I would like my advisor, Dr. Jim Davidson, thanks for his support and . mentorship throughout this project. This project would not have been possible without the support of Dr. Davidson and Dr. Kang, and I would like to express my sincere thanks for all their work, guidance, patience and support throughout this process. I would also like to Dr. Travis Wade, Dr. Hank Paxton, Dr. Mike Alles, Dr. Justin Gregory, Dr. Tony Hmelo, dr.
Ben Schmidt and Dr. Bo Choi for providing training and advice throughout the course of this project. And finally, I would like to thank my parents, who have been a constant source of emotional support.
Structure of Carbon Background
10, 11] A significant portion of the remaining charges can become trapped at defects in the dielectric layer. When exposed to an electric field, the ions will separate to the two poles of the field. Silver contacts were then deposited on top of the films for use in electrical measurements.
The seeding step was carried out on the prepared surface of the substrate in an evacuated room using a. However, the introduction of defects has a negative impact on the electrical performance of the insulator. To make a smoother surface, several of the diamond films were soldered to molybdenum and etched away the silicon substrate in KOH.
The brazing process changes the morphology of the diamond film, revealing a wavy pattern on the diamond surface. To test whether the brazing process changes the properties of the diamond film, a free-standing diamond film was grown. A thick layer of boron-doped microcrystalline diamond (~250 µm) was grown on top of the nanocrystalline layer.
However, the diamond grain cluster size in this sample is greater than 500 nm. After the films were grown, the thickness of one of the carbon films was measured with a profilometer. Resistance can be determined from the slope of the ohmic region of the I-V curve.
The diffuse nature of the depletion region indicates a fairly large number of interface traps. The probes were directly contacted to the surface with a gap of 2.3 mm between each of the probes. This indicates that there is another current path, most likely along grain boundaries on the surface of the diamond film.
The presence of surfactant on the surface makes any testing of CNTs impossible.
1.1) Aromatic Rings
1.2) Graphene
Graphene is the name given to a flat, monolayer of carbon atoms arranged in a tightly packed 2D honeycomb lattice. This honeycomb lattice consists of a large number of polycyclic benzene rings, making graphene a very large aromatic substance. 5,6] Graphene exhibits high conductivity, due to its extensive conjugated sp2 network, which is reported to be as high as ~64 mScm-1 [6] and which remains stable over a wide temperature range.
3] Graphene has been shown to achieve very large mobilities, which remain high even when the concentrations of electrons and holes are increased. 5] This suggests that if graphene were used as a channel in a transistor, it could provide an extremely fast transistor with low power consumption.
1.3) Carbon Nanotubes
In the case where p = 0, the 1-D k11 subband aligns with the K1 point at the intersection of the conical electron and hole energy bands.
1.4) Diamond
Effects of Radiation Background
Ionizing radiation can cause degradation in CMOS circuits by creating electron-hole pairs (EHPs) in the gate and insulating dielectrics. Due to the different mobilities of electrons and holes, charges can separate and cause formation. The percentage of EHPs that can escape initial recombination depends on the strength of the dielectric electric field.
While electrons can be quickly carried away to the gate electrode, as is the case with SiO2, the holes remain relatively stationary. Oxide traps occur when a net charge (usually positive) is created in the bulk region of a dielectric by trapping holes, in the case of a positive oxide trap, at defective sites in a dielectric. This is due to the large densities of intrinsic defects, in contrast to SiO2, which generally has very few "as-grown" defects, [14, 15].
In addition to oxide traps, irradiation can also change the interface trapping properties of a device. Interface traps occur directly at the interface, and have energies that lie in the Si band gap. 16] The traps in the upper half of the Si bandgap are positive, meaning they are neutral when filled and positively charged when empty.
Traps in the lower half of the bandgap are like an acceptor, so are neutral when empty and negatively charged when filled. When a single energetic particle hits a material, it generates a dense plasma of EHPs along the path of the particle that can cause a variety of single-event effects. If the energy stored in the dielectric is high enough, it can lead to a thermal runaway condition.
