Improved THz emission in c-plane InGaN due to polarization charges at the InGaN/GaN interface

There has been great progress recently in the intensity of broadband Terahertz (THz) signals utilizing surface and field effects from semiconductor materials. Typically, the Photo-Dember effect combined with optical rectification in such materials as GaAs or InAs generates the most intense broadband THz radiation from contactless THz emitters [1]. There still exists room for improvement in terms of THz emitter intensities, and several novel structures have been proposed and fabricated which include the use of large internal electric fields in polar and non-polar semiconductors as well as multiple quantum well structures, all of which show enhanced THz emission over prior materials [2-4]. Nitride semiconductors, which have both a piezoelectric and spontaneous polarization due to its wurtzite crystal structure, can have an order of magnitude larger internal electric field in e.g. InGaN/GaN MQWs than previous THz emitters utilizing piezoelectric fields to generate drift currents such as in InGaAs/GaAs MQWs [4]. It has been predicted that InGaN/GaN heterostructures with much thicker InGaN layers could be used to further enhance THz emission through large internal electric fields and high electron mobility [5]. In this paper we demonstrate that the polarization field in an InGaN/GaN surface heterostructure can be exploited for the enhancement of THz generation, and additionally opens up the possibility of tuning the band gap to wavelengths compatible with excitation by femtosecond fiber lasers.