Solid State Solutions_ Phase Diagrams_ and Phase Transitions .ppt

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					   Solid State Solutions, Phase Diagrams, and
   Phase Transitions
    Matt Highland

   ‣ Synthesis away from
   ‣ Metastable Materials
   ‣ Reactive Synthesis

Second Workshop on Photocathodes: 300nm-500nm
June 29-30 at the University of Chicago
Synthesis away from Equilibrium
‣ Typical thermodynamics gives us guide posts on synthesis near equilibrium
‣ Engineering materials with specific properties often requires synthesis
  away from equilibrium

‣ “Metastable” materials that demand non-equilibrium and kinetically
  controlled synthesis path ways
‣ Metastable synthesis requires additional stabilization during growth:

   StrainèEpitaxy      Energetic ionsèSputtering   Chemical ActivityèReactive Synthesis

    SrRuO3 and Co3O4

 Reactive Synthesis of Metastable Materials
‣ Reactive synthesis utilizes activity of chemical precursors to
  stabilize desired phases
‣ Practical example: (In,Ga)N solid solutions
‣ Band-gap tunable across solar spectrum
  by varying solid solution content

                                                LEDs for solid state lighting

InGaN: The promise and truth

‣ The promise                                                                           High-power (>1 Watt input) visible-spectrum
  • Solid-state lighting has the                                                                           LEDs

                                                    External quantum efficiency, hext
      potential reduce U.S. energy
      consumption from 3.1 to 2.1                                                                                (AlxGa1-x)0.52In0.48P
  •   Roughly the output of 250 coal                                                                            V(l)
      fired power plants

‣ The truth
  • External quantum efficiency
      drops as InN content increases                                                       Tj =
  •   Driven by problems with crystal                                                      25°C
      quality and the metastable
      nature of InN                                                                               Peak wavelength, lp (nm)

  *“Energy Savings Potential of Solid State
  Lighting in General Illumination Applications”,
The Fundamental Problem

‣ At desirable growth temperatures
  required nitrogen activity is
  equivalent to kilobars (~104 psi) of

‣ During MOCVD growth nitrogen
  activity provided by cracking

‣ Reaction we want to avoid:
                                         Ambacher et al., JVST B 14, 3532

Intermediate Chemical Species

‣ We know the overall reaction desired
  for growth

‣ However what are the intermediate
  chemical species that drive this growth
  • All we know is the precursors crack
     somehow interact

Attacking a Problem on Multiply Fronts
‣ We’re employing multiply in-situ probes and computational techniques to
  understand the details of reactive synthesis

                                                      In-situ IR spectroscopy

     In-situ X-ray Analysis

                                  Theory & modeling

Probing the Growth Environment
‣ Synchrotron x-rays are capable                           Fluorescence
  to penetrating the MOCVD                                   Detector
  environment and yield
  structural and elemental details
  in real time
  • In-situ MOCVD reactor at
      sector 12ID-D of the                        camera
                                                                Visible illumination
      Advanced Photon Source
‣ Diffraction from GaN surfaces      Scattering
                                                                              Synchrotron x-rays

  and InN crystals                    Detector

‣ X-ray Fluorescence from
  deposited Indium

‣ Measurements reveal a very
  complex growth behavior


In-InN Phase Boundaries
‣ By monitoring InN and In liquid formation we can map out an indium
  condensation phase diagram

‣ Upon increasing TMI flow
  • At higher temp, elemental In                                      In liquid
     liquid condenses                                                 droplets
   • At lower T, relaxed InN solid
     particles grow
                                           Bare GaN

                                           pNH3= 27 Torr
                                                                        InN crystals

                                            F. Jiang, et al. PRL 101, 086102 (2008)
Oscillatory Growth and Decomposition

‣ Near phase boundaries system
  can spontaneously oscillate
  • Inter-conversion between InN and
     liquid In
‣ AFM of quenched samples shows
  microstructure of distinct surface

                                       Epitaxial InN islands    Elemental In droplets

                                         F. Jiang, et al. PRL 101, 086102 (2008)
Chemical Wave Patterns

 ‣ Spatial variation between
   InN and In can be resolved
    • Dark regions: InN
    • White regions: In liquid

 ‣ Waves of InN or In liquid
    • Sweep across the sample
    • Form concentric rings
    • Spiral patterns

                                 F. Jiang, et al. PRL 101, 086102 (2008)

What Drives the Oscillatory Behavior ?

