**4.1. GaAs thin films**

directly on graphene substrates [48]. In this growth, metalorganic chemical vapor deposition (MOCVD) was used to integrate the III-V semiconductor on the graphene. The growth of InGaAs nanowires led to spontaneous separation of phase from the start of the growth, which eventually led to a well-defined InAs/In*x*Ga1–*x*As (0.2 < *x* < 1) core-shell structure. The axial growth of InAs-In*x*Ga1–*x*As (0.2 < *x* < 1) started without change in diameter of nanowires. After about 2 μm in height, those nanowires were grown in the form of core-shell structures. The shell composition of InAs-In*x*Ga1–*x*As changes as a function of indium flow; however, the thickness of the shell and core and the start of nonsegregated InAs-In*x*Ga1–*x*As are independent of the composition of indium (**Figure 3**). Moreover, the authors found out that no InGaAs phase segregation was observed when growing on MoS2 or through the Au-assisted vapor-liquidsolid (VLS) technique on graphene. This work demonstrated that QvdWE of InAs on graphene could facilitate phase segregation phenomenon causing a self-organized InAs core/InGaAs shell nanowires. This phase segregation is driven mainly by lack of strain between InAs and

**Figure 4.** The percentage of vertically well-aligned InAs nanowires (*black solid circles*) and the density of nanowires (*blue empty squares*) relationship to the rms surface roughness of the graphene substrates. Reproduced with permission

Hong et al. found out that the density of InAs nanowires arrays integrated on graphene films increases with the number of graphene layers [41]. This is mainly because the nucleation of InAs is more likely on the surface potential wells formed by surface ledges in the graphene. However, the use of single-layer graphene as a substrate will encourage the growth of vertically well-aligned InAs arrays of nanowires with high levels of strong vdW attraction (**Figure 4**). The QvdW epitaxial relationship between InAs and graphene is confirmed by the InAs's hexagonal morphology with six ZB sidewall facets, aligned in a sixfold within a single

from Ref. [41].

domain of graphene.

graphene layers interface due to the weak vdW forces at this interface.

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

The integration of III–V thin films on 2D materials opened a new and unique opportunity by releasing the lattice/thermal expansion mismatches between any 3D semiconductors and 2D material. Moreover, the weak vdW force of the layered materials allows reusing the 2D substrate multiple times by peeling off the grown semiconductors from the layered materials [42]. The demonstration of III-As-based thin films growth on graphene was reported by Alaskar et al. where ultra-smooth planar GaAs was deposited on silicon substrate using graphene as a buffer layer was performed, as presented in **Figure 5** [36]. In addition, the challenges that face the epitaxial growth of III-V semiconductors atop 2D materials, especially graphene, were highlighted. From a thermodynamics perspective of the growth, the low surface free energy of the layered materials yields an island growth mode where the surface of the layered 2D substrate is nonwetting.

The surface free energies of 3D semiconductors are two orders of magnitude higher than layered materials (**Table 1**), which make this type of epitaxial growth challenging. Indeed, engineering tricks are needed to overcome this issue [50–52].

The adsorption and migration energies of III-V-based atoms on graphene, as listed in **Ta‐ ble 2**, indicate that the growth of GaAs on graphene should be initiated with gallium for its higher adsorption and migration energies compared to arsenic, indium, or aluminum.

**Figure 5.** (a) The atomic geometry of GaAs/graphene/Si structure using (b) the schematic view of multilayered gra‐ phene used as a buffer layer for GaAs on silicon. Reproduced with permission from Ref. [36].


**Table 1.** The surface free energies of different 3D semiconductors and layered semiconductors.


*Note*: Reproduced with permission from Ref. [36].

**Table 2.** Adsorption and migration energies of Ga, As, In, and Al adatoms adsorbed on most favorable site of bilayer graphene based on density functional theory (DFT) calculation, taking into account vdW forces.

