Chemistry of Epitaxy

1
Chemistry of Epitaxy
2

• Epitaxy is an interface between a thin film and a
  substrate
• The term epitaxy describes an ordered crystalline
  growth on a monocrystalline substrate
• Epitaxial films may be grown from gaseous or
  liquid precursors
• Because the substrate acts as a seed crystal, the
  deposited film takes on a lattice structure and
  orientation identical to those of the substrate

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• Epitaxy is different from other thin film
  deposition methods which deposit polycrystalline
  or amorphous films, even on single - crystal
  substrates
• If a film is deposited on a substrate of the same
  composition, the process is called homoepitaxy
• Otherwise it is called heteroepitaxy
• Homoepitaxy is a kind of epitaxy performed with
  only one material in which a crystalline film is
  grown on a substrate or film of the same material
  
• This technology is applied to growing a more
  purified film than the substrate and fabricating
  layers with different doping levels

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• Heteroepitaxy is a kind of epitaxy performed with
  materials that are different from each other in
  which a crystalline film grows on a crystalline
  substrate or film of another material
• This technology is often applied to growing
  crystalline films of materials of which single
  crystals cannot be obtained and to fabricating
  integrated crystalline layers of different
  materials
• Examples include gallium nitride (GaN) on
  sapphire or aluminum gallium indium phosphide
  (AlGaInP) on gallium arsenide (GaAs)
• Heterotopotaxy is a process similar to
  heteroepitaxy except for the fact that thin film
  growth is not limited to two dimensional growth
• In this process, the substrate is similar only in
  structure to the thin film material

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• Epitaxy is used in silicon - based manufacturing
  processes for BJTs and modern CMOS, but it is
  particularly important for compound
  semiconductors such as gallium arsenide
• Manufacturing issues include control of the
  amount and uniformity of the deposition's
  resistivity and thickness, the cleanliness and
  purity of the surface and the chamber atmosphere,
  the prevention of the typically much more highly
  doped substrate wafer's diffusion of dopant to
  the new layers, imperfections of the growth
  process, and protecting the surfaces during the
  manufacture and handling

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• Applications of Epitaxy
• Epitaxy has applications in nanotechnology and in
  semiconductor fabrication.
• Epitaxy is the only affordable method of high
  crystalline quality growth for many semiconductor
  materials, including technologically important
  materials as silicon -germanium, gallium nitride,
  gallium arsenide and indium phosphide
• Epitaxy is also used to grow layers of pre -
  doped silicon on the polished sides of silicon
  wafers, before they are processed into
  semiconductor devices.

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• Epitaxy is one of the most vital processes in
  semiconductor device manufacturing
• This is especially true in nanotechnology, as it
  provides the means of growing very thin films in
  a controlled way to achieve the necessary
  accuracy, purity, and orientation of the film

• Chemistry plays an important role in the process
  of epitaxial layer growth
• The constituents of the film are often presented
  to the substrate in the form of compounds with
  other elements
• They must be extracted from these compounds and
  react with the substrate and possibly other
  constituents to form the epitaxial layer

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• There are many approaches to growing epitaxial
  films
• Vapor Phase Epitaxy (VPE)
• Liquid Phase Epitaxy (LPE)
• Metallorganic Chemical Vapor Deposition (MOCVD)
• Molecular Beam Epitaxy (MBE)
• Atomic Layer Epitaxy (ALE)
• Several of these methods are based on Chemical
  Vapor Deposition (CVD)

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Chemical Vapor Deposition (CVD)

• CVD is used to produce high - purity, high
  -performance solid materials, usually in the form
  of a thin film on a substrate
• In a typical CVD process, the wafer (substrate)
  is exposed to one or more volatile precursors,
  which react and/or decompose on the substrate
  surface to produce the desired deposit
• Frequently, volatile byproducts are also
  produced, which are removed by gas flow through
  the reaction chamber

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• Microfabrication processes widely use CVD to
  deposit materials in various forms, including
  monocrystalline, polycrystalline, amorphous, and
  epitaxial
• These materials include silicon, carbon fiber,
  carbon nanofibers, filaments, carbon nanotubes,
  SiO2, silicon-germanium, tungsten, silicon
  carbide, silicon nitride , titanium nitride, and
  various high - k dielectrics
• The CVD process is also used to produce synthetic
  diamonds

