All-Layer Electrodeposition of a CdTe/Hg_0.1Cd_0.9Te/CdTe Photodetector for Short- and Mid-Wavelength Infrared Detection

Abstract

CdS, CdTe, Hg_0.1Cd_0.9Te, CdTe, and Ag films were progressively electrodeposited on ITO-coated soda–lime glass to manufacture a short- and mid-wavelength infrared photodetector. A distinctive feature of the applied electrodeposition method is the use of a non-aqueous solution containing ethylene glycol (EG) as the electrolyte in a traditional three-electrode configuration for every film deposition. Using EG as a supplementary electrolyte and using the same deposition conditions with a potential below 0.75 V for all film coatings reduces their environmental incompatibility and offers a low-cost and low-energy route for fabricating the reported photodetector. The produced photodetector has a sensitivity of up to ≈957 nm with a detectivity (D*) of 2.86 × 10¹² cm Hz^1/2 W⁻¹ and a dark current density (J_dark) of 10⁻⁶ mA cm⁻². Furthermore, the manufactured photodiode exhibits self-powered performance because V_oc and J_sc are self-generated, unlike previously reported photodiodes. The presented all-layer electrodeposition assembly approach can easily be adapted to fabricate sensing devices for different applications.

1. Introduction

Cadmium chalcogenides (CCs) comprise semiconductors involving elements from groups II and VI of the periodic table. CCs possess a near-ideal direct bandgap, high light sensitivity, and high optical absorption coefficients necessary for low-cost thin-film photodiodes and solar cell applications [1,2,3,4,5,6]. In particular, owing to the bandgap tunability and high sensitivity from shortwave infrared (SWIR) to mid-wave infrared (MWIR) wavelengths, Hg_1−xCd_xTe and CdSe_xTe_1−x ternary compounds are perfect materials for next-generation airspace light detection [7,8] and terrestrial imaging in inclement weather [9]. PN, PIN, and avalanche photodiodes (APDs) are the favored structures for achieving HgCdTe-based light-sensing devices owing to their linear mode photon-counting detection ability, with an efficiency of over 90% within a wavelength range of 0.9–4.3 μm [10,11,12,13]. Many material deposition processes, such as liquid-phase epitaxy (LPE), molecular-beam epitaxy (MBE), metal–organic chemical vapor deposition (MOCVD), photolithography (PHL), and ion implantation, among others, have been combined to obtain these devices. For example, Jin Chen et al. developed a p-type HgCdTe layer doped by Hg vacancies using the liquid-phase epitaxy (LPE) method with a Cd component and boron (B) using ion implantation and an n-type HgCdTe by suppressing Hg vacancies in annealing processes. Au electrodes were deposited using lithography and etching techniques [8]. Dekang Chen et al. developed a charge and multiplied an APD using MBE. A wafer was formed into circular mesa structures using normal photolithography techniques and citric acid wet etching, as was a submicrometer metal grating array on top of the mesa using electron-beam lithography and the metal liftoff procedure [14]. P. Duke Anderson et al. used the LPE approach to generate p-type Cu-doped HgCdTe thin films and metals via plasma etching [15]. Klaudia H. et al. fabricated a p-type HgCdTd photodiode using MOCVD for the barriers and active layer, as well as photolithography and wet chemical etching for the bottom (N+ type) and top (P-type) Au contacts [16]. G. Qin et al. grew high-quality HgCdTe films with a thickness of 7–8 μm using an arsenic implantation approach on CZT(211)B substrates via MBE with a liquid Hg source at a growth temperature of 180 °C ± 1 °C. Using these films, they created a high-performance planar p-on-n HgCdTe infrared photodetector [17], among other things.

