In recent years, several well-established 3D substrates have been considered for the van der Waals epitaxy of 2D-transition metal dichalcogenides, yet require a proper surface passivation treatment. In this work, we study the structural and electronic properties of well-ordered Se-terminated $\mathrm{GaP}{\left(111\right)}_{\mathrm{B}}$ surfaces by molecular beam epitaxy. We emphasize the key role of using a combination of atomic Hydrogen and cracked phosphine at a relatively low temperature to prepare a smooth P-rich surface with a ($2\times 2$) reconstruction of $n$ and $p$ doped GaP(111). We then investigate the evolution of the surface electronic properties after a selenium flux exposiure. The $\mathrm{GaP}{\left(111\right)}_{\mathrm{B}}$ surface exhibits a systematic change in reconstruction from a ($2\times 2$) to a ($1\times 1$) pattern, while showing a change in chemical environment mainly in the outermost P-surface layer. We show the absence of a GaSe layer formation through angle-resolved photoemission spectroscopy measurements, while demonstrating that the band structure of the Se-passivated surface originates from the $\mathrm{GaP}{\left(111\right)}_{\mathrm{B}}$ one. We then emphasize the predominant effect of a low selenium flux, while demonstrating an effective partial electronic passivation of the surface through the evolution of the band bending and the surface dipole measured with ultraviolet photoemission spectroscopy. This realization of Se-terminated $\mathrm{GaP}{\left(111\right)}_{\mathrm{B}}$ substrate paves the way to new 2D/3D heterostructure development with great interest in nanoelectronic and optoelectronic applications.

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• Electronic structure
• van der Waals interaction
• Transition metal dichalcogenides
• Low-energy electron diffraction
• Molecular beam epitaxy
• X-ray photoelectron spectroscopy