ABSTRACT
Emission across the band gap is the favored solution for semiconductor materials but not the only possibility. This chapter considers two different paths of light generation without recombination across the bandgap. One is based on the incorporation of atoms with individual atomic emission properties, and the other relies on a nonequilibrium carrier distribution within the bands. Visible light with a broad spectral distribution is emitted from a junction biased toward avalanche breakdown, but breakdown is not a favorable mode of operation for a diode. For luminescence experiments, optical excitation at low temperatures is often applied (PL). For device applications, electrical excitation at room temperature is preferred (electroluminescence). The dominant electrical excitation uses diode structures for carrier injections (light-emitting diode [LED]). The Ge LED emits light of about 1.5 µm wavelength into the waveguide when the contacts are forward biased. Interestingly, similar waveguide structures can also be used for detection and absorption modulation, which facilitates integration of active photonic devices.