Using different combinations of deposition processes and growth conditions to create HgCdTe-sensing devices demands high costs, energy, and time. Furthermore, the environmental incompatibility of wet and dry growth material approaches causes degradation in layer-to-layer adhesion and increases the hydrophobicity of the CCs’ semiconducting materials [18,19]. However, electrodeposition is a straightforward, relatively easy-to-implement approach that does not need specialized, expensive equipment. It can achieve large-scale production using high-purity materials. Electrodeposition produces a coating layer on a base material through the electrochemical reduction of semiconductor/metal ions from an electrolyte. It offers a low-cost and low-energy method for the all-layer deposition of chalcogenide-based photodetectors [20,21,22]. In this study, an all-layer electrodeposition assembly process was used to create an HgCdTe-based photodetector for short- and mid-wavelength infrared detection. The process enabled the electrodeposition of every semiconductor and back metal layer on a glass/ITO-coated substrate. The constructed photodiode structure was glass/ITO/CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe/Ag, wherein the CdS, CdTe, HgCdTe, and Ag films were electrodeposited using a traditional three-electrode configuration from a nonaqueous electrolyte at potentials of 0.65 V, 0.625 V, and 0.75 V, respectively. The manufactured photodetector demonstrated a sensitivity of up to ≈900 nm with a detectivity (D*) of 4.82 × 10¹³ cm Hz^1/2 W⁻¹ and a dark current density of 10⁻⁶ mA cm⁻². The manufactured photodiode indicates self-powered performance, as it generates V_oc and I_sc, unlike other reported photodiodes, proving the viability of this electrodeposition technique for all-layer deposition.

2. Photodetector Architecture

Figure 1a depicts the constructed glass/ITO/CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe/Ag photodetector structure. A transparent glass layer measuring 25 mm × 50 mm × 1.1 mm is the substrate. A 200 nm thick layer of transparent indium tin oxide (ITO) functions as the front contact and n-electrode [23]. The QW photodiode design consists of a 300 nm layer made of a high-photoconductivity n-type CdS semiconductor, serving as a window layer. This layer must have high conductivity to minimize electrical losses [24]. An 84 nm thick layer of p-type CdTe semiconductor material acts as the primary photon absorber. A 6 nm Hg_0.1Cd_0.9Te layer and a 9.91 μm thick CdTe layer act as the QW and back-barrier for the n-p structure, respectively [25]. Next, a layer of Ag serves as the back contact. To establish good ohmic contact with the n-type material, the back contact layer must have a high work function [26]. The 6 nm Hg_0.1Cd_0.9Te layer sandwiched between the two CdTe layers forms a QW heterostructure. Figure 1b displays an energy diagram of the single photodetector, where the QW heterostructure is exhibited in the depletion zones. Additionally, the first and second energy levels in the conduction and valence bands (E_C1, E_C2, E_V1, and E_V2) of the CdTe/Hg_0.1Cd_0.9Te/CdTe QW were obtained by solving the conventional Schrödinger equation using effective mass and envelope function approximations. Hence, the effective or threshold absorption wavelength (λ_eff) of the QD was close to 742 nm. The λ_eff value was theoretically obtained using the equation ${\lambda }_{eff}=hc/[Eg\left({Hg}_{0.1}{Cd}_{0.9}Te\right)+{Ec}_1+{Ev}_1]$, where h is the Plank constant, c is the speed of light, and $Eg\left({Hg}_{0.1}{Cd}_{0.9}Te\right)=1.26 \mathrm{e}\mathrm{V}$ [27,28].

3. Materials and Methods

Electrodeposition from nonaqueous solutions was used to obtain successive thin films of cadmium sulfide (CdS), cadmium telluride (CdTe), mercury cadmium telluride (HgCdTe), and silver (Ag) on soda–lime glass substrates covered with an ITO layer. The fabrication device route includes electrode cleaning, electrolyte preparation, power source connection, and thin film and device characterization. To electrodeposit each semiconductor, different working conditions and chemical reagents were used to create electrolytes. However, ethylene glycol (EG 99%, from Sigma-Aldrich, Saint Louis, MO, USA) was used as a supporting electrolyte in all solutions. For CdS electrodeposition, we made a solution comprising 50 mL of EG, 0.036 M of cadmium chloride (CdCl₂ 99%, from Fermont, Monterrey, NL, Mexico), and 0.102 M of sulfur (S 99%, from Fermont). For CdTe, we required 20 mL of EG, 1 M of CdCl₂, 0.160 M of potassium iodide (KI 99%, from Sigma-Aldrich), and 0.010 M of tellurium chloride (TeCl₄ 99%, from Sigma-Aldrich). For the HgCdTe, the same reagents used to deposit CdTe were used with the addition of 1.99 × 10⁻⁴ M of mercury chloride (HgCl₂ 99.5%, from Sigma-Aldrich) to the electrolyte solution. For Ag, we used 15 mL of EG, 1 M silver nitrate (AgNO₃ 99%, from Sigma-Aldrich), and 1 M sodium nitrate (NaNO₃ 99%, from Sigma-Aldrich). In the electrodeposition of CdS, CdTe, and HgCdTe, a three-electrode setup was used with platinum foil as the counter electrode, a thermometer as the reference electrode, and ITO coated over the glass substrate as the working electrode. In the Ag electrodeposition, the reference electrode was not necessary; consequently, a two-electrode setup was used.

