**5. Applications of QvdWE of III/V semiconductors on layered materials**

Optoelectronic devices have been fabricated based on the integration of III/V semiconductors on 2D materials [26, 44]. In the following, two examples of such devices are presented. The first is the demonstration of a blue light-emitting diode (LED) in which an IBM group suc‐ cessfully reused the same substrate of graphene/SiC multiple times to grow GaN by depositing a nickel (Ni) stressor on top of the GaN that has stronger adhesion to the GaN than the weak vdW attraction to graphene/SiC substrate [42]. Multiple quantum wells (MQW) of InGaN/GaN sandwiched between p-GaN and n-GaN layers were grown to create to the blue LED device. The transmission electron microscope (TEM) image of the LED stack is shown in **Figure 10**(**a**– **d**). The *I–V* curve of the device shows a diodic behavior. Moreover, the electroluminescence spectrum confirming the blue emission at 440 nm of the LED is shown in **Figure 10f**–**g**.

The second application is a solar cell demonstrated by other group used radial and axial junctions of InGaAs/InAs nanowires grown epitaxially on multilayer graphene [63]. This solar cell is based on dense arrays of InGaAs nanowires, where graphene serves as the conductive back contact and growth template for vdW epitaxial assembly of vertical nanowires. In this study, three different junctions are schematically represented in **Figure 11a**–**c**. The first set shows an axial junction of n-InGAs and p-InGaAs doped by Si and Zn, respectively. The second set is fabricated using radial InGaAs p-n junction. Finally, the third set is same as the second but with a shell of p-GaAs passivation layer. The device contacts are shown in **Figure 11d**. Among the three sets, the last one demonstrates a core-shell p-n junction In0.25Ga0.75As nanowire arrays with conversion efficiency of 2.51%. The figures of merits, such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), the power conversion efficiency (PCE), and the ideality factor (*n*) for these three sets of devices are summarized in **Table 4**. Radial junction has higher short circuit currents hinting that carriers generated at various lengths along the NW are effectively swept by the p-n junction built-in electric field. Furthermore, the GaAs passivation shell quenched surface defects and the SRH recombination centers.

height and density of the ZnO nanowalls were 200–400 nm and 1010 cm2

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

diffraction rocking curves was as small as 0.8°, as shown in **Figure 9**.

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

**5. Applications of QvdWE of III/V semiconductors on layered materials**

Optoelectronic devices have been fabricated based on the integration of III/V semiconductors on 2D materials [26, 44]. In the following, two examples of such devices are presented. The first is the demonstration of a blue light-emitting diode (LED) in which an IBM group suc‐ cessfully reused the same substrate of graphene/SiC multiple times to grow GaN by depositing a nickel (Ni) stressor on top of the GaN that has stronger adhesion to the GaN than the weak vdW attraction to graphene/SiC substrate [42]. Multiple quantum wells (MQW) of InGaN/GaN sandwiched between p-GaN and n-GaN layers were grown to create to the blue LED device. The transmission electron microscope (TEM) image of the LED stack is shown in **Figure 10**(**a**– **d**). The *I–V* curve of the device shows a diodic behavior. Moreover, the electroluminescence spectrum confirming the blue emission at 440 nm of the LED is shown in **Figure 10f**–**g**.

The second application is a solar cell demonstrated by other group used radial and axial junctions of InGaAs/InAs nanowires grown epitaxially on multilayer graphene [63]. This solar cell is based on dense arrays of InGaAs nanowires, where graphene serves as the conductive back contact and growth template for vdW epitaxial assembly of vertical nanowires. In this study, three different junctions are schematically represented in **Figure 11a**–**c**. The first set shows an axial junction of n-InGAs and p-InGaAs doped by Si and Zn, respectively. The second set is fabricated using radial InGaAs p-n junction. Finally, the third set is same as the second but with a shell of p-GaAs passivation layer. The device contacts are shown in **Figure 11d**. Among the three sets, the last one demonstrates a core-shell p-n junction In0.25Ga0.75As nanowire arrays with conversion efficiency of 2.51%. The figures of merits, such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), the power conversion efficiency (PCE), and the ideality factor (*n*) for these three sets of devices are summarized in **Table 4**. Radial junction has higher short circuit currents hinting that carriers generated at various lengths along the NW are effectively swept by the p-n junction built-in electric field. Furthermore, the

GaAs passivation shell quenched surface defects and the SRH recombination centers.

, respectively. The first

**Figure 10.** (a) Cross-sectional TEM image shows (p-GaN/MQW/n-GaN) LED stacks grown on a graphene/SiC substrate (scale bar, 1 μm). (b) TEM image of a released LED stack from a graphene/SiC substrate: n-GaN/MQW/p-GaN/Ni. A selected area electron diffraction pattern is displayed in an inset. (c) Schematic of a transferred visible LED device on the tape. (d) I–V curve of a transferred LED stack. (e) Electroluminescence (EL) spectra of a transferred LED stack as a function of injection current. Reproduced with permission from Ref. [42].

**Figure 11.** (a–c) Schematic structure of the three different nanowires based solar cell junctions. (d) Schematic structure of the device including the metallic contacts. Reproduced with permission from Ref. [63].


**Table 4.** The main performance and figure of merits obtained from the NW array on graphene solar cell presented in this study.
