Characterisation of MCT using hall effect

A REPORTON
CHARACTERIZATION OF MCT USING
HALL EFFECT
Under the Guidance of
Dr. Shiv Kumar
Scientist ‘G’
SOLID STATE PHYSICS LABORATORY
(DefenceR&DOrganisation)
Timarpur,Delhi-54
Submittedinpartialfulfilmentofthe
Requirementfortheaward
Ofthedegree
BY
MAHESHSINGHNEGI
B.Tech (Electrical Engineering)
College of Technology,

G.B. Pant University of Agriculture & Technology Pantnagar Uttarakhand

CERTIFICATE
This is to certify that Mr. Mahesh Singh Negi, student of B. tech
(Electrical Engineering) from College of Technology (G. B. Pant
University of Agriculture & Technology) Pantnagar, has under gone
summer training in the MCT MBE group, Solid State Physics Laboratory
(DRDO) for a period of 4 weeks (June 1s t
to June 30th
, 2017).
He was under my guidance and was assigned the work of studying and
performing Characterization of MCT (Mercury Cadmium Telluride)
using Hall measurement method.
He has completed the assigned work successfully and in time.
Throughout the training period, he worked hard with sincerity and
dedication. He displayed the spirit of enquiry and perseverance and is
innovation in approach.
Dr. Shiv Kumar
Scientist ‘G’
MCT MBE Division
Solid State Physics Laboratory

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ACKNOWLEDGEMENT
Any job in this world, tough cannot be accomplished without the
assistance of others. I would hereby take the opportunity to express my
indebtedness to people who have helped me to accomplish this task. I
feel a deep sense of gratitude in thanking all those who helped me to
carry this training to its eventual fruition.
First of all, I wish to express my deep sense of gratitude to the
Director, SSPL, Dr. R. K. Sharma for giving me an opportunity to pursue
a training project in this prestigious institution.
I owe this movement of a great satisfaction with deep sense of gratitude
to my project guide Dr. Shiv Kumar, Scientist ‘G’ for motivating me.
His interest and guidance throughout the project has brought the project
to final stage.
I am also highly obliged to Mr. Sovinder Rana, Scientist ‘D’, Mr. Subodh
Tyagi, scientist ‘C’ and Mr. Uday Ram Meena, TO ‘A’ for their valuable tips
and for the knowledge they shared with me.
I am also thankful to Mr. Sanjeev Kumar Sharma TO ‘B’ for his help
and support during the tenure of the project.
Last but not least I thank my family members and all my friends.

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ABSTRACT
This project report in its present form is the result of the fabrication and
characterization of HgCdTe (MCT) infra-red sensors using Molecular Beam
Epitaxy(MBE).
This report begins with the introduction to basic concepts of semiconductors and
then includes the concepts of direct and indirect band-gaps, compound
semiconductors and the behaviour of chemical potential.
Subsequently the concepts of Epitaxy and Molecular Beam Epitaxy are introduced
along with the concepts of Fourier Transform Infra-Red Spectroscopy and Hall
effect. The apparatus used in the Laboratory is also discussed briefly. Then
Mercury Cadmium Telluride, its properties and growth techniques are introduced.
Finally the infra-red detection applicability of MCT is discussed and thereafter the
infra-red detector devices using MCT are given in brief details.

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Acknowledgement..................................................................................................................................
Abstract......................................................................................................................................................................................1
DRDO..........................................................................................................................................................................................5
DRDO - Vision.....................................................................................................................................................................6
DRDO - Mission..................................................................................................................................................................6
SSPL ............................................................................................................................................................................................7
ACHIEVEMENTS..................................................................................................................................................................7
AREAS OF WORK................................................................................................................................................................8
Semiconductor Basics............................................................................................................................................................9
Introduction .........................................................................................................................................................................9
Introduction to Energy Bands ..................................................................................................................................... 10
The Fermi–Dirac Distribution....................................................................................................................................... 14
Charge Carriers in Semiconductors........................................................................................................................... 16
Intrinsic and Extrinsic Semiconductors ..................................................................................................................... 19
Direct and indirect band gap....................................................................................................................................... 21
Compound Semiconductors........................................................................................................................................ 23
Behaviour of the Chemical Potential......................................................................................................................... 24
Epitaxy..................................................................................................................................................................................... 27
Molecular Beam Epitaxy (MBE)........................................................................................................................................ 27
Introduction ............................................................................................................................................27
Growth Apparatus........................................................................................................................................................... 30
Knudsen Cell..................................................................................................................................................................... 33
Advantages of MBE Technique ................................................................................................................................... 33
Analysis techniques ........................................................................................................................................................ 34
Fourier Transform Infra-Red Spectroscopy (FTIR)................................................................................................. 35

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The Sample Analysis Process .................................................................................................................................. 38
Hall Effect........................................................................................................................................................................... 40
Mercury cadmium telluride .............................................................................................................................................. 42
Properties of MCT........................................................................................................................................................... 43
Electronic properties.................................................................................................................................................. 43
Mechanical properties............................................................................................................................................... 43
Thermal properties..................................................................................................................................................... 43
Optical properties....................................................................................................................................................... 43
HgCdTe growth techniques ......................................................................................................................................... 44
Bulk crystal growth..................................................................................................................................................... 44
Epitaxial growth .......................................................................................................................................................... 44
Infrared detection ........................................................................................................................................................... 45
HgCdTe Infra-Red Detector Devices......................................................................................................................... 47
Photoconductors ........................................................................................................................................................ 48
Photodiodes................................................................................................................................................................. 48
Third Generation HgCdTe Devices........................................................................................................................ 49
Two Colour Detectors........................................................................................................................................... 49
Avalanche Photodiodes ....................................................................................................................................... 50
Hyper spectral Arrays............................................................................................................................................ 51
Summary ................................................................................................................................................................................ 51
CONCLUSION ....................................................................................................................................................................... 54
Bibliography.......................................................................................................................................................................... 55
Papers Referred............................................................................................................................................................... 55
Books Referred................................................................................................................................................................. 55
Web Section...................................................................................................................................................................... 55

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DRDO
DRDO (Defence Research & Development Organization) was formed in 1958 from
the amalgamation of the then already functioning Technical Development
Establishment (TDEs) of the Indian Army and the Directorate of Technical
Development & Production (DTDP) with the Defence Science Organization (DSO).
DRDO was then a small organization with 10 establishments or laboratories. Over
the years, it has grown multi-directionally in terms of the variety of subject
disciplines, number of laboratories, achievements and stature.
Today, DRDO is a network of 51 laboratories which are deeply engaged in
developing defence technologies covering various disciplines, like aeronautics,
armaments, electronics, combat vehicles, engineering systems, instrumentation,
missiles, advanced computing and simulation, special materials, naval systems, Life-
sciences, training, information systems and agriculture.
Presently, the Organization is backed by over 5000 scientists and about 25,000
other scientific, technical and supporting personnel. Several major projects for the
development of missiles, armaments, light combat aircrafts, radars, electronic
warfare systems etc are on hand and significant achievements have already been
made in several such technologies.

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DRDO - VISION
Make India prosperous by establishing world class science and technology base and provide our
Defence Services decisive edge byequipping themwith internationallycompetitive systems and solutions.
DRDO - MISSION
 Design, develop and lead to production state-of-the-art sensors, weapon systems,
platforms and allied equipmentfor our Defence Services.
 Provide technological solutions to the Services to optimise combat effectiveness and to promote
well-being ofthe troops.
 Develop infrastructure and committed quality manpower and build strongindigenous
technologybase.

