## 3 Results
Specific detectivity (\(D^{*}\)) is another important parameter for a photodetector, which is used to evaluate the capability of weak light detection. When the shot noise from the dark current is the major source of the total noise, specific detectivity can be expressed as \(D^{*}\) = \(RS^{1/2}\)/(2\(el_{\text{dark}}\))\({}^{1/2}\)[4; 20; 21], where \(e\) is the electron charge. Assuming the shot noise from the dark current is the major noise in our case, the specific detectivity of device 1 was calculated to be 2.26 \(\times\) 10\({}^{9}\) Jones (cm Hz\({}^{1/2}\) W\({}^{-1}\)) at \(V\) = 1 V and 3 nW illumination power. However, it is worth mentioning that, besides shot noise, flicker noise may be of some importance in MoTe\({}_{2}\)-based devices [22]. Apart from responsivity and specific detectivity, response time is also an important parameter for a photodetector. Figure 5c shows the normalized photocurrent waveform under chopped 1064 nm laser illumination. With a chopping frequency of 3989 Hz, the device presented a stable and repeatable photocurrent response. The rise (fall) time \(t_{\text{r}}\) (\(t_{\text{f}}\)) is defined as the time needed for photocurrent to rise (fall) from 10% (90%) to 90% (10%) of the maximum photocurrent in a single response cycle [16; 23; 24; 25]. A rise (fall) time of 19 \(\upmu\)s (34 \(\upmu\)s) can be acquired from Figure 5d. It is worth noting that, considering the intrinsic response time of the mechanical chopping process (\(\sim\)10 \(\upmu\)s) [8], the real response and recovery time of the device should be even shorter. In an attempt to improve the device performance, we deposited a 20 nm thick Al\({}_{2}\)O\({}_{3}\) layer on the as-fabricated devices via the ALD method to protect the devices from ambient. Figure 6a,b show the responsivity-related curves of an encapsulated device (device 2). Similarly, \(I_{\text{ph}}\) shows a saturated trend when illumination power gets higher (the black line in Figure 6b). Moreover, as illumination power increases, the responsivity decreases monotonically (the green line in Figure 6b). Notably, we obtained an ultrahigh responsivity of up to 1752 A/W at 3 nW illumination power. Meanwhile, assuming the shot noise from the dark current is the major noise, the specific detectivity of device 2 was calculated to be 5.68 \(\times\) 10\({}^{9}\) Jones (cm Hz\({}^{1/2}\) W\({}^{-1}\)) at \(V\) = 1 V and 3 nW illumination power. The higher responsivity of device 2 in the lower power range may owe to a passivation effect on the surface state-related recombination centers brought by the Al\({}_{2}\)O\({}_{3}\) layer [20; 26; 27; 28], which consequently reduced the recombination of photogenerated carriers and results in enhanced responsivity. According to the fitting results for \(I_{\text{ph}}\) and illumination power relation, we obtained \(b\) = 0.31 \(\pm\) 0.02 (\(R^{2}\) = 0.998) in the lower power range. The smaller \(b\) value indicates that the trap states play a larger role in the photoconduction gain [29]. Figure 6c,d show the temporal photoresponse performance of the device. The device presented a stable and repeatable photocurrent response under light illumination, with a chopping frequency of 3984 Hz and underestimated rise (fall) time of 16 \(\upmu\)s (42 \(\upmu\)s). Table 1 lists the main performance parameters and corresponding measuring conditions of some reported NIR photodetectors that have similar structures. The maximum responsivity and the response time of our devices are comparable to the best parameters shown in Table 1. Moreover, our devices have higher responsivity and shorter response time simultaneously, whereas only a few other studies have achieved that [2,8]. In our previous research [8], we employed the mechanically exfoliated \(n\)-type MoTe\({}_{2}\), which formed Schottky contact with \(p\)-type graphene. Therefore, the photodetector worked in a photovoltaic mechanism. In this study, we employed the CVD-grown \(p\)-type MoTe\({}_{2}\). The photodetector works in a photoconductive mechanism.