ECE 371 – Chapter 1 Crystal Structure of Solids Classifying Materials on the Basis of Their Ability to Conduct Current

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

2 . 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.

ECE 317 Chapter 1 Crystal structure of solids Classification of semiconductors

. On the basis of the periodic chart

Group IV III-V II-VI

Elemental Compound

ECE 317 Chapter 1 Crystal structure of solids Group IV semiconductors

. Consists of Carbon, Silicon and Germanium.

4 . 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.

ECE 317 Chapter 1 Crystal structure of solids III-V compound semiconductors

. 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 5 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 Group V technologies.

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

ECE 317 Chapter 1 Crystal structure of solids CD Vs DVD Vs Blue-ray

AlGaAs InGaP InGaN laser laser laser

ECE 317 Chapter 1 Crystal structure of solids II-VI semiconductors

. Mainly used in detectors made of HgCdTe. These detectors are very useful for MWIR and LWIR applications such as 7 thermal sensing and night vision. Group VI Group II

ECE 317 Chapter 1 Crystal structure of solids classification for compound semiconductors based on number of constituent elements

. 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. 8 . 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.

ECE 317 Chapter 1 Crystal structure of solids 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 9 Group III Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group V As As As As As As As As As As

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 Group III Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group V As P As As P As As P As P

ECE 317 Chapter 1 Crystal structure of solids Quaternary alloys

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

. Two group IIIs and two group Vs Ex: Al0.4Ga0.6As0.2Sb0.8 10 . 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.

ECE 317 Chapter 1 Crystal structure of solids Types of solids

. Amorphous – no order in the atoms.

. Poly-crystalline – short range order. 11 . Single crystal – Long range order.

. See fig. 1.1 in neamen.

ECE 317 Chapter 1 Crystal structure of solids Lattice and basis

. The lattice is a periodic arrangement of points in space. Each point on the lattice is called a Lattice point. (duh!) 12 . 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 See fig. one direction and b1 in another non- 1.2 colinear direction. This results in a 2-D lattice. A third translation along another non-colinear direction results in a 3-D lattice.

ECE 317 Chapter 1 Crystal structure of solids Unit Cell . 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. 13 . 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

ECE 317 Chapter 1 Crystal structure of solids Bravais Lattices

. The number of ways in which lattice Auguste Bravais points can be specified in space while maintaining translational symmetry, is 14 limited. . Auguste Bravais demonstrated 14 types of such point lattices in 1848. Nobody has come up with new ones since.

ECE 317 Chapter 1 Crystal structure of solids The 14 bravais lattices

ECE 317 Chapter 1 Crystal structure of solids Cubic lattices

. Simple cubic (SC)

. Body-centered cubic (BCC) 16 . Face centered cubic (FCC)

. See fig 1.5 in the text.

ECE 317 Chapter 1 Crystal structure of solids Class problem #1

. Calculate the packing fraction of a BCC cell assuming spherical atoms.

17 . If the interatomic distance is 5 Å what is the density of atoms in the crystal.

. Do the same for . SC . FCC

ECE 317 Chapter 1 Crystal structure of solids Defining planes (hkl)

. See Fig. 1.6 for an example of a plane.

. Miller indices are an effective nomenclature for naming planes. 18 . 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.

ECE 317 Chapter 1 Crystal structure of solids Class problems . 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.

19 . 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.

ECE 317 Chapter 1 Crystal structure of solids Expressing directions

. Fig. 1.9

. So (hkl) is the plane, [hkl] is the 20 direction.

ECE 317 Chapter 1 Crystal structure of solids Diamond structure

ECE 317 Chapter 1 Crystal structure of solids GaAs - ZincBlende

ECE 317 Chapter 1 Crystal structure of solids Atomic bonding

. Ionic bond: Na+Cl-

. Covalent bond – sharing e- to complete 23 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

ECE 317 Chapter 1 Crystal structure of solids Imperfections in solids

. Lattice vibrations

. Point defect 24 . Vacancy . Interstitial . Frenkel defect (vacancy-interstitial)

. Line dislocation

ECE 317 Chapter 1 Crystal structure of solids 25

ECE 317 Chapter 1 Crystal structure of solids Point defect

ECE 317 Chapter 1 Crystal structure of solids Impurities in solids

. Substitution

. Interstitial 27 . Doping

ECE 317 Chapter 1 Crystal structure of solids Semiconductor growth

ECE 317 Chapter 1 Crystal structure of solids From a melt

ECE 317 Chapter 1 Crystal structure of solids Epitaxy - MOCVD

ECE 317 Chapter 1 Crystal structure of solids Epitaxy -MBE

ECE 317 Chapter 1 Crystal structure of solids