1 Introduction

The appeal of low dimensional materials, particularly 2-D layered materials, stems from their ability to provide access to new physical phenomena. Additionally, their distinct chemical, mechanical, optical, and electrical properties make them highly desirable from a technological perspective. Graphene, along with other 2-D layered materials like hexagonal boron nitride (h-BN) and recently discovered 2-D nanostructures of Bi2Se3 and Bi2Te3 (known as topological insulator materials), exhibit great potential as candidates for a wide range of nanoelectronics applications in the future [1,2,3,4,5]. The family of transition metal di-chalcogenides (TMDC), which include compounds such as MoS2, WS2, WSe2, and others, is a distinct category of two-dimensional materials that has garnered growing attention within the scientific community [6,7,8,9,10,11,12]. While graphene exhibits the maximum measured mobility of over 100,000 cm2/Vs [13] among this group, its electrical band structure lacks a band gap, making it unsuitable for certain applications. However, this characteristic has paved the way for the exploration of semiconducting 2D materials such as MoS2 and others [14, 15]. The material known as MoS2 has the characteristic of being a semiconductor, and it possesses the quality of tunability of its bandgap based on its thickness. In its bulk form, MoS2 has an indirect bandgap value of 1.2 eV. The bilayer form of MoS2 demonstrates an initial indirect bandgap of 1.3 eV, which transitions to a direct bandgap of 1.8 eV in its monolayer counterpart. Additionally, it possesses an electron affinity of 4.0 eV [16, 17]. In addition, the intrinsic high absorption coefficients, exceeding 5 × 107 m−1 in the visible spectrum, position MoS2 as a material with significant potential for improving the efficacy of optical detection. The photodetection efficiency of 2D photodetectors utilising MoS2 was enhanced at specific wavelength ranges by modifying its exceptional features [18,19,20,21,22,2.3 Fabricated photodiode figures of merit measurements

2.3.1 I-V characteristics of the fabricated pn-photodiode

The primary component of the indoor arrangement for characterizing the IV photodiode is the halogen lamp. The CMP6 pyranometer/reference sensor and the Device Under Test (DUT) are securely mounted on an automated stage. The generated beam was directed towards the CMP6 pyranometer and the DUT in an alternating manner, as shown in Fig. 2. The measurement of the current–voltage (I-V) characteristics of a photodiode involves varying the load voltage while maintaining a constant irradiance (G) of 100 mW/cm2 and a fixed temperature of DUT at 25 °C. The CMP6 pyranometer is a thermopile detector used for measuring irradiance. It is classified as a First-Class instrument according to ISO 9060:1990 standards. The reaction time (63%) of the pyranometer is less than 6 s and it is capable of detecting radiation throughout the spectral range of 285–2800 nm [31]. The direct current voltage was applied to the DUT. The voltage across the DUT and the voltage across a reference shunt resistor of 100 Ω were measured using two Keithley 2010 multimeters in order to obtain the current measurement [32, 33].

Fig. 2
figure 2

IV characteristic system measurements. 1. Halogen lamp; 2. Motorized stage; 3. CMP6 Pyranometer/reference photodetector; 4. DUT; 5. Keithley 2010 (voltage reading); 6. Keithley 2010 (voltage drop across reference shunt resistance); 7. Keithley 2002 for irradiance measurement; 8. dc load power supply; 9. Personal Computer; 10. Power supply for halogen lamp

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2.3.2 Spectral power responsivity.

