DefectsDefects

Types of defectsTypes of defectsStructures of the solids considered Structures of the solids considered

have been ideal, i.e. devoid of defects.have been ideal, i.e. devoid of defects.Real materials contain structural Real materials contain structural

defects which can dominate their defects which can dominate their physical and chemical behaviour.physical and chemical behaviour.

?? Disordered?Disordered?

Definition:Definition: A structural defect is thus a A structural defect is thus a

configuration in which an atom, or configuration in which an atom, or group of atoms does not satisfy group of atoms does not satisfy the structural rules belonging to the structural rules belonging to the ideal reference state of the the ideal reference state of the materials.materials.

There are many different types of structural There are many different types of structural defects. Defects are spatially localized in some defects. Defects are spatially localized in some way, i.e. defects are structurally inhomogeneous.way, i.e. defects are structurally inhomogeneous.

Zero-dimensional (point) Zero-dimensional (point) defects:defects:

An atom may be missing from a lattice An atom may be missing from a lattice site, thereby forming an site, thereby forming an atomic atomic vacancyvacancy..

Another type of atom may be present Another type of atom may be present at a lattice site, forming a at a lattice site, forming a substitutional impurity defect.substitutional impurity defect.

An atom may be situated at an An atom may be situated at an interstritial interstritial site between lattice sites.site between lattice sites.

One-dimensional (line) One-dimensional (line) defectsdefects

TranslationaTranslational l displacemendisplacements of atoms ts of atoms (dislocations(dislocations).).

Rotational displacements (disclinations).Rotational displacements (disclinations).

Two dimensional (planar) Two dimensional (planar) defectsdefects

Low angle grain boundaryLow angle grain boundary

V-shaped crystals in CVD prepared diamond film grown on Si substrate

P.B. Barna

Amorphous cases? Amorphous cases?

Types of defects?Types of defects?

Dangling bond (positive Dangling bond (positive U) U)

It is simple a broken or unsatisfied It is simple a broken or unsatisfied

bond in a covalent solid. A simple bond in a covalent solid. A simple dangling bond normally contains one dangling bond normally contains one electron and electrically neutral. But! electron and electrically neutral. But! Under certain circumstances the Under certain circumstances the electronic occupancy can change!electronic occupancy can change!

(U is Hubbard energy; electron-(U is Hubbard energy; electron-electron interaction)electron interaction)

In amorphous silicon the atoms In amorphous silicon the atoms form a continuous random network. form a continuous random network. Not all the atoms within amorphous Not all the atoms within amorphous silicon are fourfold coordinated. Some silicon are fourfold coordinated. Some atoms have a dangling bond. These atoms have a dangling bond. These dangling bonds are defects in the dangling bonds are defects in the continuous random network, which continuous random network, which cause anomalous electrical behavior.cause anomalous electrical behavior.

Dangling bond (DB) has ESR signal!Dangling bond (DB) has ESR signal!

DD00; D; D++; D; D--

Avoid DBAvoid DB If desired, the material can be If desired, the material can be

passivated by hydrogenpassivated by hydrogen, which bonds , which bonds to the dangling bonds and can to the dangling bonds and can reduce the dangling bond density by reduce the dangling bond density by several orders of magnitude. several orders of magnitude. Hydrogenated amorphous silicon (a-Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of Si:H) has a sufficiently low amount of defects to be used within devices.defects to be used within devices.

Dangling bond (negative U)Dangling bond (negative U) VI. column elements: VI. column elements:

a. End of chains, one coordinated atoms. a. End of chains, one coordinated atoms. b. Junction, three-fold coordinated b. Junction, three-fold coordinated

atoms.atoms. No ESR signal. Electron-Phonon No ESR signal. Electron-Phonon

interaction interaction Coordination defectsCoordination defects

Wrong bondsWrong bonds A-A, B-B bonds in AB alloys instead of A-A, B-B bonds in AB alloys instead of

A-B bondsA-B bonds

Electrical Electrical conductivityconductivity

General aspectsGeneral aspects Electrical conduction of Electrical conduction of

amorphous semiconductors amorphous semiconductors consists of :consists of :

1. band conduction 1. band conduction 2. hopping conduction2. hopping conduction

1.1. Band conduction Band conduction (electron and hole)(electron and hole)

Band conduction in undoped amorphous Band conduction in undoped amorphous

semiconductor characterized bysemiconductor characterized by

бб==ббo o exp { - Eexp { - Eaa / k / kbbT}T}

where where бб andand ббoo are electrical are electrical conductivity and a prefactor. conductivity and a prefactor. EEaa, k, kbb andand T T are the activation energy, the Boltzmann are the activation energy, the Boltzmann constant and temperature.constant and temperature.

2. Hopping conductivity2. Hopping conductivity a./ nearest-neighbour hoppinga./ nearest-neighbour hopping Nearest neighbour hopping is well Nearest neighbour hopping is well

known in known in crystalline semiconductorscrystalline semiconductors, , in which electrons hop the nearest-in which electrons hop the nearest-neighbouring sites by emitting or neighbouring sites by emitting or absorbing phonons in n-type (p-type) absorbing phonons in n-type (p-type) semiconductors. semiconductors.

b./ variable-range hoppingb./ variable-range hopping

Variable-range hopping is particularly Variable-range hopping is particularly associated with tail states, in which associated with tail states, in which electrons or holes in tail states hop to electrons or holes in tail states hop to the most probable sites. This type of the most probable sites. This type of hopping conductivity is characterized hopping conductivity is characterized by temperature variation as follows:by temperature variation as follows:

ббpp==ббpo po exp { - B/ Texp { - B/ T1/41/4}}