Defects & Impurities
BW, Ch. 5 & YC, Ch 4 + my notes & research papers
“Human beings and
semiconductors are
interesting because
of their defects”*
*Peter Y.
Yu
U.C.-Berkeley
A primary reason that
Semiconductors are useful for devices:
The electronic (& other) properties can be
significantly altered by incorporating
impurities (& other defects) into the material.
• There are “good” impurities (& defects) & “bad” ones!
Good Impurities:
Are useful for device operation.
Bad Impurities:
Can make devices useless!
Semiconductors, Dielectrics, Metals
Carrier
Concentration


104  cm
Dielectrics
Advantage of Semiconductors:
Their electrical properties can be
“easily changed” by adding
impurities 
DOPANTS
1012 cm-3

Semiconductors
10-4

 cm
1020 cm-3


Resistivity
Metals
Disadvantage of Semiconductors:
Their electrical properties can be
“easily changed” by adding impurities
 CONTAMINANTS
Example: Impurities in Silicon
Very useful impurities!!
“Shallow” Impurity
Levels
Benign impurities.
Potentially dangerous
impurities!! “Deep”
Impurity Levels
Very dangerous for devices!!
“Deep” Impurity
Levels
p-type dopants: B, Al, Ga, In
n-type dopants: P, As, Sb
Oxygen, Carbon
Slowly diffusing & rare metals
La, Y, Zr, Hf, Ta, ...
Fe, Cu, Ni
Cr, Mn, Au,..
Industrial Data (~ 5 years ago): The integrated circuit
industry consumes ~> 10,000 tons of Si per year!!
Laboratory Data on Si: 10 mg of Fe is sufficient to
contaminate this amount of silicon to the level of 1011 cm-3
~10,000
tons!!
~ 10 mg
A Practical Question: With such possibilities of contamination,
how can a high purity of Si wafers be maintained in the process of
manufacturing of integrated circuits?
Example: Degradation of MOS devices by metal precipitates
Local thinning of the oxide
Trap-assisted tunneling
Effect of Iron (Fe) Contamination on MOS Devices
Threshold Iron Concentration
Note: Fe contamination isn’t the only problem! Contamination of Si
(& other materials) by most other metal atoms is also very dangerous!!
There are at least two solutions to the problem of how
to keep metal contamination low in semiconductors
1. Ultra-Clean Technology
Growth Technology
2. “Defect Engineering”
Physics!!
Metals are dangerous only if they are in the device-active region. If metals
can be removed from the devices, & localized in pre-defined regions of the
wafer, or if they can be “passivated”, they will not affect the device yield.
However, defects can be “engineered” only if we know a lot about their
PHYSICS!!
Ultra-Clean
Technology
NOTE!
• To alter the electronic properties of a semiconductor only
requires a VERY SMALL absolute impurity concentration:
(NI/NH) ~  10-6
(NI = # impurity atoms, NH = # host atoms)
• Also, impurities & defects CAN produce energy levels in
the fundamental bandgap of the perfect crystal.
 Controlling the impurity concentration is
VITAL to device performance!
• A first step to controlling them is obtaining a
Theoretical understanding
of their Physics.
Effect of Various
Substitutional
Impurities on the
Resistivity of Si
Some Measured Impurity Levels in Si & GaAs
Some Measured Impurity Levels in Ge
Shallow Donor
Levels
Shallow Acceptor
Levels
Some Measured Impurity Levels in GaAs
This shows that the measured energy for an impurity
may “depend on the measurement technique”!
Some Measured Shallow Levels in Semiconductors
Some Measured Shallow Donor
Levels in Semiconductors
Some Calculated & Measured Donor Levels in Si
Some Calculated & Measured Acceptor Levels in Si & Ge
Defects & Impurities
• From the data, impurity & defect levels in semiconductor
bandgaps are diverse, varied, & complicated, even
for “simple” substitutional impurities!
• In addition to impurity levels, there can also be
bandgap levels due to complex defects.
• It is now known that the bandgap levels can be
understood as being “signatures” of defects & impurities.
NOTE
• Whole books have been written on defects &
impurities in semiconductors!
– So, we will just discuss the highlights.
Classification of Defects & Impurities
Classification by Level Depth
• One obvious way to classify impurities & defects
is by their level “depth” in the bandgap.
“Shallow” Impurities
• These produce bandgap levels near the conduction
or valence band edges.
• These can be accurately calculated by Effective Mass
Theory (“Effective Hydrogen Atom Theory”). We will
describe this theory in some detail.
“Deep” Impurities
• All others. We will describe a theory of these in detail.
Classification by Spatial Extent
• Another way to classify impurities & defects is by the
spatial extent of their potential and their wavefunction.
Point Defects
• These are isolated atoms or small groups of atoms
(complexes). This kind is all that we’ll discuss here.
• Point defects can be either good or bad for the material,
depending on the individual material and defect.
Line Defects
• These are defects in which rows or planes of atoms are
involved (such as dislocations). These are usually bad
for the material. We won’t discuss these here.
Types of Point Defects & Impurities
• Vacancy: A missing atom at a lattice site.
– The symbol is VA for a missing atom of type A.
• Interstitial: An atom in between lattice sites.
– It is possible to have a self-interstitial.
– The symbol is IA for an atom of type A at an interstitial site.
• Substitutional Impurity: An impurity atom C
replacing a host atom A.
– The symbol is CA for an atom of type C replacing an atom of
type A.
• Antisite Defect: In compounds only. A host atom B
occupying a site that should have had a host atom A on it.
– The symbol is BA for an atom of type B on an A site.
Types of Point Defects & Impurities
• Complexes: Combinations of some point defects.
– For example, a vacancy - interstitial pair: VA-IA
Other Classifications
• Intrinsic or Native Defects: No matter what the
growth process is, these cannot be completely eliminated.
– Examples: Vacancies, Antisite Defects, Self-interstitials.
• Extrinsic Defects: Impurities or impurity
complexes of some sort.
Point Defects & Impurities
• Our main interest in this discussion will be
Electrically Active Defects
• Donors: Contribute electrons to the host material.
• Acceptors: Accept electrons from the host.
Or donate Holes to the host.
• Isoelectronic Impurities: Are substitutional
impurities from the same column of the periodic
table as the host atom being replaced.
Consider Si (or any column IV atom material)
Some Single Donor Impurities:
• These are impurities from column V of the
periodic table (P, As, …)
Some Single Acceptor Impurities:
• These are impurities from column III of the
periodic table (B, Al, Ga, ..)
• There are also Double Donors or Double
Acceptors, etc. which donate or accept two
electrons.