Figure (3) schematically shows the experimental setup for measuring the spectral responsivity. The fabricated photodiodes spectral responsivity has been evaluated using different Light Emitting diodes (LED) at certain wavelengths (430 nm, 463 nm, 481 nm, 535 nm, 545 nm, 645 nm, 650 nm, 744 nm, 769 nm) as an optical source. As shown in Fig. (3), The reference detector measures the power at the chosen wavelengths; this is followed by the DUT measuring the signal in its focus. DUT measures dark current when the shutter is closed, and when it is opened the signal is measured. By means of a transitional stage the reference detector is allowed to detect the dark current and the signal level at the same wavelength as DUT, at the previously stated wavelengths of LED the process is repeated [34]. DUT spectral responsivity RDUT(λ) has been calculated by analogy with the reference detector [35,36,37], by the aid of (1):

Fig. 3
figure 3

Schematic layout of the experimental setup for spectral responsivity measurements at NIS: (1) Light Emitting Diode (LED), (2) Power supply, (3) Reference detector, (4) DUT, (5) Digital multimeter, (6) (x,y,z) Stage controller, (7) PC software automation

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$${R}_{DUT}\left(\lambda \right)=\frac{{S}_{DUT}\left(\lambda \right)}{{S}_{REF}\left(\lambda \right)}\times {R}_{REF}\left(\lambda \right)\times G,$$
(1)

The symbol SDUT (λ) represents the signal photocurrent of the DUT at a certain wavelength (λ). SREF (λ) denotes the signal photocurrent measured by the reference detector, while RREF (λ) represents the spectral responsivity data of the reference detector in units of amperes per watt (A/W). In our particular situation, the amplifier's gain is denoted by G and has a value of 1. The diameter of the incident beam on the detector was approximately adjusted to 3 mm. The photocurrent response of both the reference detector and the detector under calibration was recorded at each wavelength. The measurements were conducted in the laboratory facilities of NIS, where the temperature of the detector under examination was maintained at 23.0 °C ± 1.0 °C. The temperature sensor included within the transfer detector was utilized to regulate the reading process. The temperature was controlled using the thermoelectric cooler (TEC) and recorded both prior to and following each measurement at a certain wavelength. Adequate time was allowed to ensure that the detector and the device under test (DUT) reached thermal equilibrium.

2.3.3 Response Time measurement

The structure Fig. (4) is intended to obtain the fast response time for an MSM type photodetector by providing a directional current path. A fast response time to light is one of the essential factors for evaluating a photodetector, using a mechanical chopper adjusted at 25 Hz as an optic modulator.

Fig. 4
figure 4

Schematic diagram representing time response measurement 1. Halogen lamp; 2. Power supply 3. optical chopper; (4) Reference detector, (5) DUT, (6) Digital multimeter, (7) (x,y,z) Stage controller, (8) PC software automation

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2.3.4 Quantum efficiency (Q.E.)

Quantum efficiency (Q.E.) is a quantifiable measure of the effectiveness of incident radiant flux in producing measurable quantities, such as temperature or current, within a sensor. The effects under consideration can be categorized as either internal quantum efficiency (IQE) or external quantum efficiency (EQE), depending on the specific portion of light involved in their manifestation. IQE of a photodiode refers to the ratio of the number of electron–hole pairs generated to the number of absorbed photons. On the other hand, EQE represents the ratio of the number of electron–hole pairs formed to the number of incident photons [38]. QE is commonly represented as a mathematical function that relates to certain characteristics such as wavelength, absorption coefficient, frequency, and so on. Furthermore, in the scenario where incident photons are fully absorbed and all resulting minority carriers actively participate in the current generation process at a certain wavelength, QE is considered to be equal to one. The parameters that contribute to QE reduction below unity include optical reflection losses, carrier recombination, defects, passivating layer thickness, and do** level [39].

2.3.4.1 External quantum efficiency \(\left({{\varvec{\eta}}}_{{\varvec{e}}}\right)\)

Incident radiant flux on a photodiode undergoes reflectance, absorbance and transmittance events. The percentage of the incident photons that influence to photocurrent is expressed as \({\eta }_{e}\) [38, 40, 41] and is calculated using Eq. (2),

$${\eta }_{e}=\frac{R\left(\lambda \right)hc}{\lambda e}$$
(2)

where, \(R\left(\lambda \right)\) is the responsivity, h is Planck’s constant, \(\lambda \) is the wavelength, e is the electronic charge

2.3.4.2 Internal quantum efficiency \(\left({{\varvec{\eta}}}_{{\varvec{i}}}\right)\)

Is obtained by the aid of responsivity and reflectance [40] as given by Eq. (3),

$${\eta }_{i}=\frac{{\eta }_{e}}{1-\rho \left(\lambda \right)}$$
(3)

where, \(\rho \left(\lambda \right)\) is the reflectance of the photodiode.

