1. Defects & Impurities BW, Ch. 5 & YC, Ch 4 + my notes & research papers
“Human beings &
semiconductors are
interesting because
of their defects”*
*Peter Y. Yu
U.C.-Berkeley

2. Semiconductors are useful for devices is:
A primary reason that
Semiconductors are useful for devices is:
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: Useful for device operation.
Bad Impurities: Can make devices useless!

3. Semiconductors, Dielectrics, Metals
Carrier
Concentration
Dielectrics
104  cm
1012 cm-3
Semiconductors
10-4  cm
1020 cm-3
Metals
Resistivity

4. Semiconductors, Dielectrics, Metals
Carrier
Concentration
Advantage of Semiconductors: Their electrical properties can be “easily changed” by adding impurities
 Dopants
Dielectrics
104  cm
1012 cm-3
Semiconductors
10-4  cm
1020 cm-3
Metals
Resistivity

5. Semiconductors, Dielectrics, Metals
Carrier
Concentration
Advantage of Semiconductors: Their electrical properties can be “easily changed” by adding impurities
 Dopants
Dielectrics
104  cm
1012 cm-3
Semiconductors
Disadvantage of Semiconductors: Their electrical properties can be “easily changed” by adding impurities
 Contaminants
10-4  cm
1020 cm-3
Metals
Resistivity

6. Example: Impurities in Silicon
Very useful impurities!!
“Shallow” Impurity
Levels
p-type dopants: B, Al, Ga, In n-type dopants: P, As, Sb

7. Example: Impurities in Silicon
Very useful impurities!!
“Shallow” Impurity
Levels
p-type dopants: B, Al, Ga, In n-type dopants: P, As, Sb
Oxygen, Carbon
Benign impurities.

8. Example: Impurities in Silicon
Very useful impurities!!
“Shallow” Impurity
Levels
p-type dopants: B, Al, Ga, In n-type dopants: P, As, Sb
Oxygen, Carbon
Benign impurities.
Potentially dangerous
impurities!! “Deep”
Impurity Levels
Slowly diffusing & rare metals La, Y, Zr, Hf, Ta, ...

9. Example: Impurities in Silicon
Very useful impurities!!
“Shallow” Impurity
Levels
p-type dopants: B, Al, Ga, In n-type dopants: P, As, Sb
Oxygen, Carbon
Benign impurities.
Potentially dangerous
impurities!! “Deep”
Impurity Levels
Slowly diffusing & rare metals La, Y, Zr, Hf, Ta, ...
Very dangerous for devices!!
“Deep” Impurity
Levels
Fe, Cu, Ni
Cr, Mn, Au,..

10. The integrated circuit industry consumes ~> 10,000 tons
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
1 g of Si to the level of 1011 cm-3

11. 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!!

12. 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

13. 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?

14. Devices by Metal Precipitates
Example: Degradation of MOS
Devices by Metal Precipitates
Local Thinning of
the Oxide
Trap-Assisted
Tunneling

15. Devices by Metal Precipitates
Example: Degradation of MOS
Devices by Metal Precipitates

16. Effect of Iron (Fe) Contamination
on MOS Devices
Threshold Iron
Concentration
Fe contamination
isn’t the only problem!
Contamination of Si (& other materials) by
most other metal atoms is also dangerous!!

17. 1. Ultra-Clean Technology
There are at least two solutions to the problem of how
to keep metal contamination low in semiconductors
1. Ultra-Clean Technology
Growth Technology

18. 1. Ultra-Clean Technology
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!!

19. 1. Ultra-Clean Technology
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 won’t affect device yield. However, defects can be
“engineered” only if we know a lot about their Physics!!

20. Ultra-Clean
Technology

21. (NI = # impurity atoms, NH = # host atoms)
NOTE!
Altering 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.

22. Theoretical Understanding
 So, controlling the impurity concentration is VITAL to device performance!
A first step to controlling them is obtaining a
Theoretical Understanding
of their Physics.

