The authors have studied the effect of substrate atomic steps on the azimuthal alignment of crystals in polycrystalline pentacene films. The substrates used were Si (111) with a low miscut angle and these were annealed at high temperature in ultrahigh vacuum before the pentacene deposition to produce surfaces with atomically flat terraces and arrays of parallel atomic steps. By depositing pentacene on these heated samples, at a low deposition rate, the authors have succeeded in obtaining significant azimuthal alignment of the pentacene crystals relative to the atomic steps.

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Atomically flat surfaces can be obtained by high-temperature annealing in UHV of specially patterned silicon samples. Thin silicon oxide layers were grown by dry oxidation on three types of surfaces: (a) atomically flat surfaces, (b) normal (stepped) surfaces cleaned in UHV by the same high-temperature annealing and (c) normal wafer surfaces, which underwent just an RCA chemical cleaning before oxidation. Atomic force microscopy (AFM) was performed to reveal the topography of the surfaces. Aluminum pads were deposited on these oxidized surfaces using photolithography techniques. The leakage current through the oxide was measured for all three cases. Our results show that the smoother the surface before oxidation, the smaller the leakage current.

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We have studied the effect of substrate atomic steps on the azimuthal alignment of vapor-deposited pentacene crystals. Si(111) substrates with a low miscut angle were annealed at high temperature in ultra-high vacuum before the pentacene deposition; this produced surfaces with atomically flat terraces and arrays of parallel atomic steps. AFM analysis shows that pentacene deposited on these heated samples, at a low deposition rate, results in significant alignment of the pentacene crystals along the atomic steps.