Fabrication and Study of Metal- Organic Semiconductor-Metal Structures for Applications in Electronics

Abstract

Organic electronics is a rapidly growing technology that is expected to compete with conventional microelectronics in the near future. Organic active materials based devices such as junction diodes, sensors, FETs, solar cells, OLEDs, RFIDs and memories will enable future generations of electronics products that may ultimately enter the mainstream electronics market. The interest of researchers and industry in organic electronics is mainly for its advantages of offering inexpensive, environmental friendly flexible and large manufacturing area processes. electronic In the devices with research work undertaken in this dissertation, methyl-red (MR), 5,10,15,20-Tetrakis(3’,5’-di-tert- butylphenyl) porphyrinatocopper(II) (TDTPPCu) and polymethylsilsesquioxane (PMSSQ) nanocomposite are investigated as active materials for their potential applications in organic electronics. These organic materials belong to the families of azo dyes, macrocyclic compounds and polymer nanocomposites. Employing these materials, sensors, a field effect transistor and non-volatile memory cells are fabricated and characterized. Using methyl-red thin film based metal/organic/metal structures are investigated. The potential applications of methyl-red for organic rectifying junctions and humidity sensors are demonstrated successfully. The methyl-red based surface- type diodes fabricated with different electrodes exhibited high rectification ratio. In the case of Au/methyl-red/Ag surface-type diode, the rectification ratio of the order of 105 is achieved, which is the 2nd highest rectification ratio ever attained for organic diodes. Various electrical parameters of these methyl-red based devices have been determined using conventional current-voltage (I-V) method, Abstract Cheungs' functions and modified Norde’s function. The electrical parameters obtained using different techniques are found in reasonable agreement. The humidity-dependent-characteristics of methyl-red are also studied in the surface- type capacitive and resistive sensors. Significant changes in the capacitance and resistance of the sensors are observed under the effect of different relative humidity (RH) levels. The methyl-red based sensors exhibit fast response/recovery time and very small hysteresis. All these interesting features make methyl-red a promising material for electronics applications. To determine the potential of TDTPPCu for electronics applications, n- Si++/TDTPPCu/Al device is fabricated and its electrical properties are investigated. The current-voltage characteristics of the device are observed nonlinear and asymmetric. Using these characteristics, the junction parameters are calculated. Subsequently, TDTPPCu is studied as an active material for multifunctional capacitive sensors and photo-field effect transistor (photo-FET). The TDTPPCu based light, humidity and temperature sensors are studied in surface-type structures. The capacitance of the photo sensor is increased by 4.7 times from the dark condition under an illumination of 3850 lx. In the case of the humidity sensor the capacitance of the sensor changed 9.5 times with the increase in relative humidity (RH) from 30% to 95%. No change in capacitance appeared in the temperature sensor below 120 °C. Based on the experimental results, a mathematical model for the multifunctional sensors has been developed which explains the basic sensing mechanisms of the sensors. The sensors are simulated using this model. The simulated results match well with the experimental results. The effect of light on the output characteristics of a very simple and novel structured photo-FET is also investigated. The transistor is fabricated by employing the TDTPPCu as a channel material. Light, instead of the gate voltage, is used for controlling the drain-source current. Significant Abstract change in the drain-source current is observed under illumination. The photo- FET structure might lead to advancement in the understanding of photo-physics and electronic processes in organic semiconductors that can direct to efficient devices, modified to the specific requirement without the limitations imposed by conventional semiconductor technology. Employing a nanocomposite of PMSSQ non-volatile memory (NVM) cells are fabricated for the first time. The electrical behavior of PMSSQ nanocomposite thin films and possibility of nano-traps formation and charge storage in the films have been studied. Later, a nano-trapped NVM using the nanocomposite is investigated. Capacitance-voltage (C-V) analysis is performed to examine the memory effect. A wide clockwise hysteresis window of 12 V is observed, which indicates the high charge storage capability of the nanocomposite film. A model for charge trapping mechanism and potential distribution in the NVM cells have been proposed which elucidates the charge trapping and detrapping mechanism in the composite. The programmed/erased characteristics of the composite based memory cells show the great potential of this nanocomposite for practical applications. The proposed model will help in further understanding in the charge trapping and detrapping mechanism in the nanocomposite based memories.