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

2. 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

3. Band gap engineering of III-nitride semiconductors

4. 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 !

5. Time-resolved second-harmonic generation

6. Femtosecond laser pump and probe technique
t = 2Dl/c t BS To
Sample
Probe
Pump
Femtosecond
Laser pulse
Mirror

7. Time-resolved second-harmonic generation
Probed
SHG
Signal AC
Delay Time (t)
Probed SHG AC
Pump t
Probe
Sample
Femtosecond temporal resolution
No-contact, no-damage, remote, and all optical configuration
Better surface / interface sensitivity than other optical techniques

8. TRSHG can probe carrier and phonon dynamics in semiconductors
Second Harmonic Generation:
Modulation of ceff(2) due to the pump pulse:
Carrier Dynamics
Phonon Dynamics

9. Coherent phonon spectroscopy: GaAs as example
Bulk LO phonon
8.8 THz
Fourier Power Spectrum
Time-Resolved Second-Harmonic Generation (TRSHG) measurement

10. What is coherent phonon spectroscopy ?

11. 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

12. 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 < Phonon oscillation period
Raman / IR active phonon mode
Sample with built-in electric / piezoelectric field

13. t = 0 t > 0 t < 0 EF CB VB Edc>0 E=hv Depletion Region Edc~ 0
Femtosecond laser photoexcited carrier dynamics in the depletion region of GaAs EF CB VB
Edc>0
E=hv
Depletion Region
t = 0
Edc~ 0 EF CB VB
Depletion Region
t > 0
Edc>0 EF CB VB
Depletion Region
t < 0

14. Coherent phonon generation in the depletion region of GaAs(100)
Field
screening
by free
carrier
injection
E~0
E field
Edc EF CB VB
Depletion
Region
TIME
[100]

15. Coherent phonon spectroscopy: GaAs as example
Fourier Power Spectrum
Time-Resolved Second-Harmonic Generation (TRSHG) measurement

16. Semiconductor nanostructures
Quasi-2DEG
Schottky Interface
Single Quantum Well
GaAs 9 nm
AlGaAs nm
Si d-doping layers
AlGaAs 22 nm
GaAs 1.5 mm
Au 10 nm
GaP n-type 30 mm
GaP substrate
Metal electrode
InGaP 200 nm
GaAs nm
InGaP 500 nm
GaAs buffer layer

17. Coherent phonon spectroscopy of InN
- Coherent LO phonon and plasmon coupling in the near surface region of InN

18. InN sample structure and its physical properties
This InN sample is n-type and its bulk carrier concentration is nd=3.7x1018 cm-3 determined by Hall measurement.
The electron mobility is measured as me=1150 cm2/V·sec 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 l=800 nm is ~ 150 nm.
InN
AlN
Si3N4
Si (111) substrate
Sample Structure
Provided by
Prof. S. Gwo, NTHU

19. 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 < Phonon oscillation period
Raman / IR active phonon mode
Sample with built-in electric / piezoelectric field

20. Electron accumulation in the near surface region of InN

22. EF

23. Huge electric field
E~4.7x106 V/cm

24. Electron accumulation in the near surface region of InN

25. Coherent Phonon Spectroscopy of InN Sample

26. Spontaneous Raman spectroscopy of InN

27. Coherent phonon spectrum vs. CW Raman spectrum

28. Identification of surface optical phonon
Y.M. Chang and et. al., APL v90, (2007)

29. The phonon peak at 16.2 THz : a surface optical phonon ?!
InN
Sample A
2 nm LT-GaN
InN
Sample B

30. Sample dependence
SHG Polarization dependence

31. The vibration mode of InN surface optical phonon
A1(LO)-like (bulk-terminated) surface phonon mode

32. 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, (2007)

33. Coherent phonon spectroscopy : pump power dependence
Photo-injected carrier density
nex~2x1018 /cm3
nex~1x1018 /cm3
nex~6x1017 /cm3
nex~2x1017 /cm3

34. LO phonon-plasmon coupling in polar semiconductors
Dielectric Function
LO-plasmon
coupling modes

35. Dielectric function: determine the LOPC frequencies

36. Coherent phonon spectroscopy : pump power dependence
Photo-injected carrier density
nex~2x1018 /cm3
nex~1x1018 /cm3
nex~6x1017 /cm3
nex~2x1017 /cm3

37. Coherent LO phonon-plasmon coupling modes
A1(LO)
A1(TO)

38. 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)

39. Determination of the effective mass of electron along the c-axis of wurtzite InN
Y.M. Chang and et. al., APL v90, (2007)

40. Coherent LO phonon-plasmon coupling modes
A1(LO)
A1(TO)

41. Determination of InN effective mass (m*//) along the c-axis

42. Determination of the plasma relaxation time (the following slides are deleted for confidential reason)
Y.M. Chang and et. al., in preparation (2007)

43. Coherent phonon spectroscopy of InN ultrathin films (the following slides are deleted for confidential reason)
Y.M. Chang and et. al., in preparation (2007)

44. 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 : 200~700 fsec
 involving phonon-phonon, phonon-carrier, and phonon-defect scatterings
(2) Plasma damping time constant : 50~150 fsec
 involving carrier-carrier and carrier-defect scatterings
(3) Surface electron accumulation : > 1020 /cm3
(4) Bulk carrier concentration is overestimated by Hall measurement
 inhomogenous spatial distribution of carrier concentration
(5) InN effective mass (along c axis): ~ me
 nonparabolic G conduction band

45. 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 ~ 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.