Passivation of Exfoliated Black Phosphorus Transistors Against Ambient Degradation*

Unencapsulated exfoliated black phosphorus field-effect transistors are found to rapidly degrade upon exposure to ambient conditions, causing large increases in threshold voltage after only 6 h in ambient, followed by a $\sim$ 10$^{3}$ decrease in FET on/off ratio and mobility after 48 h. Careful investigation into the cause of this degradation suggests that H$_{2}$O irreversibly reacts with unprotected, exfoliated BP to form oxidized phosphorus species, as observed by AFM, TEM, XPS, Fourier transform infrared spectroscopy, and electrostatic force microscopy. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers as opposed to hydrophilic SiO$_{2}$, implicating an edge-based intercalation of O$_{2}$ saturated H$_{2}$O in BP as the cause of degradation. Atomic layer deposited AlO$_{\mathrm{x}}$ overlayers were found to suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of $\sim$ 10$^{3}$ and mobilities of $\sim$ 100 cm$^{2}$/(Vs) for over one month in ambient, demonstrating the effective passivation of BP flakes against ambient degradation [1]. \\[4pt] [1] J.D. Wood, S.A. Wells \textit{et al., Nano Lett}. \textbf{ASAP}, DOI: 10.1021/nl5032293