Field-effect transistors based on organic single crystals grown by an improved vapor phase method

High-quality organic single crystals are produced directly onto the substrates using an improved vapor phase method. Unlike the conventional vapor phase methods, the present method is characterized by forming a large-sized crystal to which semiconductor devices can readily be made. The relevant method requires small space of only a 10-cm cube in which a couple of plates are put in close proximity. The crystal growth is carried out nearly at the thermodynamic equilibrium within the narrow space surrounded with the two plates. Thin single crystals of several hundreds of micrometers in size are grown on one of those plates. For the organic materials to be crystallized, we have chosen 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) and 5,5-diphenyl-2,2′:5′,2″:5″,2:5,2-quinquethiophene (P5T) from among thiophene/phenylene co-oligomers. The resulting crystals are well-defined polygons, each side reflecting the specific crystallographic orientation. In particular, those grown on self-assembled monolayers are exceedingly flat and free from cracks. We have directly fabricated top-contact field-effect transistors on these crystals. The devices exhibit the excellent performance and keep it both in air and in vacuum for a maximum of a hundred days.     

High performance p-type organic thin film transistors with an intrinsically photopatternable,ultrathin polymer dielectric layer

A high-performing bottom-gate top-contact pentacene-based oTFT technology with an ultrathin (25–48 nm) and electrically dense photopatternable polymeric gate dielectric layer is reported. The photosensitive polymer poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) is patterned directly by UV-exposure (λ = 254 nm) at a dose typical for conventionally used negative photoresists without the need for any additional photoinitiator. The polymer itself undergoes a photo-Fries rearrangement reaction under UV illumination, which is accompanied by a selective cross-linking of the macromolecules, leading to a change in solubility in organic solvents. This crosslinking reaction and the negative photoresist behavior are investigated by means of sol–gel analysis. The resulting transistors show a field-effect mobility up to 0.8 cm² V⁻¹ s⁻¹ at an operation voltage as low as −4.5 V. The ultra-low subthreshold swing in the order of 0.1 V dec⁻¹ as well as the completely hysteresis-free transistor characteristics are indicating a very low interface trap density. It can be shown that the device performance is completely stable upon UV-irradiation and development according to a very robust chemical rearrangement. The excellent interface properties, the high stability and the small thickness make the PNDPE gate dielectric a promising candidate for fast organic electronic circuits.     

Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

It has been demonstrated that the modification of electrodes with self-assembled monolayers (SAMs) reduces the contact resistance and improves the device performances of organic field-effect transistors (OFETs). However, it has been difficult to judge if the contact resistance was reduced by the change in the electronic properties or by the change in the morphology of the metal–organic interface caused by the SAM modification because they have been difficult to be separately assessed. We have directly investigated the local impedance and the potential difference at the electrode–channel interfaces of the OFETs with and without modification of the electrodes by a pentafluorobenzenethiol SAM using frequency-modulation scanning impedance microscopy (FM-SIM). The potential profile measurement and the FM-SIM measurement at the interface showed that the improvement of the field-effect mobility in the SAM-modified OFET was caused by the reduction of the energy level mismatch, namely, the hole injection barrier at the source–channel interface, presumably with the reduction of the hole trap sites at the source–channel interface.     

Stabilization of space charge polarization in ion-dispersed gate dielectric layer of organic transistors by ultraviolet illumination for write-once read-many memory

In this work, a new type organic field effect transistor (OFET) based write-once read-many memory (WORM) device was developed. The device uses an ultraviolet (UV) cross-linkable matrix polymer mixed with ionic compounds to form an ion-dispersed gate dielectric layer. Under an applied gate voltage bias, migration of cations and anions in opposite directions forms space charge polarization in the gate dielectric layer, resulting in change of the electrical characteristics. It is shown that, with UV illumination to cross-link the matrix polymer, the formed space charge polarization can be stabilized. Therefore, the OFET can be operated as a WORM with the applied voltage bias to define the polarization and in turn the stored data, and the UV illumination to stabilize the stored data.     

Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors

We have modeled the dependence on the gate voltage of the bulk contact resistance and interface contact resistance in staggered polycrystalline organic thin film transistors. In the specific, we have investigated how traps, at the grain boundaries of an organic semiconductor thin film layer placed between the metal electrode and the active layer, can contribute to the bulk contact resistance. In order to the take into account this contribution, within the frame of the grain boundary trapping model (GBTM), a model of the energy barrier E_B, which emerges between the accumulation layer at the organic semiconductor/insulator interface and injecting contact, has been proposed. Moreover, the lowering of the energy barrier at the contacts interface region has been included by considering the influence of the electric field generated by the accumulation layer on the injection of carriers at the source and on the collection of charges from the accumulation layer to the drain contact. This work outlines both a Schottky barrier lowering, determined by the accumulation layer opposite the source electrode, as well as a Poole-Frenkel mechanism determined by the electric field of the accumulation layer active at the drain contact region. Finally it is provided and tested an analytical equation of our model for the contact resistance, summarizing the Poole-Frenkel and Schottky barrier lowering contribution with the grain boundary trapping model.     

Preparation and properties of thin parylene layers as the gate dielectrics for organic field effect transistors

We report on the fabrication and characterization of parylene C thin layers for organic electronic devices passivation and gate dielectric of organic field effect transistors (OFETs) development. The investigated thin parylene layers were deposited from the vapour phase in thickness ranging from 3 to 800 nm at room temperature. The thickness and surface morphology of parylene layers were characterized by ellipsometry and AFM technique. The quality of parylene structures were analysed by X-ray reflectivity and diffraction as well as micro-Raman spectroscopy. The measurements confirmed perfect homogeneity and structural properties of parylene layers. Two types of pentacene OFETs were prepared on the silicone dioxide and parylene surface with bottom contact structures. The results demonstrated that using parylene, as the gate dielectric layer is an effective method to fabricate OFETs with improved electric characteristics.     

