Yu-Ming Chang ( ??? )

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Carrier and Phonon Dynamics in InN and its
Nanostructures
Yu-Ming Chang ( ??? ) Center for Condensed
Matter Sciences National Taiwan University April
12, 2007 Institute of Physics, NCTU
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Outline

• Motivation
• Time-resolved second-harmonic generation
  (TRSHG)
• What is coherent phonon spectroscopy
• Coherent phonon spectroscopy of InN and its
  nanostructures
• Identification of surface optical phonon
• Direct observation of LO phonon and plasmon
  coupling
• Determination of the InN effective mass along
  the c-axis
• Determination of the InN plasma relaxation time
• Coherent phonon spectroscopy of InN ultrathin
  films
• Conclusion

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Band gap engineering of III-nitride semiconductors
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Question What can we play in this ball game
? Our research strategy Try to explore the
transient carrier and phonon dynamics in InN and
its nanostructures !
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Time-resolved second-harmonic generation
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Femtosecond laser pump and probe technique
t 2Dl/c t
BS
To Sample
Probe
Pump
Femtosecond Laser pulse
Mirror
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Time-resolved second-harmonic generation
Probed SHG Signal
AC
AC
Pump
Probe
Sample

• Femtosecond temporal resolution
• No-contact, no-damage, remote, and all optical
  configuration
• Better surface / interface sensitivity than
  other optical techniques

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TRSHG can probe carrier and phonon dynamics in
semiconductors
Second Harmonic Generation
Modulation of ceff(2) due to the pump pulse
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Coherent phonon spectroscopy GaAs as example
Fourier Power Spectrum
Time-Resolved Second-Harmonic Generation (TRSHG)
measurement
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What is coherent phonon spectroscopy ?
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Coherent phonon spectroscopy
Coherent lattice oscillation
where A, T, f, and ? are the oscillation
amplitude, dephasing time, frequency, and initial
phase respectively.
Impulsively driving force can be ..

• Raman scattering / electronic transition process
• Transient depletion / piezoelectric field
  screening process
• (c) Transient local strain induced by thermal
  absorption

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Driving force for launching coherent phonon
Driving Mechanisms

• Impulsive stimulated Raman scattering
• Transient electric field screening
• Displasive excitation due to electronic
  transition
• Impulsive thermal excitation

Some Criterions

• Laser pulse width lt Phonon oscillation period
• Raman / IR active phonon mode
• Sample with built-in electric / piezoelectric
  field

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Femtosecond laser photoexcited carrier dynamics
in the depletion region of GaAs
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Coherent phonon generation in the depletion
region of GaAs(100)
E field
100
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Coherent phonon spectroscopy GaAs as example
Fourier Power Spectrum
Time-Resolved Second-Harmonic Generation (TRSHG)
measurement
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Semiconductor nanostructures
Single Quantum Well
Schottky Interface
Quasi-2DEG
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Coherent phonon spectroscopy of InN
- Coherent LO phonon and plasmon coupling in the
near surface region of InN
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InN sample structure and its physical properties

• This InN sample is n-type and its bulk carrier
  concentration is nd3.7x1018 cm-3 determined by
  Hall measurement.
• The electron mobility is measured as me1150
  cm2/Vsec at room temperature.
• The X-ray diffraction study shows that this
  sample is a high-quality wurtzite structured InN
  epitaxial layer formed with its c axis
  perpendicular to the substrate surface.
• The photoluminescence spectrum indicates the
  band gap Eg 0.7 eV.
• The absorption length at l800 nm is 150 nm.

InN
AlN
Si3N4
Si (111) substrate
Sample Structure
Provided by Prof. S. Gwo, NTHU
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Coherent Phonon Generation in InN
Driving Mechanisms

• Impulsive stimulated Raman scattering
• Transient electric field screening
• Displasive excitation due to electronic
  transition
• Impulsive thermal excitation

Some Criterions

• Laser pulse width lt Phonon oscillation period
• Raman / IR active phonon mode
• Sample with built-in electric / piezoelectric
  field

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Electron accumulation in the near surface region
of InN
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Coherent Phonon Spectroscopy of InN Sample
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Spontaneous Raman spectroscopy of InN
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Coherent phonon spectrum vs. CW Raman spectrum
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Identification of surface optical phonon
Y.M. Chang and et. al., APL v90, 072110 (2007)
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The phonon peak at 16.2 THz a surface optical
phonon ?!
InN
Sample A
2 nm LT-GaN
InN
Sample B
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Sample dependence
SHG Polarization dependence
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The vibration mode of InN surface optical phonon
A1(LO)-like (bulk-terminated) surface phonon mode
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Direct observation of coherent A1(LO)
phonon-plasmon coupling modes
Y.M. Chang and et. al., APL v85, 5224 (2004)
Y.M. Chang and et. al., APL v90, 072111 (2007)
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Coherent phonon spectroscopy pump power
dependence
Photo-injected carrier density
nex2x1018 /cm3
nex1x1018 /cm3
nex6x1017 /cm3
nex2x1017 /cm3
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LO phonon-plasmon coupling in polar semiconductors
Dielectric Function
LO-plasmon coupling modes
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Dielectric function determine the LOPC
frequencies
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Coherent phonon spectroscopy pump power
dependence
Photo-injected carrier density
nex2x1018 /cm3
nex1x1018 /cm3
nex6x1017 /cm3
nex2x1017 /cm3
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Coherent LO phonon-plasmon coupling modes
Plasmon
A1(LO)
A1(TO)
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Large electron concentration in the surface region
Large electron concentration in the near surface
region
InN
Y.M. Chang and et. al., APL v85, 5224 (2004)
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Determination of the effective mass of electron
along the c-axis of wurtzite InN
Y.M. Chang and et. al., APL v90, 072111 (2007)
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Coherent LO phonon-plasmon coupling modes
Plasmon
A1(LO)
A1(TO)
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Determination of InN effective mass (m//) along
the c-axis
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Determination of the plasma relaxation time
(the following slides are deleted for
confidential reason)
Y.M. Chang and et. al., in preparation (2007)
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Coherent phonon spectroscopy of InN ultrathin
films(the following slides are deleted for
confidential reason)
Y.M. Chang and et. al., in preparation (2007)
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Summary
Coherent A1(LO) phonon-plasmon coupling modes of
InN are observed for the first time. We obtain
the following important physical properties
(1) A1(LO) phonon dephasing time 200700 fsec
? involving phonon-phonon, phonon-carrier, and
phonon-defect scatterings (2) Plasma damping
time constant 50150 fsec ? involving
carrier-carrier and carrier-defect scatterings
(3) Surface electron accumulation gt 1020 /cm3
(4) Bulk carrier concentration is overestimated
by Hall measurement ? inhomogenous spatial
distribution of carrier concentration (5) InN
effective mass (along c axis) 0.033 me ?
nonparabolic G conduction band
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Conclusion

• Time-resolved second-harmonic generation
  (TRSHG) is capable of probing the carrier and
  phonon dynamics in InN and its heterostructures
• Surface optical phonon at 16.2 THz is observed
  and characterized for the first time.
• We directly observe the coherent A1(LO) phonon
  and plasmon coupling in the near surface region
  of InN.
• The effective mass (m//) of InN electron is
  determined to be 0.033 me by fitting the
  upper-branch of bulk A1(LO) phonon-plasmon
  coupling mode.
• Coherent phonon spectroscopy of InN ultrathin
  film are carried out for comparison. The
  carrier and phonon dynamics are very different
  from those of the InN thick films. The analysis
  is in progress now.