Utilizing the superior kinetic properties of gallium, the growth was initiated by two mono‐ layers of Ga atoms at room temperature. This was followed by arsenic overpressure while elevating the growth temperature to 400°C and the deposition of 25 nm GaAs atop graphene at a rate of 0.15 Å/s. The GaAs morphology showed a smooth surface with an RMS value of 0.6 nm. The crystal quality was characterized using both Raman spectroscopy and X-ray diffraction (XRD) [40]. The two GaAs Raman signature peaks at 268 (TO) and 292 (LO) cm−1 can be seen in the micro-Raman spectrum (**Figure 6a**). **Figure 6b** shows the rocking curve full width at half maximum (FWHM) value for the GaAs(111) plane around 245 arcsec (0.068°), indicating high defect density. This is still encouraging since the direct growth of GaAs with the same thickness shows much higher defect density [2].

**Figure 6.** (a) Room temperature micro-Raman spectrum of GaAs on graphene/silicon shows TO and LO peaks at 268 and 292 cm−1, respectively, where the TO peak is the dominant one. (b) XRD rocking-curve of GaAs (111) peak. Repro‐ duced with permission from Ref. [36].

#### **4.2. GaN thin films**

**Materials Surface free energy (mJ m−2)**

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

**Table 1.** The surface free energies of different 3D semiconductors and layered semiconductors.

Gallium 1.5 0.05

Arsenic 1.3 0.21

Indium 1.3 0.06

Aluminum 1.7 0.03

graphene based on density functional theory (DFT) calculation, taking into account vdW forces.

**Table 2.** Adsorption and migration energies of Ga, As, In, and Al adatoms adsorbed on most favorable site of bilayer

Utilizing the superior kinetic properties of gallium, the growth was initiated by two mono‐ layers of Ga atoms at room temperature. This was followed by arsenic overpressure while elevating the growth temperature to 400°C and the deposition of 25 nm GaAs atop graphene

**Atom Favored adsorption site Adsorption energy** *E***b (eV) Migration energy** *E***m (eV)**

Si (111) 1467 [53]

GaAs (111) 1697 [53]

Graphene 48

Multilayer graphene (MLG) 52

*Note*: Reproduced with permission from Ref. [36].

*Note*: Reproduced with permission from Ref. [36].

Bismuth selenide (Bi2Se3) 180 [54]

So far, the thin-film growth of nitride-based semiconductors on graphene is more successful and promising compared to III-As semiconductors. This is mainly because of the high adsorption energy of nitrogen on graphene of (4.6 eV) compared to arsenic or phosphide [55, 56].

An IBM research group demonstrated the QvdWE growth of high-quality GaN on graphene/ SiC substrates. The weak vdW forces between GaN and graphene help to reuse an expensive graphene/SiC substrate for multiple transfers and direct bonding to GaN/Si substrate en‐ hanced by atomically smooth-released interface [42]. Compared to the conventional laser liftoff process, this novel technique is more advantageous since the extra surface treatment after releasing GaN layer on graphene is not required for further repetition. In addition, the high roughness of released surfaces out of laser lift-off process does not allow the released layer for direct bonding to other substrates. By using graphene grown on silicon carbide (SiC) substrate, the edged steps on the graphene surface were used to nucleate GaN on the inert graphene (**Figure 7**).

**Figure 7.** (a) Schematic showing the edged steps on the graphene/SiC surface. (b) Schematic of the final structure after GaN growth on graphene/SiC substrate. Reproduced with permission from Ref. [42].

**Figure 8.** (a) AFM image of graphene/SiC surface. (b) SEM image of GaN films grown on graphene by two-step growth (580 and 1150°C). (c) SEM image of GaN films grown on graphene by one-step growth at 1100°C. (d) SEM image of GaN films grown on graphene by modified two-step growth (1100 and 1250°C). (e) AFM image of GaN films using the modified two-step method with RMS roughness of 3 Å. (scale bar, 10 μm). Reproduced with permission from Ref. [42].


**Table 3.** Comparison of GaN/graphene crystalline quality to GaN films grown on other substrates.

Direct QvdWE of high-quality single-crystalline GaN on epitaxial graphene/SiC substrates was performed through an optimized two-step growth temperature. Using low nucleation temperature (580°C) will form 3D-faceted GaN clusters while using one-step growth of high temperature (1100°C) will form continuous GaN stripes aligned along the SiC steps. To obtain a smooth GaN surface, the nucleation preferentially formed at 1100°C along the periodic step edges, that are 5–10 nm height and 5–10 μm apart, then the growth advanced laterally to coalesce at a higher temperature of 1250°C (**Figure 8a**–**d**). The resulted GaN has a root mean square surface roughness of 3 Å (**Figure 8e**). The average dislocation density of the grown GaN on graphene was reported to be ~1 × 109 cm−2, which is comparable to AlN-buffer-assisted GaN films that were grown on other substrates using MOCVD (**Table 3**).

