In1 − xGaxAs Double Metal Gate-Stacking Cylindrical Nanowire MOSFET for Highly Sensitive Photo Detector

This paper proposed a highly sensitive Double Metal Gate-stacking Cylindrical Nanowire-MOSFET (DMG CL-NWMOSFET) photosensor by using In1 − xGaxAs. For the best control of short channel effects (SCEs), a double metal gate has been utilized and for efficient photonic absorption, III-V compound has been utilized as channel material. The currently available Conventional Filed-Effect-Transistors (CFET) based photosensor have been used threshold voltage as parameter for the calculation of sensitivity, but in the proposed photosensor, change in subthreshold current has been used as the detecting parameters for sensitivity (Iillumination/Idark). The scientifically electrons study and the photo-conductive characteristics of In1 − xGaxAs CL-NWMOSFET are taken through Silvaco Atlas Tools with two work-functions 4.83 eV and 4.41 eV of metal 1 and metal 2 respectively. After the analysis of In1 − xGaxAs dual Metal Gate Stacking Cylindrical NWMOSFET responds to detectable spectrum (~ 450 nm), incidents light with constant, reversible and fast response by responsivity (4.3 mAW− 1), high Iillumination/Idark (1.36 * 109) and quantum-efficiency (1.12 %). The obtained results of In1 − xGaxAs DMG CL-NWMOSFET based photodetectors have the potential in optoelectronics applications.


Introduction
Nanowires (NW) comprises of Indium gallium arsenide (InGaAs) is favorable semiconductor material due to its excellent properties such as emission wavelength over large spectral ranges, tunable band gap, variable Schottky barrier heights, high saturation velocities, one dimensional conduction, with high absorption coefficients [1][2][3][4] and it additionally gives epitaxial combination on silicon substrate [5][6][7][8]. In current Nanoscale industry, huge number of electronic and photonic devices is designing based on devices such as photonic crystal laser [9], nanolasers [10], light-emitting diodes (LED) [11], and solar cells [12], as well as tunnel diodes [13] and short channel three dimensional (3D) transistors [14]. The photosensor based on Metal Oxide Semiconductor Field Effect Transistor (MOSFET) [15] and Metal Semiconductor Field Effect Transistor (MESFET) have variety of merits such as high input impedance, low power dissipation and low noise as well as its high stability towards temperature as compared to the conventional photodetectors such as pn junction photodiode, avalanche photodiode and pin photodiode. The main drawback of pn junction based photodiode is having lowquantum efficiency, but pin photodiode is faster than pn photodiode with high sensitivity [16], but its speed of operation is limited by transit time (T t ) of photogenerated carriers. Similarly, high sensitivity can be also achieved using Avalanche photodiode, but it faces the problem of higher noise [17][18][19]. Nowadays, CMOS based image sensors is mostly used due to various merits such as random sensor, low power-consumption, and design flexibility [20][21][22]. MOSFET is better as compared to MESFET due to its integration methodology and promising device due low power consumption and low Dark Current (I dark ). Further, dark current can also reduces using non planar devices like Cylindrical (CL) gate MOSFET, because of better gate control as compared to single gate (SG) bulk MOS transistors. This paper presents a highly sensitive Double Metal Gate Stacking Cylindrical Nanowire (NW) MOSFET photosensor by using In 1 − x Ga x As including the Double Metal Gate and III-V compound has been utilized as channel material. The different metal work function has been used for better carrier transport proficiency and designing of CL-NWMOSFET. The higher work-function has been utilized to accelerate the charge carrier in the channel [23][24][25][26] and lower workfunction at drain side for the reduction of peak electric field, which also results in reduced Hot -Carrier-Effect [27]. To fulfill the requirement of effective absorption of light in the desired spectrum, III-V compound material has been used as the channel material such as InGaAs. InGaAs has absorbed light more efficiently, due to director bandgap material as compared to indirect bandgap materials. In this present work, photosensor has been biased in subthreshold area as reported in [28], which results in realizing highly sensitive photodetector based on low power.

Device Structure
The designed 3D view of DMG CL-NWMOSFET structure under incident radiation is shows in Fig. 1a and crosssectional view in Fig. 1b. The dimensional parameters such as channel/gate length (L), source/drain length are taken as 20 nm and diameter (D) taken as 5nm. The maintained the Equivalent Oxide Thickness (EOT) in the DMG CL-NWMOSFET designing, high-k and low-k materials such as HfO 2 and SiO 2 respectively are used for gate stacking [29] as well as the value of EOT is taken as 0.8 nm. The film of III-V material such as In. 53 Ga. 47 is kept lightly doped/undoped for the reduction of mobility degradation. The gate work-function of metal 1 (Φ M1 ), near source at L1 is higher than the gate work function of metal 2 (Φ M2 ), near drain at L2 (Φ M1 > Φ M2 ). The total channel/gate length is equal to summation of L1 and L2.

