Epitaxial Deposition

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					Epitaxial Deposition
               Daniel Lentz

                             EE 518
            Penn State University
                   March 29, 2007
          Instructor: Dr. J. Ruzyllo
 Introduction
 Mechanism of epitaxial growth

 Methods of epitaxial deposition

 Properties of epitaxial layers

 Applications of epitaxial layers
Epitaxial Growth
   Deposition of a layer on
    a substrate which              Ordered,
    matches the crystalline        growth;
    order of the substrate         NOT
   Homoepitaxy
       Growth of a layer of the
        same material as the
       Si on Si                   growth:
   Heteroepitaxy
       Growth of a layer of a
        different material than
        the substrate
       GaAs on Si
   Epitaxial growth is useful for applications that place
    stringent demands on a deposited layer:
       High purity
       Low defect density
       Abrupt interfaces
       Controlled doping profiles
       High repeatability and uniformity
       Safe, efficient operation
   Can create clean, fresh surface for device
General Epitaxial Deposition
   Surface preparation
       Clean surface needed
       Defects of surface duplicated in epitaxial layer
       Hydrogen passivation of surface with water/HF
   Surface mobility
       High temperature required heated substrate
       Epitaxial temperature exists, above which deposition is
       Species need to be able to move into correct
        crystallographic location
       Relatively slow growth rates result
           Ex. ~0.4 to 4 nm/min., SiGe on Si
General Scheme

         Modified from http://www.acsu.buffalo.edu/~tjm/MOVPE-GaN-schematic.jpg
   Specific thermodynamics varies by process
       Chemical potentials
       Driving force
   High temperature process is mass transport controlled, not very
    sensitive to temperature changes
   Steady state
   Close enough to equilibrium that chemical forces that drive growth
    are minimized to avoid creation of defects and allow for correct
   Sufficient energy and time for adsorbed species to reach their lowest
    energy state, duplicating the crystal lattice structure
   Thermodynamic calculations allow the determination of solid
    composition based on growth temperature and source composition
   Growth rate controlled by kinetic
       Mass transport of reactants to surface
       Reactions in liquid or gas
       Reactions at surface
       Physical processes on surface
           Nature and motion of step growth
           Controlling factor in ordering
   Specific reactions depend greatly on method
Kinetics Example
                                                        Atoms can bond to flat surface,
                                                         steps, or kinks.
                                                            On surface requires some critical
                                                            Easier at steps
                                                            Easiest at kinks
                                                        As-rich GaAs surface
                                                            As only forms two bonds to
                                                             underlying Ga
                                                            Very high energy
                                                        Reconstructs by forming As dimers
                                                            Lowers energy
                                                            Causes kinks and steps on surface
                                                        Results in motion of steps on
                                                            If start with flat surface, create step
                                                             once first group has bonded
                                                            Growth continues in same way

Vapor Phase Epitaxy
   Specific form of chemical vapor deposition (CVD)
   Reactants introduced as gases
   Material to be deposited bound to ligands
   Ligands dissociate, allowing desired chemistry to
    reach surface
   Some desorption, but most adsorbed atoms find
    proper crystallographic position
   Example: Deposition of silicon
       SiCl4 introduced with hydrogen
       Forms silicon and HCl gas
       Alternatively, SiHCl3, SiH2Cl2
       SiH4 breaks via thermal decomposition
Precursors for VPE
   Must be sufficiently volatile to allow
    acceptable growth rates
   Heating to desired T must result in pyrolysis
   Less hazardous chemicals preferable
       Arsine highly toxic; use t-butyl arsine instead
   VPE techniques distinguished by precursors
Varieties of VPE
   Chloride VPE
       Chlorides of group III and V elements
   Hydride VPE
       Chlorides of group III element
           Group III hydrides desirable, but too unstable
       Hydrides of group V element
   Organometallic VPE
       Organometallic group III compound
       Hydride or organometallic of group V element
   Not quite that simple
       Combinations of ligands in order to optimize
        deposition or improve compound stability
       Ex. trimethylaminealane gives less carbon
        contamination than trimethylalluminum

Other Methods
   Liquid Phase Epitaxy                     Fast, inexpensive
       Reactants are dissolved in           Not ideal for large area
        a molten solvent at high              layers or abrupt interfaces
        temperature                          Thermodynamic driving
       Substrate dipped into                 force relatively very low
        solution while the               Molecular Beam Epitaxy
        temperature is held
                                             Very promising technique
                                             Elemental vapor phase
       Example: SiGe on Si
           Bismuth used as solvent
                                             Beams created by
           Temperature held at
                                              evaporating solid source in
       High quality layer
Doping of Epitaxial Layers
   Incorporate dopants during deposition
       Theoretically abrupt dopant distribution
       Add impurities to gas during deposition
       Arsine, phosphine, and diborane common
   Low thermal budget results
       High T treatment results in diffusion of dopant into
       Reason abrupt distribution not perfect
Properties of Epitaxial Layer
   Crystallographic structure of film reproduces that of
   Substrate defects reproduced in epi layer
   Electrical parameters of epi layer independent of
       Dopant concentration of substrate cannot be reduced
       Epitaxial layer with less dopant can be deposited
   Epitaxial layer can be chemically purer than
   Abrupt interfaces with appropriate methods
   Engineered wafers
       Clean, flat layer on top of
        less ideal Si substrate
       On top of SOI structures
       Ex.: Silicon on sapphire
       Higher purity layer on lower
        quality substrate (SiC)
   In CMOS structures
       Layers of different doping
       Ex. p- layer on top of p+
        substrate to avoid latch-up
More applications
                                                                 Bipolar Transistor
                                                                     Needed to produce
                                                                      buried layer
                                                                 III-V Devices
                                                                     Interface quality key
                                                                     Heterojunction Bipolar
                                                                     LED
                                                                     Laser
   Deposition continues crystal structure
   Creates clean, abrupt interfaces and high
    quality surfaces
   High temperature, clean surface required
   Vapor phase epitaxy a major method of
   Epitaxial layers used in highest quality wafers
   Very important in III-V semiconductor
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