Generation of THz radiation by (100), (110), and (111)A-oriented multiple pseudomorphic InGaAs/GaAs quantum wells and photoconductive antennas

We report the effect of the built-in electric field emerging in elastically strained multiple InGaAs/GaAs quantum wells on the THz generation efficiency upon irradiation of the surface of these heterostructures, as well as photoconductive antennas based on them, with femtosecond optical laser pulses. The built-in field arises as a result of the piezoelectric effect in heterostructures with multiple {InGaAs/GaAs}×10 quantum wells grown on GaAs substrates with (110) and (111)A crystallographic orientations. Terahertz radiation produced under the same excitation conditions from films with the same composition, but grown on substrates with different orientations, is compared. The most intense THz radiation is obtained from the surface of the {In_0.2Ga_0.8 As/GaAs}×10 heterostructure on a GaAs (110) substrate. Among photoconductive antennas, the highest efficiency of THz generation is demonstrated by antennas made on (110) and (100)-oriented {In_0.2Ga_0.8As/GaAs}×10 heterostructures. In this case, the influence of the substrate orientation, which is clearly manifested upon generation of THz radiation directly by the surface of the films, is much weaker for photoconductive antennas on the same films.