Graphene and MoS2 Transferring-Free Top-Gated Transistors with Dielectric Layers Fabricated by Using the Atomic Layer Deposition

In this thesis, we focus on the fabrication of transferring-free top-gated transistors using two-dimensional material: graphene and MoS2 as the channel. The growth of top-gate dielectric is critical in the fabrication process. Therefore, we have optimized the growth conditions of high-k Al2O3 on graphene by the atomic layer deposition technique (ALD). Then, this technique is also applied to top-gated MoS2 transistors. Additionally, a sequential investigation of dual-gated transistor is carried out to investigate the influence of the top dielectric layer to bottom-gated transistors and the difference between top- and bottom-gated transistors. This thesis is divided into two parts. The first part is the fabrication of transferring-free graphene transistors. The ALD growth recipe is first investigated. The best recipe is applied to fabricate the transferring-free graphene transistor. The device shows an hole mobility of 15.4 cm2V-1s-1 and electron mobility of 13.6 cm2V-1s-1 with a hysteresis less than 2 V and a Dirac point at 0 V. Then, the dual-gated graphene transistor is made to investigate the difference between top- and bottom-gated transistors. The performance enhancement is found right after the oxide deposition on bottom-gated transistor, showing the increment of hole mobility 660 cm2V-1s-1to 993 cm2V-1s-1from and electron mobility from 469 cm2V-1s-1 to 911 cm2V-1s-1. We believe that the oxide layer acts as a passivation layer to prevent contaminations from the environment to the graphene channel. However, after the top-gate metal is deposited, the performance of both top- and bottom- gated transistors are degraded. We believe that the heat introduced during e-beam evaporation is responsible for the degradation, which enables graphene to interact with the oxygen atom of the Al2O3 layer and form graphene oxide. In this case, lower conductivity than graphene and thus decreased mobility are observed for the device. In the second part, the oxide growth technique using ALD is applied to the fabrication of transferring-free top-gated MoS2 transistor. The electron mobility is 0.13 cm2V-1s-1. Dual-gated MoS2 structure is also made to compare top-gated transistor with bottom-gated transistor after the MoS2 film transferred to a SiO2/Si substrate. The performance enhancement of bottom-gated transistor is also observed after the oxide is grown on bottom-gated transistor, the mobility values increase from 0.0097 to 0.045 cm2V-1s-1. However, after the top-gate metal is deposited, the performance of bottom-gated transistors is even further improved. We believe that the heat introduced during the metal deposition is responsible for the improved performance, which would enable MoS2 to interact with the oxygen atom of the Al2O3 layer and form Mo oxides. The higher conductivity of the material will induce the mobility value enhancement. On the other hand, the top-gated transistor has worse performance comparing with transferring-free top-gated transistor. This difference further proves the advantage of transferring-free process that avoids the contamination during transfer process.