Terahertz time-domain spectroscopy (THz-TDS) applications are driven by the development of fiber-coupled THz devices and subsystems sought to resolve practical issues: compactness, robustness and sensitivity to surrounding noises. With the introduction of fiber-based femtosecond lasers, ultrafast photoconductive THz antenna materials with enough optical absorption become necessary. Due to its small bandgap, InGaAs received attention first. Well-engineered implantation damage complexes in InGaAs are known to be crucial in obtaining such photoconductors. However, large resistivity values, required for strong THz-TDS signal-to-noise ratios, are difficult to get (i.e. MΩ/square) while maintaining sub-ps carrier lifetimes. Switching for thin layers of absorbing InGaAsP quaternary alloys grown on resistive InP substrates could address this issue since slightly larger bandgaps are possible.

We discuss our results obtained on 1.5 μm-thick 1310 and 1560 nm quaternary alloys modified with high Fe ion fluence (10¹⁵ Fe/cm²). MeV Fe ions were generated using a 1.7 MV tandem accelerator for multi-energy implantation at -190, 100 and 200 °C. The implanted InGaAsP layers were processed with rapid thermal annealing (RTA) to adjust and stabilize their electrical properties which were measured by Hall effect. The RTA process tends to reduce the density of active traps in the ion-bombarded material: its influence on the carrier trapping times has been investigated using pump-probe reflectivity measurements. Our results show that it is possible to preserve sub-ps trapping times on high-resistivity materials. This post-growth process allows for manufacturing ideal substrates for THz photoconductive antennae to be coupled with the new commercially available laser-fiber sources.