1. CHARACTERIZATION OF MCT USING
HALL EFFECT
SUBMITTED TO
Dr. Shiv Kumar
Scientist ‘G’
SUBMITTED BY
Mahesh Singh Negi &
Bhuwan Chandra
B.Tech (Electrical Engineering
SOLID STATE PHYSICS LABORATORY
(Defence R & D Organization)
Timarpur, Delhi-54
2. CONTENT
• Semiconductors: Introduction
• Types OF Semiconductors
• Intrinsic
• Extrinsic
• N-Type Material
• P-Type material
• Current flow in semiconductor
• MCT (Mercury Cadmium Telluride )
• MBE: Introduction
• What is Epitaxy?
• Epitaxy Techniques
3. • Working Principle of MBE
• MBE process & Epitaxial growth
• Working conditions
• Operation
• Control Mechanisms
• Benefits and Drawbacks of MBE
• Applications
• Hall Effect
• X-ray crystallography
• Scanning electron microscope
• Secondary ion mass spectrometry
• FTIR (Fourier Transform Infrared)
4. SEMICONDUCTORS
• Material which exhibit the characteristics
between conductor and an insulator.
Ex : Silicon and Germanium
Difference in conductivity
7. EXTRINSIC
Created when animpureatom is diffused or implanted into anintrinsic
semiconductor.
Theprocess of diffusion or implantation impuritiesan the atom is knownas
doping.
Thepurpose of doping is to increase the numberof electron/holes to improve
the conductivity of extrinsic
8. Extrinsic Semiconductors: n type
• Add a small amount of phosphorus (P:
3s23p3) to Silicon (Si: 3s23p2)
(generally, a group V element to a
group IV host) P replaces a Si atom
and it donates an electron to the
conduction band (P is called the donor
atom). The periodic potential is
disrupted and we get a localized energy
level, D.
• This is an n-type semiconductor – more electrons around that
can be mobile; and the Fermi energy is closer to the
conduction band.
9. Next suppose Si atom is replaced with Boron (B: 2s22p) to
Silicon (Si: 3s23p2). Again, we have a perturbed lattice and
a localized E-level created.
Boron is missing an electron and accepts an
electron from valence band, creating a hole.
Therefore doping with B increases hole
concentration. We call this p-type doping,
the electron concentration n is reduced.
Extrinsic Semiconductors: p type
F
c
v
A
10. Increasing conductivity by temperature
150 200 250 300 350 400 450 500
100
1 10
3
1 10
4
1 10
5
1 10
6
1 10
7
1 10
8
1 10
9
1 10
10
1 10
11
1 10
12
1 10
13
1 10
14
1 10
15
1 10
16
1 10
17
Carrier Concentration vs T emp (in Si)
Temperature (K)
IntrinsicConcentration(cm^-3)
ni
T
T
Therefore the conductivity of a semiconductor is influenced by
temperature
As temperature increases, the number of free electrons and
holes created increases exponentially.
16. CURRENT FLOW IN SEMICONDUCTOR
N-typeMaterials P-typeMaterials
•Occursimultaneously in two ways:
i. Flow of free electron in the
conduction band which is the
majority currentcarrier
ii. Flow of holes in valence band
which is the minority current
carrier
•Occursimultaneously in two ways:
i. Flow of holes in the valence band
which is the majority current
carrier,
ii. Flow of free electron in the
conduction band which is the
minority currentcarrier.
18. • N-Types extrinsicsemiconductor connected to a voltage source. Electrons move
to positive terminal of the voltage source. Holes left behind because of the
movement of the electron in the valence band seem to move to the negative
terminal of the voltage source.
• In N-types material, most of current is due to electron because they are majority
carrier. While holes are the minority carrier in N-types materials, thus the
current due to holes is minimal.
• When the temperature of the semiconductor is increased, more holes will be
formed due to thermal energy. The minority current from holes will increase.
20. • Holes repelled by the positive terminal of the voltage source will move
towards the negative terminal of the voltage source. Free electron will
move to the positive terminal of the voltage source.
• Current in N-type materials is greater than the current inthe P-type
materials because the electron movement in the conduction band is
much easier compared to holes movement in the valence band.
22. INTRODUCTION
Mercury Cadmium Telluride (MCT) is recognized
as one of the most promising semiconductor
material used for IR(infrared) photon detection
over a wide spectral range.