Radiation characteristics of wide-angle H-plane sectoral horn loaded with dielectric of multiangular shape

Radiation characteristics of H-plane sectoral horns can be improved or modified by inserting a proper dielectric loading. A study of the radiation of wide-angle sectoral horns with a multiangular dielectric insert that behaves as a lens is presented. Two types of loading are considered, the first in the waveguide feed, the second in the aperture. The method of analysis is the domain product technique utilizing Mathieu function expansions. Numerical investigation has shown that both configurations can provide improvements, but the first lens possesses certain advantages in comparison with the second one. Specifically, it has smaller dimensions and allows minimization of the reflected wave amplitude. Proper choice of the shape of the first insert and material of the dielectric can ensure reduction in the beamwidth, lowering the side lobe level and low return losses in a broad band of frequencies.     

Bottom-contact small-molecule n-type organic field effect transistors achieved via simultaneous modification of electrode and dielectric surfaces

Low-voltage, n-type organic field effect transistors (OFETs) with simultaneously modified bottom-contact (BC) electrodes and dielectric were compared to their top-contact (TC) counterparts. The devices modified with 6-phenoxyhexylphosphonic acid (Ph6PA) self-assembled monolayer (SAM) showed similar performance, morphology, and contact resistance. Electron mobility of C₆₀ devices were 0.212 and 0.320 cm² V⁻¹ s⁻¹ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) devices were 0.04 and 0.06 cm² V⁻¹ s⁻¹ for TC and BC devices, respectively. Low contact resistance between 11 and 45 kΩ cm was found regardless of device architecture or n-type semiconductor used. This work shows it is possible to fabricate solution processable low-voltage bottom-contact devices with performance that is similar or better than their top-contact counterparts without the addition of complex and time-consuming processing steps.     

Holography and plasma oxidation for uniform nanoscale two dimensional channel formation of vertical organic field-effect transistors with suppressed gate leakage current

Vertical organic field-effect transistors (VOFETs) with nanoscale channel openings have been fabricated using pentacene as an active layer material. To achieve uniform nanoscale two-dimensional channel openings, a laser holography lithography has been introduced. Uniformly distributed and well-aligned holes with 250 nm diameter were successfully obtained with the laser holography lithography. VOFET devices with these channel openings have shown high on/off ratio of about 10³ without any further treatment. Gate leakage current was also decreased with an additional insulating layer generated on the gate electrode sidewall via plasma oxidation.     

Length scaling of the Double Gate Tunnel FET with a high-K gate dielectric

In this paper, the length scaling of the silicon Double Gate Tunnel Field Effect Transistor (DG Tunnel FET) is studied. It is found that scaling limits are reached sooner by Tunnel FETs with an SiO₂ gate dielectric, while those with a high-K dielectric can be scaled further before threshold voltage, and average and point subthreshold swing are affected. It is demonstrated that the scaling of the high-K Tunnel FET is completely different than that of conventional MOS transistors. An outstanding feature of the Tunnel FET switch is that length scaling has a much weaker impact on device characteristics than does gate control (e.g. the use of a high-K dielectric), which primarily dictates the tunneling barrier width and consequently, device conduction. This paper demonstrates that while some improvements are observed, the length scaling does not dramatically affect switch figures of merit such as subthreshold slope, I_on and I_off down to about 20 nm, and an optimized device design can be extended over a much larger window of sub-micron dimensions, compared to the MOSFET. A discussion of the length dependence of the transconductance, g_m, and output conductance, g_ds of the Tunnel FET is presented for the first time.     

Analysis of CZT crystals and detectors grown in russia and the ukraine by high-pressure bridgman methods

Sandia National Laboratories (SNL) is leading an effort to evaluate vertical high pressure Bridgman (VHPB) Cd_1−xZn_xTe (CZT) crystals grown in the former Soviet Union (FSU) (Ukraine and Russia), in order to study the parameters limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04<x<0.25, has been determined by methods such as proton-induced x-ray emission (PIXE), x-ray diffraction (XRD), microprobe analysis and laser ablation ICP mass spectroscopy (LA-ICP/MS). Other methods such as triaxial double crystal x-ray diffraction (TADXRD), infrared transmission spectroscopy (IR), atomic force microscopy (AFM), thermoelectric emission spectroscopy (TEES) and laser induced transient charge technique (TCT) were also used to evaluate the material properties. We have measured the zinc distribution in a CZT ingot along the axial direction and also its homogeneity. The (Cd+Zn)/Te average ratio measured on the Ukraine crystals was 1.2, compared to the ratio of 0.9–1.06 on the Russian ingots. The IR transmission showed highly decorated grain boundaries with precipitates and hollow bubbles. Microprobe elemental analysis and LA-ICP/MS showed carbon precipitates in the CZT bulk and carbon deposits along grain boundaries. The higher concentration of impurities and the imperfect crystallinity lead to shorter electron and hole lifetimes in the range of 0.5–2 and 0.1 μs, respectively, compared to 3–20 and 1–7 μs measured on U.S. spectrometer grade CZT detectors. These results are consistent with the lower resistivity and worse crystalline perfection of these crystals, compared to U.S.-grown CZT. However, recently grown CZT from FSU exhibited better detector performance and good response to alpha particles.