Electronic structure and band alignment of 9,10-phenanthrenequinone passivated silicon surfaces

a r t i c l e i n f o In this work we demonstrate that the room-temperature deposition of the organic molecule 9,10-phenanthrenequinone (PQ) reduces the surface defect density of the silicon (100) surface by chemically bonding to the surface dangling bonds. Using various spectroscopic measurements we have investigated the electronic structure and band alignment properties of the PQ/Si interface. The band-bending at the PQ-passivated silicon surface is negligible for both n-and p-type substrates, demonstrating a low density of surface defects. Finally we show that PQ forms a semiconducting wide-bandgap type-I heterojunction with silicon. There is considerable interest in integrating carbon-based organic semiconductors with inorganic semiconductors like silicon [1–3]. While the organic semiconductors can be easily layered with different bandgaps and are potentially low-cost, traditional silicon technology has higher carrier mobilities. Integrating the two to form hybrid organic/silicon interfaces may offer a novel way to improve the device capabilities. For example, a wide-bandgap heterojunction realized using organic/silicon interfaces can reduce parasitic carrier recombi-nation in crystalline silicon solar cells [4], be used to detect gases [5], and enable bio-compatible sensors [6]. Amorphous organic thin films and crystalline silicon surfaces are inherently dissimilar materials, and from an electrical perspective are difficult to integrate. The surface silicon atoms do not chemically react with many classes of organic molecules that might be used to form an organic/silicon interface, and so the silicon valencies can remain unsatisfied. The resulting dangling-bonds lead to mid-bandgap energy-levels at the silicon surface, which degrade device performance by increasing carrier recombination and preventing modulation of the Fermi-level [7]. Recently the electronic passivation of the silicon (100) surface was demonstrated [8] using the organic molecule, 9,10 phenanthrenequi-none (PQ) (Fig. 1), which can react with surface silicon atoms by a cycloaddition " redox " process. Minority carrier lifetime measurements yielded surface-state densities of only ~10 11 cm − 2 at the PQ-silicon interface, rivaling the electronic quality of silicon-dioxide passivated surfaces. Further, measurements of metal/insulator/PQ/silicon devices proved that the Fermi level at the PQ-passivated silicon surface is not " pinned " and can be easily modulated over a wide energy range. This is useful for hybrid devices because once the surface is electronically passivated, further organic films can be deposited to tailor the heterojunction energy levels. Furthermore, the π-conjugated aromatic structure of PQ imparts semiconducting properties, so the PQ layer can act as a non-insulating bridge between the crystalline and …