Top-contact Geometry Research Articles

Design of Pentacene Thin-Film Transistor Based Hydrogen Gas Sensor with High-K Dielectric Materials for High Sensitivity

Electrical properties of an organic field-effect transistor were modelled in top gate top contact (TGTC) geometry and H2 gas sensors were designed for increased sensitivity based on the structure. Safety concerns related to hydrogen usage must be addressed; these hazardous characteristics include a wide flammable range (4%–75%) that results in a rapid burning velocity, a low minimum ignition energy (0.017 mJ), a high heat of combustion (143 kJ g−1), and the high diffusivity of hydrogen gas (0.61 cm2 s−1 in the air). These characteristics make it impossible to control hydrogen combustion after a specific time. All simulations were performed in the Silvaco TCAD ATLAS tool. We analysed the driving principle of gas sensors and introduced gas sensing properties in OFET using platinum metal at the gate electrode for H2 gas detection. IOFF, ION, and VTH are sensitivity parameters that alter when the metalwork function of the gate changes with respect to the gas present on it. The designed sensor was analysed for different dielectric materials. Results demonstrate that the increase in sensitivity for OFET-based H2 sensors is 73.4%, 80.7%, 90.5%, and 95.6% when the work function changes by 50, 100, 150, and 200 meV for Pt gate electrodes with an increase in dielectric value of insulating layer from SiO2 (3.9) to La2O3 (27). Results were compared with the In1-xGaxAs CGNWFET-based H2 sensor as the work function varies at 200 meV,the sensitivity enhancement with OFET-based H2 sensors is 8.09%.

Au-MoS2 contacts: Quantum transport simulations using a continuum description

We present a novel method of modeling the contact between a metal and a two-dimensional semiconductor. Using Au on MoS2 as an example, we self-consistently solve the Schrödinger and Poisson equations and obtain the charge density in the contact. We consider open boundary conditions using the quantum transmitting boundary method and model the electron current through the contact region. We then investigate the effect of effective Schottky barrier height, electrostatic doping, and length of the overlap region on the contact resistance in a top contact geometry. By using data from experiments or from ab initio calculations for the fitting of parameters, such as the effective Schottky barrier height, the model can be used to efficiently obtain the contact resistance and, therefore, the quality of the contact. Furthermore, we investigate the effect of sampling of the Brillouin zone in the transverse direction on the numerical calculation of key quantities, such as contact resistance and free charge density. Additionally, we show that the boundary conditions applied to the Poisson equation during the calculation of the free charge density have a significant impact on the calculated contact resistance and that the impact is more pronounced in heterostructures with a larger Schottky barrier. We found that the contact resistance may be significantly underestimated, by up to one order of magnitude, when the height of the simulation domain is not large enough.

Edge contacts accelerate the response of MoS2 photodetectors.

We use a facile plasma etching process to define contacts with an embedded edge geometry for multilayer MoS2 photodetectors. Compared to the conventional top contact geometry, the detector response time is accelerated by more than an order of magnitude by this action. We attribute this improvement to the higher in-plane mobility and direct contacting of the individual MoS2 layers in the edge geometry. With this method, we demonstrate electrical 3 dB bandwidths of up to 18 MHz which is one of the highest values reported for pure MoS2 photodetectors. We anticipate that this approach should also be applicable to other layered materials, guiding a way to faster next-generation photodetectors.

Extraction of intrinsic contact resistance in organic thin-film transistors with single channel length and high capacitance density

Accurate characterization of the intrinsic contact resistance is important for the optimization of organic thin-film transistors (OTFTs), especially in devices with high gate capacitance density. This study reports on a method for the extraction of the intrinsic, or gate-voltage independent, contact resistance from OTFTs. In contrast to the transfer length method (TLM), this method can be applied to devices with single geometry. The method is an extension of the previously reported Y-function method and combines the model proposed by Luan and Neudeck to analyze the effect of the gate voltage on the contact resistance. This method is first applied to OTFTs with a bottom-gate top-contact geometry having varying gate capacitance density values. The intrinsic contact resistance is found to decrease with increased gate capacitance density with values ranging between 4 kΩ cm and under 1 kΩ cm when the gate capacitance is changed from 36.6 to 231.7 nF/cm2, respectively. These values are in good agreement with those obtained using TLM experiments. This method is also shown to be applicable to OTFTs with top-gate bottom-contact geometry.

