Also, the rate of evaporation could not be increased by adding more heat unless the equilibrium vapor pressure was also increased by this action. The net evaporation flux is the difference between the collision rates of the two fluxes. Knudsen Cell The evaporation flux into a solid angle of dw from a source area of dAe.
The evaporative mass deposited per unit area of the condensing surface is .. the surface area is directly proportional to the cosine of the angle.
CNDL
Physical Vapor Deposition
- Evaporation
Feature of Evaporation Method
Shadowing or step coverage
Inert-Gas Condensation
Laser Ablation
Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is struck by sufficiently energetic particles.
Advantages
Sputtering
Application of Sputtering
Parameters influencing to sputtering yield
Chemical Vapor Deposition
- Process Principle
At high temp., decomposition and homogeneous gas phase reaction intermediate decomposition and/or chemical reaction. At low temperature, absorbed on the substrate, heterogeneous reactions occur at the interface between gas and solid.
The effect of the temperature on the deposition rate
Thermodynamic aspects I
Where A and B are supplied with gas, C is a solid product (deposition) Gas D is diffused away from the substrate during the process. Where A and B are supplied with gas, C is a solid product (deposition) Gas D is diffused away from the substrate during the process.
Thermodynamic aspects II
Efficient contact between the gas phase and the growing surface
Advantages & Disadvantages
Similar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction. ALD film growth is self-limited and based on surface reactions, enabling atomic-scale control of deposition to be achieved. By keeping the precursors separated throughout the coating process, atomic layer thickness control of films grown can be achieved as fine as atomic/molecular scale per monolayer.
Definition of ALD
Atomic Layer Deposition
Release successive pulses of precursor gas to deposit a film on the substrate one layer at a time. The precursor gas is introduced into the process chamber and produces a monolayer of gas on the wafer surface. A second precursor of gas is then introduced into the chamber which reacts with the first precursor to produce a monolayer film on the wafer surface.
Because each pair of gas pulses (one cycle) produces exactly one monolayer film, the thickness of the resulting film can be precisely controlled by the number of deposition cycles. Trimethylaluminum (TMA) reacts with the adsorbed -OH groups, TMA does not react with itself, causing the reaction to terminate later. After the TMA and methane reaction product are pumped out, water vapor (H2O) is pulsed into the reaction chamber.
H2O reacts with the dangling methyl groups on the new surface and forms aluminum oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse. The reaction product methane is pumped away, Excess H2O vapor does not react with the hydroxyl surface groups, again causing perfect passivation down to one atomic layer.
ALD reactors
Carbon Nanotubes CNDL
- Introduction
- Structures of CNTs
- Properties of CNTs
- Synthesis of CNTs
- Introduction CNDL Carbon allotropes
- Introduction CNDL
- What are CNTs?
- Discovery of CNT: misunderstanding and truth
- Introduction CNDL 4. Classification of CNT
- Structures of CNTs CNDL
However, commercial applications have developed rather slowly, mainly due to the high production costs of the best quality nanotubes. Single-walled carbon nanotubes (SWCNTs). one graphite sheet seamlessly wrapped into a cylinder. typical radius 1nm, length up to mm.
Chiral Vector, C h
Three Types of Nanotubes
- Structures of CNTs CNDL Structural Properties of CNTs
- Properties of CNTs CNDL 1. Electronic properties of CNTs
- Electronic properties of CNTs
- Properties of CNTs CNDL
- Mechanical properties of CNTs
- Magnetic properties of CNTs
- Properties of CNTs CNDL Magnetic properties of CNTs
- Synthesis of CNTs CNDL 1. Arc discharge
- Synthesis of CNTs CNDL
- Arc discharge
- Synthesis of CNTs CNDL 2. Laser ablation
- Laser ablation
- Chemical vapor deposition
The electronic properties of nanotubes derived from the dispersion relationship of a graphite sheet with the wave vectors (kx, ky). The intertube coupling causes a small bandgap for certain metallic tubes, but a reduced bandgap by 40% for semiconducting tubes in SWNT. These structures are molecular mimics of 2- or 3-terminal heterojunctions that connect two or three different nanotubes in the form of A-B or A-B-C, where A, B and C can be a metallic or semiconducting tube.
Localized states appear at the contact interface containing pentagons and heptagons and have also been observed at the closed ends of SWNTs and MWNTs. Electronic properties have been related to mechanical, chemical, biological, thermal, and magnetic interactions with nanotubes, so extended electromechanical, electrochemical, thermal electronic, and electromagnetic properties have been associated with the use of CNTs in sensors, actuators, field emissions, batteries, fuel cells, capacitors and many others. Young's modulus is independent of the tube chirality but depends on the tube diameter. coaxial intertube coupling or van der Waals force).
In asymmetric tubes, any strain will cause asymmetric σ-π rehybridization and therefore change in electronic properties. Magnetic properties such as anisotropic g-factor and susceptibility of nanotubes are expected to be similar to those of graphite, while some unusual properties may exist for nanotubes. A metal-insulator transition and band gap shift for semiconductor tubes under magnetic field parallel to the tube axis. similar to the electrical response of nanotubes to mechanical deformation).
Driven by magnetic or voltage field, the Fermi level will move away from the original position, and this leads to the band gap change pattern in Fiqure 1.15. Similar response can also be observed when magnetic field or strain field is perpendicular to tube axis. Carbon groups from the anodic graphite rod caused by the collision are cooled to low temperature and condensed on the surface of the cathode.
The carbon clusters from the graphite target are cooled, adsorbed and condensed on the Cu collector at low temperature. It involves heating the catalyst material to high temperatures in a tube furnace and flowing hydrocarbon gas through a tube reactor. The CVD process involves transition metal-catalyzed dissociation of hydrocarbon molecules and the dissolution and saturation of carbon atoms in metal nanoparticles.
Kinds of CVD
Plasma enhanced CVD
Thermal CVD
Synthesis of CNTs CNDL Vapor phase growth
- Vapor phase growth
- Other methods
The VPG method does not use a substrate, but reacts in a chamber by supporting both the reaction gas and the metal catalyst in the chamber.
HiPCO (The High Pressure Carbon Monoxide Process)
Growth Mechanism of CNTs
Synthesis of CNTs CNDL 6. Issues in the synthesis
Applications of CNTs CNDL
Potential applications and limitations
Electronic devices
FET (Field-effect transistors)
FET (Field-effect transistors): MOSFET
FET (Field-effect transistors): CNT-FET
TFT (Thin film transistors) for flexible display
TFT (Thin film transistors) for flexible display Optical performance: visible transmittance
Applications of CNTs CNDL 1. Electronic devices
Onchip thermal management
Energy applications
Lithium-ion batteries
- Applications of CNTs CNDL 2. Energy applications
Fuell cells
Mechanical applications
CNT-filled polymer composites
Applications of CNTs CNDL 4. Field emission display
- Field emission display
The mechanism of electron emission is in principle similar to that of a lightning rod: the sharp points center the electrons and enable electron emission. When the gate and emitter are energized, electrons are released, and when an intense electric field is applied to the space between the two glasses, the electrons are accelerated and maintain high kinetic energy.
