Terahertz Emission Mechanisms in III–V Semiconductors …
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Fig. 1 Band bending at the surface of an n-doped semiconductor, which has a bandgap of E g and
whose Fermi level is E FB away from the valance band edge E V . E a and E d represent the position
of acceptor and donor levels, respectively, above E V . The quantity qφ BB represent the maximum
extent of band bending (at the surface) measured from the conduction band edge E c in bulk
levels determine its position. Due to this, there is a mismatch between the Fermi
levels at the bulk and the surface. This mismatch leads to band bending, charge
separation, and establishment of the electric field near the surface (depletion region).
This phenomenon is called the Fermi-level pinning [2] (Fig. 1).
Ultrafast excitation of the semiconductor surface introduces new carriers in the
depletion region, and these carriers accelerate due to the surface electric field. These
transient drift photocurrents lead to radiation in the THz frequency range. Widebandgap semiconductors like GaAs and InP have higher Fermi-level mismatch and
stronger surface fields. Hence, drift currents constitute the prominent emission mechanism. Drift currents form the basis of popular THz source called the photoconductive
antenna (PCA). PCAs are made using wideband semiconductors with biased electrodes on the surface where the external electric field drives the drift photocurrent.
PCAs, compared to bare surface emitters, offer better flexibility in terms of dipole
strength, dipole orientation, spectral tunability, etc.
However, low-bandgap materials do not form strong surface fields. InAs, which
is a low-bandgap material, shows the highest amplitude THz emission among III–V
semiconductors at 800 nm excitation, and the emission is due to carrier diffusion. In a
low-bandgap material that has a short absorption length at 800 nm, when excited with
an ultrafast near-infrared (NIR) pulse, a dense pool of hot carriers is created close to
the surface. The gradient in carrier density between the bulk and surface drives the
diffusion of carriers into the bulk. The hole has a higher effective mass and lower
mobility compared to electrons. Hence, the inhomogeneous carrier distribution created by ultrafast optical excitation evolves with a net movement of electrons toward
171
Fig. 1 Band bending at the surface of an n-doped semiconductor, which has a bandgap of E g and
whose Fermi level is E FB away from the valance band edge E V . E a and E d represent the position
of acceptor and donor levels, respectively, above E V . The quantity qφ BB represent the maximum
extent of band bending (at the surface) measured from the conduction band edge E c in bulk
levels determine its position. Due to this, there is a mismatch between the Fermi
levels at the bulk and the surface. This mismatch leads to band bending, charge
separation, and establishment of the electric field near the surface (depletion region).
This phenomenon is called the Fermi-level pinning [2] (Fig. 1).
Ultrafast excitation of the semiconductor surface introduces new carriers in the
depletion region, and these carriers accelerate due to the surface electric field. These
transient drift photocurrents lead to radiation in the THz frequency range. Widebandgap semiconductors like GaAs and InP have higher Fermi-level mismatch and
stronger surface fields. Hence, drift currents constitute the prominent emission mechanism. Drift currents form the basis of popular THz source called the photoconductive
antenna (PCA). PCAs are made using wideband semiconductors with biased electrodes on the surface where the external electric field drives the drift photocurrent.
PCAs, compared to bare surface emitters, offer better flexibility in terms of dipole
strength, dipole orientation, spectral tunability, etc.
However, low-bandgap materials do not form strong surface fields. InAs, which
is a low-bandgap material, shows the highest amplitude THz emission among III–V
semiconductors at 800 nm excitation, and the emission is due to carrier diffusion. In a
low-bandgap material that has a short absorption length at 800 nm, when excited with
an ultrafast near-infrared (NIR) pulse, a dense pool of hot carriers is created close to
the surface. The gradient in carrier density between the bulk and surface drives the
diffusion of carriers into the bulk. The hole has a higher effective mass and lower
mobility compared to electrons. Hence, the inhomogeneous carrier distribution created by ultrafast optical excitation evolves with a net movement of electrons toward