Fabrication of Copc and PTCDI-Ph Based low -operating voltage organic field-effect Transistors

We studied the growth of active channels on top of the dielectric surfaces and optimized the growth for the fabrication of OFETs with better properties. We investigated the effect of substrate temperature on the active channel growth and performance of the OFETs.

Introduction

Organic Semiconductors

Organic Semiconducting Molecule

• Metal Phthalocyanines (MPcs)
• Perylene Derivatives

Growth of Organic Thin‐Film

• Study of Rough Surface

For most purposes, a more useful measure of the roughness is the root-mean-square (rms) value (often called the interface width (W) in the literature). To study the dynamics of the growth process, it is important to study the behavior of local slope m(t).

Organic Field‐Effect Transistors

Basic Design and Operation of OFETs

The conductive channel is formed in a few nm thin layers at the semiconductor/dielectric interface. The transmitted characteristics (IDS vs. VGS at constant VDS) and the output characteristics (IDS vs. VDS at constant VGS) of the devices are measured to.

Hysteresis Effect in OFETs

The hysteresis effects are very well observed in OFETs and cause instability in device operation. Polarization of the dielectric (ferroelectric materials as dielectric or metastable .. quasi-ferroelectric” polarization in the dielectric).

Gate Dielectric Layer

The main advantage of the above mentioned metal oxides is that they can be dissolved. In the following chapters, we have presented the details of synthesis and importance of different metal oxides Al2O3, TiO2, BaTiO3, SrTiO3 and BaSrTiO3 via characterization of fabricated OFETs.

Horowitz, Physics of Organic Field-Effect Transistors, Semiconducting Polymers, Wiley-VCH Verlag GmbH & Co.

Experimental Techniques

Film Deposition Techniques

• Developed Deposition Systems
• Standard Techniques

We have chosen a final dielectric layer thickness of 75 nm for OFETs fabrication. We have used the same dielectric layer thickness as the MIM system to fabricate the OFETs.

Microscopic Characterization Techniques

• Atomic Force Microscopy
• Electron Microscopy

Spectroscopic Characterization Techniques

• Ultra‐Violet Visible Spectroscopy
• Fourier Transform Infra‐Red Spectroscopy
• X‐Ray Diffraction Technique

However, the increasing nature of ξwith t, as shown in Figure 3.3(b), indicates the lateral growth of the islands. In this chapter, we studied the growth kinetics of the active organic channel grown on the dielectric polymer layer. We studied how the OFET parameters, such as carrier mobility and threshold voltage, depend on the growth of the films.

We have demonstrated the performance of the OFETs with operating voltages as low as 1.2 V for the devices based on PTCDI-Ph.

Electrical Characterization Techniques

• Probe Station
• Semiconductor Characterization System

Kinetic Roughening in CoPc Thin Film Growth

• Introduction
• Experimental Details
• Results and Discussions
• Coverage Dependent Growth of CoPc Thin Films
• Temperature Dependent Growth of CoPc Thin Films
• Growth of CoPc on Mica (001) Substrate
• Conclusions
• References

We observed equal diffusion activation energy for lateral and vertical diffusion of the molecules. To study the optical response of the CoPc microstrips, we performed UV-Vis. In general, organic semiconducting active channels of the OFETs are grown on the dielectric gate surfaces.

We have observed roughness in CoPc films due to instability in the growth mechanism induced by local diffusion of the molecules.

CoPc microstructures based OFETs with higher carrier mobility…

Introduction

It is well known that the performance of the organic semiconductor is controlled by how molecules or polymer chains assemble in the solid state [17, 18]. The field-effect carrier mobility (µFE) of OFETs depends not only on the organic semiconductors and their molecular arrangement within the active channel of the device, but also on the gate dielectric that controls the charge flow through the channel. As a result, a better capacitive coupling between active channel and the gate dielectric is expected to improve the charge carrier mobility of the devices.

However, anodized Al2O3 films are generally too rough to increase the semiconductor channel at the top of the film.

Experimental Details

• Growth of CoPc Microstructures
• Fabrication Process of CoPc Wires Based OFETs

In this chapter, we described the growth of CoPc microwires by organic vapor deposition (OVPD) method. Tested with multiple substrate surfaces to achieve long CoPc microwires that can be used in the fabrication of wire-based OFETs as an active channel. A portion of the Al film was then anodized to form an Al2O3 layer to be used as the gate dielectric layer.

The stack of Al2O3 and PVA layers was used as the gate dielectric layer for OFET fabrications.

Results and Discussions

• CoPc Microstructure Growth on Various Substrates
• CoPc Micro‐Wires Based OFET Fabrication

As the growth temperature increases, the length of the strips also increases in one direction (Figure 4.3 (a-c)). 100 µm which are not clearly visible in the FESEM images shown in Figure 4.6(f), because only part of the strips are exposed on the surface. The average length of the microstrips is confirmed by optical micrograph, as shown in Figure 4.7(b).

The crystalline nature of the wires was further confirmed by high-resolution transmission electron microscopy (HRTEM) as shown in Figure 4.11(c).

Summary & Conclusions

Field effect mobility was further improved by the appropriate choice of the dielectric materials and the thickness where the leakage current is significantly low.

The field-effect carrier mobility is also dependent on the work function of the metal contacts [39, 40]. We did not observe significant changes in the performance of the OFETs fabricated with TiOx/Al2O3 and BTO/Al2O3 as dielectric layer. Most of the experimental works on the bias voltage (BS) phenomenon are focused on p-type organic transistors with silicon dioxide (SiO2) layer as gate dielectric.

Morpurgo, Effect of Gate Leakage Current on the Stability of Organic Single-Crystal Field-Effect Transistors, Appl.

