Large Scale Multi-layer MoS₂ Growth by Reactive Suttering for Light Absorber Applications

Transition metal dichalcogenides (TMDCs) are renowned for their high light absorption coefficient, high in-plane carrier mobility, and strong in-plane Young’s modulus, but at the same time low absorptivity in monolayer [1,2]. Multi-layer TMDCs, on the other hand, have strong absorptivity even with their indirect bandgap nature and can be suitable for certain applications that necessitate higher carrier mobility and light absorptivity. Research on their applications is limited so far due to challenges in fabricating multi-layer TMDCs because of weak van der Waals interaction that hinders effective formation of nuclei for n-th layer growth [3]. Alternative growth methods seem necessary to exploit the merit of multi-layer TMDCs for optoelectronic applications.
Reactive sputtering can be suitable for multi-layer growth with controlled orientation [4]. In reactive sputtering, films with desired property can be fabricated relatively easily as each precursor flux can be controlled individually. In this research, we use reactive sputtering system equipped with a hot lip cell filled with sulfur powder. By optimizing the growth conditions, we succeeded in sputtering laterally oriented epitaxially aligned 20 nm thick MoS₂ film in 4 cm² area. This research first reports the growth condition that enables laterally oriented epitaxial multi-layer growth and then suggests light trapping strategies for absorptivity enhancement.
20 nm thick laterally oriented epitaxially aligned MoS₂ can be fabricated on c-plane sapphire at high sulfur to molybdenum flux ratio at high substrate temperature. XRD analysis shows that MoS₂ film and sapphire substrate show R30°/90° relationship. This means that epitaxial alignment with the sapphire substrate guides the growth of MoS₂, which is not observed on other substrates, such as silicon or glass. Sulfur to molybdenum flux ratio is controlled by working pressure, sulfur hot lip cell temperature, and molybdenum target RF power. Working pressure as low as 1.5 Pa at sulfur hot lip cell temperature of 160 °C and molybdenum target RF power of 10 W ensure lateral growth with no out-of-plane vertical growth. Growth temperature is important as low substrate temperature leads to poor crystallinity. Substrate temperature is set as 550 °C in this study.
Light trapping strategies can enhance the absorptivity of multi-layer MoS₂. Calculation by transfer matrix method shows that absorptivity enhancement merely by increasing the thickness is limited due to strong reflection from MoS₂. Use of appropriate back mirror and anti-reflection coating can improve the absorptivity of MoS₂ significantly. Among palladium, gold, molybdenum, and silver, silver is the best back mirror for enhancement in absorptivity by MoS₂. For the ease of comparison, photocurrent density generation under AM 1.5G illumination is calculated using the absorptivity of MoS₂ in different configurations assuming 100 % photocarrier collection. Single 20 nm thick MoS₂ film can produce 7.6 mA/cm² whereas 20 nm thick MoS₂ on Ag back mirror can produce 20.6 mA/cm². 80 nm thick SiO₂ anti-reflection coating on top can further enhance photocurrent generation 1.15 times with Fary-Pérot interference.
In this research, we demonstrate 20 nm thick laterally oriented epitaxially aligned MoS₂ film growth by reactive sputtering and strategies to further enhance its absorptivity. Unidirectional multi-layer MoS₂ thickness of 20 nm is the thickest value reported as far as we found, and even with 20 nm thick MoS₂ film, photocurrent density of 23.7 mA/cm² under AM1.5 G illumination can be achieved with the help of anti-reflection coating and back mirror. We expect our findings to pave a way toward further exploitation of multi-layer MoS₂ for optoelectronic applications.
References
[1] Manzeli et al., Nat. Rev. Mater., 2 (2017). 1.
[2] An et al., Adv. Funct. Mater., 32 (2022). 2110119.
[3] Samad et al., ACS Nano, 10 (2016). 7039.
[4] Jang et al., Nanotechnology, 31 (2020). 225206.