Evidence of Charging and Storage at the MoS2/Si Interface

Abstract

Non-volatile memory chips are important in devices for data storage, where demand kept increasing and the demand of large capacity of storage grew with the development in technology. Flash memory is one of the non-volatile memory devices that has a simple structure that is based on CMOS basic structure. Flash memory was introduced by Toshiba in 1984, which is an electrically erasable/programable non-volatile memory device [1]. Due to the development new technology the demand for size scale down of the memory structure was high, at which the size was kept on being reduced. At sizes less than 65nm flash memory started to show different problem including charge loss and the stress-induced leakage current, due to the tunnelling oxide thickness reduction. [2,3]. The structure of the flash memory has been developed to overcome the issues due to size reduction by introducing nanoparticles to the floating gate, which showed huge development. This development made researchers explore different nanoparticles including 2D materials. 2D materials has distinctive electrical, optical, chemical, and mechanical properties that makes them interesting to be implemented in new technology [4]. One of the interesting 2D materials is Molybdenum disulfide (MoS2). MoS2 have a multitude of features such as high strength, flexibility and has better quantum yield than bulk MoS2 [5]. 2D films of MoS2 can be widely used in various devices for their electronic, optical, and catalytic properties [6]. In this work MoS2 nanoflakes were bonded to silicon to study the charge transport at the interface.The initial bulk MoS2 was in crystal form, which was used to prepare the MoS2 nanoflakes. The preparation procedure for the MoS2 flakes was by using mechanical exfoliation technique. Nitto SPV224 PVC tape had been used to exfoliate bulk MoS2, where part of the bulk MoS2 crystal is placed on the sticky part of the tape. The tape is then folded on the MoS2 piece with the tape surrounding the crystal on the top and bottom. The tape is then separated from each other, which results in exfoliation of the MoS2 into flakes. The more the steps are repeated of this process, the thinner the flakes are. The flakes on the tape were transferred to a p-Si wafer on a hot plate at 50C and bonded to create the MoS2/Si interface.The achieved MoS2 nanoflakes on p-Si wafer sample had been imaged using AFM topography and conductivity measurements. The AFM images were collected to identify the distribution of the flakes. AFM images showed clear flakes on the p-Si wafer at which they were distributed randomly over the surface. The thickness of single monolayer flakes measured was 0.65nm and two monolayer flakes were 1.3 nm. Furthermore, conductive AFM was employed to obtain the IV curve over the flake and obtain their electrical properties, where forward and reverses scanning was done showing a hysteresis loop. The hysteresis observed showed a voltage window gap of around ∆Vth=0.5 V. This indicates electron tunneling and charge storage at the MoS2/Si interface or in the MoS2. Finally, the results, highlight the potential use of 2D MoS2 flakes in future no power memory devices.