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SSPL
Solid State Physics Laboratory (sspl), one of the establishments under the DEFENCE RESEARCH AND
DEVELOPMENT ORGANISATION (DRDO), Ministry of Defence, was established in 1962 with the
broad objective of developing an R&D base in the field of Solid State Materials, Devices and Sub-
systems.
The Laboratory has a vision to be the centre of excellence in the development of Solid State Materials,
Devices and has a Mission to develop and characterize high purity materials and solid state devices
and to enhance infrastructure, technology for meeting the futuristic challenges.
ACHIEVEMENTS
The Laboratory has contributed immensely on the growth of materials and development of devices.
Some of the achievements are:
 SPST Switch
 GaAs MMIC technology
 Remotely activated acoustic warning system (RAAWS)
 Silicon Photo diodes & Silicon Quadrant Detectors
 GaAs Gunn Diodes for W-band applications
 Thermo – Electric Coolers

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AREAS OF WORK
Over the years, the Laboratory has developed core competence in the design and
development in the following areas:-
 GaAs based Microwave devices and circuits
 SAW devices & sensors
 MEMs components
 IR devices and Materials Development & Characterization

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SEMICONDUCTOR BASICS
INTRODUCTION
A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor
and an insulator. This means conductivity roughly in the range of 10-2 to 104
Siemens per centimetre (S⋅cm-1). Semiconductors are the foundation of modern electronics,
including radio, computers, and telephones. Semiconductor-based electronic components
include transistors, solar cells, many kinds of diodes including the light-emitting diode (LED), the
silicon controlled rectifier, photo-diodes, and digital and analog integrated circuits.
Semiconductor solar photovoltaic panels directly convert light energy into electricity. In a metallic
conductor, current is carried by the flow of electrons.
Common semiconducting materials are crystalline solids—chips, but amorphous and liquid
semiconductors are also known. These include hydrogenated amorphous silicon and mixtures of
arsenic, selenium and tellurium in a variety of proportions. Such compounds share with better known
semiconductors intermediate conductivity and a rapid variation of conductivity with temperature, as
well as occasional negative resistance. Such disordered materials lack the rigid crystalline structure
of conventional semiconductors such as silicon and are generally used in thin film structures, which
do not require material of higher electronic quality, being relatively insensitive to impurities and
radiation damage.
Silicon is used to create most semiconductors commercially. Dozens of other materials are used,
including germanium, gallium arsenide, and silicon carbide. A pure semiconductor is often called an
“intrinsic” semiconductor. The electronic properties and the conductivity of a semiconductor can be
changed in a controlled manner by adding very small quantities of other elements, called “dopants”,
to the intrinsic material. In crystalline silicon typically this is achieved by adding impurities of boron
or phosphorus to the melt and then allowing it to solidify into the crystal. This process is called
"doping" and the semiconductor is "extrinsic".

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INTRODUCTION TO ENERGY BANDS
In solid-state physics, the electronic band structure (or simply band structure) of a solid
describes those ranges of energy, called energy bands, that an electron within the solid may have
("allowed bands"), and ranges of energy called band gaps ("forbidden bands"), which it may not have.
Band theory models the behaviour of electrons in solids by postulating the existence of energy
bands. It successfully uses a material's band structure to explain many physical properties of solids,
such as electrical resistivity and optical absorption.
When two valence electron atomic orbitals in a simple molecule such as hydrogen combine to form a
chemical bond, two possible molecular orbitals result. One molecular orbital is lowered in energy
relative to the sum of the energies of the individual electron orbitals, and is referred to as the
'bonding' orbital. The other molecular orbital is raised in energy relative to the sum of the energies
of the individual electron orbitals and is termed the 'anti-bonding' orbital.
In a solid, the same principles apply. If N valence electron atomic orbitals, all of the same energy,
are taken and combined to form bonds, N possible energy levels will result. Of these, N/2 will be
lowered in energy and N/2 will be raised in energy with respect to the sum of the energies of the N
valence electron atomic orbitals.
However, instead of forming N/2 bonding levels all of the exact same energy, the allowed energy
levels will be smeared out into energy bands. Within these energy bands local differences
between energy levels are extremely small. The energy differences between the levels within the
bands are much smaller than the difference between the energy of the highest bonding level and
the energy of the lowest anti-bonding level. Like molecular orbitals, each energy level can contain at
most two electrons of opposite spin.

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The allowed energy levels are so close together that they are sometimes considered as being
continuous. It is very important to bear in mind that, while this is a useful and reasonable
approximation in some calculations, the bands are actually composed of a finite number of very
closely spaced electron energy levels.
If there is one electron from each atom associated with each of the N orbitals that are
combined to form the bands, then because each resulting energy level can be doubly occupied,
the 'bonding' band, or valence band will be completely filled and the 'anti- bonding' band, or
conduction band will be empty. This is depicted schematically in the picture above by the grey
shading of the valence band.
Electrons cannot have any values of energy that lie outside these bands. An electron can only move
('be promoted') from the valence band to the conduction band if it is given an energy at least as
great as the band gap energy. This can happen if, for example, the electron were to absorb a
photon of sufficiently high energy.
If, as in the above one-dimensional schematic, a band is completely filled with electrons, and the
band immediately above it is empty, the material has an energy band gap. This band gap is the
energy difference between the highest occupied state in the valence band and the lowest unoccupied
state in the conduction band. The material is either a semiconductor if the band gap is relatively
small, or an insulator if the band gap is relatively large.
Electrons in metals are also arranged in bands, but in a metal the electron distribution is
different - electrons are not localised on individual atoms or individual bonds. In a simple metal
with one valence electron per atom, such as sodium, the valence band is not full, and so the highest
occupied electron states lie some distance from the top of the valence band. Such materials are
good electrical conductors, because there are empty energy states available just above the
highest occupied states, so that electrons can easily gain energy from an applied electric field and
jump into these empty energy states.
The distinction between an insulator and a semiconductor is less precise. In general, a material
with a band gap of less than about 3 eV is regarded as a semiconductor. A material with a band gap of
greater than 3 eV will commonly be regarded as an insulator. A number of ceramics such as silicon
carbide (SiC), titanium dioxide (TiO2), barium titanate (BaTiO3) and zinc oxide (ZnO) have band
gaps around 3 eV and are regarded by ceramicists as semiconductors. Such ceramics are often

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referred to as wide-band-gap semiconductors.

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The distinction between semiconductors and insulators arises because in small band gap
materials at room temperature a small, but appreciable, number of electrons can be excited from the
filled valence bands into the unfilled conduction bands simply by thermal vibration. This leads to
semiconducting materials having electrical conductivities between those of metals and those of
insulators.
The picture we have sketched here is only a very simple qualitative picture of the electronic structure
of a semiconductor designed to capture essential aspects of the band structure in semiconductors
relevant to this TLP. More precise and quantitative approaches exist - see Going Further. Such
quantitative approaches are generally quite complex and require an understanding of quantum
mechanics. Fortunately, the very simple qualitative picture described above for semiconductors is
all that we need to be able to build upon and develop in this TLP.
An extension of the simple band energy diagram with only the vertical axis labelled as energy,
with the horizontal axis unlabelled, is to plot the energy vertically against wave vector, k. From
de Broglie's relationship p = k where p is momentum and is Planck's constant, h, divided by 2π.
Such plots therefore relate energy to momentum. The reason why such plots are useful lies in the
more quantitative methods referred to above, from which we shall simply quote useful results.
The energy of a classical, non-quantum, particle is proportional to the square of its momentum.
This is also true for a free electron, as in the simplest picture possible of valence electrons in metals
where the electrostatic potential from the nuclei is ignored. However, in a real crystalline solid the
periodicity of the lattice and the electrostatic potential from the nuclei together mean that in the
quantum world in a crystalline material the electron energy, E, is not simply proportional to the square
of the momentum, and so is not proportional to the square of the wave vector, k.
In these E-k diagrams, often called band diagrams, plotted in what is referred to as a reduced
zone scheme, the momentum that is plotted is actually a quantity called crystal momentum. The
distinction between momentum and crystal momentum arises from the periodicity of the solid.
Fortunately, this distinction is not important for understanding this TLP on semiconductors.
There are usually many different values of electron energy possible for any given value of the electron
momentum. Each possible energy value lies in one of the energy bands.

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When plotted against the wave vector, k, the bands of allowed energy are not really flat. This means
that bands can overlap in energy, as the maximum value in one band may be higher than the
minimum value in another band. In this case the relevant maximum and minimum will occur for
different values of k because energy bands never cross over each other. This is one way in which
metals can have partially filled energy bands. The available energy states are filled with electrons
starting with those lowest in energy. Such overlapping of bands as a function of k does not occur in
semiconductors.