Ultrahigh Anisotropic Transport Properties of Black Phosphorus Field Effect Transistors Realized by Edge Contact

AbstractHighly anisotropic black phosphorus (BP) has recently attracted significant interest for electronic and optoelectronic devices. To date, in‐plane anisotropic properties of BP field effect transistors (FETs) have been reported only with top contact. However, the 2D top contact geometry is unable to measure the in‐plane electrical conductance precisely, due to the presence of the out‐of‐plane conductance, resulting in underestimation of anisotropy. Here, 1D edge contact method is employed to measure the in‐plane conductance precisely along the armchair and zigzag directions of BP without the contribution of out‐of‐plane conductance. The conductance and mobility anisotropies for BP FETs are measured with edge contact at 300 K to be ≈5.5 and ≈7.5, respectively. The results further show that the mobility of BP FETs with edge contact weakly depends on temperature, indicating that the edge roughness scattering limits the mobility. In contrast, the mobility of BP FETs with top contact strongly depends on temperature, showing that the impurity scattering and phonon scattering limit the mobility at below and above 150 K, respectively. Finally, a scattering phase diagram is demonstrated to understand the role of different scattering mechanisms on the modulation of mobility anisotropy in BP FETs with edge and top contacts.

Extreme Orientational Uniformity in Large-Area Floating Films of Semiconducting Polymers for Their Application in Flexible Electronics.

Layer-by-layer fabrication of uniformly oriented thin films over large areas by cost-effective solution-based approaches can open new horizons for the realization of high-performance organic circuits in various applications. In this work, fabrication of a large-area ≈40 cm2 film with uniform orientation is reported for poly(3,3‴-dialkylquaterthiophene) (PQT) using a unidirectional floating film transfer method (UFTM). Orientation characteristics and charge transport anisotropy were analyzed using polarized UV-vis spectral mapping and fabrication of bottom-gated organic field-effect transistors (OFETs) from different regions. Films were found to be highly oriented with an optical dichroic ratio of ca. 15. Orientation characteristics reveal that films were highly oriented along the width of the film, covering >70% of the area, and angle-dependent field-effect mobilities are in good agreement with the orientation of the polymer backbones. These highly oriented films resulted in charge transport anisotropy of 8.9. An array of bottom-gated OFETs fabricated along the length of single large-area (≈15 × 2.5 cm2) thin film demonstrated the average field-effect mobility of 0.0262 cm2/(V s) with a very narrow standard deviation of 12.6%. We also demonstrated that film thickness can be easily tuned from 5.6 to 45 nm by increasing the PQT concentration, and field-effect mobility is highly reproducible even when the film thickness is 10 nm. Microstructural characterization of the thus-prepared large-area thin films revealed the edge-on stacked polymer backbones and surface roughness of <1 nm as probed by grazing incidence X-ray diffraction and atomic force microscopy, respectively. Flexible OFETs with bottom-gate top-contact geometry were also fabricated, having average field-effect mobility of 0.0181 cm2/(V s). There was no considerable change in mobility after bending the flexible devices at different radii.

Reliable inkjet contact metallization on printed polymer semiconductors for fabricating staggered TFTs

Even though the fundamental benefits of the staggered bottom-gate top-contact geometry in organic thin-film transistors (TFTs) have been fully demonstrated for enhancing the charge injection efficiency, most printed organic TFTs with inkjet-printed source/drain electrodes have only been fabricated in bottom-contact configurations, probably due to the difficulty of printing with metal-nanoparticle ink on soluble organic semiconductor (OSC) without film deformation/dissolution. Here, we present the printing process and the electrical characteristics of inkjet-printed top contact OSC in bottom-gate TFTs. We first fabricated polymer bottom-gate TFTs by printing in two different top- and bottom-contact configurations. The physical carrier mobility of the two TFTs was extracted from a gated-contact organic TFT model to exclude the effect of geometric contact resistance RC. When compared to assess the OSC film damage from the printing metallization, the two mobility values were almost identical. This result indicates that the metal nanoparticle ink used in this work formed a top metal contact on the OSC film without significant chemical damage. Furthermore, the printed top-contact TFT exhibited I–V characteristics almost identical to those of a thermally evaporated Ag top-contact TFT. This study suggests the possibility of inkjet-printed top metal contacts for organic thin-film devices such as transistors, solar cells, and diodes.

