Strain-Balanced InGaAs/InAlAs Superlattices for 1550 nm-based Terahertz Photoconductive Antennas

Abstract

Terahertz (THz) technology has developed as a transformative tool with critical applications in spectroscopy, imaging, and high-speed communication across diverse fields. Recent advancements have facilitated a transition from conventional photoconductors, driven by laboratory-scale and costly Ti:Sapphire lasers operating at 800 nm wavelengths, toward compact, cost-effective, and industrially viable THz time-domain spectroscopy systems utilizing a fiber-optic platform. Despite these advances, developing high-performance photoconductive materials remains a significant challenge, especially for efficient THz generation and broadband detection at the telecom wavelength of 1550 nm.

This research investigates the growth, characterization, and photoconductive properties of strain-balanced InGaAs/InAlAs superlattices grown on InP substrates as promising photoconductive materials for fiber-compatible THz photoconductive sources and detectors. First, fundamental principles underlying photoconductivity, including carrier generation, transport, and recombination, are reviewed, highlighting their critical role in determining overall performance. Recent material engineering strategies targeting THz operation at telecom wavelengths are also discussed. Despite recent advancements, there remains a need for photoconductive materials simultaneously exhibiting enhanced optical absorption, superior carrier transport, and ultrafast recombination lifetimes to enable efficient broadband THz operation.

Based on these principles, a theoretical framework describing carrier dynamics for transient THz photocurrent generation and detection is reviewed. This theoretical foundation is complemented by comprehensive experimental studies, including time-resolved pump-probe spectroscopy, Hall effect measurements, optical absorption spectroscopy, and band structure simulations, enabling precise quantification of carrier lifetime, mobility, and optical absorption coefficients.

Molecular beam epitaxy was employed to implement a stationary (non-rotating) substrate growth method, enabling precise lateral control of structural parameters across the wafer. The primary contribution lies in strategically balancing compressive strain within InGaAs wells with tensile strain in InAlAs barriers, thereby achieving a net-zero stress condition in each superlattice period. This strain-balancing approach systematically addresses limitations inherent to conventional lattice-matched epitaxy, resulting in significantly enhanced crystal quality in strain-induced superlattices, improved optical absorption at 1550 nm, and optimized electronic transport properties. Experimental results confirm these structural optimizations improve carrier dynamics, essential for high-performance THz devices.

Moreover, low-temperature-grown Be-doped strain-balanced superlattices are comprehensively characterized, revealing sub-picosecond carrier recombination lifetimes, increased mobility, and enhanced optical absorption. Detailed structural analyses through high-resolution X-ray diffraction, atomic force microscopy, and transmission electron microscopy identify notable Be-induced interdiffusion effects at interfaces. Lastly, modulation doping strategies are explored to further refine photoconductive properties. By systematically controlling dopant distribution within the superlattice layers, this study reveals the complex interplay among doping, defect formation, and strain, enabling precise tuning of carrier transport and recombination dynamics critical for advanced THz photoconductive applications.

This research advances the fundamental understanding of carrier dynamics and transport in strain-balanced photoconductive superlattices and offers practical guidance for developing high-performance, telecom-compatible THz photoconductive materials well-suited for portable pulsed THz spectroscopy and imaging systems.