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THE FERMI–DIRAC DISTRIBUTION
Electrons are an example of a type of particle called a fermion. Other fermions include protons
and neutrons. In addition to their charge and mass, electrons have another fundamental property
called spin. A particle with spin behaves as though it has some intrinsic angular momentum. This
causes each electron to have a small magnetic dipole. The spin quantum number is the projection
along an arbitrary axis (usually referred to in textbooks as the z-axis) of the spin of a particle
expressed in units of . Electrons have spin ½, which can be aligned in two possible ways, usually
referred to as 'spin up' or 'spin down'.
All fermions have half-integer spin. A particle that has integer spin is called a boson. Photons, which
have spin 1, are examples of bosons. A consequence of the half-integer spin of fermions is that
this imposes a constraint on the behaviour of a system containing more than one fermion.
This constraint is the Pauli Exclusion Principle, which states that no two fermions can have the exact
same set of quantum numbers. It is for this reason that only two electrons can occupy each electron
energy level – one electron can have spin up and the other can have spin down, so that they have
different spin quantum numbers, even though the electrons have the same energy.
These constraints on the behaviour of a system of many fermions can be treated statistically. The
result is that electrons will be distributed into the available energy levels according to the Fermi Dirac
Distribution:
Where f(ε) is the occupation probability of a state of energy ε, kB is Boltzmann's constant, μ (the
Greek letter mu) is the chemical potential, and T is the temperature in Kelvin.
The distribution describes the occupation probability for a quantum state of energy E at a

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temperature T. If the energies of the available electron states and the degeneracy of the

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states (the number of electron energy states that have the same energy) are both known, this
distribution can be used to calculate thermodynamic properties of systems of electrons.
At absolute zero the value of the chemical potential, μ, is defined as the Fermi energy. At room
temperature the chemical potential for metals is virtually the same as the Fermi energy
– typically the difference is only of the order of 0.01%. Not surprisingly, the chemical potential for
metals at room temperature is often taken to be the Fermi energy. For a pure undoped
semiconductor at finite temperature, the chemical potential always lies halfway between the
valence band and the conduction band. However, as we shall see in a subsequent section of this
TLP, the chemical potential in extrinsic (doped) semiconductors has significant temperature
dependence.
In order to understand the behaviour of electrons at finite temperature qualitatively in metals and
pure undoped semiconductors, it is clearly sufficient to treat μ as a constant to a first
approximation. With this approximation, the Fermi-Dirac distribution can be plotted at several
different temperatures. In the figure below, μ was set at 5 eV.
From this figure it is clear that at absolute zero the distribution is a step function. It has the value
of 1 for energies below the Fermi energy, and a value of 0 for energies above. For finite temperatures
the distribution gets smeared out, as some electrons begin to be thermally excited to energy
levels above the chemical potential, μ. The figure shows that at room temperature the distribution
function is still not very far from being a step function.

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CHARGE CARRIERS IN SEMICONDUCTORS
When an electric field is applied to a metal, negatively charged electrons are accelerated and carry
the resulting current. In a semiconductor the charge is not carried exclusively by electrons. Positively
charged holes also carry charge. These may be viewed either as vacancies in the otherwise filled
valence band, or equivalently as positively charged particles.
Since the Fermi-Dirac distribution is a step function at absolute zero, pure semiconductors will have all
the states in the valence bands filled with electrons and will be insulators at absolute zero. This is
depicted in the E-k diagram below; shaded circles represent filled momentum states and empty
circles unfilled momentum states. In this diagram k, rather than k, has been used to denote that the
wave vector is actually a vector, i.e., a tensor of the first rank, rather than a scalar.
If the band gap is sufficiently small and the temperature is increased from absolute zero, some
electrons may be thermally excited into the conduction band, creating an electron- hole pair. This
is as a result of the smearing out of the Fermi-Dirac distribution at finite temperature. An
electron may also move into the conduction band from the valence band if it absorbs a photon that
corresponds to the energy difference between a filled state and an unfilled state. Any such photon
must have an energy that is greater than or equal to the band gap between the valence band and the
conduction band, as in the diagram below.

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Whether thermally or photonically induced, the result is an electron in the conduction band and a
vacant state in the valence band.
If an electric field is now applied to the material, all of the electrons in the solid will feel a force
from the electric field. However, because no two electrons can be in the exact same quantum state,
an electron cannot gain any momentum from the electric field unless there is a vacant momentum
state adjacent to the state being occupied by the electron. In the above schematic, the electron
in the conduction band can gain momentum from the electric field, as can an electron adjacent
to the vacant state left behind in the valence band. In the diagram below, both of these electrons

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are shown moving to the right.

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The result of this is that the electrons have some net momentum, and so there is an overall
movement of charge. This slight imbalance of positive and negative momentum can be seen in
the diagram below, and it gives rise to an electric current.
The vacant site in the valence band which has moved to the left can be viewed as being a particle
which carries positive electric charge of equal magnitude to the electron charge. This is therefore a
hole. It should be appreciated that these schematics do not represent electrons 'hopping' from
site to site in real space, because the electrons are not localised to specific sites in space. These
schematics are in momentum space. As such, holes should not be thought of as moving through
the semiconductor like dislocations when metals are plastically deformed – it suffices to view them
simply as particles which carry positive charge.
The opposite process to the creation of an electron-hole pair is called recombination. This occurs
when an electron drops down in energy from the conduction band to the valence band. Just as

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the creation of an electron-hole pair may be induced by a photon,

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recombination can produce a photon. This is the principle behind semiconductor optical devices
such as light-emitting diodes (LEDs), in which the photons are light of visible wavelength.
INTRINSIC AND EXTRINSIC SEMICONDUCTORS
In most pure semiconductors at room temperature, the population of thermally excited charge
carriers is very small. Often the concentration of charge carriers may be orders of magnitude
lower than for a metallic conductor. For example, the number of thermally excited electrons
cm–3 in silicon (Si) at 298 K is 1.5 × 1010. In gallium arsenide (GaAs) the population is only 1.1 ×
106 electrons cm–3. This may be compared with the number density of free electrons in a typical
metal, which is of the order of 1028 electrons cm–3.
Given these numbers of charge carriers, it is no surprise that, when they are extremely pure, silicon
and other semiconductors have high electrical resistivities, and therefore low electrical conductivities.
This problem can be overcome by doping a semiconducting material with impurity atoms. Even
very small controlled additions of impurity atoms at the 0.0001% level can make very large
differences to the conductivity of a semiconductor.
It is easier to begin with a specific example. Silicon is a group IV element, and has 4 valence electrons
per atom. In pure silicon the valence band is completely filled at absolute zero. At finite temperatures
the only charge carriers are the electrons in the conduction band and the holes in the valence
band that arise as a result of the thermal excitation of electrons to the conduction band. These
charge carriers are called intrinsic charge carriers, and necessarily there are equal numbers of
electrons and holes. Pure silicon is therefore an example of an intrinsic semiconductor.
If a very small number of atoms of a group V element such as phosphorus (P) are added to the
silicon as substitutional atoms in the lattice, additional valence electrons are introduced into the
material because each phosphorus atom has 5 valence electrons. These additional electrons are
bound only weakly to their parent impurity atoms (the bonding energies are of the order of
hundredths of an eV), and even at very low temperatures these electrons can be promoted into
the conduction band of the semiconductor. This is often represented schematically in band
diagrams by the addition of 'donor levels' just below the bottom of the conduction band, as in the
schematic below.

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The presence of the dotted line in this schematic does not mean that there now exist allowed
energy states within the band gap. The dotted line represents the existence of additional
electrons which may be easily excited into the conduction band. Semiconductors that have been
doped in this way will have a surplus of electrons, and are called n-type semiconductors. In such
semiconductors, electrons are the majority carriers.
Conversely, if a group III element, such as aluminium (Al), is used to substitute for some of the atoms
in silicon, there will be a deficit in the number of valence electrons in the material. This introduces
electron-accepting levels just above the top of the valence band, and causes more holes to be
introduced into the valence band. Hence, the majority charge carriers are positive holes in this case.
Semiconductors doped in this way are termed p-type semiconductors.
Doped semiconductors (either n-type or p-type) are known as extrinsic semiconductors. The
activation energy for electrons to be donated by or accepted to impurity states is usually so low that
at room temperature the concentration of majority charge carriers is similar to the concentration of
impurities. It should be remembered that in an extrinsic semiconductor there is a contribution
to the total number of charge carriers from intrinsic electrons and holes, but at room
temperature this contribution is often very small in comparison with the number of charge carriers
introduced by the controlled impurity doping of the semiconductor.

