ALD/ALE 2022 Session NS-WeA2: 2D Materials II

Seamless integration of two-dimensional (2D) semiconductors with bulk materials is essential for preserving and exploiting their outstanding optoelectronic properties within functional devices. In this respect, ALD has proven to be a critical tool for the dielectric integration of 2D materials by tailoring substrates and interfaces.[1] A major challenge that prevents harnessing the full potential of 2D materials is to contact mono- and few-layer systems with metals without introducing defects or otherwise impeding interfacial charge transport. Here, we demonstrate the encapsulation and doping of monolayer MoS₂ with van der Waals (vdW) bonded aluminum oxide (AlO_x) and aluminum oxynitride (AlO_xN_y) by ALD. This is accomplished at low substrate temperature (40 °C) via sequential exposure to TMA and ozone or TMA and N₂ plasma, respectively. Unique to the field of 2D materials, we utilize in situ spectroscopic ellipsometry to assess the effects of adsorbed reactants and film formation on the dielectric function and excitonic properties of a vdW material during ALD, thus allowing optimization of film growth and adlayer modulation doping in real-time.

Current-voltage measurements of monolayer MoS₂ field-effect transistors (FETs) reveal that the nanometer-thin AlO_x coating increases the carrier concentration (from 1×10¹² cm^-2 to 2×10¹³ cm^-2), while it also protects MoS₂ from defect creation during metallization and processing. Complementary Raman spectroscopy and atomic force microscopy characterization reveal the reversibility of modulation doping induced by the AlO_x adlayer. Encapsulated monolayer MoS₂ FETs exhibit a lower contact resistance and an order of magnitude larger maximum drive current, I_ON. By alleviating the effects from the contact interfaces, we were able to reliably determine a field-effect room-temperature mobility of ~10 cm²/Vs for the applied monolayer MoS₂, synthesized by chemical vapor deposition on a large scale (6Carbon Techn.).

Overall, this work demonstrates the scalable and damage-free encapsulation and doping of 2D materials with weakly bonded and ultrathin AlO_x and AlO_xN_y by ALD near room temperature, as well as the fabrication of tunnel contacts, readily compatible with polymer and lift-off processing. Beyond the demonstrated application as a contact interfacial layer, the nanometer-thin conformal coatings are potentially relevant for surface functionalization in chemical sensors and modulation doping of 2D and organic materials implemented in optoelectronic devices.

[1] Grünleitner, T.; Henning*, A.; Bissolo, M.; Kleibert, A.; Vaz, C.A.F.; Stier, A.; Finley, J.J..; Sharp*, I.D.: Adv. Funct. Mater. 2022, 2111341.

2D transition metal dichalcogenides have attracted interest in recent years for their unique optical and electrical properties. Tungsten disulfide (WS­₂)in particular, has gained attention for its applications as channel material for next generation FETs¹ and catalysis.² Popular methods such as mechanical exfoliation and chemical vapor deposition have been demonstrated to synthesize crystalline films with grain sizes of up to a few microns and show good electrical properties, but lack scalability and precise layer thickness control, respectively.³ Atomic layer deposition (ALD) is an ideal technique for achieving highly conformal and uniform films with the layer by layer thickness control needed for these applications, but faces challenges in achieving high crystallinity. Recent work on ALD WS₂ has achieved films with superior electrical properties by improving film crystallinity, either by inducing substrate inhibited growth⁴or by post-deposition annealing.⁵ However, growing crystallites of the order of a few microns remains a challenge and new processes are needed.

In this work, we report ALD of WS₂ using bis(t-butylimido)bis(trimethylsilylmethyl)tungsten (WSN-4) and H₂S. 200 cycles of a 1/5/10/0.1/5/10 s WSN-4/soak/N₂/H₂S/soak/N­₂ pulse sequence shows film growth at temperatures above 290 °C. Grazing incidence x-ray diffractograms of as-deposited films show a strong peak at 13.9° near the dominant 14.32° (002) peak of 2H polytype of WS₂. While no characteristic Raman signal is seen for as-deposited films, x-ray photoelectron spectroscopy reveals the presence of sulfur-deficient WS₂ at 290 °C with improved film quality at a deposition temperature of 350 °C. Post-deposition elevated temperature anneals in elemental sulfur produce a significant improvement in crystallinity at temperatures as low as 600 ⁰C, with SEM images revealing multi-layered WS₂ pyramids with sizes of up to ~ 1 µm. The presence of WS₂ in sulfur-annealed films is further confirmed by the signature Raman 2LA(M), E­¹_2g and A­_1g peaks.

Further details on the ALD process, sulfur annealing, and electrical properties will be presented at the meeting.

1. D. Lin et al., in 2020 IEEE International Electron Devices Meeting (IEDM) (2020), p. 3.6.1
2. D. Voiry, et al., Nat Mater 12, 850 (2013).
3. M. Mattinen et al., Adv. Mater. Interfaces 8, 2001677 (2021).
4. B. Groven, et al., Chem. Mater. 30, 7648 (2018).
5. H. Yang et al., Research 2021, (2021).

In this work, we describe a novel method to deposit high-κ dielectrics on graphene through an in-situ-prepared protective aluminum nitride (AlN) seed-layer. The process is performed in an Oxford Instruments Atomfab^TM plasma ALD system.¹ Short and low power remote plasma conditions were used to directly grow a thin layer of AlN on graphene, followed by deposition of high-quality aluminum oxide (Al₂O₃) by remote plasma ALD.

For the development of graphene-based devices, such as transistors, photodetectors, or optical modulators, a deposition of a high-quality dielectric film on graphene is required. However, this deposition is challenging because nucleation on pristine graphene is difficult. While defect-induced nucleation, for example through plasma exposure, improves nucleation, it also decreases the quality of the graphene layer. Recently we reported dielectric deposition using remote plasma ALD, without observable damage, by protecting the graphene by hexagonal boron nitride (hBN).² However, using hBN involves additional transfer processes, which may complicate the fabrication and introduce contamination, defects, and wrinkles.

Inspired by this process, we developed a new process using an in-situ deposited AlN seed-layer to protect the graphene effectively, which enables plasma-assisted deposition of Al₂O₃ without damaging the graphene. Raman measurements demonstrate that the wafer encapsulated by PEALD without AlN shows damage to the graphene, while the wafer protected by the AlN seed layer shows negligible damage. This result confirms that a thin layer of AlN provides sufficient protection for the graphene against the O₂ plasma in the subsequent Al₂O₃ deposition step. The N₂ based plasma conditions for the AlN layer were such to allow AlN growth but not lead to observable damage to the graphene. In this contribution, we will furthermore discuss electrical and device properties for this scalable wafer-level production method.

Acknowledgment: This project has received funding from the European Union's Horizon 2020 research and innovation program 2D-EPL (952792) and German BMBF project GIMMIK (03XP0210).

References:

1. Knoops et al., JVST A 39 (6), 2021
2. Canto et al., Adv. Mater. Technol. 6 (11), 2021