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Abstract
The article delves into the effects of bismuth (Bi) doping on the optoelectronic functionality of cadmium oxide (CdO) thin films and their integration into silicon-based heterojunctions. Key topics include the structural analysis of undoped and Bi-doped CdO thin films, the optical characteristics and band gap variations, and the electrical properties of Al/Bi:CdO/p-Si heterojunctions. The study also explores the photodiode behavior and performance metrics such as rectification ratio, photosensitivity, and detectivity under different light intensities. The findings reveal that Bi doping enhances the optical transmittance and band gap of CdO, leading to improved photoresponse and detectivity in heterojunction devices. The research concludes that Bi-doped CdO thin films show significant potential as efficient electron transport layers and transparent electrodes in next-generation optoelectronic devices.
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Abstract
CdO and bismuth (Bi)-doped CdO (Bi:CdO) thin films were prepared on glass and p-Si substrates using a spin-coating technique. Energy-dispersive X-ray (EDX) spectra confirm the elemental composition of Bi-doped CdO films, revealing the presence of cadmium, oxygen, and bismuth in appropriate atomic percentages. Atomic force microscopy (AFM) analysis indicates that the surface roughness of CdO films can be effectively controlled through Bi doping. X-ray diffraction (XRD) measurements demonstrate that all CdO thin films possess a cubic CdO crystal structure. No diffraction peaks associated with Bi-related impurity phases are observed in the Bi-doped CdO patterns, indicating that Bi incorporation does not alter the CdO crystal structure. Furthermore, variations in the optical band gap (Eg) are observed as a function of Bi doping concentration. The electrical characteristics of Al/Bi:CdO/p-Si heterojunctions with CdO thin films containing 0.0, 0.5, 1, and 2 wt% Bi were investigated using current–voltage (I–V) measurements under dark conditions and various illumination intensities. The results demonstrate that both the undoped and Bi-doped CdO/p-Si diodes exhibit good photodiode behavior. The photodetector achieves a detectivity of 1.15 × 109 Jones, while exhibiting a photoresponsivity in the 10⁻3 A/W range.
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1 Introduction
Over the past years, transparent conductive oxides (TCOs) have been studied due to their wide range of technical applications, such as transparent electrodes in photovoltaic and imaging devices, transparent conductive windows, supercapacitors, ultraviolet semiconductor laser, and sensors. Cadmium (Cd) is highly toxic and still used as important components in battery production, corrosion protection in steel, coloring pigments, and barriers that control neutrons in nuclear fission [1, 2]. The properties of cadmium oxide (CdO) can be modified by adding appropriate dopant atoms with ionic radii that are the same as or smaller than the ionic radius of the Cd ion (ionic radius 0.95 Å), without distorting the CdO lattice [3]. CdO crystallizes in the cubic rock salt (NaCl) structure. The formation of a non-degenerate or degenerate n-type semiconductor may vary depending on the preparation or processing conditions. Ionized donors have an impurity energy level below the conduction band. The impurity energy level can lie in the conduction band in CdO, leading to metallic or quasi-metallic properties [4]. Numerous structures of CdO in nanoscale have been reported as nanowires, nanoparticles nanoneedles and nanocrystals [5].
The size of the additive element and the preparation conditions have a great influence on the electrical resistivity and mobility. The vapor transport method (chemical vapor deposition, CVD) was used to prepare undoped and indium (In)-doped CdO films. The structure, surface, electric, and magnetic properties of the produced thin films were investigated. While the cubic CdO phase was obtained, no peak of In or indium oxide was observed in the In-doped CdO pattern. While the resistance value determined for the undoped CdO films was 1.56 × 102 Ωcm, this value could be reduced to 2.31 × 10–2 Ωcm with In doping [6]. Dakhel [7] prepared Eu-doped CdO thin films (0.4%, 0.5%, 0.8%, and 1.1%) on glass and Si wafer substrates using a vacuum evaporation method. Eu-doped CdO thin films were analyzed by UV–Vis–NIR absorption spectroscopy, X-ray diffraction, X-ray fluorescence and DC electrical measurements. The results indicate that Eu3+ doping slightly stresses the CdO structure and changes the electrical and optical characteristics. The bandgap was narrowed by doping with Eu3+ ions. This was attributed to the bandgap shrinkage effect. Additionally, for 0.8% Eu-doped CdO, its conductivity is increased by 40 times, its mobility is increased by approximately 3.5 times, and its carrier concentration is increased by 11 times compared to the undoped CdO film.
