Growth and Characterization of 2D Semiconductors: MoS2 and Application to Photodetectors

Advancements in technology have increased the demand for nanoelectronic and microelectronic devices. Molybdenum disulfide (MoS₂) is an emerging, two-dimensional semiconducting material that is widely used in nanoelectronic and microelectronic component fabrication. It is used in devices like solar cells by layering the MoS₂ with silicon (Si) and aluminum (Al) to facilitate an electrically efficient circuit. It has medical applications such as biosensors that use the MoS₂ to complete a circuit when the comes in contact with a specific sensor. MoS₂ is also used in Internet of Things devices: the transistors that make up the circuitry often use MoS₂ because of its large band gap and easiness to control whether the transistor is on or off.

This project developed a simple chemical vapor deposition (CVD) reaction between Molybdenum trioxide (MoO₃) and Sulfur (S) to grow a monolayer of MoS₂. The original procedure for device fabrication involved growing the MoS₂ on an arbitrary target substrate, usually Si or Si-SiO₂, and then transferring the MoS₂ onto the material that would be ultimately used in the device. The new procedure had the MoS₂ grown directly onto the “final” material, which is often a n-doped/p-doped Si or Silicon nitrate (SiNO₃). Thus eliminating the complicated layer transfer steps from the overall process. This optimization has significant cost reduction potential because it eliminates the equipment and labor expenses for the material transfer step. Raman spectroscopy was used to detect the inelastic scattering of monochromatic radiation of the 2D MoS₂. It was observed that not only did the growths yield larger MoS₂ crystal domain sizes, but they also yielded larger monolayer regions of MoS₂

To demonstrate the applications of MoS₂ to electrical engineering, solar cells and photodetectors were created using the MoS₂ samples that were grown in the previous stage. The MoS₂ was grown onto specific p-doped silicon substrates and underwent Al deposition processes. The devices were developed to show that the material growth process is a valid solution. The goal of this project was not necessarily to create devices that could high generate high amounts of electrical energy from solar energy. The electrical efficiencies of the devices were higher than anticipated, but were not enough to where it could be manufactured on a large scale. However, this proof of concept confirms that these devices can be created using the optimized procedure, and with a few tweaks to equipment and procedure, solar cells can be made very easily and quickly. As this project progresses, the growth chamber will be built on a much larger scale to allow larger amounts of MoO₃ to react with more S thus yielding even larger amounts of the 2D MoS₂ to make more efficient solar cells and photodetectors.