3.1. CdS Electrodeposition

The CdS film was applied to a soda–lime glass substrate coated with ITO. First, the substrate was cleaned in an ultrasonic bath containing acetone and isopropyl alcohol, each for 300 s at a power of 50 W, followed by drying at room temperature. The preparation of the electrolyte solution involved dissolving all reagents in EG, using magnetic stirring on a hot plate (SH-2 from ANPOOZ, Berlin, Gremany) at 40 °C until a homogeneous solution was created. The temperature was then raised to 140 °C, and electrodes were attached to power supply terminals (DP832 from RIGOL, Portland, OR, USA). The electrodeposition process lasted 35 min with a constant current density of 15 mA/cm² at 140 °C. Subsequently, the substrate containing the electrodeposited CdS film was ultrasonically cleaned in isopropyl alcohol for 60 s at 50 W to eliminate any remaining residue. This post-electrodeposition process helped to guarantee the purity and ultimate quality of the CdS film. Figure 2a,b depict the substrate being submerged in the electrolyte and the substrate with the CdS electrodeposited layer, respectively. The electrolyte solution turns brown when additional ions leave the solution to produce the binary compound in the substrate. Figure 2c displays the substrate with the electrodeposited CdS film following the post-electrodeposition ultrasonic cleaning process.

3.2. CdTe/HgCdTe/CdTe Electrodeposition

The CdTe/HgCdTe/CdTe heterostructure was deposited on a previously coated glass/ITO/CdS substrate. The substrate containing coated CdS was cleaned using isopropyl alcohol, and a thermal tape mask was subsequently placed in the area where the CdTe was not required. The electrolyte solution was produced by dissolving all chemicals in EG, using magnetic stirring on a hot plate at 40 °C to ensure a homogeneous solution. In addition, the temperature was raised to 90 °C, the magnetic stirring was stopped, and the electrodes were linked to the source terminals. The electrodeposition time was 20 min, maintaining a constant current density and temperature of 2.5 mA/cm² and 90 °C, respectively. Furthermore, the deposited heterostructure passed through a heat treatment at 180 °C for 300 s to enhance the adhesion between the formed films. The HgCdTe film electrodeposition process was comparable to the process used to deposit CdTe film. The only difference was the addition of 1.99 × 10⁻⁴ M of HgCl₂ to the electrolyte solution with 10 min of electrodeposition time. Once the HgCdTe film was deposited, a second layer of CdTe was also deposited to create a glass/ITO/CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe-type arrangement. Figure 3a–c depict the substrate submerged in the electrolyte, the substrate with the thermal mask out of the electrolyte, and the substrate with electrodeposited CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe films, respectively.

3.3. Ag Electrodeposition

The Ag film was applied following CdTe/Hg_0.1Cd_0.9Te/CdTe heterostructure electrodeposition. A thermal tape mask was collocated to demarcate the area where Ag was placed. To create the electrolyte solution, all reagents were dissolved in EG using magnetic stirring on a hot plate at 40 °C until a homogeneous solution was obtained. Subsequently, both electrodes were submerged in the electrolyte, and the power supply was turned on. The electrodeposition process was 20 min at room temperature, maintaining a constant potential difference of 3 V between the electrodes. Subsequently, a heat treatment at 180 °C for 60 s was performed on the deposited Ag film to enhance the adhesion between the CdTe and Ag layers. The heat treatment improved the bond adhesion between the layers, optimizing the quality and integrity of the depositions. Figure 4a,b depict the coated substrate submerged in the electrolyte and the substrate with electrodeposited Ag film, respectively. An uncolored electrolyte is shown, which does not change when ions leave the solution to form the Ag film in the substrate.

