Rare-earth arsenide III-V nanocomposites for heterodyne terahertz generation
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Room-temperature, broadly tunable, continuous-wave, terahertz sources operating in the range from 0.3 to 3 THz are required for applications such as high-resolution spectroscopy and medical imaging. Optical heterodyne conversion, or photomixing, is a technique for generating terahertz frequencies by modulating the photoconductance of a material via the interference of two incident lasers. For efficient terahertz generation, the photoconductive material must simultaneously possess high dark resistivity, high carrier mobilities, and short carrier lifetimes. Superlattice based nanocomposites of rare-earth arsenide nanoparticles epitaxially embedded into In [subscript 0.53] Ga [subscript 0.47] As are attractive candidate materials for terahertz generation via photomixing. In addition to the superior charge transport properties of InGaAs relative to GaAs, InGaAs-based devices can leverage the well-developed fiber-optic technology infrastructure available at 1550 nm, enabling low-cost, compact, room-temperature terahertz sources. Unfortunately, due to the alignment of the Fermi level near the InGaAs conduction band in previously investigated ErAs-InGaAs nanocomposites, photomixers based on these materials currently exhibit high leakage currents resulting in poor device performance. High depositions of ErAs (up to 1.75 monolayers per superlattice period) and beryllium counter-doping can partially overcome this limitation by adjusting the Fermi level closer to the midgap. However, carrier mobilities suffer, reaching a high of only 202 cm²/V-s, as a result of carrier scattering from the high counter doping required. Through an exploration of other rare-earth species, we were able to discern the importance of the structural quality of the III-V host matrix overgrowth. By improving the material overgrowth through surfactant-mediated epitaxy and nanoparticle morphology optimizations, we were able to increase the total amount of deposited rare-earth arsenide to 3.2 monolayers per superlattice period, while maintaining good structural quality of the overall nanocomposite. The properties of the resulting photoconductive material were also significantly improved, achieving Fermi level alignments near 240 meV below the conduction band and high dark resistivities of 4.2 Ω-cm, a record for a 1550 nm absorbing nanocomposite material without beryllium counter doping. Mobilities remained above 2700 cm²/V-s, however, the carrier lifetimes of these enhanced nanocomposites leveled off at 1.45 ps, above the terahertz threshold of 1 ps. An alternate path to improve the properties of nanocomposites is to change the Fermi level alignment of the nanoparticle/matrix interface by using alloy compositions of In [subscript 0.53] Ga [subscript 0.47] As and GaAs [subscript 0.5] Sb [subscript 0.5]. Since the Fermi level aligns closer to the valence band in GaAsSb and to the conduction band in InGaAs, the Fermi level can be tailored toward the midgap. We demonstrate rare-earth arsenide nanocomposite materials based on these quaternary alloys that achieve 2× improvement in carrier lifetimes over previous nanocomposites without any counter-doping, 0.73 ps, while maintaining carrier mobilities at least 4× higher than any other material based on counter-doping, over 4000 cm²/V-s, and modest dark resistivities of 0.8 Ω-cm. Although the InGaAsSb alloy matrix is a potentially elegant solution, the growth of these materials is very challenging due to the miscibility gap that exists for this quaternary alloy. With further optimization, RE-As-InGaAsSb nanocomposite systems show great potential for the development of 1550 nm based photomixing devices.