lecture 4

Question 1

why diff

Answer 1

yo enhance properties of materials, they are often
heat-treated e.g. steel hardening , sintering, plating, …

Question 2

diff occurs in

Answer 2

Diffusion occurs in gases (scents)
and liquids (ink in water)
Our main focus here is solidstate diffusion

Question 3

types of diffusion

Answer 3

Solid State Diffusion
self diffusion
diffusion of elements such as hocn

Question 4

solid state diffusion

Answer 4

After a high-temp. heat treatment
(atoms move from larger concentration to lower conc.)

Also called interdiffusion or impurity diffusion

Question 5

self diffusion

Answer 5

In an elemental solid, atoms migrate as well
It can not be observed by noting change in chemical composition

Question 6

diffusion mechanisms

Answer 6

vacancy diffusion
interstitial diffusion

Question 7

vacancy diffusion

Answer 7

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

Question 8

interstitial diffusion

Answer 8

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

Question 9

what is diffusion flux and how do we measure it

Answer 9

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

Question 10

ficks first law

Answer 10

a formula for when Steady, linear, state diffusion i.e. concentration profile doesn’t change with time

Question 11

ficks 2nd law

Answer 11

Non-steady state diffusion
* The concentration of diffusing species is a function of both time and
position C=C(x,t)

Question 12

Diffusion Rate and Temperature

Answer 12

refer to formula
* Diffusion coefficient increases with increasing T

Question 13

Diffusion in Semiconducting Devices

Answer 13

Doping – Diffusion of very small concentrations of atoms of an impurity
(e.g., P) into the semiconductor silicon.

Question 14

Doping process

Answer 14

1. Deposit P rich
   layers on surface
2. Heat treat
   the sample to
   drive in P
3. Result is P doped
   semiconductor
   regions

Question 15

Diffusion FASTER for…

Answer 15

Diffusion FASTER for…
* open crystal structures
* materials w/secondary
bonding
* smaller diffusing atoms
* lower density materials

Question 16

Diffusion SLOWER for…

Answer 16

Diffusion SLOWER for…
* close-packed structures
* materials w/covalent
bonding
* larger diffusing atoms
* higher density materials