Metallic structures with feature sizes small compared to the wavelength of optical radiation have become extremely useful for photonic devices. Surface plasmons have been exploited for various applications including molecular sensing, light focusing, near-field optical microscopy and enhanced near-field semiconductor absorption. The integration of metallic nanoparticles (e.g. silver, gold, etc.) is currently limited to <I>ex situ</I> deposition on the device periphery, as most metal systems cannot be epitaxially integrated into semiconductors. An alternate class of materials, which is compatible with the growth of photonic devices, is required to reach the full potential of nanostructured metallic features. The rare-earth monopnictides are rocksalt semimetals that can be embedded epitaxially into III-V semiconductors. To date, most studies of ErAs nanoparticles have focused on power conversion devices and terahertz emission sources where high optical quality overgrowth in close proximity to the nanoparticles has not been of paramount importance. For monolithic integration with other photonic devices, particularly long-wavelength vertical-cavity surface-emitting lasers (VCSELs), a careful analysis of the overgrowth material must be conducted.
GaAs-based tunnel junctions, with ErAs nanoparticles embedded at the p<SUP>+</SUP>/n<SUP>+</SUP> interface, were grown by solid-source molecular beam epitaxy (MBE) on silicon-doped GaAs (100) substrates. Silicon and beryllium were used as the n-type and p-type dopants, respectively. Surface roughness and local conductivity were measured by atomic force microscopy (AFM) for tunnel junctions of varying p-type capping layer thickness. The RMS surface roughness versus capping layer thickness demonstrated some surface roughening during the initial stages of overgrowth. However, the RMS roughness eventually recovered to ~1 monolayer after sufficiently thick GaAs overgrowth. From the conductive AFM measurements, it was clear that the roughening was not correlated with the location of the ErAs nanoparticles.
In addition to surface morphology, the optical quality of the overgrown layers was investigated with photoluminescence (PL). PL structures were grown on semi-insulating GaAs (100) substrates, under conditions nominally identical to those of the tunnel junctions. PL structures with and without ErAs nanoparticles exhibited comparable optical emission from the overgrown In<SUB>0.1</SUB>Ga<SUB>0.9</SUB>As quantum wells. This demonstrates for the first time that the ErAs nanoparticles can be overgrown with high-quality III-Vs.
Epitaxial integration of ErAs nanoparticles compatible with III-V optical devices appears quite promising for incorporation into a number of (nano)photonic devices, including long-wavelength VCSELs where smooth interfaces are critical for the quantum wells, as well as, the distributed Bragg reflector mirrors. In addition. the availability of plasmonic nanomaterials that can be monolithically integrated with III-Vs is exciting for subwavelength photonic devices and circuits for future applications.
This work was supported by Dr. Mike Gerhold of ARO and DARPA through a Young Faculty Award.