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Theoretical modeling of short pulse laser interaction with condensed matter
Tzveta Apostolova
Institute for Nuclear Research and Nuclear Energy,
Bulgarian Academy of Sciences
Compared to continuous lasers, ultra-fast pulsed laser systems damage targets by
localized effects before the heated electrons produced during the interaction have time to
diffuse away. Optical and transport properties of semiconductors determine their applicability
to devices such as photodetectors, transistors, and light emitters. The optical responsivity of
interband photodetectors decreases greatly when they are exposed to an intense laser field due
to the incoherent creation of a large number of free conduction electrons. Therefore, it is of
great importance to describe the microscopic processes taking place when a semiconductor
gets irradiated by an intense laser field. The governing microscopic processes occur within the
time scale of the laser pulse so that the appropriate dynamical models have to explicitly
account for them. We conduct theoretical investigation of the many laser damage effects in
semiconductor devices due to high power, ultra-fast pulse laser fields. By comparing our
numerical results with experimental data we hope to gain insight into the microscopic
mechanisms for laser-induced damage in semiconductor devices.
We derive a quantum kinetic theory for laser-induced damage in semiconductors based
on a generalized Boltzmann-type equation, including energy-drift and anti-diffusion effects,
interband excitation, Coulomb scattering, thermal exchange with the lattice, etc. The kinetic
Fokker-Planck equation describes the physics of these processes and can be used to explain
how they lead to a laser-induced breakdown (LIB) of the semiconductor materials. In the
theoretical picture, LIB of semiconductors directly results from electron production in the
conduction band (CB) by photoionization via coherent absorption of photons from the
incident laser field followed by incoherent interband Coulomb scattering through induced
collisional ionization with excited conduction electrons. Collisional ionization can occur if
electrons in the CB gain energy until they reach a threshold kinetic energy equal to the bandgap. These excited electrons then may collide with valence electrons and promote them into
the CB by energy exchange. In order to reach this threshold energy for collisional ionization,
conduction electrons have to gain energy both classically and quantum-mechanically from the
incident laser. It is well known that conduction electrons cannot directly absorb incident laser
photons without assistance from defects. However, it can be shown that for conduction
electrons the energy gain can come from either classical absorption of laser power via Joule
heating or field-assisted phonon absorption.
The damage due to high intensity, ultra-fast pulse laser fields is due to the production
of huge amounts of hot electrons which form an electron plasma leading to the breaking of
chemical bonds, and the creation of defects inside the target materials, to name just a few of
the possible effects.
We define the various high-intensity, ultra-fast pulse laser damage mechanisms. For
the “electron production mechanism,” the maximum absorbed laser intensity is defined by the
point at which an incident laser beam becomes completely reflected by the electron plasma
generated during the interaction. Reflection occurs when the plasmon frequency (proportional
to the free electron concentration) exceeds the laser frequency. In addition to this reflection
effect, there could be a current surge effect that may occur when the generated free electron
concentration exceeds the maximum intrinsic concentration within the semiconductor,
possibly resulting in the burnout of any electronic circuit designed for room-temperature
operation. For the “structural damage mechanism,” when the average kinetic energy of the
generated hot electrons (proportional to the electron temperature) exceeds the band-gap
energy of the semiconductor material, the chemical bonds become unstable and free electrons
are released from these bonds.
The created model numerically predicts semiconductor material damage using
computational codes parameterized by the incident laser pulse, including pulse width and
peak intensity, and by the properties of the target materials, including band-gap, mobility,
initial lattice temperature, dielectric constant, and effective mass of electrons. The energy
spectra of the electron distribution function and the time dependence of the electron density
are calculated to illustrate how different laser and material parameters influence the
conduction electron dynamics. The electron distribution is used to evaluate average electron
energy and temperature.