Vol.:(0123456789) 1 3 Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-019-02723-w Growth of uniform MoS 2 layers on free-standing GaN semiconductor for vertical heterojunction device application Pradeep Desai 1  · Ajinkya K. Ranade 1  · Mandar Shinde 1  · Bhagyashri Todankar 1  · Rakesh D. Mahyavanshi 1  · Masaki Tanemura 1  · Golap Kalita 1,2 Received: 7 August 2019 / Accepted: 8 December 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract The feasibility of van der Waals (VdW) heteroepitaxy of molybdenum disulphide (MoS 2 ) layers on gallium nitride (GaN) semiconductor has attracted signifcant interest in heterojunction optoelectronic device applications. Here, we report on the growth of uniform MoS 2 layers on free-standing GaN semiconductor for vertical heterojunction device application. A uniform MoS 2 layer was directly grown on the n-type GaN wafer by sulphurization process of molybdenum oxide thin layer. Raman and scanning electron microscopy (SEM) analyses showed homogenous growth of the few-layers MoS 2 forming a continuous flm, considering the suitability of GaN semiconductor substrate. The fabricated MoS 2 /GaN vertical heterojunction showed excellent rectifying diode characteristics with a photovoltaic photoresponsivity under monochromatic light illumination. The X-ray photoelectron spectroscopy (XPS) studies showed the conduction and valence band ofset values are around 0.44 and 2.3 eV with type II band alignment in the fabricated heterojunction device. This will facilitate efective movement of photo- excited electrons across the MoS 2 –GaN junction, while a large valence band ofset will prevent movement of holes towards the GaN, resulting in low recombination loss to obtain a photovoltage in the heterojunction device. Our study revealed the formation of large-area homogenous MoS 2 layers on GaN wafer for vertical heterojunction device application. 1 Introduction Transition metal dichalcogenides (TMDCs) layers have attracted signifcant interest over the past few years for its fundamental studies and also for its opportunities in opto- electronics device applications [1–8]. Compared to other known layered materials, TMDCs show sizeable bandgap of 1–2 eV with intriguing electronic, optical and structural properties. The relative earth abundance and the suitable light absorption wavelength of the TMDC layers are consid- ered to be alternative materials for high-efciency optoelec- tronic devices [9–13]. A typical layered structure of TMDCs consists of two chalcogen’s hexagonal layers separated by a metal layer. Each of these TMDC layers are held by weak van der Waals force of attraction in the layered crystal. The overall symmetry of TMDCs is hexagonal or rhombohedral, and the metal atoms have octahedral or trigonal prismatic coordination [1, 14–18]. The changes in absorption, pho- toluminescence and photoconductivity are observed with change from bulk to two-dimensional (2D) monolayer in molybdenum disulphide (MoS 2 ) and other TMDCs [14, 19, 20]. Thus, the possibility of modulating energy bandgap with changes in number of layers has been exploited for photoresponse/photodetection at diferent wavelengths [21]. Additionally, due to the absence of the dangling bonds, when compared to many of the bulk semiconductors, TMDC lay- ers have signifcant prospect in integrating with other three- dimensional (3D) semiconductors such as gallium nitride (GaN) for fabricating 2D/3D heterostructures [12, 13, 22, 23]. Recently, growth of MoS 2 layers on GaN and their inter- face characteristics have been explored to develop a novel Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10854-019-02723-w) contains supplementary material, which is available to authorized users. * Pradeep Desai desai.pradeep@zoho.com * Golap Kalita kalita.golap@nitech.ac.jp 1 Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan 2 Frontier Research Institute for Material Science, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan