Wide Band Gap coating for Enhancement of THz radiation in GaAs
Rafael B. Jaculbia1, Nemesio S. Mangila IV1, Jefferson M. Abrenica1, Elmer S. Estacio1, Arnel A. Salvador1, Armando S. Somintac1*
1National Institute of Physics, University of the Philippines - Diliman, Diliman, Quezon City, Philippines
* presenting author:Armando Somintac, email:asomintac@nip.upd.edu.ph
Due to the wide applications of terahertz (THz) waves, the search for intense THz sources is a promising research field. GaAs is one of the most widely used THz emitter especially for THz time domain spectroscopy (THz-TDS). In this work, we report on the enhancement of THz radiation from GaAs coated with wide band-gap materials (WBM). The wide band materials used in this work are aluminium nitride (AlN) grown via RF magnetron sputtering [1] and aluminum doped zinc oxide (AZO) grown via spray pyrolysis [2]. The THz emission properties are investigated using standard THz-TDS setup in reflection geometry. We found that the addition of either AlN or AZO on nGaAs improves its THz emission intensity. More than a factor of three increase in THz emission intensity was observed for AlN/n-GaAs sample while more than a factor of five was observed for the AZO/n-GaAs sample both compared to the bare substrate. At the pump fluence used in these experiments, the dominant THz emission mechanism from GaAs is due to photocarrier drift as driven by the surface electric field. For such a case, one of the contributing factors to the THz emission is the electric field experienced by the photocarriers. It is therefore worthwhile to assess the electric fields of our samples. We employ photoreflectance (PR) spectroscopy to determine the electric field at the surface of GaAs and at the interface of the WBM and n-GaAs. From PR measurements, we were able to observe that for the AlN/nGaAs sample, the electric field is stronger by 2.4 times while for the AZO/nGaAs sample, the electric field is stronger by 2.6 times. Here we can see that the enhancement in the THz intensity is primarily due to the increase in the electric field upon deposition of the WBM. The stronger electric field is brought about by the large difference in the bandgaps of the WBM and GaAs. Notice however that despite the fact that the increase in electric field for both samples are almost the same, a stronger THz enhancement is observed for the AZO/n-GaAs sample. Apart from the electric field, the absorption of GaAs can also contribute to the enhanced THz emission. A reduction of the reflectance at the pump beam wavelength is observed for both samples, where a larger decrease is observed for the AZO/n-GaAs sample, suggesting that it has a more improved absorption compared to the AlN/n-GaAs sample Hence, this explains the reason why the THz enhancement is larger for the AZO/n-GaAs than the AlN/n-GaAs despite close values of electric field measured. Our results demonstrate a simple method to improve the THz emission intensity of GaAs which can possibly be employed for future low cost and intense THz emitters.

1. Jaculbia, R. B., Balgos, M. H. M., Mangila, N. S., Tumanguil, M. A. C., Estacio, E. S., Salvador, A. A., and Somintac, A. S., "Enhanced terahertz emission from GaAs substrates deposited with aluminum nitride films caused by high interface electric fields." Applied Surface Science 303, 241-244, 2014.
2. Jaculbia, R. B., Abrenica, J.M., Estacio, E. S., Salvador, A. A., and Somintac, A. S., “Terahertz emission from aluminum-doped ZnO–nGaAs heterostructure investigated using reflection-mode terahertz time-domain spectroscopy.” Applied Physics Express 8 (12), 122101-1 – 122101-3, 2015.

Keywords: Terahertz, GaAs, Electric field, wide band gap materials