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Bismuth (Bi) consists of 6 coordination numbers and exhibits different oxidation states to be Bi3+—0.103 nm and Bi5+—0.076 nm. The radius of Bi3+ is 0.103 nm [8]. However, Bi can locate or substitute at interstitial sites in CdO lattice. Velusamya et al. [9] synthesised Bi-doped CdO thin films (0.25-, 0.50-, 0.75-, and 1.0-wt% Bi) by pyrolytic spray process. The effects of Bi doping concentration in CdO thin films were systematically investigated from optoelectronic, microstructural, and gas sensing perspectives. Bi ratio that gives the highest response to formaldehyde is determined. All Bi-doped CdO thin films show an improved transmittance, carrier mobility, and an enhanced optical band gap compared to the undoped CdO thin films. CdO nanostructures with various Bi doping levels (1%, 3%, and 5%) were produced by the microwave irradiation technique. It was reported that Bi doping had various effects on the structure, band gap, size-shape of the crystallites, the morphology and antimicrobial behavior of CdO. Bi doping level with smaller particle size causes 78% toxicity for G. positive pathogens and 89% for Gram-negative pathogens [10].
Bi-doped CdO (Bi:CdO) thin films with Bi concentrations of 0.5%, 1%, and 2% were investigated to elucidate their size-dependent efficacy. The cluster size was observed to decrease with increasing Bi content. The corresponding shift in the optical band gap is attributed to the quantum size effect [11]. Cerium (Ce)–CdO nanoparticles for different doping concentrations of Ce are synthesized using jet nebulizer spray pyrolysis (JNPS) technique. The absence of peaks of Ce+3 ions indicates that the CdO crystal structure is not affected by the addition of Ce+3 with a larger radius of 114 pm compared to Cd+ (95 pm) [12]. Lowering the reflection of the film or enhancing the porosity of the film helps to improve the optoelectronic performance of the photovoltaics. Hence, by absorption, current conversion efficiency in photovoltaics is increased due to high light gathering ability and low light scattering ability. Adding Ce to the CdO changes the microstructure and makes the surface more porous and fine-grained. This means that it can be used for optoelectronic devices [13].
The current–voltage (I–V) measurements showed that Al/Cu:CdO/p-Si heterojunctions exhibited photodiode behavior in dark and under various illumination intensities. It was seen that the diode parameters of the Al/Cu:CdO/p-Si heterojunctions could be optimized by suitably choosing the wt% doping [14]. It is considered whether the doping strategy is suitable for Bi ions with radii larger than Cd2+. Previous studies on Bi doping of CdO have investigated the morphological, structural, optical, antimicrobial behavior on human organs and sensing properties that change with Bi dopant content. The photoconductivity mechanism is elucidated by creating a Si-based junction instead of conventional Schottky diodes. It will be interesting to investigate whether Bi:CdO/p-Si heterojunctions can be used for photonic applications over a wide range of solar photon densities. The aim of this work is to investigate the impacts of Bi incorporation by synthesizing CdO thin films with unique properties for electro-optic applications which constitute the novelty of the present work.
In this work, undoped CdO and CdO with various Bi doping levels (0.5%, 1%, and 2%) were synthesised by sol–gel method. Al/CdO/p-Si and Al/Bi:CdO/p-Si heterojunctions were formed, focusing on metal–semiconductor contacts for photodetection and optoelectronic applications. To date, there have been no reports on Al/Bi:CdO/p-Si heterojunctions and the impact of Bi doping, but to the best of our knowledge, there have been a few reports on the structural, morphological, electrical, optical and antimicrobial activity.
