
Carrier mobility is one of the most important
properties in semiconductor devices, such as high electron mobility
transistor (HEMT) and fieldeffect transistor (FET), both of which are
capable for realizing terahertz (10^{12} Hz) ultrahighspeed
operation in telecommunication and optical memory. The carrier mobility
is determined by µ = eτ/m^{*}, where e
denotes
electron
charge, τ is the carrier relaxation time introduced in the
relaxation time approximation, and µ represents
electron effective mass in ntype semiconductors. Here, the carrier
relaxation time τ includes all scattering processes, such as
electronhole scattering, electronphonon scattering (Frohlich and
deformation potentials), and scattering by defects and disorders. In
general, the time scale of τ is less than a few hundred
femtoseconds. While the carrier mobility has recently been obtained by
using THz timedomain spectroscopy with the fit by Drude
function, the accessible range of the carrier density was
limited to lower density than an order of 10^{16} cm^{3},
due to
nonDrude behavior originating from nonthermal carrier distribution at
the high carrier density region.
In doped semiconductors, such as ntype
GaAs, it is well known that the plasmon and the longitudinal optical
(LO) phonon form coupled modes through Coulomb interactions, and the
frequencies of the LO phononplasmon coupled (LOPC) modes depend on the
carrier density N_{e} through the relation of the
plasma frequency. Using Raman spectroscopy Nakashima and Harima have
developed characterization of carrier mobility in SiC. They observed
plasmonlike LOPC modes in frequency domain and fit the spectra with
the line profile determined by frequencydependent amplitude and
dielectric function. As the results of the fitting, they obtained
bandwidth of the plasmonlike LOPC mode, and put it into the formula
µ = e/m^{*}γ, where γ is a damping rate
of plasmon. The derived carrier mobility matched very well to that
obtained by Hall measurements. Although Raman spectroscopy is a
promising tool to estimate carrier mobility without any mechanical
contact onto a sample, one need fitting of the line shape using a
number of equations.
Here, the carrier
(electron) mobility was determined from the dephasing time of the
plasmonlike coherent LOPC mode (L_{+}) in nGaAs, which is
obtained by mapping the timefrequency dynamics of the LOPC modes by
the use of the wavelet analysis. The electron mobility extracted from
the coherent phonon spectroscopy decreases with increasing the
photodoping levels, indicating the suppression of the mobility by
enhanced electronhole scattering. The availability of this technique
will spread over the polar semiconductors, such as SiC, GaN, under the
condition that the photodoping level assure that the LOPC mode is
plasmonlike.


