MOCVD  &  MBE  CEE135  

Wen0ng  Hou  2013-­‐12-­‐03  

compound  semiconductor  growth  techniques  

The three basic and most commonly used compound semiconductor growth techniques: •  Liquid Phase Epitaxy (LPE)

•  Metal Organic Chemical Vapour Deposition (MOCVD)

•  Molecular Beam Epitaxy (MBE)

MOCVD  (Metal  Organic    Chemical  Vapor  Deposi0on)  

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This involves the forced convection of the metal organic vapour species over a heated substrate. Those molecules striking the heated crystal release the desired species, resulting in crystal growth. The chemical process involved is quite simple in that an alkyl compound for the group III element and a hydride for group V element decompose in the 500 °C to 800 °C temperature range to form the III-V compound semiconductor.

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Metal Organic Vapor Phase Epitaxy （MOVPE）

Common sources for group V elements are AsH3 or PH3 while for group III these are trymethyl gallium (TMGa / Ga(CH3)3), trimethyl aluminium (TMAl / Al(CH3)3) and trimethyl indium (TMIn / In(CH3)3) or triethyl gallium (TEGa / Ga(C2H5)3), triethyl aluminium (TEAl / Al(C2H5)3), and triethyl indium (TEIn / In(C2H5)3) respectively.

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MOCVD mechanism This involves the forced convection of the metal organic vapour species over a heated substrate. Those molecules striking the heated crystal release the desired species, resulting in crystal growth. The alkyl compound for the group III element and a hydride for group V element decompose in the 500 °C to 800 °C temperature range to form the III-V compound semiconductor.

GaN could be grown in a reactor on a substrate by introducing Trimethygallium ((CH3)3Ga) and ammounium (NH3). Formation of the epitaxial layer occurs by final pyrolysis of the constituent chemicals at the substrate surface.

http://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy

MOCVD Growth of GaN

TMGa  molecules  deposit  and  react  on  surface  

NH3  molecules  react  on  surface,  leaving  N  to  react  and  form  GaN  and  CH4  

MOCVD:  Working  condi0ons  

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This takes place not in a vacuum, but from the gas phase at moderate pressures (10 to 760 Torr). As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, and it has become a major process in the manufacture of optoelectronics.

T too high: desorption, nitrogen vacancies. T too low: impurities incorporation, low surface mobility, structure defects; Usually temperature ~1050 °C

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•  Advantages – Faster growth than MBE, can be a few microns per hour; multi-wafer capability easily achievable – Higher temperature growth; growth process is thermodynamically favorable •  Disadvantages – Difficult to monitor growth rate exactly (no Rheed possible due to higher pressure) – Not as abrupt a process as MBE due to gas flow issues and memory effects – Toxic gases are to be handled

Benefits  and  Drawbacks  of  MOCVD  

Molecular  Beam  Epitaxy  (MBE)  

What  is  Epitaxy?  •  Epitaxy:  Deposi0on  and  growth  of  

monocrystalline  structures/layers.    •  Greek  root:  epi  means  “above”  and  taxis  

means  “ordered”.  

•  Grown  from:  gaseous  or  liquid  precursors.  

•  Substrate  acts  as  a  seed  crystal:  film  follows  that    

 •  Epitaxial  growth  results  in  monocrystalline  

layers  differing  from  deposi0on  which  gives  rise  to  polycrystalline  and  bulk  structures.  

•  Epitaxy  types:  –  Homoepitaxy:  Substrate  &  material  are  of  same  

kind.      (Si-­‐Si)  

–  Heteroepitaxy:  Substrate  &  material  are  of  different  kinds.  (Ga-­‐As)  

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* Veeco Instruments ©2008, http://www.veeco.com

** Dan Connelly, ©2007, http://oz.irtc.org

MBE growth mechanism *

MBE growth mechanism **

MBE:  Working  Principle  

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A typical MBE system*

* “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi

Ø  Epitaxial growth: Due to the interaction of molecular or atomic beams on a surface of a heated crystalline substrate. Ø  The solid source materials sublimate Ø  They provide an angular distribution of

atoms or molecules in a beam. Ø  The substrate is heated to the necessary

temperature. Ø  The gaseous elements then condense on

the wafer where they may react with each other.

Molecular Beam Epitaxy**

** http://iramis.cea.fr/en/Phocea/Vie_des_labos/Ast/ast_sstechnique.php?id_ast=494

Ø  Atoms on a clean surface are free to move until finding correct position in the crystal lattice to bond.

Ø  Growth occurs at the step edges formed: More binding forces at an edge.

Ø  The term “beam” means the evaporated atoms do not interact with each other or with other vacuum chamber gases until they reach the wafer.

MBE:  Working  Condi0ons  

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§  The mean free path (λ) of the particles > geometrical size of the chamber (10-5 Torr is sufficient)

§ Ultra-high vacuum (UHV= 10-11Torr) to obtain sufficiently clear epilayer.

§ Gas evolution from materials has to be as low as possible. Pyrolytic boron nitride (PBN) is chosen for crucibles (Chemically stable up to 1400°C)

§ Molybdenum and tantalum are widely used for shutters.

§ Ultrapure materials are used as source.

Mean free path for Nitrogen molecules at 300 K *

* “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi

MBE:  Results  and  Control  Mechanisms  

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RHEED oscillations *

* “Basics of molecular beam epitaxy (MBE)” by Fernando Rinaldi

Ø  Control of composition and doping of the growing structure at monolayer via computer controlled shutters Ø  Growth rates are typically on the order of a few A°/s and the beams can be

shuttered in a fraction of a second allowing nearly atomically abrupt transition from one material to another.)

Ø  Independent heating of material sources Ø  RHEED (Reflection High Energy Electron Diffraction) for monitoring the

growth of the crystal layers.

Ø  Mass spectrometer for monitoring the residual gases and checking source beams for leaking

Ø  A cryogenic screening around the substrate as a pump for residual gases.

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MBE Growth of GaN

GaN molecular beam epitaxy (MBE) growth is a non-equilibrium process where a Ga vapor beam from an effusion cell and an activated nitrogen beam from a plasma source are directed toward a heated substrate. Under suitable conditions, layer-by-layer deposition of Ga and N atomic planes is possible.

Benefits  and  Drawbacks  of  MBE  Advantages Disadvantages n  Clean surfaces, free of an oxide layer n  Expensive (106 $ per MBE chamber)

n  In-situ deposition of metal seeds, semiconductor materials, and dopants

n  ATG instability

n  Low growth rate (1µm/h) n  Very complicated system n  Precisely controllable thermal evaporation n  Epitaxial growth under ultra-high vacuum

conditions

n  Seperate evaporation of each component n  Substrate temperature is not high n  Ultrasharp profiles

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Applica0ons  •  Novel  structures  as  quantum  devices  •  Silicon/Insulator/Metal  Sandwiches  •  Superla]ces  •  Microelectronic  Devices    

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TEM image of MBE Growth of Ultra-Thin InGaAs/AlAsSb Quantum Wells*

* http://www.photonics.ethz.ch/research/core_competences/technology/epitaxial_growth/mbe

Conclusions  of  MBE  •  Typically  in  ultra-­‐high  vacuum  •  Deposi0on  rates  are  very  low  (1monolayer/second)  •  Very  well  controlled  (Shu^ering:  0.1s)  •  Grow  films  with  good  crystal  structure  •  O_en  use  mul0ple  sources  to  grow  alloy  films  •  Deposi0on  rate  is  so  low  that  substrate  temperature  

doesn’t  need  to  be  as  high  •  Expensive  •  Sophis0cated  system  

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MBE  vs.  MOCVD  

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