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ECE 371 – Chapter 1. Crystal Structure of solids. Classifying materials on the basis of their ability to conduct current. Conductor – allows for flow of current ex: copper Insulator – prevents flow of current ex: rubber

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ece 371 chapter 1

ECE 371 – Chapter 1

Crystal Structure of solids

classifying materials on the basis of their ability to conduct current
Classifying materials on the basis of their ability to conduct current.

ECE 317 Chapter 1 Crystal structure of solids

Conductor – allows for flow of current ex: copper

Insulator – prevents flow of current ex: rubber

Semiconductor - A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current.

classification of semiconductors
Classification of semiconductors

Group IV

III-V

II-VI

Elemental

Compound

ECE 317 Chapter 1 Crystal structure of solids

On the basis of the periodic chart

group iv semiconductors
Group IV semiconductors

ECE 317 Chapter 1 Crystal structure of solids

Consists of Carbon, Silicon and Germanium.

Silicon is the dominant semiconductor material.

Germanium has certain niche uses in high speed electronics, optoelectronics and photovoltaics.

Carbon semiconductor research is currently being conducted with very promising results with carbon nanotube, diamond and graphene based semiconductors.

iii v compound semiconductors
III-V compound semiconductors

Group III

Group V

ECE 317 Chapter 1 Crystal structure of solids

Consists of group III and group V elements.

This class of material is considered as alloys.

III-N also referred to as nitrides are the basis of most visible light emitting diodes and lasers in the blue to green range. Ex: Blue-ray DVD players

III-P alloys are called phosphides – mainly used for red lasers and solar cells.

III-As are referred to as arsenides used for a variety of near-IR opto-electronic and electronic technologies.

III-Sb alloys are called antimonides these are used for high speed electronics and mid-IR technologies like countermeasures lasers and thermal cameras.

cd vs dvd vs blue ray
CD Vs DVD Vs Blue-ray

AlGaAs

laser

InGaP

laser

InGaN

laser

ECE 317 Chapter 1 Crystal structure of solids

ii vi semiconductors
II-VI semiconductors

Group II

Group VI

ECE 317 Chapter 1 Crystal structure of solids

Mainly used in detectors made of HgCdTe. These detectors are very useful for MWIR and LWIR applications such as thermal sensing and night vision.

classification for compound semiconductors based on number of constituent elements
classification for compound semiconductors based on number of constituent elements

ECE 317 Chapter 1 Crystal structure of solids

Binary: One group III and one group V. Simplistic model consists of one layer of group III and one layer of group V. Group III and V atomic site are mutually exclusive to their respective elements. Ex: GaAs, InP.

Ternary: Three elements in all. Could be two group IIIs and one group V or vice-versa. Again group III sites and group V sites are exclusive thus in ternary with two group III species the group III atoms divide the spots up amongst themselves.

Ex1: Al0.7Ga0.3As. Here 70% of the group III sites are occupied by Al and the rest by Ga and 100% of the group V sites are taken by As.

Ex2: GaAs0.6P0.4. Here 100% of the group III sites are occupied by Ga and 60% of the group V sites are occupied by As and the rest and 40% of the group V sites are taken by P.

binaries and ternaries cont
Binaries and ternaries (cont.)

GaAs – Binary alloy

Group III

Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga

Group V

As As As As As As As As As As

Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga

Group III

As As As As As As As As As As

Group V

Al0.7Ga0.3As – Ternary alloy

Group III

Al Ga Al Al Ga Al Al Ga Al Al

Group V

As As As As As As As As As As

GaAs0.6P0.4 – Ternary alloy

Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga

Group III

As P As As P As As P As P

Group V

ECE 317 Chapter 1 Crystal structure of solids

quaternary alloys
Quaternary alloys

ECE 317 Chapter 1 Crystal structure of solids

Three group IIIs one group V Ex: Al0.3Ga0.3In0.4As

Two group IIIs and two group Vs Ex: Al0.4Ga0.6As0.2Sb0.8

One group III and three group Vs. Ex: GaAs0.8Sb0.1P0.1

Verify this yourself – in the above examples all the group III constituents add to give a 100% and all the group V constituents add to give 100%.

Can you think of a quintinary (5 element) alloy? Is Al0.1Ga0.9In0.1As0.7Sb0.2 a valid composition? (hint: its not ). Feel free to change the compositions of this alloy to make it correct.

types of solids
Types of solids

ECE 317 Chapter 1 Crystal structure of solids

Amorphous – no order in the atoms.

Poly-crystalline – short range order.

Single crystal – Long range order.

