A photodetector based on p-GaN/N-MoS2 QDs heterojunction with high responsivity

Molybdenum disulfide (MoS2) is the most thoroughly investigated for photodetection applications with direct bandgap transition in low-dimensional structures, high light–matter interaction, and good carrier mobility. In this work, MoS2 quantum dots was synthesis by liquid exfoliation and characterized using scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fluorescence emission spectra (FES), UV–vis spectroscopy, and Photoluminescence (PL). The average size is ∼ 3.6 nm with strong absorption in the UV region and a band gap of 4.49 eV. Moreover, a novel structure of N- MoS2 QDs \ p-GaN heterojunction photodetector was deposited by spray coating. The PL of N- MoS2 QDs \ p-GaN emission spectra expanded from UV to visible light with high responsivity to the visible light of 7.06 mA W−1 and detectivity of 1.24 × 1010 jones.


Introduction
Broadband photodetector with high performance is receiving remarkable attention in different fields in scientific and industrial applications such as imaging, optical communications, and sensing.Inorganic semiconductor thin film-based broadband photodetectors are challenging due to the relatively high fabrication and processing costs, slowness responsiveness, layer lattice mismatch, and limited absorption coefficient throughout the whole spectrum.A significant necessity for unique functional materials has led to the existence of developments in the fields of electronics.Two-dimensional (2D) materials have tremendous potential for various applications.An unusual degree of flexibility is possible in 2D materials due to their unique layered structure and remarkable characteristics [1].Graphene and transition metal dichalcogenides (TMDs) series are recently used for optoelectronic devices such as WS 2 , and MoS 2 [2].Molybdenum disulfide (MoS 2 ) is an inorganic compound of the transition metal dichalcogenides (TMDs) family consisting of one Molybdenum atom and two sulfur atoms.MoS 2 abilities and structure made it appealing for a wide range of electronics, sensors, microwaves, and terahertz applications.Moreover, MoS 2 structures are noticeably different from 3D, 2D, 1D, or dot structures Its properties and applications vary depending on its dimensions; it can be semiconductor, metallic, or superconducting.Also, it exists in different 2D structures like nanosheets, nanoribbons, or 1D structures such as nanowires and nanotubes, or 0D structures such as quantum dots and nanoplatelets.MoS 2 has extensive uses in various fields.It has been used in biosensing and bio-applications such as DNA, cancer, and Corona Virus detection [3][4][5][6].Also used in optoelectronic devices like solar cells [7][8][9], and photodetectors [10][11][12].MoS 2 QDs are commonly extracted from MoS 2 crystal or bulk powder by syntheses such as exfoliation with solvent, solvothermal treatment, hydrothermal synthesis, or combining two techniques.The synthesis impacts size, size distribution, and the amount of yield.Exfoliation with solvent is a facile synthesis that sonicates and/or stirs MoS 2 with solvent where organic solvents such as N-Methyl-2-Pyrrolidone (NMP) and dimethylformamide (DMF) are more effective than other solvents where the average size for DMF is 3 nm compared to 7 nm for toluene, followed by sort light and small QDs using centrifugal force where the size of QDs can be easily control by optimize centrifugal speed, for instance, the average particle sizes corresponding to centrifugation speeds of 15000, 14000 and 1200 rpm were 2, 5 and 7 nm respectively [13,14].According to Shengjie Xu et al n-MoS2 QDs were successfully prepared by combining this synthesis with solvothermal treatment where the QDs average size was 3.3 nm [15].To avoid using strong organic solvents, solvothermal treatment is an alternative method to synthesize n-MoS2 QDs by using ethanol and sodium hydroxide (NaOH) presence where an average size of 5.5 nm was achieved [16].Hydrothermal synthesis is a typical synthesis that uses a tofeln lined autoclave reactor.However, the results of this synthesis are low yield and the average size was 2.27 nm [17].Moreover, other n-MoS2 QDs syntheses include chemical exfoliation, thermal ablation, electrochemical, and emulsion method [18].Therefore, the synthesis technique of MoS2 QDs plays a critical factor to fabricate high-performance devices.However, to improve the performances of photodetectors, fabricating a p-n heterojunction is an excellent method to enhance the separation efficiency of photo-generated electron-hole pairs by providing a built electric field.Meanwhile, GaN, one of the third-generation semiconductors employed with MoS 2 , is an appropriate match for PDs due to its large direct bandgap (3.4 eV), excellent radiation hardness, and high conductivity [19].As well as some impressive device performances have been exhibited.In this work, we have successfully fabricated the first MoS 2 QDs based on GaN photodetectors.Electrical properties show N-MoS 2 QDs/p-GaN photodetector with high responsivity and detectivity which makes MoS 2 QDs promising for optoelectronic devices.

