Thermally-evaporated organic semiconductors on Ga2O3 for hybrid organic–inorganic self-powered UV-C detectors

The fast detection of UV-C light is of great importance in various civil and military applications [1]. Ga2O3 is a material with the appropriate bandgap and absorption coefficient for the use as a photon absorber in solar-blind UV-C photodetectors [2]. It has been demonstrated that the metastable κ-Ga2O3 polymorph can be a valid solution for realizing solar-blind UV-C photodetectors [3]. The main advantage of κ-Ga2O3 over the thermodynamically stable β polymorph is the higher crystallographic symmetry, in principle resulting in lower intrinsic anisotropy of several physical properties and in better epitaxial compatibility with standard substrates [4]. Our study is focused upon UV-C photodetectors based on p/n heterojunctions between Ga2O3 and organic semiconductors, which may work without external bias (self-powered mode) thanks to the photovoltaic effect induced by UV-C light, with the added benefit of the generally better cost-effectiveness and processing simplicity of organic semiconductors. Several examples of such devices where the organic part is deposited by spin coating have already been reported [5,6]. In this work, various organic matrices and co-dopants were deposited by vacuum-assisted thermal evaporation on top of n-type κ-Ga2O3 epilayers (weakly doped with Si) grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) on c-plane sapphire substrates. In particular, BPAPF:C60F48 was deposited on κ-Ga2O3 in order to build a photodiode with a custom-made interdigitated electrodes geometry. The effective functioning of such UV-C photodetector architecture in self-powered mode was confirmed by I-V profiling under dark and light along with measurements of spectral responsivity and current under transient illumination. Moreover, the fabricated devices retained their performances for a few months. In conclusion, the results obtained so far pave the way for the further optimization of the device’s performance by improving the encapsulation process and testing other types of organic semiconductors, different electrode geometries and different types of Ga2O3 layers, such as different polymorphs of this family as well as amorphous films. [1] E. Monroy et al 2003 Semicond. Sci. Technol. 18 R33 https://doi.org/10.1088/0268-1242/18/4/201 [2] Pearton, S. J.; Yang, J.; Cary, P. H.; Ren, F.; Kim, J.; Tadjer, M. J.; Mastro, M. A. A review of Ga2O3 materials; processing; and devices. Appl.Phys.Rev. 2018, 5, 011301/1-55. https://doi.10.1063/1.5006941. [3] Carmine Borelli et al, Electronic properties and photo-gain of UV-C photodetectors based on high-resistivity orthorhombic κ-Ga2O3 epilayers, Materials Science and Engineering: B, Volume 286, 2022, 116056, ISSN 0921-5107, https://doi.org/10.1016/j.mseb.2022.116056. [4] M. Bosi et al., “Ga2O3 polymorphs: tailoring the epitaxial growth conditions,” J. Mater. Chem. C, 2020,8, 10975-10992, https://doi.org/10.1039/D0TC02743J [5] C. Wu et al, A general strategy to ultrasensitive Ga2O3 based self-powered solar-blind photodetectors, Materials Today Physics, Volume 23, 2022, 100643, ISSN 2542-5293, https://doi.org/10.1016/j.mtphys.2022.100643. [6] Zuyong Yan et al, J. Mater. Chem. C, 2020,8, 4502-4509, https://doi.org/10.1039/C9TC06767A