## 3 Results
The high responsivity and high speed of our heterostructure devices are because of the excellent optical and electrical properties of graphene and MoTe\({}_{2}\), as well as the vertical channel of the heterostructure. Compared with photodetectors that have lateral channels, the vertical heterostructure guarantees a large photosensitive area, together with a shorter carrier transit path. Thus, the vertical heterostructure can overcome the common trade-off between high responsivity and high speed of photodetectors. It is worth noting that even higher responsivity can be expected in our vertical heterostructure devices if we use lower incident illumination power, apply larger \(V\), or enlarge the effective photosensitive area. In addition, our vertical heterostructure devices have low energy consumption in view of the ultrahigh responsivity acquired at the low \(V\) (no more than 1 V) and low illumination power (as low as several nanowatts). Moreover, in this study, we demonstrated a practical approach to fabricating photodetector arrays using large-area CVD-grown MoTe\({}_{2}\) and graphene, showing the application potential of our devices in integrated circuits. Figure 4: The Raman and atomic force microscope (AFM) characterizations of the graphene and MoTe\({}_{2}\): (a) the Raman spectra measured at graphene, MoTe\({}_{2}\), and their heterostructure regions, labeled by the green, red, and blue dots, respectively, of a representative device (the upper inset). The white dashed lines delineate the outlines of the bottom (vertical) and top (level) graphene. The light-blue area is MoTe\({}_{2}\). The Raman spectrum of MoTe\({}_{2}\) shows the Raman characteristic peak of 2H-MoTe\({}_{2}\) at 234 cm\({}^{-1}\). The Raman spectrum of graphene consists of the Raman characteristic peaks of graphene at 1593 cm\({}^{-1}\) and 2681 cm\({}^{-1}\). In the heterostructure region, the Raman spectrum shows Raman characteristic peaks of both graphene and MoTe\({}_{2}\). The middle and lower insets show the Raman mapping images acquired at 234 cm\({}^{-1}\) and 2681 cm\({}^{-1}\), respectively, taken in the area labeled by the orange box in the upper inset. The scale bar is 30 \(\upmu\)m. It is generally observed that both the overlapping and non-overlapping regions are uniform. Meanwhile, the Raman intensity of the overlapping region is much (slightly) lower than that of the non-overlapping region for graphene (MoTe\({}_{2}\)); (b) the AFM image of the MoTe\({}_{2}\) and the surface height profile along the white dashed line. The thickness of the MoTe\({}_{2}\) is about 12 nm. The scale bar is 5 \(\upmu\)m.  
Figure 5: The photoresponse performance of device 1: (**a**) the photocurrent (\(l_{\text{ph}}\))–voltage (\(V\)) curves under 1064 nm laser illumination with different illumination power (\(P\)) from 3 nW to 531 nW. Inset: The current (\(I\))–\(V\) curve under dark conditions; (**b**) the \(I_{\text{ph}}\) and illumination power relation (the black line), and the responsivity (\(R\)) and illumination power relation (the green line) measured at \(V=1\) V. The red line is the fitted curve with the equation \(l_{\text{ph}}=a^{\text{p}b}\), where \(a\) and \(b\) are fitted constants; (**c**) the normalized photocurrent waveform under chopped 1064 nm laser illumination. The chopping frequency is 3989 Hz; (**d**) a single normalized response cycle. The rise (fall) time \(t_{\text{r}}\) (\(t_{\text{f}}\)) of 19 μs (34 μs) is acquired. Figure 6: The photoresponse performance of an Al\({}_{2}\)O\({}_{3}\) (20 nm thick)-encapsulated vertical heterostructure device (device 2): (**a**) the \(I_{\text{ph}}\)–\(V\) curves under 1064 nm laser illumination with different illumination power from 3 nW to 531 nW.