**2. vdWE vs. conventional epitaxy**

Thanks to the two-dimensional (2D)-layered materials whose unique and promising proper‐ ties enable to open up a new route of heteroepitaxy without the aforementioned constraints. Recently, the so-called van der Waals epitaxy (vdWE), a new paradigm of epitaxial growth for III–V semiconductors, has been the focus of significant research interest, which is evidenced by a search in Google Scholar, leading to 12,200 hits in the years 2011 through 2015 [16–21]. Needless to say, this new research area of heteroepitaxy is mainly dictated by the surface properties of emerging 2D materials, which also have many novel electrical, optical, thermal,

44 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Recently, different research groups already used few 2D atomic-layered materials as a semiconductor substrate for the growth of 3D semiconductors [35–37]. Among them, gra‐ phene, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), and topo‐ logical insulator materials are the most frequently used ones. The main reason why these 2D materials are advantageous to be used as a semiconductor substrate is that the overgrown film materials float on top of the dangling-bonds-free 2D substrate surface instead of being rigidly bound to it, thereby mitigating lattice and thermal mismatch between 3D materials and the underlying 2D substrate. Besides, due to the crystalline surface with triangular lattice or honeycomb atomic arrangement, 2D materials are structurally compatible with many 3D semiconductors of zinc-blend (ZB), wurtzite (WZ), and diamond crystal structures. Further‐ more, most of the 2D materials are thermally stable, which exhibit high decomposition temperature, thus making it an ideal material of choice for many fabrication processes.

The integration of III–V semiconductors on 2D materials is feasible using the epitaxial growth by molecular beam epitaxy (MBE) or metal organic chemical vapor phase deposition (MOCVD). Layered materials can be grown directly on the target substrates using either chemical vapor deposition (CVD) or MBE [38, 39]. Then, the layered material can be mechan‐ ically transferred onto any substrate after being grown if needed. Graphene is found to be one of the most popular 2D materials for van der Waals (vdW) epitaxial growth. A number of studies are undertaken to achieve high-quality MBE-grown GaAs nanowires (NWs) on a graphene/silicon substrate. High-density, vertical, coaxially heterostructured InAs/InGaAs NWs on graphene over a wide tunable ternary compositional range is demonstrated using

Using graphene as a 2D material for relieving thermal/lattice mismatch, the proof-of-principle demonstrations for the epitaxial growth of GaAs [36, 40], InAs [41], and GaN [23, 42, 43] on silicon substrates are already presented in several studies. Hence, these early demonstrations show a great promise of vdW heteroepitaxy for integrating III–V semiconductors on silicon using 2D materials as a buffer layer. Both nanostructures and thin films of these compound semiconductors are addressed by these preliminary studies. Most importantly, the results obtained from these studies motivate to realize high-quality epitaxial thin film or nanostruc‐ tures of other III–V semiconductors, for example, InP, GaSb, InAs on silicon using graphene buffer layers. In such a way, the epitaxial growth on 2D material platform will have significant implications for a wide variety of optoelectronic devices, such as light-emitting devices [26,

42] and a new generation of solar cells for flexible applications [44].

and mechanical properties [22–34].

MOCVD.

VdWE was first introduced by Koma in 1984 where he successfully grew NbSe2/MoS2 system using vdW weak interactions [18, 45]. In this type of epitaxy, the growth is driven by the vdW forces between the 2D layers with neither surface dangling bonds nor passivation. These weak vdW interactions between the layers during the epitaxy releases any stress stored in the strained films, yielding high-quality material with reduced threading dislocation densities. This kind of epitaxy is different from the conventional growth where lattice-matching conditions are very important to have high-quality single-crystal semiconductors. As an intermediate type of epitaxy between vdWE and the conventional epitaxy, quasi van der Waals epitaxy (QvdWE) emerged recently by integrating 3D conventional semiconductors on layered substrates. In QvdWE, the interface interactions are not purely based on the vdW interaction but a midpoint where the dangling bonds of the grown 3D semiconductor have an effect on the epitaxial growth [46]. Importantly, the concept of QvdWE can be applied for the deposition of different types of grown structures including nanowires and thin films, as schematically shown in **Figure 1**. In the following sections, several demonstrations by different research teams worldwide for such structures utilizing QvdWE are discussed.

**Figure 1.** Different types of growth structures.
