Terahertz Electromodulation Spectroscopy for Characterizing Electronic Transport in Organic Semiconductor Thin Films

Terahertz (THz) spectroscopy is a well-established tool for measuring the high-frequency conductance of inorganic semiconductors. Its application to organic semiconductors, however, is challenging, because of the low carrier mobilities in organic materials, which rarely exceed 10cm(2)/Vs. Furthermore, low charge carrier densities in organic field-effect devices lead to sheet conductivities that are often far-below the detection limits of conventional THz techniques. In this contribution, we present the application of THz electromodulation spectroscopy for characterizing charge transport in organic semiconductors. Pulses of THz radiation are transmitted through organic field-effect devices and are time-resolved by electro-optic sampling. A differential transmission signal is obtained by modulating the gate voltage of the devices. This controls charge injection into the semiconductors, where the charge carriers reduce the THz transmission by their Drude response. Advantageous is that a nearly noise-free differential transmission can be obtained. Furthermore, electromodulation allows to sense specifically either injected electrons or holes. Because the method exclusively probes transport of mobile carriers, it provides access to fundamental transport properties, which are difficult to access with conventional characterization methods, such as conductance measurements of organic field-effect transistors. The outstanding property that a relative differential signal is measured allows to obtain charge carrier mobilities with high reliability. Mobilities as small as 1cm(2)/Vs can be probed, which makes THz electromodulation spectroscopy an attractive tool for studying charge transport in most technologically relevant organic semiconductors

Molecular semiconductors and the Ioffe–Regel criterion: A terahertz study on band transport in DBTTT

Terahertz electromodulation spectroscopy provides insight into the physics of charge carrier transport in molecular semiconductors. The work focuses on thin-film devices of dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene. Frequency-resolved data show a Drude-like response of the hole gas in the accumulation region. The temperature dependence of the mobilities follows a T1/2 power law. This indicates that the thermal mean free path of the charge carriers is restricted by disorder. Only a fraction of approximately 5% of the injected carriers fulfills the Ioffe–Regel criterion and participates in band transport.info:eu-repo/semantics/publishe

Probing the momentum relaxation time of charge carriers in ultrathin semiconductor layers

We report on a terahertz time-domain technique for measuring the momentum relaxation time of charge carriers in ultrathin semiconductor layers. The phase sensitive modulation technique directly provides the relaxation time. Time-resolved THz experiments were performed on n-doped GaAs and show precise agreement with data obtained by electrical characterization. The technique is well suited for studying novel materials where parameters such as the charge carriers' effective mass or the carrier density are not known a priori

Impact of surface roughness on conduction in molecular semiconductors

The interface roughness between gate insulator and semiconductor is expected to reduce the conductance of molecular field-effect transistors. This study merges atomic force microscopy data of layer topographies with self-consistent calculations of charge carrier densities and conductances within the channel region. It is found that a roughness equivalent to one monolayer reduces the conductance by nearly 50%. Currents flow mainly within the first monolayer of the semiconductor and along percolation pathways, where charges rarely undergo transfers between adjacent monolayers

Field-induced migration of gold in molecular semiconductors

We report on the degradation of field-effect devices due to the migration of gold from injection contacts into the channel region. The experimental results are obtained by THz spectroscopy on devices with a migration distance of 50 nm. The dependence of the degradation on gate voltage, as well as the partial reversibility, indicates that degradation is caused by field-induced transport of gold ions. The transport is found to be thermally activated with an activation energy independent of the field strength, which suggests that lattice deformations of the molecular semiconductor support the migration of gold

Band Transport and Trapping in Didodecyl[1]benzothieno[3,2-b][1]benzothiophene Probed by Terahertz Spectroscopy

Terahertz electromodulation spectroscopy provides insight into the material-inherent transport properties of charge carriers in organic semiconductors. Experiments on didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT-C12) devices yield for holes an intraband mobility of 9 cm2 V-1 s-1. The short duration of the THz pulses advances the understanding of the hole transport on the molecular scale. The efficient screening of Coulomb potentials leads to a collective response of the hole gas to external fields, which can be well described by the Drude model. Bias stress of the devices generates deep traps that capture mobile holes. Although the resulting polarization across the device hinders the injection of mobile holes, the hole mobilities are not affected.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Reliable Surface Analysis Data of Nanomaterials in Support of Risk Assessment Based on Minimum Information Requirements

The minimum information requirements needed to guarantee high-quality surface analysis data of nanomaterials are described with the aim to provide reliable and traceable information about size, shape, elemental composition and surface chemistry for risk assessment approaches. The widespread surface analysis methods electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were considered. The complete analysis sequence from sample preparation, over measurements, to data analysis and data format for reporting and archiving is outlined. All selected methods are used in surface analysis since many years so that many aspects of the analysis (including (meta)data formats) are already standardized. As a practical analysis use case, two coated TiO2 reference nanoparticulate samples, which are available on the Joint Research Centre (JRC) repository, were selected. The added value of the complementary analysis is highlighted based on the minimum information requirements, which are well-defined for the analysis methods selected. The present paper is supposed to serve primarily as a source of understanding of the high standardization level already available for the high-quality data in surface analysis of nanomaterials as reliable input for the nanosafety community