Model Used
The various models are used for simulation of DMG CL-NWMOSFET, such as Shockey Read Hall (SRH) [30] for generation and recombination of carrier, Bohm Quantum Model (BQP) for quantum mechanical effect, which cannot be ignored in Nanoscale [31]. In BQP model the values of alpha and gamma are taken as 0.5 and 1.2 respectively [32]. The physical parameters of DMG CL-NWMOSFET are illustrates in Table 1.

Method Used
The block method has been used for numerical solution during to simulation process of In 1 − x Ga x As Double Metal Gate Stacking Cylindrical Nanowire-MOSFET.

Calibration with Reported Paper
The simulation results of the In 0 . 53 Ga 0 . 47 As DMG CL-NWMOSFET are validated using Silvaco TCAD simulator  with the reported research paper [33] for different parameters such as channel/gate length (L), oxide thickness (t ox ), gate work function (Φ), radius of Nanowire (R), channel doping (N A ) and source/drain doping (N D + ) are taken as 1 µm, 1.5 nm, 5.05 eV, 7.5 nm, 1e 14 cm − 3 and 5e 17 cm − 3 respectively at V gs -V t = 0.5, where as V gs respresents the gate-source voltage and V t represents the threshold voltage. The extraction of data has been performed by graph digitizer and plotted with simulated results. The result shows that the better agreement with reference [33]. The calibration curve of DMG CL-NWMOSFET with reported data are shown in Fig. 2.
The structure of DMG CL-NWMOSFET has been designed by using Silvaco Atlas 3D simulator. The metal work-function of source side (Φ M1 ) and drain side (Φ M2 ) are taken as 4.83 eV and 4.41 eV respectively. The work-function has been tuned to get required threshold voltage (V t ). The Gate Drive Voltage (Vgt) is the important factor for defining the region of operation, which is equal to Vgt = V gs -V t .

Optical Beam
In order to archive the accurate characteristics of device under dark/ incident radiations are obtained through SILVACO tools (3D). To calculate the photo generation rate at defined meshing points, advance optical device simulator (LUMINOUS-3D) incorporates under the incident radiation with the help of Ray Trace method. The parameters of DMG CL-NWMOSFET which is utilized for simulation work are given in Table 1. The various optical parameters such as wavelength, location and radiation intensity for incident radiation are located by using BEAM keyword, which is incorporated in optical simulator module that is LUMINOUS-3D.