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DIRECT AND INDIRECT BAND GAP
The band gap represents the minimum energy difference between the top of the valence band and
the bottom of the conduction band, however, the top of the valence band and the bottom of the
conduction band are not generally at the same value of the electron momentum. In a direct band
gap semiconductor, the top of the valence band and the bottom of the conduction band occur at
the same value of momentum, as in the schematic below.
In an indirect band gap semiconductor, the maximum energy of the valence band occurs at a
different value of momentum to the minimum in the conduction band energy:
The difference between the two is most important in optical devices. As has been mentioned in the
section charge carriers in semiconductors, a photon can provide the energy to produce an electron-
hole pair.

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Each photon of energy E has momentum , where c is the velocity of light. An optical
photon has energy of the order of 10–19 J, and, since c =3 × 108 ms–1, a typical photon has a very
small amount of momentum.
A photon of energy Eg, where Eg is the band gap energy, can produce an electron-hole pair in a
direct band gap semiconductor quite easily, because the electron does not need to be given very
much momentum. However, an electron must also undergo a significant change in its momentum
for a photon of energy Eg to produce an electron-hole pair in an indirect band gap semiconductor.
This is possible, but it requires such an electron to interact not only with the photon to gain energy,
but also with a lattice vibration called a phonon in order to either gain or lose momentum.
The indirect process proceeds at a much slower rate, as it requires three entities to intersect in order
to proceed: an electron, a photon and a phonon. This is analogous to chemical reactions, where,
in a particular reaction step, a reaction between two molecules will proceed at a much greater rate
than a process which involves three molecules.
The same principle applies to recombination of electrons and holes to produce photons. The
recombination process is much more efficient for a direct band gap semiconductor than for an
indirect band gap semiconductor, where the process must be mediated by a phonon.
As a result of such considerations, gallium arsenide and other direct band gap semiconductors are
used to make optical devices such as LEDs and semiconductor lasers, whereas silicon, which is an
indirect band gap semiconductor, is not. The table in the next section lists a number of different
semiconducting compounds and their band gaps, and it also specifies whether their band gaps are
direct or indirect.

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COMPOUND SEMICONDUCTORS
In addition to group IV elements, compounds of group III and group V elements, and also
compounds of group II and group VI elements are often semiconductors. The common feature
to all of these is that they have an average of 4 valence electrons per atom.
One example of a compound semiconductor is gallium arsenide, GaAs. In a compound
semiconductor like GaAs, doping can be accomplished by slightly varying the stoichiometry, i.e., the
ratio of Ga atoms to As atoms. A slight increase in the proportion of As produces n- type doping,
and a slight increase in the proportion of Ga produces p-type doping.
The table below list some semiconducting elements and compounds together with their
bandgaps at 300 K.
Material
Direct / Indirect
Bandgap
Band Gap Energy
at 300 K (eV)
Elements
C (diamond)
Ge
Si
Sn (grey)
Indirect
Indirect
Indirect
Direct
5.47
0.66
1.12
0.08
Groups III-V
compounds
GaAs
InAs
InSb
GaP
GaN
InN
Direct
Direct
Direct
Indirect
Direct
Direct
1.42
0.36
0.17
2.26
3.36
0.70
Groups IV-IV
compounds
α-SiC Indirect 2.99
Groups II-VI
compounds
ZnO
CdSe
ZnS
Direct
Direct
Direct
3.35
1.70
3.68

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BEHAVIOUR OF THE CHEMICAL POTENTIAL
The Fermi-Dirac distribution was introduced in the section The Fermi-Dirac Distribution. The
relevant equation to describe the distribution is
so that for a chemical potential, μ, of 5 eV, the distribution takes the formas a function of
temperature.
One feature that is very important about the Fermi-Dirac distribution is that it is symmetric about
the chemical potential. Hence for a simple intrinsic semiconductor, which has equal numbers of
electrons in the conduction band and holes in the valence band, and where the density of states is
also symmetric about the centre of the band gap, the chemical potential must lie halfway between
the valence band and the conductance band, regardless of the temperature, because each
electron promoted to the conduction band leaves a hole in the valence band. This is shown in the
band diagram below in which energy is plotted vertically against temperature horizontally.

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[Note that if the density of states is not exactly symmetric about the centre of the band gap, then the
chemical potential does not have to be exactly in the centre of the band gap. However, under
such circumstances, it will still be extremely close to the centre of the band gap whatever the
temperature, and for all practical purposes can be considered to be in the centre of the band gap.]
For an extrinsic semiconductor the situation is slightly more complicated. At absolute zero in an n-
type semiconductor, the chemical potential must lie in the centre of the gap between the donor level
and the bottom of the conduction band. At low temperatures in such a semiconductor there are
more conduction electrons than there are holes. If the donor level is more than half full, the chemical
potential must lie somewhere between the donor levels and the conduction band. At higher
temperatures, when the donor level is completely depleted of electrons, and the contribution
from intrinsic electrons to the overall electrical conductivity becomes more substantial, the
chemical potential tends towards that for an intrinsic semiconductor, i.e., halfway between the
conduction and valence bands, and therefore in the middle of the band gap.

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For p-type semiconductors the behaviour is similar, but the other way around, i.e., the chemical
potential starts midway between the valence band and the acceptor levels at absolute zero and
gradually increases in energy as the temperature increases, so that at high temperatures it too is in
the middle of the band gap.

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EPITAXY
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where the
overlayer is in registry with the substrate. In other words, there must be one or more preferred
orientations of the overlayer with respect to the substrate for this to be termed epitaxial
growth. The overlayer is called an epitaxial film or epitaxial layer. The term epitaxy comes from the
Greek roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to
arrange upon". For most technological applications, it is desired that the deposited material form
a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal
structure (single-domain epitaxy).
Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed
crystal, the deposited film may lock into one or more crystallographic orientations with respect to
the substrate crystal. If the overlayer either forms a random orientation with respect to the
substrate or does not form an ordered overlayer, this is termed non-epitaxial growth. If an epitaxial
film is deposited on a substrate of the same composition, the process is called homoepitaxy; otherwise
it is called heteroepitaxy.
MOLECULAR BEAM EPITAXY (MBE)
Molecular beam epitaxy (MBE) is one of several methods of depositing single crystals. It was
invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.
Molecular beam epitaxy is a technique for epitaxial growth via the interaction of one or several
molecular or atomic beams that occurs on a surface of a heated crystalline substrate. In Fig. given
below a scheme of a typical MBE system is shown. The solid sources materials are placed in
evaporation cells to provide an angular distribution of atoms or molecules in a beam. The
substrate is heated to the necessary temperature and, when needed, continuously rotated to
improve the growth homogeneity.