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2 Experimental details
The starting materials were cadmium acetate (Cd(CH3COO)22H2O), monoethanolamine (MEA) (C2H7NO), 2-methoxyethanol(C3H8O2), and Bi (from Sigma-Aldrich). The sol–gel process was used for producing CdO. The precursor material was dissolved in 2-methoxyethanol (C3H8O2). Then, bismuth and monoethanolamine were added. The solution molarity is 0.5 mol/L. The prepared solution was stirred for 2 h at 65 °C using a hot plate with magnetic stirrer. Then, solutions were aged at room temperature for 72 h with aim of evaporating all the solvent. CdO thin films were prepared at different proportions of Bi (0.5-, 1.0-, and 2.0-wt% Bi). Glass substrates were cleaned ultrasonically with deionized (DI) water for 15 min with an ultrasonic mixer. They were dried in pure nitrogen gas for 5 min. Then, substrates were immersed in methanol and acetone, respectively, and cleaned ultrasonically at 55 °C for 15 min. Finally, the glass substrates rinsed with DI water were dried in an oven at 90 °C for 1 h. Si wafers (100 ± 0.5° orientation, 350 μm thickness, doping concentration of 2.4 × 1016 cm−3) substrates were cleaned ultrasonically with RCA cleaning processing [10-min bubbling up in NH3 + H2O2 + 6H2O, cleaning ultrasonically with DI water for 5 min, 10 min immersing in HCl + H2O2 + 6H2O at 325 K (60 °C), cleaning ultrasonically with DI water for 5 min]. Finally, the surfaces of the wafers were etched chemically using HF. The cleaned substrates were dried in pure nitrogen gas for 5 min. CdO thin films were coated onto the substrates at 800 and 1400 rpm for 50 s. The process was repeated 5 times. After each step, the sample was heated at 150 °C degrees for 10 min. The samples were annealed at 450 °C for 1 h and allowed to cool to room temperature (to avoid peeling-off or cracking) (Scheme 1). Al contacts were obtained by a molybdenum mask with a circular shaped holes (diameter 1 mm) in a vacuum chamber of pressure about 1.3 × 10–6 Torr. Thus, Al/Bi:CdO/p-Si heterojunction structures were fabricated. Scheme 2 shows a schematic cross-section of the Al/CdO/p-Si structure. The reproducible synthesis using sol–gel method was selected. The surface analysis and the elemental composition were examined using the scanning electron microscopy (SEM) and the energy-dispersive spectroscopy (EDX). The surface morphologies of CdO and Bi:CdO thin films were obtained from atomic force microscopy (AFM) analysis. A thickness of 189 ± 05 nm was estimated from AFM analysis. Optic and crystallographic data of the thin films were provided by the aid of UV–visible (UV–Vis) spectrophotometer (Shimadzu UV-VISNIR 3600) and X-ray diffraction (XRD, Rigaku) with Cu Kα radiation, respectively. Dark and photovoltaic current–voltage (I–V) curves were collected using a Keithley 2400 voltage source.
Scheme 1
Flow chart of synthesis procedure for CdO thin films
3.1 The microstructure analysis of undoped and Bi-doped CdO thin films
Figure 1a shows the SEM images at high and low magnifications for undoped CdO and Bi-doped CdO samples. The images of undoped CdO and Bi-doped CdO samples consist of small and large cauliflower-like nanostructures in a ball-like configurations. Bi-doped CdO thin films exhibit a regular morphology consisting of unequally shaped and non-dense grains. SEM images reveal a three-dimensional architecture with a variation in the component size. The change of doping-induced morphology has been observed previously for Cu-doped CdO nanostructures [15] and Zn-doped CdO nanoparticles [16]. Figure 1b shows the energy-dispersive X-ray (EDX) spectra, verifying the elemental identification of Bi-doped CdO (atomic percentage of cadmium, oxygen, and Bismuth). Peaks due to Bi atoms are observed in the data, confirming that the samples are Bi-doped CdO at different concentrations. Bi ratio (from EDX results) increases as the addition of Bi increases.
Fig. 1
a The SEM images and b EDX results for CdO thin films with various Bi wt% contents (0.0, 0.5, 1.0, 2.0 at.%, respectively)
Figure 2 displays typical AFM measurements of undoped CdO and Bi-doped (Bi concentration: 0.0, 0.5, 1.0 and 2.0 wt%) CdO nanostructures. 2D and 3D images of different surface morphology and roughness are determined by AFM. Bi doping has an effect on the nanostructured surface. The grain size and surface roughness of CdO films are modified by Bi doping. Surface roughness is a component that describes whether the shape of a surface is in its ideal form. The root mean square (Rq) is related to the roughness. The surface roughness is important for the penetration of light due to scattering. AFM gives information about the statistics of the surface heights, such as root mean square roughness (Rq), average roughness of the surface (Ra), peak to valley roughness (Rpv), and average maximum height (Rz). The values of the Ra were determined to be 31.754 nm, 48.895 nm, 56.183 nm, and 25.195 nm for undoped CdO and Bi-doped (Bi concentration: 0.5, 1.0, and 2.0 wt%) CdO nanostructures, respectively.
