GaN and AlGaN/AlN Nanowire Ensembles for Ultraviolet Photodetectors: Effects of Planarization with Hydrogen Silsesquioxane and Nanowire Architecture

The interest in nanowire photodetectorsstems from theiruniqueproperties, such as high sensitivity, fast response times, and compatibilitywith integrated circuit technology, making them well suited for diverseapplication domains, including solar cells, cameras, biomedical sensors,and communication systems. Implementing devices based on nanowireensembles requires a planarization process, which must be conceivedto preserve the advantages of the nanowire geometry. This is particularlychallenging in the ultraviolet (UV) range, where spin coating withhydrogen silsesquioxane (HSQ) appears as an interesting approach interms of transmittance and refractive index. Here, we report a comprehensivestudy on UV photodetectors based on GaN or AlGaN/AlN nanowire ensemblesencapsulated in HSQ. We show that this material is efficient for passivatingthe nanowire surface, and it introduces a compressive strain in thenanowires and preserves their radiative efficiency. We discuss thefinal performance of planarized UV photodetectors based on three kindsof nanowire ensembles: (i) non-intentionally doped (nid) GaN nanowires,(ii) Ge-doped GaN nanowires, and (iii) nid GaN nanowires terminatedwith an AlGaN/AlN superlattice. The incorporation of the superlatticeallows tuning the spectral response with bias, which can enhance thecarrier collection from the AlGaN/AlN superlattice or from the GaNstem. In all the cases, the performance of the planarized devicesremains determined by the nanowire nature since their characteristicsin terms of linearity and spectral selectivity are closer to thosedemonstrated in single nanowires than those of planar devices. Thus,the visible rejection is several orders of magnitude, and there isno indication of persistent photocurrent, which makes all the samplessuitable for UV-selective photodetection applications.