1. Crystal Properties and Growth of Semiconductors

statementdizzyeyedSemiconductor

Nov 1, 2013 (3 years and 9 months ago)

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1

1. Crystal Properties and
Growth of Semiconductors


Electrical behavior of solids


Arrangement of atoms


1.1. Semiconductor Materials



Electrical conductivity intermediate between


metals and insulators

2

What distinguishes semiconductors?


-

resistance/resistivity

-

crystalline structure


The resistance of a bar of material with dimensions
L
,
W
,
t:








Resistivity



is:

metals:



<10
-
3

-
cm

insulators:



>10
2

-
cm

semiconductors:

10
-
3

-
cm <


<10
2

-
cm


W

t

L

3

Figure 1:
Electrical conductivities of some common
materials.

4

*Conductivity of semiconductor material can
be varied over orders of magnitude by
changes in

-
temperature

-
optical excitation

-
impurity content

Periodic table:

Column IV

Si, Ge
Elemental semiconductors

III
-
V

Compound semiconductors
example:GaAs

II
-
VI

Compound semiconductors
example:CdTe

5

The Portion of Periodic Table Where
Semiconductors Occur

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Binary

compounds: GaAs, GaP, GaN, CdTe, InP etc.

Ternary

compounds: AlGaAs, GaAsP, HgCdTe, etc.

Quaternary

compounds: InGaAsP, AlGaInAs,etc.

Applications:

*transistors, integrated circuits (Si)

*light emitting diodes (LEDs) (GaAs,GaN, GaP)

*lasers (AlGaInAs, InGaAsP, GaAs, AlGaAs)

*light detectors(Si, InGaAsP, CdSe, InSb, HgCdTe)

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1.2. Crystal Lattices

Solids classified according to atomic arrangement:


Crystalline


Amorphous


Polycrystalline


(a) Crystalline


(b) Amorphous


(c) Polycrystalline


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The atoms making up the CRYSTAL

are arranged in a periodic fashion called
LATTICE.

UNIT CELL is a representative of the entire lattice and is
generally repeated throughout the structure.

PRIMITIVE CELL is the smallest unit cell that can be
repeated to form the lattice.

Crystal lattice determines:



Density of solid (mechanical property)



Allowed energy bands of electrons (electrical property).


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Cubic Lattices

Simple (SC) Body
-
centered (BCC) Face
-
centered (FCC)

a



lattice constant

10

Diamond Lattice

Diamond lattice is an FCC plus placing atoms ¼
a
, ¼
a
, ¼
a

(in x, y and z direction) from each atom in the FCC.


Construction of a diamond lattice from two interpenetrating
FCC sublattices.

Top view

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Diamond Lattice of Si from Different Angles of View

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*Diamond (zincblende) lattice
-

typical of most of the
commonly used semiconductors.

*Characteristic of Si, Ge, III
-
V compounds.

*Possible to vary the composition of alloy by choosing
the fraction of numbers of the atoms, par example:


Al
x
Ga
1
-
x
As (x % of Al, (1
-
x)% of Ga)

In
x
Ga
1
-
x
As
y
P
1
-
y


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Conclusion


Atomic arrangement in certain planes in
crystal is important to many of the
mechanical, metallurgical, and chemical
properties of the material.


Crystals cleaved along certain atomic planes,
resulting in exeptionally planar surfaces


Chemical reactions
-
etching of a crytal often
take place preferentially along certain
directions