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The principle underlying MBE growth is relatively simple: it consists essentially of atoms or
clusters of atoms, which are produced by heating up a solid source. They then migrate in an UHV
environment and impinge on a hot substrate surface, where they can diffuse and eventually
incorporate into the growing film. Despite the conceptual simplicity, a great technological effort
is required to produce systems that yield the desired quality in terms of material purity, uniformity
and interface control.
The choice of MBE and other growth techniques depends on the desired structure and needs.
For example, in the case of mass production, MBE suffers from a lower yield, compared to other
techniques such as Liquid Phase Epitaxy (LPE) and Metal organic Vapour Phase Deposition (MOCVD),
due to a lower growth rate and wafer capability (currently, GaAs based MBE production systems
are capable of up to 4X6” diameter wafers, compared to5X10” of MOCVD). MBE, instead, is the
proper technique when some particular requirements are needed, such as abruptness and control
of interfaces and doping profiles, thanks to the lower growth temperature and growth rate. Besides,
the control on the vacuum environment and on the quality of the source materials allows a
much higher material purity, compared to non-UHV-based techniques, especially in Al-containing
semiconductors for applications in high-mobility, high-speed devices. Finally, the UHV environment
allows the use of electron diffraction probes, which provide fundamental information on the growth
mechanisms.
Molecular beam epitaxy takes place in high vacuum or ultra-high vacuum (10−8 Pa). The most

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important aspect of MBE is the slow deposition rate (typically less than 1000 nm per

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hour), which allows the films to grow epitaxially. The slow deposition rates require
proportionally better vacuum to achieve the same impurity levels as other deposition techniques.
In solid-source MBE, elements such as gallium and arsenic, in ultra-pure form, are heated in
separate quasi-Knudsen effusion cells until they begin to slowly sublime. The gaseous elements
then condense on the wafer, where they may react with each other. In the example of gallium
and arsenic, single-crystal gallium arsenide is formed. The term "beam" means that evaporated
atoms do not interact with each other or vacuum chamber gases until they reach the wafer, due to
the long mean free paths of the atoms.
During operation, reflection high energy electron diffraction (RHEED) is often used for monitoring
the growth of the crystal layers. A computer controls shutters in front of each furnace, allowing
precise control of the thickness of each layer, down to a single layer of atoms. Intricate structures
of layers of different materials may be fabricated this way. Such control has allowed the
development of structures where the electrons can be confined in space, giving quantum wells or
even quantum dots. Such layers are now a critical part of many modern semiconductor devices,
including semiconductor lasers and light-emitting diodes.
In systems where the substrate needs to be cooled, the ultra-high vacuum environment within
the growth chamber is maintained by a system of cryopumps, and cryopanels, chilled using liquid
nitrogen or cold nitrogen gas to a temperature close to 77 Kelvin (−196 degrees Celsius). Cryogenic
temperatures act as a sink for impurities in the vacuum, so vacuum levels need to be several orders
of magnitude better to deposit films under these conditions. In other systems, the wafers on
which the crystals are grown may be mounted on a rotating platter which can be heated to several
hundred degrees Celsius during operation.
Molecular beam epitaxy is also used for the deposition of some types of organic
semiconductors. In this case, molecules, rather than atoms, are evaporated and deposited onto the
wafer. Other variations include gas-source MBE, which resembles chemical vapor deposition.
Lately molecular beam epitaxy has been used to deposit oxide materials for advanced electronic,
magnetic and optical applications. For these purposes, MBE systems have to be modified to
incorporate oxygen sources.

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GROWTH APPARATUS
A schematic drawing of a generic MBE system is presented in Fig. 1. Some basic components can be
identified: The vacuum system consists in a stainless-steel growth chamber, UHV- connected to a
reparation chamber, where substrates are degassed prior to growth and a load-lock module for
transfer to and from air (not shown). All the components of the growth chamber must be able to
resist bake-out temperatures of up to 200ºC for extended periods of time, which are necessary to
minimize outgassing from the internal walls.
A commercial MBE system delivered by VG Semicon

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The pumping system must be able to efficiently reduce residual impurities to a minimum. Typical
MBE growth rates for III-V type semiconductors are of the order of 1 m/h (˜ 1ML/sec), obtained for
group III partial pressures of ~10-6Torr. With atomic densities in the crystal of about 10 22cm-3,
this means that to reduce the impurity concentrations below 1015cm-3, the impurity partial
pressures must be reduced below ~10-13Torr, assuming a unity sticking coefficient [1]. In practice,
base pressure is reduced to the 10 -11-10-12 Torr range, with the residual gas being essentially
H2. The pumping system usually consists of ion pumps, with auxiliary Ti-sublimation and
cryogenic pumps, for the pumping of specific gas species. Liquid N2 cryopanels surround internally
both the main chamber wall and the source flange. Since MBE is a cold wall technique, cryopanels
prevent re-evaporation from parts other than the hot cells. Besides, they provide thermal isolation
among the different cells, as well as additional pumping of the residual gas.
Effusion cells are the key components of an MBE system, because they must provide excellent flux
stability and uniformity, and material purity. Furthermore, being the parts that must withstand the
highest temperatures (up to 1400ºC) for the longest periods, they are often responsible for
machine downtime. Therefore a careful choice of elements, materials and geometry must be taken.
The cells (usually six to ten) are placed on a source flange, and are co-focused on the substrate
heater, to optimise flux uniformity. The flux stability must be better than 1% during a work day, with
day-to-day variations less than 5% . This means that the temperature control must be of the order
of ±1ºC at 1000ºC . Furthermore, the cell geometry must be chosen in a way that the material flux
does not drift appreciably as the source is depleted. The first analytical studies on flux distribution
were performed on the so called Knudsen cells, with small orifices that ensure thermodynamic
equilibrium between the melt and the vapour in the cell. As a matter of fact, however,
Langmuir-type (i.e., no equilibrium) effusion cells are used in MBE growth. Due to the large orifice in
these real cells, a given flux to the substrate can be reached with a lower cell temperature, resulting
in lower power consumption and in a reduction of thermal generation of impurities.

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Illustration of the Deposition Chamber
Picture of the high-mobility Applied EPI Gen II MBE system installed at TASC-INFM National
Laboratoryin Trieste, Italy

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KNUDSEN CELL
In crystal growth, Knudsen Cells are often used as sources evaporators for relatively low partial
pressure elementary sources (e.g. Ga, Al, Hg, As). It is easy to control the temperature of evaporating
content and commonly used in molecular-beam epitaxy.
A typical Knudsen cell contains a crucible (made of pyrolytic boron nitride, quartz, tungsten or
graphite), heating filaments (often made of metal tantalum), water cooling system, heat shields and
orifice shutter.
ADVANTAGES OF MBE TECHNIQUE
I. Clean growth environment.
II. Precise control of the beam fluxes and growth condition.
III. Easyimplementation of in situ diagnostic instruments.
IV. Compatibility with other high vacuum.
V. Thin-film processing methods (metal evaporation, ion beam milling, ion implantation).

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ANALYSIS TECHNIQUES
The most popular in-situ analysis technique for MBE-grown layers is reflection high energy
diffraction (RHEED), see Figure 1. Electrons of energy 5 - 40 keV are directed towards the sample.
They reflect from the surface at a very small angle (less than 3°) and are directed onto a screen.
These electrons interact with only the top few atomic layers and thus provide information about the
surface. Figure 2 shows a typical pattern on the screen for electrons reflected from a smooth
surface, in which constructive interference between some of the electrons reflected from the lattice
structure results in lines. If the surface is rough, spots will appear on the screen, also. By looking at
the total intensity of the reflected electron pattern, an idea of the number of monolayers deposited
and how epilayers grow can be obtained. The island-type growth shown in this figure is an area of
intense interest. These oscillations in intensity are gradually damped as more layers are grown,
because the overall roughness of the surface increases.
Figure 1 Schematic illustrating the formation of a RHEED pattern.

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Figure 2 RHEED diffraction pattern of a GaAs surface. Adapted from images by the MBE
Laboratory in the Institute of Physics of the ASCR.
FOURIER TRANSFORM INFRA-RED SPECTROSCOPY (FTIR)
In infrared spectroscopy, IR radiation is passed through a sample. Some of the infrared
radiation is absorbed by the sample and some of it is passed through (transmitted). The
resulting spectrum represents the molecular absorption and transmission, creating a molecular
fingerprint of the sample. Like a fingerprint no two unique molecular structures produce the
same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis.
FTIR can provide us with the following information:
• It can identify unknown materials
• It can determine the quality or consistency of a sample
• It can determine the amount of components in a mixture