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• Types of chemical vapor deposition
• A number of forms of CVD are in wide use and are
  frequently referenced in the literature
• These processes differ in the means by which
  chemical reactions are initiated (e.g.,
  activation process) and process conditions

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• These processes can be classified by operating
  pressure
• Atmospheric pressure CVD (APCVD) - CVD processes
  at atmospheric pressure
• Low-pressure CVD (LPCVD) - CVD processes at
  subatmospheric pressures
• Reduced pressures tend to reduce unwanted
  gas-phase reactions and improve film uniformity
  across the wafer
• Most modern CVD process are either LPCVD or
  UHVCVD
• Ultrahigh vacuum CVD (UHVCVD) - CVD processes at
  a very low pressure, typically below 10 -6 Pa (
  10 -8 torr)

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• Classified by physical characteristics of vapor
• Aerosol assisted CVD (AACVD) - A CVD process in
  which the precursors are transported to the
  substrate by means of a liquid/gas aerosol, which
  can be generated ultrasonically.
• This technique is suitable for use with
  nonvolatile precursors
• Direct liquid injection CVD (DLICVD) - A CVD
  process in which the precursors are in liquid
  form (liquid or solid dissolved in a convenient
  solvent)
• Liquid solutions are injected in a vaporization
  chamber towards injectors (typically car
  injectors).
• The precursor vapors are then transported to the
  substrate as in classical CVD process
• This technique is suitable for use on liquid or
  solid precursors
• High growth rates can be reached using this
  technique

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• Microwave plasma-assisted CVD (MPCVD)
• Plasma-Enhanced CVD (PECVD) - CVD processes that
  utilize a plasma to enhance chemical reaction
  rates of the precursors
• PECVD processing allows deposition at lower
  temperatures, which is often critical in the
  manufacture of semiconductors
• Remote plasma-enhanced CVD (RPECVD) - Similar to
  PECVD except that the wafer substrate is not
  directly in the plasma discharge region
• Removing the wafer from the plasma region allows
  processing temperatures down to room temperature
• Atomic layer CVD (ALCVD) Deposits successive
  layers of different substances to produce
  layered, crystalline films
• Hot wire CVD (HWCVD) - Also known as Catalytic
  CVD (Cat-CVD) or hot filament CVD (HFCVD)
• Uses a hot filament to chemically decompose the
  source gases

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• Metallorganic chemical vapor deposition (MOCVD) -
  CVD processes based on metallorganic precursors
• Hybrid Physical-Chemical Vapor Deposition (HPCVD)
  - Vapor deposition processes that involve both
  chemical decomposition of precursor gas and
  vaporization of solid a source
• Rapid thermal CVD (RTCVD) - CVD processes that
  use heating lamps or other methods to rapidly
  heat the wafer substrate
• Heating only the substrate rather than the gas or
  chamber walls helps reduce unwanted gas phase
  reactions that can lead to particle formation
• Vapor phase epitaxy (VPE)

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• Polysilicon
• Polycrystalline silicon is widely used as the
  gate oxide in MOSFETs
• Polycrystalline silicon is deposited from silane
  (SiH4), using the following reaction
• This reaction is usually performed in LPCVD
  systems, with either pure silane feedstock, or a
  solution of silane with 70-80 nitrogen
• Temperatures between 600 and 650 C and pressures
  between 25 and150 Pa yield a growth rate between
  10 and 20 nm per minute. An alternative process
  uses a hydrogen - based solution
• The hydrogen reduces the growth rate, but the
  temperature is raised to 850 or even 1050 C to
  compensate

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• Polysilicon may be grown directly with doping, if
  gases such as phosphine, arsine or diborane are
  added to the CVD chamber
• Diborane increases the growth rate, but arsine
  and phosphine decrease it

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TEOS

• TEOS is a material commonly used to grow silicon
  dioxide layers on semiconductors
• It stands for Tetra - Ethyl - Ortho - Silicate,
  or equivalently tetra - ethoxy - silane

