Er₂O₃ Top Gate MoS₂ FET with EOT Lower than 1 nm

Two-dimensional (2D) molybdenum disulfide (MoS₂) has emerged as a promising channel material for the next generation of electronics, offering a solution to the scaling limitations of silicon due to its atomic thinness and the maintained mobility the layered structure brings. To achieve effective gate control of MoS₂-based field-effect transistors (FETs) while meeting scaling requirements, the integration of high dielectric (high-k) oxides onto the MoS₂ surface is essential. However, the absence of dangling bonds at the MoS₂ surface significantly hinders conventional atomic layer deposition (ALD) method for high-k oxides, which relies on an active surface as a nucleation site. Alternative techniques, like van der Waals integration, are considered as ideal for minimizing interface traps between the insulator and the MoS₂ channel. Nonetheless, both the choice of materials and the integration mechanisms are requiring further investigation from the viewpoint of real application. The judicious selection of materials and suitable integration methods is, therefore, of paramount importance for the prospective application of MoS₂ FETs in the future.<br/>While the thermal evaporation method is widely employed for thin film fabrication, the thermal evaporation of high-k materials faces a significant challenge due to their exceptionally high melting points, often exceeding 2000°C. This makes it nearly impossible to attain the desired partial pressure for deposition. Instead, the evaporation of a metal, followed by subsequent oxidation to form high-k oxides, offers a potential solution. Herein, erbium (Er), with its oxidation product Er₂O₃, was selected as the high-k material based on a well-designed differential partial pressure system, allowing for the separated evaporation and oxidation by meticulously controlling the vapor pressure of the Er source and the partial pressure of the oxygen supply. As the result, extremely low deposition rate of approximately 0.16 Å/s was achieved. This low deposition rate is expected to have minimal impact on the intrinsic properties of the MoS₂ surface. Furthermore, the thickness of the deposited Er₂O₃ can be precisely controlled by adjusting the deposition time, offering the possibility of reducing the equivalent oxide thickness (EOT).<br/>In this study, a dual-gated MoS₂ FET with a 3.5 nm Er₂O₃ stack was fabricated using the differential partial pressure deposition system. The top gate capacitance was evaluated through I-V characteristics, utilizing a sweep of both top gate voltage and back gate voltage. The results revealed negligible threshold shifts and on/off switching within a 1 V range, confirming a high top-gate capacitance with <i>k</i> = ~13, resulting in an EOT of 0.94 nm. Low-temperature photoluminescence (LT-PL) and Raman characterizations indicated a lack of defect-bound exciton response, as well as no significant Raman peak shift or broadening, suggesting that the high-k deposition is non-destructive.<br/>Moreover, the stability of Er₂O₃ and the electrical reliability of the Er₂O₃/MoS₂ gate stack were investigated over an extended period, up to 100 days. Although the reduction of the dielectric constant was observed for the Er₂O₃/MoS₂ stack, the Er₂O₃/SiO₂ stack exhibited no reduction in dielectric constant over the same duration, suggesting that stability concerns were attributed to the MoS₂ surface rather than the high-<i>k</i> Er₂O₃. On the contrary, the interface properties, as exemplified by the subthreshold swings (S.S.), were improved over time, which may be attributed to the relaxation of strain introduced during the high-k deposition process. These findings regarding the degradation of bulk property (k) and the enhancement of S.S. over time highlight that improving the initial interface quality between high-k materials and 2D materials will lead to enhanced bulk properties.