Fig. 2
The AFM images of CdO thin films with various Bi wt% contents (0.0, 0.5, 1.0, 2.0 at.%, respectively)
Figure 3 shows the Rietveld refinement profile of the X-ray diffraction (XRD) patterns of synthesized undoped and Bi-doped CdO thin films. The XRD patterns have been indexed on the basis of data, which agrees with the JCPDS card no. JCPDS: 05-0640. Three prominent peaks were indexed as (111), (200), and (220) planes (the values of the Bragg’s angle 2θ) corresponding to cubic CdO phases (Table 1). The figure shows that there is no shift toward higher positions for (111), (200), and (220) peaks. This excellent fit is due to the coherent effective ionic radius of Bi (Bi3+~ 0.93 Å) [17] and Cd2+ ion. The peaks corresponding to (111) and (200) planes are more intense in both undoped and Bi-doped CdO [18]. Peak intensity is characterized by full width at half maximum (FWHM). The broadening of the diffraction peaks and the decrease in intensity with increasing Bi content are related to the crystal structure.
Fig. 3
XRD spectra of undoped and Bi-doped CdO thin films deposited on glass substrates
d and a values of undoped and Bi-doped CdO samples for (111) peak are given in Tables 1 and 2. No variation in the lattice constant due to Bi was determined in the XRD patterns, indicating that the addition of Bi does not affect CdO nanocrystalline structure.
Table 2
Some optical, structural, and diode parameters based on undoped and Bi-doped CdO thin films
Bi-doped CdO
(0.0%)
(0.5%)
(1.0%)
(2.0%)
Eg (eV)
2.10
2.34
2.29
2.15
The number of crystallites/m2 (1015)
9.12
6.91
8.04
7.66
Crystallite size (D) (nm)
21.20
23.26
22.11
22.46
Lattice constant (a) (nm)
0.468
0.467
0.467
0.469
Dislocation density (δ) × 1013 lines/m2
2.225
1.848
2.046
1.982
Strain (ε) × 10−3
1.77
1.556
1.637
1.611
BH (eV) TE
0.85
0.74
0.80
0.87
BH (eV) Norde
0.86
0.75
0.80
0.88
n
1.39
1.61
1.36
1.61
RR
1.81 × 104
0.45 × 103
0.12 × 104
7.79 × 104
Rs (kOhm)
2.36 × 103
52.31
242.81
3.6 × 103
D* (Jones)
1.62 × 108
1.15 × 109
1.00 × 108
1.61 × 108
The crystallite size (D) values for the CdO films are estimated using Scherrer’s relation according to the broadening of the peak in crystallography [20]:
where λ (= 1.5418 Å) is the X-ray wavelength and the correlation factor is 0.94. θ is the Bragg/diffraction angle. β is the FWHM measured in radians (the broadening of diffraction line) as follows:
βinst is instrumental broadening and βobs is the value measured (observed) from XRD. The D values are given in Table 2. The crystallite size value increases for 0.5- wt.% Bi doping level compared to that of undoped CdO and then decreases to ~ 22 nm with doping levels of 1- and 2-wt% Bi. The reduction in crystallite size observed in Bi-doped CdO is attributed to the stabilization of crystallites with minimal interfacial energy, whereas the subsequent increase is primarily governed by the disparity in ionic radii between Bi species and Cd2⁺.
The dislocations density (δ) (the unity of lines/m2) and micro-strain (ε) are determined using X-ray analysis [21]:
$$\delta = \frac{1}{{D^{2} }},$$
(4)
$$\varepsilon = \frac{\beta \cos \theta }{4}.$$
(5)
The values of the δ (the unity of lines/m2) and ε are listed in Table 2. Both the δ and ε values decrease with Bi doping concentration. The strain for undoped and Bi-doped CdO samples is on the order of 10−3. The defects and grain size vary depending on the FWHM. The strain is defined as a defect caused by the strain or crystal’s imperfection on the lattice. It can be said that the film quality improves with the addition of Bi in the CdO structure. V-doped CdO thin films were synthesized by a vacuum evaporation method. The structural strain of 4 × 10−4 was determined for CdO with 18.5 wt% V doping level [22]. The dislocation density is 1.3 × 1011 line/m2 for Ga-doped CdO thin films with 3 wt% Ga doping level [23]. Although the Cd2+ ion was replaced by the dopant (Bi3+), which has a larger ionic radius compared to Cd2+, the strain in the CdO lattice is partially reduced by Bi doping. This is attributed to the strain compensation. The strain has been eliminated by the incorporation of boron to SiGe (Si1−x−yGexBy) films [24]. Strain compensation was demonstrated by adding Ge to boron-doped Si [25]. The doping of an element with a larger radius compared to the host structure plays an important role in balancing the compressive stress in the perovskite lattice. The local residual compressive strain for Formamidinium (FA+)-based organic–inorganic hybrid perovskite materials was achieved by the addition of dimethylammonium (DMA+) with a slightly larger radius of 272 pm compared to FA+ (253 pm) [26].
