Elsevier

Microelectronic Engineering

Volume 111, November 2013, Pages 210-213
Microelectronic Engineering

Nonvolatile organic memory devices with CdTe quantum dots

https://doi.org/10.1016/j.mee.2013.03.157Get rights and content

Highlights

  • A nonvolatile organic memory devices were fabricated using CdTe QDs.

  • The charging and discharging behavior of CdTe QDs as MIS structure was measured through CV characteristics.

  • The memory devices using CdTe QDs showed a large memory window and good retention characteristics.

Abstract

The nonvolatile organic memory devices were fabricated utilizing CdTe quantum dots (QDs). QDs were used as a hole-trapping component in the memory device. We analyzed the electrical properties of the memory device fabricated with CdTe QDs by measuring the capacitance–voltage characteristics and retention time. A number of holes were trapped in CdTe from pentacene, which formed band bending between pentacene and QD layer. We observed large hysteresis at capacitance–voltage response during the operation of the device. The long retention of programmed state time of 104 s can be potentially useful in practical applications of non-volatile memory.

Introduction

Numerous studies have showed much interest on conventional nonvolatile memories over the last decade, due to their extensive usage in electronic equipment such as USB, computer DRAM memory chip, etc. However, organic non-volatile memory devices are also coming up with greater attention [1]. These organic memory devices are fabricated with organic semiconductors, but their electrical performances cannot catch up with those of inorganic memory device based on silicon. In most of the cases, memory devices which are necessary in complicated machines usually use inorganic semiconductors based on silicon. However, organic memory devices bear some advantages such as low temperature fabrication, cost effectiveness, and mechanical flexibility. Hence, organic electronics has become a popular field of study to many engineers [2], [3], [4], [5]. Especially mechanical flexibility of organic electronics enables the manufacture of electronics that are different in shape. The reason why inorganic memory device is not flexible lies in the use of solid substrate. Fabricating organic electronic device in low temperature let plastic substrates endure the heat, which is impossible in the fabrication of inorganic memory device. These features can be used in high-technology industry fields such as wraparound computer, flexible display TV, etc.

Several types of gate dielectrics enable reversible trapping of charges upon application of a gate field, for instance polymer electrets [6], dielectrics with embedded metallic [7] or semiconducting nanoparticles (NPs) [8] with permanent and/or switchable electrical dipoles.

Among these technologies, organic memories based on NPs have attracted a great deal of interest because of their potential applications in next-generation memory devices. In general, metal NPs such as Au, Ag, and Cu have been utilized as charge-trapping elements for non-volatile memory devices [7], [9], [10]. In comparison to the metal NPs-based memory devices, the devices using semiconductor NPs as charge traps possess many intriguing properties and are therefore a powerful tool for the development and fabrication of materials with novel functions.

Among the various organic memory devices, semiconducting nanoparticles (NPs)-based organic memory is a promising candidate for the future nonvolatile memory [11], [12]. In semiconducting nanoparticles-based organic memory, thin layer of NPs acts as a charge-trapping element by trapping holes from the pentacene layer [7], [9]. Holes in the pentacene layer move to the layer of NPs due to the tunneling effect. As the holes enter the layer of NPs, nanoparticles capture the holes and change the CV characteristics of the device. As various types of NPs exist, memory characteristics also can be controlled upon the structure and the type of NPs. A few significant studies were reported about NPs-based memory device with many interesting properties.

The utilization of NCs in floating gate offers several important advantages. First, NCs permit the use of a thinner tunneling oxide layer, which can lead to long term charge retention [7], [8]. In addition, the thinner oxide layer can lower program/erase voltage, which results in less damage, larger endurance for write/erase cycles, and improved reliability. Second, band structure engineering methods, such as tuning band-offsets and barriers, are possible through a combination of various NCs [6], [7], [9]. Especially the electrical properties of memory device fabricated with CdSe NPs were studied recently. Kim et al., showed different kind of NPs, constructs different form of energy band which changes the electrical property of memory device [13]. However, the electrical properties of these memory devices according to the type of NPs have been still ambiguous.

Here, we present the performance variation of the organic memory device based on CdTe NPs. We analyzed the hole trapping effect by measuring CV characteristics and retention time. Organic memory device used in this study is based on pentacene, an organic semiconductor. A unique feature of our device is that unlike the traditional pentacene-based device configuration, it has no tunneling insulator inside it. A thin layer of CdTe NPs were spin-coated as a single layer which directly connects the pentacene layer and PMMA layer, creating type II junctions [14].

Section snippets

Experimental

Fig. 1 shows a schematic illustration of an organic memory device comprising a layer of CdTe QDs beneath a pentacene layer and energy band structure of the organic memory with CdTe QDs. CdTe QDs were prepared by hot injection method with some modifications [15]. CdO (0.39 mmol), trioctylphosphine oxide (5.2 mmol), and tetradecylphosphonic acid (1.1 mmol) were degassed at 110 °C for 1 h and then heated under nitrogen to 315 °C so as to completely dissolve the precursors. TOPSe (0.25 mmol Se dissolved

Results and discussion

To study the performance of organic floating-gate memory device using CdTe QDs which were localized beneath the pentacene, control devices were first studied. The control device fabricated without CdTe QD appeared no hysteresis when the bottom voltage (Vbottom) was swept from +10 to −10 V and then back to +10 V and even ±30 V (Fig. 2). No hysteresis indicates the absence of any charge trapping events inside PMMA and its interface. On the other hand, memory device containing CdTe NPs appeared

Conclusion

The nonvolatile organic memory devices were fabricated using CdTe QDs. We studied the electrical properties of the CdTe QDs based memory device by measuring the CV characteristics and retention time. Through measured CV characteristic, the charging and discharging behavior of CdTe QDs as MIS structure was observed. The capacitance shift stands for hole injection in CdTe QDs as a function of increasing applied positive or negative bottom voltage stress. The memory devices using CdTe QDs

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