• MCT is alloy of CdTe & HgTe
• CdTe is semiconductor with bandgap of approx. 1.5eV
at room temperature.
• HgTe is semimetal with bandgap is zero.
24. PHYSICAL PROPERTIES OF MCT
• Properties discussed in this section are related
to the lattice structure and band structure of
HgCdTe. Mercury telluride is a semimetal &
cadmium Telluride is a semiconductor. Both
type of behavior of properties exist in solid
solution of two known as MCT.
A zinc blende unit cell
25. ELECTRONIC PROPERTIES
The electron mobility of HgCdTe with a large Hg
content is very high. Among common
semiconductors used for infrared detection, only
InSb and InAs surpass electron mobility of
HgCdTe at room temperature. At 80 K, the
electron mobility of Hg0.8Cd0.2Te can be several
hundred thousand cm2/(V·s). Electrons also have a
long ballistic length at this temperature; their
mean free path can be several micrometres.
26. MECHANICAL PROPERTIES
• HgCdTe is a soft material due to the weak bonds
Hg forms with tellurium. It is a softer material
than any common III-V semiconductor. The
Mohs hardness of HgTe is 1.9, CdTe is 2.9 and
Hg0.5Cd0.5Te is 4. The hardness of lead salts is
lower still.
27. THERMAL PROPERTIES
• The thermal conductivity of HgCdTe is low; at
low cadmium concentrations it is as low as 0.2
W/(Km). This means that it is unsuitable for
high power devices. Although infrared light-
emitting diodes and lasers have been made in
HgCdTe, they must be operated cold to be
efficient. The specific heat capacity is 150
J·kg−1K−1.[5]
28. OPTICAL PROPERTIES
• HgCdTe is transparent in the infrared at
photon energies below the energy gap. The
refractive index is high, reaching nearly 4 for
HgCdTe with high Hg content.
30. WHAT IS EPITAXY?
• Method of depositing a mono crystalline film i.e deposition
and growth of mono crystalline layers.
• Epitaxial : Growing crystalline layers on a crystalline
substrate.
• Greek root : ‘epi’ means ‘above’ and ‘taxis’ means ‘ordered’
31. MOLECULAR BEAM EPITAXY IS A TECHNIQUE FOR
EPITAXIAL GROWTH VIA THE INTERACTION OF ONE
OR MORE MOLECULAR OR ATOMIC BEAMS THAT OCCUR
ON A SURFACE OF A HEATED CRYSTALLINE SUBSTRATE.
32. Epitaxy types:
1.Homoepitaxy: Substrate & material are of
same kind, means same composition.(Si-Si)
To fabricate layers with different doping levels.
2.Heteroepitaxy: Substrate & material are of
different kinds, means different composition
(Ga-As).
To fabricate integrated crystalline layers of
different materials
33. EPITAXY TECHNIQUES
• Vapor-Phase Epitaxy (VPE)
• Modified method of chemical vapor deposition (CVD).
• Undesired polycrystalline layers
• Growth rate: ~2 µm/min.
• Liquid-Phase Epitaxy (LPE)
• Hard to make thin films
• Growth rate: 0.1-1 µm/min.
• Molecular Beam Epitaxy (MBE)
• MBE is an ultra high vacuum(UHV) based technique for producing high qualit
epitaxial structures with mono layer (ML) control.
• “Beam” molecules do not collide to either chamber walls or existent gas atom
• We do MBE In a vacuum chamber (pressure: ~10-11 Torr).
• Growth rate: 1µm/hr.
34. WHY WE GO FOR MBE ?
• This technique for producing epitaxial layers
of metals, insulators , and super conductors
as well as both at the research and industrial
production level.
35. 1.MBE PROCESS: THE TERM ‘BEAM’ MEANS THE
EVAPORATED ATOMS DO NOT INTERACT WITH EACH
OTHER OR WITH OTHER VACUUM CHAMBER GASES UNTIL
THE REACH THE WAFER.
ULTRA PURE ELEMENTS ARE HEATED IN SEPARATE
QUAI- EFFUSION CELLS (EG. GA AND AS) UNTIL
THEY BEGIN TO SLOWLY SUBLIMATE.