• TEOS slowly hydrolyzes into silicon dioxide and
  ethanol when in contact with ambient moisture

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• The key to understanding the difference between
  TEOS and silane is to note that in TEOS the
  silicon atom is already oxidized
• The conversion of TEOS to silicon dioxide is
  essentially a rearrangement rather than an
  oxidation reaction, with much reduced changes in
  free enthalpy and free energy

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• The basic overall reaction for the deposition of
  silicon dioxide requires the removal of two
  oxygen atoms

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• While gas phase reactions can occur, particularly
  at the high end of the temperature range,
  deposition is probably the result of TEOS surface
  reactions
• TEOS chemisorbs onto silanol groups (Si-OH) at
  the surface, as well as strained surface bonds

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• TEOS will not adsorb onto the resulting
  alkyl-covered surface, so deposition is probably
  limited by removal of the surface alkyl groups
• These groups can undergo elimination reactions
  with neighboring molecules to form Si-O-Si bridges

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• This process proceeds in an inert atmosphere
  TEOS can be its own oxygen source, and SiO2 can
  be deposited from TEOS in nitrogen
• However, addition of oxygen increases the
  deposition rate, presumably through providing an
  alternative path for removal of the ethyl groups
  from the surface
• TEOS/O2 is generally performed in tube reactors
  at pressures of a few Torr

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• Silicon dioxide
• Silicon dioxide (SiO2) is commonly used in
  integrated circuits and nanodevices as an
  insulator and as a capacitor dielectric
• Silicon dioxide may be deposited by several
  different processes
• Common source gases include silane and oxygen,
  dichlorosilane (SiCl2H2) and nitrous oxide (N2O),
  or tetraethylorthosilicate (TEOS Si(OC2H5)4)
• The reactions are as follows
• SiH4 O2 ? SiO2 2H2
• SiCl2H2 2N2O ? SiO2 2N2 2HCl
• Si(OC2H5)4 ? SiO2 byproducts

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• The choice of source gas depends on the thermal
  stability of the substrate for instance,
  aluminum is sensitive to high temperature
• Silane deposits at temperatures between 300 and
  500 C, dichlorosilane at around 900 C, and TEOS
  between 650 and 750 C, resulting in a layer of
  Low Temperature Oxide (LTO)
• However, silane produces a lower-quality oxide
  than the other methods (lower dielectric
  strength, for instance), and it deposits
  nonconformally
• Any of these reactions may be used in LPCVD, but
  the silane reaction is also done in APCVD
• CVD oxide invariably has lower quality than
  thermal oxide, but thermal oxidation can only be
  used in the earliest stages of IC manufacturing

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• Silicon dioxide may also be grown with impurities
  (alloying or "doping") for one of two purposes
• (1) During further process steps that occur at
  high temperature, the impurities may diffuse from
  the oxide into adjacent layers (most notably
  silicon) and dope them
• Oxides containing 5 to 15 impurities by mass
  are often used for this purpose
• (2) silicon dioxide alloyed with phosphorus
  pentoxide ("P-glass") can be used to smooth out
  uneven surfaces
• P-glass softens and reflows at temperatures above
  1000 C
• This process requires a phosphorus concentration
  of at least 6, but concentrations above 8 can
  corrode aluminum
• Phosphorus is deposited from phosphine gas and
  oxygen
• 4PH3 5O2 ? 2P2O5 6H2

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• Glasses containing both boron and phosphorus
  (borophosphosilicate glass, BPSG) undergo viscous
  flow at lower temperatures around 850 C is
  achievable with glasses containing around 5
  weight  of both constituents, but stability in
  air can be difficult to achieve
• Phosphorus oxide in high concentrations interacts
  with ambient moisture to produce phosphoric acid
• Crystals of BPO4 can also precipitate from the
  flowing glass on cooling
• These crystals are not readily etched in the
  standard reactive plasmas used to pattern oxides,
  and will result in circuit defects in integrated
  circuit manufacturing

