(Invited) Recent Progress in Large Area MOCVD Growth of 2D Materials

Abstract

Metal-organic chemical vapor deposition (MOCVD) has developed to a mature production technology for state-of-the-art (opto)electronic devices such as LEDs, lasers, solar cells and transistors. Current multi-wafer MOCVD systems allow for scalable processes with precise control of growth parameters ensuring excellent uniformity of deposition rates, dopant concentrations and layer thicknesses. All of these properties are of crucial importance for reproducibly obtaining high-quality ultra-thin films required for next-generation nano-electronic devices. One of the most promising materials for the fabrication of such devices are layered transition-metal dichalcogenides (TMDCs), e.g. molybdenum disulfide (MoS2) or tungsten disulfide (WS2), graphene (Gr) or hBN. Due to their 2D structure, these materials possess unique optical, electrical and magnetic characteristics, and monolayers are considered as material system for several future applications. Moreover, the properties and functionality of a single monolayer can be even further improved by forming heterostructures of several two-dimensional materials. Such complex structures which require control on the atomic level strongly limit the number of available fabrication options, especially for volume production, leaving MOCVD as the most reliable industrial deposition method.In this work, we present wafer-scale MOCVD of high-quality monolayers of MoS2, WS2, hBN and graphene (Gr) on single-crystalline C-plane (0001) sapphire deposited in commercial AIXTRON reactors. Our upscaling effort from 2 inch wafer up to growth on 200 mm wafer supports the industrial demand. Molybdenum hexacarbonyl, tungsten hexacarbonyl and di-tert-butyl sulfide are employed as precursors. By optimizing growth conditions, fully-coalesced MoS2 and WS2 samples without parasitic carbon-related Raman peaks were obtained. XPS analysis indicates only Mo-S and W-S bonds for MoS2 and WS2 samples, respectively, with no indications of any side reactions with either the substrate, impurities or oxygen and demonstrates a high layer quality suitable for research on all kind of devices including advanced transistors and memresistive devices. Samples were characterized using Raman/photoluminescence spectroscopy, SEM and AFM to understand the correlation of growth parameter and material properties. As an example a WS2 growth duration series at 700°C temperature was performed. The intensity ratio of about 2 for the Raman E2g/A1g peaks for the optimized WS2 samples suggests deposition of monolayer WS2 with some admixture of bilayer, as confirmed by AFM. The WS2 monolayer film growth has been successfully upscaled from 2” to 200mm sapphire wafers. hBN thin films have been also grown from borazine precursor both on 2” and 200 mm sapphire substrates. Thickness uniformity of 9 nm hBN films was 3% on 200 mm wafers, as determined by XRR. High-quality monolayer graphene layers grown from CH4/H2 on 2” and 200mm sapphire wafers demonstrated good performance by Raman (2D/G peak ratio >1.5 and low D/G peak ratio (<0.25), as well as low sheet resistance (~1300-1500 Ohm/sq). The grown films had smooth surface morphology with the expected graphene wrinkles visible by AFM). These results demonstrate the applicability of AIXTRON systems for the growth of TMDC, hBN and graphene films on large size substrates. The growth could be easily upscaled from 2” to 200 mm wafer growth with only minor adjustment of the growth parameters. More details on the growth process and material properties as well as results on hBN/Gr, TMDC/Gr and TMDC/hBN heterostructures will be discussed in detail. Figure 1