Metallic and alternative plasmonic materials are useful for a broad range of applications: enhanced sensing, waveguiding, hyperbolic metamaterial components, and so forth. [1][2][3] The majority of the successful implementation of plasmonics so far has been limited to shorter wavelengths, including visible and near-infrared (NIR). However, the search for plasmonic materials in the mid-infrared (MIR) to long-wave infrared (LWIR) range (3-15 µm) could also advance technology in many fields ranging from security to health-related technologies and including thermal imaging and molecular sensing. [4,5] Currently, there are a few challenges in the search for plasmonic materials at longer wavelengths. First, the use of current plasmonic materials at shorter wavelengths cannot be simply expanded to longer wavelengths. For example, noble metals such as gold and silver have been traditionally used as plasmonic materials in the visible and NIR regimes, but their permittivities become too large and negative to be practically useful in the MIR frequency range. [4,6] Another challenge is the difficulty in integrating the plasmonic The group III-V semiconductor photonic system is attractive to photonics engineers because it provides a complete set of photonic components. A plasmonic material that can be epitaxially integrated with the group III-V photonic system will potentially lead to many applications leveraging plasmonics and metamaterials. In this work, the shortest plasma wavelength ever reported in a III-V-based material is demonstrated by epitaxially embedding ErAs into GaAs. This composite material acts as a tunable plasmonic material across the technologically important 2.68-6 µm infrared window. The growth window of this material is demonstrated to be much wider than other current heavily doped III-V plasmonic materials. Additionally, it is shown that the scattering rate can be reduced by increasing the growth temperature. The wide growth temperature range, designer plasmonic response, and the ease of epitaxial integration with other III-V semiconductor devices demonstrate the potential of ErAs:GaAs nanocomposites for the creation of a new type of metamaterial and other novel optoelectronic and nanophotonic applications.