See fig. 1.1 in neamen.

lattice and basis
Lattice and basis

See fig. 1.2

ECE 317 Chapter 1 Crystal structure of solids

The lattice is a periodic arrangement of points in space. Each point on the lattice is called a Lattice point. (duh!)

The basis consists of the simplest arrangement of atoms which is repeated at every point in the lattice to build up the crystal structure.

Translation to produce the lattice: Each lattice point can be translated by a1 in one direction and b1 in another non-colinear direction. This results in a 2-D lattice. A third translation along another non-colinear direction results in a 3-D lattice.

unit cell
Unit Cell

ECE 317 Chapter 1 Crystal structure of solids

Mathematical Definition (from P.K. Bhattacharya): A unit cell is the region of a crystal defined by vectors a, b and c and the angles α, β and γ such which when translated by integral multiples of those vectors reproduce a similar region of the crystal.

OR A unit cell is a small volume of the crystal that can be used to reproduce the entire crystal.

See fig. 1.3

Translation property:

r = ha + kb + lc

a,b,c are basis vectors.

r is the translational vector.

a, b and c could be inter-atomic distances in which case they are called lattice-constants.

Primitive Cell: A primitive cell is the smallest unit cell in volume that can be defined for a specific lattice. See fig. 1.4

bravais lattices
Bravais Lattices

Auguste Bravais

ECE 317 Chapter 1 Crystal structure of solids

The number of ways in which lattice points can be specified in space while maintaining translational symmetry, is limited.

Auguste Bravais demonstrated 14 types of such point lattices in 1848. Nobody has come up with new ones since.

the 14 bravais lattices
The 14 bravais lattices

ECE 317 Chapter 1 Crystal structure of solids

cubic lattices
Cubic lattices

ECE 317 Chapter 1 Crystal structure of solids

Simple cubic (SC)

Body-centered cubic (BCC)

Face centered cubic (FCC)

See fig 1.5 in the text.

class problem 1
Class problem #1

ECE 317 Chapter 1 Crystal structure of solids

  • Calculate the packing fraction of a BCC cell assuming spherical atoms.
  • If the interatomic distance is 5 Å what is the density of atoms in the crystal.
  • Do the same for
    • SC
    • FCC
defining planes hkl
Defining planes (hkl)

ECE 317 Chapter 1 Crystal structure of solids

See Fig. 1.6 for an example of a plane.

Miller indices are an effective nomenclature for naming planes.

Miller indices refer to the integers (hkl). Ex: (110), (111), (100) See fig. 1.7

All parallel planes have the same indices and are equivalent to each other. So avoid planes through the origin.

class problems
Class problems

ECE 317 Chapter 1 Crystal structure of solids

Example 1.3, see fig. 1.8

Problem #2: TYU E 1.3

Determine the distance between the nearest (110) planes in a SC lattice with a lattice constant of ao = 4.83 Å.

Problem #3: TYU E 1.4

The lattice constant of a FCC structure is 4.75 Å. Calculate the surface density of atoms for (a) a (100) plane and (b) a (110) plane.

expressing directions
Expressing directions

ECE 317 Chapter 1 Crystal structure of solids

Fig. 1.9

So (hkl) is the plane, [hkl] is the direction.

diamond structure
Diamond structure

ECE 317 Chapter 1 Crystal structure of solids

gaas zincblende
GaAs - ZincBlende

ECE 317 Chapter 1 Crystal structure of solids

atomic bonding
Atomic bonding

ECE 317 Chapter 1 Crystal structure of solids

  • Ionic bond: Na+Cl-
  • Covalent bond – sharing e- to complete an octet
    • H need only one atom to complete the octet and therefore we only have H2.
    • Silicon needs 4 e- and so can bond to four other Si atoms, forming a crystal.
  • Metallic bond
  • Van der Waals
imperfections in solids
Imperfections in solids

ECE 317 Chapter 1 Crystal structure of solids

  • Lattice vibrations
  • Point defect
    • Vacancy
    • Interstitial
    • Frenkel defect (vacancy-interstitial)
  • Line dislocation
point defect
Point defect

ECE 317 Chapter 1 Crystal structure of solids

impurities in solids
Impurities in solids

ECE 317 Chapter 1 Crystal structure of solids

Substitution

Interstitial

Doping

semiconductor growth
Semiconductor growth

ECE 317 Chapter 1 Crystal structure of solids

from a melt
From a melt

ECE 317 Chapter 1 Crystal structure of solids

epitaxy mocvd
Epitaxy - MOCVD

ECE 317 Chapter 1 Crystal structure of solids

epitaxy mbe
Epitaxy -MBE

ECE 317 Chapter 1 Crystal structure of solids