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the bulk. This is called the photo–Dember effect. The transient diffusion current thus
generated results in THz emission. Carriers attain high excess kinetic energy and
carrier temperature in lower-bandgap materials, making diffusion stronger in them
[4].
Drift and diffusion photocurrents coexist in semiconductor systems. However,
their relative and absolute strengths depend on many factors, including the bandgap,
the position of satellite valleys, mobility, doping density, pump photon energy, etc.
Guessing the THz emission mechanisms in a semiconductor system is not easy, as
one needs to consider a large number of factors that are at play. For instance, when
excited with ultrafast pulses with photon energy ~1.5 eV (λ ~ 800 nm), GaAs with
a bandgap, E g of ~1.42 eV, is a surface field-assisted drift current emitter [5]. On
the other hand, studies have shown GaAs transforming into a diffusion emitter when
excited at 3.1 eV due to high carrier excess energy [4]. Following this reasoning,
one might be tempted to conclude that pumping a system with higher photon energy
would force stronger diffusion leading to the photo–Dember effect. However, in the
case of InAs (E g ~ 0.35 eV), pumping with photon energies above ~1.6 eV decreases
photo–Dember emission. When excited high up to the conduction band, electrons
encounter an increased probability of scattering to the low-mobility satellite valleys
[6]. Similar reasoning explains the higher photo–Dember THz emission from InAs
compared to the InSb at 1.5 eV excitation, even though InSb has a lower bandgap
[7].
Similarly, one would also expect heavy doping as a way to create a higher surface
field and leading to higher drift currents. Even though high doping density enhances
the field strength, the depletion region becomes very narrow, and hence a large
fraction of photocarriers are generated outside the depletion region where there is
no field to accelerate the charges. Moreover, the increased screening [8, 9] and high
carrier–carrier scattering [10] sharply reduced the THz emission efficiency at high
doping levels.
The excitation density of the pump beam is a vital factor in determining the
emission mechanism. Let us look at THz emission from InAs. At low intensities, the
THz emission happens via the photo–Dember effect [11]. However, at higher fluence,
the surface field-assisted OR takes over as the dominant mechanism [12]. Moreover,
the drift and diffusion currents tend to saturate at higher fluences (photocarrier–
carrier density). The nonlinear phenomenon, on the other hand, might get stronger
and show saturation only at much higher excitation densities.
In short, the THz emission from semiconductors is a complex phenomenon that
cannot be easily generalized. Multiple emission mechanisms can coexist and compete
in a single material system. One has to study the physical system and THz emission
carefully to elucidate the underlying emission mechanisms. Such investigations are
vital in the pursuit of the identification and design of efficient broadband sources
of THz radiation, which have the potential for spectroscopic and technological
applications.
R. N. Kini and C. P. Vaisakh
the bulk. This is called the photo–Dember effect. The transient diffusion current thus
generated results in THz emission. Carriers attain high excess kinetic energy and
carrier temperature in lower-bandgap materials, making diffusion stronger in them
[4].
Drift and diffusion photocurrents coexist in semiconductor systems. However,
their relative and absolute strengths depend on many factors, including the bandgap,
the position of satellite valleys, mobility, doping density, pump photon energy, etc.
Guessing the THz emission mechanisms in a semiconductor system is not easy, as
one needs to consider a large number of factors that are at play. For instance, when
excited with ultrafast pulses with photon energy ~1.5 eV (λ ~ 800 nm), GaAs with
a bandgap, E g of ~1.42 eV, is a surface field-assisted drift current emitter [5]. On
the other hand, studies have shown GaAs transforming into a diffusion emitter when
excited at 3.1 eV due to high carrier excess energy [4]. Following this reasoning,
one might be tempted to conclude that pumping a system with higher photon energy
would force stronger diffusion leading to the photo–Dember effect. However, in the
case of InAs (E g ~ 0.35 eV), pumping with photon energies above ~1.6 eV decreases
photo–Dember emission. When excited high up to the conduction band, electrons
encounter an increased probability of scattering to the low-mobility satellite valleys
[6]. Similar reasoning explains the higher photo–Dember THz emission from InAs
compared to the InSb at 1.5 eV excitation, even though InSb has a lower bandgap
[7].
Similarly, one would also expect heavy doping as a way to create a higher surface
field and leading to higher drift currents. Even though high doping density enhances
the field strength, the depletion region becomes very narrow, and hence a large
fraction of photocarriers are generated outside the depletion region where there is
no field to accelerate the charges. Moreover, the increased screening [8, 9] and high
carrier–carrier scattering [10] sharply reduced the THz emission efficiency at high
doping levels.
The excitation density of the pump beam is a vital factor in determining the
emission mechanism. Let us look at THz emission from InAs. At low intensities, the
THz emission happens via the photo–Dember effect [11]. However, at higher fluence,
the surface field-assisted OR takes over as the dominant mechanism [12]. Moreover,
the drift and diffusion currents tend to saturate at higher fluences (photocarrier–
carrier density). The nonlinear phenomenon, on the other hand, might get stronger
and show saturation only at much higher excitation densities.
In short, the THz emission from semiconductors is a complex phenomenon that
cannot be easily generalized. Multiple emission mechanisms can coexist and compete
in a single material system. One has to study the physical system and THz emission
carefully to elucidate the underlying emission mechanisms. Such investigations are
vital in the pursuit of the identification and design of efficient broadband sources
of THz radiation, which have the potential for spectroscopic and technological
applications.