Electronic and optoelectronic devices based on heterostructure with two-dimensional materials

Abstract

Two-dimensional (2D) materials have attracted much attention as a ‘helping hand’ to conventional silicon semiconductor technology because of their superb electronic and photonic properties as well as excellent mechanical flexibility. 2D materials are layered materials, which consist of covalent bond at in-plane direction and van der Waals bond at out-of-plane direction. This unique structure makes it possible to form the van der Waals heterojunction, which is not capable of conventional bulk semiconductor technologies. For example, we can fabricate not only two-dimensional 2D-2D heterojunction but also three dimensional bulk semiconductor with two-dimensional semiconductor of 3D-2D heterojunction. These van der Waals heterostructure have several advantages compared to conventional heterostructure. First, we don’t have to concern the lattice mismatching issues between two semiconductors because of van der Waals bond. Second, the unique interface characteristics can be obtained. For example, we can easily fabricate the heterostructure which has sharp doping profile as well as rapidly changed charge distribution. In addition, 2D-2D semiconductor van der Waals heterojunction has much smoother interface than Si/$SiO_2$. If we utilize the characteristics of 2D semiconductor based van der Waals heterojunction, we can fabricate excellent electronic and optoelectronic devices. In this thesis, the exfoliation method for thin 2D materials and the transfer technique is developed for experiments, and the research of electronic and optoelectronic devices based on 2D van der Waals heterostructure have been conducted. In electronic devices, we studied for 3D-2D vertical tunneling field effect transistor (FET) based on three layers of $MoS_2$ and highly doped p-type Si. 8-nm $Al_2O_3$ gate dielectric was deposited on $Si-MoS_2$ heterojunction using atomic layer deposition (ALD). Fabricated vertical tunneling FET shows the minimum subthreshold swing (SS) of 15 mV/dec and the average SS of 77 mV/dec for 4 decades. The current On/Off ratio is $10^7$, and the band-to-band tunneling mechanism was confirmed by low-temperature I-V measurement. In addition, we studied for high performance field effect transistor (FET) based on GaS-$MoS_2$ heterostructure (2D-2D). $MoS_2$ acts as the channel and gallium sulfide (GaS) acts as the gate insulator. Fabricated device shows the field effect mobility of 83 $cm^2$/Vs, SS of 63 mV/dec and the current On/Off ratio of $10^6$. Furthermore, Logic gates including the inverter and NAND gate were demonstrated. In optoelectronic devices, we performed the study of the photodetector based on Si-$MoS_2$ heterostructure to improve the responsivity. The resulting 48-nm $MoS_2$/Si device shows the maximum responsivity of 76.1 A/W and the detectivity of 1012 Jones. Furthermore, the depletion layer thickness was theoretically calculated by the depletion layer model. Low-frequency noise is performed and a low noise equivalent power (NEP) of $7.82 \times 10^{-15} W Hz^{-1/2}$ is achieved. In addition, we conducted the study of the phototransistor based on $WS_e2-MoS_2$ heterostructure in order to improve the responsivity. $MoS_2$ acts as the main conduction channel whereas WSe2-MoS2 heterostructure acts as the charge transfer layer. The out-of-plane PN junction of $WSe_2$ and $MoS_2$ separates the electron-hole pairs by the built-in electric field. This hinders the recombination of photo-generated carriers, increasing the carrier life time and photoresponsivity. The device shows the maximum responsivity of 2700 A/W and detectivity of $10^{11}$ Jones. We have studied the electronic and optoelectronic devices based on van der Waals heterostructure with 2D materials. Through these studies, the limitation of the silicon based electronic and optoelectronic device could be mitigated and contribute to practical applications with the technology development of large area 2D material synthesis.