4. Results and Discussion

The chemical content, elemental morphology, and light absorbance of the electrodeposited CdS, CdTe, HgCdTe, and Ag films were investigated using an Empyrean X-ray diffraction system (XRD) from Malvern Panalytical (Cambridge, UK), a Lyra3XMU field emission microscope (EDS) from TESCAN (Kohoutoice, Czech Republic), and a GENESIS^TM UV-vis-NIR scanning spectrophotometer from Thermo Fisher Scientific (Potrero, CA, USA). Figure 5a displays the XRD pattern of the CdS coated on the ITO layer, and the associated structural data are summarized in

Table 1. CdS, CdTe, HgCdTe, and Ag structure parameters (crystalline size, dislocation density, and lattice micro-strain).

Material	2θ (Degree)	(hkl)	FWHM (°)	D (Å)	δ (nm−2)	ε (nm−2)
CdS	26.53	002	1.03	82.60	14.65 × 10−3	19.24 × 10−3
CdTe	22.30	100	0.96	88.00	12.91 × 10−3	21.20 × 10−3
Hg0.1Cd0.9Te	30.00	111	1.60	53.60	34.80 × 10−3	26.00 × 10−3
Ag	38.00	111	0.197	445.2	5.04 × 10−4	2.50 × 10−3

. This figure shows that the significant ITO and the CdS XRD peaks (002) correspond to 2(θ) angles of 29.18° and 26.53°, respectively. The lattice characteristics of CdS are well matched with conventional JCPD (card no. 41–1049) [29]. Additionally, the structure parameter verified that the CdS film is polycrystalline, as explained in Table 1. The crystallite size, D (nm), was calculated using Scherrer’s equation, $D=\lambda K/\beta cos\theta$ [30,31], where K is 0.94 (Scherrer’s constant), λ is 0.15406 nm (wavelength of the X-ray source), β is the full width at half-maximum (FWHM), and θ is the Bragg angle or peak location in radians. The dislocation density (δ) and lattice micro-strain (ε) were calculated using the equations $\delta =1/D^2$ and $\epsilon =\beta /4tan\theta$, respectively [32,33,34]. These findings are consistent with previously published research [35] and with the EDS analysis displayed in Figure 5b, showing no impurity peaks except in a small carbon (C) peak generated by traces of organic chemicals such as EG or CO₂ taken from the surroundings [36,37]. Additionally, the absorbance spectra of the deposited film were measured over wavelengths ranging from 350 nm to 800 nm using the scanning spectrophotometer. An incident light intensity of 100 mW/cm² was established for the measurement using a xenon lamp with appropriate filters, offering good overlap with the standard AM1.5G. The measured absorbance (Figure 5c) was plotted as a function of the wavelength. For CdS, a high absorbance was obtained below 500 nm, and a low absorbance was found at 550 nm. Figure 5d displays an amplification of the deposited CdS film surface. This figure shows a homogeneous deposition without micro-cavities or -cracks, ensuring no leaks during the subsequent film deposition; additionally, the grain shape and grain boundaries of the deposited film are noted.