Electrical contact resistance in graphite–graphene contacts from ab initio methods

We study the ballistic transmission and the contact resistance (Rc) of a graphite–graphene contact in a top contact geometry from first principles. We find that the calculated Rc’s depend on the amount of graphene–graphite overlap, but quickly saturate for transfer lengths of the order of a few tens of Å. For contacts overlapping more than this transfer length, the Rc can be lower than the 100 mark. On the other hand, edge graphite–graphene contacts are expected to have very low contact resistance.

3D reconstruction of pentacene structural organization in top-contact OTFTs via resonant soft X-ray reflectivity

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.

Highly Sensitive Ammonia Gas Sensor Based on Single-Crystal Poly(3-hexylthiophene) (P3HT) Organic Field Effect Transistor.

A highly sensitive organic field-effect transistor (OFET)-based sensor for ammonia in the range of 0.01 to 25 ppm was developed. The sensor was fabricated by employing an array of single-crystal poly(3-hexylthiophene) (P3HT) nanowires as the organic semiconductor (OSC) layer of an OFET with a top-contact geometry. The electrical characteristics (field-effect mobility, on/off current ratio) of the single-crystal P3HT nanowire OFET were about 2 orders of magnitude larger than those of the P3HT thin film OFET with the same geometry. The P3HT nanowire OFET showed excellent sensitivity to ammonia, about 3 times higher than that of the P3HT thin film OFET at 25 ppm ammonia. The ammonia response of the OFET was reversible and was not affected by changes in relative humidity from 45 to 100%. The high ammonia sensitivity of the P3HT nanowire OFET is believed to result from the single crystal nature and high surface/volume ratio of the P3HT nanowire used in the OSC layer.

Poly(Vinyl Alcohol) Gate Dielectric Treated With Anionic Surfactant in C60 Fullerene-Based n-Channel Organic Field Effect Transistors

We report on the preparation and performance enhancement of n-type low-voltage organic field effect transistors (FETs) based on cross-linked poly(vinyl alcohol) (cr-PVA) as gate dielectric and C60 fullerene as channel semiconductor. Transistors were prepared using bottom-gate top-contact geometry and exhibited field-effect mobility (µFET) of 0.18 cm2V-1s-1. Treatment of the gate dielectric surface with an anionic surfactant, sodium dodecyl sulfate (SDS), passivates the positively charged defects present on the surface of cr-PVA, hence resulting in overall transistor performance improvement with an increase in µFET to 1.05 cm2V-1s-1 and additional significant improvements in dielectric capacitance, transistor on/off current ratio and transconductance.

3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides

The transition metal dichalcogenides (TMDs) are two-dimensional layered solids with van der Waals bonding between layers. We calculate their Schottky barrier heights (SBHs) using supercell models and density functional theory. It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S = 0.3, a similar value for both top and edge contact geometries. This arises because there is direct bonding between the contact metal atoms and the TMD chalcogen atoms, for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials. The Schottky barriers largely follow the metal induced gap state (MIGS) model, like those of three-dimensional semiconductors, despite the bonding in the TMDs being largely constrained within the layers. The pinning energies are found to be lower in the gap for edge contact geometries than for top contact geometries, which might be used to obtain p-type contacts on MoS2.

Low-resistance contacts of electroless-plated metals with high-mobility organic semiconductors: Novel organic field-effect transistors with solution-processed electrodes

The establishment of a reliable vacuum-free method for the formation of electrical contacts on high-performance organic semiconductors has become an urgent task due to rapid progress made in the development of solution-processable high-mobility organic field-effect transistors (OFETs). We have recently proposed that electroless plating, a standard technology to mass produce wirings in currently commercialized electronic devices, is suited for high-performance solution-crystallized OFETs. A low contact resistance at the source and drain electrodes is necessary with organic semiconductors for high-speed device operation; therefore, we have evaluated the contact resistance using the transfer line method. A top-contact geometry with sufficient contact area is employed to achieve stable carrier injection, which has enabled contact resistances as low as 1.4 kΩ cm on a polyethylene naphthalate substrate at a gate voltage of −10 V. This marks outstanding performance among the solution-processed metal electrodes reported for OFETs, particularly on plastic substrates. The result indicates that high-quality boundaries with minimized trap densities are realized due to the mild conditions of the electroless plating process at room temperature.

