表面预处理对石墨烯上范德瓦耳斯外延生长GaN材料的影响

Effect of surface pretreatment on GaN van der Waals epitaxy growth on graphene

基于范德瓦耳斯外延生长的氮化镓/石墨烯材料异质生长界面仅靠较弱的范德瓦耳斯力束缚,具有低位错、易剥离等优势,近年来引起了人们的广泛关注.采用NH_3/H_2混合气体对石墨烯表面进行预处理,研究了不同NH_3/H_2比对石墨烯表面形貌、拉曼散射的影响,探讨了石墨烯在NH_3和H_2混合气氛下的表面预处理机制,最后在石墨烯上外延生长了1.6μm厚的GaN薄膜材料.实验结果表明:石墨烯中褶皱处的C原子更容易与气体发生刻蚀反应,刻蚀方向沿着褶皱进行;适当NH_3/H_2比的混合气体对石墨烯进行表面预处理可有效改善石墨烯上GaN材料的晶体质量.本研究提供了一种可有效提高GaN晶体质量的石墨烯表面预处理方法,可为进一步研究二维材料上高质量的GaN外延生长提供参考.

Due to the weak van der Waals interaction between GaN epitaxial layer and graphene substrate, GaN grown on graphene has attracted considerable attention in recent years, benefited from the possibility to grow epitaxial material without any necessity to satisfy the requirement for the lattice matching between the epitaxial materials and underlying materials, and the unique facility of transferring GaN epitaxy to other substrates. However, clusters formed in GaN grown on graphene lead to poor crystalline quality, deteriorating the applications of GaN epilayer on graphene. It is observed that preferential nucleation occurs primarily at the sites of defects and along the step edges of graphene. In order to study the effects of NH3/H2 ratio on the graphene/sapphire template and properties of GaN epilayer, the growth of GaN by metal organic chemical vapor deposition on the graphene/sapphire template pretreated with the mixed gas of NH3 and H2 is investigated.Prior to the deposition of GaN, five samples with different NH3/H2 flow ratios（0, 0.2, 0.5, 1 and 2, respectively）are pretreated at 1030℃ while the H2 flow rate is fixed at 3.6 mol/min. The surface topographies and Raman spectra of the pretreated graphene are investigated, and the chemical reaction mechanism is studied. It is found that the graphene is etched at the wrinkle firstly and then along the direction of wrinkles where there is bigger contact interface with NH3 and H2, and graphene decomposition is enhanced with the increase of NH3/H2 flow ratio. The pretreatment mechanisms of different mixed gases are also discussed. Owing to the weak bond energy, NH3 is easier to decompose than H2.The reaction between graphene and H, NH2 which are produced by the decomposition of NH3, enhances the etching of graphene.Finally GaN film is deposited on graphene/sapphire template pretreated by different NH3/H2 flow ratios. The quality of GaN was improved on graphene pretreated by appropriate NH3/H2 flow ratio and verified through highresolution X-ray diffraction.The lowest（002） and（102） full widths at half maximum（FWHM） of GaN obtained on graphene/sapphire template are 587 arcsec and 707 arcsec respectively, while the root-mean-square（RMS） of GaN is 0.37 nm. The stress of GaN is characterized by Raman spectra at room temperature. The co-presence of characteristic peaks of sapphire, graphene and GaN suggests that GaN has deposited on graphene/sapphire template. The E2-high Raman peak is used to estimate the residual stress in GaN material as described elsewhere. The E2-high peak of GaN grown on graphene is around 566.7 cm^（-1）, while the value of strain-free GaN is 566.2 cm^（-1）. Thus, there is subtle compressive stress in the GaN grown on graphene, which can be calculated from the relationship： ？ωγ= 4.3 · σ（χχ）cm^（-1）·GPa^（-1）, giving a value of 0.11 GPa of GaN obtained on graphene/sapphire template.This study provides an effective pretreatment technique to improve the crystal quality of GaN epilayer deposited on graphene/sapphire template, which gives guidance in well crystallizing three-dimensional materials on two-dimensional materials.