Dielectric Materials Background
21] Thus, by using it as a substrate for radiation testing purposes, it would be possible to view the damage on the CNT or graphene rather than on the underlying substrate. Because the nature and properties of the substrate play such a major role in electron transport, it is important to understand the substrate itself. These conditions were used for the seed layer to stimulate nucleation growth, creating a uniform film over the entire substrate.
Lowering the methane level stunts growth and the intense plasma conditions etch away some of the methane. In some cases the features were not sharp enough for the gold pattern on top of the photoresist to be discontinuous with the gold deposited on the film. This filter was then transferred to the surface of the desired substrate and placed under pressure while baking at 90°C for 45 minutes.
As shown in Fig 3.8, none of the three nucleation techniques shown resulted in a cohesive, pinhole-free film. The sample grown after sonication shows areas of "good" diamond growth, but also has large areas of the film which are sparsely nucleated. This size would completely obscure the bottom gate structure as it would bridge the crack.
Some of the films were used to test the material properties of the films while the others were used to test their electrical properties. The surface of the substrate must be as smooth as possible to avoid adding stress to the graphene sheet or nanotube that would rest on top of it. This can be problematic, as the diamond can begin to fail as a dielectric for some of the voltages we would need to use in our radiation testing.
These defects would have a negative impact on the mobility of the carbon devices that would be supported by the diamond film, and efforts must be made to minimize them if we want to use polycrystalline diamond films as substrates. Once the transistor devices have been fabricated, the nature of the carbon-diamond interface will be measured using a variety of techniques. By nature, polycrystalline diamond films contain a number of imperfections, specifically at the interfaces between the individual crystals, i.e. grain boundaries.
Charge carrier transport phenomena in amorphous SiO2: Direct measurement of drift mobility and lifetime.
Material Characterization
Diamond as a Dielectric
The diamond films were characterized by their current-voltage (I-V) behavior using a Hewlett Packard 4156 Semiconductor Parameter Analyzer and the setup shown in Figure 4.3. With so little current flow through the diamond insulator, the structure should essentially act as an ideal oxide. This can be attributed to the presence of highly ordered pyrolytic graphene (HOPG) in the diamond film.
Capacitance and voltage measurements were not performed on the above diamond films; however, C-V measurements were performed on previous films grown under similar conditions. Using this as the capacitance of the diamond foil, we calculate a dielectric constant of 0.209, which is well below the expected value of 5.5. Our diamond films are no longer in the ohmic range around -2 volts, while it does not accumulate to -10 volts.
At these voltages, excessive current leaks through the device, and so we expect a smaller capacitance. This observed large shift in the flatband voltage indicates the existence of a significant amount of oxide charges. This value does not correspond to the high resistance of 10^13 Ωcm and above that was measured earlier through the film.
The measured plate resistances are quite low and could prove problematic if trying to use them as dielectrics in MOSFETs. The resistivity of the whole diamond film was also measured using the four-point probe technique to check whether the brazing process introduced additional defects in the film after soldering. The all-diamond film had a slightly higher plate resistance, by a factor of 2, but much more than that.
Transfer of Carbon Nanotubes
In Figure 4.8b, a much larger amount of CNTs appears to have been transferred to the surface, but the surfactant has not been removed, obscuring possible electrical access to the transferred CNTs. They have been shown to have a number of favorable properties for use as an insulating substrate. These features strongly support the use of diamond films to characterize the radiation response of carbon nanostructures.
Although very promising as substrates for radiation testing, these particular films would make very poor insulators due to the large number of dielectric and interfacial traps. Adjustments to the process need to be refined so that a greater volume of CNTs is transferred to the diamond surface. Graphene will be grown using CVD methods, or exfoliated and transferred to the surface.
This will be done in several ways, including weakening the vacuum, reducing the filter size, and other related actions. The conductance and mobility will be determined using four-point probe and Hall Effect measurements. The results derived from graphene on diamond will be compared with graphene transistors formed in the same way, but with oxide (SiO2) as substrate.
After the graphene on diamond devices have been characterized, they will be exposed to radiation in the form of X-rays, electrons and protons. We can involve additional types of radiation (eg alpha particles, heavy ions) if we can access the necessary sources. 34;Total-dose Radiation-hard Diamond-based Hydrogen Sensor.". 34;High-frequency, scaled graphene transistors on diamond-like carbon.".
Future Work