Effects of post-lift-off annealing conditions on contact oxidation of Ti–Au top-contacts in In-Sn–Zn–O TFT

Thin film transistors (TFT) with an indium based mixed oxide semiconductor are investigated for titanium–gold top-contacts. It is noticed that upon post annealing, in order to remove chemical residuals from top-contact lift-off steps, oxidation of titanium occurs depending on the annealing conditions. Mobility of the TFT is strongly affected by contact oxidation arising from this post lift-off annealing process. Oxidation of the top-contact is facilitated by adsorbed surface oxygen or out-diffusing oxygen from the semiconductor depending on the post lift-off annealing conditions. A passivation layer that binds effectively to surface vacancies and removes adsorbed oxygen species from the semiconductor surface is demonstrated. The combinations of this passivation layer with relatively low temperature and short post lift-off annealing in an oxygen deficient environment result in significantly reduced contact oxidation and subsequently better transistor performance. Contact resistance as low as 90Ωcm and mobility as high as 5.3cm2/Vs are obtained for solution processed mixed metal oxide semiconductor in top-contact geometry.

Contact-enhanced transparent silver nanowire network for all solution-based top-contact metal-oxide thin-film transistors.

In this paper, we investigate contact-enhanced transparent silver nanowire (Ag NW) network for solution-processed metal-oxide thin-film transistors (TFTs). Mechanical roll pressing was applied to a bar-coated Ag NW film to enhance the inter-nanowire connectivity. As a result, the sheet resistance of the Ag NW film was decreased from 119.5 ψ/square to 92.4 ψ/square, and more stable and enhanced TFT characteristics were achieved when the roll-pressed Ag NW was employed as source/drain electrodes. In addition, a non-acidic wet etching method was developed to pattern the Ag NW electrodes to construct top-contact geometry indium-gallium-zinc oxide TFTs. From the results, it is believed that the mechanical roll pressing and non-acidic wet etching method may be utilized in realizing all solution-based transparent metal-oxide TFTs.

Thienothiophene-benzotriazole-based semicrystalline linear copolymers for organic field effect transistors

AbstractA series of thienothiophene-benzotriazole-based semicrystalline copolymers, PTTBTz, PTTBTz-F, and PTTBTz-OR, were synthesized by considering chain linearity, planarity and inter-chain packing by virtue of non-covalent attractive interaction. Fluorine and alkoxy substituents were introduced to modulate the intra- and inter-chain coulombic interactions and crystalline ordering. The fluorine and alkoxy-substituted PTTBTz-F and PTTBTz-OR showed pronounced inter-chain packing with edge-on orientation confirmed by UV-vis absorption and X-ray diffraction measurements. The well-resolved diffraction patterns were obtained for PTTBTz-F and PTTBTz-OR, showing (100)∼(500) inter-lamellar scattering peaks (d-spacing, 17∼18 Å) in the out-of-plane direction and a π-π stacking peak (d-spacing, 3.5∼4.1 Å) in the in-plane direction. Organic field effect transistor (OFET) devices were fabricated with a bottom gate and top contact geometry. PTTBTz-F (μh = 4.49 × 10–2 cm2 V–1 s–1, on/off ratio = 1.13 × 107) and PTTBTz-OR (μh = 8.39 × 10–3 cm2 V–1 s–1, on/off ratio = 2.98 × 104) showed nearly 3 and 2 orders of magnitude higher hole mobility upon annealing at 305 and 260 °C, with compared to the unsubstituted PTTBTz.

