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 ∆V_th=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.

References

[1] G. Lawton, "Improved flash memory grows in popularity," in Computer, vol. 39, no. 1, pp. 16-18, Jan. 2006.

[2] J. De Blauwe, “Nanocrystal nonvolatile memory devices,” IEEE Transactions on Nanotechnology, vol. 1, no. 1, pp. 72–77, 2002.

[3] J.-S. Lee, “Review paper: Nano-floating gate memory devices,” Electronic Materials Letters, vol. 7, no. 3, pp. 175–183, 2011.

[4] S. Das, H.-Y. Chen, A. V. Penumatcha, and J. Appenzeller, “High performance multilayer MOS2 transistors with scandium contacts,” Nano Letters, vol. 13, no. 1, pp. 100–105, 2012.

[5] R. Ganatra and Q. Zhang, “Few-Layer MoS2: A Promising Layered Semiconductor,” ACS Nano, vol. 8, no. 5, pp. 4074–4099, 2014.

[6] L. Muscuso, S. Cravanzola, F. Cesano, D. Scarano, and A. Zecchina, “Optical, Vibrational, and Structural Properties of MoS2Nanoparticles Obtained by Exfoliation and Fragmentation via Ultrasound Cavitation in Isopropyl Alcohol,” The Journal of Physical Chemistry C, vol. 119, no. 7, pp. 3791–3801, 2015.2012.