**Figure 7.** (a) Schematic showing the edged steps on the graphene/SiC surface. (b) Schematic of the final structure after

**Figure 8.** (a) AFM image of graphene/SiC surface. (b) SEM image of GaN films grown on graphene by two-step growth (580 and 1150°C). (c) SEM image of GaN films grown on graphene by one-step growth at 1100°C. (d) SEM image of GaN films grown on graphene by modified two-step growth (1100 and 1250°C). (e) AFM image of GaN films using the modified two-step method with RMS roughness of 3 Å. (scale bar, 10 μm). Reproduced with permission from Ref. [42].

**dislocation density (cm−3)**

GaN GaN 0 3 × 106 1.18 34.86 90 [57] GaN Al2O3 14 9 × 108 1.74 34.54 220 [58, 59] GaN/AlN Al2O3 14 6 × 108 2.14 34.22 380 [60] GaN/AlN Si(111) 17 3 × 109 >6.0 34.56 380 [60, 61] GaN/AlN SiC 3 2 × 109 1.80 34.55 200 [62] GaN Graphene/SiC 23 1 × 109 2.98 34.57 222 [42]

**AFM RMS roughness (Å)** **XRD (0002) 2***θ***/ω scan 2***θ* **peak (°) FWHM (arcsec)**

**Threading**

**Table 3.** Comparison of GaN/graphene crystalline quality to GaN films grown on other substrates.

Direct QvdWE of high-quality single-crystalline GaN on epitaxial graphene/SiC substrates was performed through an optimized two-step growth temperature. Using low nucleation temperature (580°C) will form 3D-faceted GaN clusters while using one-step growth of high temperature (1100°C) will form continuous GaN stripes aligned along the SiC steps. To obtain a smooth GaN surface, the nucleation preferentially formed at 1100°C along the periodic step

GaN growth on graphene/SiC substrate. Reproduced with permission from Ref. [42].

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

**Material Substrate Epitaxial**

*Note*: Reproduced with permission from Ref. [42].

**mismatch (%)**

**Figure 9.** X-ray diffraction (XRD) scans of GaN films grown on (a) CVD graphene/SiO2 substrates, (b) grown on exfoli‐ ated graphene layer/SiO2 and (c) grown directly on SiO2. (d) XRD rocking curve of GaN films grown on CVD gra‐ phene/SiO2. (e) ϕ-Scans of GaN films grown on CVD graphene SiO2, on exfoliated graphene layers/SiO2, and directly on SiO2 without graphene layers. Reproduced with permission from Ref. [43].

Chung et al. reported the growth of GaN layer on graphene/fused silica substrate. Compared to the growth of GaN on fused silica directly, GaN grown on graphene using interlayer ZnO nanowalls as a buffer layer showed a significant improvement [43]. To facilitate the growth on graphene, it was exposed to oxygen plasma at 30 W and 100 mT of oxygen for 1 s. This treatment increased the wettability of the graphene surface for a proceeding GaN/ZnO layers. In this study, MOCVD was used to grow ZnO walls first followed by three-step growth of GaN. The height and density of the ZnO nanowalls were 200–400 nm and 1010 cm2 , respectively. The first step of GaN was at a low temperature 560–600°C to prevent the reaction between GaN and underlayer ZnO. Second step was to grow at a higher temperature of 1100°C to promote lateral growth of GaN under hydrogen ambient gas. Finally, the growth at 1200°C was to smoothen the surface and achieve a good-quality GaN layers. The crystal quality of GaN grown on graphene, GaN grown on exfoliated graphene and GaN grown on SiO2 were examined by XRD. XRD spectra of the GaN/graphene peaks correspond to the (002) and (004) orientations of WZ GaN. However, multiple peaks were observed when no graphene layer was used. For GaN films grown on the substrates with CVD graphene films, the FWHM value of the X-ray diffraction rocking curves was as small as 0.8°, as shown in **Figure 9**.