3 Results and discussion:

3.1 Characterization of (DUT) fabricated photodiode

3.1.1 FTIR analysis

Optical characterization of the DUT was performed with FTIR as shown in Fig. 5. FTIR represents different peaks that are attributed to the different layer compositions of DUT. It shows small peak range between 3113 and 3370 cm−1 attributed to Si- wafer substrate [42]. As well silicon lattice phonons assigned at ~ 480 cm−1 [43]. The small peak ranged at ~ 2130 cm−1 attributed to middle Ag-layer [44]. However, the upper layer content MoS2 was attributed by broad band ranged at 860–1300 cm−1 representing stretching vibration bond of MoS2 and two peaks at 1616, 1424 cm−1 which may be ascribed to presence of 1T-MoS2 phase [45].

Fig. 5
figure 5

FTIR results of the DUT fabricated device

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3.1.2 SEM and map**

Scanning electron microscopy (SEM) was utilized to characterize the surface morphology of the investigated films, fig. (6:a→c) shows a representative SEM image of nanostructured MoS2 thin film deposited by RF-magnetron sputtering Also, the fine quality of the deposited MoS2 layer was clearly verified when going down to 1 μm scale bar. Thickness was measured and was shown in Fig. 6b, 6c. SEM investigation also verifies that the film has smooth surface without pinholes, the particles are distributed uniformly throughout the MoS2 thin film, which indicates a homogeneous film surface. RF-sputtering offers a smooth shiny surface observed by the eye. Elemental constituents of the deposited thin film is confirmed by map** analysis Fig. (7) and Table (1). Table (1) shows the concentration percentage of materials in all thin film layers, the concentration depends mainly on the thickness of each layer.

Fig. 6
figure 6

a SEM Image @ 1 μm scale bar showing surface details & quality. b MoS2 thickness. c Ag thickness

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Fig. 7
figure 7

Element Overlay

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Table 1 represents map** smart quant results for the fabricated device as given by the scanning electron microscope
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3.2 Fabricated photodiode performance parameters

3.2.1 I-V characteristics

Figure 8 illustrates the current–voltage (I-V) characteristics of a MoS2 pn-photodiode with a Cu top electrode (referred to as the Cu-device) under reverse bias conditions. The Ag electrode is negatively biased in both the absence of light (dark condition) and when the device is illuminated. It is observed that the magnitude of electric currents is significantly greater when the system is exposed to illumination compared to when it is in a dark environment. As an illustration, when the voltage (Vd) across the device is 3.9 V, the current (Idark) in the absence of illumination is measured to be 0.56 mA. Conversely, when the device is exposed to an illumination intensity of 100 mW/cm2, the current (Iillumination) is found to be 2.54 mA, which is 4.5 times greater than the dark current. The primary factor influencing the current in the presence of light illumination and a specific reverse bias voltage is the density of charge carriers created by photons. In conditions of low light intensities, characterized by a low density of photo carriers, it is seen that only shallow traps are occupied, resulting in a correspondingly low trapped charge density. Consequently, the rate at which the photocurrent increases with increasing light intensity is found to be high. Nevertheless, when the intensity of light grows, the density of photo charge carriers also increases. Consequently, deep traps become occupied, resulting in trapped charge limiting conduction. As a result, the rate at which the photocurrent increases with light intensity is attenuated [46, 47].

