Semi Conducting & Magnetic Materials Week 3 Feb 7 2012

8/2/2019 Semi Conducting & Magnetic Materials Week 3 Feb 7 2012

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Semi-conducting & Magnetic Materials

Prof S. B. Sant

Department of Metallurgical & Materials EngineeringIIT Kharagpur

MT41016




Emission of light

Electron – excited state – reverts back to lower energy state.

Happens with emission of photons (light) & phonons (heat).

Luminescence

If this occurs in nanoseconds – fluorescenceIf microseconds or milliseconds – phosphorescencePhotoluminescence – photons impinge on a material – remits light at lower energy

Electroluminescence – emission of light as a consequence of electric field

Cathodoluminescence – light emission by the showering of electrons of high energy.




Stimulated Emission - LASER Light Amplification by Stimulated Emission of R adiation

E1

E2

21

υ h

Two energy levels, E1 and E2

Higher energy level – more electrons

Assume electrons made to stay some time at that level.In time, one of them, will E2 E1 emitting a photon

with energy 21υ h

This photon might stimulate another electron to E2 E1

The photons can be in phase – coherent emission - Avalanche effect

Highly Intense – Monochromatic – Strongly Collimated

How – Optical Pumping – external light absorption –

Xe flashlamps – pulse lasers




Stimulated Emission - LASER

Device current ~ 5 mA at ~ 3V

Input power ~ 15 mW,

η 33%-95% (L.D.)Power density ~ 300-600 W/cm2

Sunlight intensity ? ~100 mW/cm2

2 major Designs for Laser Diodes




Energy Bands in Crystals…contd.




Energy Bands in Crystals…contd. pn junction

Electrons drift and diffuse across a junction between unlike materials





V.B.

V.B.

C.B.

C.B.

Difference in Fermi Energies before contact establishes a band bending edge and

a contact potential V bi after contact.





Band diagrams for (a) n-type whose surface is negatively charged

(b) p-type whose surface is positively charged.

Assume that the surface of an n-type SC has somehow been negatively charged – willrepel the free electrons near the surface leaving +vely charged holes behind. Any

electron that drifts towards the surface “feels” this repelling force. Far fewer free

electrons at surface compared to the interior – depletion region – is a potential barrier

for electrons.




Metal-Semiconductor junctions Need to know:

How do electrons flow on making contact.

Fermi energies of electrons in both.

– Work Function, Φ – measures positionof Fermi Energy w.r.t vacuum level

(lowest energy to raise electron to

escape from surface)

– well-defined constant for metals.

– It also measures Fermi level for

Semiconductors – varies with doping !

But, CB & VB Edges, w.r.t. vacuum,

do not depend much on doping

– better choices.Therefore, Electron Affinity, χ s is used.

– defined as CBE relative to vacuum level

Challenge: Experiments determine WF – but, SC is doped – need to convert to

doping-independent Electron Affinity, χ s - difficult – debate – papers!!!




Metal-Semiconductor junctions

Doping-independent values good for

tables – device design, but, doping must

be known to understand device performance!

Let us then assume that actual Fermi

level of the Semiconductor & Metal are

precisely know – leave thedetermination to Experiments!

When contact is made between a Semiconductor & Metal - electrons flow

– depending on the respective Fermi Levels.

– The flow can lead to Depletion or Enhancement of the majority carriers.

– Four possible situations to consider.




Metal-Semiconductor junctions

Work Function of Metal

What does High & Low Work Function of a Metal mean?

– All it means is whether the Fermi Levels of the Metal lies above or below the

Fermi Level of the Semiconductor Before Contact is made.

– Let us look at the Four possible situations.

Doped Semiconductor

Low High

n-type p-type



Semi-conducting & Magnetic MaterialsMetal-Semiconductor contacts

After contact:

Two produce Ohmic contacts (metallization) while the other two produce Schottky or rectifying contacts (now replaced by p-n diodes)

M Semi Semi Semi SemiMMM




Electron Flow Into n-type semiconductor or Out of p-type – Increases Majority carrier

concentration of the semiconductor - increases conductivity of the Semiconductor near

the junction - Ohmic contacts (metallization). Electrons flow from Metal into SC

Difficult to find correct metals with appropriate WF for Wide-Band GapSemiconductors


I

V

Ohm’s Law

Ohmic contacts (metallization).





Band diagrams for metal & n-type semiconductor (a) before contact

(b) After contact. φM > φS. Potential barrier: Heavy lines. χ is Electron Affinity.




Electron Flow: When charge flowing Into the semiconductor is opposite sign as the

majority carriers type, the majority carriers are depleted near the junction forming a

resistive depletion layer.

- An applied bias voltage appears across this relatively insulating depleted region – induces current flow.





