Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Yu-Ming Chang ( 張玉明 ) Center for Condensed Matter Sciences National Taiwan University April 12, 2007 Institute of Physics, NCTU Carrier and Phonon Dynamics."— Presentation transcript:

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

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  l  c   l Femtosecond Laser pulse Probe Pump BS Mirror To Sample

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

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

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

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

12 Driving force for launching coherent phonon  Impulsive stimulated Raman scattering Transient electric field screening  Transient electric field screening  Displasive excitation due to electronic transition  Impulsive thermal excitation  Laser pulse width < Phonon oscillation period  Raman / IR active phonon mode Sample with built-in electric / piezoelectric field  Sample with built-in electric / piezoelectric field Some Criterions : Driving Mechanisms :

13 Femtosecond laser photoexcited carrier dynamics in the depletion region of GaAs E dc >0 EFEF CB VB Depletion Region  < 0 EFEF CB VB E dc >0 E=hv Depletion Region  = 0 E dc ~ 0 EFEF CB VB Depletion Region  > 0

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

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

16 Semiconductor nanostructures InGaP 200 nm GaAs 7 nm InGaP 500 nm GaAs buffer layer Single Quantum Well Schottky Interface Quasi-2DEG GaAs 9 nm AlGaAs 10 nm Si  -doping layers AlGaAs22 nm GaAs 1.5  m Au 10 nm GaP n-type 30  m GaP substrate Metal electrode

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 n d =3.7x10 18 cm -3 determined by Hall measurement. The electron mobility is measured as  e =1150 cm 2 /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 E g ~ 0.7 eV. The absorption length at =800 nm is ~ 150 nm. InN AlN Si 3 N 4 Si (111) substrate Sample Structure Provided by Prof. S. Gwo, NTHU

19 Coherent Phonon Generation in InN  Impulsive stimulated Raman scattering Transient electric field screening  Transient electric field screening  Displasive excitation due to electronic transition  Impulsive thermal excitation  Laser pulse width < Phonon oscillation period  Raman / IR active phonon mode Sample with built-in electric / piezoelectric field  Sample with built-in electric / piezoelectric field Some Criterions : Driving Mechanisms :

20 Electron accumulation in the near surface region of InN

21

22 EFEF

23 Huge electric field E~4.7x10 6 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 Sample B 2 nm LT-GaN

30 Sample dependence SHG Polarization dependence

31 The vibration mode of InN surface optical phonon A 1 (LO)-like (bulk-terminated) surface phonon mode

32 Direct observation of coherent A 1 (LO) phonon-plasmon coupling modes Y.M. Chang and et. al., APL v90, (2007) Y.M. Chang and et. al., APL v85, 5224 (2004)

33 Coherent phonon spectroscopy : pump power dependence n ex ~2x10 18 /cm 3 n ex ~1x10 18 /cm 3 n ex ~6x10 17 /cm 3 n ex ~2x10 17 /cm 3 Photo-injected carrier density

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

35 Dielectric function: determine the LOPC frequencies

36 Coherent phonon spectroscopy : pump power dependence n ex ~2x10 18 /cm 3 n ex ~1x10 18 /cm 3 n ex ~6x10 17 /cm 3 n ex ~2x10 17 /cm 3 Photo-injected carrier density

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

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) Plasmon

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 Coherent A 1 (LO) phonon-plasmon coupling modes of InN are observed for the first time. We obtain the following important physical properties : (1) A 1 (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 : > /cm 3 (4) Bulk carrier concentration is overestimated by Hall measurement  inhomogenous spatial distribution of carrier concentration (5) InN effective mass (along c axis): ~ m e  nonparabolic  conduction band Summary

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 A 1 (LO) phonon and plasmon coupling in the near surface region of InN.  The effective mass (m * // ) of InN electron is determined to be ~ m e by fitting the upper-branch of bulk A 1 (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.


Download ppt "Yu-Ming Chang ( 張玉明 ) Center for Condensed Matter Sciences National Taiwan University April 12, 2007 Institute of Physics, NCTU Carrier and Phonon Dynamics."

Similar presentations


Ads by Google