Effect of growth temperature on the Performance of PTCDI­Ph …

Experimental Details

• PTCDI‐Ph Based OFET Fabrication on SiO 2 Substrate
• OFET Fabrication on Organic Inorganic Dielectric Surface

PTCDI-Ph is one of the Perylene derivatives synthesized, purified and used as an active material for the manufacture of the OFETs. Highly boron (B) doped silicon (Si) (Resistance Ω-cm) with thermally grown oxide layer of 300 nm wafers was used to fabricate the devices. In general, gold, aluminum, silver, and chromium metals are used as contact materials for OFETs to provide good electron injection into the active channel.

However, all the OFETs recovered performance immediately and showed better electrical properties under vacuum.

Results and Discussions

• Evolution of Surface Morphology
• Electrical Characterizations
• Film Morphology and Device Properties

This can be attributed to a better molecular self-assembly within the film due to improved diffusion of the molecules at higher substrate temperatures. The formation of pores can also be due to desorption of the molecules at higher substrate temperature. The evolution of the surface morphology can be quantified by the variation of ξ and W with substrate temperature.

However, the formation of a smooth film is essentially driven by the lateral diffusion of molecules.

Summary & Conclusions

This is the signature of smooth film growth as the value of ξ and W decreases with increasing temperature. In this case, the crystalline quality of the film can be improved due to the enhanced diffusion of the molecule, but at the same time, the film becomes rough and discontinuous due to the formation of a large number of holes as shown in the AFM images in Fig. . 5.6 (d). The rapid hardening by pore formation in the films at the substrate temperature of 120 °C can be attributed to lateral inhomogeneities in the molecular arrangement in the film at the higher growth temperature.

Therefore, OFETs fabricated at 120 °C substrate temperature are expected to exhibit poor carrier mobility as observed.

However, we have used chemical synthesis root along with anodization method to fabricate the dielectric layers of the OFETs. In the case of SiO2 as dielectric layer, the operating voltage of the OFETs was around 30 V. In this section, we described the results of the OFETs fabricated with BaSrTiOx (BST)/Al2O3.

The stability of low operating voltage OFETs based on PTCDIPh 139 confirms the stability of PTCDI-Ph based OFETs fabricated with STO/Al2O3 as the dielectric system.

PTCDI­Ph Based Low–Operating Voltage OFETs with Metal –

Experimental Details

• Synthesis of Metal‐Oxide Precursor Sols
• OFET Fabrication

Another stock solution was based on titanium isopropoxide with 2-methoxyethanol as solvent, giving a precursor concentration of 1.0 mol/dm3. Before the experiments, the stock solutions were mixed in molar ratios of 1:1 and stirred for 10 min to obtain a concentration of 0.50 mol/dm3 in the final BaTiOx (BTO) precursor solution. In the next step, they were diluted with 2-methoxyethanol to reach a concentration of 0.1 mol/dm3 in the final BTO precursor solution.

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Results and Discussions

• Fabrication of OFETs using TiO x /Al 2 O 3 as bilayer gate dielectric…
• OFETs based on BaTiO x / Al 2 O 3 as dielectric layer
• OFETs based on SrTiO x / Al 2 O 3 as dielectric layer
• OFETs based on BaSrTiO x / Al 2 O 3 as dielectric layer

As can be seen from Figure 6.4, the double-layer dielectric system exhibits a current density of 10-7 A/cm2 at ±3 V. The surface morphological images of STO/Al2O3 and PTCDI-Ph bilayer films grown on the bilayer dielectric system are shown in the figure. 6.11 (a-b). From the output characteristics shown in Figure 6.17(a), one can see the continuous displacement of the drain current near the origin.

The same phenomenon has been reflected in the transfer characteristic (shown in Figure 6.17(b)) in terms of high threshold voltage (5 V).

Summary & Conclusions

Marks, High-k Organic, Inorganic, and Hybrid Dielectrics for Low-Voltage Organic Field-Effect Transistors, Chem. Okuzaki, Low-voltage organic field-effect transistors fabricated on self-assembled-monolayer-free SrTiO3 insulator, Jpn. Xu, A low-temperature solution-processed high-k dielectric for low-voltage, high-performance organic field-effect transistors (OFETs), J.

Chem, High-Performance Low-Voltage Organic Field-Effect Transistors Prepared on Electropolished Aluminum Wires, ACS Appl.

Stability of Low­Operating Voltage OFETs based on PTCDI­Ph

• Introduction
• Experimental Details
• Results and Discussions
• Hysteresis in Transistor Output & Transfer Currents
• Reliability of Organic Field Effect Transistors
• Bias – Stress Effect on Transistors
• Stability after periodic application of bias‐stress
• Summary and Conclusions
• References

To study the stability of these devices, we considered (i) the hysteresis effect on the OFET output characteristics, (ii) the effect of bias stress on OFET performance, and (iii) degradation of the devices in the over time as they are exposed to environmental factors. multiple days. Hysteresis effects are often observed in organic transistors during gate voltage swings (VGS). The origin of the hysteresis could be (i) the effect of mobile charge on the channel, (ii) the effect of the resulting polarization in the gate dielectric or (iii) charge injection into the gate electrode [8]. Stability of low operating voltage OFETs based on PTCDIph 137 exposure time under ambient conditions.

However, the reduction factor was only 9% for the OFETs based on STO/Al2O3 double layer dielectric system.

Summary and Conclusions

We have successfully fabricated OFETs with high carrier mobility on the order of 1.1 cm2/Vs with CoPc as active materials. To study the stability under bias stress and exposure in air, we have performed measurement on one of the best devices manufactured above. This result is consistent with the high reliability of the OFETs with high stability under bias stress.

The observed exponents do not fit any of the existing growth models for inorganic film growth.