Figure 6a shows the XRD pattern for the deposited CdTe, suggesting a WZ phase. The intensity of the diffraction peaks at 22.30° (100) and 46.38° (110) can be attributed to the dependence of the Te/Cd ratio during the electrodeposition process [38,39]. Table 1 displays the structural characteristics of the CdTe film, confirming its polycrystalline nature. Figure 6b displays an EDS image of the deposited CdTe film. Similar to that for CdS, this image shows no impurity peaks except in a small carbon (C) peak generated by traces of organic chemicals (such as EG or CO₂) taken from the surroundings. Figure 6c shows the light absorbance of the electrodeposited CdTe, which decays faster than that of CdS for wavelengths less than 500 nm; the PL is centered around 780 nm. Figure 6d displays an amplification of the deposited CdTe film surface. Similar to that for the CdS film, this figure shows a uniform deposition without micro-cavities or -cracks, ensuring no leaks during the subsequent film deposition. Furthermore, the grain shape and grain boundaries of the deposited film are noted. Figure 6e shows the XRD spectra of the Hg_0.1Cd_0.9Te film deposited over the ITO/CdS/CTe substrate. Since the binary compounds CdTe and HgTe possess cubic structures with almost identical lattice constants (a_CdTe = 0.6478 nm and a_HgTe = 0.6465 nm), the diffraction peaks of the ternary HgCdTe exhibit some similarities. The reflection peaks near 2θ = 30.00° (111), 50.00° (220), and 59.50° (311) planes can be attributed to CdTe/HgTe collectively, and the corresponding peak at 2θ = 94.00° can be credited to the (422) planes of HgTe [40]. Figure 6f shows an EDS image of the deposited HgCdTe layer, indicating the presence of Hg, Te, and Cd in the deposited film. Figure 6g displays the light absorbance of the electrodeposited Hg_0.1Cd_0.9Te, which has a maximum absorbance of around 957 nm, agreeing with λ_eff, as determined in Section 2. After the position of its maximum, the absorbance decays faster, and a new peak linked to the HgTe combination arises at about 1300 nm. Figure 6h displays a uniform deposition without micro-cavities or -cracks, ensuring no leaks in the subsequent film deposition. Additionally, the grain shape and grain boundaries of the deposited film are noted, and the crystallinity parameter is summarized in Table 1. Figure 7 shows the surface profile of the deposited Hg_0.1Cd_0.9Te layer. A Bruker Contour GT optical profiler (VSI, Vertical Scanning Interferometry) was used to perform the measurement. Figure 7 shows a surface with an average width of around 6 nm.

Figure 8a displays the XRD patterns of the deposited Ag film. The intensity of the diffraction peaks at both 38.00° (111) and 44.00° (200) exhibits a face-centered cubic structure well matched with the standard JCPDS (card no. 65–2871) [41]. Figure 8b shows an EDS analysis of the deposited Ag film, displaying no impurity peaks except in agreement with previously published findings [42]. Figure 8c depicts the light absorbance of the electrodeposited HgCdTe, which decays quickly between 300 and 400 nm. Figure 8d displays a uniform deposition without micro-cavities or -cracks, ensuring no leaks in the subsequent film deposition. Additionally, the grain shape and grain boundaries of the deposited film can be seen, and the crystallinity parameter is summarized in Table 1.

The photoelectrical properties and spectral response of the fabricated photodiode are shown in Figure 9. The photocurrent density (J_ph) was measured at an illumination power of P_in = 100 mW/cm²; the wavelengths were measured using a spectrometer SP215i from Princeton Instruments (Tenton, NJ, USA); and the optical power was measured using a PM100D from Thorlabs Inc. (Jessup, MD, USA). The fabricated photodiode exhibited a significant detectivity and responsivity window from 1 to 4.5 μm. Beyond 4.5 μm, no significant phenomes were observed. Specifically, the solid and dotted lines in Figure 9a display the spectral detectivity [D*(λ)] and responsivity [R(λ)], respectively. R(λ) and D*(λ) were calculated using Equations (1) and (2), respectively [41,42,43,44].

$$R\left(\lambda \right)={\displaystyle \frac{J_{ph}}{P_{in}}}$$

(1)

$$D^{*}\left(\lambda \right)=R\left(\lambda \right)\times {\left({\displaystyle \frac{rA\left(\lambda ,V\right)}{4kT}}\right)}^{1/2}$$

(2)

$$EQE={\displaystyle \frac{hv}{q}}{R}_{\lambda }$$

(3)

