  The interest in nanowire photodetectors stems from their potential to improve
the performance of a variety of devices, including solar cells, cameras,
sensors, and communication systems. Implementing devices based on nanowire
ensembles requires a planarization process which must be conceived to preserve
the advantages of the nanowire geometry. This is particularly challenging in
the ultraviolet (UV) range, where spin coating with hydrogen silsesquioxane
(HSQ) appears as an interesting approach in terms of transmittance and
refractive index. Here, we report a comprehensive study on UV photodetectors
based on GaN or AlGaN/AlN nanowire ensembles encapsulated in HSQ. We show that
this material is efficient for passivating the nanowire surface, it introduces
a compressive strain in the nanowires and preserves their radiative efficiency.
We discuss the final performance of planarized UV photodetectors based on three
kinds of nanowire ensembles: (i) non-intentionally-doped (nid) GaN nanowires,
(ii) Ge-doped GaN nanowires, and (iii) nid GaN nanowires terminated with an
AlGaN/AlN superlattice. The incorporation of the superlattice allows tuning the
spectral response with bias, which can enhance the carrier collection from the
AlGaN/AlN superlattice or from the GaN stem. In all the cases, the performance
of the planarized devices remains determined by the nanowire nature, since
their characteristics in terms of linearity and spectral selectivity are closer
to those demonstrated in single nanowires than those of planar devices. Thus,
the visible rejection is several orders of magnitude and there is no indication
of persistent photocurrent, which makes all the samples suitable for
UV-selective photodetection applications.