Result and Discussion
To study the performance of In 0 . 53 Ga 0 . 47 As DMG CL-NWMOSFET as a photosensor application, the device has been presented with monochromatic beam of light in vertical nature and corresponding results are recorded. Figure 3 represents the drain current (I d ) variation with respect to gate-source voltage (V gs ) of In 0 . 53 Ga 0 . 47 As DMG CL-NWMOSFET under dark and illuminated conditions with different light intensity and incident power (P i ) respectively, by taking wavelength (λ) is equal to 0.450 µm, EOT = 1.2 nm and N A = 1×10 16 cm − 3 at drain-source  voltage (V ds ) is equal to 0.1 V. If the value of incident power is increases, then the generation of electron-hole pair increases which also increases the conductivity of the channel (In 0 . 53 Ga 0 . 47 As) under incident power (P i ).Due to effect of photo-gating and photo-dopping [34] higher current are obtained under incident radiations. This change in illumination current (I illumination ) with respect to power changes is more suitable in subthreshold area as compared to other regions such as linear and saturation. hence proposed In 0 . 53 Ga 0 . 47 As DMG CL-NWMOSFET works good agreement as a ultraviolet visible photosensor in this area like subthrehold area/region. According to Fig. 3, it is clear that if the power increases from picowatts (pW) towards microwatts (µW), the photocurrent increases and maximum saturation are obtained at P i =16.5 µW. The further increase in power after this value, the performance of the proposed device has been degraded and also consumes more power during their operation. Figure 4 shows the curve between available photocurrent (I ap ) and wavelength (λ) under illumination by taking L = 20 nm, radius of Nanowire (R) = 2.5 nm, N A = 1×10 16 cm − 3 and fixed incident power (P i ) = 16.5 µW. The data extraction has been performed at V gs = 0.0 V, V ds =0.1 V, EOT = 1.2 nm, and obtained maximum available photocurrent (I ap ). According to Fig. 4, the peak value of I ap has observed around λ = 0.450 µm, because of at this λ value maximum light is absorbed from source to channel that is In 0 . 53 Ga 0 . 47 As. Another two important parameters such as Responsivity (Re) and Quantum Efficiency (Qe) [35] can be evaluated by using following formula as shown in Eqs. (1) and (2) Fig. 6 Curve between percentage Quantum Efficiency (Q e ) and Wavelength (λ) of DMG CL-NWMOSFET Figure 5 shows the responsivity (R e ) of the of proposed device with wavelength ranging from 0.0 µm to 1.2 µm by taking same parametric values which are used to calculate the I ap; with different values of work-function for metal 1(Φ M1 ) and metal 2 (Φ M2 ) such as 4.83 eV and 4.41 eV respectively. The peak value of R e has been observed at wavelength (λ) = 0.450 µm and it lies in the visible region of spectrum as shown in Fig. 5. Hence, proposed DMG CL-NWMOSFET works powerfully as a visible photodetector, which is highly recommended for various biological and sensing applications [36,37]. Figure 6 show the curve between percentage Quantum Efficiency (Q e ) and different wavelength (λ) of the channel. It is observed that the value of Q e increases with the increase in λ from 0.2 µm to 0.45 µm. If the value of λ increases from 0.45 µm, then percentage Q e decreases with the increase in λ for R = 2.5 nm at V gs = 0.0 V, V ds =0.1 V. Hence the carefully Quantum Efficiency first increases with increase in wavelengths and then decreases after attaining a maximum value at λ = 0.450 µm for R = 2.5 nm. Hence DMG CL-NWMOSFET can be utilized as a vastly efficient photosensor in visible range of spectrum.
The sensitivity (S) is the important parameter of photodetector and it is defined as the ratio of obtained current under illumination (I illumination ) to dark condition (I dark ) as shown in Eq. (3).
The sensitivity (S) versus incident Wavelength (λ) curve of proposed device under illumination is shown in Fig. 7. The data extraction has been performed at V gs = 0.0 V, V ds =0.1 V, EOT = 1.2 nm with physical parameters of proposed device such as L = 20 nm, radius of Nanowire (R) = 2.5 nm, N A = 1×10 16 cm − 3, Φ M1 = 4.83 eV, Φ M2 = 4.41 eV and fixed incident power (P i ) = 16.5 µW. It is observed that the value of S is small at lower spectrum and maximum at near about 0.45value of λ. After crossing the λ = 0.45, the sensitivity further decreases at higher spectrum. Hence the proposed DMG CL-NWMOSFET can be effectively utilized as a highly sensitive low power photosensor in visible region of spectrum.

Conclusion
The device DMG CL-NWMOSFET has been successfully demonstrated with single In 0.53 Ga 0.47 As Nanowire (NW) and analyses photoelectric performances of the device. The proposed In 1 − x Ga x As Double Metal Gate-stacking Cylindrical Nanowire-MOSFET has efficiently responds to near about 0.45 µm in terms of visible spectrum incident light with stable and fast response. After the variation of different parameters with respect to wavelength, the best optimum value for responsivity (R e ), Quantum efficiency (Q e ) and high photo Sensitivity (S) are observed such as 4.3 mAW − 1 , 1.12% and 1.36x10 9 respectively. The performance and characteristics of DMG CL-NWMOSFET has been better than conventional detectors based on telluride, selenide and sulfide. Although, In 0.53 Ga 0.47 As NW structure based on onedimensional (1D) have been improved significantly, but many challenges have reside. Exact control of NW width, including imperfections, traps and surface states for basically building up reliable devices.
& A double metal gate has been utilized and for efficient photonic absorption, III-V compound has been utilized as channel material. & Proposed photosensor, change in subthreshold current has been used as the detecting parameters for sensitivity (I illumination /I dark ). Cylindrical NWMOSFET responds to detectable spectrum (~450 nm), incidents light with constant, reversible and fast response by responsively (4.3 mAW − 1 ), high I illumination /I dark (1.36 * 10 9 ) and quantum-efficiency (1.12 %). & The results of In 1 − x Ga x As DMG CL-NWMOSFET based photodetectors have the potential in optoelectronics applications.