Here, J_ph = I_ph/A, where I_ph is the photocurrent and A is the sensitivity area of the pn junction. k is Boltzmann’s constant, T is the temperature, and rA is the resistance-area product of the photodetector, expressed as rA = (dJ(λ,V)/dV)⁻¹. Maximum detectivity (D*max) was found at peaks 1.28 μm and 2.31 μm at 2.86 and 2.80 × 10¹² cmHz^1/2/W, respectively. Figure 9a shows that the D* spectrum stays nearly constant across a wide range of wavelengths, making the electrodeposited photodetector a solid option for short- and mid-wavelength infrared detection. The maximal responsivity (Rmax) was obtained at a 3.8 μm peak, indicating mid-wavelength infrared detection. Because detectivity was obtained as the inverse of the noise-equivalent power (noise affected detectivity more than responsivity), the discrepancy between the cutoff ranges of detectivity and responsivity can be attributed to weak signals (including noise) detected by the photodiode as frequency increased. The external quantum efficiency (EQE) versus wavelength was calculated using Equation (3) [45,46]. Figure 9b shows the curve of J_ph and the curve of EQE versus wavelength. Figure 9b shows that the EQE is directly proportional to spectral responsivity. A maximal EQE of 35% was obtained around the peak of the maximal R(λ), consistent with their direct relationship.

Using a light source with a wavelength range between 350 and 1700 nm from a 1000 W/m² xenon lamp and a Keithley meter system, the fabricated glass/CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe/Ag photodiode was characterized (at zero-bias). Figure 9b displays the produced J-V transfer curve. The findings indicate that the photodiode operates in self-powered mode without an external power supply. This operation mode shows promise for wireless and autonomous PD applications. The open circuit voltage (V_oc) is 0.91 V, with a short circuit current density (J_sc) of 27.3 mA/cm², providing a power conversion efficiency (η) of 12.57% with a fill factor of 60.57%. The characterization parameters can be attributed to the good crystallinity of the electrodeposited materials and good acquired film interface matching. The good crystallinity of the materials minimizes losses in material interfaces and boosts light absorption and photocurrent generation.

The bandgap (B_g) of the CdTe/Hg_0.1Cd_0.9Te/CdTe QW was determined using the Tauc plot of Figure 9b and the equation α = (K/hv)[hv − E_g]ⁿ. Here, α is the absorption coefficient, K is a constant, hv is the photon energy, E_g is the bandgap, and n is a constant (n equals ½ or 2 for a direct or indirect semiconductor, respectively). In the Tauc plot, the QW arrangement has an absorption linear slope at around 1.36 eV, the effective bandgap of the Hg_0.1Cd_0.9Te, and a second absorption linear slope at around 1.58 eV, the bandgap of CdTe. Hence, the photodetector has the potential for photodetection in mid-wavelength infrared ranges. Furthermore, the manufactured photodetector is appealing owing to its inexpensive construction. The electrodeposition procedures function at a modest temperature, without the high temperature or vacuum-based fabrication steps required by conventional photodetector manufacturing processes.

The device’s performance is summarized in

Table 2. Summary of characteristic performance of the HgCdTe-fabricated photodiode and previously reported HgCdTe-based photodiodes.

Heterojunction	Fabrication Technique	Detection Range (μm)	Reported Performance	Ref.
Hg1−xCdxTe	MBE	0.9–4.3	Photon detection efficiency > 70% Dark count rate of <250 Hz at 110 K	[7]
HgCdTe/CdZnTe	LPE	0.8–5.0	g factor 5.7018 Dmax = 2 × 1014 cm Hz1/2 W−1 at the bias voltage −7.1 V	[8]
HgCdTe/CdZnTe	LPE	0.9–4.3	High dynamic resistance times Active area (R0A) product, 2 × 106 Ω-cm2, Jdark = 4 nA/cm2 Gain > 5500 at −8 V and 80 K.	[11]
HgCdTe/CdTe	LPE	0.4–4.5	Gain of ~6100 (reverse bias of 14.9 V at 1.55 µm illumination) Reduction factor of 4 in ROIC glow-induced dark counts Photoelectron jitter > 1.5 ns	[15]
HgCdTe/CdTe	MOCVD	0.9–8.0	Jdark = 1.06 × 10−3 A/cm3 Dmax = 9.83 × 1011 cm Hz1/2 W−1	[16]
HgCdTe	LPE/MBE	0.9–6.0	Jdark = 1 × 10−4 A/cm3, EQE = 74%	[43]
CdHgTe/CdTe	Electrodeposition	0.9–5.0	Jdark = 1.1 × 10−6 A/cm3 Dmax = 2.86 × 1012 cm Hz1/2 W−1 Voc = 0.91 V Jsc = 27.3 mA/cm2	This work