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Infrared spectroscopy has been a workhorse technique for materials analysis in the laboratory for
over seventy years. An infrared spectrum represents a fingerprint of a sample with absorption peaks
which correspond to the frequencies of vibrations between the bonds of the atoms making up the
material.
Because each different material is a unique combination of atoms, no two compounds produce
the exact same infrared spectrum. Therefore, infrared spectroscopy can result in a positive
identification (qualitative analysis) of every different kind of material. In addition, the size of the peaks
in the spectrum is a direct indication of the amount of material present. With modern software
algorithms, infrared is an excellent tool for quantitative analysis.
The original infrared instruments were of the dispersive type. These instruments separated the
individual frequencies of energy emitted from the infrared source. This was accomplished by the use
of a prism or grating. An infrared prism works exactly the same as a visible prism which separates
visible light into its colours (frequencies). A grating is a more modern dispersive element which
better separates the frequencies of infrared energy. The detector measures the amount of energy at
each frequency which has passed through the sample. This results in a spectrum which is a plot of
intensity vs. frequency.
Fourier transform infrared spectroscopy is preferred over dispersive or filter methods of infrared
spectral analysis for several reasons:
• It is a non-destructive technique
• It provides a precise measurement method which requires no external calibration
• It can increase speed, collecting a scan every second
• It can increase sensitivity – one second scans can be co-added together to ratio out random
noise
• It has greater optical throughput
• It is mechanically simple with only one moving part
Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome the
limitations encountered with dispersive instruments. The main difficulty was the slow scanning
process. A method for measuring all of the infrared frequencies simultaneously, rather than
individually, was needed. A solution was developed which employed a very simple optical device

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called an interferometer. The interferometer produces a unique type of signal which has all of the
infrared frequencies “encoded” into it. The signal can be measured very

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quickly, usually on the order of one second or so. Thus, the time element per sample is
reduced to a matter of a few seconds rather than several minutes. Most interferometers employ
a beamsplitter which takes the incoming infrared beam and divides it into two optical beams.
One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat
mirror which is on a mechanism which allows this mirror to move a very short distance (typically a
few millimetres) away from the beamsplitter. The two beams reflect off of their respective
mirrors and are recombined when they meet back at the beamsplitter. Because the path that
one beam travels is a fixed length and the other is constantly changing as its mirror moves, the
signal which exits the interferometer is the result of these two beams “interfering” with each other.
The resulting signal is called an interferogram which has the unique property that every data point (a
function of the moving mirror position) which makes up the signal has information about every
infrared frequency which comes from the source.
This means that as the interferogram is measured; all frequencies are being measured simultaneously.
Thus, the use of the interferometer results in extremely fast measurements. Because the analyst
requires a frequency spectrum (a plot of the intensity at each individual frequency) in order to make
identification, the measured interferogram signal cannot be interpreted directly. A means of
“decoding” the individual frequencies is required. This can be accomplished via a well-known
mathematical technique called the Fourier transformation. This transformation is performed by the
computer which then presents the user with the desired spectral information for analysis.

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THE SAMPLE ANALYSIS PROCESS
The normal instrumental process is as follows:
1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes
through an aperture which controls the amount of energy presented to the sample (and, ultimately,
to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes
place. The resulting interferogram signal then exits the interferometer.
3. The Sample: The beam enters the sample compartment where it is transmitted through or
reflected off of the surface of the sample, depending on the type of analysis being accomplished.
This is where specific frequencies of energy, which are uniquely characteristic of the sample, are
absorbed.
4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are
specially designed to measure the special interferogram signal.
5. The Computer: The measured signal is digitized and sent to the computer where the Fourier
transformation takes place. The final infrared spectrum is then presented to the user for
interpretation and any further manipulation.
Because there needs to be a relative scale for the absorption intensity, a background spectrum
must also be measured. This is normally a measurement with no sample in the beam. This can
be compared to the measurement with the sample in the beam to determine the “per cent

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transmittance.”
This technique results in a spectrum which has all of the instrumental characteristics removed.

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Thus, all spectral features which are present are strictly due to the sample. A single background
measurement can be used for many sample measurements because this spectrum is characteristic of
the instrument itself.
Some of the major advantages of FT-IR over the dispersive technique include:
• Speed: Because all of the frequencies are measured simultaneously, most measurements by FT-IR
are made in a matter of seconds rather than several minutes. This is sometimes referred to as
the Felgett Advantage.
• Sensitivity: Sensitivity is dramatically improved with FT-IR for many reasons. The detectors
employed are much more sensitive, the optical throughput is much higher (referred to as the
Jacquinot Advantage) which results in much lower noise levels, and the fast scans enable the co-
addition of several scans in order to reduce the random measurement noise to any desired level (
referred to as signal averaging).

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• Mechanical Simplicity: The moving mirror in the interferometer is the only continuously
moving part in the instrument. Thus, there is very little possibility of mechanical breakdown.
• Internally Calibrated: These instruments employ a HeNe laser as an internal wavelength
calibration standard (referred to as the Connes Advantage). These instruments are self-
calibrating and never need to be calibrated by the user.
These advantages, along with several others, make measurements made by FT-IR extremely accurate
and reproducible. Thus, it is a very reliable technique for positive identification of virtually any
sample. The sensitivity benefits enable identification of even the smallest of contaminants. This
makes FT-IR an invaluable tool for quality control or quality assurance applications whether it is
batch-to-batch comparisons to quality standards or analysis of an unknown contaminant. In addition,
the sensitivity and accuracy of FT-IR detectors, along with a wide variety of software algorithms,
have dramatically increased the practical use of infrared for quantitative analysis. Quantitative
methods can be easily developed and calibrated and can be incorporated into simple procedures for
routine analysis.
Thus, the Fourier Transform Infrared (FT-IR) technique has brought significant practical advantages
to infrared spectroscopy. It has made possible the development of many new sampling techniques
which were designed to tackle challenging problems which were impossible by older technology.
It has made the use of infrared analysis virtually limitless.
HALL EFFECT
The Hall effect’s is traditionally used to characterise carrier transport in semiconductors by
allowing determination of electrical parameters such as the carrier type, concentration, and mobility,
thus providing an immediate indication of material quality and eventual device performance. The
basic principle behind the Hall’s effect is best explained by considering a slab of conducting material
through which a uniform current density flows under the presence of an applied magnetic field
directed perpendicular to the current flow.

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Fig 1.Scheme illustrating the Hall’s effect phenomenon.
Using Figure 1 as reference, under an applied electric field, EX along the x-axis, charge carriers
will flow along the same axis with an average drift velocity, proportional to the electric field.
The proportionality constant, μ is the carrier mobility and is independent of electric field for small
field intensities. When a uniform magnetic field, Bz is applied along the z-axis, a force termed the
Lorentz force, FB, which is perpendicular to the applied magnetic field and drift velocity, acts to
deflect moving charge carriers to one side of the sample. An electrostatic force, FE is then set up in
such a way as to oppose the deflection by the Lorentz force creating a measurable electric field, Ey
along the y-axis. This field, also known as the Hall field, is proportional to the current density, Jx
and the magnetic field, Bz. The proportionality constant is known as the Hall coefficient, RH where
Ey = −RH.Jx. Bz (1)
Assuming hole carriers with density, p, the resulting current density, Jx can be expressed as
Jx = q.vx .p (2)
The Lorentz force, FB exerted on the carriers is given by
FB = q.Bz.vx (3)
Under steady state conditions, the Lorentz and electrostatic forces balance resulting in zero current in
the y direction and a constant Hall voltage (Vy), such that
−q.Ey = q.Bz.vx (4)
Substituting Equation 1into 4, the Hall coefficient is found to be directly related to the carrier density
by
(5)
Where the sign of RH denotes whether the carriers are electrons (negative) or holes

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(positive).Equation (5) implies that the larger the carrier density, the smaller the Hall

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coefficient due to a smaller Hall voltage being measured. Resistivity, r of the sample can be defined
from the above equations using the following expression for conductivity, σ
Where
(6)
Since the Hall coefficient and resistivity at zero magnetic field are directly measurable, the carrier
type, density, and mobility can easily be calculated from the above expressions. More specifically,
experimental data on Hall’s effect and resistivity over a wide temperature range (e.g. 4 K to 300 K)
can be analysed to give information concerning impurities, dopant activation energies, material
imperfections and uniformity, and carrier scattering mechanisms.
The above analysis makes several assumptions about the sample, the most restrictive of which is
that there is a single mobile carrier. This is often not the case for HgCdTe due to the presence of
multi-carrier effects, and can cause significant errors in determination of transport parameters. This
problem can essentially be overcome by incorporating Hall measurements as a function of magnetic
field into the analysis.
MERCURY CADMIUM TELLURIDE
HgCdTe or mercury cadmium telluride (also cadmium mercury telluride, MCT or CMT) is an alloy of
CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological
importance after silicon and gallium (III) arsenide. The amount of cadmium (Cd) in the alloy (the
alloy composition) can be chosen so as to tune the optical absorption of the material to the desired
infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room
temperature. HgTe is a semimetal; hence its bandgap energy is zero. Mixing these two substances
allows one to obtain any bandgap between 0 and 1.5 eV.
HgCdTe is usually referred to as MerCad Telluride, MerCadTel, or simply MerCaT in the infrared
sensors community.