3.2 The optical characteristics of undoped and Bi-doped CdO thin films
Figure 4a,b shows the optical results in the UV–Vis–NIR spectral regions for undoped CdO and Bi-doped CdO films. All samples show a transmittance above 75% in the range of 553–800 nm (Fig. 4a). The transmittance shows reduction in two regions, at approximately 540 nm and 350 nm, relating to different emission bands and transitions. Transmittance decreases with Bi doping (0.5-wt% Bi doping level), especially in the UV region, while it increases exhibiting the highest value in the visible region. The variation in transmittance of Bi-doped CdO compared to undoped CdO was attributed to the density of carriers [11]. Figure 4b shows the absorption spectra as a function of wavelength for undoped CdO and Bi-doped CdO samples. The absorption curves of all samples do not change at long wavelengths (500–800 nm) except for two different regions (480 nm and 330 nm). The absorption increases with Bi (0.5-wt% Bi doping level) doping in the range of 200–490 nm, while it shows larger values for the undoped sample at longer wavelengths. The ionic radius of doping element compared to the host lattice may play a key role in absorbance variation, i.e., substituting Cd2+ ion with the larger ionic radii of dopant (Bi3+) compared to Cd2+. The absorption edge shifts to higher wavelength for CdO prepared by doping with Sb, a semi-metal element like Bi [27]. Figure 4b inset shows the (αhγ)2‐hν plots for undoped CdO and Bi-doped CdO. The band gap energy (Eg) is evaluated by examining the absorption coefficient as a function of photon energy in the high absorption region and determined by the following equation [28]:
$$\alpha h\nu = B(h\nu - E_{{\text{g}}} )^{m} ,$$
(6)
where B is constant, h is the Planck’s constant, ν is the frequency, and m is a constant which determines the transition process of optical absorption. Theoretically, m is equal to 2 for indirect allowed transition and 1/2 for direct allowed transition. The optical band gap values estimated from Fig. 4b inset for CdO thin films are given in Table 2. Low-Eg value of undoped CdO films is in agreement with the Eg value of Ga-doped CdO thin films prepared with vacuum evaporation method by Dakhel [23]. The Eg value in CdO thin films increases initially with Bi doping, but it decreases with further increase in Bi doping content. First, increase in the Eg with Bi doping is in agreement with the variation of band gap value of Bi-doped CdO thin films prepared at a previous work [11]. The spectral lines will shift toward shorter (blue) wavelengths, known as blue shifting, as the doping content increase. Bi doping narrows the band gap of CdO by the formed structural point defects. The energy levels of the point defects are close to or overlapped with the bottom of the conduction band, causing an effective decrease in band gap. The Eg reduces with an increase in the doping concentration of Fe to the TiO2. The red shifts, which known band gap narrowing (BGN) for Fe-doped TiO2 nanoparticles can be attributed to the overlapping conduction bands due to the Ti of TiO2 and 3d electrons of Fe+3 ions, resulting in the decrease of the energy band gap of the TiO2 [29]. As the absorption edge of the doped semiconductor shifts to higher energy, Eg increases due to the filling of all states close to the conduction band. This situation, where the Fermi level rises above the conduction band, is explained by the Burstein–Moss (BM) effect, resulting in increase of the electron carrier concentration (Ne) by Bi doping [30]. The increasing further Bi concentration appears to decrease the Eg. The Burstein–Moss shift is as follows [31]:
where Ne is the carrier density and \(\hbar\) is the reduced Planck’s constant (\(\hbar\) = h/2π). m* is the effective electron mass. \(1/m^{*} = 1/m_{{\text{v}}}^{*} + 1/m_{{\text{c}}}^{*}\); \(m_{{\text{c}}}^{*}\) = 0.21 ± 0.01m0 (m0; free electron mass, mv and mc; the effective mass for valence and conduction band, respectively). The carrier density can be determined by Ne = βIexcitation/hwexcitation, where β is the absorption coefficient. The undoped CdO has the electron concentration as 3.21 × 1018 m−3. The electron concentration is related to the Bi doping level. A high carrier concentration results in an upward of the valence band edge and a downshift of the conduction band edge, showing the band gap renormalization. Therefore, the electron–hole plasma (EHP)-induced band gap renormalization (BGR) effect will reduce the band gap [31‐35].
