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Strain incorporation in InGaAs/InAlAs superlattices enhances terahertz emission

By Shweta Agarwala June 19, 2019
The amplitude of the emitted THz power spectra (a) and the waveforms of THz pulses (b) for the unstrained InGaAs/InAlAs and the strained InGaAs/InAlAs superlattices at an excitation wavelength of 800 nm. Reprinted with permission from D.S. Ponomarev et al., J. Appl. Phys. 125, 151605 (2019); ©2019/AIP Publishing.

Terahertz (THz) radiation has been of interest due to its potential applications in imaging in the medical sciences, for security, surveillance, and communication systems. THz waves can penetrate through many optically opaque nonpolar dielectrics, such as paper, textile, plastic, and ceramics, with low attenuation. InGaAs especially is an attractive candidate for fabricating cost-effective and portable THz emitters, due to efficient optical absorption at wavelengths of 1550 nm and 1030 nm. A research group at the Institute of Ultra High Frequency Semiconductor Electronics RAS, Moscow and their colleagues at other institutions in Russia have now successfully reduced photocarrier relaxation time in InGaAs using a new approach involving strained superlattices. In their recent publication in the Journal of Applied Physics the researchers demonstrated ultrashort photocarrier relaxation times of approximately 1.7 ps through the introduction of strained superlattices.

Both unstrained and strained samples were fabricated in the study and their morphology, residual strain estimation, and THz time-domain spectroscopic measurements were compared. The unstrained samples showed relaxation time of 4.37 ps, and inducing strain lowers it to 1.7 ps. The new design of strained InGaAs/InAlAs superlattice resulted in increased THz emission, both in terms of electric field and bandwidth. The fabricated structures emit an enhanced THz spectrum with frequencies lower than 1 THz. Through a thorough theoretical evaluation of the energy band structures, the researchers attribute the enhanced THz emission to a slight decrease in the energy bandgap in InGaAs due to residual strain in the superlattice.

Pernille Klarskov Pedersen of Aarhus University, whose research focuses on THz technologies and its applications, says that compared to present commercial InGaAs-based photoconductive antennae (PCAs) for fiber-coupled THz systems—that are limited in efficiency due to the material’s low resistivity and high dark currents—the strained InGaAs/InAlAs superlattice structure is expected to be able to contribute to more efficient PCAs for THz emission and detections.

The lead author of the article, Dmitry Ponomarev, emphasizes that the novelty of the work comes from incorporating epitaxial stresses in InGaAs/InAlAs superlattices. This optimizes the parameters of InGaAs photoconductive layers, which can be further used in PCAs. He says, “We have showcased that these stresses can efficiently reduce photocarriers lifetime and affect the energy bandgap of the photoconductive layer. The latter can then be used to enhance the intensity of the THz radiation in the InGaAs-based surface emitters.”

Read the abstract in Journal of Applied Physics.