UV assisted non-volatile memory behaviour using Copper (II) phthalocyanine based organic field-effect transistors

Highlights

• Demonstrated non-volatile memory characteristics using CuPc OFETs.

• Programming with electric pulse under UV-light photo-illumination substantially increases the memory window.

• Switching ratio ~10² and massive memory window >45V for −50V device operation.

Abstract

In this report, we have demonstrated the optical non-volatile memory characteristics using CuPc OFET. The memory operation was comprehensively demonstrated with different programming conditions. It was found that the programming of CuPc OFET with an electric pulse at the gate terminal under UV-light photo-illumination compared to other programming conditions, could substantially increase the memory window due to massive charge trapping in the polymer electret layer, which causes shift in the device transfer characteristics from low-conduction state (“OFF state”, or logic 0) to high conduction state (“ON state”, or logic 1) at V_GS = 0V. From device operation at −50V, a memory window of greater than 45V could be achieved by applying a programming voltage of +70 V at the gate terminal under UV-light photo-illumination. Moreover, it was completely erased by applying −100 V at the gate terminal in dark.

Introduction

Organic field-effect transistors (OFETs) based memory devices have been the subject of interest research [1,2] over the past few years because of several advantages such as cost-effectiveness, light weight, mechanical flexibility [3,4]. In the existing literature, OFETs have been explored for memory applications in several ways such as: (i) polymer electret based OFET memory [5,6], (ii) ferroelectric OFET memory [7,8], and (iii) floating gate OFET memory [9,10]. Among all these methodologies, polymer electret based OFET memory technology is commonly preferred because of simplicity, compatibility with integrated circuits, and ease of reading operation. Memory operation in polymer electret based OFET devices was achieved due to charge trapping and de-trapping in the chargeable polymer electrets. Conventionally, a high electric gate voltage (>150 V) has been used to enhance the charge trapping in the polymer electret, which in turn assists to achieve a massive memory window for memory operation in polymer electret based OFET [5]. The requirement of higher gate voltage for memory operation could be mitigated by employing the programing of OFET devices with electric gate pulse under photo-illumination conditions instead of dark [5]. Photo-illumination on organic semiconductor materials causes the generation of excitons which eventually dissociated into free electron and hole pairs [11,12] under external electrical biasing conditions. These photo-generated electrons could be motivated by applying external positive electric pulse at the gate terminal, enabling abundant amount of charge trapping in the polymer electret; which in turn will assist to achieve a massive memory window. In the recent past, physicists have explored organic photo-transistors for memory applications [13,14]. Though several organic semiconductors with higher charge-carrier mobility have been explored for OFET devices and their memory applications, Copper (II) Phthalocyanine (CuPc) offers wide absorption spectrum in UV–Vis spectrum [15,16], and long exciton diffusion length (8 nm–68 nm) [17,18]. OFETs have been explored for non-volatile memory applications mostly using following two programming conditions with electric pulse at the gate terminal either in the dark, or under UV light illumination. Moreover, reports on the comprehensive comparison of enhancement in memory window by employing the different programming conditions such as only pulse, only UV illumination, and combination of Pulse under UV illumination are scarce [19].

In this work, we have demonstrated the UV-assisted non-volatile optical memory using CuPc OFETs. The non-volatile memory operation of CuPc OFETs was comprehensively explored with three different programming conditions (i) programming with an electric pulse in dark, (ii) programming with illumination of UV-light source, and (iii) programming with an electric pulse under illumination of UV-light source; and the corresponding change in the device transfer characteristics were examined. We found that, the programming of CuPc OFETs by applying an electrical pulse at the gate terminal in presence of UV-light photo illumination causes prominently higher shift/change in device transfer characteristics compared to other programming cases, resulting in a change in device conduction state (from low-conduction state to high conduction state) of representative CuPc OFET for memory applications. The memory window and switching ratio achieved of the representative CuPc OFET for optical non-volatile memory operation was 46.9 V, and ~10² respectively. The deposited CuPc thin film was also studied for various thin film properties such as film crystallinity, and uniformity.