Electrical and photo-electrical properties of MoS2 nanosheets with and without an Al2O3 capping layer under various environmental conditions

Abstract

The electrical and photo-electrical properties of exfoliated MoS₂ were investigated in the dark and in the presence of deep ultraviolet (DUV) light under various environmental conditions (vacuum, N₂ gas, air, and O₂ gas). We examined the effects of environmental gases on MoS₂ flakes in the dark and after DUV illumination through Raman spectroscopy and found that DUV light induced red and blue shifts of peaks (E¹_{2 g} and A_{1 g}) position in the presence of N₂ and O₂ gases, respectively. In the dark, the threshold voltage in the transfer characteristics of few-layer (FL) MoS₂ field-effect transistors (FETs) remained almost the same in vacuum and N₂ gas but shifted toward positive gate voltages in air or O₂ gas because of the adsorption of oxygen atoms/molecules on the MoS₂ surface. We analyzed light detection parameters such as responsivity, detectivity, external quantum efficiency, linear dynamic range, and relaxation time to characterize the photoresponse behavior of FL-MoS₂ FETs under various environmental conditions. All parameters were improved in their performances in N₂ gas, but deteriorated in O₂ gas environment. The photocurrent decayed with a large time constant in N₂ gas, but decayed with a small time constant in O₂ gas. We also investigated the characteristics of the devices after passivating by Al₂O₃ film on the MoS₂ surface. The devices became almost hysteresis-free in the transfer characteristics and stable with improved mobility. Given its outstanding performance under DUV light, the passivated device may be potentially used for applications in MoS₂-based integrated optoelectronic circuits, light sensing devices, and solar cells.

Keywords: 100 Materials; 105 Low-Dimension (1D/2D) materials; 200 Applications; 201 Electronics / Semiconductor / TCOs; 212 Surface and interfaces; 40 Optical; DUV light; MoS2; Raman spectroscopy; electrical property; magnetic and electronic device materials; photoresponse parameters; transition metal dichalcogenides.

Graphical abstract

Figure 1.

Few-layer MoS₂ FET. (a) Optical image of FL-MoS₂ flake serving as a conducting channel in the transistor. (b) Optical image of the final device based on the flake shown in (a). The two contacts were used for electrical transport measurements. (c) Raman spectra of the FL-MoS₂ pristine (black), O₂ treated in the dark (red), N₂ treated in the dark (blue), N₂ treated under DUV (green), and O₂ treated under DUV (pink). (d) Schematic view of the structure of the passivated (10 nm Al2O3 layer) FL-MoS₂ FET under DUV illumination. A FL-MoS₂ was transferred on a p-doped silicon substrate with a 300-nm-thick SiO₂ capping layer. The substrate served as a back gate.

Figure 2.

Electrical transport of FL-MoS₂ FET before passivation. (a) Transfer characteristics of the FL-MoS₂ FET device in the dark. Transfer characteristics were in vacuum, N₂, air, and O₂ environments. (b) Hysteresis in transfer characteristics of the FL-MoS₂ FET device in the dark. Transfer characteristics were obtained under various environmental conditions (vacuum, N₂ gas, air, and O₂ gas). (c) Transfer characteristics of the FL-MoS₂ FET under DUV illumination in vacuum, N₂, air, and O₂ environments. The power intensity and wavelength of light were 11 mW cm^-2 and 220 nm, respectively. (d) Field-effect mobility of the FL-MoS₂ FET device in the dark and in the presence of DUV illumination under various environmental conditions.

Figure 3.

Drain-source characteristics of the device in the dark. The output characteristics (I _D-V _ds) of the FL-MoS₂ FET at fixed V _g in (a) vacuum, (b) N₂, (c) air, and (d) O₂ environments. V _g ranges from –20 V to + 60 V with a step of 20 V.

Figure 4.

Photocurrent response in various environments. (a) Time-dependent photocurrent response of the device in different environments at V _ds = 1 V and V _g = 0 V. DUV light was first turned on for 60 s and then turned off for 60 s. (b) Photocurrent saturation of the FL-MoS₂ FET in vacuum and N₂ gas (left axis and bottom axis), and in air and O₂ gas (right axis and top axis) at (V _ds = 1 V and V _g = 0 V). (c) Saturated photocurrent values in vacuum, N₂ gas, air, and O₂ gas. (d) Modulation of photocurrent response parameters; maximum responsivity (left axis) and detectivity (right axis) in different environments.

Figure 5.

Light detection response. Crucial factors to be calculated in vacuum, N₂, air, and O₂ for (a) external quantum efficiency and (b) linear dynamic range. The data are obtained from Figure 4(b), (c), and (d).

Figure 6.

Photocurrent decaying behavior in different environments. Transient property of FL-MoS₂ FETs under different conditions and data fitting (red line) with a single exponential equation. (a) Decay of drain current in vacuum. (b) Decay of drain current in N₂. The device exhibits a slow decay with a large time constant. (c) Decay of drain current in air and O₂. The drain current decays with a small time constant. (d) Relaxation time constants of the device in vacuum, N₂, air, and O₂.

Figure 7.

Electrical transport measurements of passivated FL-MoS₂ FET. (a) Transfer characteristics of the FL-MoS₂ FET device in the dark. Transfer characteristics are measured in vacuum. The linear scale is on the right axis, and the logarithm scale is on the left axis. (b) Hysteresis in transfer characteristics after passivation. The measurements were performed in the dark under different environmental conditions with V _g from –60 V to +80 V. (c) Transfer characteristics under DUV illumination. The measurements were carried out in vacuum at V _ds = 1 V. The power intensity and wavelength of light were 11 mW cm^–2 and 220 nm, respectively. (d) Field–effect mobility of the passivated FL-MoS₂ FET device in the dark and under illumination. The measurement was conducted in vacuum.

Figure 8.

Photocurrent response. (a) Time-dependent photocurrent response of the pristine and passivated devices under vacuum at V _ds = 1 V and V _g = 0 V. DUV light was initially turned on for 60 s and then turned off for 60 s. (b) Photocurrent saturation of FL-MoS₂ FETs in vacuum for the pristine and passivated devices at V _ds = 1 V and V _g = 0 V. (c) Photocurrent response parameters, including maximum responsivity (left axis) and detectivity (right axis), of the pristine and passivated devices. The data were obtained from Figure 8(b). (d) Decaying behavior of the pristine and passivated devices when the light was turned off after photocurrent saturation. The red line presents the fitting by a single exponent equation.