Ashutosh Tripathi

Space charge limited transport and time of flight measurements in tetracene single crystals: A comparative study

We report on a systematic study of electronic transport in tetracene single crystals by means of space charge limited current spectroscopy and time of flight measurements. Both I–V and time of flight measurements show that the room-temperature effective hole mobility reaches values close to μ≃1 cm2/V s and that, within a range of temperatures, the mobility increases with decreasing temperature. The experimental results further allow the characterization of different aspects of the tetracene crystals. In particular, the effects of both deep and shallow traps are clearly visible and can be used to estimate their densities and characteristic energies. The results presented in this article show that the combination of I–V measurements and time of flight spectroscopy is very effective in characterizing several different aspects of electronic transport through organic crystals.

Kramers-Kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene

The Kramers-Kronig relations between the real and imaginary parts of a response function are widely used in solid-state physics to evaluate the corresponding quantity if only one component is measured. They are among the most fundamental statements since only based on the analytical behavior and causal nature of the material response [Phys. Rev. 104, 1760-1770 (1956)]. Optical losses, for instance, can be obtained from the dispersion of the dielectric constant at all wavelengths, and vice versa [Handbook of optical constants of solids, Vol. 1, p. 35]. Although the general validity was never casted into doubt, it is a longstanding problem that Kramers-Kronig relations cannot simply be applied to anisotropic crystalline materials because contributions from different directions mix in a frequency-dependent way. Here we present a general method to identify frequency-independent principal polarizability directions for which the Kramers-Kronig relations are obeyed even in materials with lowest symmetry. Using generalized spectroscopic ellipsometry on a single crystal surface of triclinic pentacene, as an example, enables us to evaluate the complex dielectric constant and to compare it with band-structure calculations along the crystallographic directions. A general recipe is provided how to proceed from a macroscopic measurement on a low symmetry crystal plane to the microscopic dielectric properties of the unit cell, along whose axes the Kramers-Kronig relations hold.

Mechanical and Electronic Properties of Thin-Film Transistors on Plastic, and Their Integration in Flexible Electronic Applications

The increasing interest in flexible electronics and flexible displays raises questions regarding the inherent mechanical properties of the electronic materials used. Here, the mechanical behavior of thin-film transistors used in active-matrix displays is considered. The change of electrical performance of thin-film semiconductor materials under mechanical stress is studied, including amorphous oxide semiconductors. This study comprises an experimental part, in which transistor structures are characterized under different mechanical loads, as well as a theoretical part, in which the changes in energy band structures in the presence of stress and strain are investigated. The performance of amorphous oxide semiconductors are compared to reported results on organic semiconductors and covalent semiconductors, i.e., amorphous silicon and polysilicon. In order to compare the semiconductor materials, it is required to include the influence of the other transistor layers on the strain profile. The bending limits are investigated, and shown to be due to failures in the gate dielectric and/or the contacts. Design rules are proposed to minimize strain in transistor stacks and in transistor arrays. Finally, an overview of the present and future applications of flexible thin-film transistors is given, and the suitability of the different material classes for those applications is assessed.

Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors Part I: Long-Channel Devices

Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numerical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent analysis based on experiments, numerical simulations, and analytical modeling is provided.

Low-voltage gallium–indium–zinc–oxide thin film transistors based logic circuits on thin plastic foil: Building blocks for radio frequency identification application

In this work a technology to fabricate low-voltage amorphous gallium-indium-zinc oxide thin film transistors (TFTs) based integrated circuits on 25 µm foils is presented. High performance TFTs were fabricated at low processing temperatures (<150 °C) with field effect mobility around 17 cm2 /V s. The technology is demonstrated with circuit building blocks relevant for radio frequency identification applications such as high-frequency functional code generators and efficient rectifiers. The integration level is about 300 transistors.

Multilevel Information Storage in Ferroelectric Polymer Memories

Multibit memory devices based on the ferroelectric copolymer P(VDF-TrFE) (poly-(vinylidenefluoride-trifluoroethylene)) are presented. Multilevel microstructures are fabricated by thermal imprinting of spin-coated ferroelectric polymer film using a rigid Si template. Multibit storage in capacitors and thin-film transistor memory is realized by implementing imprinted ferroelectric polymer films as the insulator and gate dielectric layers, respectively. Copyright

Correlation between ambipolar transport and structural phase transition in diindenoperylene single crystals

Diindenoperylene (DIP) single crystals were grown by vapor phase sublimation growth technique, yielding crystals with preferential growth in the ab crystallographic plane. Time-of-flight (TOF) and x-ray measurements were performed over the temperature range between 300 and 405K. By TOF the mobility for electrons is found to be higher by one order of magnitude than for holes over the studied temperature range. The temperature dependent mobility behavior indicates the effect of structural phase transition in accordance with temperature dependent x-ray measurements. Observation of electron and hole transport in DIP single crystals makes this compound interesting for ambipolar organic electronics.

Infrared spectroscopy on the charge accumulation layer in rubrene single crystals

To get access to the intrinsic properties of organic semiconductors, investigations on single crystals are essential. The authors report on far- and mid-infrared spectroscopies of the charge accumulation layer in an organic field-effect transistor fabricated on a rubrene single crystal. By charge modulation spectroscopy in the range between 70 and 4750cm−1, they were able to detect the Drude response of the accumulated charges in the channel. From this they can extract important intrinsic transport parameters such as the mobility, the plasma frequency, the effective mass, and the scattering rate.

Electrical Characterization of Flexible InGaZnO Transistors and 8-b Transponder Chip Down to a Bending Radius of 2 mm

In this paper, we present the fabrication and characterization of highly flexible indium-gallium-zinc-oxide (IGZO)-based thin-film transistors (TFTs) and integrated circuits on a transparent and thin polymer substrate. Mechanical reliability tests are performed under bending conditions down to a bending radius of 2 mm. All the TFT parameters show only a weak dependence on mechanical strain. TFTs can withstand bending strain up to 0.75% without any significant change in the device operation. Mechanical reliability is further demonstrated to a higher TFT integration level by ring oscillators and 8-b transponder chips operating at a bending radius of 2 mm.

Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors—Part II: Contact Resistance in Short Channel Devices

Short-channel zinc oxide (ZnO) thin-film transistors (TFTs) are investigated in a wide range of temperatures and bias conditions. Scaling down the channel length, the TFT performance is seriously affected by contact resistances, which depend on gate voltage and temperature. To account for the contact resistances, the transistor is ideally split in three parts. The contact regions are modeled as two separate transistors with a fixed channel length and an exponential distribution of localized states, whereas the channel is treated as reported in Part I. The overall model reproduces the measured characteristics at different channel length, with a single set of physical and geometrical parameters. It can be readily implemented in a circuit simulator. Numerical simulations confirm the validity of the model approach and are used to evaluate the impact of nonidealities at the electrode/semiconductor interface.