Direct Growth of Graphene on Dielectric Materials by Low Temperature Plasma-Enhanced CVD

Graphene offers outstanding electronic and optical properties, such as high electrical conductivity and high transparency in the UV-VIS spectral range. The most developed method for growing high quality graphene is chemical vapor deposition (CVD) on copper (Cu) substrates, where Cu has a catalytic effect on the growth. Practical applications, however, require a subsequent transfer of the graphene onto target substrates like, e.g., dielectric materials. This transfer process induces defects and contaminations, which in turn leads to reduced performance of the intended applications [1]. Thus, a direct growth on dielectric substrates, such as sapphire and (Al,Ga)N is preferable, enabling, e.g., the use of graphene as a transparent electrode for (Al,Ga)N-LEDs [2], 2D-photodetectors [3] or biosensors [4] without any complex transfer process.<br/><br/>We developed a low temperature plasma-enhanced CVD (PECVD) process for depositing graphene on dielectric substrates in a transfer-free approach. The PECVD process was conducted in an industrially relevant 4-inch system with methane (CH₄) as the precursor. For growing graphene on (Al,Ga)N, the conventionally used hydrogen (H₂) carrier gas was replaced by nitrogen (N₂) for preventing surface decomposition of the (Al,Ga)N [2]. Under optimized conditions, i.e., a gas mixture of CH₄/N_2,a plasma power of 40 W, a growth time of 60 min and a growth temperature of 670 °C, graphene layers with Raman intensity ratios I_D/I_G ≈ 1.6 and I_2D/I_G ≈ 1.4, respectively, have been realized. The sheet resistance could be reduced to &lt; 5 kΩ/sq and the transmittance of the optimized graphene film was ≥ 90% in the UV-VIS spectral range. For graphene growth on sapphire with different crystal orientations (c-plane, ca-plane, a-plane and r-plane), no distinct dependence of the graphene quality on the crystal orientation is found, in contrast to findings for thermal CVD as reported in literature [5]. Under optimized conditions, the I_D/I_G ratio was as low as 0.7 and a sheet resistance down to 1.65 kΩ/sq could be achieved. Apparently, the PECVD process seems to provide a flexible approach for depositing graphene on dielectric substrates, where the balance between sheet resistance and optical transmittance can be willingly adjusted by the growth parameters.<br/><br/>[1] Wang et al., Adv.Mater.2016, <b>28</b>, 4956–4975<br/>[2] Mischke et al., 2D Mater. <b>7</b> (2020) 035019<br/>[3] Munoz et al., npj 2D Materials and Applications <b>7</b>, 57 (2023)<br/>[4] Xu et al. Applied Surface Science <b>427</b>, 1114–1119 (2018)<br/>[5] Ueda et al., Appl. Phys. Lett. <b>115</b> (2019) 013103