environmental exposure. C is the capacitance, which is determined by the permittivity and the thickness of the oxide layer.
Interestingly, the measured mobility of the same device showed no significant change at the same exposure time (~60 h), indicating that the electrical channel interface at BP-SiO2. The observed predominant degradation near the electrodes can be attributed to the different water diffusion behavior from the Au and SiO2 surface. Since the BP decomposition is not uniform as shown in Figure 3.5, the performance of the electrical device, such as the charge carrier mobility, will be mainly affected by the decomposition behavior near the electrodes.
Conclusion
In particular, the bubbles on the perfluorooctane phosphonic acid substrate, which is the most hydrophobic, were much larger than those on 3-P. As shown in Figure 4.5, there is no significant difference in the on/off ratio on the PEI substrate even after 94 h exposure, but it suddenly increased after 80 h exposure on the perfluorooctane phosphonic acid substrate. On the other hand, the mobility of the device fabricated on the hydrophobic substrate dropped sharply after 80 hours and eventually completely lost its function.
We point out that the exposed BP on the device comes into contact with both the substrate and the electrode. We observed that the degradation is slow and uniform on the hydrophilic substrate, but fast and non-uniform on the hydrophobic substrate. We attribute the observed phenomena to the different diffusion rate of water molecules depending on the hydrophobicity of the substrate.
Degradation of black phosphorus on various substrates
Introduction
As previously discussed, BP degradation with ambient exposure is one of the main challenges to study various important properties as well as future electrical applications. These studies have shown that surface passivation can be an effective way to eliminate the degradation of BP. The detailed study of BP degradation and its mechanism will therefore pave the way to prevent its degradation by another fundamental process.
A previous result indicated that BP degradation is strongly influenced by the surface on which BP flakes exfoliate and is subsequently exposed to environmental conditions. To study the effect of a substrate on BP degradation behavior in detail, we prepare BP flakes of similar thickness on various substrates and monitor their degradation. The electrical measurement as well as AFM imaging, Raman spectroscopy, TEM cross-section imaging clearly show that BP degradation is indeed strongly influenced by the surface chemistry of substrate.
Substrates preparation
BP surface measurement by Atomic Force Microscopy
- AFM image analysis method
- Results and discussion
Next, the original image was converted to an 8-bit image and the part of the area to be analyzed was selected. Please note that we performed the comparison experiments for all seven substrates and Figure 4.2 only shows results on three substrates. Interestingly, BP flakes on PEI substrate, which is the most hydrophilic, showed very few bubbles even after exposure to air for 48 h.
Fig.4.3 clearly shows the correlation between the water contact angles of the substrates and the size of the bubbles on the BP surface. Bubble size tends to be larger for hydrophobic substrates, while bubble formation was not observed on PEI (40 degree contact angle). Although we did not observe bubble formation in PEI, we cannot rule out the possibility that BP undergoes degradation from the buried channel without surface change.
Electrical Properties
On the other hand, on the perfluorooctane phosphonic acid, the transfer curve shows a strong sharp change in shape around 50 hours of exposure. From the previous exposure result in Chapter 3, we can assume that the degradation of BP is faster on perfluorooctane phosphonic acid compared to PEI. This faster degradation to perfluorooctane phosphonic acid by electrical measurement is consistent with AFM and optical imaging results.
4.5 (a) On/off ratio (b) and hole mobility as a function of exposure time for PEI and perfluorooctanephosphonic acid devices. The mobility of the BP device on the PEI substrate was reduced to approximately 10 cm2V-1s-1 after exposure for 94 hours, but the device still functioned. From the geometry of the device, we must be careful that the electrical degradation of the BP device on PEI may be affected by the degradation near the electrodes.
TEM cross section observation
Conclusion
The low-dimensional material is a system in which at least one of the three dimensions is limited, and therefore the material properties are significantly changed compared to conventional bulk [1]. The furnace temperature is slowly raised to 1020 °C, followed by annealing for 30 minutes or more. Fig. 5.3 is an optical microscope image and a transfer curve of the fabricated graphene device.
5.4 (a) Optical microscope image of GeSe device. b) A plot of gate sweep varying with light response. For the fabricated devices, the thickness of the GeSe channel material was about 100 nm or more, which was a very thick bulk crystal. A gate sweep was performed while applying the +1V discharge voltage to measure the electrical characteristics of the fabricated GeSe device.
Other materials
Introduction
That is, the properties can vary greatly depending on the type of material in contact with the surface of these materials. By using these properties of low-dimensional materials, it is possible to create new physical properties that have never been observed by the combination of low-dimensional materials. In this paper, low-dimensional materials such as graphene, GeSe, and AgCN were investigated in addition to BP.
Graphene
- Graphene synthesis
- Properties of graphene
At this time, the hydrogen gas is continuously flowed to remove impurities on the Cu foil surface. After CH4 gas flows after annealing, graphene starts to form on the Cu foil surface. Graphene is synthesized by maintaining these gas flow conditions for 15 minutes and the furnace is then slowly cooled.
The graphene sample is mostly monolayer, as shown in the blue Raman spectrum. When the multilayer graphene is grown as marked by the red dot of the optical image, the width of the 2D peak broadens and the G peak/2D peak intensity ratio changes [5].
Germanium selenide (GeSe)
- Properties of GeSe
- Conclusion (GeSe)
We find that the on/off ratio was very small because the GeSe flame was so thick. The device characteristics measured with the light off were a mobility of 0.82 cm2/V·s and an on/off ratio of 2.16. Characteristics measured under white light illumination shifted the transfer curve with high photocurrent.
On the other hand, GeSe has a high photoresponsivity and can have an interesting optoelectronic device properties.
Silver cyanide (AgCN)
- AgCN growth method
- Properties of AgCN
- Conclusion (AgCN)
The AgCN (wire) device was fabricated similarly to other device manufacturing methods introduced in Chapter 2. Although the drain voltage was 100 V, only a very weak current of less than 5 nA flowed. 5.9 (a) Optical and SEM images of pristine hexagonal AgCN devices. b) Optical and SEM images after exposure to the electric field in SEM equipment.
At the beginning of the measurement, the current does not flow as in wired AgCN devices, but when the 10V drain voltage is continuously applied for a long time, the current is observed at some point (Figure 5.9 (c)). The detailed mechanisms for this electrical disturbance require further attention, but we attribute this observation to the formation of conductive pathways (such as filaments) under the high electric field. Through our conducted experiment, we found that the AgCN is originally an insulator, but the electrically conductive path can be formed under the continuously applied voltage condition.
Summary and Conclusions
