Defects & ImpuritiesBW, Ch. 5 & YC, Ch 4 + my notes & research papers

“Human beings and semiconductors are interesting because

of their defects”*

*Peter Y. YuU.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!

10-4 cm

104 cm

1020 cm-3

1012 cm-3

Resistivity

Carrier Concentration

Metals

Semiconductors

Dielectrics

Advantage of Semiconductors:Their electrical properties can be

“easily changed” by adding

impurities DOPANTS

Disadvantage of Semiconductors:Their electrical properties can be

“easily changed” by adding impurities

CONTAMINANTS

Semiconductors, Dielectrics, Metals

Very useful impurities!!

“Shallow” ImpurityLevels

p-type dopants: B, Al, Ga, Inn-type dopants: P, As, Sb

Benign impurities. Oxygen, Carbon

Potentially dangerous impurities!! “Deep” Impurity Levels

Slowly diffusing & rare metals

La, Y, Zr, Hf, Ta, ...

Very dangerous for devices!!

“Deep” ImpurityLevels

Fe, Cu, Ni Cr, Mn, Au,..

Example: Impurities in Silicon

Industrial Data (~ 5 years ago): The integrated circuitindustry consumes ~> 10,000 tons of Si per year!!

Laboratory Data on Si: 10 mg of Fe is sufficient tocontaminate this amount of silicon to the level of 1011 cm-3

~ 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?

~10,000 tons!!

Example: Degradation of MOS devices by metal precipitates

Local thinning of the oxide

Trap-assisted tunneling

Note: Fe contamination isn’t the only problem! Contamination of Si

(& other materials) by most other metal atoms is also very dangerous!!

Effect of Iron (Fe) Contamination on MOS DevicesThreshold Iron Concentration

There are at least two solutions to the problem of howto keep metal contamination low in semiconductors

1. Ultra-Clean Technology

2. “Defect Engineering”

Growth Technology

PhysicsPhysics!!!!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 VariousSubstitutional

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

This shows that the measured energy for an impurity

may “depend on the measurement technique”!

Some Measured Impurity Levels in GaAs

Some Measured Shallow Levels in Semiconductors

Some Measured Shallow DonorLevels 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 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 of Defects & Impurities

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.

• 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.

Types of 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.

Point Defects & 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.