Sse Lecture1

8/22/2019 Sse Lecture1

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SOLID STATE

ELECTRONICS


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CRYSTAL PROPERTIES ANDGROWTH OFSEMI CONDUCTORS


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Basic Semiconductors

(Properties & Growth)

Solid State Device - a component whose operationdepends on the control of electrical or magnetic

phenomena in solids, such as a transistor, diode, or

ferrite.

We will concentrate on semiconductor devices.

What distinguishes semiconductors?- resistance/resistivity

- crystalline structure


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RL

Wt

The resistance of a bar of material with dimensions L,

W, t and resistivity is:

metals: < 10-3-cminsulators: >102-cmsemiconductors: 10-3-cm < < 102-cm

W

Lt


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Properties of Semiconductor

MaterialsSC is a group of materials having electrical conductivityintermediate between metals and insulators.

Conductivity can be varied by order of magnitude withchanges in temperature, optical excitations and impuritycontents. Materials are found in column IV and neighboringcolumns of periodic table Energy band gap of SC materials is too different fromconductors and insulators


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Different semi conducting materials have differentband gap. This property is specifically used inconstructing light emitting diodes and lasers ofdifferent wavelengths over a broad range ofinfrared and visible region of spectrum.

The band gap and consequently electronic and

optical properties of SC materials are stronglyaffected by the impurities which may be added inprecisely controlled amounts.


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Conclusion:

As slight variation in the electronic structure due toimpurities can drastically change the properties of SC

Study of electronic and crystalstructure is very important


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TYPES OF

SEMICONDUCTORS

Elemental

Semiconductors

Group IV elements

e.g., Si, Ge etc.

Compound

Semiconductors

Group III-V compounds

Group IIVI compounds


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Semiconductors are found in columns II-VI

of the periodic table. Why is silicon so

popular if there are so many to choosefrom?


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elemental semiconductors: Si, Gecompound semiconductors: GaAs, InP

ternary semiconductors: AlGaAs, HgCdTequaternary semiconductors: InGaAsP, InGaAlPCommon semiconductor materials: (a) the portion of the

periodic table where semiconductors occur; (b) elemental and

compound semiconductors.


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Elemental Semiconductors:

Si and Ge are the most commonly used used SC, initially fordiodes and transistors

Now Si is used for majority of rectifiers, transistors and ICs

Compound Semiconductors:

Mostly usedin high speed devices requiring the emission orabsorption of light


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Practical Examples

Light emitting Diode:

III-V Compounds like GaN, GaP, GaAs are common lightemitting diodes (LEDs)

TV Screen Fluorescent Materials:

ZnS, II-VI compunds

Light Detectors:

InSb, CdSe, PbTe, HgCdTe light detectors

Si and Ge infrared and Nuclear Radiation detectors


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Gunn Diodes:

Microwave devices made of GaAs and InP

Semiconductor Lasers:

GaAs, AlGaAs and other ternary and quaternary compounds


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Some characteristics of semiconductors

1) melting temperature

2) relative abundance/cost3) band gap (light generation/absorption)4) ease of doping5) ease of growth6) ease of processing

7) performance


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CRYSTAL STRUCTURE

Crystalline solids (Periodic structure):

A solid in which the atoms are arranged in periodic fashion mostlysingle crystals

Amorphous Solids:

Solids in which there is no periodic structure like glass

Polycrystalline materials:

Solids composed of many small regions of single crystal materials


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Figure 11Three types of solids, classified according to atomic arrangement:(a) crystalline and (b) amorphous materials are illustrated bymicroscopic views of the atoms, whereas (c) polycrystalline structure isillustrated by a more macroscopic view of adjacent single-crystallineregions, such as (a).


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Lattice: Periodic arrangement of atoms in a crystal iscalled LATTICE Lattice is defined as a set of points in which

all the points are connected with some atoms or molecules.

