Thin-Film Deposition of Surface Passivated Black Phosphorus

A single-step, direct silicon-substrate growth of black phosphorus (BP) crystals is achieved in a self-contained short-way transport technique under low-pressure conditions (<1.5 MPa). A 115 nm-thick BP hero single crystal is formed with lateral dimensions of 10 × 85 μm. The synthesis proceeds with Sn, SnI₄, and red phosphorus and has a well-defined phosphorus phase dependency on the SnI₄ concentration. Furthermore, in situ Sn passivation of BP occurs. This allows long-term stability with no sign of any degradation after 4 months of exposure to ambient conditions. Single-crystal BP flakes and multigrain flakes with high- and low-angle grain boundaries are achieved. Electron backscatter diffraction determined crystal growth to be independent of the substrate, which is further supported by successful growth on various substrates, including sapphire, silicon nitride, silicon, and silicon oxide. Cross-sectional transmission electron microscopy of a single crystal flake provides valuable insight into the growth mechanism. Elemental Sn encapsulates BP crystals, and crystalline SnI _x inclusions are found to be scattered throughout the BP crystal. It is suggested that SnI _x inclusions may provide the dominant mechanism for seeding vertical growth. IR absorption measurements for thin and bulk BP recipes show an equal response below E_g dominated by free carrier absorption. FET devices fabricated from thin-film and bulk BP recipes show improved device performance compared to unpassivated BP films of equal thickness with an on/off current ratio >10².