Final answer:
Momentum conservation in radiative recombination of direct band gap semiconductors involves complex quantum mechanical processes, as evidenced by principles like the Compton effect, which demonstrated momentum conservation in photon-electron interactions.
Step-by-step explanation:
In direct band gap semiconductors, the conservation of momentum during radiative recombination can be complex due to quantum mechanical effects. The process involves an electron transitioning from the conduction band to the valence band, emitting a photon in the process. Conservation of momentum is a fundamental principle that states that the total momentum of a closed system remains constant if no external forces act upon it. This is true in quantum mechanics, as evidenced by experiments like the Compton effect.
Arthur H. Compton's experiments with x-ray photons and electrons showed that when a photon is scattered by an electron, both energy and momentum are conserved in the interaction. In the case of radiative recombination in direct band gap semiconductors, the recombining electron and the hole can be considered a system, and the conservation of momentum must be considered when a photon is emitted. Conservation of momentum in such quantum processes can involve not just the particles themselves, but also their interactions with the crystal lattice or other quasi-particles such as phonons.