 ‣ The key to this complex growth behavior is local nitrogen activity

 ‣ NH3 impinges on the hot sample surface, cracks and forms some highly
   active chemical species (NHx)
 ‣ These active species either interact with In and form InN or react to
   eventually form N2 and leave the surface.
 ‣ The efficiency with which NH3 is cracked and the residence time of the
   intermediate species determines which material grows

Oscillatory Growth Mechanism

NH3 cracks on the GaN      Critical amount of
                             liquid In metal         Liquid In metal
   of InN surface and                             evaporates to expose
                            condenses which
        forms the                                 GaN surface and InN
                         accelerates conversion
 intermediate species     of NH3to N2 and InN      growth starts again
that allow InN to grow    starts to decompose
Intermediate Chemical Species

 ‣ The local intermediate chemical species dictate growth behavior
 ‣ Different surfaces catalytically crack NH3 differently and possibly change
    residence time of intermediate species

 ‣ If we can understand which intermediate species enable InN growth, then
    we can better stabilize and encourage its formation

 ‣ What are the intermediate
    nitrogen species?
    • First principle calculations
    • Additional in-situ probes

 First-principle Calculations
‣ We can calculate the lowest energy
  configurations of NH3, NH2, NH, N, and H on        (2x2) surface unit cell
  a GaN and InN surface                              - 4 H3 “hollow” sites
‣ We can then create a phase diagram                 - 4 T1 “on top” Ga sites
  predicting the equilibrium coverage species        - 4 T4 “on top” N sites
  for given conditions                               - 12 br “bridge” site
   • “We” = Peter Zapol, Weronika Walkosz, and Xin

 Predicted Phase Diagram
                               Predicted structures on GaN surface

‣ Lowest energy surface
 species differ depending
 temperature and nitrogen

‣ One of these configuration
 maybe be what enable InN

‣ Can we find these phases
 experimentally ?

                                    N-rich                      Ga-rich

                               W. Walkosz, et al. PRB 85, 033308 (2012)
Surface and Crystal Truncation Rods
‣ An abrupt crystalline surface in real-
  space creates an extended rod of
  scattering in reciprocal space
‣ Scattering that occurs along this Crystal
  Truncation Rod (CTR) away from the
  Bragg peaks is very sensitive to surface
                                              (10L) CTR

                                                          (00L) CTR

 Predicted CTRs
                                                      Predicted structures on GaN surface

‣ First Principle can
  be used to predict
  CTRs for each

‣ Can we see these
  changes with in-situ
  x-rays ?

               W. Walkosz, et al. PRB 85, 033308 (2012)    N-rich                Ga-rich
Experimentally measured CTRs
‣ With different amounts of NH3, N2, and H2 in the sample environment we
  see large changes at anti-Bragg conditions
‣ Modeling shows that CTR changes are consistent with a number of
  predicted surface structures
   • Uniqueness problem: Modeling generates a number of structures that fit equally well.
          Surface studies of GaN at 450°C as a function of chemical environment

                    20L Rod

In-situ Surface Chemistry
‣ How can we get information about the intermediate chemical species on
   the surface ?
    • X-rays are great at looking at the In phases (the heavy stuff), but how about highly
      reactive surface species (the light stuff)?

‣ Photons of a different length: in-situ Reflection-Absorption IR Spectroscopy
    • Can distinguish between NH3, NH2, and NH
    • Can penetrate MOCVD environment

RAIRS: Challenges to Overcome

‣ Heater is IR Source
   • Solution: Bandpass filtering
       to mask black body radiation

‣ Surface vs. Gas species
   • Solution: Polarize emitted
   • Gas species are isotropic
   • Surface species show
     polarization dependence
                                      01L rod of ZrN

‣ Metallic Surface:
   •   Solution: ZrN
   •   10% lattice mismatch to InN
   •   0.6% lattice mismatch to GaN
   •   Stable in MOCVD

Future Plans
‣ By combining Reflection-Absorption IR Spectroscopy with grazing incidence surface x
   -ray scattering we correlate InN structure, surface chemical species, and theoretical
   surface structure predictions we will understand what are the intermediate chemical
   process the allow InN to form and grow

                                                             In-situ IR spectroscopy

      In-situ X-ray Analysis

                                     Theory & modeling

‣ We hope to use this knowledge to design new synthesis pathways and improve the
   quality of InN and InGaN alloys

‣ Synthesis of Metastable Materials requires we exploit kinetically limited and
    non-equilibrium pathways.
‣   We’ve shown that the synthesis of InN with highly reactive chemical species is a
    complex interplay of surface chemistry and structure
‣   Through a fundamental understanding of these metastable path ways we may
    be able to push the boundaries of the materials we synthesis and properties we
    can engineer
Edith Perret, Materials Science Division, Argonne National Laboratory
Weronika Walkosz, Chemical Sciences and Engineering Division, Argonne National Laboratory
Xin Tan, Chemical Sciences and Engineering Division, Argonne National Laboratory
Kedar Manandhar , Department of Physics, University of Illinois at Chicago
Paul Fuoss, Materials Science Division, Argonne National Laboratory
Carol Thompson, Department of Physics, Northern Illinois University
Peter Zapol, Chemical Sciences and Engineering Division, Argonne National Laboratory
Stephen Streiffer, Physical Sciences & Engineering, Argonne National Laboratory
Mike Trenary, Department of Physics, University of Illinois at Chicago
Brian Stephenson, Advanced Photon Source, Argonne National Laboratory

Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences, under contract No. DE-AC02-06CH11357


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