2.Epitaxial growth: Epitaxial growth takes place
Due to the interaction of molecular or atomic
beams on a surface of a heated crystalline
substrate.
They provide an angular distribution of atoms or
molecules in a beam.
The substrate is heated to the necessary
temperature.
Atoms on a clean surface are free to move until
finding correct position in the crystal lattice to
bond.Molecular Beam Epitaxy**
MBE: WORKING PRINCIPLE
37. MBE: WORKING CONDITIONS
The mean free path (l)
of the particles >
geometrical size of the
chamber.
That is easily fulfilled if the total pressure does not exceed 10-5
torr.
It is also the condition for growing a sufficiently clean epilayer.
Mean free path for Nitrogen molecules at 300 K *
39. MBE: OPERATION
• The vacuum system consist in a
stainless-steel growth chamber.
• The pumping system usually
consists of ion pumps, cryogenic
pumps for the pumping of specific
gas species.
• Ultra high vacuum is used to
obtain sufficiently clear epilayer
40. BENEFITS AND DRAWBACKS OF MBE
Benefits Draw backs
Clean surfaces, free of an oxide layer. Expensive (106 $ per MBE chamber)
good control of layer thickness and
composition.
• Very complicated system
Low growth rate (1μm/h),
So, we get high material purity.
Epitaxial growth under ultra-high
vacuum conditions
Precisely controllable thermal
evaporation.
Seperate evaporation of each
component takes place.
Substrate temperature is not high.
41. APPLICATIONS
• Hetero junction bipolar transistors(HBT’s) used in satellite
communications.
• Electronic and optoelectronic devices (LED’s for laser printers,
CD and DVD players).
• Used in the construction of quantum wells , dots and wires for
use in lasers.
• To build a solar cell by depositing a Thin film of a photo voltaic
material
• Low temperature Super conductor.
42.
43. DEFINITION
The Hall effect is the production of a
voltage difference (the Hall voltage)
across a current carrying conductor (in
presence of magnetic field),
perpendicular to both current and the
magnetic field.
44. DISCOVERY
The Hall effect was discovered in 1879
by Edwin Herbert Hall while working on
his doctoral degree at the Johns Hopkins
University in Baltimore, Maryland, USA.
46. THEORY
• A static magnetic field has no effect on a
charged particle unless it is moving.
• When charges flow, a mutually perpendicular
force (Lorentz force) is induced on the charge.
• Now electrons and holes are separated by
opposite force.
47. THEORY
•This produces a electric field which
depends upon cross product of magnetic
intensity [H] and current density [J]
Eh=R(JxH)
•R is called Hall Coefficient
•Consider a Semiconductor bar along X-
axis, Magnetic field along Z-axis. Thus Eh
will be along Y-axis.
53. X-RAY CRYSTALLOGRAPHY
X-ray crystallography is a technique used for
determining the atomic and molecular structure of
a crystal, in which the crystalline atoms cause a beam of
incident X-rays to diffract into many specific directions.
By measuring the angles and intensities of these
diffracted beams, a crystallographer can produce a
three-dimensional picture of the density
of electrons within the crystal. From this electron
density, the mean positions of the atoms in the crystal
can be determined, as well as their chemical bonds,
their disorder, and various other information.
55. SCANNING ELECTRON
MICROSCOPE
A scanning electron
microscope (SEM) is a
type of electron
microscope that
produces images of a
sample by scanning the
surface with a focused
beam of electrons. M. von Ardenne's first SEM
56. SECONDARY ION MASS
SPECTROMETRY
Secondary ion mass
spectrometry (SIMS) is a
technique used to analyse the
composition of solid surfaces
and thin films by sputtering the
surface of the specimen with a
focused primary ion beam and
collecting and analysing ejected
secondary ions
Old magnetic sector SIMS, model IMS 3f, succeeded
by the models 4f, 5f, 6f, 7f and most recently, 7f-Auto,
launched in 2013 by the manufacturer CAMECA.
57. FTIR (FOURIER TRANSFORM
INFRARED)
FTIR (Fourier Transform Infrared) spectrometer is a
obtains an infrared spectra by first collecting an
interferogram of a sample signal using an interferometer,
then performs a Fourier Transform on the interferogram
to obtain the spectrum.
An example of an
FTIR
spectrometer with
an attenuated
total
reflectance (ATR)
attachment.
