Optical Properties and Semiconductor Photonics
Absorption and Emission
Fundamental optical processes
Optical absorption in semiconductors occurs when photons promote electrons across the band gap or within impurity and excitonic states. The absorption coefficient depends on the joint density of states, optical matrix elements, and selection rules. Direct-gap semiconductors usually exhibit much stronger near-edge absorption than indirect-gap materials.
Selection rules
Optical transitions are constrained by momentum conservation, symmetry, and polarization. In indirect materials, phonons often supply the missing crystal momentum.
Excitons and Many-Body Effects
Bound electron-hole states
Excitons are Coulomb-bound electron-hole pairs that can dominate optical spectra near the band edge, especially in low-dimensional systems and materials with weak dielectric screening. Their binding energy can be strongly enhanced in quantum wells, nanowires, and two-dimensional semiconductors.
Why are excitons often more prominent in low-dimensional semiconductors?
Reduced dimensionality and dielectric screening increase exciton binding.
Correct answer: Because screening is typically weaker and Coulomb interactions are stronger
What is the main difference between direct and indirect optical transitions?
The distinction is determined by the band extrema locations in -space.
Correct answer: Direct transitions conserve momentum without a phonon, whereas indirect transitions require phonon assistance to satisfy crystal-momentum conservation.
Photon Detection and Light Emission Devices
Photodiodes and LEDs
Photodiodes exploit optical generation of electron-hole pairs and their separation by built-in or applied fields. LEDs and semiconductor lasers rely on efficient radiative recombination, carrier confinement, and optical cavity design. Device performance is shaped by nonradiative losses, injection efficiency, and thermal management.
A key reason direct-gap semiconductors are preferred for LEDs is that they:
Direct-gap materials allow radiative transitions without phonon assistance, improving emission efficiency.
Correct answer: Support efficient radiative recombination
Name one limitation on LED or laser efficiency at high current density.
Any of these can reduce internal quantum efficiency under strong injection.
Correct answer: Auger recombination, self-heating, carrier leakage, or nonradiative defect recombination.