Probing Local Electronic Transport at the Organic Single‐Crystal/Dielectric Interface

Abstract

Electrostatic doping in field-effect transistor (FET) configuration is a powerful tool for investigating electronic transport properties of complex materials such as correlated oxides or organic semiconductors.[1] Key issues in electrostatically doped organic semiconductors are carrier mobility, contact resistance, structural defects, and interfacial phenomena.To disentangle intrinsic transport features from other contributions, scanning probe microscopy potentiometric techniques can be used for investigations of the local electric potential under bias, but were so far restricted to sub-100 nm thick evaporated and solution-processed polycrystalline [2,3] and semi-crystalline [4][5][6] organic thin film FETs. Bulk (i.e., lm to mm thick) single-crystal organic field-effect transistors [7][8][9] (SCOFETs) offer alternative and attractive paths for probing ultimate performances of defect-free molecular semiconductors, with charge carrier mobility values as large as 10-40 cm 2 V -1 s -1 for rubrene [10][11][12][13] and pentacene. [14] While the nanostructure and surface electronic states of organic single crystals have been successfully probed by scanning tunnelling microscopy (STM) and its spectroscopic modes, [15] to our knowledge, there are no reports of local investigations under electrostatic doping on operating SCOFETs. A recent report of high mobility values in "ultra-thin" (thicknesses in the 100 nm-1 lm range) SCOFETs [16] prompted us to check the possibility of investigating the local electronic transport under gate and source-drain bias on sub-lm thick single-crystals. We here demonstrate that scanning Kelvin probe microscopy [17] (SKPM), an atomic force microscopy (AFM)-based potentiometric technique, [18] allows for an in-depth exploration of the local electrical potential at the organic/dielectric interface in sub-lm thick organic single-crystals (OSCs), [16] opening novel perspectives for a deeper understanding of intrinsic charge transport and interfacial effects in semi-conducting singlecrystals. [7,19,20] Electrical ( Fig. 1) and SKPM (Figs. 2 and 3) investigations were carried out at ambient conditions on electrostatically bonded sub-lm thick rubrene single-crystals on Si back-gated FET structures with SiO 2 dielectric and metal source-drain contacts recessed in the oxide (see the Experimental section).[21] A member of the oligoacene family, [20] e.g., rubrene, was chosen as a model organic semiconductor, both for its high charge carrier mobility values and for its low sensitivity to contamination by oxygen in ambient conditions. The quality of rubrene single-crystals [22,23] was confirmed by polarized optical microscopy (POM) experiments [10,16] for which large birefringence effects were observed under cross-polarised light and by AFM characterisations of the surface morphology which displays flat terraces with single mono-layers steps (Fig. 3b). However, for some SCOFET devices (Fig. 2c), the surface morphology probed by AFM over the electrostatically bonded single-crystals presents irregularities...

Self-assembly of Rubrene on Copper Surfaces

“…33,39 A hole mobility as high as 2.5 cm 2 /Vs has recently been achieved in rubrene thin film transistors. 40 The excellent charge transport properties of rubrene have motivated several fundamental studies of rubrene single crystals, [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] thin films, [43][44][45][46][47] and self-assembled structures. 28,[48][49][50] Rubrene is a nonplanar and flexible molecule consisting of a tetracene backbone with two pairs of phenyl substituents symmetrically attached on either side of the backbone ( Figure 1).…”

Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy

“…Previous studies of rubrene single crystal (SC) OTFTs utilized SKPM data, specifically the slope of the potential profile in the device channel (V/µm), to directly calculate the intrinsic charge mobilities of individual devices (µ). 8 Mobilities calculated as such provide a more direct measurement of the intrinsic charge transport in the organic crystals themselves, and are typically higher than those derived from conventional I-V measurements of FET performance. 3 B y nature, conventional I-V measurements yield charge mobility values that are a convolution of several factors including the intrinsic charge mobility of the organic material itself, charge injection effects at the source and drain contacts, and structural variations in the active organic material such as grain boundaries, interface states, defects, etc.…”

“…© 2010 American Institute of Physics Scanning Kelvin probe microscopy (SKPM) has demonstrated utility in correlating the relationship between film structure and charge transport in organic thin-film transistor (OTFT) devices as well as providing a detailed view of charge injection at the source and drain contacts. [1][2][3][4][5][6][7] While traditional field effect transistor (FET) I-V (performance) measurements yield the average mobility of the device, SKPM offers the advantage of probing the local mobility of the device. Previous studies of rubrene single crystal (SC) OTFTs utilized SKPM data, specifically the slope of the potential profile in the device channel (V/µm), to directly calculate the intrinsic charge mobilities of individual devices (µ).…”

Influence of film structure and light on charge trapping and dissipation dynamics in spun-cast organic thin-film transistors measured by scanning Kelvin probe microscopy

“…[11][12][13][14] Proximal probe techniques, such as scanning Kelvin probe microscopy (SKPM), allow us to fully explore the structure-property relationships in working OTFT structures. [14][15][16][17][18][19][20][21][22][23][24][25] SKPM provides the ability to monitor changes in charge transport phenomena in both space and time, a capability not afforded by traditional electrical performance measurements a Author to whom correspondence should be addressed. Electronic mail: lucile.teague@srnl.doe.gov alone.…”