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• Besides these intentional impurities, CVD oxide
  may contain byproducts of the deposition process.
• TEOS produces a relatively pure oxide, whereas
  silane introduces hydrogen impurities, and
  dichlorosilane introduces chlorine
• Lower temperature deposition of silicon dioxide
  and doped glasses from TEOS using ozone rather
  than oxygen has also been explored (350 to 500
  C)
• Ozone glasses have excellent conformality but
  tend to be hygroscopic -- that is, they absorb
  water from the air due to the incorporation of
  silanol (Si-OH) in the glass
• Infrared spectroscopy and mechanical strain as a
  function of temperature are valuable diagnostic
  tools for diagnosing such problems

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• Silicon Nitride
• Silicon nitride is often used as an insulator and
  chemical barrier in manufacturing ICs
• Silicon nitride
• The following two reactions deposit nitride from
  the gas phase
• 3SiH4 4NH3 ? Si3N4 12H2
• 3SiCl2H2 4NH3 ? Si3N4 6HCl 6H2
• Silicon nitride deposited by LPCVD contains up to
  8 hydrogen.
• It also experiences strong tensile stress , which
  may crack films thicker than 200 nm
• However, it has higher resistivity and dielectric
  strength than most insulators commonly available
  in microfabrication (1016 Ocm and 10 MV/cm,
  respectively)

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• Another two reactions may be used in plasma to
  deposit SiNH
• 2SiH4 N2 ? 2SiNH 3H2
• SiH4 NH3 ? SiNH 3H2
• These films have much less tensile stress, but
  worse electrical properties (resistivity 106 to
  1015 Ocm, and dielectric strength 1 to 5 MV/cm)

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• Vapor-phase Epitaxy (VPE)
• In VPE, one or more materials to be deposited are
  transported to the substrate as compounds in
  vapor form
• In this manner, single materials, doped
  materials, or compounds may be deposited in
  single crystal form
• Once the materials reach the substrate, they are
  extracted from the compound and attach themselves
  to the surface atoms on the substrate
• One of the most common examples of VPE is the
  growth of a doped silicon film on a silicon
  substrate
• This process can be used to fabricate individual
  transistors and to fabricate transistors and
  isolation regions on integrated circuits

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• There are four major chemical sources of silicon
  for commercial epitaxial deposition
• 1) silicon tetrachloride (SiCl4)
• 2) trichlorosilane (SiHCl3)
• 3) dichlorosilane (SiH2Cl2)
• 4) silane (SiH4)
• Each of the chemical sources mentioned above may
  be described by an over-all reaction equation
  that shows how the vapor phase reactants form the
  silicon epitaxial film
• For example, the over-all reaction for silicon
  epitaxy by silane reaction may be written as
  follows SiH4 ? Si 2H2

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• Silicon is most commonly deposited from silicon
  tetrachloride in hydrogen at approximately 1200
  C
• SiCl4(g) 2H2(g) ? Si(s) 4HCl(g)
  
• This reaction is reversible, and the growth rate
  depends strongly upon the proportion of the two
  source gases
• Growth rates above 2 ?m/minute produce
  polycrystalline silicon, and negative growth
  rates (etching) may occur if too much hydrogen
  chloride byproduct is present
• An additional etching reaction competes with the
  deposition reaction
• SiCl4(g) Si(s) ? 2SiCl2(g)

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• The reaction is actually a complex series of
  reactions that ultimately result in the
  deposition of pure silicon

SiCl4 H2 ? SiHCl3 HCl SiHCl3 H2 ? SiH2Cl2
HCl SiH2Cl2 ? SiCl2 H2 SiHCl3 ? SiCl2
HCl SiCl2 H2 ? Si 2HCl
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• Silicon VPE may also use silane, dichlorosilane,
  and trichlorosilane source gases
• For instance, the silane reaction occurs at
  650 C in this way
• SiH4 ? Si 2H2
• This reaction does not inadvertently etch the
  wafer, and takes place at lower temperatures than
  deposition from silicon tetrachloride
• However, it will form a polycrystalline film
  unless tightly controlled, and it allows
  oxidizing species that leak into the reactor to
  contaminate the epitaxial layer with unwanted
  compounds such as silicon dioxide

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VPE (vapor phase epitaxy)