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Energy gap as a function of cadmium composition.
PROPERTIES OF MCT
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.
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.
THERMAL PROPERTIES
The thermal conductivity of HgCdTe is low; at low cadmium concentrations it is as low as 0.2
W.K−1m−1. This means that it is unsuitable for high power devices. Although infraredlight-
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.[1]
OPTICAL PROPERTIES
HgCdTe is transparent in the infrared at photon energies below the energy gap. The refractive index

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is high, reaching nearly 4 for HgCdTe with high Hg content.

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HGCDTE GROWTH TECHNIQUES
BULK CRYSTAL GROWTH
The first large scale growth method was bulk recrystallization of a liquid melt. This was the main
growth method from the late 1950s to the early 1970s.
EPITAXIAL GROWTH
Highly pure and crystalline HgCdTe is fabricated by epitaxy on either CdTe or CdZnTe substrates.
CdZnTe is a compound semiconductor, the lattice parameter of which can be exactly matched to
that of HgCdTe. This eliminates most defects from the epilayer of HgCdTe. CdTe was developed as an
alternative substrate in the '90s. It is not lattice-matched to HgCdTe, but is much cheaper, as it can be
grown by epitaxy on silicon (Si) or germanium (Ge) substrates.
Liquid phase epitaxy (LPE), in which a substrate is repeatedly dipped into a liquid melt, gives the best
results in terms of crystalline quality, and is still a common technique of choice for industrial
production.
In recent years, molecular beam epitaxy (MBE) has become widespread because of its ability to
stack up layers of different alloy composition. This allows simultaneous detection at several
wavelengths. Furthermore, MBE, and also MOVPE, allow growth on large area substrates such as CdTe
on Si or Ge, whereas LPE does not allow such substrates to be used.
The HgCdTe energy band structure has three key features that make it the nearly ideal IR detector
material:
(i) tailor-able energy band gap over the 1–30 μm range,
(ii) large optical absorption coefﬁcients that, together with long diffusion lengths, enable high
quantum efﬁciencies (approaching 100% in most cases),
(iii) favourable inherent recombination mechanisms that lead to long carrier lifetimes, low thermal
generation rates, high operating temperatures, and long diffusion lengths.
An additional ideal feature of the HgCdTe band structure, only fully realized and exploited within
the past 10 years, is that it truly enables ideal electron-initiated avalanche photodiodes, with
single-carrier multiplication and no excess noise.

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The favourable material properties of HgCdTe include:
I. the ability to grow bandgap engineered ﬁlms, with excellent lateral spatial uniformity and low EPD
(<1 × 105cm−2),by several epitaxial methods (LPE, MBE, MOVPE) onto IR-transparent lattice-matched
CdZnTe substrates,
II. the ability to grow ﬁlms of useful quality by MBE and MOVPE on alternative substrates such as silicon,
germanium and gallium arsenide,
III. residual background carrier concentrations as low as 1 × 1014 cm−3,
IV. convenient n-type and p-type dopants,
V. versatile methods for forming mesa and planar homojunctionsxxviii FOREWORD and
heterojunctions,
VI. a low dielectric constant for low junction capacitance,
VII. a small change (0.3%) in lattice constant over the entire alloy range, and
VIII. a native CdTe passivation that provides photodiodes with low 1/f noise and high radiation
tolerance.
This combination of energy band structure and material properties has enabled a diverse family of
high-performance quantum IR detectors, including photoconductors and both single-colour and
two-colour photodiodes, which has led to large-format photovoltaic arrays that are the basis for a
widely applicable hybrid focal plane array (FPA) technology.
INFRARED DETECTION
HgCdTe is the only common material that can detect infrared radiation in both of the
accessible atmospheric windows. These are from 3 to 5 µm (the mid-wave infrared window,
abbreviated MWIR) and from 8 to 12 µm (the long-wave window, LWIR). Detection in the MWIR
and LWIR windows is obtained using 30% [(Hg0.7Cd0.3)Te] and 20% [(Hg0.8Cd0.2)Te] cadmium
respectively. HgCdTe can also detect in the short wave infrared SWIR atmospheric windows of 2.2 to
2.4 µm and 1.5 to 1.8 µm.
HgCdTe is a common material in photodetectors of Fourier transforms infrared spectrometers. It is
also found in military field, remote sensing and infrared astronomy research. Military technology
has depended on HgCdTe for night vision. In particular, the US air force makes extensive use of

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HgCdTe on all aircraft, and to equip airborne smart bombs. A variety of heat-seeking missiles are
also equipped with HgCdTe detectors. HgCdTe detector arrays can also be found at most of the
world’s major research telescopes including several satellites.

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Many HgCdTe detectors (such as Hawaii and NICMOS detectors) are named after the
astronomical observatories or instruments for which they were originally developed.
The main limitation of LWIR HgCdTe-based detectors is that they need cooling to temperatures
near that of liquid nitrogen (77K), to reduce noise due to thermally excited current carriers (see
cooled infrared camera). MWIR HgCdTe cameras can be operated at temperatures accessible to
thermoelectric coolers with a small performance penalty. Hence, HgCdTe detectors are relatively
heavy compared to bolometers and require maintenance. On the other side, HgCdTe enjoys much
higher speed of detection (frame rate) and is significantly more sensitive than some of its cheaper
competitors.
HgCdTe is often a material of choice for detectors in Fourier-transform infrared (FTIR)
spectrometers. This is because of the large spectral range of HgCdTe detectors and also the high
quantum efficiency.
HgCdTe can be used as a heterodyne detector, in which the interference between a local source and
returned laser light is detected. In this case it can detect sources such as CO2lasers. In
heterodyne detection mode HgCdTe can be uncooled, although greater sensitivity is achieved by
cooling. Photodiodes, photoconductors or photoelectromagnetic (PEM) modes can be used. A
bandwidth well in excess of 1 GHz can be achieved with photodiode detectors.
The main competitors of HgCdTe are less sensitive Si-based bolometers (see uncooled infrared
camera), InSb and photon-counting superconducting tunnel junction (STJ) arrays. Quantum well
infrared photodetectors (QWIP), manufactured from III-V semiconductor materials such as GaAs and
AlGaAs, are another possible alternative, although their theoretical performance limits are inferior
to HgCdTe arrays at comparable temperatures and they require the use of complicated
reflection/diffraction gratings to overcome certain polarization exclusion effects which impact array
responsivity. In the future, the primary competitor to HgCdTe detectors may emerge in the form
of Quantum Dot Infrared Photodetectors (QDIP), based on either a colloidal or type-II superlattice
structure. Unique 3- D quantum confinement effects, combined with the unipolar (non-exciton
basedphotoelectric behaviour) nature of quantum dots could allow comparable performance to
HgCdTe at significantly higher operating temperatures. Initial laboratory work has shown
promising results in this regard and QDIPs may be one of the first significant nanotechnology
products to emerge.

