lecture 4 Flashcards
why diff
yo enhance properties of materials, they are often
heat-treated e.g. steel hardening , sintering, plating, …
diff occurs in
Diffusion occurs in gases (scents)
and liquids (ink in water)
Our main focus here is solidstate diffusion
types of diffusion
Solid State Diffusion
self diffusion
diffusion of elements such as hocn
solid state diffusion
After a high-temp. heat treatment
(atoms move from larger concentration to lower conc.)
Also called interdiffusion or impurity diffusion
self diffusion
In an elemental solid, atoms migrate as well
It can not be observed by noting change in chemical composition
diffusion mechanisms
vacancy diffusion
interstitial diffusion
vacancy diffusion
Need
adjacent vacant site: ↑ temp. è ↑ # of vac.
* Need energy to break atomic bond and jump: ↑ temp.
* Both self-diffusion and inter-diffusion can occur through this mechanism
refer to formula
interstitial diffusion
Small atoms such as hydrogen, carbon, nitrogen & oxygen
* Requires energy to squeeze past atoms
* Typically faster than vacancy diffusion:
* More site available
* Less energy required
what is diffusion flux and how do we measure it
quantify the amount or rate of diffusion
Measured empirically
* Make thin film (membrane) of known surface area
* Impose concentration gradient
* Measure how fast atoms or molecules diffuse through the membrane
refer to formula
ficks first law
a formula for when Steady, linear, state diffusion i.e. concentration profile doesn’t change with time
ficks 2nd law
Non-steady state diffusion
* The concentration of diffusing species is a function of both time and
position C=C(x,t)
Diffusion Rate and Temperature
refer to formula
* Diffusion coefficient increases with increasing T
Diffusion in Semiconducting Devices
Doping – Diffusion of very small concentrations of atoms of an impurity
(e.g., P) into the semiconductor silicon.
Doping process
- Deposit P rich
layers on surface - Heat treat
the sample to
drive in P - Result is P doped
semiconductor
regions
Diffusion FASTER for…
Diffusion FASTER for…
* open crystal structures
* materials w/secondary
bonding
* smaller diffusing atoms
* lower density materials
