Growth of layered semiconductors by molecular-beam epitaxy: Formation and characterization of GaSe, MoSe₂, and phthalocyanine ultrathin films on sulfur-passivated GaP(111)

Studies of the sulfur passivation of GaP(lll) surfaces with solution treatments, and the subsequent molecular-beam epitaxy growth and characterization of ultrathin films of layered semiconductors on those surfaces are presented in this article. Simple solution passivation procedures allow formation of a S-GaP(lll) semiconductor surface which will support the growth of highly ordered layers of GaSe (Eband gap⁼2.05 eV). These thin GaSe layers then permit the further growth of ordered layers of a semiconductor like MoSe₂ (Eband _gap⁼l.0 eV) or an organic dye such as a phthalocyanine. Reflection high energy electron diffraction and angle-resolved x-ray photoelectron spectroscopy have been used to determine the structure and composition of the surface layers formed at each stage of the deposition process. Absorption spectra are used to characterize the band edge and A_1,2exciton energies of the MoSe₂/GaSe S-GaP(lll) thin films, and the position and shape of the phthalocyanine Q band.