Van der Waals Epitaxy: A new way for growth of III-nitrides
Van der Waals Epitaxy: A new way for growth of III-nitrides
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范德华外延：III族氮化物生长的新方法
DOI：
10.1007/s11431-019-1488-y
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发表时间：
2020-03
影响因子：
4.6
通讯作者：
Li Dabing
中科院分区：
文献类型：
--
作者：
Chen yang;Jia Yuping;Shi Zhiming;Sun Xiaojaun;Li Dabing
无机半导体在当今的技术进步中发挥着关键作用[1 - 4]。与其他有机半导体[5，6]和二维（2D）金属硫化物[7 - 10]相比，III族氮化物作为直接带隙半导体在商业应用中取得了巨大的成功。由于其优异的物理性质和通过其家族中的组成设计从紫外到近红外范围的可调带隙[11 - 13]，III族氮化物在许多领域中显示出巨大的潜力，包括但不限于紫外（UV）发光二极管（LED）和光电探测器。由于缺乏合适的衬底，III族氮化物通常通过金属有机化学气相沉积（MOCVD）在异质衬底上外延生长[14，15]。外延层和衬底在晶格常数和热膨胀系数方面的固有差异导致大的残余应力和高缺陷密度，因此降低了III族氮化物基器件的性能[16]。尽管在降低III族氮化物中的缺陷密度方面取得了很大成就，但仍然存在对改善晶体质量的进一步要求。此外，衬底和外延层之间的强共价键使得难以从衬底剥离III族氮化物膜以进一步制造柔性和可穿戴设备。最近，一种称为"货车德瓦尔斯（vdW）外延"的新方法提供了解决上述困难的希望。通过在衬底和外延层之间引入2D材料，由于2D缓冲层可以改善晶体质量，并且由于弱的vdW相互作用，通过机械剥离可以容易地释放外延层。在二维材料家族中，石墨烯是研究最广泛的vdW外延材料。图1（a）显示了ZnO涂覆的石墨烯作为种子层以生长GaN外延层和随后的LED结构，其可以容易地转移到其他异质衬底上[17]。薄层石墨烯不能屏蔽像GaAs这样的一些衬底的强势场，并且外延层的生长仍然由下面的同质外延衬底控制[18]。在石墨烯涂覆的蓝宝石和非晶硅氧化物上生长的高质量GaN和AlN已经通过vdW外延工艺实现[19 - 21]。由于III族氮化物和石墨烯之间的弱vdW相互作用，外延层受到下面衬底的裸露限制，然后实现低缺陷密度和松弛应力。由于原始石墨烯是一种集成的二维平面结构，缺乏悬挂键，导致III族氮化物的低成核密度和三维岛状生长模式。对原始石墨烯引入N2等离子体或NH3处理（在MOCVD系统中）以产生CN键合作为III族氮化物成核位点（图1（B））。除了石墨烯之外，hBN也已被实验性地用作缓冲层以生长无应力和可转移的III族氮化物外延层[22]。根据最近的报道，在hBN上的III-氮化物vdW外延的晶体质量劣于在石墨烯上生长的晶体质量[23，24]。为了探索有效的vdw外延策略，构建与III族氮化物vdw匹配的2D材料衬底，
Inorganic semiconductor plays a key role for today’s technological progress [1–4]. Comparing with other organic semiconductors [5, 6] and two-dimensional (2D) metal sulfides [7–10], III-nitrides as direct bandgap semiconductors have achieved enormous success in commercial applications. Owing to their excellent physical properties and adjustable bandgap from ultraviolet to near-infrared range by composition design in their family [11–13], III-nitrides have shown great potential in many fields including but not limited to ultraviolet (UV) light-emitting diodes (LEDs) and photodetectors. Due to the deficiency of suitable substrate, III-nitrides are usually epitaxial on foreign substrates by metalorganic chemical vapor deposition (MOCVD)[14, 15]. The intrinsic difference between epitaxy layer and substrate in lattice constant and thermal expansion coefficient induces large residual stress and high defect density, and therefore degrading the performance of III-nitride based devices [16]. In spite of great achievement in reducing defect density in III-nitrides, further requirement for improvement in crystal quality is still remained. In addition, the strong covalent bond between substrate and epitaxy layer makes it difficult to exfoliate III-nitride films from substrate for further fabrication in flexible and wearable devices. Recently, a novel method termed as “van der Waals (vdW) epitaxy” provides promise to solve both above-mentioned difficulties. By introducing 2D materials between substrate and epitaxy layer, the crystal quality could be improved due to the 2D buffer layer and the release of epitaxy layer is facile by the mechanical exfoliation owing to the weak vdW interaction. Among 2D material family, graphene is the most extensively investigated material for vdW epitaxy. Figure 1 (a) shows ZnO-coated graphene as seed layer to grow GaN epilayer and subsequent LEDs structure, which could be easily transferred onto other foreign substrates [17]. The thin layer graphene could not screen the strong potential field of some substrate like GaAs and the growth of epilayer is still controlled by the underneath homoepitaxial substrate [18]. High-quality GaN and AlN grown on graphene-coated sapphire and amorphous silicon oxide have been realized by vdW epitaxial process [19–21]. Thanks to the weak vdW interaction between III-nitrides and graphene, epilayer suffers bare restriction from the underneath substrates then realizes low defect density and relaxed stress. As pristine graphene is an integrated 2D flat structure which lacks of dangling bonds, resulting in low nucleation density of III-nitrides and three-dimensional island growth mode. The N2 plasma or NH3 treatment (in MOCVD system) are introduced for pristine graphene to create CN bonding as III-nitride nucleation sites (Figure 1 (b)). In addition to graphene, hBN has also been experimentally applied as buffer layer to grow stress-free and transferable III-nitrides epilayer [22]. According to recent reports, the crystal quality of III-nitrides vdW epitaxy on hBN is inferior to that grown on graphene [23, 24]. To explore efficient vdWepitaxy strategy, the construction of 2D materials’ substrates matched with III-nitrides vdW
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