HIGH TEMPERATURE PHOTOCONDUCTIVITY OF 2D LAYERS OF MoS2

Author

DUSHMANTHA KUMARA DE SILVA GUSTHIGNGNHADURAGE, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2025

Degree Name

Master of Science

Department

Physics

First Advisor

Talapatra, Saikat

Abstract

In recent years atomically thin, layered two-dimensional (2D) Transition Metal chalcogenides (TMDCs) are investigated heavily due to their exotic electronic and optoelectronic behavior. Apart from exhibiting fundamental electronic phenomenon, such as 2D Metal-Insulator transition, several of these materials have shown tunable optical properties that can be utilized for applications such as photo detection. Molybdenum Disulfide (MoS2), a widely researched Transition Metal dichalcogenides (TMDCs) fall in this category. To date multiple studies have shown the potential of these materials to act as efficient Photodetectors, however most of these studies report optoelectronic properties at below or near room temperatures. In this thesis, we mainly focus on high temperature photo response behavior of photodetector devices fabricated using mechanically exfoliated MoS2 layers. Multiple bottom contacted devices were fabricated by transferring a few layers of MoS2 flakes on pre-patterned gold electrodes. The electronic and optoelectronic properties of the exfoliated 2D layers of MoS2 flakes were investigated within the temperature (T) range of (290 K < T < 360 K). Temperature-dependent photoconductivity measurements were performed using a continuous laser source of λ = 640 nm (Energy E = 1.94 eV) over a range of effective illuminating laser power intensities, Peff (0.1 μW < Peff < 0.8 μW). The data obtained was utilized to determine some of the key figures of merit of a photodetector, such as photoconductive responsivity (R) as well as detectivity (D). For the measurements performed on four different exfoliated devices we found that the responsivity values ranged between ~ 0.14 A/W and 1.26 A/W, with corresponding detectivity values ranging between ~ 0.2 x 1010 Jones, and 1.3 x 1010 Jones. These values were significantly improved upon annealing these devices ~ 423 K for 2 hours in an inert gas atmosphere. Following the annealing process on two of such devices, we found that the responsivity improved significantly. The values obtained for, the two annealed devices were found to be 8.88 A/W and 52.67 A/W while the detectivity also increased to 6.1 x 10^10 Jones and 1.8 x 10^10 Jones respectively. Furthermore, a systematic investigation of the variation of photocurrent (I_Ph) as a function of the effective incident light power (P_eff) at various temperatures was thoroughly examined for both the as exfoliated devices as well as the annealed devices. We found that I_Ph follows a power law dependence of the following nature, I_Ph α P_eff^γ , where γ is the power dependence factor. We found that the values of γ ranged between 1.0 – 0.5 and less than 0.5. We found that increase the temperature resulted in a corresponding gradual decrease in γ factor, indicating trap assisted photocurrent generation. These experimental data obtained on the dependence of I_Ph at elevated temperatures for the MoS2 devices studied was analyzed in the light of established theoretical model. The results of these thesis indicate the possibility of developing high temperature photodetector devices based on layered 2D semiconductors.