• All reactants in vapor phase, deposited on heated
  substrate
• Halide or hydride process
• I GaAs (s) HCl (g) ? GaCl (g) ¼ As4 (g) ½
  H2 (g)
• II 3 GaCl (g) ½ As4 (g) ? 2 GaAs (s) GaCl3
  (g)
• III GaCl (g) ¼ As4 (g) ½ H2 (g) ? GaAs (s)
  HCl (g)
• Advantage fast rate (.1 - .5 mm.min), easy, safe
  (w/o arsine process)
• Disadvantage Al compounds difficult, thickness
  resolution

halide AsCl3, H2, dopantshydride AsH3, H2,
dopants
Reducing atmosphere
As4
II, III
I
HCl
substrate
halide AsCl3, H2hydride HCl, H2
Ga metal
GaAs
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Epitaxial growth
direction of sliding

• LPE (liquid phase epitaxy)
• Thermodynamic equilibrium growth
• saturated melt (As in Ga)
• cool which reduces solubility of As, so GaAs
  deposits
• can do in bath melt, or slider technique
• advantage inexpensive, easy
• disadvantages
• no in situ diagnostics
• gt binaries hard x x(t)
• surface morphology
• thickness control not very precise

H2 reducing atmosphere
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• Metallorganic Chemical Vapor Deposition (MOCVD)
• Metallorganic Chemical Vapor Deposition (MOCVD)
  is a method of epitaxial growth of materials,
  especially compound semiconductors, from the
  surface reaction of organic compounds or
  metallorganics and metal hydrides containing the
  required chemical elements
• For example, indium phosphide could be grown in a
  reactor on a substrate by introducing
  Trimethylindium ((CH3)3In) and phosphine (PH3)

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• Formation of the epitaxial layer occurs by final
  pyrolisis of the constituent chemicals at the
  substrate surface.
• In contrast to molecular beam epitaxy (MBE) the
  growth of crystals is by chemical reaction and
  not physical deposition.
• This takes place not in a vacuum, but from the
  gas phase at moderate pressures (2 to 100 kPa)
• As such this technique is preferred for the
  formation of devices incorporating
  thermodynamically metastable alloys
• It has become the dominant process for the
  manufacture of laser diodes, solar cells, and LEDs

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Growth Process of MOCVD
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MOCVD Reactor Block Diagram
42

• Reactor Components
• A reactor is a chamber made of a high -
  temperature material that does not react with the
  chemicals being used
• The chamber is composed of reactor walls, a
  liner, a susceptor, gas injection units, and
  temperature control units
• The reactor walls are typically made from
  stainless steel or quartz
• To prevent overheating, cooling water must flow
  through the channels within the reactor walls
• Special glasses, such as quartz or ceramic, are
  often used as the liner in the reactor chamber
  between the reactor wall and the susceptor

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• A substrate sits on a susceptor which is held at
  a controlled temperature.
• The susceptor is made from a material resistant
  to the metalorganic compounds used, such as
  graphite
• For growing nitrides and related materials, a
  special coating on the graphite susceptor is
  necessary to prevent corrosion by ammonia (NH3)
  gas

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• Gas inlet and switching system
• Gas is introduced via devices known as
  'bubblers'.
• In a bubbler a carrier gas (usually nitrogen or
  hydrogen) is bubbled through the metallorganic
  liquid, which picks up some metallorganic vapor
  and transports it to the reactor
• The amount of metallorganic vapor transported
  depends on the rate of carrier gas flow and the
  bubbler temperature
• Allowance must be made for saturated vapors

45

• Gas Exhaust and cleaning System
• Toxic waste products must be converted to liquid
  or solid wastes for recycling (preferably) or
  disposal
• Ideally processes will be designed to minimize
  the production of waste products

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MOCVD Process

• Basic reaction for GaAs
• Ga(CH3)3AsH3 ? GaAs3CH4
• Al(CH3)3AsH3 ? AlAs3CH4
• For GaN
• Ga(CH3)3NH3 ? GaN3CH4
• Process
• MO sources and hydrides mixed inside reactor and
  transferred to the substrate
• high temperature of substrate results in the
  decomposition of sources, forming the film
  precursors.
• film precursors transport absorb on the growth
  surface
• precursors diffuse to the growth site,
  incorporate
• by-products of the surface reactions absorb from
  surface

