Strain-Balanced InGaAs/InAlAs Superlattices on InP(111)B for Terahertz Photoconductive Antennas

Abstract

Terahertz time-domain spectroscopy (THz-TDS) systems hold great promise for next generation communication, imaging, sensing, and metrology applications, all of which demand stringent performance requirements. Extending the deployment of THz-TDS systems beyond research laboratories into practical field applications requires the development of cost-effective and portable systems compatible with 1550 nm femtosecond fiber-coupled lasers. InGaAs/InAlAs photoconductors grown on InP(001) have emerged as strong candidates for efficient THz signal generation and detection. However, further material modifications are necessary to optimize their performance. One promising strategy involves growing InGaAs/InAlAs superlattices (SLs) along crystal orientations other than [001], such as [111], which can significantly influence their electronic and optical properties. Despite this potential, the growth and characterization of InGaAs/InAlAs SLs on (111)-oriented InP substrates remain underexplored due to intrinsic growth complexities and financial constraints. Addressing this gap, the present work investigates the molecular beam epitaxy (MBE) growth of InGaAs/InAlAs SLs on InP(111)B substrates. Through rigorous theoretical design, controlled MBE growth, and detailed structural and electrical characterization, we successfully achieved an atomically smooth, strain-balanced InGaAs/InAlAs SL on InP(111)B at 450°C for the first time. Leveraging the polar nature of the [111] orientation and strain engineering, simulations revealed a strong piezoelectric field of 153 kV/cm across the SL. This systematic approach enabled a detailed analysis of how structural parameters— such as indium composition and layer thickness—affect carrier dynamics, evaluated using time-resolved pump-probe spectroscopy at 1550 nm. Complementary absorption measurements indicated an enhanced absorption coefficient reaching 5195 cm⁻¹, while Hall effect characterization showed carrier mobility as high as 2756 cm²/Vs. These findings mark a crucial step toward achieving low-temperature-grown InGaAs/InAlAs structures with subpicosecond carrier lifetimes. While high-quality SLs were realized at 450°C, the impact of lower growth temperatures on structural quality remained unclear and warranted further investigation. To this end, two 50-period InGaAs/InAlAs SLs—one nominally lattice-matched and the other strain-balanced—were grown on InP(111)B substrates, with the growth temperature systematically reduced from 450°C to 200°C. Cross-sectional scanning transmission electron microscopy (STEM) revealed that lowering the temperature from 450°C to 400°C led to the formation of various defects and stacking faults within the SL grown at 400°C. Further temperature reduction resulted in spatial modulation of interfaces, the formation of microtwins, and phase separation in both the InGaAs and InAlAs layers. A comparative study of strain-balanced SLs grown on (001)- and (111)-oriented substrates under identical conditions showed that while high crystalline quality could be preserved on InP(001), maintaining structural integrity on InP(111) requires careful temperature-specific optimization. Building on these findings, we evaluated the sensitivity of InGaAs and InAlAs crystal quality to the temperature using two novel SLs with modulated growth temperature profiles. In the first structure, InGaAs and InAlAs layers were grown at 450°C and 200°C, respectively; in the second, the respective growth temperatures were reversed. The first SL exhibited a well-defined structure, although the InGaAs-on-InAlAs interfaces appeared slightly more diffuse than the InAlAs-on-InGaAs counterparts. These results suggest that growing InGaAs at 450°C can help mitigate interface roughness introduced by low-temperature InAlAs growth, thereby preserving SL integrity throughout the growth process. Despite the structural imperfections observed in SLs grown entirely at low temperatures on InP(111)B, these structures remained functional for THz photoconductive applications. This promising behavior led to a further investigation of carrier dynamics using strain-balanced InGaAs/InAlAs SLs grown on stationary substrates with varied indium compositions in the wells and barriers. Remarkably, trapping times as short as 1 ps and carrier lifetimes as fast as 4 ps were achieved without relying on complex Be doping schemes. Collectively, these advancements provide new insight into the controlled growth of InGaAs/InAlAs SLs on InP(111)B substrates, paving the way for the development of high performance electronic and photonic devices operating at telecommunication wavelengths.