Tracking speed bumps in organic field-effect transistors via pump-probe Kelvin-probe force microscopy

Murawski, Jan; Mönch, Tobias; Milde, Peter; Hein, Moritz; Nicht, S.; Zerweck-Trogisch, U.; Eng, Lukas M.

doi:10.1063/1.4938529

Cited by 20 publications

(25 citation statements)

References 38 publications

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“…The vast majority of SKPM measurements on OFETs use a statically biased device, but there is much to learn by incorporating time‐dependent techniques, especially when investigating properties like switching characteristics. Murawski et al devised a pump‐probe SKPM (pp‐KPFM) method that pushes time resolution into the nanosecond regime . The pump is a voltage pulse delivered to the drain electrode while the probe is a much shorter voltage pulse applied to the SKPM tip in addition to the sinusoidal driving voltage.…”

Section: Characterizing Contact Resistance In Ofetsmentioning

confidence: 99%

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Waldrip

Jurchescu

Gundlach

et al. 2019

Adv Funct Materials

238

261

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Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto-)electronic applications. One example is the organic field-effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.semiconductors with superior intrinsic charge transport properties, there is a stringent need to improve the device properties in order to allow organic devices to fulfill their technological prospectives. In the organic semiconductor community, contact resistance (R C ) has come under increased scrutiny as it is apparent that the final device performance is often domi nated by carrier injection, rather than the transport through the semiconductor layer. Contact resistance can impact OFET development in multiple ways. First, if not accounted for, a high contact resistance may lead to inaccurate extraction of the device parameters. Second, any inaccuracies in parameter extraction can have significant consequences on material development, from generating incorrect structure-property relationships to discarding organic semiconductors whose efficient intrinsic electrical properties have been masked by inefficient contacts. Beyond OFET and semiconductor characterization, analog, low power, and high frequency applications require a greater level of device parameter control than has been obtained in typical DC, high-voltage OFETs. For analog applications, e.g., integration of conditioning circuits such as local sense amps for distributed flexible sensor arrays, nonlinearity of the transistor response inhibits proper functioning and limits applicability of common models used to design circuitry. Low power device development for novel materials are hindered by the high voltage turn-on and current suppression in devices with large R C . Considering high-frequency applications, Klauk suggest...

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Section: Characterizing Contact Resistance In Ofetsmentioning

confidence: 99%

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Waldrip

Jurchescu

Gundlach

et al. 2019

Adv Funct Materials

238

261

View full text Add to dashboard Cite

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“…These EFM photocapacitance experiments stand in contrast to scanning probe microscopy-based variants of optical pump probe techniques, which exploit a nonlinearity to infer ultrafast dynamics by measuring differences in a time-averaged quantity versus a pulse time, delay, or frequency [47,48]. Recent experiments along these lines have measured the surface photovoltage [49][50][51][52] and charge moving through a transistor [53,54] with ultrafast time resolution. In contrast, the origin of sub-cycle time resolution in single-shot, transient EFM experiments is not clearly understood.…”

Section: Introductionmentioning

confidence: 99%

Lagrangian and Impedance-Spectroscopy Treatments of Electric Force Microscopy

2019

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Scanning probe microscopy is often extended beyond simple topographic imaging to study electrical forces and sample properties, with the most widely used experiment being frequency-modulated Kelvin probe force microscopy. The equations commonly used to interpret this frequency-modulated experiment, however, rely on two hidden assumptions. The first assumption is that the tip charge oscillates in phase with the cantilever motion to keep the tip voltage constant. The second assumption is that any changes in the tip-sample interaction happen slowly. Starting from an electro-mechanical model of the cantilever-sample interaction, we use Lagrangian mechanics to derive coupled equations of motion for the cantilever position and charge. We solve these equations analytically using perturbation theory, and, for verification, numerically. This general approach rigorously describes scanned probe experiments even in the case when the usual assumptions of fast tip charging and slowly changing samples properties are violated. We develop a Magnus-expansion approximation to illustrate how abrupt changes in the tip-sample interaction cause abrupt changes in the cantilever amplitude and phase. We show that feedback-free time-resolved electric force microscopy cannot uniquely determine sub-cycle photocapacitance dynamics. We then use first-order perturbation theory to relate cantilever frequency shift and dissipation to the sample impedance even when the tip charge oscillates out of phase with the cantilever motion. Analogous to the treatment of impedance spectroscopy in electrochemistry, we apply this approximation to determine the cantilever frequency shift and dissipation for an arbitrary sample impedance in both local dielectric spectroscopy and broadband local dielectric spectroscopy experiments. The general approaches we develop provide a path forward for rigorously modeling the coupled motion of the cantilever position and charge in the wide range of electrical scanned probe microscopy experiments where the hidden assumptions of the conventional equations are violated or inapplicable. arXiv:1807.01219v2 [cond-mat.mes-hall]

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“…This technique was subsequently used by various groups for the characterization of organic devices [ 23 – 26 ] and, by using bias modulation (BM) KPFM, also for the measurement of the minority carrier lifetime in epitaxial Si solar cell materials [ 27 ]. In a variation of the bias or light modulation approach, a bias-based pump–probe approach (pp-KPFM) was used to measure the charge-carrier dynamics with a time resolution of 2 μs in pentacene-based OFETs [ 28 – 29 ]. Similarly, light-based pp-KPFM was used to measure a charge carrier lifetime in low-temperature grown GaAs of ≈1 ps, currently the best time-resolution that has been demonstrated experimentally for KPFM [ 30 ].…”

Section: Introductionmentioning

confidence: 99%

Artifacts in time-resolved Kelvin probe force microscopy

Sadewasser

Nicoara

Solares

2018

Beilstein J. Nanotechnol.

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Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.

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Tracking speed bumps in organic field-effect transistors via pump-probe Kelvin-probe force microscopy

Cited by 20 publications

References 38 publications

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Lagrangian and Impedance-Spectroscopy Treatments of Electric Force Microscopy

Artifacts in time-resolved Kelvin probe force microscopy

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