Basis: Atoms, set of atoms or molecules placed at everypoint of lattice are known as basis

Crystal Lattice + BasisBasis Vectors: For three dimensional lattice the set ofthree orthogonal vectors a, b, c which can form a representationof any point of lattice are known as basis vectors


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Figure 1

2A two-dimensional lattice showing

translation of a unit cell by

r = 3a + 2b.


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Primitive unit cell: A smallest possible cell in the periodicstructure is know as primitive cell.

Unit Cell: A unit cell is a volume that is repeated throughoutthe crystal. A unit cell need not to be primitive all the time

A two-dimensional lattice showing translation of a unit cell by

r = 3a + 2b.a and b are known as basis vectors

Points within the lattice are indistinguishable if the vectorbetween the points is given by

r = pa + qb + sc

Where p, q, s are integers


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Figure 13

Unit cells for three types of cubic lattice

structures.


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Figure 14

Packing of hard spheres in an fcc lattice.


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PACKING FRACTION OR FILLING FACTOR

FCC STRUCTUREAssuming the max packing (all nearest neighbors touching each other) Consider one face, a is the length of side of cube (lattice constant)

a ac

ac a

2

2

2

22

2

2

4

1

22

2

4aThe radius of the spheres =

each corner contributes 1/8(8 corners) 1 sphereeach face contributes 1/2(6 faces) 3 spheresatoms/cell = 4


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Volume of each sphere = a3 21/2 / 24

Max. fraction of filled cells =(No. of spheres vol. of each sphere)total volume of each cell

= (4 a3 21/2 ) / 24

a3

= 21/2 = 74 % filled6


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Planes and Directions

Specify the planes and directions within the lattice Use a set of three integers to describe the position or direction of a

plane within the lattice

1. Find the intercepts of the plane with the crystal axes and expressthese as integral multiple of basis vectors (the plane can e move inor out from origin retaining its orientation until such an integralintercept is discovered)

2. take the reciprocal of these intercepts and reduce it to smallest setof integers h, k, and l, which have the same ratio to each other as

three reciprocals3. Label the planes (hkl)

h, k, and l called miller indices


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Figure 1

5A (214) crystal plane.


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Figure 1

6Equivalence of the cube faces ({100} planes) by

rotation of the unit cell within the cubic lattice.


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Miller indices in cubic lattice:

Looking at three faces miller inices (hkl) for different faces

Are hkl for blue upper face = (001)hkl for dark grey face = (100)hkl for light grey front face = (010)

Now due to symmetry of the cubic structure which is

rotationally invariant all these directions specify the sameface.

All such face are described by hkl in braces {hkl}

{100} describes the set of six faces of cube


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Directions:

Specified with three integers with the same relationshipas the components of vector in that direction

vector describing the upper plane is 0i + 0j + kor [001]describe the direction of plane (001) [010] (010)

[100] (100)

For cubic structure all these directions are equivalent anddescribed by

LOOK CAREFULLY is perpendicular to {100} for cubic lattice


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(a) (b)

Figure 17

Crystal directions in the cubic lattice.


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DI AMOND STRUCTURE

Basic lattice structure for majority of semiconductors is diamondstructure Characteristic os Si and Ge For compound semiconductors basic structure is diamond but different

atoms are placed at alternate sites.

The diamond lattice is thought of as an fcc structure with anextra atom placed at a/4 + b/4 + c/4 from each of the fccatoms. Two interpenetrating fcc lattices with co-ordinates shifted

by (1/4,1/4 ,1/4)


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Figure 18

Diamond lattice structure: (a) a unit cell of the diamond lattice constructed by

placing atoms from each atom in an fcc; (b) top view (along any

direction) of an extended diamond lattice. The colored circles indicate one fcc

sublattice and the black circles indicate the interpenetrating fcc.