Fig. 8
figure 8

MoS2 pn-photodiode I-V characteristics in dark and illumination

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3.2.2 Device spectral responsivity

Photoresponse curve for the fabricated pn-photodiode Fig. (9) is presented and it was found that its maximum responsivity was \(42.21{\text{mA}}/{\text{W}}\) @ 535nm and this is attributed to huge amount of charge carriers accumulated upon the silver layer by surface plasmon resonance (SPR) by the collective oscillation of conduction band electrons that are in resonance with the oscillating electric field of incident light when illuminating the top layer of the upper electrode with the selected wavelengths. This will produce energetic plasmonic electrons through non-radiative excitation that in turn enhances the current flow after the (e–h) pair recombination.

Fig. 9
figure 9

pn-photodiode photoresponsivity

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3.2.3 Time response measurement

The photoresponse time refers to the duration it takes for a photosensitive material or device to exhibit a measurable reaction to incident light. The response of \({\tau }_{response}\) is dependent on the rise time (τrise) and fall time (τfall), which represent the durations for the photocurrent to grow from 0 to 90% of maximum photocurrent and decrease from 100 to 0% of maximum photocurrent accordingly. The measurement of \({\tau }_{response}\) for a thin film pn-photodiode based on MoS2 was conducted at a switching frequency of 3 s and a power density of 0.8 mW/cm2, as depicted in Fig. 10. The obtained value for \({\tau }_{response}\) was determined to be 1.748 ms. The sensitivity of the response time of atomically thin MoS2 to surface interaction and defect states is widely acknowledged in the scientific community. The reduction in carrier lifespan will result in a significant decrease in response time. There is a significant improvement in response time of our device compared to previously prepared MoS2 devices. The rapid reaction of the manufactured pn-photodiode can be attributed to the electron cloud generated by the silver layer underlying the MoS2 layer. This electron cloud generates a significant number of holes, resulting in a downward shift of the Fermi level. In addition, this phenomenon will result in a change in the threshold voltage and a decrease in the carrier lifetime, ultimately leading to the suppression of its inherent n-type characteristics [42].

Fig. 10
figure 10

MoS2 pn-photodiode time response

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3.2.4 Specular reflectance measurement ρ(λ) and Q.E.

Specular reflectance for the fabricated device Fig. (11) was measured using UV–VIS-NIR scanning spectrophotometer (UV-3101PC) and maximum peaks was determined for pn photodiode to be @ 421nm, 653 nm also \({\eta }_{e}\) and \({\eta }_{i}\) for the fabricated photodiode was determined Figs. (12, 13) as the largest E.Q.E was 9.8% @ λ = 535 nm and largest I.Q.E. was 10.5% @ λ = 535 nm.

Fig. 11
figure 11

MoS2 photodiode specular reflectance from the device surface

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Fig. 12
figure 12

pn photodiode external Q.E

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Fig. 13
figure 13

pn photodiode internal Q.E

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4 Conclusion

A photodiode utilising a p–n junction has been successfully manufactured using MoS2 as the semiconductor material which formed by RF-sputtering PVD method yielded 1T metallic phase. This phase has a tetragonal symmetry with octahedral coordination and a one-layer unit cell. 1T MoS2 exhibits great potential as a material for electrical devices and energy conversion processes. A comprehensive evaluation has been conducted to examine the performance and properties of the fabricated device. The measured dark current \(\left({I}_{d}\right)\) at a voltage Vd of 3.9 V was found to be 0.56 mA. The measured value of the illumination current \(\left({I}_{illumination}\right)\) was 2.54 mA. The highest recorded responsivity was 42.21 mA/W at a wavelength of 535nm. Additionally, the measured response time \(\left({\tau }_{response}\right)\) was found to be 1.748 ms, resulting in an external quantum efficiency (Q.E) of up to 9.8% at a wavelength (λ) of 535 nm, and an internal quantum efficiency of up to 10.5% at the same wavelength (λ = 535 nm). The measured transient period was found to be relatively small. This observation was attributed to the building of free electron clouds caused by the surface plasmonic phenomena generated by the Ag thin film layer beneath the MoS2 thin film. This accumulation induces a large number of holes and results in a downward shift of the Fermi level. In addition, this phenomenon will result in a shift in the threshold voltage and a decrease in the carriers’ lifetime.