Transient Photocurrent in InAs/GaAs Pyramidal Quantum Dots observed by Terahertz Spectroscopy
Jorge Michael M. Presto1, Elizabeth Ann P. Prieto1, Karim M. Omambac1, Jessica Pauline C. Afalla1, Deborah Anne O. Lumantas1, Arnel A. Salvador1, Armando S. Somintac1, Elmer S. Estacio1*, Kohji Yamamoto2, Masahiko Tani2
1National Institute of Physics, University of the Philippines, Diliman, Quezon City, Philippines
2Research Center for Development of Far-Infrared Region, University of Fukui, Fukui, Japan
* presenting author:Elmer Estacio, email:eestacio@nip.upd.edu.ph
Terahertz (THz) time-domain spectroscopy (TDS) has been revolutionizing the advancement of THz science. THz-TDS has not only been useful in providing spectral fingerprint of various materials in the THz regime but has also been effective in providing unambiguous demonstration of carrier transport in various bare semiconductors such as in InAs/GaAs quantum dot (QD) structures which theoretically has zero degrees of freedom. Previously, THz enhancement in InAs/GaAs QDs grown via molecular beam epitaxy was demonstrated to be due to the current surge from the acceleration of GaAs carriers by the interfacial strain field upon excitation at 800 nm wavelength [1]. The THz emission of the QD sample significantly increased as compared to the GaAs sample and was as high as 70% that of a bulk p-InAs reputed, to date, as the most intense bare semiconductor THz surface emitter. Recently, THz-TDS of InAs/GaAs QDs revealed photocarrier transport exclusively from InAs QDs by using two excitation wavelengths at 800- and 910-nm and observing the magnetic (B)-field polarity variation of the THz signal [2]. The photo-carrier drift in the QD upon ultrafast excitation but before carrier recombination, due to the existing permanent dipole within the QD that spatially separates the electron and hole, resulted to the emission of the THz radiation. This was established by the anomalous B-field polarity characteristics of the QD sample at 910 nm excitation wavelength where only the InAs layers were probed. A 180°-phase shifting in the THz signal, upon the reversal of an applied B-field direction, should lend proof to the presence of Lorentz force-driven carriers [3]. For the QD sample, only partial flipping was observed. The cyclotron radius of the electron was much larger than the dimension of the QD hence the incomplete reversal of the THz signal as the applied B-field direction was reversed. The results demonstrated the underlying confined photocarrier transport from the apex to the base of the pyramidal QD hindering the complete circular revolution of the electron upon B-field polarity variation.

References:
1. E. Estacio, M. H. Pham, S. Takatori, M. Cadatal-Raduban, T. Nakazato, T. Shimizu, N. Sarukura, A. Somintac, 
M. Defensor, F. C. B. Awitan, R. B. Jaculbia, A. Salvador, and A. Garcia, “Strong enhancement of terahertz 
emission from GaAs in InAs/GaAs quantum dot structures,” Appl. Phys. Lett. 94, 232104 (2009).
2. J. M. P. Presto, E. A. P. Prieto, K. M. Omambac, J. P. C. Afalla, D. A. O. Lumantas, A. A. Salvador, A. S. Somintac, E. S. Estacio, K. Yamamoto, M. Tani, “Confined photocarrier transport in InAs pyramidal quantum dots via terahertz time-domain spectroscopy,” Optics Express 23 (11), 14533-14540 (2015).
3. M. Migita and M. Hangyo,“Pump-power dependence of THz radiation from InAs surfaces under magnetic fields excited by ultra-short laser pulses,” Appl. Phys. Lett. 79, 3437–3439 (2001).


Keywords: Terahertz spectroscopy, InAs Quantum Dots, Nanostructures