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

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!

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

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

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

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

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.

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, ...

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,..

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

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!!

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

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?

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

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

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!!

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

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!!

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!!

Ultra-Clean Technology

(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.

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.

Effect of Various Substitutional Impurities on the Resistivity of Si

Some Measured Impurity Levels in the Fundamental Band Gap of Ge

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

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

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

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

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

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”!

Some Measured Shallow Levels in Semiconductors

Some Measured Shallow Donor Levels in Semiconductors

Some Calculated & Measured Donor Levels in Si

Calculated & Measured Acceptor Levels in Si & Ge

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.

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.

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.

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

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.

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.

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

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.

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.

Types of Point Defects & Impurities

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

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.

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.

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.

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

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.

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.

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

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

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.

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: 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 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, ..)

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.