Fig. 4
The optics spectroscopy results for undoped and Bi-doped CdO thin films a transmittance and b absorbance (inset shows the plots of (αhγ).2 vs. hγ)
3.3 The electrical characteristics of Al/Bi:CdO/p-Si heterojunctions
The energy band diagram of the CdO/Si heterojunction can be constructed using the Anderson model. Electron injection from CdO into p-Si is significantly higher than hole injection from p-Si into CdO, as electrons encounter a smaller potential barrier during charge transfer. Consequently, a potential barrier is formed at the CdO/Si interface [36]. Figure 5(a–d) shows semi-log plots of I‐V characteristics for Al/Bi:CdO/p-Si heterojunctions containing the 0.0-, 0.5-, 1-, and 2-wt% Bi doping. Measurements were carried out in the dark and the light intensity range of 20–100 mW/cm2 in 20 mW/cm2 steps. All heterojunctions appear to exhibit good rectifying behavior. The rectification ratio (RR), is known as the positive ratio of currents of the I‐V data at negative and positive bias voltages, V (RR(V) = − I(− V)/I(V) = − IReverse/IForward). IReverse and IForward are the reverse and forward bias currents, respectively. The RR is 7.79 × 104 at ± 1.7 V for heterojunction with the 2-wt% Bi doping, reaching a high ratio. Al/Bi:CdO/p-Si diode shows a low leakage current of 6.08 × 10–9 A at -1.7 V. The RR is given as a function Bi doping content (Table 2). Although the RR varies depending on the wt% Bi doping, it is clear that there is no particular trend. The rectification ratio is higher than that of the nanocluster n-CdO/p-Si heterojunction diode prepared by the sol–gel method (37 at ± 2 V) [37]. Additionally, the RR is also related to the light intensity.
Fig. 5
The light-dependent I–V characteristics of Al/Bi:CdO/p-Si diodes a 0-, b 0.5-, c 1.0-, and d 2.0-wt% Bi doping
where \(\varphi_{{\text{b}}}\), q, n, Rs, A, k, A*, and T are defined in our previous study [38]. The voltage drop is represented by the V−JRsA term. The barrier height (\(\varphi_{{\text{b}}}\), BH) and ideality factor (n) are given by following equations:
\(\varphi_{{\text{b}}}\) and n are processed by the straight-line region of the log I vs. V plot (the y-axis intercept and the slope, respectively). The values of \(\varphi_{{\text{b}}}\) and n are given in Table 2 for various Bi contents. They vary with Bi doping compared to undoped CdO/p-Si heterojunction, showing some fluctuations. The spatial distribution of interface states also plays a significant role in the observed behavior of the ideality factor. Charge transport through regions with locally reduced Schottky barrier height (SBH) allows electrons to preferentially traverse lower barrier patches, leading to an increased ideality factor and a reduced rectification ratio (RR).
Ideality factor values greater than 1 show the presence of non-thermionic emission mechanism in current transport mechanism [39]. To determine the transport properties of CdO/p-Si heterojunction, the I‐V measurements were plotted on log–log scale (Fig. 6). It has been observed that at low voltages, an ohmic relationship is I α V and the current increases with the relationship I α Vm as the voltage increases. The slope (using a least-square fit) m, which is related to the distribution of trap centers, depends on the injection level. In the region where m > 2, the increase in current is linear, the distribution of trapping centers changes exponentially. This is attributed to a space charge limited current (SCLC) model in which traps are controlled by exponential distribution [40].