Methodology
We have successfully synthesized n-MoS 2 QDs through the liquid phase exfoliation method, figure 1(a) illustrates the steps of MoS 2 QDs synthesis.First, 0.1 g of bulk powder was mixed with 10 ml of DMF (Fisher Scientific U.K) and sonicated using (Grant XUBA 10, 200 W, 38 kHz) ultrasonic bath at room temperature for 3 h.The sonicated solution was again stirred at (90-100-110) °C for 12 h.After that, all samples were cooled at room temperature.Then the mixture was centrifuged for 5000 rpm for 10 min to filter out the sediments.The top 3/4 resultant fine yellow color supernatants containing QD suspensions were used for further characterization and fabrication.Meanwhile, the solution was diluted with DMF in a ratio of 1:3 for UV-vis spectroscopy.Commercial P-GaN film was deposited on a sapphire substrate (Finewinwafers Co, Ltd,) were diced into 1.5 × 1.5 cm 2 .GaN substrate was cleaned through three steps for 30 min long with acetone, ethanol, and DI water.Airbrush spray coater was used for the n-MoS 2 QDs deposition by spraying 10 ml of n-MoS 2 QD for roughly 25 min and under a temperature of 80 °C followed by annealing under 100 °C for 30 min.The thermal evaporation method was utilized for contacting 100 nm Ag and Au electrodes with a shadow mask on the GaN, the Fabrication process is shown in figure 1(b).To clarify the p-n junction electric transport of figure 1(c), shows the device's energy band alignment.The voltage bias is applied to the bottom electrode, where Au contact on the top of GaN film, which is carrier flowing to Ag contact on the top of MoS 2 QDs film.The output curve exhibits excellent conversion characteristics with a threshold voltage of 1 V, enabling carrier transport to occur at a reverse bias.

Structure properties
The Scanning electron microscopy (SEM) images in figure 2

Optical properties
Fluorescence is a significant feature of MoS 2 QDs for promising applications in a broad range of fields.The prepared MoS 2 QDs of size varied from 1.7-6.4nm with an average size of 3.6 nm exhibited fluorescence excitation spectra lie in between 390 nm and 420 nm when excited under 357 nm as shown in figure 4(a).Previous studies of MoS 2 QDs emitted blue color when exposed to UV light at 365 nm [10][11][12], which is consistent with our result shown in figure 4(b).Furthermore, photoluminescence shows emissions spectra of n-MoS 2 QDs in the visible range.While the emission spectra of p-GaN were found at 342 nm as shown in figures 4(c)-(d).Thus, the p-GaN/n-MoS 2 QDs emission was expanded to the whole visible light region in addition to UVA as shown in figure 4(e) with strong intensity in this region at the peak at 550 nm which agreed with expected values corresponding to previous studies, making the photodetector effective for visible light detection [20].The UV-VIS absorption spectrum of n-MoS 2 QD figure 3(f) exhibits a prominent blue shirt and a significant absorption peak at approximately 280 nm is found in the UV region which is similar to that previously described 41 .Hence Tauc plot was used to obtain the band gap of the three samples.The best samples shown at 90 °C have a bandgap of 4.49 eV.However, the absorption peak shows a strong blue shift for n-MoS 2 QDs dissolved in DMF in the literature due to quantum confinement and size [21][22][23].
3.3.I-V characteristic I-V characteristic curves in figure 5(a) show the relationship between the current and voltage applied under the light of intensity = 1000 w/m 2 using sun simulator was done from -2.5 to 2.5 V.The responsivity and detectivity (R) can be calculated using the formulas R= , ∆ where I ∆ is the difference between the photo-current and the dark current, P indicates incident power density, and S is the effective area that is illuminated by the light source.where q is the electron charge ( ´-C 1.6 10 19 ) and J d denotes the dark current density.The responsivity value obtained is R= 7.069 mA W −1 , and the detectivity value D * = 1.24 10 10 Jones.The time response of the p-GaN/N-MoS 2 QDs photodetector is shown in    figure 5(b), which was evaluated by the light pulse (on/off cycles) using a switch, each for 10 s at a reverse bias voltage of 2.5 V.The results indicate rise time = 1.34 s and fall time = 2.08 s that the photodetector has periodic performance repeated with time.

Conclusion
In this study we success synthesis low-cost high quality of n-MoS 2 QDs from bulk MoS 2 powder by utilized liquid phase exfoliation which is recommended to be the most efficient technique.MoS 2 QDs samples were prepared at different temperatures (90-100-110) °C, with 90 °C achieving excellent results with an average size d of 3.6 nm, and band gap of E g = 4.49 eV with high absorbance, and a fluorescence excitation wavelength of = 340 nm.Moreover, the I-V characteristic of an n-MoS 2 QDs/p-GaN photodetector demonstrated a remarkable performance in the visible light region, with a high responsivity of 7.069 mA/W and a detectivity of 1.24 × 10 10 Jones.Temperature and fabrication methods are important factors that influenced both surface morphology and device performance.Based on its exceptional PL results, the n-MoS 2 QDs/p-GaN photodetector is recommended as a UV photodetector.
(a) shows MoS 2 nanosheets resulting of sonication which leads to rupture of large flakes and formation.After stirring for 12 h under optimal temperature the MoS 2 nanosheets are break down, and together with intercalation of solvent exfoliated into QDs.SEM images of MoS 2 QDs film shown in figures 2(b)-(d), with QDs sizes ranging from ∼ 1.7-12 nm with spherical shape and homogenous distribution.The three samples were prepared at different temperatures of (b) 90 °C (c) 100 °C (d) 110 °C.Clearly, figure 2(b) demonstrated the most exceptional results regarding the size and distribution of the quantum dots.However, Poor findings were obtained from figures 2(c)-(d) of samples 100 °C and 110 °C, with higher sizes up to 12.27 nm.The results conclude that temperature gradients may strongly influence quantum dot structure and fabrication during synthesis.n-MoS 2 QDs were also present throughout all three specimens, with approximately the same sizes.To further investigate the size of the QDs, TEM images were carried out and shown in figures 3(a)-(b) with sizes ranging from 1.7 nm−6.4 nm and average size distribution of 3.6 nm confirming of the full exfoliation of all nanosheets.

Figure 5 .
Figure 5. (a) Current as a function of voltage for p-GaN/N-MoS 2 heterojunction photodetector (b) On/Off for p-GaN/N-MoS 2 QDs photodetector.