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Diamond lattice:If all the atoms are similar on both interpenetrating lattices

e.g, Si, Ge then the lattice is known as diamond lattice

Zincblend Lattice:If one sublattice in fcc structure is of Ga and other sublattice

is formed by As then the structure is known as Zincblend lattice. Most of the

compound semiconductor have this kind of structure except few from groupII-VI compounds

Wurtzite lattice:

Few compounds from II-VI form wurtzite lattice


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Example 1-3

Calculate the densities of Si and GaAs from the lattice

constants (appendix III), atomic weights, and avogadros

number. Compare the result with densities given in appendix

III. The atomic weight of Si, Ga, and As are 28.1, 69.7 and

74.9 respectively

For Si: a = 5.43 10-8 cm, 8 atoms/cm3For GaAs: a = 5.65 10-8 cm, 4 each As and Ga atoms/cm3


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For III-V compounds, mixture of elements on each of twointerpenetrating lattices of zincblend structure could be variedin any composition

AlxGa1-x As refers to a ternary alloy in which one of thesublattice will be occupied partly by Al and partly by Ga atoms,whereas the other sublattice will be entirely filled by As atoms

Growth of crystals over entire composition range from x = 0to x = 1. Compounds f wide range of properties can be grown.


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Figure 19

Diamond lattice unit cell, showing the four nearest neighbor structure. (FromElectrons and Holes in Semiconductors by W. Shockley, 1950 by Litton

Educational Publishing Co., Inc.; by permission of Van Nostrand Reinhold

Co., Inc.)


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Bulk Crystal Growth

1948 Invention of transistor

1950 pure single crystal Si

Recently Si crystals grown for devices have impuritylevel as less than 1 part in ten billion


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Starting Materials

SiO2 + 2C Si + 2CO ~ 1800o C

Sand + coke MGS (metallurgical grade Si)~ 1001000 ppm impurities like Fe, Al etc.

MGS is refined to EGS (electronic grade Si)Impurity level is 1ppb (1ppb = 5 1013 cm-3 )Si + 3HCl SiHCl3 + H2 ~32

o BP

trichlorosilaneSiHCl3 is separated through fractional distillation

2SiHCl3 + 2H2 2Si + 6HCl


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Figure 110

Pulling of a Si crystal from the melt (Czochralski method): (a) schematicdiagram of the crystal growth process; (b) an 8-in. diameter, oriented Si

crystal being pulled from the melt. (Photograph courtesy of MEMC Electronics

Intl.)


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Czochralski method (Growth of single crystal Ingots):

EGS is melted in quartz lined graphite crucible by resistive

heatingPolycrystalline EGS to single crystal Si ingots This technique is used to grow Si, Ge and other compoundcrystals e.g., GaAs, As is volatile, layer of molten B2O3 is floated onthe molten to prevent vaporization

Technique is known as liquid encapsulated Czochralski(LEC) growth


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Figure 111

Silicon crystal grown by the Czochralski method. This large single-crystal ingot

provides 300 mm (12-in.) diameter wafers when sliced using a saw. The ingotis about 1.5 m long (excluding the tapered regions), and weighs about 275 kg.

(Photograph courtesy of MEMC Electronics Intl.)


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Wafers:

single crystal ingots are grounded mechanicallygrounded cylinder Most ingots are grown along direction From si Ingots, Si wafers are cut by using the diamond

tipped inner-hole blade saw to get ~ 775 micron thick The si wafers are then rounded along the edges. Finally wafers undergo chemical-mechanical polishingusing a slurry of fine SiO2 particles in a basic NaOHsolution to give the front surface a mirror like finish the wafers are ready for IC circuit fabrication.


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Figure 112

Steps involved in manufacturing Si wafers: (a) A 300 mm Si cylindrical ingot, with a

notch on one side, being loaded into a wire saw to produce Si wafers; (b) a technician

holding a cassette of 300 mm wafers. (Photographs courtesy of MEMC Electronics Intl.)


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Doping:

To change the electronic properties of the Si, we addintentional impurities or dopants to molten Si Distribution co-efficient kd, which is the ratio of theconcentration of the impurity in the solid Cs to the

concentration in the liquid CL at equilibrium

kd = Cs / CL

This is the function of the material, the impurity, the

temperature of the solid-liquid interface and the growthrate.


