crystal properties and growth of semiconductors-lecture-1

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    1. Crystal Properties and

    Growth of Semiconductors

    Electrical behavior of solids

    Arrangement of atoms

    1.1. Semiconductor Materials

    Electrical conductivity intermediate betweenmetals and insulators

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    What distinguishes semiconductors?

    - resistance/resistivity

    - crystalline structure

    The resistance of a bar of material with dimensionsL, W,t:

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

    RL

    Wt

    W

    tL

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    Figure 1: Electrical conductivities of some commonmaterials.

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    *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-VCompound semiconductors example:GaAs

    II-VICompound semiconductors example:CdTe

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    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

    AmorphousPolycrystalline

    (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

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    Diamond Lattice

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

    AlxGa1-xAs (x % of Al, (1-x)% of Ga)InxGa1-xAsyP1-y

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    Conclusion Atomic arrangement in certain planes in

    crystal is important to many of the

    mechanical, metallurgical, and chemicalproperties of the material.

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

    Chemical reactions-etching of a crytal oftentake place preferentially along certaindirections