Nanoscale Surface Morphology and Rectifying Behavior of a Bulk Single-Crystal Organic Semiconductor

Single-crystal organic semiconductors represent perfect systems not only for studying basic science associated with transport of polaronic charge carriers but also for investigating the upper limits of mobilities in thin-film organic devices for flexible displays and other emerging electronic applications. The field-effect transistor provides an important tool to explore the transport of field-induced charge carriers at the surface of organic semiconductors. Special care, however, must be taken during the device fabrication to avoid degradation of the critical interface between the semiconductor and the dielectric. High material purity, excellent crystalline quality, and nanoscale morphological smoothness are among the characteristics of this interface that are crucial to obtaining high-performance devices and test structures for studies of intrinsic phenomena. Local surface measurements of organic semiconductors at the active interfaces of conventional transistors are difficult to perform, if not impossible, due to the inaccessibility of the semiconductor/gate dielectric interface by many classes of useful probes. A new method for building transistors on the pristine surfaces of bulk single crystals uses free-space dielectrics (i.e., vacuum or air) to eliminate completely any contact or processing of the active surface of the semiconductor. This technique, which provides a reversible ability to assemble and dissemble the devices repetitively, recently enabled observation of intrinsic transport and transport anisotropy in rubrene. Other groups, using different techniques, have observed similarly high mobilities and orientational anisotropy. Unlike organic monolayers and thin films for which several scanning tunneling microscopy (STM) studies have been reported in the past, bulk organic crystals with thicknesses ranging from a few micrometers up to millimeters cannot be imaged by STM because of their inherent high electrical resistivity. We found that the only STM study of a bulk organic crystal was reported in 1988 by Sleator and Tycko on tetrathiafulvalene–tetracyanoquinodimethane (TTF–TCNQ), later developed by Wang et al.. These authors were able to image the surface of TTF–TCNQ crystals down to molecular resolution. However, being a charge-transfer salt, TTF–TCNQ possesses a metallic conductivity and therefore is much different from an organic semiconductor in terms of charge transport. Furthermore, this early study is limited to surface morphology and does not report on local transport properties by scanning tunneling spectroscopy (STS). In this paper, we report on a nanoscale investigation of the critical interface between bulk single crystals of rubrene and air (i.e., the gate dielectric in the above-mentioned transistors) by means of STM and current–voltage (I–V) spectroscopy. In spite of dimensions as large as a few millimeters, the high conductivity and structural quality of undoped rubrene single crystals allows the imaging of their surfaces down to molecular resolution. STM images, combined with atomic force microscopy (AFM) and X-ray diffraction (XRD), reveal directly the position and orientation of individual molecules in the a–b plane which is used for fabrication of high-mobility field-effect transistors. Besides, local I–V curves recorded by STS in the dark and under light on the rubrene single crystals indicate a strongly rectifying p-type behavior. These nearly ideal I–V characteristics observed at the nanoscale are also observed on point-contact diodes on top of the rubrene crystals. Such remarkable transport properties indicate that the rubrene crystals are free of the surface transport channels that are commonly observed in inorganic semiconductors owing to dangling bonds. High-quality single crystals of rubrene were grown by physical-vapor transport in hydrogen. Figure 1A shows an optical image of a typical crystal with an elongated hexagonal plate shape and dimensions of 4 mm × 1.5 mm × 0.5 mm. The orientation of the crystal axis was determined by XRD using Cu Ka radiation. Figure 1C shows the 2h X-ray (45 kV, 40 mA, 2 s per angle step) intensity data collected on this crystal. The polar plots show that, as typically observed for crystals belonging to the orthorhombic pyramidal point group, the c-axis is normal to the top surface (slow-growing face) and the b-axis C O M M U N IC A TI O N S