Fig. 6
The light-dependent logI–logV graphics of Al/Bi:CdO/p-Si diodes: a 0-, b 0.5-, c 1.0-, and d 2.0-wt% Bi doping
Norde function is applied to the diode structure’s all forward bias I–V data. Thus, it is defined an alternative approach for calculating diode parameters (\(\varphi_{b}\), Rs) [41]:
where \(\gamma\) is the integer greater than n (ideality factor) and dimensionless. Figure 7 shows the plots of F(V) vs. voltage. The \(\varphi_{{\text{b}}}\) and n values for Al/Bi:CdO/p-Si heterojunctions were determined as a function of Bi doping (0.0-, 0.5-, 1.0-, 2.0-wt% Bi) (Table 2). It is estimated that n values varying with Bi content are > 1, indicating non-ideal behavior in the main mechanism of charge transport. For diodes with n > 1, various minority carrier injection models are considered, taking into account the series resistance, interface film layer, inhomogeneous barrier distribution, tunneling effect and interface states, minority carrier injection, and recombination [42‐45]. The ideality factor (n) of the Al/CdO/p-Si heterojunction diode initially increases with Bi doping; however, the CdO/p-Si diode with 1.0-wt% Bi exhibits the lowest n value. Although no monotonic trend is observed with increasing Bi concentration, near-ideal diode behavior is achieved at an optimal Bi content.
The values of the barrier height are higher compared to the values determined on the basis of the thermionic emission (TE) theory. Also, the values of the Rs estimated are listed in Table 2. It is observed that Rs decreases approximately 45 times with the initial doping of Bi. CdO/p-Si with 2.0-wt% Bi doping shows the highest value of Rs, on the order of kΩ. It is observed that the series resistance does not increase proportionally with increasing Bi doping. This behavior is attributed to the influence of interface states, interfacial oxide layers, defect states, and charges trapped at the interface.
There is no significant change in the forward bias current as the light intensity (P) increases. But, an in the reverse bias current increases, exhibiting the photodiode behavior. Figure 8 shows the RR variation as a function of the light intensity. The RR decreases with increasing light intensity. Although the RR value for the Al/CdO/p-Si heterojunction diode initially decreases with Bi doping, further increasing the Bi (2.0 wt%) content appears to increase the RR. The rectification efficiency is related to the turn-on voltage (Von) and the series resistance (Rs). The photodiode sensor converts incident light into an electrical signal. The photosensitivity (PS) as the ratio of the photocurrent to the dark one is defined by the following relation [46]:
where Idark is the dark current and Iph is the photocurrent (Iph = total current I−dark current Id). Figure 9 shows the photosensitivity vs. the light intensity (P) as a function of Bi content. The photosensitivity decreases as Bi content increases (from 0.5 wt% to 1.0 wt%). Further increasing Bi content appears to increase the photosensitivity, attributing to the structural changing. Dark current may arise from leakage currents that are independent of light excitation. The photodiode with higher Bi content exhibits a relatively lower dark current compared to the other devices, resulting in enhanced photosensitivity.
Fig. 8
The RR variation as a function of light intensity for the heterojunction diodes
There is a power law relationship between the photocurrent (Iph) and light intensity (P) [47]:
$$I_{{{\text{ph}}}} = BP^{\alpha } ,$$
(16)
where B is a constant and α is the illumination coefficient (the empirical value). Figure 10 shows the variation in the double logarithmic form of logIph–logP plots. The result of α > 1, < 0.5, and 0.5–1 is attributed to the occurrence of sensitising centres or recombination centers located in the structure due to Bi dopant. The α usually takes value > 1, < 0.5, and 0.5–1, defining the photoconductivity phenomenon and has an impact on interface defect state density [48]. The value of α is 0.54 for heterojunction diode with 1.0% Bi-doped CdO thin films. The photoconductivity mechanism is based on the continuity of localized states in which carrier transport is improved. The value of α is higher than 1 (α > 1) for heterojunction diode with 0.5% Bi-doped CdO thin films. This is attributed to unfilled traps that vary with Bi content [49]. The α values ranging from 0.85 to 1 indicate bimolecular recombination at polymer/fullerene-based solar cells [50].