23. Effect of Various
Substitutional
Impurities on the
Resistivity of Si

24. Some Measured Impurity Levels in the Fundamental Band Gap of Ge

25. Some Measured Impurity Levels in the Fundamental Band Gap of Ge
Shallow Donor
Levels
Shallow Acceptor Levels

26. Some Measured Impurity Levels in the Fundamental Band Gap of Ge
“Deep” Levels
Shallow Donor
Levels
Shallow Acceptor Levels
“Deep” Levels

27. Some Measured Impurity Levels in the Fundamental Band Gap of Si & GaAs

28. Some Measured Impurity Levels in the Fundamental Band Gap of Si & GaAs
Shallow
Donor
Levels
Shallow Acceptor Levels
Shallow
Donor
Levels
Shallow Acceptor Levels

29. Some Measured Impurity Levels in the Fundamental Band Gap of Si & GaAs
Shallow
Donor
Levels
“Deep”
Levels
Shallow Acceptor Levels
Shallow
Donor
Levels
“Deep”
Levels
Shallow Acceptor Levels

30. Some Measured Impurity Levels in the Fundamental Band Gap of GaAs
This shows that the measured energy in the band gap for an
impurity may “depend on the measurement technique”!

31. Some Measured Shallow Levels in Semiconductors

32. Some Measured Shallow Donor Levels in Semiconductors

33. Some Calculated & Measured Donor Levels in Si

34. Calculated & Measured Acceptor Levels in Si & Ge

35. Defects & Impurities
From the data, impurity & defect levels in semiconductor band gaps are diverse, varied, & complicated, even for “simple” substitutional impurities!
In addition to impurity levels, there can also be bandgap levels due to complex defects.

36. Defects & Impurities “Signatures” of defects & impurities.
From the data, impurity & defect levels in semiconductor band gaps 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.

37. Whole books have been written on defects & impurities
NOTE:
Whole books have been written on defects & impurities
in semiconductors!
So, we will just discuss the highlights.

38. 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

39. 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 band gap levels near the conduction or valence band edges.
These can be accurately calculated by Effective Mass Theory (“Effective Hydrogen Atom Theory”). We’ll describe this theory in some detail.

40. 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 band gap levels near the conduction or valence band edges.
These can be accurately calculated by Effective Mass Theory (“Effective Hydrogen Atom Theory”). We’ll describe this theory in some detail.
“Deep” Impurities/Defects
All others. We’ll describe a theory of these in detail.

41. Classification by Spatial Extent
Another way to classify impurities/defects is by the spatial extent of their potential & their wavefunction.

42. Classification by Spatial Extent
Another way to classify impurities/defects is by the spatial extent of their potential & 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 material & the defect.

43. Classification by Spatial Extent
Another way to classify impurities/defects is by the spatial extent of their potential & 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 material & the 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.

44. Types of Point Defects & Impurities

45. Types of Point Defects & Impurities
Vacancy: A missing atom at a lattice site.
The symbol is VA for a missing atom of type A.

46. 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.

47. 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.

48. 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: Compounds only. Host atom B occupying a site that should have had host atom A on it.
The symbol is BA for an atom of type B on an A site.

49. Types of Point Defects & Impurities
Complexes: Combinations of some point defects.
For example, a vacancy - interstitial pair: VA-IA

50. Types of Point Defects & Impurities Other Classifications
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.

51. Types of Point Defects & Impurities Other Classifications
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.

52. Point Defects & Impurities Electrically Active Defects
Our main interest in this discussion will be
Electrically Active Defects

53. Point Defects & Impurities
Our main interest in this discussion will be
Electrically Active Defects
Donors: Contribute electrons to the host material.

54. 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.

55. 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.

56. Consider Si (or any column IV atom material)

57. Some Single Donor Impurities:
Consider Si (or any column IV atom material)
Some Single Donor Impurities:
These are impurities from column V of the periodic table (P, As, …)

58. Some Single Donor Impurities:
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, ..)

59. Some Single Donor Impurities:
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.