, including the type of heterojunction, the fabrication method, and the detection range of the manufactured photodiode and other previously published HgCdTe-based photodiodes. The previously published HgCdTe photodiodes were made using various methods (MBE, LPE, and MOCVD) and architectures (pn, pin, and APD) and are employed from the 0.8 to 5 μm region of the spectrum (SWIR, MWIR, and part of LWIR). The most frequently reported performance metrics include gain, GBW, impulse response time, dark current, avalanche gain, etc. Notably, the reported detectivity (D*) and J_dark range from 2.0 × 10¹⁴ to 9.83 × 10¹¹ cmHz^1/2W⁻¹ and 4.0 × 10⁻⁹A/cm³ to 1.06 × 10⁻³ A/cm³, respectively, while the greatest D* and dark density current (J_dark) achieved in this work are 2.86 × 10¹² cmHz^1/2W⁻¹ and 1.1 × 10⁻⁶ A/cm³, respectively. Accordingly, the produced photodiode achieved η > 12 with balance in all its performance parameters. Furthermore, the synthesized HgCdTe-based photodiode exhibits self-powered performance since it produces V_oc and I_sc, in contrast to other documented photodiodes. The demonstrated self-powered performance of the electrodeposited CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe structure can be attributed to its self-powered performance due to the passivation of the thin CdHg_0.4Te_0.6 layer in the implemented heterostructure.

5. Conclusions

In this study, we propose progressively electrodepositing CdS, CdTe, HgCdTe, CdTe, and Ag films on ITO-coated glass as an easy, inexpensive, and efficient method of fabricating a short- and mid-wavelength infrared photodetector. A distinct feature of the proposed method is the use of EG in a non-aqueous solution for every film deposition. Using EG as a supporting electrolyte for all-layer deposition reduces environmental incompatibility and degradation in layer-to-layer adhesion and lessens the hydrophobicity of the deposited materials. Uniform films without micro-cavities or -cracks were obtained, ensuring no leaks in the subsequent film deposition. An analysis of the optical and electrical properties confirmed the functionality of this photodetector. Its effective optical bandgap of 1.2963 eV and threshold absorption wavelength of 0.957 μm can be attributed to the constructed QW structure using the passivation of a thin Hg_0.1Cd_0.9Te layer between CdTe barriers. In addition, the fabricated photodetector has a significant detectivity and responsivity window, from 0.957 to 4.5 μm (from 0.275 to 1.296 eV). Hence, the optical photodetector works for the short- and mid-wavelength infrared ranges.

The deposition time, current density, and temperature control the preparation and coating quality. For CdS deposition, a working time of 35 min with a constant current density of 15 mA/cm² at 140 °C produced the ideal conditions. However, in the CdTe and HgCdTe deposition, 20 min of processing at a steady current of 2.5 mA/cm² at 90 °C was sufficient. To deposit the HgCdTe, 1.99 × 10⁻⁴ M of HgCl₂ was added to the electrolyte solution, unlike the electrolyte preparation used to deposit CdTe. Furthermore, for Ag film deposition, a high temperature was not required; 40 °C during the electrolyte preparation was enough to guarantee the solubility of the reagents in EG. However, it was crucial to maintain a steady potential difference of 3 V between the electrodes. Furthermore, the fabricated photodetector exhibits self-powered performance, as it produces V_oc and I_sc, in contrast to other documented photodiodes. The self-powered performance of the electrodeposited CdS/CdTe/Hg_0.1Cd_0.9Te/CdTe structure can be attributed to the passivation of the thin CdHg_0.4Te_0.6 layer in the implemented heterostructure.