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50 | P a g e
In HgCdTe, detection occurs when an infrared photon of sufficient energy kicks an electron from the
valence band to the conduction band. Such an electron is collected by a suitable external readout
integrated circuits (ROIC) and transformed into an electric signal. The physical mating of the
HgCdTe detector array to the ROIC is often referred to as a "focal plane array".
In contrast, in a bolometer, light heats up a tiny piece of material. The temperature change of the
bolometer results in a change in resistance which is measured and transformed into an electric signal.
Mercury zinc telluride has better chemical, thermal, and mechanical stability characteristics than
HgCdTe. It has a steeper change of energy gap with mercury composition than HgCdTe, making
compositional control harder.
HGCDTE INFRA-RED DETECTOR DEVICES
The popularity of HgCdTe detectors is made possible by their flexibility in spectral response over a
wide span of the infrared regions of interest. HgCdTe spectral flexibility is illustrated in Fig which
shows the spectral quantum efficiency of a variety of HgCdTe devices, including photoconductors
(PC), photodiodes (PV), and avalanche photodiodes (APDs). Photodiode technology is being
vigorously extended to wavelengths beyond 12 µm. In the next few years, photodiodes will largely
replace photoconductors at wavelengths out to about 20 μm. Single element photoconductors used
in spectrometers to about 25 μm at liquid nitrogen temperature will continue to occupy that niche
application.
Fig: Spectral quantum efficiency for HgCdTe devices without antireflection coating.
Photodiodes span all but the longest wavelengths where photoconductors are still commonly
used. At short wavelengths, avalanche photodiodes are in development. Three

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generations of HgCdTe devices have been successively developed. Photoconductors, the first
generation of HgCdTe devices, entered production in the late 70s.
PHOTOCONDUCTORS
First generation HgCdTe detectors consist of linear arrays of photoconductive devices. Good quality
photoconductors can be fabricated by applying metal electrodes to pure n-type material, thinned
to approximately 10 μm. The basic photoconductor device structure is illustrated:
Fig: Cross section of a basic HgCdTe photoconductor. The n-type layer of HgCdTe is approximately
10 μm thick. Typical photoconductors are passivated with anodic oxide and antireflection coated with
zinc sulphide.
Characteristics of a basic LWIR photoconductive HgCdTe are:
• 50-100 Ω impedance per square.
• 105 V/W at 1 mA bias for a 50_50 μm device.
• D* about 80% of background limit.
• Photon noise level of a few nV/Hz.
LWIR HgCdTe photoconductors have also been widely used in NASA applications for a variety
of earth satellite missions.
PHOTODIODES
Second-generation HgCdTe devices are two-dimensional arrays of photodiodes. Photodiodes
having modest impedance (resistance-area product or R0A) of 10 Ωcm2 can be mated to silicon
readout arrays with indium bump bonds at the pixel level. First demonstrated in the mid-70s,
indium bump bonding of readout electronics provides for multiplexing the signals from
thousands of pixels onto a few output lines, greatly simplifying the interface between the vacuum-
enclosed cryogenic sensor and the system electronics. In a general sense, the signal-to-noise ratio of
a sensor will improve with the square root of the number of detector elements in an array – to the
extent that they can collect proportionally more signal from the scene. Today, millions of
pixels are connected to millions of

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amplifiers/integrators in the unit cells of readout circuits. Figure shows a region of pixels on a 1024 X
1024 HgCdTe array with indium bumps deposited for bump bonding to readout.
HgCdTe photovoltaic arraywith indium bumps. This scanning-electron microscope image shows a
portion of an array of 1024 X 1024 pixels on 17 μm centres
In spite of the tremendous impetus offered by large PV arraydevelopment, photovoltaic HgCdTe took
many years to emerge from laboratorydemonstrations.
THIRD GENERATION HGCDTE DEVICES
The definition of third-generation devices is not particularly well established. Here it is taken to mean
device structures that have substantially enhanced capabilities over an ordinary photodiode. We
will describe three such examples:
• Two colour detectors.
• Avalanche photodiodes.
• Hyper-spectral arrays.
TWO COLO UR DETECTORS
The virtues of colour vision are easily appreciated in the visible because colour is a powerful
discriminator of everyday objects. For infrared systems, sensitivity in dual spectral bands has been
demonstrated to have similar virtues.
Two-colour detectors are made with a stack of two detector layers separated by a common electrode,
in the case of HgCdTe, a p-type layer. Figure illustrates the structure :

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The Band 1 and Band 2 alloy compositions can be any two x-values as long as Band 1 has a higher x-
value than Band 2. Although this structure can be grown by LPE methods, vapour phase growth is
the preferred method.
AVAL ANCHE PHOTODIODES
Short-wavelength HgCdTe avalanche photodiodes (APDs) are able to exploit a very favourable
property in the band structure of the alloy when the bandgap is about 0.90 eV. For materials of this
alloy composition, the energy required to excite an electron from the top of the valence band to the
bottom of the conduction band is identical to the energy for the excitation of an electron from the
top of the split-off valence band to the top of the valence band.
Band structure of HgCdTe for alloy compositions at which the bandgap is about 0.90 eV,
corresponding to 1.37 μm. At this composition, the split-off band is separated from the top of the
conduction band by the same energy. This resonance condition allows very favourable multiplication
of holes.
The avalanche effect in the high-field region of an avalanche photodiode multiplies the number
of photo-excited carriers by the avalanche gain. This raises the signal level, which

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itself may be highly useful for raising low signal levels above the amplifier noise. A second
consideration is the amount by which the noise is increased by the avalanche process. Here it is
advantageous to have a large asymmetry between the avalanche gain of holes and electrons. The
band structure of HgCdTe gives k-values of 0.1 or less – a highly favourable ratio of hole to
electron multiplication during avalanche conditions, resulting in very little noise gain.
A 25 element of HgCdTe Avalanche photodiodes
HYPER SPECTR AL ARRAYS
When a second-generation array is combined in a scanning imager having a means to
selectively illuminate each row with a different spectral band we have a hyper-spectral imager.
Such instruments can image a scene in hundreds of spectral bands per frame, generating a
hypercube image.
Such a capability is anticipated to revolutionize disciplines such as land resource utilization which
today rely on just a handful of spectral bands.
EXPERIMENTAL WORK

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Annealed @260C;n-type, cc~7x10
14
cm
-3
)
As-grown;p-type, cc~1.5x10
17
cm
-3
)
Sample # MCT-65
Layer thickness ~8.9 m
Composition ~0.298
60
50
40
30
20
10
0
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm
-1
)
An MBE grown MCT epitaxial layer was characterised using FTIR and Hall
measurements in as-grown condition and after annealing in Hg-ambient @ 260oC
for 17 hours. Following table gives the data obtained from these
measurements.
K
bility
V.s)
The thickness and composition of Hg1-xCdxTe epilayer was determined from FTIR
Transmission(%)
Sample #
MCT-65
FTIR
Thickness Composition
T (m) (x)
As-grown 8.9 0.298 1.5x1017 210
Annealed 8.9 0.298 7x1014 15,000

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Thickness was calculated from the interference fringes using formula
T = 10000/(2*n*),
where n is refractive index of the layer and is the wavenumber difference (cm-
1)between any two maxima or minima of interference fringes.
THE COMPOSITION OF THE EPILAYER WAS DETERMINED FROM 50% TRANSMISSION CUT-OFF
VALUE. THIS METHOD IS EMPIRICALLY ESTABLISHED METHOD OF DETERMINING THE
COMPOSITION OF THE GROWN MCT EPILAYERS.
SUMMARY
HgCdTe has emerged as the most widely used infrared detector today because of its excellent
properties, including:
• The alloy composition can be optimised for any wavelength in the range of 0.7–20 μm.
• Quantum efficiency is very high.
• Minimal cooling is required because the detection mechanism relies on photo-excitation across
an intrinsic bandgap.
• The R0A product (or inversely the leakage current) responds to cooling.
• Growth technology has matured.
• Sophisticated device structures can be grown by MBE in future.

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CONCLUSION
The industrial training at defence research and development organisation
(DRDO), Delhi has given me an opportunity to acquire the knowledge regarding
the most advanced technology used to fabricate semiconductors and MCT which is
used in infra-red detectors.
The summer training has given me great knowledge and tremendous confidence.

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Bibliography
PAPERS REFERRED
1. Basics of Molecular Beam Epitaxy (MBE) byFernandoRinaldi
2. HgCdTe Infra-red detectors by P. NORTON*
3. MOLECULAR BEAM EPITAXY: PRINCIPLES AND APPLICATIONS G. Biasiol1 and L.
Sorba1,2
4. Introduction to Fourier Transform Infrared Spectrometry by Thermo Nicolet
Corporation
BOOKS REFERRED
1. VLSI Technology By S.M.SZE
WEB SECTION
1. www.google.com
2. www.wikipedia.com
3. Dissemination of IT for the Promotion of Materials Science (DoITPoMS), Cambridge
University

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