MoS2-Based MOS Capacitor for In-Memory Light Sensing
Authors: Kumar Dayanand , Rezk Ayman , Nayfeh Ammar , El-Atab Nazek 
Published: September 2023

Recently, two-dimensional transition metal dichalcogenide (2D-TMD) materials have shown great attention due to their special physical characteristics and their applications in nanodevices [1]. The 2D-TMD materials are possible to develop via chemical vapor deposition (CVD) and physical and chemical exfoliation methods. The mechanical exfoliation method is very promising to the growth of high quality TMDs like MoS2, WS2, ZrS2, MoSe2, NbSe2, and WSe2 [2]. Along with other 2D materials, MoS2 has been used as a channel material in field effect transistors due to its transition from indirect bandgap to direct bandgap. This bandgap is sufficient to balance the limitations of the zero-bandgap graphene for electronic devices [3]. Moreover, researchers have reported that the threshold voltage of the memory or transistor could be shifted when optical light is illuminated. The memory window of the transistor is modulated using the different wavelengths of light. The phenomenon is primarily based on the application of photo-induced charge carriers. The light induced threshold voltage shifting with bias voltage confirms the possibility to use photo-generated carriers in in-memory light sensors [4].
MOS capacitor was fabricated on Si substrate. A 3 nm thick Al2O3 tunneling layer was deposited by plasma enhanced atomic layer deposition (PE-ALD) at 2500 C. Next, a 70-nm thick MoS2 was deposited using the drop casting. A 7 nm thick Al2O3 was again deposited by PE-ALD as a blocking layer at 2500 C. Finally, 50 nm thick Al film was deposited as a top electrode by sputter using a metal shadow mask. The complete fabrication process of the Al/Al2O3/MoS2/Al2O3/P+-Si MOS devices is shown in Fig. 1.
The charging properties of the device were measured, as shown in Fig. 2. The device was operated with sweep voltage of +6/-6 V in forward and reverse conditions, as depicted in Fig. 2 (a). The memory window of the device was examined of about 2.5 V. To confirm the long-term stability, the endurance of the device was measured with the programming and erasing voltages of +6/-6 and -6/+6, respectively (Fig 2 (b)). The device demonstrates good endurance for more than 1000 cycles. We also measured the memory window of the device by increasing the sweeping voltages from +4/-4 to +8/-8 during the programming and erasing conditions, which is depicted in Fig. 2(c). The memory window was increased with increasing the sweeping voltages and the noted maximum was about 5 V at +8/-8 V, suggesting superb charge trapping effect. The optical characteristics of the device were examined by illuminating light onto the device. The optical features of the device were determined using the different wavelength of light from 600 nm to 450 nm. When the device was in dark, the memory window was observed to be about 2.5 V. When the optical light (Intensity: 2 mW/cm2) was turned on for 1 s onto the device with 600 nm wavelength during the programming of +6/-6 V, the threshold voltage was increased from 2.5 V to 3.2 V. We further checked the effect of different wavelengths of light from 550 nm to 450 nm with the interval of 50 nm using the same programming voltage, same intensity, and same illumination time. We got an enormous shift in threshold voltage from 3.2 V to about 5 V. The large threshold voltage shift confirms the MoS2 layer traps more electrons for 450 nm wavelength.
The MOS device displays a decent memory window (2.5 V) and good endurance (1000cycles) without any humiliation. The MOS device reveals a great memory window when the operating voltage was varied from 4/-4 to 8/-8. The memory window of the MOS device was increased from 2.5 V to about 5 V when optical light with different wavelengths was induced onto the device. These decent features of the device make it highly applicable for storage and in memory light sensing in the near future.
References

Konstantatos et al, “Hybrid graphene–quantum dot phototransistors with ultrahigh gain. Nature Nanotech”, vol 7, pp. 363–368, 2012

Shanmugam et al, “A Review of the Synthesis, Properties, and Applications of 2D Materials. Particle, vol 39, 2200031, 2022

F. Mak et al, “Atomically Thin MoS2: A New Direct-Gap Semiconductor”, Phys. Rev. Lett, vol 105, 136805, 2010.

Hu et al, “Dependency of organic phototransistor properties on the dielectric layers”, Appl. Phys. Lett. vol 89, 072108, 2006

Fig. 1. Schematic of complete fabrication process of the MOS device
Fig. 2. (a) C-V curve of the device. (b) Endurance of the device. (c) C-V curve with the sweeping voltages of +4/-4 to +8/-8 in the programming and erasing.

Figure 1