The responsivity of a photodiode (PR) is defined as the ratio of generated photocurrent and incident/ absorbed light (optical power) [51]:
$$PR = \frac{{I_{{{\text{ph}}}} }}{PA},$$
(17)
where A is the area exposed to the light. Figure 11 inset depicts the photoresponsivity vs. light intensity (P) of Bi:CdO/p-Si diodes as a function of the Bi ratio. The photoresponsivity is often thought of as optical power-dependent quantity. Bi:CdO/p-Si photodiode with 0.5% Bi-doped CdO thin films has the highest responsivity about 5.35 × 10–3 A/W at ~ 1.7 V under the 100 mW/cm2 light power. Bi:CdO/p-Si photodiode shows a good performance with a photoresponse time of ~ 47 ms and a decay time of ~ 1 ms. The result is improved compared to the VO2(B) photodetector, which shows a photoresponse time of 83 ms [52]. The photoresponsivity for photodiode prepared with Cu(II) complex thin layer onto p-Si is about 2 × 10–3 A/W at 3 V under the 100 mW/cm2 illumination [53]. The photoresponsivity for organic semiconductor photodiode based on indigo carmine/n-Si is 1.52 × 10–3 A/W at 3 V [54]. Organic–inorganic heterojunctions are fabricated by forming the thin films of an azo-azomethine-based ligand (H2L) and its transition-metal complexes on n-Si substrate. The photoresponsivity has an optimum value of 11.07 × 10–4 A/W at 2 V at intensity of 100 mW/cm2 [55]. The results are comparable with the literature. It is ideal to operate a photodetector in a spectral region where its sensitivity is close to the possible value. This results in high sensitivity and signal-to-noise ratio, thus leading to low detection noise.
Fig. 11
The PR variation as a function of light intensity for the heterojunction diodes
The detectivity (D*) of the photodiode to the visible light is extracted quantitatively as follows [56]:
$$D^{*} = \frac{R\sqrt A }{{\sqrt {2qI_{{\text{d}}} } }},$$
(18)
where q is the absolute value of electron charge (1.6 × 10–19 Coulombs), A is the surface area, R is the photoresponsivity, and Id is the dark current (very low-optical power). The equation highlights the direct correlation between the dark current and photocurrent and thus to the specific detectivity. Bi:CdO/p-Si photodiode affords a specific detectivity (in Jones) in the order of 109 (Table 2). The detectivity is significantly enhanced through Bi doping compared to undoped CdO, and the results obtained in this work are competitive with those of state-of-the-art photodetectors. Tannic acid (C76H52O46) in n-Si heterojunction is used as adsorbent material for visible and UV lights (365 nm and 395 nm). The maximum detectivity of 3.2 × 109 Jones has been observed at −0.42 V, showing a more stable voltage dependence at higher voltages [57]. Wadsley B-phase vanadium oxide (VO₂(B)) was synthesized using a solution-based method, and its specific detectivity was calculated to be 6.02 × 109 Jones [52]. In addition, several studies have reported the detectivity performance of CdO-based photodiodes. For instance, Ag-doped CdO photodiodes have demonstrated enhanced detectivity values of up to ~ 1.10 × 1011 Jones, indicating substantial improvements in sensitivity through optimized doping [58]. Although focused on CdO:Zn, this study reports a specific detectivity D ~ 4.90 × 109 Jones for a p-Si/CdO–Zn–La photodetector, useful for comparative context [59]. These discrepancies are primarily ascribed to differences in preparation conditions, calculation methodologies, the spatial distribution of interfacial trap states, and the doping levels of acceptor and donor atoms.
4 Conclusion
EDX analysis confirmed the presence and atomic percentages of Cd, O, and Bi, verifying the successful incorporation of Bi into the CdO lattice. Compared with undoped CdO, Bi:CdO films exhibit a highly textured surface morphology, which reduces carrier scattering, enhances light trapping, and leads to improved photoresponse. XRD measurements reveal that all CdO films are highly crystalline and retain the cubic CdO crystal structure, indicating that Bi incorporation does not degrade crystallinity. Optical characterization shows that Bi doping increases the optical transmittance in the visible region (≈495–800 nm). In addition, a pronounced blue shift in the Eg is observed with increasing Bi content, which is attributed to an increase in electron carrier concentration resulting from BM effect. Electrical measurements of Bi:CdO/p-Si heterojunctions demonstrate excellent rectifying behavior, with the RRs on the order of ~ 104 at ~ 1.7 V. Device performance is strongly dependent on Bi doping level and processing conditions, with optimized Bi incorporation leading to significant enhancements in diode characteristics. Under illumination, Bi:CdO/p-Si diodes exhibit markedly enhanced the Iph, PR, and PS. Although Bi doping causes fluctuations in the dark current by one to two orders of magnitude, the devices achieve a high specific detectivity of up to ~ 10⁹ Jones. Overall, the superior properties of CdO and Bi:CdO thin films highlight their strong potential as efficient electron transport layers and transparent electrodes on silicon substrates, making them promising candidates for next-generation optoelectronic devices.
Declarations
Conflict of interest
The authors declare no competing interests.
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