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Epitaxial Growth

The technique of growing an oriented single crystal layer on asubstrate wafer is called epitaxial growth.

For the device applications we grow a thin layer on a wafer(substrate) of compatible crystal.

The substrate could be same material or some other materialwith same lattice structure

Substrate is acting like seed crystal

The growing layer will maintain the crystal structure ofsubstrate

Epitaxial growth can be performed at temperature below themelting temperature of substrate


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Growth Techniques

Chemical Vapor deposition (CVD)

Liquid phase epitaxy (LPE)

Vapour phase epitaxy (VPE)

Molecular beam epitaxy (MBE)


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Lattice matching in epitaxial growth

For high quality single crystals, there should be latticematching of the substrate and the growth layer.

If it is desirable to obtain epitaxial layers that differ from thesubstrate, then we call it heteroepitaxy

Some advanced techniques allow the growth of very thin(~100Ao) layers of lattice mismatched crystal, resulting incompression or tension along the surface plane. Such a layer isa called pseudomorphoic.

For mismatched lattices if thickness of the layer t < tc thecritical thickness, then formation of dislocations will start

known as misfit dislocations


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Figure 113

Relationship between band gap and lattice constant for alloys in the InGaAsP and AlGaAsSb systems. The

dashed vertical lines show the lattice constants for the commer-cially available binary substrates GaAs and

InP. For the marked example of In x Ga 1 x As, the ternary composition x 0.53 can be grown lattice-

matched on InP, since the lattice constants are the same. For quaternary alloys, the compositions on both the

III and V sub-lattices can be varied to grow lattice-matched epitaxial layers along the dashed vertical lines

between curves. For example, In x Ga 1 x As y P 1 y can be grown on InP substrates, with resulting

band gaps ranging from 0.75 eV to 1.35 eV. In using this figure, assume the lattice constant a of a ternary

alloy varies linearly with the composition x.


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Figure 114

Heteroepitaxy and misfit dislocations. For example, in heteroepitaxy of a SiGelayer on Si, the lattice mismatch between SiGe and Si leads to compressive

strain in the SiGe layer. The amount of strain depends on the mole fraction of

Ge. (a) For layer thicknesses less than the critical layer thickness, tc,

pseudomorphic growth occurs. (b) However, above tc, misfit dislocations form

at the interface which may reduce the usefulness of the layers in device

applications.

Liquid Phase Epitaxial Growth


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Liquid Phase Epitaxial Growth

Fig. Liquid-phase epitaxial growth of AlGaAs and GaAs layers on a substrate:(a) cross section of the sample in contact with a Ga-rich melt containing Aland As; (b) carbon slider used to move the GaAs substrate between various

melts. In this case, two pockets are provided, containing melts for AlGaAsand GaAs growth. The GaAs substrate on the slider is moved first into theAlGaAs growth chamber; after growth of this layer (shown in part a), theexcess melt is wiped off as the slider moves the substrate to the next growthchamber.


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Vapour Phase Epitaxy:


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Figure 1

15

A barrel-type reactor for Si VPE. These are atmospheric pressure systems.The Si wafers are held in slots cut on the sides of a SiC-coated graphitesusceptor that flares out near the base to promote gas flow patternsconducive to uniform epitaxy.

Vapour Phase Epitaxy:

SiCl4 + 2H2 Si + 4HCl

i


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(b)(a)

Figure 116

Crystal growth by molecular beam epitaxy (MBE): (a) evaporation cells inside a

high vacuum chamber directing beams of Al, Ga, As, and dopants onto a GaAssubstrate; (b) scanning electron micrograph of the cross section of an MBE-growncrystal having alternating layers of GaAs (dark lines) and AlGaAs (light lines).Each layer is four monolayers (4 xa/2 = 11.3) thick. (Photograph courtesy of BellLaboratories.)

Molecular Beam epitaxy

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