Band diagrams for metal & p-type semiconductor (a) before contact

(b) After contact. φM < φS.

Metal-Semiconductor contacts





Band Edge Diagrams for Biased Junctions

http://www.iue.tuwien.ac.at/phd/ayalew/node54.html




Biasing-Metal-Semiconductor contacts – Schottky Diodes – n-type SC

http://www.iue.tuwien.ac.at/phd/ayalew/node54.html





Band diagrams for (a) n-type whose surface is negatively charged

(b) p-type whose surface is positively charged





Band diagrams for metal & n-type semiconductor (a) before contact

(b) After contact.




Band diagrams for metal & p-type semiconductor (a) before contact

(b) After contact.





Work Function of Metals

4.14Lead

4.5Iron

5.1Gold

4.7Copper 5.0Cobalt

2.1Cesium

4.81Carbon

2.9Calcium

4.07Cadmium

5.0Beryllium

4.08Aluminum

Work Function (eV)Element

4.3Zinc

3.6Uranium

2.28Sodium

4.73Silver

5.11Selenium

6.35Platinum

2.3Potassium

4.3 Niobium

5.01 Nickel

4.5Mercury

3.68Magnesium

Work Function (eV)Element





(a) before contact (b) After contact.




Metal-Semiconductor ContactsElectron Affinity, χ s

6H-SiC: 3.5 eV 3C-SiC: 4.0 eV 4H-SiC: 4.17 eV




Metal-Semiconductor ContactsElectron Affinity, χ s

6H-SiC: 3.5 eV 3C-SiC: 4.0 eV 4H-SiC: 4.17 eV

Assignment:

Find appropriate metal contact

to form

(a) Ohmic contacts and

(b) Schottky contacts

on n-type and p-type

6H-SiC

3C-SiC4H-SiC

Provide calculations to justify

the selection.

Due: Feb 12, 2009




Crystal Growth




Thin FilmsWhat is a "thin film" ?thin = less than about one micron ( 10,000 Angstroms, 1000 nm)

film = layer of material on a substrate

(if no substrate, it is a "foil")

Applications:•microelectronics - electrical conductors, electrical barriers, diffusion barriers . . .

•magnetic sensors - sense I, B or changes in them

•gas sensors, SAW devices

•tailored materials - layer very thin films to develop materials with new properties

•optics - anti-reflection coatings•corrosion protection

•wear resistance

Special Properties of Thin Films: different from bulk materials

Thin films may be:

•not fully dense•under stress

•different defect structures from bulk

•quasi - two dimensional (very thin films)

•strongly influenced by surface and interface effects

•This will change electrical, magnetic, optical, thermal, and mechanical properties.




Thin FilmsTypical steps in making thin films:1.emission of particles from source ( heat, high voltage . . .)

2.transport of particles to substrate (free vs. directed)

3.condensation of particles on substrate (how do they condense ?)

Simple model:

How do the variables effect film structure and properties ?




Thin FilmsWhat physics is in all this ?

thermodynamics and kinetics

phase transition - gas condenses to solid

nucleation

growth kinetics activated processes

• desorption

• diffusion

allowed processes and allowed phases

solid state physics crystallography

defects

bonding

electricity and magnetism

optics conductivity - resistivity

magnetic properties

mechanics

stresses in films

friction and wear




Thin FilmsThree types of defects:

1. Planar defects

- grain boundaries interfaces between two single crystal regions of different orientation

atoms at grain boundaries tend to be loosely bound

=> more reactive (corrosion) and accelerated diffusion along grain boundaries

typical grain sizes: 0.01 mm - 100 mm (micron)

How many atoms in a solid are at grain boundaries ?




Thin Films

Rough model:

Assume grains are all cubes with sides of length l

l = grain size

a = atomic lattice parameter

n = number of atoms in one row of the grain

then, l = na

What would be the result for spherical grains of diameter, l ?

for l = 0.1 micron (1000 Angstroms) and a = 3 Angstroms

about 10 atoms out of 1000 are at grain boundaries (1 %)




Thin Films Number of grain boundaries in film (grain size) depends

on deposition rate and substrate temperature.

generally:

lower T => smaller grains => many boundaries

higher T => larger grains => fewer boundaries

grain size is often proportional to film thickness

(thinner films tend to have smaller grains)

2. line defects - dislocations

example:

edge dislocation - from inserting an extra row of atoms

distorts lattice => stresses (compression and tension)

very common:

often 1010 - 1012 dislocations/ cm2 in filmsform from:

film growth process

dislocations in substrate continuing into film

contamination on substrate




3. point defects

self interstitial - extra atom

vacancy - missing atom

substitutional impurity - impurity atom in latticeinterstitial impurity - impurity atom not in regular lattice site