Benzothienobenzothiophene-Based Conjugated Oligomers as Semiconductors for Stable Organic Thin-Film Transistors

Two benzothienobenzothiophene (BTBT)-based conjugated oligomers, i.e., 2,2'-bi[1]benzothieno[3,2-b][1]benzothiophene (1) and 5,5'-bis([1]benzothieno[3,2-b][1]benzothiophen-2-yl)-2,2'-bithiophene (2), were prepared and characterized. Both oligomers exhibit excellent thermal stability, with 5% weight-loss temperatures (T(L)) above 370 °C; no phase transition was observed before decomposition. The highest occupied molecular orbital (HOMO) levels of 1 and 2 are -5.3 and -4.9 eV, respectively, as measured by ultraviolet photoelectron spectroscopy. Thin-film X-ray diffraction and atomic force microscopy characterizations indicate that both oligomers form highly crystalline films with large domain sizes on octadecyltrimethoxysilane-modified substrates. Organic thin-film transistors with top-contact and bottom-gate geometry based on 1 and 2 exhibited mobilities up to 2.12 cm(2)/V·s for 1 and 1.39 cm(2)/V·s for 2 in an ambient atmosphere. 1-based devices exhibited great air and thermal stabilities, as evidenced by the slight performance degradation after 2 months of storage under ambient conditions and after thermal annealing at temperatures below 250 °C.

Synthesis, characterization and semiconducting properties of oligo(2,6-naphthalene)s

Oligo(2,6-naphthalene)s (nNs) with the number of repeating unit (n) of 3–5 and dioctyl substituted trimer (DO-3N) and tetramer (DO-4N) were synthesized and characterized. Except of DO-4N that decomposed at 331°C, all other oligomers display good thermal stability with 5% weight loss temperatures beyond 350°C. For 4N and DO-4N, only one phase transition corresponding to melting can be found. However, DO-3N and 5N exhibit multiple phase transitions. The film absorption maximum of the oligomers at the long wavelength exhibits a red-shift with increasing the number of repeating unit. Meanwhile, the highest occupied molecular orbital (HOMO) levels increase from −5.96eV of 3N to −5.91eV of 4N and then −5.85eV of 5N. The introduction of octyl group has negligible effect on the photophysical properties. Highly ordered films of 4N, 5N, DO-3N and DO-4N in which the molecules orient with their long axis standing on the substrates can be prepared by vacuum deposition. Organic thin-film transistors (OTFTs) with top-contact and bottom-gate geometry have been fabricated based on these films. The introduction of octyl groups results in the films with higher order and therefore higher field-effect mobilities (μ) of OTFT devices. Among these oligomers, DO-3N exhibits the highest mobility of 0.50cm2/Vs when the film deposited on the octyltrichlorosilane (OTS) modified substrate.

Interfacial insertion of a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) layer between the poly(3-hexyl thiophene) semiconductor and cross-linked poly(vinyl alcohol) insulator layer in organic field-effect transistors

The role of a thin layer of conductive poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT : PSS), inserted between the gate dielectric and the active layer in poly(3-hexylthiophene)-based transistors was investigated. The devices were fabricated in the bottom-gate top-contact geometry by using cross-linked poly(vinyl alcohol) as the dielectric, whereas the PEDOT : PSS layer was prepared by using an aged aqueous dispersion with addition of different amounts of dimethyl sulfoxide (DMSO) as a secondary dopant. Under these conditions, both a significant reduction in the number of electrically active traps at the interface with the semiconductor and an improvement in the field-effect mobility were obtained, whereas the low power consumption was preserved. The threshold voltage was also displaced by approximately −1 V.

An easily made thienoacene comprising seven fused rings for ambient-stable organic thin film transistors

A novel easily made thienoacene-based organic semiconductor, i.e., dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2), was synthesized in high yield, and its thermal stability, electrochemical properties, thin-film morphology and field-effect mobility were investigated. Ph5T2 exhibit excellent thermal stability with a decomposition temperature (Td) of 427°C. Thin-film X-ray diffraction (XRD) and atomic force microscopy (AFM) characterizations indicate that Ph5T2 can form highly ordered films with large domain size on the para-sexiphenyl (6P)-modified substrates. Organic thin-film transistors (OTFTs) with top-contact geometry based on Ph5T2 exhibit mobilities up to 1.2cm2V−1s−1 in ambient. The devices are highly stable and exhibit almost no performance degradation during 3months storage under ambient conditions with relative humidity up to 80%.