Terahertz emitters based on intracenter transitions in semiconductors

Terahertz emitters are important for fundamental studies in an interesting frequency regime and for applications ranging from medical diagnostics to see-through imaging. A simple approach to THz emission from semiconductors is based on intracenter optical transitions in dopants and impurities in semiconductors. The centers can be excited either electrically or optically, and the THz emission occurs when carriers in the dopant upper energy states relax toward the ground state. Both n-type and p-type dopants as well as deep impurities can be used for THz emission from many host semiconductors including silicon, SiC, and GaN. Unlike with conventional p-n junction devices, the centers for THz emission must be occupied and not thermally ionized, which suggests the need for deep energy levels and/or low temperature operation. Significant center occupation at elevated temperatures favors the wide bandgap semiconductors such as SiC and GaN, in which the dopant ionization energy can greatly exceed the thermal energy k(B)T at room temperature. For example, electrically pumped THz emitters fabricated from nitrogen-doped SiC can operate at temperatures to about 250 K in pulse mode. The SiC emission spectra had peaks from 5 to 12 THz (20 to 50 meV), and these surface-emitting devices produced a peak power density of 30 milliwatt-cm(-2) at 77 K, which is suitable for a wide range of high power THz applications. We report the characteristics and limitations of electrically pumped dopant-transition THz emitters, and their performance in several semiconductor systems.