In principle you can eliminate all of these except vacancies

Thin Films




Vacancies arise from thermodynamics (entropy)

•fraction of vacancies (f)

where k B

= Boltzmann's constant = 1.381 x 10-23 J/K

typically Ef is about 1 eV

at room temperature, f is about 10-17

point defects often arise from•fast deposition

•low substrate temperatures

=> no time for atoms to move to crystal lattice sites

T k E B f

e f /−

=

Thin Films




Thin Films

Surface Roughness

Films always have some statistical distribution of thickness across the film

in the worst case:

generally observed less roughness

d d =Δ




Crystal Growth




Crystal GrowthSi crystal growth

Starting material is ? Sand

(SiO2) –

reduced in an arc furnace with coal

– 98%Si

Powdered raw Si – reacted with

HCl acid forming trichlorosilane

Si + HCl →SiHCl3 + H2

This is fractionally distilled to

purify and then reduced with H2.

SiHCl3 + H2 →Si + 3HCl

Polycrystalline




Crystal Growth

(a) Czochralski (b) Float zone (c) Bridgman (d) 12-inch Si single crystal




Crystal Growth(a)Czochralski

• Si melted in SiO2 crucible

inside a

graphite crucible.• Seed single crystal – held

on a rod,

touches the melt& slowly

lifted

– 1 mm/min

• Crucible & rod – rotate in

opposite• Direction – 50 rpm

• Vacuum or protective

atmosphere.

S i d ti & M ti M t i l




Crystal Growth





Crystal Growth

(a)Czochralski

Si melted in SiO2

crucible inside a graphite crucible.

Seed single crystal – held on a rod, touches the melt& slowly lifted

– 1 mm/min

Crucible & rod – rotate in opposite direction – 50 rpm

Vacuum or protective atmosphere.





Crystal Growth

(b) Float-zone

Polysilicon rod with a seed

crystal at the end is put in avacuum chamber.

A ring type induction furnace

slowly moves along the length

melting – solidifying

small portions from the bottom up

– single crystal – high purity.





Crystal Growth

Semi conducting & Magnetic Materials




Single Crystal Growth

Floating Zone Method Czochralski Method

Dopants: n-type – use PH3, for p-type B2H6

Higher purity

Oxygen content <1016 atoms/cm3 ~1018 atoms/cm3

Higher resistivity

200 Ω cm 50 Ω cm

Reverse voltage > 750-1000V

Mechanically weaker Majority of IC - # of temp steps in

Si wafer processing





Crystal Growth

(c)Bridgman method – rarely for Si, but for GaAs.

Polycrystalline material – melted in a Si3 N4 coated graphite or quartzCrucible inside a sealed quartz tube.

Traveling furnace, 2 temperature zones, gradually melts the seed

single crystal and the poly extra arsenic in low temperature zone





Crystal Growth

(c)Bridgman method – rarely for

Si, but for GaAs.

Polycrystalline material –

melted in a Si3 N4 coated

graphite or quartz

Crucible inside a sealed quartztube.

Traveling furnace, 2 temperature

zones, gradually melts the

seedsingle crystal and the poly extra

arsenic in low temperature

zone





Crystal Growth

Liquid Encapsulate Czochralski Growth





Crystal Growth





Thin Film Growth

Molecular Beam Epitaxy (MBE)

Metal-Organic Chemical Vapour Deposition (MOCVD)

Chemical Vapour Deposition (CVD)

Magnetron Sputtering

Pulsed Laser Deposition (PLD)

Ion Beam Deposition (IBD) – amorphous or fine grains?

- LPCVD, APCVD, PECVD

Atomic Layer Deposition (ALD) – amorphous or fine grains?

Single Crystal - Epitaxy

Poly-Crystal





Crystal Growth





Semi conducting & Magnetic MaterialsCrystal Growth





g gCrystal Growth





Crystal Growth





Crystal Growth


Organo-Metallic Vapour Phase Epitaxy (OMVPE)




Crystal Growth





Crystal Growth


Semi-conducting & Magnetic MaterialsC l G h



Crystal Growth


(a) Emcore – Veeco Turbo-disc (b) Thomas Swan showerhead

(c) Aixtron planetary (d) EMF Vector flow reactor

Semi-conducting & Magnetic MaterialsC t l G th



Crystal Growth


Semi-conducting & Magnetic MaterialsC t l G th



Crystal Growth




Semi-conducting & Magnetic MaterialsCrystal Growth



Crystal Growth

Magnetron Sputtering

Semi-conducting & Magnetic MaterialsCrystal Growth



Crystal Growth

Pulsed Laser Deposition (PLD)




Thin Film Growth

Veeco – Ion Beam Deposition




Thin Film Growth

Semi Conducting & Magnetic Materials Week 3 Feb 7 2012

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