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47
MOCVD System
47
48

• Molecular Beam Epitaxy
• The Molecular Beam Epitaxy System is used to grow
  and characterize thin crystalline films of oxides
  and ceramics
• Molecular beam epitaxy (MBE), is one of several
  methods of depositing single crystals
• MBE takes place in high vacuum or ultra high
  vacuum (10-8 Pa)
• The most important aspect of MBE is the slow
  deposition rate (typically less than 1000 nm per
  minute), which allows the films to grow
  epitaxially
• However, the slow deposition rates require
  proportionally better vacuum in order to achieve
  the same impurity levels as other deposition
  techniques

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• In solid - source MBE, ultra - pure elements such
  as gallium and arsenic are heated in separate
  quasi-Knudsen effusion cells until they begin to
  slowly sublimate
• 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" simply means that evaporated
  atoms do not interact with each other or any
  other vacuum chamber gases until they reach the
  wafer, due to the long mean free paths of the
  atoms

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• During operation, RHEED (Reflection High Energy
  Electron Diffraction) 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 in this manner
• 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

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• 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 oK (-196 oC)
• However, cryogenic temperatures act as a sink for
  impurities in the vacuum, and 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 oC
  during operation

52

• 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

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MBE
53
54
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Growth process

• UHV (lt 10-8)
• Knudsen sources
• As flux, sticking coeff. lt 0.5
• growth JIII excess JV
• high As/Ga flux, low T - As stabilized
• low As/Gas flux, high T - Ga stabilized

Congruent sublimation Tcs (C)GaAs 650AlAs 850A
lP gt700GaP 670InP 363InAs 380 if T lt Tcs,
group V stable if T gt Tcs, group III stable
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Comparison of Epitaxial Methods
limit
features
time
Growth method
Limited substrate areas and poor control over the
growth of very thin layers
Growth form supersaturated solution onto
substrate
1963
LPE (Liquid phase epitaxy)
No Al contained compound, thick layer
Use metal halide as transport agents to grow
1958
VPE (Vapor phase epitaxy
Hard to grow materials with high vapor pressure
Deposit epilayer at ultrahigh vacuum
1958 1967
MBE (Molecular Beam Epitaxy)
Some of the sources like AsH3 are very toxic.
Use metallorganic compounds as the sources
1968
MOCVD (Metal-Organic Chemical Vapor Deposition)
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Atomic Layer Epitaxy (ALE)

• Atomic layer epitaxy (ALE), or Atomic Layer
  Deposition (ALD), is a specialized form of
  epitaxy that typically deposit alternating
  monolayers of two elements onto a substrate,
  making it ideal to generate nanostructures
• The crystal lattice structure achieved is thin,
  uniform, and aligned with the structure of the
  substrate
• The reactants are brought to the substrate as
  alternating pulses with "dead" times in between.
  ALE makes use of the fact that the incoming
  material is bound strongly until all sites
  available for chemisorption are occupied
• The dead times are used to flush the excess
  material

58

• Atomic layer epitaxy (ALE) or atomic layer
  deposition (ALD) is a technique mostly used in
  semiconductor fabrication to grow thin films of
  thickness of the atomic order
• The main approach used for this technique is the
  use of a self limiting chemical reaction to
  control in a very accurate way the thickness of
  the film deposited
• Compared to basic CVD for example, chemical
  reactants are pulsed alternatively in a reacting
  chamber and then chemisorb on to the surface of
  the substrate in order to form the monolayer
• The reaction is very easy to set up and doesnt
  require that many restrictions over the
  reactants, allowing the use of a wide range of
  materials

59

• ALE introduces two complementary precursors (e.g.
  Al(CH3)3 and H2O) alternatively into the reaction
  chamber.
• Typically, one of the precursors will adsorb onto
  the substrate surface, but cannot completely
  decompose without the second precursor.
• The precursor adsorbs until it saturates the
  surface and further growth cannot occur until the
  second precursor is introduced
• Thus the film thickness is controlled by the
  number of precursor cycles rather than the
  deposition time as is the case for conventional
  CVD processes
• In theory ALCVD allows for extremely precise
  control of film thickness and uniformity